A New One-trial Test for Neurobiological Studies of Memory in Rats_I_Behavioral Data

8/3/2019 A New One-trial Test for Neurobiological Studies of Memory in Rats_I_Behavioral Data

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Behavioural Brain Research, 31 (1988) 47-59 47ElsevierBBR 00844

A n ew one-tr ia l test for neurobiolog ical s tudies o f m em ory inrats. 1" Be ha vio ral da taA . E n n a c e u r a n d J . D e l a c o u r

Laboratoire de Psychophysiologie, Universit~ Paris VII , Paris (France)(Receiv ed 20 Januari 1988)

(Revised vers ion received 15 March 1988)(Acc epted 16 Ma rch 1988)

Key words." E xplora t ion ; Memory- recogn i t ion ; Hab i tua t ion ; Ra t

In this pape r we describe a new me mo ry tes t in ra ts , based on the dif ferent ial explora t io n of famil iar and new objects . In af irs t t r ia l (T1), ra ts are exposed to one or to two ident ica l objects (samples) and in a second t r ia l , to two diss imilar objects , afamil iar ( the sample) and a new one. For short inter t r ia l intervals (~ 1 min), most ra ts discrim inate between the two o bjectsin T2: they spend more t ime in exploring the new object than the famil iar one. This tes t has severa l interes t ing characteris t ics :(1) i t is similar to visual recognit ion tes ts wideley used in subhum an primates , this a l lows interspecies com parisons ; ( 2)i t isen t i re ly bas ed on the s pon taneous behav io r o f ra ts and can be cons ide red a s a ' pu re ' work ing- memory t e s t comple te ly f ree o fre f e rence memo ry com ponen t ; (3 ) i t does no t invo lve p r imary re in f o rcement s uch a s f ood o r e lect r i c shocks , th is mak es i tcomparab le to memory t e s t s cu r ren t ly us ed in man .

I N T R O D U C T I O N

According to recent advances in c l in ica lneuropsych ology, a t leas t two types of memor iesshould be d is t inguished f rom the ana lys is of theglobal amnes t ic syndromes: the f i r s t i s spared ,whi le the second i s deeply d is turbed . There i s nogenera l agreem ent on the def in it ion of these twok ind o f memor i e s . Depend ing on t he au tho r s,type I ( spared ) i s the semant ic , the re ference orthe proce dura l m emory , type I I (d is turbed) is theepisodic , the working or the dec lara t ive mem-ory3-5,23,26,31,36,38,40,41,43.

A s imila r hypothes is was a t the bas is o f anexper imenta l research developed in monkeysyears ago by Mishkin and Delacour25: a cr i t ical

fac tor in the pa thology of memory would be therepe t i t ion . Memory of a unique event would bemuch more vu lne r ab l e t han t ha t ba sed on t herepe t i t ion of some condi t ions , such as the asso-c ia t ion of s t imulus or responses to a re inforcer .Fro m th is hypothes is , a t r ia l -unique mem ory test ,ba sed on a de l ayed ma tch ing o r non -ma tch ingprocedure , was developed in monkeys25 andproved to be sensi t ive to temporal lobe st imula-tion 8 o r lesi on s 9,26.

However, this test is not completely sat isfac-tory: i t is not purely one tr ial s ince i t is based onthe learn ing of a ru le (matching or no n-match ingto sample) through the repe t it ion of s t imulus orrespons e reward assoc ia t ions dur ing a pre t ra in ings tage . Al te ra t ion of per forman ces m ay be due to

Correspondence: A. Ennaceu r, Labor atoire d e Psychophy siologie , Univers i t6 Paris VII , 7 Quai Saint B ernard, B~tt . B, 5 e t . 75251Paris Ced ex 05, France.

0166-4328/88/$03.50 1988 Elsevier Science Publishers B.V. (Biomedical Divis ion)


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48dif ferent causes: ei ther to ef fects on the one t r ialcomp onent , o r on the l earn ing o f the rule or onboth .

A rece nt ' rat ' vers ion of this t ask ~'35 suffersf rom the same d raw bac ks ; in th is respec t i t i s no tsuperior to the delayed reinforced al ternat ion, thenaost popular working memory test in the rat .

