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D iv isio n o f P ha rm ac olo gy G la xo R es ea rc h In stitu te R es ea rc hTriangle Park N orth Carolina; 2Baylor College of M edicine C enter for
E xp er im e nt al T he ra pe uti cs H o us to n Tex as ;a nd 3 La bo ra to ry of Cell Biology Na tional Institute of M ental H ealth N ational Institutes of HBethesda Maryland
Introduction
R eceptors: an operational definitionFunctional tissue system s
Pharm acological criteria for classification
A . R eco gn itio n1. Antagonists2 . A gon ists
Transduction
M olecular b iology relevant to receptor characterizationB asis for a m olecular nom enclature
Parallel pharm acological and m olecular nom enclaturesSuggested ru les for nam ing receptors
ConclusionsReferences
35 1
0031.6997/92/4403-0351$03.00/0PH A R M A C O L O G IC A L R E V IE W S
Copyright 1992 by The A merican Society for Pharm acology and Experimental Therapeutics
I.
II .III.
IV .
V .V I.
VII.VIII.
IXX
II D efin ition of P harm acological R ecep tors
TER RY P. K ENA K IN , RIC HA RD A. BO ND ,2 A N D TO M I. BO NN ER3
Vol. 44, N o. 3Printed in U S A
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I Introduction
TRADITIONALLY, different receptors have been distin-
guished by drugs (chem icals) for w hich the receptorsvary in their responsiveness. This has led to the gradual
recognition of additional receptors for the sam e ligands.
A s the structures of the ligands have been refined , thishas allow ed the distinguishing of m ultiple subtypes of
receptors. The central problem in identify ing new recep-
tors, thus far, has been in defining the significance of
variations in pharm acological properties, i.e., w hat is the
m inim um difference that is necessary to establish a new
subtype. The recent cloning of num erous receptor m ole-
cules for m any of the m ajor neurotransm itters has m ade
it clear that there are often m ore clearly distinct receptor
m olecules expressed in a single m am m alian species than
can be easily distinguished w ith availab le drugs. A l-
though these m olecules can be expected to eventuallyprovide the basis for identifying the corresponding recep-tors and m ay allow the developm ent of m ore selective
drugs, the current lack of sufficiently selective drugs
m eans that it is d ifficu lt to estab lish the physio logical
functions that m ore selective drugs m ight target.
N evertheless, it is clear that m any m ore such receptorsw ill be identified in the near future and that to m inim ize
confusion in the literature, it w ill be necessary to arriveat a generally accepted defin ition of w hat constitu tes
new receptors, as w ell as a m eans of nam ing them . These
issues have been central to the deliberations of the In-
ternational U nion of Pharm acology C om m ittee on R e-
ceptor N om enclature and D rug C lassification. Inpaper, w e sum m arize the current pharm acological
m olecular approaches to the definition of the criteriaw hich alterations in cellular function resulting frompresence of a ligand can be ascribed to specific recep
for that ligand and present suggestions for the nam
of these receptors.
II R ecep tors A n O perational D efinition
W e w ill discuss som e criteria by w hich alterations
cellular function resulting from the presence of a lcan be ascribed to specific receptors for that ligand
the bio logical system . It w ill be axiom atic that thecriteria for qualification for the operational term recare the functions ofrecognition an d t ransduct ion B y thisdefinition, a receptor m ust recognize a distinct chementity and translate inform ation from that entity i
form that the cell can read to alter its state according
e.g., by a change in m em brane perm eability, activationof a guanine nucleotide regulatory protein, an alterain the transcription of D N A .
Langley, a pioneer in the field of receptor researdefined a receptor (in his w ords, receptive substanc
as being the site of com petition for agonists and aonists (recognition) and the vehicle for the transm issio
of the stim ulus of agonist interaction to the cell (trduction) for the production of ap hysio lo gica l respo nseThe criterion of transduction m ay require furthercussion for processes know n to produce a biochem i
change in a cell w ith an as yet unknow n physio logic
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Am plif ica tion of St imulus
r s s u r Drug
L Affini ty
35 2 K EN A K IN ET A L .
resu ltant. T hese criteria w ould ex clude sites that are
concerned w ith the bio logical disposition of m olecu les
bu t not cellu lar m etabolism (i.e., silent recep tors). In-
cluded in this category w ould be the neuronal up tak e site
for catecho lam ines (targets for tricy clic antidepressants)
and sites such as the clearance recep tors fo r atrial natri-
uretic factor (M aack et al., 1987). For these to be in -
cluded , m odif ications to the criteria of transductionw ould need to be m ade. T his def inition also distinguishes
bind ing sites w ith no k now n phy sio log ical sign if icance
(i.e ., accep tors; Furchgott, 1972 ; A riens, 1984; G reen
and M aay an i, 1987 ; L aduron , 1987) f rom recep tors. T h is
def in ition does not dif ferentiate entities curren tly
thought to be pharm aco log ical recepto rs f rom enz y m es.
T o dif feren tiate these, w e w ould stipu late that the rec-
ognition unit should not chem ically alter the endogenous
ligand. T his w ould separate enz y m e substrates from hor-
m ones, neurotransm itters, and other agon ists for drug
receptors.
I I I . F u n ct ion a l T issu e System s
T he relativ e m erits, strengths, and w eak nesses of d if -
feren t approaches to recep tor pharm aco logy hav e been
debated. In general, m ulticellular sy stem s such as iso-lated tissues hav e the adv an tages resu lting f rom stim ulus
am plif ication m echan ism s, because these allow the quan-
tif ication of agon ist ef f icacy . A lso, they are better p red ic-
to rs of drug response in hum ans. H ow ev er, by their
natu re, these sy stem s necessitate indirect approaches to
receptors that binding does not requ ire. A schem atic of
the relativ e m erits of v arious sy stem s used to study drugs
is g iv en in f igure 1.A relativ ely new dev elopm ent in th is debate is the
discov ery that recepto rs m ay in teract w ith each other on
the lev el o f the cell m em brane by sharing ef fector cou-
p lers. For ex am ple, a com m on coupling G protein m ay
be shared by opioid and a2-adrenoceptors in N G 1O 8-15
cells (L ee et al., 1988), rabb it locus coeruleus neurons
(A ghajanian and W ang, 1987; C hristie et al., 1987), and
rabb it cortex (L im berger et al., 1988). U nder these cir-
cum stances, isolated tissues w ould be better predicto rs
of organ responses to drugs than subcellu lar or reconsti-
tu ted sy stem s. H ow ev er, f rom the point o f v iew of recep-
M em brane S ing le C ell Iso lated In Vivo In Vivo
Fragm ents C ells C ulture T issues (im paired R ef lex es) (Intact)
[I C on tro l D ru g C on ce nt rat io nNo R ef lex O bserv ed
Integratedii I1
M easu re R elativ eEff icacy
M easu re Prim aryResponse
F I G . 1. R ep r esen ta t ion of d iffer en t system s forth e pharmaco log ica l
stu d y of drug r ecep tor s. Som e th eor et ica l ad van ta ges of va r iou s test
sys tems (fr om K en ak in , 1987a).
tor classif ication, these interactions m ay prov e to b
se rio us c om p licatio n.
Perhaps m ore relev an t is the potential fo r isom eriza-
tion of receptors in d if feren t sy stem s (both phy siologica
and artif icial such as ex pression sy stem s) by com pl
form ation w ith m em brane-bound pro teins (i.e ., G
teins). T h is is a w ell-k now n phenom enon for m any
nists, m ak ing them unreliab le tools fo r recep tor classication. T ernary com plex form ation and recep tor iso
eriz ation are assumed not to occur w ith an tagonists.
