CHEMICAL SYNAPTIC TRANSMISSION BETWEEN TYPE I VESTIBULAR HAIR CELLS AND THE PRIMARY AFFERENT NERVE CHALICE: AN INTRACELLULAR STUDY UTILIZING HORSERADISH PEROXIDASE

Abstract

This work addresses the morpho-functional relationship between the presynaptic receptor hair cell (HC) and its postsynaptic primary afferent nerve terminal with particular consideration given to the synapse between the type I HC and its nerve chalice. This relationship is of interest because in our attempt to understand the role of the calyx, key features such as the mode of transmission and means of action potential initiation are undefined at present.;The reptile, Calotes versicolor, was used in this experiment. Computer analyzed spike trains with subsequent intracellular injection of these physiologically characterized units with horseradish peroxidase (HRP), demonstrated that irregularly discharging afferents (coefficient of variation (c.v. > 0.4) terminated on one or more contiguous type I hair cells via a calyceal termination (termed multicalyces if there were more than a single type I HC incorporated), while regular fibers (c.v. < 0.4) terminated either solely on multiple type II HCs via bud-shaped endings or combinations of both bud-shaped endings on type II HCs and calyceal terminations on type I HCs.;Electrophysiological and pharmacological properties of subthreshold events in irregular primary afferents (c.v. > 0.4) were observed that support a chemical mode of transmission across the calyceal synapse. These properties include: (1) Observation of subthreshold events underlying rising phases of all action potentials. (2) Frequency of subthreshold events was modulated along with action potential frequency in response to caloric disfacilitation and facilitation. (3) Hyperpolarizing current injected through the microelectrode modulated psp amplitude, hyperpolarizing increasing depolarizing decreasing amplitude. (4) Subthreshold events were tetrodotoxin (ttx) resistant, action potentials were blocked by ttx and further, caloric stimuli continued to modulate psp frequency in the ttx treated axon. (5) Application of cobalt chloride to the perilymph caused a reversible inhibition of spontaneous spiking and decreased the frequency of subthreshold events. The amplitude of the remaining events varied around a value of .85mV. These subthreshold events are interpreted as miniature or quantal synaptic events coming from the type I hair cell. Serially sectioned HRP injected irregular afferents consistently demonstrated membrane associated presynaptic specializations, ribbons, with associated small, clear vesicles.;These results were applied to a hypothesis addressing the generation of irregular and regular firing in vestibular primary afferents.