The task described in this paper i s real ly aone-trial task. It does not involve at all the learn-ing of a rule since i t i s enti rely base d on the spon-t aneous explora tory behavior of ra t s t owardsobjec t s .M A T E R I A L S A N D M E T H O D SSubjects

A total of 220 male W istar rats weighing200 -25 0 g were used . They were hous ed in in -dividual cages during the test ing period. Thel igh t -dark cyc le was 12" 12 (07 .00-19 .00 h) andthe ambient t empera ture w as 23 + 1 C. Dur ingthe en t ire exper iment they had f ree access to foo dand water .Apparatus

T h e ap p a ra t u s u sed w as an o p en b o x m ad e o fwo od 65 x 45 x 45 cm (height) . The obje cts to bedi scr imina ted were ma de of g l ass , p l as ti c o r m eta land existed in dupl icate. Their weight was suchtha t t hey could no t be d i sp laced by ra t s . Theappara tus was p laced in a sound- i so la t ed roomwhere the sound- insu la t ion was a ided by amasking whi t e no i se of 70 dB above the hum anthreshold . One l igh t bu lb f as t ened in the upperpar t o f t he room provided a cons tan t i ll umina t ionof abou t 40 lux a t t he l eve l o f t he t es t appara tus .E X P E R I M E N T 1SubjectsA to ta l o f 90 ra t s t ha t had rece ived only onemedium dose of a noot ropic drug 10 days beforewere randomly a l loca ted to 4 groups .Behavioral testing

Animals were handled and weighed each day .A da y before t es ting they were a l lowed to explorethe box for 2 min. Th ey were then given two test -

65 c m

4 5 c m

J

Fig. 1. Repre senta tion of experim ental condition s in TI ,Expt. 1.

ing se ssions sepa rated by a 48 h delay.Throu ghout the exper iment no c l ean ing of the boxwas al lowed, in order to saturate i t wi th ol factorystimuli.

Each sess ion was com pri sed of two t ri a ls . Inthe f i rst tr ial (T1), one o bject -st imu lus, the samp le(A) , was p laced as ide the rear wal l o f t he box ina loca t ion equid i s t an t f rom the b ack co rners of t hebox (see Fig. 1) . During the second t r ial (T2), anew ob jec t (B) was adde d (see Fig . 5 ). Here , eachobjec t was p laced in a back corner . The objec t (A)presen ted dur ing T2 w as a dupl i ca te of the sam plepresented in T1 in order to avoid ol factory t rai l s .From ra t t o ra t , t he ro le (sample or new objec t )and the pos i t ion o f the two o bjec t s dur ing T2 wascounterba lanced and randomly permuted . A d i f -f e ren t pa i r o f ob jec t s was used for each sess ion .We shal l cal l O 1 and 02 , t he two objec t s o f eachpair . For ha l f o f t he ra t s , O 1 was the sample (A)and 0 2 the new objec t (B) . For the o ther ha l f, t hero le of O1 and 02 was the oppos i t e . Theseprecaut ions were taken to reduce objec t and p lacepreference ef fects . I t should be st ressed that theobjec t s apparen t ly had no na tura l s igni f icance forra t s and had never been assoc ia t ed to re inforce-ment .At the begining of each t rial, the rats w ereplaced near the cen ter o f t he f ron t wal l o f t he box ,thei r heads oriented in the opposi te di rect ion tothe ob jec t. The respec t ive dura t ion of T1 and T2


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wa s 5 and 3 min. R ats for which the intertr ialin te rva l was grea te r t han 1 min were t aken bac kto thei r home cages during this interval .

According to the group, the intert r ial delaywa s: 1 min (G1, n = 23); 1 h (G 2, n = 23); 4 h(G 3, n = 22); 24 h (G 4, n = 22).Measu remen t s and s ta t is t ica l t rea tmen t s

The bas ic measure was the to t a l t ime spent byrats in exploring an object during T1 or T2. Ex-p lora t ion o f an ob jec t was def ined as fo l lows:di rect ing the no se at a distan ce < 2 cm to theobjec t and/or touching i t wi th the nose . Turn ingaroun d or s i tt ing on the ob jec t was n o t cons ideredas an explorat ion.Let (a) be the t ime spe nt in exploring the sam pledur ing T1, (a ' ) and (b) the t ime spent respec t ive lyin explor ing the sam ple a nd the new o bjec t dur ingT2. The fo l lowing var i ab les were cons idered :el = (a), e2 = (a' + b), hl = (el - e2),d l = (b - a ' ) and d2 = (d l /e2) .

