Potentiation in the first visual synapse of the fly compound eye

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, photo-receptor axon terminals, and the postsynaptic first order visual inter-neurons (LMCs). The dark recovery, the shifts in intensity depen-dence, 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 poten-tiation 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 mecha-nism that after light adaptation rapidly recovers the sensitivity loss of the visual system. However, this kind of mechanism, being an intrin-sic property of graded potential transmission, may be quite wide-spread among graded synapses, and the phenomenon demonstrates that functional plasticity is also a property of graded synaptic transmission