H ow ev er, antagon ism is a pharm aco log ical def initio
bestow ed upon a drug because it block s the ef fects o
agonist and does no t produce a m easurab le response.
com pletely ru le out recepto r isom eriz ation , it m ust
show n that the ex perim ental sy stem possesses the
ported properties and am plif ication characteristics
dem onstrate low lev els of agon ism . In general, there
no reason a prio ri to assum e that ternary com plex
m ation am ong a ligand, receptor, and G protein w
sum m arily prom ote dissociation of thea j3 and y sub-
un its o f the G pro tein and, thus, rev eal agonism . R ecdata concerning an tagon ists that activ ely destab iliz e
nary com plex form ation (C osta and H erz , 1989 ; Costa
a ., 1992) show that the observ ed af f inity can be grea
dependen t on G proteins in m em brane sy stem s. U n
these circum stances, the env ironm ent of the receptor
determ ine m easured af f in ity , thereby in troducing th
retical and practical p rob lem s w ith the use of such
tagonist af f in ity data for recepto r classif ication . T h
ideas are d iscussed further in relation to the concep ts
negativ e ef f icacy and recep tor precoup ling (v ide inf ra
IV . P h a r m a cologica l C r iter ia for C la ss ifica t ion
A p r er eq u isit e fo r a m ea n in gfu l d iscu ssion of p ossibcriteria for receptor def inition should be the presen tation
of the lim itations of the m ethods used to estim ate
param eters on w hich the criteria are based.
A Recogni t ion
D iscussion of the def in ition of receptors inev itab l
centers on the criteria fo r classif ication of recep tors
M uch has been w ritten about the pharm acological
teria for recep tors, bo th f rom the point o f v iew of bind
and iso lated tissue studies (Furchgo tt, 1972 ; B lack , 1
M olinof f et al., 1981; L aduron , 1984; L aduron, 19
G reen and M aay an i, 1987 ; K enak in , 1987a; N ahorsk i
1987; B raestrup and A ndersen, 1989 ; G reen, 1990).term s of receptor binding stud ies, it is generally agre
that ligands m ust hav e selectiv ity , bind in a satu rab l
m anner, and, w here appropriate , dem onstrate stereose
lectiv ity for the proposed recep tor. It is ex pected t
there be a class of chem icals that bind selectiv elyto
recep tors, in a satu rab le m anner, that can be d isplaced
com petitiv ely by other chem icals or rem ov ed by w ash
w ith drug-f ree m edia. Often added to th is list is the need
for an endogenous agonist in additionto selectiv e b ind ing
w ith foreign ligands. T h is w ould ex clude classif ications
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F I G . 2. Potent ia l Schild regression s an d possible experimental con-
di t ions that w ou ld cau se d ev iat ion from sim p le com p et it iv e b eh av iou r(from Kenakin, 1987b).
PH A RM AC OLO G ICA L R ECEPTO RS 35 3
such as cannabinoid receptors for w hich, currently, there
is no know n endogenous agonist. In such cases, this
condition has less to do w ith definition of the receptorthan w ith its proper identification . A lthough the princi-
p le of nam ing a receptor after its endogenous agonist
should be m aintained w herever possib le, the w ithholdingof receptor status from an entity, w hich otherw ise satis-
fies all criteria for defining a receptor, could be counter-productive. Such receptors should be accepted as re-
ceptors w ith the understanding that the nam e should bechanged w hen the identity of the endogenous ligand
becom es clear. O ne potential error in doing so is that w e
m ight be giv ing tw o nam es to the sam e receptor, if itsendogenous ligand turns out to be one for receptors that
already have been identified. A related error, e.g ., theidentification of subtypes of substance P receptors before
the identification of the related peptides, neurokin in Aand neurokinin B , is that w e incorrectly give one agonist
nam e to a group of receptors that actually has a group
of closely related endogenous agonists. O ccasional errors
of both types are unavoidable in a useful nom enclaturesystem.
The traditional m ethod of receptor classification has
been by the relative potencies and selectivities of antag-onists and agonists. Specifically , the follow ing criteriahave been used: a affin ity of antagonists, b potency
ratio of agonists, c affinity of agonists, and d relative
in trinsic efficacy of agonists. It is w orth considering thesem ethods and how they are used in the receptor classifi-
cation process.1 . Antagon i st s. The m easurem ent of antagonist po-
tency in functional assays by the Schild technique (A run-
lakshana and Schild , 1959) has yielded an abundance ofdata for receptor classification . U nder these conditions,the abscissa in tercept of a Schild regression (under
equilibrium conditions w ith com pounds interacting at asingle site) w ith the antagonist tested over a concentra-
tion range yields an im portant param eter for receptorclassification . If the regression is linear w ith unit slope,th is param eter is defined as an estim ate of the equilib-
rium dissociation constant of the antagonist-receptor
com plex. If one assum es that antagonists bind com peti-
tively to the agonist-binding site, d ifferences in the equi-
librium dissociation constants of antagonist-receptor
com plexes m easured w ith this m ethod can be used todefine receptor subtypes. A suggested standard for theproposal of a novel receptor subtype is for the dissocia-tion constant K B of a com pound (or ideally tw o corn-
pounds w ith chem ically diverse structures) to exhib it a1 log unit or greater d ifference from its value(s) at know n
receptors. In general, the m inim al requirem ents for ac-curate estim ation ofKB values by th is m ethod are as
follows.1 . M easurem ents m ust be m ade under equilibrium
conditions (Furchgott, 1972, 1978). There is m uch data
to show how nonequilibrium conditions can give the
appearance of equilibria and sim ple com petitive antagnism (K enakin, 1987b) yet y ield erroneous estim ates
KB (fig . 2). T hree m ajor objectives in estab lish inglibrium conditions are: a elim ination of sites of losincluding bio logical processes such as uptake sys
and degradative enzym es, as w ell as pro tectionphysicochem ical breakdow n (i.e., au tooxidation of
chols) of the ligands interacting w ith the receptor; bthe elim ination of the possible in terference from en
enously released ligands that interact w ith the recepan d c the achievem ent of tem poral equilibrium .
general, these conditions are of m uch greater concernagonists than antagonists, particu larly w hen using
lated tissue preparations.2 . The Schild regression is based on the assum ptio
that the observed antagonism is of the sim ple com petit
type. U nder these conditions, a slope that is d ifferfrom unity has no m eaning in term s of the chem i
constant of interaction betw een an antagonist areceptor. T herefore, if the slope is not statistically
ferent from unity , it should be constrained to u(M acK ay, 1978) and the in tercept u tilized as an estim
of the KB. This is based on the assum ption thatinfin ite num ber of assays w ould yield a slope exaequal to unity.
3 . The regression should be linear w ith unit slopea considerable concentration range (m inim ally 3100-fo ld w here possib le). There are instances in w
physio logical antagonism (e.g., $-adrenoceptor-m ediatedinhibition of cholinergic contraction in guinea pig
chea; K enakin, 1982) can produce linear Schild re
sions of unit slope. This usually cannot be dem onstrate
over large ranges of concentration.
