The Journal of Neurophysiology Vol. 83 No. 4 April 2000, pp. 2103-2112
Copyright ©2000 by the American Physiological Society
Potentiation in the First Visual Synapse of the Fly Compound Eye
R. O.
Uusitalo1,2 and
M.
Weckström1,3
1Department of Physiology,
2Department of Ophthalmology, and
3Department of Physical Sciences/Division of
Biophysics, University of Oulu, FIN-90220 Oulu, Finland
Uusitalo, R. O. and
M. Weckström.
Potentiation in the First Visual Synapse of the Fly Compound Eye. J. Neurophysiol. 83: 2103-2112, 2000. In the first visual synapse of the insect compound eye, both
the presynaptic and postsynaptic signals are graded, nonspiking changes
in membrane voltage. The synapse exhibits tonic transmitter release
(even in dark) and strong adaptation to long-lasting light backgrounds,
leading to changes also in the dynamics of signal transmission. We have
studied these adaptational properties of the first visual synapse of
the blowfly Calliphora vicina. Investigations were done in situ by
intracellular recordings from the presynaptic photoreceptors,
photoreceptor axon terminals, and the postsynaptic first order visual
interneurons (LMCs). The dark recovery, the shifts in intensity
dependence, and the underlying processes were studied by stimulating
the visual system with various adapting stimuli while observing the
recovery (i.e., dark adaptation). The findings show a transient
potentiation in the postsynaptic responses after intense light
adaptation, and the underlying mechanisms seem to be the changes in the
equilibrium potential of the transmitter-gated conductance (chloride)
of the postsynaptic neurons. The potentiation by itself serves as a
mechanism that after light adaptation rapidly recovers the sensitivity
loss of the visual system. However, this kind of mechanism, being an
intrinsic property of graded potential transmission, may be quite
widespread among graded synapses, and the phenomenon demonstrates that
functional plasticity is also a property of graded synaptic transmission.
