G-PROTEIN IS COUPLED TO PRESYNAPTIC GLUTAMATE AND GABA RECEPTORS IN LOBSTER NEUROMUSCULAR SYNAPSE

We have examined the effects of L-glutamate and γ-aminobutyric acid (GABA) on the presynaptic membrane of spiny lobster by the use of intra-axonal recording near the nerve terminals. Application of glutamate to the synaptic region produced hyperpolarization in the presynaptic membrane but depolarization in the postsynaptic membrane. The presynaptic glutamate potential (PGP) is generated by an activation of K+ channels, as evidenced by its dependence on external K+ concentration. The PGP was not affected by a spider toxin (JSTX), which blocks the postsynaptic glutamate receptor. By contrast, pertussis toxin (IAP) effectively blocked the PGP without affecting the resting conductance channels or action potentials in the presynaptic membrane. Guanosine 5'-O-(3-thiotriphosphate) (GTPγS), a hydrolysis-resistant analogue of GTP, blocked the PGP, suggesting the involvement of a G protein in the generation of K+ current. Application of GABA induced depolarization or hyperpolarization in the presynaptic axons depending on the resting membrane potential. By reducing external Cl-, GABA-induced hyperpolarizations were converted to depolarizations, indicating that they are mainly mediated by Cl-. In contrast to GABA, baclofen consistently induced hyperpolarization in low Cl- solution as well as in normal solution. Baclofen-induced hyperpolarization was blocked by IAP, indicating the mediation of G protein. These results suggest that the presynaptic membrane of lobster neuromuscular synapse has entirely different types of amino-acid receptors from those in the postsynaptic membrane. Both the excitatory and the inhibitory axonal membrane have glutamate ('glutamate(B)') and GABA(B) receptors, which activate K+ channels via G protein. These metabotropic receptors, along with GABA(A) receptor in the excitatory axon, may regulate transmitter release in the crustacean neuromuscular synapse.