4 . O ne possible criterion for defin ing receptor diences via Schild analysis is a difference in the eleva
of regression lines, i.e., a difference in location along
elope
= i(Spurious
a . C he mi ca l In te rf er en ce
tk bm p oral lneuilsbriumC A ntag on isl R em ov alS a t u r a t i o n
a. C onco m itant R eceptor an dU pt ak e B lo ck ad e
t Te mp or al l ne gu il sb rs um( di ff us io n l im it in g )
0 123
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354 K EN A K IN ET A L .
log an tagon ist ax is that w ould indicate a dif ference in
KB (S nedecor and C ochran, 1967). B ecause regression
lines are used to characteriz e antagonist po tency , a sta-
tistical com parison w ould rem ov e sub jectiv ity (K enak in
and B lack , 1978). T h is also w ould elim inate character-
iz ations such as receptor sub ty pe X -lik e w hen tw o
regressions are sim ilar but no t co inciden t.
A potential com plicating facto r in S child analy sis isthe case in w hich recep tors are substan tially precoupled
to G proteins in the absence of agon ists (C osta et al.,
1990, 1992). U nder these circum stances, there w ould be
a heterogeneity of b ind ing sites av ailable to the antago-
nist, if the precoupled recep tor has a d if feren t confor-
m ation than the uncoupled one. T his also raises the
specter of negativ e ef f icacy for an tagonists that m ay
stabilize the guanosine diphosphate-G pro tein com plex .
T he potency of such an tagon ists then w ould be sub ject
to the relativ e proportions of recep tor and G protein and,
thus, w ould be tissue dependent (W regget and D eL ean ,
1984; C osta and H erz , 1989 ; C osta et a ., 1992). T he
degree of precoup ling of recep tors can produce a sin istral
shif t o f the S child regression but no detectab le non line-
arity (C osta et a ., 1992). T herefore, no obv ious ind ica-
tion of coupling pro tein ef fects w ould be seen by S child
analy sis. S uch drugs w ould v ary in ef fectiv eness in d if -
feren t sy stem s of v ary ing degrees of receptor precoupling ,
w hich, in tu rn, has been show n to v ary w ith ex perim ental
cond itions such as cellu lar integrity (C osta et a ., 1992).
Presen tly , it is unclear to w hat ex tent th is is a sign if icant
haz ard in classif ication of recep tors w ith an tagonists, but
dif ferences in observ ed antagonist af f inity w ith cellular
integrity and /or cy clic nucleo tide lev els w ould be m di-
cators of a po tential p rob lem .
A lthough a case cou ld be m ade for a purely pharm a-
co log ical def inition of receptors based on the quantitativ e
scale of potency of antagonists, a po int to be considered
is the possibility that dif feren tbinding doma ins o f re ce p-
tors then m igh t be classif ied as un ique receptors. It is
k now n from deletion m utation stud ies that dif ferent
binding dom ains for agonists and antagonists can be
dif feren tiated for agonists and antagonists. T hese data
and that o f others open the possibility that m any drugs
thought to be com petitiv e w ith agonists for com m on
bind ing sites m ay , in fact, be allosteric effectors binding
to sites phy sically rem ov ed f rom the agonist-binding site.
A possib ly usefu l techn ique to dif ferentiate true com -petitiv eness at a com m on site on the receptor and allo-
steric interaction isresultant analysis w ith w hich the
an tagonism of a test antagonist is m easured in con-
junction w ith that of a reference antagonist (B lack et
a ., 1986; K enak in and B eek , 1987; L eff and M orse, 1987).
T h is m ethod is sim ilar to the additiv e dose ratio m ethod
to determ ine com petitiv eness (Paton and R ang, 1965)
but has the added adv an tage of com pensating for sec-
ondary ef fects of the test antagon ist. T his is because the
contro l dose-response curv e is determ ined in the pres-
ence of the test antagon ist, thereby canceling secondar
ef fects on the b lock ade produced by the reference an t
onist thereaf ter. R esultan t analy sis can be used to
ferentiate apparen t sim ple com petitiv e an tagon ism f
allosteric in teraction (K enak in and B oselli, 1989).
T here are data to suggest that the agonist b ind ing
at least som e G pro tein-link ed receptors is con tained
w ithin the sev en m em brane-spanning portions ofreceptors and that these regions are the m ost high
conserv ed in term s of am ino acid hom ology across spe
and tissues. If allosteric ef fectors are used for recepto
classif ication and these drugs b ind to the heterogeneous
outer portions of recepto rs rem ov ed f rom the endogenou
agonist-binding site , then it w ould be possible that
erogeneous b ind ing across species and tissues cou ld
su it not f rom dif ferences in the endogenous agonis
binding dom ain but, rather, f rom dif ferences in alloster
m odulation sites. U nder these circum stances, it w ould
possib le that recep tor heterogeneity w ould not be
tected by the in teraction of the recep tor w ith endogenouagonists but on ly w ith foreign ligands. T hus, it cou ld
conceiv ed that the classif ication of recep tors on the b
of ailosteric sites m igh t open a Pandoras box of b
erogeneity because, conceiv ab ly , recepto rs constructed
v arious cells f rom dif feren t species w ould be d if feren
due to the av ailability of dif feren t m aterials for recept
construction . In practical term s, such a Pandoras b
m ight translate into a cornucopia of po ten tial pharm a
cological selectiv ity . H ow ev er, the danger fo r therapeutic
pharm acology w ould be that structure-activ ity relation
ships, based on such regions in one test sy stem used
screening m olecu les for bio logical activ ity , m ight not
hav e a counterpart in the hum an receptor.
T here now is m ounting ev idence that, ev en w hen
k now n nonequilibrium conditions are ef fectiv ely
traliz ed or allow ed for, there still is considerable heter
geneity in estim ated KB v alues as m easured in a func-
tional sy stem . O ne daunting prospect is the possibility
of v ariable af f inity of ligands for recepto rs resulting f
d if ferences in m em brane com ponen ts (B ev an et
1989). It is w ell k now n that dif feren tial purif ication
receptors can produce dif ferences in binding prof iles
agonists and an tagon ists. O f po tentially greater conce
are data to suggest thatflexibility is a characteristic of
recep tors required for their function w ith respecttransduction. U nder these circum stances, the
arrangem ent of subunit structure w ould be a m ajor
term inan t of ligand af f inity and this, in turn , w ould
sensitiv e to the env ironm ent for that recep tor (W ei
stein, 1987). T his pro tean nature of receptors sugges
that un ifo rm classif ication , based so lely upon liga
recogn ition, w ould be m ore v ariab le than assum ed , if
recep tor w ere sim ply a recognition un it w ith no tran
ducing properties. T he poten tial for negativ e ef f icacy
recep tor precoup ling should introduce caution in assum
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PH A R M A C O L O G IC A L R EC EPT O R S 355
in g that ligand af f inity fo r recepto rs w ill prov ide the go ld
standard for classif ication .
