Spike frequency adaptation mediates looming stimulus selectivity in a collision-detecting
neuron
Peron and Gabbiani – Nature Neuroscience, March 2009
In Houston at Rice and Baylor
Terminology• Spike Frequency Adaptation (SFA) - firing
frequency to decrease during a train of action potentials.
• Translating Stimuli
• Looming Stimuli
• XY = Y dependent X Channels
• Kca = Calcium dependent Potassium Channels
Escape Behavior and Locomotion
• Escape Movement occurs at angle threshold (when triangle is sufficiently large enough to cover the needed number of retinal receptors)• Retinal neurons - LGMD interneurons – Lobula
Giant Movement Detector To DCMD – Descending Contralateral Movement Detector To muscles. Usually a 1:1 correspondence from LGMD to DCMD.
Methods - Differences
• In vivo - Locus chosen because had to present visual stimuli in vivo. Larger input currents , 10+ nA (e-9) compared to pico (e-12) amps (our lab). Use Matlab instead of Igor. Locus saline instead of ACSF.
Locus – a model organism
Methods - Alan Lloyd Hodgkin• Goldman-Hodgkin-Katz
• Hodgkin–Huxley model - 1963 Nobel Prize with Huxley - model that describes how action potentials in neurons are initiated and propagated
Cm = capaticance – lipid bilayerGn and Gl = liner and non-linear ion flowIp = current source = Na/K Pump
The Locus escape mechanism is activated in response to looming stimuli but not translating stimuli. How does this occur?
LGMD responds to looming stimuli
• Maximum frequency in dynamic range - 27spikes/sec versus 300spikes/sec. Current was 10nA for locus.
0 100 200 300 400 500 6000
5
10
15
20
25
30AP Firing Frequency vs Current Strength
I (pA)
# o
f A
Ps
LGMD Response - Looming versus Translating
• 300/sec versus 50spikes/sec (6x) with lower sustained firing rates
• When looking at last .4 sec of the 1 sec pulse, a difference of 100 spikes looming versus 10 spikes for translating (10x)
• Translating stimuli show spike frequency adaptation while looming stimuli do not. SFA leading to repressed translational stimuli response
• Looming correlated with motor response. Correlation is not causation.
SK-Like Potassium conductance mediates the spike-frequency adaptation in translating stimuli
• SK - Small conductance K+ channel – activated by calcium – too slow to contribute to repolarization but SK currents contributes to medium after-hyperpolarization.
• How was this conclusion reached? - BAPTA
BAPTA addition
• Calcium Chelator BAPTA – Forms a chelate complex with calcium - sequesters calcium
• Calcium Channel blocker Cadmium (Cd2+)• Chelator sequesters calcium while Cadmium prevents
Calcium entry
• RESULT : Post BAPTA administration - steady translating stimuli frequency, No adaptation
• SFA is calcium-dependent
Singling out SK from the Kca Class
• Kca – SK (small-conductance), BK, IK (=IsAHP?)
• Use of Charybdotoxin – blocks BK and IK but not SK – Reduced spike frequency and broadened the action potential but no effect on final spike frequency, SFA still observed.
• However, Apamin – traditional SK antagonist - and other SK blockers had no effect on SFA. SFA was still present.
SK-like Conductance responsible for SFA
• SFA seems to be mediated by SK – confirmed calcium dependence and all other Kca ruled out.
• But - aunique SK showing “nontraditional properties.”
• “SK-like” – something new?
A side investigation into BAPTA
• BAPTA abolishes the 1:1 LGMD – DCMD ratio.
• Chemical Synapse Review: Calcium diffuses into synaptic terminal in response to action potential repolarization leading to release of neurotransmitters from calcium sensitive vesicles.
• Shows BAPTA is able to diffuse throughout the LGMD – all the way to the LGMD –DCMD electrical-chemical synapse.
• Hypothesis Reconfirmation - Increased firing frequency observed in response to translating stimuli post BAPTA treatment.
BAPTA effect on Looming Stimuli
• “Overall course of looming stimuli response was nearly unaffected”
• Angle threshold for number of receptor activated by looming stimuli was not changed.
Spike Initiation Zone = Axon Hillock?
• Proximal conductance has more influence than distal conductance. SFA maximum inhibitory effect when located as close as possible to Spike Initiation Zone where input currents are summated at axon hillock.
• SK localized to points close to SIZ through Calcium Indicator Oregon Green BAPTA-1. Because BAPTA complexes with Calcium, can see where Calcium is localized. Fluorescence observed near SIZ.
BAPTA and the time constant
• Addition of BAPTA resulted in a decreased time constant for calcium
• In other words – there is a faster decline in free calcium when BAPTA is around.
• Time constant for Calcium needed to be obtained for use in mathematical calculations in MATLAB to confirm results
Mathematical Confirmation based on the Hodgkin–Huxley model
• Based Gn and Gl on SK-Like Kca and modified for the morphology of the LGMD and Calcium localization. Also add BAPTA constraints – Plug into MATLAB
• Result: SK-Like Kca conductance can almost perfectly account for the observed translational adaptation.
Important Results
• End result – New mechanism for neuron stimuli tuning – SK Channel mediated SFA in the LGMD is responsible for repression of translational stimuli.
• Because SK channels are responsible for medium AHP, Increasing numbers of activated SK channels gives a longer after hyperpolarization – resulting in a longer period of hyperpolarization – end result is a lowered firing frequency through adaptation.
• Locus achieves looming stimuli sensitivity through translational stimuli repression.
• Remaining Questions – Why were the “SK-Like” channels unaffected by apamin and or other known SK blockers?
• SK channel mutation or new channel class?