Spike frequency adaptation mediates looming stimulus selectivity in a collision-detecting neuron

1. 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

2. 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

3. 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.

4. 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

5. 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 bilayer Gn and Gl = liner and non-linear ion flow Ip = current source = Na/K Pump

6. The Locus escape mechanism is activated in response to looming stimuli but not translating stimuli. How does this occur?

7. LGMD responds to looming stimuli • Maximum frequency in dynamic range - 27spikes/sec versus 300spikes/sec. Current was 10nA for locus.

8. 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.

9. 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

10. 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

11. Singling out SK from the KcaClass • 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.

12. SK-like Conductance responsible for SFA • SFA seems to be mediated by SK – confirmed calcium dependence and all other Kcaruled out. • But - aunique SK showing “nontraditional properties.” • “SK-like” – something new?

13. 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.

14. 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.

15. 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.

16. 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

17. Mathematical Confirmation based on the Hodgkin–Huxley model • Based Gnand Gl on SK-Like Kcaand modified for the morphology of the LGMD and Calcium localization. Also add BAPTA constraints – Plug into MATLAB • Result: SK-Like Kcaconductance can almost perfectly account for the observed translational adaptation.

18. 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.

19. Remaining Questions – Why were the “SK-Like” channels unaffected by apamin and or other known SK blockers? • SK channel mutation or new channel class?