Activation of G proteins by neuropeptide Y and gamma-aminobutyric acid(B) receptor agonists in rat cerebral cortical membranes through distinct modes of action.

The neuropeptide Y (NPY)-elicited increase in high-affinity GTPase activity in the rat cerebral cortical membranes was assayed and compared with the gamma-aminobutyric acid (GABA)(B) receptor-mediated response, representative of the conventional receptor-dependent mode of G protein activation. GABA and a selective GABA(B) receptor agonist, (+/-)-baclofen, stimulated the high-affinity GTPase activity in a concentration-dependent and saturable manner, with a strict Mg(2+) dependence. On the other hand, NPY (10 microM)-stimulated high-affinity GTPase activity was detectable even in the absence of Mg(2+). The concentration-response curve for NPY-induced increase in high-affinity GTPase activity in the presence of 2 mM MgCl(2) revealed a biphasic pattern, and NPY (100 nM)-stimulated activity was dependent on MgCl(2). In the presence of 2 mM MgCl(2), the increase in high-affinity GTPase activity by 100 nM NPY was almost fully inhibited by a selective NPY Y-1 receptor antagonist, (R)-N(2)-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]argininami de (BIBP3226), whereas the effect of 10 microM NPY was only partially antagonized by this compound. The increase in the activity by 10 microM NPY in the absence of MgCl(2) was not at all inhibited by BIBP3226. The high-affinity GTPase activity was augmented by [Leu(31),Pro(34)]NPY (porcine) but not by desamido-NPY, NPY(13-36) (porcine), or rat pancreatic polypeptide at submicromolar concentrations. These results indicate that NPY activates G proteins through two distinct modes of action: the conventional receptor-mediated pathway through NPY Y-1 receptor subtype dominant in the presence of the lower concentrations of NPY and receptor-independent, direct G protein activation driven by the higher concentrations of NPY.