Effects of guanine nucleotides on kainic acid binding and on adenylate cyclase in chick optic tectum and cerebellum.

Adenylate cyclase activity and binding of neurotransmitters to some receptors can be modulated simultaneously by guanine nucleotides. Furthermore it has been shown, in different neurotransmitter systems, that the ability of GTP to inhibit agonist binding is related to the capacity of the transmitter to modulate adenylate cyclase activity. In the present report we show that in chick optic tectum and cerebellum the effects of guanine nucleotides on kainic acid binding and on adenylate cyclase activity can be dissociated. In lysed membrane preparations, GTP, GDP, and GMP, or their analogs, displace binding of kainic acid with the same efficiency, whereas only GTP stimulates adenylate cyclase. In vesicle preparations, all three nucleotides inhibit binding of kainic acid without modifying adenylate cyclase activity. The present results suggest that, if adenylate cyclase is indeed coupled to this particular type of excitatory amino acid receptor, the coupling mechanism would be probably different from those operating in other neurotransmitter systems and also that the displacement of kainic acid by GDP and GMP (and even perhaps by GTP) is not likely to depend on the interaction between the receptor and a Gs-protein-mediated effector system.