Nonlinear Models of the First Synapse in the Light-Adapted Fly Retina.
JUUSOLA, M., M. WECKSTROM , R.O. UUSITALO, M.J. KORENBERG AND A.S. FRENCH.
Department of Physiology and Biophysics, Dalhousie University, Halifax, NS,
Canada, Department of Physiology, University of Oulu, Oulu, Finland,
Department of Electrical Engineering, Queen's University, Kingston, Ontario,
Canada.
APStracts 2:0219N, 1995.
SUMMARY AND CONCLUSIONS
1 . Randomly modulated light stimuli were used to characterize the nonlinear
dynamic properties of the synapse between photoreceptors and large monopolar
neurons (LMC) in the fly retina. Membrane potential fluctuations produced by
constant variance contrast stimuli were recorded at eight different levels of
background light intensity. 2 . Representation of the photoreceptor-LMC
input-output data in the form of traditional characteristic curves, indicated
that synaptic gain was reduced by light adaptation. However, this
representation did not include the time-dependent properties of the synaptic
function, which are known to be nonlinear. Therefore, nonlinear systems
analysis was used to characterize the synapse. 3 . The responses of
photoreceptors and LMCs to random light fluctuations were characterized by
second-order Volterra series, with kernel estimation by the parallel cascade
method. Photoreceptor responses were approximately linear, but LMC responses
were clearly nonlinear. 4 . Synaptic input-output relationships were measured
by passing the light stimuli to LMCs through the measured photoreceptor
characteristics to obtain an estimate of the synaptic input. The resulting
nonlinear synaptic functions were well characterized by second-order Volterra
series. They could not be modelled by a linear-nonlinear-linear cascade, but
were better approximated by a nonlinear-linear-nonlinear cascade. 5 . These
results support two possible structural models of the synapse, the first
having two parallel paths for signal flow between the photoreceptor and LMC,
and the second having two distinct nonlinear operations, occurring before and
after chemical transmission. 6 . The two models were each used to calculate
the synaptic gain to a brief change in photoreceptor membrane potential. Both
models predicted that synaptic gain is reduced by light adaptation.
Received 17 April 1995; accepted in final form 25 July 1995.
APS Manuscript Number J260-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.