J. exp. Biol. 140, 535-548 (1988) 535 Printed in Great Britain © The Company of Biologists Limited 1988 THE ACTIONS OF L-GLUTAMATE AT THE POSTSYNAPTIC MEMBRANE OF THE SQUID GIANT SYNAPSE BY D. J. ADAMS 1 AND J. I. GILLESPIE 2 The Laboratory of the Marine Biological Association, Plymouth PL1 2PB, UK, 1 Department of Pharmacology, University of Miami School of Medicine, PO Box 016189, Miami, FL 33101, USA and 2 Department of Physiological Sciences, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, UK Accepted 22 June 1988 Summary The actions of L-glutamate on the postsynaptic membrane of the squid giant synapse were investigated using two methods of application: ionophoresis and bath perfusion. Bath perfusion of lOmmoll" 1 sodium glutamate did not produce an appreciable depolarization of the postsynaptic membrane but reversibly blocked the neurally evoked postsynaptic potential (PSP). The postsynaptic membrane depolarized when L-glutamate was applied ionophoretically. The sensitivity to glutamate application was not uniform, but sharply localized to sites which may correspond to synaptic contacts made by branching colaterals from the postsynaptic axon. The relationship between membrane potential and amplitude of the glutamate-activated postsynaptic potential (PSP) examined under current- clamp conditions was linear over the voltage range studied (—110 to —60 mV) with an extrapolated reversal potential of — 36 mV. The amplitude of the glutamate- activated PSP was reduced either by replacing Na + in the external solution with Tris + (Na + -free) or by raising the extracellular K + concentration to 20mmoll~' and was abolished by removing both Na + and Ca 2+ from the bath solution. The PSP amplitude was insensitive to changes in the extracellular Mg 2+ concentration. The extrapolated reversal potential of the glutamate PSP was shifted to more positive potentials in both Na + -free and raised-K + bathing solutions and was unchanged by anion substitution. The depolarization induced by L-glutamate increased with increasing ionophor- etic current and reached a maximum with large pulses. Double logarithmic plots of the coulomb dose-response relationship gave a limiting slope in the range 1-7-2-2, suggesting that two glutamate molecules are required for receptor activation. The time course of desensitization of the glutamate response was studied using a double-pulse method. The initial decrease in the ratio, PSP 2 /PSP!, is followed by a slower time-dependent recovery of the postsynaptic response with a time constant of 8-5 s. Prolonged perfusion of the squid giant synapse with concanavalin A failed to abolish desensitization of the glutamate-evoked PSP. Key words: glutamate, squid giant synapse, calcium, dose-response, desensitization.