(e l ) and (e2) measu re the explora t ion of bo thobjec t s dur ing T1 and T2 respec t ive ly ; (h l ) maybe cons ide red as an index of habi tua tion of theexplora tory behavior and (d l ) , (d2) as amnes icindexes ref lect ing the discriminat ion between thenew and the f ami l i a r ob jec t s .

Within groups stat i s t ical tests were appl ied tov a r i ab l e s (h l ) an d (d l ) . T h ey w e re b a sed o n t h epaired Student t - test . For instance, s ignif icance of(h i ) fo r a g iven group was t es t ed by compar ingthe mean va lue of (e l ) and (e2) for t ha t g roup .B e t w een - g ro u p s co m p ar i so n s w e re b a sed o n

49A N O V A an d p o s t - h o c co m p ar i so n s acco rd i n g t oWin er 42. The signif icance thr esho ld wa s 0.05.R esu l t s

One ra t o f G2 was exc luded f rom the exper i -me nt b eca use of i ts lack of explorat ion aci tivity.

(1) As indicated in Table I , there were no grou pdif ferences in the t ime of explorat ion of thesample in T1 dur ing each sess ion . So we cancons ider tha t t he d i f f e ren t g roups were comp ara-b le wi th respec t t o explora tory behavior . Whenra t s were re -exposed to the sample and a newobject , they did not show a signif icant di f ferencebetween groups in the to t a l t ime spent for t heexplora t ion o f bo th ob jec t s dur ing T2.

(2) The me an va lue of (a ) was com parab le forthe two sess ions , and for e i ther ob jec t O1 and 0 2of the same pa i r use d as sample (see Mater i a l sand Metho ds) . Thi s me ans tha t t he 4 ob jec t s ( twopai rs ) used in the two sess ions e l ic i ted compa ra-b le explora tory behavior .

(3) Al l rats , in depe nden t ly of thei r res pect ivegroups, spent in T2 more t ime in exploring thenew objec t t han the sample in each sess ion , asindicate d in Table II . Th e highly signif icant ' t 'ob ta ined for each group m eans tha t for each de lay ,the new objec t was d i s tingui shed f rom the sample .

(4) Accord ing to the overa ll ANO VA , the index(d 1) which ref lects the discrim inat ion betw een thefami l i a r and the new objec t ( see Mater i a l s andMethods) shows only a t endency to s ign i f i canceF~.85 = 2.58, P < 0.057.

TABLE IMea n value (+ S .E .M .) of the total exploration time (s) during the first trial (el) and the second trial (e2)Gro ups : G1 = 1 min, G2 = 1 h, G3 = 4 h and G4 = 24 h intertrial delays.

Groups G1 (n = 23) G2 (n = 22) G3 (n = 22) G4 (n = 22)(el) (e2) (el) (e2) (el) (e2) (el) (e2)

Se ss io n 1 31.17 49.43 24.09 37.86 30.23 38.32 30.13 34.32+ 2.74 + 3.33 + 2.17 + 3.12 + 3.16 + 3.26 _+ 2.51 + 2.23

Se ss io n 2 26.13 41.09 30.95 40.73 34.50 40.36 30 40.54+ 2.25 + 2.94 + 2.84 + 3.75 + 2.51 + 2.66 + 2,50 + 2.71

Se ss io n 1 + 2 57.30 90.52 55.05 78.59 64.73 78.68 60,13 74.86+ 1.93 + 4.99 + 4.40 + 6.03 + 5.33 + 4.85 + 4,36 + 4.28


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5OTABLE I1Comparisons within each group (paired Stud ent's t-test, two-tailed) o f the time (s) spent b y rats in exploring the samp le(a') and the new object (b) in T2, dl = (b-a')Groups: G I = 1 min, G2 = 1 h, G3 = 4 h and G4 = 24 h intertrial delays.Groups Session 1

(dl) t P

A New One-trial Test for Neurobiological Studies of Memory in Rats_I_Behavioral Data

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