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

The neuropeptide Y (NPY)-elicited increase in high-affinity GTPase activity in the rat cerebral cortical membranes was assayed and compared with the g amma-aminobutyric acid (GABA)(B) receptor-mediated response, representative of the conventional receptor-dependent mode of G protein activation. GABA and a selective GABAB receptor agonist, (+/-)-baclofen, stimulated the high -affinity GTPase activity in a concentration-dependent and saturable manner , with a strict Mg2+ dependence. On the other hand, NPY (10 mu M)-stimulate d high-affinity GTPase activity was detectable even in the absence of Mg2+. The concentration-response curve for NPY-induced increase in high-affinity GTPase activity in the presence of 2 mM MgCl2 revealed a biphasic pattern, and NPY (100 nM)-stimulated activity was dependent on MgCl2. In the presen ce of 2 mM MgCl2, the increase in high-affinity GTPase activity by 100 nM N PY was almost fully inhibited by a selective NPY Y-1 receptor antagonist, ( R)-N-2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]argininamide (BIBP3226), whereas the effect of 10 mM NPY was only partially antagonized by this com pound. The increase in the activity by 10 mM NPY in the absence of MgCl2 wa s not at all inhibited by BIBP3226. The high-affinity GTPase activity was a ugmented by [Leu(31),Pro(34)]NPY (porcine) but not by desamido-NPY, NPY(13- 36) (porcine), or rat pancreatic polypeptide at submicromolar concentration s. These results indicate that NPY activates G proteins through two distinc t 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 hi gher concentrations of NPY.