Actions of excitatory amino acid agonists and antagonists on the primary afferents of the vestibular system of the axolotl ( Ambystoma mexicanum)

Abstract

In order to determine the nature of the transmitter in the synapse between hair cells and primary afferent fibers, both resting and evoked spike activity of vestibular system afferents were recorded. Excitatory amino acid agonists and antagonists were applied by micro perfusion. Excitatory amino acid agonists consistently increased the firing rate of these afferents. The rank order in potencies of the agonists tested was:kainate⩾quisqualate> d-aspartate⩾ l-glutamate⩾ l-aspartate> N-methyl d-aspartate. Blockade of synaptic transmission with high-Mg 2+ and low-Ca 2+ solutions did not seem to affect the responses to the excitatory amino acid agonists indicating their postsynaptic action. Excitatory amino acid antagonists inhibit both resting and physiologically evoked activity. The rank order of inhibitory potency was:kynurenate> l-glutamate diethyl ester> d,l-2- amino-4-phosphono-butyrate> d-α-amino adipate> d,l-2-amino-5-phosphonovalerate. This findings suggest that an amino acid-related compound may be transmitter at this synapse. The relative potencies of agonists and antagonists tested provide evidence that the transmitter released from the hair cell's basal pole in the axolotl vestibular system interacts with postsynaptic kainic/quisqualic type receptors.