Presynaptic enhancement of signal transients in photoreceptor terminals in the compound eye

Abstract

Intracellular recordings show that photoreceptor axons in the first optic ganglion of the blowfly have characteristic electrical properties not seen in the soma. In the axon, a fast depolarizing transient (FDT) occurs on the rising phase of the photoresponse. The FDT is elicited by depolarizing current injections, can follow a sudden relaxation from hyperpolarization, has a distinct voltage threshold, and is accompanied by a rapid transient increase in membrane conductance. Blocking voltage-sensitive potassium conductances in photoreceptors with TEA unmasks spontaneous, long-lasting (ca. 50-150 ms) action potentials. These voltage-sensitive effects and measurements of membrane resistance suggest that the FDT is caused by a voltage-dependent increase in conductance, localized in the photoreceptor axon terminals. The transient amplifies the peak presynaptic response at least twofold, and appears to generate the fast, initial component of the transient postsynaptic response. Thus the FDT contributes to the synaptic amplification and boosting of high frequencies that enhance signals at the visual system's first synapse.