2 . Ag on is ts . R ecep tors also hav e been classif ied phar-
m acologically w ith agonists, but the possib le problem s
with this appr oach ar e com pounded by the addeddrugproperty of intrinsic ef f icacy . S pecif ically , agon ists pos-
sess the properties of af f inity (lik e an tagonists) and also
efficacy, the ability to im part a change of state in therecep tor. H isto rically , agonist po tency ratios hav e been
the estab lished m ethod of receptor classif ication by ag-
onists, the m ost w ell-k now n ex am ple being the classif i-
cation of adrenoceptors in to a and /3 s ub ty pes by A hlqu ist
(1948). T he prem ise is that the potency of an agonist
depends upon the drug factors of intrinsic eff icacy and
af f in ity and, also, the tissue facto rs of recep tor density
and the ef f iciency w ith w hich the receptors are coupled
to ef fecto r m echan ism s. In a giv en tissue in w hich tw o
agonists are com pared directly , the tissue facto rs cancel
and their relativ e potency depends only upon the drug
param eters ef f icacy and af f in ity . U nder these circum -
stances, the relative potency of agon ists becom es a recep-
tor f ingerprint w hich can be utiliz ed in receptor clas-
sif ication. S om etim es, this m ethod is degraded to the use
of the relativ e order of po tency for classif ication, but this
can lead to serious errors. G iv en the prerequisite that
the relativ e potencies of agon ists depend only upon drug-
related param eters, agonist potency ratios should be
useful m easures of recep tor sub ty pes. H ow ev er, recent
ev idence indicates that the m agnitude ofbo th the af f inity
and ef f icacy of agon ists m ay not be independent of tissue
factors.
In term s of the af f inity of agon ists fo r recepto rs, ev en
w hen care is tak en to attain equilibrium conditions iniso lated tissue preparations, theoretical m odeling and
ex perim ental data hav e show n that the scales of potency
and selectiv ity of agonists all can be obfuscated in intact
systems (M acK ay , 1987; K enak in, 1989). If the agonist
induces a change in the recep tor to w hich it is b ind ing
(e.g ., as in an ion channel f rom a closed to an open state,
the form ation of a ternary com plex w ith a m em brane-
coup ling protein ), then the ex isting m ethods for the
estim ation of the af f in ity of agonists fo r recep tors y ield
an apparent af f in ity that m ay characteriz e the com plete
m echan ism of receptor activ ation (C olquhoun, 1987).
T his w ould include m em brane com ponents in add ition
to the receptor, and, therefore, these data w ould not be
useful for recep tor classif ication (M acK ay , 1978; L ef f
and H arper, 1989; L ef f et a ., 1990 ; M acK ay , 1990; K en-
ak in et al., 1990). Errors in the estim ation of agonist
aff inity w ould lead to concom itant errors in agon ist
potency ratios.
B. Tr an sd uctio n
B y def inition , the transduction aspect o f recep to r
m echanism s suggests selectiv e interaction of the recep tor
w ith unique m em brane or cy tosolic com ponents of the
cell that then carry the m essage im parted by the dr
T heoretically , part of the receptor classif ication proce
could include in form ation about the transducing rec
nition sites on the receptor and ev en to the ov erall
observ ed phy sio log ical response. In term s of agon ist
tiv ity , there are tw o possibilities for recepto r classif ica
tion : the type or the magnitude of the response.
T he ty pe of response clearly is inadequate as a toolclassification. D ata f rom a num ber of sources dem on-
strate considerable cross-talk betw een receptors a
ef fecto r sy stem s. It has been show n in reconstitu tion
studies that m any recep tors are capab le of in teracting
w ith m ore than one ty pe of G protein , and there
suggestive ev idence that this m ay occur phy siolog ically
in cells (K enak in, 1988, 1990). C lassif ication by phy sio-
logical function is not practical f rom the poin t o f v
that a g iv en biochem ical p rocess m ay hav e d if feren
ef fects in d if ferent cells depend ing on the processing
the b iochem ical signal. A lso , som e agonists are k now n
be pleiotropic, producing m any biochem ical ef fects
cells that m ay also be d if ferentially coupled to cy toso
processes. T herefore, it is conceiv ab le that agonists
low intrinsic ef f icacy w ould activ ate only the m ost
ciently coupled of these processes and produce a prof i
o f activ ity d if ferent f rom that of a pow erfu l high-ef f icac
agonist. U nder these conditions, agonists o f d if ferin
intrinsic ef f icacy could produce d if ferent pharm acologi-
cal ef fects by activ ating thesame recep tor. For ex am ple,
opioid receptors m ediate the inhib ition of adeny late
clase and the stim ulation of G T Pase in N G 1O 8-15 c
but the latter ef fect is m ore sensitiv e to dim inution
recep tor density (sm aller ef fectiv e receptor reserv
(C osta et al., 1988). Presum ably , the stim ulation oG T Pase also w ould be less sensitiv e to activ ation by
ef f icacy agonists. G iv en these constrain ts, classif ication
by ef fector w ould seem to be unsu itab le.
T he other possib ility is to use the m agnitude of
sponse to a giv en agonist f or classif ication. It is clear
that the in trinsic activ ity of agon ists is not a usef
param eter for recepto r ty p ing , because its m agnitude
varies w ith the ef f iciency of stim ulus-response couplin
T here are num erous ex am ples of low -ef f icacy agoni
that p roduce direct responses on ly in highly ef f icien tl
coup led tissues and act on ly as an tagon ists in oth
sy stem s. T herefore, nu ll m ethods m ust be used to fa
ou t the tissue effects and , hopefu lly , y ield param eters
relating only to the agonist-recep tor pairs. T heoretically ,
the m easurem ent of relativ e ef f icacy of agon ists fu l
this function , bu t again, the com plex ity of m em bran
dynam ics needs tobe considered. M easurem ents of ef f
cacy and relativ e potency w ould be inv alid m easures
recep tor selectiv ity in cases in w hich recep tors are p
m iscuous w ith respect to the ef fector coup ling pro tei
w ith w hich they interact (K enak in, 1988; K enak in and
M organ, 1989). For ex am ple, in IM R -32 tum or cells,
M 3 m uscarinic acety lcholine receptor sub ty pe selectiv e
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35 6 K EN A K IN ET A L .
couples to phosphoinositide hy droly sis and the M 4 sub-
type to inhibition of adeny late cy clase, but in cells con-
tam ing M 2 subty pe m R N A , the ex pressed M 2 recep tor
couples to both responses (Pink as-K ram ask i et al., 1990).
A n ex am ple of w here recep tor prom iscuity , w ith respect
to receptor-ef fector coupling m echan ism s, m ay occur in
nativ e m em branes is in rat pancreatic acinar cells. In
th is sy stem , cho lecy stok inin functionally interacts w ithsix G 5 pro teins and w ith G i, G 12 , and G 13 pro teins as
w ell (S chnefel et a ., 1990). A nother m ay be adenosine
A 1 receptors f rom bov ine brain w hich copurify w ith G 11,
G 2, and G 0 w hen activ ated by an agonist (M unsh i et al.,
1991). T he ability of th is receptor to in teract w ith all
th ree G pro teins w ith agonist activ ation w as conf irm ed
in reconstitution studies. U nder these circum stances,
dif f erent agonists cou ld pred ispose the form ation of dif -
feren t ternary com plex es, m ak ing the relativ e av ailab ility
of G pro teins in v arious m em branes a determ inant of
ef f icacy . In these sy stem s, the relativ e agonist po tency
and/or intrinsic ef f icacy w ould not be usefu l scales for
re ce pto r c las sif ic atio n.
G iv en these po ten tial pitfalls to the use of agonists for
recep to r classif ication, the follow ing w ould be a m inim al
list o f requirem ents fo r their use. a A s f or an tagon ists,
equilibrium conditions should be attained. For agonists,
this m ay be m ore d if f icu lt in v iew of b iochem ical m ech-
an ism s in tissues that are designed to rem ov e endogenous
agonists from the receptor com partm ent (i.e., degrada-
tiv e enz y m es, uptak e processes). It is a prerequisite for
recep to r classif ication stud ies that the concen tration of
drug at the receptor be k now n accurately . b Potency
ratios should be used and not rank order of potency . T he
correct use of agonist potency ratios alw ay s ensurescorrect rank orders, bu t the conv erse is not alw ay s true.
c If a recepto r sub ty pe is being classif ied , the endoge-
nous agonist fo r that receptor sub ty pe m ust be included
in the analy sis. d A gonists shou ld dem onstrate stereo-
selectiv ity at the receptor w here appropriate .
