Desensitization and potentiation during glutamate application to locust skeletal muscle

Abstract

1. Desensitization of glutamate receptor populations at excitatory synapses on locust leg muscle fibres have been studied using bath and iontophoretic application of L-glutamate. 2. Double-barrelled and single-barrelled iontophoretic electrodes were used for studying the effects of conditioning pulses of L--glutamate on the responses of excitatory synapses to constant test pulses of this amino acid. With small conditioning pulses test responses were potentiated but with larger conditioning pulses desensitization masked any potentiation that might have occurred. 3. Following the conclusion of a small conditioning pulse a further brief period of test response potentiation ensued. This post-conditioning potentiation was not seen following large conditioning pulses. 4. The effect of bath application of 5 × 10 −5 M-5 × 10 −2 M L-glutamate on the effective membrane resistance ( R eff) of locust leg muscle fibres was investigated. Initially L-glutamate caused a decrease in R eff, but R eff slowly recovered during continuing application of this ligand, e.g. with 5 × 10 −2 M glutamate, R eff initially decreased and then increased towards , steady value below the control R eff with a half-decay time of about 65 sec and a full-decay time of about 600 sec. 5. Following bath application of L-glutamate a period of supranormal R eff was observed. Injection of glutamate iontophoretically into a muscle fibre also increased R eff and slightly hyperpolarized the locust fibre. 6. Evidence was obtained for the contribution of calcium in both activation and desensitization of glutamate receptors on locust muscle fibres. However, since this evidence was from bath application studies it was not possible to identify the receptor populations involved.