Abstract

The neuropeptide Y (NPY)-elicited increase in high-affinity GTPase activity in the rat cerebral cortical membranes was assayed and compared with the γ-aminobutyric acid (GABA)_Breceptor-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 Mg²⁺ dependence. On the other hand, NPY (10 μM)-stimulated 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 presence of 2 mM MgCl₂, the increase in high-affinity GTPase activity by 100 nM NPY was almost fully inhibited by a selective NPY Y-1 receptor antagonist, (R)-N²-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]argininamide (BIBP3226), whereas the effect of 10 μM NPY was only partially antagonized by this compound. The increase in the activity by 10 μM NPY in the absence of MgCl₂ was not at all inhibited by BIBP3226. The high-affinity GTPase activity was augmented by [Leu³¹,Pro³⁴]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.