impact of presynaptic voltage transients on postsynaptic ...via.i-networx.de/docs/nwg_2009.pdf ·...

Literature1) Haag and Borst (2004) Neuronal mechanism underlying complex receptive field properties of motion-selective interneurons. Nat Neurosci 7:628-635

2) Beckers et. al. (2007) Synapses in the fly motion vision pathway: evidence for a broad range of signal amplitudes and dynamics. J Neurophysiol 97:2032-2041

3) Haag and Borst (2008): Electrical coupling of lobula plate tangential cells to a heterolateral motion-sensitive neuron in the fly. J Neurosci 28(53):14435–14442

To assess how the occurrence and timing of V1 spikes is affected by VS spikes we monitored V1

spiking activity when single VS spikes were elicited by brief current injections (H). V1 spikes were

tightly coupled to the injected current pulses in time (I). On occurence of a VS spike a V1 spike fol-

lowed with much higher probability than when the current pulse remained subthreshold, i.e. when it

failed to elicit a VS spike (J). The larger a VS spike is, the higher is the probability that a V1 spike is

triggered (K).

Voltage clamping of one VS

cell during rest (i.e. in the ab-

sence of visual motion) left

the postsynaptic spike rate

unaffected. However, the in-

terspike interval distribution of V1 spikes was affected. While in the non-clamped situation (L) many

doublet V1 spikes (marked by the X) occurred, the occurence of doublet V1 spikes was significantly

reduced when one VS cell was voltage-clamped (M, N).

When a presynaptic VS cell is voltage clamped V1 spikes are accompanied by a brief hyperpolari-

zing current transients in the clamped VS cell (C). Some V1 spikes (3.1%) are not accompanied by

a detectable current transient (D-E). The amplitu-

des of these transients are not uniform (F). Some

VS2/3 cells showed a clear bimodal distribution of

current transient amplitudes (G).

The occurence of ‘failures’ (V1 spikes not accompanied

by a current transient in VS), the different amplitudes

of the current transients, and in particular the bimo-

dal distribution, speak against the assumption that the

observed current transients result mainly from electri-

cal coupling between VS and V1. We therefore propo-

se that due to electrical coupling among the VS cells

the current transients are caused by the occurrence

of spikes in neighboring VS cells.

We conclude the spike rate of the postsynaptic V1-neuron to be controlled by both, graded voltage

modulations and spikes of the presynapic VS cells. Whereas the graded presynaptic component re-

gulates the postsynaptic spike rate to a large extent, the presynaptic spikes predominantly control

the actual timing of postsynaptic spikes. Presynaptic spikes play a role for ensemble synchroniza-

tion, as is evident from the decrease in the rate of postsynaptic spike doublets when one VS cell is

prevented from spiking. Signal transmission from VS to V1 is largely mediated by chemical synap-

ses, but an additional contribution from electrical coupling cannot be excluded and an eleclectrical

coupling between VS1 and V1 has recently been shown3.

Introduction Results

Results

Conclusions

Ulrich Beckers, Martin Egelhaaf and Rafael KurtzDepartment of Neurobiology, Bielefeld University, 33501 Bielefeld, Germanyubeckers1@uni-bielefeld.de

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vcmode

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ML N

Information transfer between synaptically coupled neurons

can be mediated by spikes, by graded signals, or by combi-

nation of both types of signals. We simultaneously recorded

from identified visual motion sensitive cell pairs of the fly (A).

The presynaptic VS cells convey both graded and spike si-

gnals (VS membrane potential: blue traces) to their common

purely spiking postsynaptic target cell V1 (V1 spikes indicated

as red dashes). The VS cells are electrically coupled among

one another1. Synaptic transmission of graded signals from

VS to V1 is linear over a broad signal range2. Postsynaptic

spikes were elicited by voltage clamp (C) or injecting brief

current pulses (H) into one VS cell. In this study we analyzed

the time course of the holding current (C-G) to monitor the input received by the presynaptic cell and

the correlation of V1 spike activity to single VS spikes (H-K). The role of fast signals for postsynaptic

synchronization was studied by functionally removing one VS cell from the input ensemble (L-N).

Contribution to the 8th NWG Göttingen Meeting 2009

Impact of presynaptic voltage transients on postsynaptic spike

timing at a graded synapse in the fly motion vision system

intracellular recording electrode in VS axon

extracellularrecording electrodein V1's output region

registeredspikes

VS-V1 synapse

null direction preferred direction

resting potential

VS cell retinotopic input region

V1

VS1VS2VS3VS4

A

B

C

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n=21

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n=6