TONIC TRANSMITTER RELEASE IN A GRADED POTENTIAL SYNAPSE

1. We studied graded synaptic transmission in the fly photoreceptor-in terneuron synapse by using intracellular in situ recordings from pre- and postsynaptic cells. 2. A large presynaptic hyperpolarization after Light adaptation, caused by the activation of the electrogenic Na+/K pump, drastically reduced the conspicuous postsynaptic dark noise. At the same time, the postsynaptic neurons depolarized, with an increase of input resistance of 5-10 M Omega. 3. The spectral characteristics of the postsynaptic membrane noise in dark and during noise reduction, together with the other results, suggested that the transmitter relea se decreased dramatically similar to 12 mV below the resting potential of the presynaptic photoreceptors. 4. During the postsynaptic noise r eduction, the saturated and subsaturated first-order visual interneuro n responses were increased up to 9 mV with a time constant of recovery of similar to 10 s. This increase was shown to be caused by the negat ive shift of the reversal potential of the transmitter-gated (mainly C l-) conductance, caused apparently by the reduced transmitter input. 5 . The results strongly suggest that the photoreceptor transmitter rele ase in fly is tonic, even in dark, and further support the modulation of the synaptic voltage transfer by postsynaptic Cl-extrusion.