V M olecular B iology R elevant to R eceptorCharacterization
T w o prob lem s hav e plagued a purely pharm acological
def in ition of recep tor subty pes. T he f irstis th e q uestio n
of w hether observ ed pharm aco log ical dif ferences repre-
sen t d if ferent recep tor m olecules or sim ply cell-specif ic
dif ferences in the env ironm ent of a com m on recep torm olecu le. T his question has arisen in sev eral fo rm s in
the preceding d iscussion . T he second is the prob lem of
k now ing w hether there is a pure receptor population in
a giv en tissue, a prob lem that m ay generally hav e been
underestim ated. T he cloning of recep tors has rev ealed
that there are of ten m ore d if ferent recep tor m olecules
for a giv en ligand than generally recognized pharm aco-
log ical subty pes. A lthough one m igh t be inclined to ex -
pect that, therefo re, there w ill be a d istinct recep tor
m olecu le ev ery w here there is a reasonable suggestion of
a pharm acological sub ty pe, one should ex pect that
som e cases there w ill be few er d istinct m olecules th
there are apparent pharm acolog ical sub ty pes, because
cell o r species-specif ic v ariations in the properties
w hat should be considered a sing le receptor m olecule
section IV for a discussion of species v ariation). A
ollary of the ex istence of prev iously unrecognized
ty pes is the observ ation that the occurrence of m ultipsub ty pes in a tissue, or ev en a cell line, is m ore com m o
than had been generally recogn iz ed . T he introduction
m olecular b iology in the form of cloned receptors p
v ides a basis for allev iating these prob lem s.
T he characteriz ation of cloned recep tors shou ld ev
tually prov ide the inform ation on w hich to base a
m enclatu re that truly ref lects the m olecular identity
the receptors so that a particular subty pe of receptor w
refer to a specif ic pro tein (or set o f proteins for m ultip l
subunit receptors). In particular, the ex pression of clon
receptors in cell lines w ill prov ide pure populations
recep tors in identical cellular env ironm ents so that
factors that allow the dif ferent recep tors to be operation
ally distinguished can be iden tif ied. T o prov ide a p
population of receptors, the untransfected host cell line
in to w hich the cloned receptors are in troduced m ust
produce m easurable am ounts of any ofthe receptor sub-
ty pes in question . A lthough it is relativ ely easy to
such cell lines, it is no t y et clear how m any or w h
specif ic cell lines should be used to hav e an adequat
representation ofthe cellu lar env ironm ents that the re
ceptor experiences in v iv o.For ex am ple, the f ib rob last
cell lines w hich m olecular bio log ists hav e used for in i
characteriz ation of G protein -coup led recep tors m ay
contain signif icant am ounts of specif icG proteins thatin teract w ith the receptors in their natu ral cellu larv ironm ent. B ecause w e do not k now w ith w hich spec
G pro teins indiv idual recepto rs in teract or ev en w hat
fu ll repertoire of G proteins is, the cho ice of the m
appropriate host cell lines is not im m ediately obv iou
H ow ev er, som e pharm aco logical properties of these
ceptors, such as antagonist af f inities, m ay not depend
w hich G proteins are present so that the choice of h
cell line w ill no t be critical.
T he cloned receptors also prov ide new m ethods
determ in ing w hich recep tor m olecules are presen t i
g iv en cell o r tissue. T he D N A sequences of the clon
can prov ide probes of h igh sub ty pe specif icity fordetection of receptor m R N A s by a v ariety of m ethod
such as northern blots and in situ hy bridiz ation . T hese
m ethods vary in the sensitiv ity for detecting low leve
of m R N A , as w ell as in their spatial resolution. F
ex am ple, in situ hy brid ization is capab le of identify ing
ind iv idual cells in tissue sections, w hereas northern bl
ty p ically use R N A ex tracted f rom a w hole tissue. T h
if there is a low lev el o f m R N A ex pressed in m any c
of a g iv en tissue, a northern b lo t that sum s the sign
from al l of the cells w ill be m ore sensitiv e than in sit
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PH A RM A CO L O G IC A L R ECEPT OR S 357
hy bridiz ation , w hereas in situ hy brid iz ation w ill be m ore
sensitiv e if there are only a few cells ex pressing m ore
substantial levels ofm R N A . O ther m ethods such as
solution hy bridization coupled w ith probe protection as-
says or rev erse transcription coupled w ith polym erase
chain reaction am plif ication of fer po tentially greater
sensitiv ity , although generally not w ith the single-cell
resolu tion of in situ hy brid ization. U nfortunately , therelativ e am ounts of tw o m R N A s m ay not ref lect the
relativ e am ounts of recep tor protein in a giv en tissue for
tw o reasons. First, there can be large d if ferences in the
ef f iciency w ith w hich dif feren t m essages are conv erted
in to pro tein. S econd , at least fo r tissues con tain ing neu-
ronal cells, in w hich the m R N A is located in the cell
body but the recep tors are located predom inan tly on
ax onal and dendritic p rojections, there m ay be a sub-
stantial dif ference betw een the location of the m R N A
and the receptor pro tein . A lthough not y et w idely av ail-
able, subty pe-specif ic an tibod ies against portions of the
cloned receptor sequences shou ld allow m ore accurate
determ ination of recep tor distribu tions and heterogene-
ity . For ex am ple, antibodies specif ic f or the dif f erent
m uscarinic acety lcho line receptors hav e been m ade using
as antigens proteins deriv ed f rom ex pression inEsche-
r ichia coli of m ost of the dom ain connecting the f if th and
six th transm em brane dom ains (L ev ey et a ., 1991).
A ll o f the receptors that hav e been cloned and se-
quenced belong to one of a sm all num ber of fam ilies of
structurally sim ilar p roteins, such as the ligand-gated
ion channels, w hich hav e sev eral dif ferent but related
subunits, and the G pro tein-coupled recep tors, w hich are
sing le-subunit recep to rs characteristically hav ing sev en
m em brane-spanning dom ains. For som e ligands, such as
acety lcho line, there are recep tors, nicotinic and m usca-
rinic, that belong to m ore than one structu ral fam ily .
O ther ex am ples are G A B A A * v ersus G A B A B and 5-H T 3
v ersus 5-H T 1 and 5-H T 2. T he dif ferent structures gen-
erally correspond to distinctly dif feren t m odes of signal
transduction and can be used as a prim ary m eans of
subd iv id ing receptors for a giv en ligand . T here is, how -
ev er, a second structural class of recepto rs that can
interact w ith G proteins. T hese are the receptors for
peptides such as insulin and insu lin-lik e grow th factors
(O k am oto et a ., 1990). T hey consist o f tw o identical
subunits, each of w hich has only a sing le m em brane-
spanning dom ain .V I B asis for a M olecular N om enclatur e
T he m ost im portant con tribution of m olecu lar b iology
to receptor characterization is that it allow s the identi-
f ication of the actual m olecu les that underlie the phar-
m acology and prov ides an ev olu tional perspectiv e w ith
w hich to iden tify species-specif ic v ariation in recep tor
pharm acology , w hich is essential fo r ex trapolating re-
su lts in an im al m odels to hum an therapeutics.* Abbreviat ions G A B A , y -am inobuty ric acid; 5-H T , 5-hy drox y -
tryptamine.
