Influence of Agonists on Desensitization of Glutamate Receptors on Locust Muscle

1. The desensitization, by ionophoretically and bath-applied L-glutamate and agonists, of excitatory post-junctional receptor populations on locust extensor tibiae muscle was investigated. 2. The kinetics of onset of and recovery from desensitization were determined ionophoretically either with trains of glutamate/agonist pulses of different frequencies ('conditioning trains') followed by a single 'test' pulse at different intervals after cessation of the conditioning train or with trains of constant frequency drug pulses superimposed upon steady 'conditioning' doses. Both methods gave quantitatively similar results. 3. For approximately equipotent doses, L-glutamate and agonists produced different rates of desensitization onset. Qualitatively similar results were obtained when changes in input conductance of single muscle fibres were measured during bath application of the amino acids. 4. The time courses of recovery from desensitization for receptor populations activated by glutamate and agonists could be described adequately by single exponentials and were independent of the concentration of amino acid except when desensitization exceeded ca. 90%. 5. The rate of recovery from desensitization was different for each agonist.