TRANSFER OF GRADED POTENTIALS AT THE PHOTORECEPTOR INTERNEURON SYNAPSE

To characterize the transfer of graded potentials and the properties o f the associated noise in the photoreceptor-interneuron synapse of the blowfly (Calliphora vicina) compound eye, we recorded voltage respons es of photoreceptors (R1-6) and large monopolar cells (LMC) evoked by: (a) steps of light presented in the dark; (b) contrast steps; and (c) pseudorandomly modulated contrast stimuli at backgrounds covering 6 l og intensity units. Additionally, we made recordings from photorecepto r axon terminals. Increased light adaptation gradually changed the syn aptic signal transfer from low-pass to band-pass filtering. This was a ccompanied by decreased synaptic delay and increased contrast gain, bu t the overall synaptic gain and the intrinsic noise (i.e., transmissio n noise) were reduced. Based on these results, we describe a descripti ve synaptic model, in which the kinetics of the tonic transmitter (his tamine) release from the photoreceptor axon terminals change with mean photoreceptor depolarization. During signal transmission, tonic trans mitter release is augmented by voltage-dependent contrast-enhancing me chanisms in the photoreceptor axons that produce fast transients from the rising phases of the photoreceptor responses and add these enhance d voltages to the original photoreceptor responses. The model can pred ict the experimental findings and it agrees with the recently proposed theory of maximizing sensory information.