T he m ajor questions concern ing a m olecu lar def initio
of recep to r sub ty pes are how dif ferent does the sequen
of a m olecule have to be to justif y a new nam e, and h
can one establish the m olecular identity of the receptor
that m ediates a phy siological response. If w e iden tify
recep tors by their pro tein sequences, w hich seem s to
the m ost appropriate m olecular identif ier, w e shoul
attach the sam e nam e to all m inor variants in sequencesuch as naturally occurring alleles or in v itro created
m utants. Ifsuch variants dif fer in any im portant recepto
properties, then they w ould be referred to as specif
v ariants, e.g., the A la197 allele or m utant of a giv e
recepto r. D if ferences in g ly cosy lation cou ld be referr
to as dif f erent glycosy lation states if glycosy lation prov
to be im portant. H ow ev er, ev idence based on m utating
the glycosy lation sites of the f2-adrenoceptor (R ands et
a ., 1990) and the m 2 m uscarinic acety lcholine recep t
(v an K oppen and N athanson, 1990) indicates that
of any or all gly cosy lation sites has little ef f ect on
pharm acology . L ik ew ise, v arian ts resulting f rom al
nativ e sp licing , as in the case of the dopam ine D 2 rec
tor, should be considered as length v ariants of a single
recepto r sub ty pe. T he tw o D 2 sequences dif fer by
insertion or deletion of 29 am ino acids near the m iddl
of the cy toplasm ic dom ain connecting the f if th and s
transm em brane dom ains. B ecause this dom ain has be
im plicated in the specif icity of G protein binding
other G protein-linked receptors, it has been suggested
that the tw o form s m ay bind to dif ferent G proteins
(E idne et a ., 1989; G iros et al., 1989). If th is w ere to
the case, variant nam es should be used to distinguish
tw o m olecules. H ow ev er, the sequence d if ference oc
in the portion of the cy top lasm ic dom ain that candeleted f rom both adrenoceptors and m uscarinic acety
choline receptors w ithout af fecting functional respon
(S trader et a ., 1987; S hapiro and N athanson, 1989
T hus, the biological signif icance of this dif f erence
m ains to be determ ined. O n the other hand, biologically
signif icant alternative splicing does occur in the glut
m ate receptor genes G lu-A , -B , -C , and -D (S om m er
a ., 1990). T he alternativ ely spliced form s of these rec
tors, w hich prov ide dif f erent sequences for a 38-am ino
acid region preceding the fourth transm em brane dom a
hav e been referred to as the f lip and f lop form
A lthough these m ay not be the m ost suitable nam es
these alternativ e fo rm s of the proteins, it is clear th
they should be giv en dif ferent nam es because they im pa
dif f erent channel properties.
M ore substantial sequence variations, in the approx i-
m ate range of 1 to 10% , such as those that occur betw een
m am m alian species during the course of evolution
receptor m olecules, also should be included under a sing
nam e. T his assum es that it is clear that the m olecules
are true hom ologs, i.e., the sole descendants of the sam
m olecule in the m ost recent com m on ancestor of the
species being considered. T he rationale fo r do ing s
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35 8 K EN A K IN ET A L .
that function is generally conserved during ev olution so
that subty pes hav ing the sam e nam e could be ex pected
to hav e the sam e physiological functions in dif f erent
species. T he ev olution of protein sequences for fam ilies
of related proteins, such as the hem oglobins, has been
the subject of study for m ore than 20 y ears and prov ides
the foundation for identif y ing w hich sequences are true
hom ologs. If there are the sam e num ber of functionalgenes for dif ferent subty pes of a given receptor in the
tw o species, it is generally not dif f icult to identif y w hich
pairs of receptors, one from each species, are the true
hom ologs. T hus, assum ing that all of the subty pe genes
ex isted in the ancestor of all m am m als, one can expect
that the sequences for a single subtype in all m am m als
w ill be m ore closely related to each other than the various
subty pes in any one species are to each other. How ev er,
if there hav e been gene duplications since the evolution-
ary div ergence of the tw o species and if the duplicated
gene has acquired a function, then the correspondence
m ay not be clear because there are now tw o genes derived
f rom the ancestral gene in one species but only one in
the other species. O ne can expect that the tw o genes in
the f irst species w ill be m ore sim ilar to each other than
they are to the single gene in the second species. T he
question of gene duplication w ill have to be resolved for
each gene fam ily , i.e ., each set of receptors for the sam e
ligand and hav ing the sam e general receptor structure.
In general, this com plication can be ex pected to be rela-
tiv ely infrequent am ong m am m als but m ore serious w hen
com paring m am m alian recep tors to recep tors f rom other
vertebrates or ev en inv ertebrates.
In princip le, if one has, for ex am ple, a rat recepto r
sequence and w ants to k now w hether it is the hom olog
of one of the k now n hum an subtype sequences, one w ould
need the sequences of all the hum an subty pes for that
ligand to determ ine w ith certainty w hich w as the m ost
closely related to the rat sequence. In practice, one prob-
ably w ould have a potentially incom plete set of hum an
subty pe sequences to com pare and, therefore, w ould w ant
to know w hether the rat sequence is the hom olog of one
of the know n sequences or represents a new subtype. If
the rat sequence is not signif icantly m ore closely related
to one of the hum an sequences than to the others, it
alm ost certain ly represen ts a new subty pe. If it d if fers
by 1 to 10% from one of the sequences, it is probably the
hom olog of that sequence. For G protein-coupled recep-tors for w hich the sam e receptor has been cloned from
sev eral species such as cow , pig, rat, m ouse, ham ster, and
hum an, the hom ologous genes typically hav e 85 to 95%
am ino acid sequence identity . T he m em brane-spanning
dom ains of these receptors are quite conserv ed betw een
subtypes, but there are regions (the am ino term inal to
the beginning of the f irst transm em brane dom ain, be-
tween the f if th and six th transm em brane dom ains, and
follow ing the seventh transm em brane to the carbox y l
term inal) that show little conservation not only in se-
quence but in their leng th. Ex am ination of such reg io
can of ten be m ore usefu l than the ov erall sequence w
getting an indication of w hether tw o sequences are sp
cies hom ologs. W hen com paring sequences, one shoul
be alert to the possibility of sequencing erro rs. T he m
troublesom e error is a f ram e-shif t error, w hich causes
incorrect translation of an ex tensiv e portion the D N
sequence into am ino acid sequence w hen a sing le nucltide is erroneously deleted or inserted. S uch errors a
not uncom m on. If there is a second com pensating err
that resu lts in resto ration of the correct read ing f ram
the errors are of ten not uncovered until sequences a
com pared betw een dif ferent laboratories or betw een
cies (G uyer et a ., 1990).
