Actions Of Putative Amino Acid Neurotransmitters On The Neuropile Arborizations Of Locust Flight Motoneurones

To characterize the receptors for putative amino acid neurotransmitters present on the dendritic arborizations of flight motoneurones in Locusta migratoria, the effects of pressure applications of glutamate, γ-aminobutyric acid (GABA), aspartate, taurine, glycine and cysteine were studied using an animal preparation in which neuropile intracellular recordings could be made during expression of the flight motor output. A majority of cells responded to glutamate, GABA, aspartate and taurine. At resting potential, glutamate and GABA caused, in different cells, a depolarization, a hyperpolarization or, in a few cells, a biphasic response, all accompanied by a decrease in the size of the evoked and spontaneous postsynaptic potentials (PSPs). At spiking threshold, the responses were always hyperpolarizing. Activation of a chloride conductance mediated the effects of both glutamate and GABA. In some cells, the response to glutamate or GABA desensitized during long-lasting applications, but in most cells the amplitude of the response did not decrease during applications lasting several minutes. Responses to aspartate and glutamate had identical reversal potentials and cross-desensitized. Responses to GABA and taurine had more negative reversal potentials and did not cross-desensitize with those elicited by glutamate or aspartate. Only a few neurones responded to applications of glycine or cysteine at resting potential; they responded with an inhibition of spiking at depolarized potentials. These data suggest that a variety of amino acid receptors are present on the neuropile arborizations of flight motoneurones.