title: Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors.
abstract: In order to investigate the localization and the type(s) of excitatory amino acid receptors in the frog vestibular system, the exogenous amino acid agonists Quisqualic acid, Kainic acid and N-methyl-D-aspartic acid were tested on the sensory organ of semicircular canals. Intracellular recordings of the resting discharge from single afferents showed that these agonists exerted a complex excitatory action consisting in a rapid and brief increase in frequency of both EPSPs and spikes, followed by a slower and longer lasting membrane depolarization. The progressive impairment of natural transmitter release achieved by adding Mg2+ or Co2+ in the bath caused a dose-dependent decrease of the agonist-induced afferent discharge, without substantially affecting axonal depolarization. These results suggest that the exogenous amino acid agonists act both pre- and postsynaptically on the vestibular organs. Quisqualic acid and kainic acid were much more potent than N-methyl-D-aspartic acid in inducing excitatory effects, suggesting that the amino acid receptors located on both hair cells and afferent endings are mainly of the non-NMDA type. The present findings, while not excluding that an excitatory amino acid may be the afferent transmitter, highlight its possible function as a presynaptic modulator of the afferent transmission in the frog vestibular system.
authors.affiliation: Institute of General Physiology, University of Pavia, Italy.
authors.forename: I
authors.initials: I
authors.lastname: Prigioni
authors.forename: G
authors.initials: G
authors.lastname: Russo
authors.forename: P
authors.initials: P
authors.lastname: Valli
authors.forename: S
authors.initials: S
authors.lastname: Masetto
mesh_terms: D000344:Afferent Pathways / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005971:Glutamates / Q000502:physiology*; D066298:In Vitro Techniques; D007608:Kainic Acid / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D016202:N-Methylaspartate; D010069:Oxadiazoles / Q000494:pharmacology*; D016318:Quisqualic Acid; D011893:Rana esculenta; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter / Q000187:drug effects* / Q000502:physiology; D013569:Synapses / Q000187:drug effects / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D014722:Vestibule, Labyrinth / Q000187:drug effects* / Q000502:physiology
chemical_list: D005971:Glutamates; D010069:Oxadiazoles; D017470:Receptors, Glutamate; D017981:Receptors, Neurotransmitter; D001224:Aspartic Acid; D016202:N-Methylaspartate; D016318:Quisqualic Acid; D007608:Kainic Acid