In som e cases, it m ay be dif f icu lt to decide w hethe
tw o sequences are species hom ologs w ithou t supp lem en
tary inform ation. T he m ost useful inform ation is pr
v ided by a D N A hy brid ization test, w hereby one de
m ines w hat hum an gene is m ost closely related by
bridiz ing a probe deriv ed f rom the rat sequence to
S outhern b lo t of restriction enz y m e digests of hum
genom ic D N A and ask ing w hether the m ost stable (w
respect to tem perature) bands of hy bridiz ation corr
spond to those characteristic of one of the k now n hum
genes. T his test requires that the probes used to def in
the characteristic patterns of the k now n hum an gen
represen t as closely as possible the sam e reg ion of
am ino acid sequence as the rat p robe so that the hom o
ogous rat and hum an probes w ould detect p recisely
sam e restriction f ragm ents. Just as one shou ld use 5
era d rugs to characteriz e a recep tor, one should u
sev eral restriction enz y m es for such an analy sis to avthe possibility that tw o dif ferent genes m ight g iv e
sam e restriction f ragm ents fo r a single random ly chos
enz y m e. A nother fo rm of corroborating ev idence w o
be w hether the receptor (either protein or m R N A )
the sam e tissue distribution in the tw o species, assum ing,
of course, that the distributions are suff iciently distinct
to distinguish am ong subtypes.
B ecause the recep tors hav e not ev olv ed to d istinguish
am ong v arious sy n thetic ligands and changing ev en
single am ino acid in the receptor can cause substantial
changes in ligand af f inities (S ury anaray ana et a ., 199
there is reason to ex pect that there w ill be species d if f
ences for som e synthetic ligands (ev en though the aff i
ity for the endogenous ligand m ay be unchanged)w hat w e w ould identify as a sing le m olecular sub ty p
S uch dif ferences w ill need to be noted in the pharm acological def initions attached to the m olecular nam es a
are clearly of m ajor im portance in selecting anim a
m odels for testing drugs intended for hum an use. T he
H T 1 B and 5-H T 1D receptors prov ide a clear case of suc
species variation. Even before the recent cloning of the
recep to rs, it appeared p lausible that they m ight represen
such species dif ferences of a single m olecule based
their sim ilar distributions and functional properties b
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PH A R M A C O L O G IC A L R EC EPT O R S 359
apparen t m utual ex clusion in d if ferent species. From the
cloning of these receptors, it is now clear that there are
tw o receptors in hum ans w ith 5-H T 1D pharm acological
p roperties, w hereas the rat and m ouse hom ologs of one
of these tw o genes has 5-H T 1B pharm aco log ical p roper-
ties (H artig et al., 1992). T he adv antage of identify ing
such species v arian ts as the sam e recep tor is that func-
tion can be ex pected to be largely conserv ed betw eenspecies, ev en if there are changes in structu re that alter
its in teraction w ith sy nthetic ligands.
Ind iv idual sub ty pes of m ultiple-subunit recepto rs can
be m olecularly def ined by identify ing each of the subunits
that are present, e.g., the R i receptor cou ld be def ined
as con taining the al, b3 , c2 and d5 subunits. A m ajor
unresolv ed question is w hich of the m ultitude of possib le
com binations of subunits actually occurs to form in v iv o
receptors.
T he question of identify ing w hich m olecule m ediates
a particular phy sio log ical response w ill largely depend
on hav ing d iscrim inating drugs av ailab le, bu t in som e
cases, such as cell lines or tissue sam ples, m olecu lar too ls
such as specif ic D N A probes for hy brid iz ation to m R N A
or subty pe-specif ic antibod ies generated f rom clone-de-
riv ed an tigens m ay be helpfu l in iden tify ing w hich sub-
ty pes are present or absent. H ow ev er, absence is alw ay s
diff icu lt to estab lish and m R N A distribution in tissue
m ay not coincide w ith the recep tor distribution. It m ay
also be possible to iden tify the phy siolog ical function of
specif ic subty pes by purely genetic m eans, such as gene
inactiv ation or replacem ent in transgen ic an im als or the
inh ibition of translation of specif ic m R N A s by the use
of antisense R N A s. T o the ex ten t that there are k now n
m olecu lar sub ty pes for w hich discrim inating drugs hav eno t y et been identif ied , caution w ould dictate that the
recep tor be identif ied as, for ex am ple, m olecu lel o r
possibly m olecu le3. A tten tion shou ld be g iv en to the
possibility that m ore than one recep tor ty pe is inv olv ed
w hich necessitates using m ore than one or tw o drugs to
m ak e the identif ication.
V I I . P a r a llel P h a r m a cologica l a n d M olecu la r
N o m e n c l a t u r e s
T he cloned receptors that are presen tly av ailable hav e
no t y et been stud ied thoroughly enough to allow a full
def inition for any set o f recep tors that w ould allow theunam biguous assignm ent of a m olecular nam e to an in
v iv o receptor, especially in the case in w hich m ultiple
related receptors m ay be present. N ev ertheless, the char-
acterization of cloned adrenoceptors and m uscarin ic ace-
ty lcholine receptors has adv anced far enough that w e
can predict w ith a reasonab le am ount of conf idence the
basic facto rs that w ill be im portan t in def in ing sub ty pes
of G pro tein-coupled recep tors. S im ilarly , the cloning of
a substantial num ber of subunits of n ico tin ic acety lcho-
line recep tors and G A B A A recep tors has def ined the
problem s but has no t y et prov ided the solu tions for
def ining m olecular sub ty pes of ligand-gated ion chann
recep tors. T he task of the In ternational U nion of Ph
m acology C om m ittee on R ecep tor N om enclatu re
D rug C lassif ication is to f ind a unifo rm fram ew ork
def ining receptor nam es that w ill allow all recepto rs
be def ined accord ing to the sam e princip les so that
subcom m ittees dev oted to particu lar receptor ty pes
form ulate nom enclature proposals w ith a com m onm at. T o m inim iz e changes in the nom enclature and
the associated def in itions w ith the adv ent of new in
m ation, especially new subty pes, w e should try to pred
w hich factors are essen tial to the def in ition of sub ty p
and w hich factors are suf f icien tly v ariab le (e.g., w
cellular env ironm ent) that they shou ld no t be considere
as parts of the prim ary def in ition. T he latter facto
m igh t or m ight no t be included as secondary inform ation
in m uch the sam e w ay as a dictionary f requently includ
in form ation distinguishing the usage of w ords w ith
ilar m ean ings. T o the ex ten t that one cannot accurately
pred ict all the factors, a f ram ew ork m ust be designe
that allow s the incorporation of new inform ation
the def in itions w ithout drastically altering the nom e
clatu re. H ow ev er, substantial changes in def inition w
retain ing the sam e nam es m ay generate som e confusio
G iv en our present state of k now ledge, w e need a tr
sitional sy stem such as the form at suggested by J.
