Potassium-evoked release of endogenous primary amine-containing compounds from the trout saccular macula and saccular nerve in vitro

Abstract

An in vitro preparation of the trout saccular macula, containing a large number of hair cells, served as a potential source of neurotransmitter(s) released at the acousticolateralis hair cell-afferent nerve synapse. An in vitro preparation of the saccular nerve, maintained in parallel, served to indicate the potential neural contribution to overall release from the macula. Efflux of 27 primary amine-containing compounds from the macula and nerve fractions was monitored by cation-exchange HPLC with fluorescence detection, and release by 53.5 mM potassium was determined at 1.45 mM calcium, 0.35 mM magnesium or 0 mM calcium, 10.1 mM magnesium. Taurine was released from the saccular macula in the greatest amount, accounting for 72% of the total evoked release of primary amine-containing compounds. Its release was calcium dependent and its time course prolonged. The contribution by myelinated nerve and associated Schwann cells within the macula to overall release of taurine from the macula in the presence of calcium, as determined from the saccular nerve preparation, was only 2%. Other components specifically released from the macula included ethanolamine, phosphoserine, β-alanine, and glycine. Glutamate and aspartate were released from both the macula and saccular nerve fractions by potassium in the presence of calcium and in a ratio of 6:1 (glutamate:aspartate) for the macula and 7.5:1 for the nerve. The release of aspartate, but not that of glutamate, was lowered in saline containing 0 mM calcium, 10.1 mM magnesium. The calculated contribution from neural elements to overall release from the macula was 10% for aspartate and 18% for glutamate. These studies demonstrate that both the macula and saccular nerve fractions release the ‘excitatory neurotransmitter’ candidates aspartate and glutamate. Calcium-dependent, potassium-evoked release of taurine appears to be specific to the hair cell-supporting cell population of the saccular macula, and taurine may, therefore, be involved directly or indirectly in hair cell neurotransmission in labyrinthine organs. This study represents the first detailed biochemical characterization of efflux and release for an in vitro hair cell system of relatively high purity with respect to hair cells.