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

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?