G reen (1990), o r as used in theTr ends i n Phar macol ogi cal
Science nom enclature supp lem ent (v o l. 12 , 1991), w h
can incorporate m ost of the observ ationally relev a
inform ation to prov ide parallel pharm aco log ical
structu ral (or m olecu lar) def initions. S uch a form at
low s the correspondence betw een these tw o def in ition
to be indicated . H ow ev er, to av oid m isinterpretation,there should be an ex p licit assessm ent of the reliab ilit
of the correspondence betw een m olecular and pharm
cological subty pes. O ne should not ind icate that a part
ular m olecular sub ty pe corresponds to a giv en pharm
cological sub ty pe sim ply because its pharm acological
prof ile m ost closely m atches that of the pharm acological
subty pe, w hen , in fact, sev eral m olecu lar subty pes
no t be distingu ished by the pharm acological def initio
O ne w ould hope that, as add itional data becom e av ailab
concern ing the pharm aco log ical p roperties of cloned
ceptors such that the pharm acological def initions can
ref ined , the in form ation that does no t allow unam biguous distinction of the m olecular identities w ill be m ov e
to foo tno te status, and the pharm aco log ical and m ol
ular def initions w ill coalesce. T here is general agreem en
that recepto rs shou ld be def ined as a ligand-binding
w ith a ligand-m ediated functional response, the respon
being biochem ical, electrical, o r m echanical in its m
festation . T here is also general agreem ent that the
m ary label in the nam es shou ld be the endogenous lig
(o r, p rov isionally , ano ther ligand w hen the endogenou
ligand is unk now n). W hether the com pendium of re
tors should list the ligands alphabetically or alphabeti
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360 K EN A K IN ET A L .
cally w ithin chem ical ty pes (such as am ines, am ino acids,
pep tides, etc.) is open for discussion. T hey could easily
be listed one w ay w ith an index listing them the o ther
w ay . A secondary classif ication , w hich is w ell g rounded
in m olecu lar structure and is observ ationally distingu ish-
ab le, w ould be on the basis o f structural ty pe of recep tor,
e.g., G pro tein-coupled or ligand-gated ion channel. S uch
a distinction has already been m ade in som e cases, suchas nico tinic v ersus m uscarin ic acety lcholine receptors or
G A B A A v ersus G A B A B recep tors, but not in other cases,
such as the 5-H T 3 receptor w hich is not giv en a nam e
distinctly d if feren t f rom the 5-H T 1, 5-H T 2, and 5-H T 4
recep tors. T h is is one subgroup ing of recep tors for the
sam e ligand that is not lik ely to change w ith add itional
info rm ation and , therefore, could safely and usefully be
incorporated in to the nom enclature.
V I I I . Su ggested R u les for N a m in g R ecep tor s
W e are faced w ith the tw in prob lem s of receptor no-
m enclatu re and classif ication. T o the ex ten t that they
can be d istinguished , nom enclature im plies a set of
nam es w ith def in itions that allow s a one-to -one corre-
spondence betw een nam es and objects (such as receptors)
w ith specif ic properties, w hereas classif ication im plies a
hierarch ical o rdering of a group of objects based on their
degree of sim ilarity to each other. T here is clearly a
pressing need for an agreed upon set o f nam es and a set
o f criteria fo r def ining w hat constitu tes a new recep tor
sub ty pe, but it is no t clear that there is a m ajor need for
these nam es to ref lect an ex tensiv e classif ication schem e.
T he ex isting tendency tow ard classif ication in recep tor
nom enclatu re is a natu ral consequence of the process inw hich new subty pes generally hav e been identif ied
th rough the use of new drugs, w hich could distingu ish
d if ferent recep to rs w ith in a prev iously unreso lv ed sub-
ty pe. In contrast, w e can ex pect that in the future m any
new subty pes w ill be identif ied as d istinct m olecules
th rough cloning and, in m any cases, the drugs capable of
d istinguish ing the subty pes w ill hav e to be dev eloped
subsequently . G iv en our relativ ely incom plete k now ledge
of the num ber of recep tors and of the fu ll range of the
properties of those receptors that hav e already been
iden tif ied, it w ould probably be a m istak e in term s of
stability of the nom enclature to incorporate m ore than
the m ost rudim entary classif ication, i.e ., the ligand and
the structu ral ty pe (G protein -coup led, ligand-gated ionchannel, etc.), into the nom enclature.
C lassif ication neu tral labels such as 1, 2, 3,... are
preferred to labels such as 1A , 1B , 2A , 2B ,..., w hich
im ply d if fering degrees of sim ilarity , because the per-
ceiv ed relationsh ips betw een sub ty pes can change w ith
the adv ent of new subty pes or new drugs. It is inev itable
that such changes w ill occur if the relationships are based
on a few properties of the receptors. T his is clearly the
case in the tax onom y of p lants and anim als in w hich, if
one considers on ly a few specif ic characteristics, one can
deriv e dif feren t phy logenetic trees depend ing on w h
characteristics are used . T he case of the 5-H T 1c recep to
is ev idence that the sam e th ing can easily happen w
receptors. It w as originally classif ied as a 5-H T 1 subty p
based on a sing le drug but subsequently appeared to
m ore sim ilar to 5-H T 2 both pharm acologically and
com parison of the cloned sequences. If relationships
not im plied by the nam es, the relatedness of d if ferensubty pes cou ld be included as supplem entary inform
tion so that it can be m odif ied w ithou t m odify ing
nom enclature. A s a rule, the m ore subty pes there a
the m ore d if f icu lt it w ill be to def ine m eaningfu l r
tionships. C learly , there w ill be cases, such as adrenocep-
tors, in w hich a nom enclatu re that is not classif ication
neu tral is so w e estab lished that it w ould no t m ak
sense to change the w hole sy stem unless it w ere
becom e unw ieldy because of the iden tif ication of e
m ore subty pes.
A lthough it w as ex ped ient fo r the a2-adrenocep tors,
the nam ing of m olecular sub ty pes af ter their chrom o
som al location is no t desirable. G enerally , the hum a
chrom osom al location is not estab lished for som e ti
af ter a cloned receptor is published , especially if it is
a hum an clone. Furtherm ore, there m ay be m ore th
one sub ty pe on the sam e chrom osom e as in the case
the m 1 and m 4 m uscarin ic acety icho line recep tors.
I X. C on clu sion s
For the presen t, tw o ongoing sy stem s of recep tor d
inition , structural and pharm acological, appear to
necessary . O ne w ould be based upon receptor structure
w ith po ten tially incom plete pharm acological criteriA f ter pharm acological criteria hav e been fulf illed, th
recep tors w ould be def ined in bo th sy stem s. T he o t
w ould def ine recep tors by pharm acolog ical criteria (
endogenous agonist, antagonists). A gain , it w ould
conceiv able that there w ould be incom plete data for so
recep tors def ined in this category , therefo re, preclud ing
def in ition in the o ther. A com pletely def ined recepto
w ould possess a un ique pharm acological p rof ile based
agonist and an tagonist data, a k now n endogenous ligan
and a d istinct am ino acid sequence. It m ay be th
recep tor identif ication by structure w ill p rov e to b
technically m ore sim ple task than identif ication by
gand pharm aco logy if recep tor env ironm ent (i.e ., lip
coup ling proteins) p lay s a signif icant ro le in def iningpharm aco logy of som e recep tors. In these cases, recep to
pharm aco logy w ould v ary w ith the tissue or cellul
sy stem but no t be ind icativ e of d if ferent recepto r ty p
T he reso lution of the question of w hether recepto r
v ironm ent is a signif icant factor in receptor identif ica-
tion and/or classif ication w ill require the use of sy stem
in w hich the structu ral iden tity of the recep tor is w
defined.
T he nom enclature sy stem w ill clearly ev olv e at a r
p roportional to the diligence of all concerned in reco
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P H A R M A C O L O G IC A L R E C E P T O R S 361
ciling the structural and pharm acological definitions. A san initial fram ew ork for im plem enting these parallel sets
of definitions in a form that facilitates their reconcilia-tion , the In ternational U nion of Pharm acology C om m it-tee on R eceptor N om enclature and D rug C lassification
has adopted a tabular form at closely resem bling theTrends in Pharmacological Sciences receptor nom encla-
tu re su pp le me nt.
Acknowledgments. W e ack n ow led ge t h e in -d ep th com m en ts con cern -ing this paper by D r. D on Jenkinson, University College, London,
U n it ed K in gd om .
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