Role of Nitric Oxide in Alpha-melanocyte-stimulating Hormone-induced Hypotension in the Nucleus Tractus Solitarii of the Spontaneously Hypertensive Rats

Pro-opiomelanocortin (POMC) is expressed in the nucleus tractus solitarii (NTS) of the brainstem, where nitric oxide (NO) plays an important role in cardiovascular regulation. The POMC-derived neuropeptides and their receptors are important regulators of energy homeostasis and cardiovascular functions in the central nervous system. In this study, we investigated the cardiovascular effect of alpha-melanocyte-stimulating hormone (alpha-MSH), a POMC-derived neuropeptide, and its relationship with NO pathway in the NTS of spontaneously hypertensive rats (SHR). Unilateral microinjection of alpha-MSH (0.3-300 pmol) into the NTS resulted in a dose-dependent hypotension and bradycardia in urethane-anesthetized SHR. The alpha-MSH-induced hypotension was abolished by pretreatment with the antagonist of melanocortin-3/4 receptor (MC-3/4R), Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (SHU9119). Blockade of cAMP/protein kinase A (PKA), the downstream effector of melanocortin receptors, by previous injection of N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H89) also ablated the cardiovascular effect of alpha-MSH. To elucidate the role of NO pathway in alpha-MSH-evoked hypotension, pretreatment with Nomega-nitro-L-arginine methyl ester, a universal inhibitor of nitric-oxide synthase (NOS), partially reversed the depressor and bradycardic effects of alpha-MSH. Furthermore, previous application of the inducible NOS (iNOS) inhibitor, aminoguanidine, but not the neuronal NOS inhibitor, 7-nitroindazole, attenuated the cardiovascular effect of alpha-MSH. Histological analysis revealed the colocalization of MC-4R, but not MC-3R, with iNOS in the NTS of SHR. In summary, intra-NTS injection of alpha-MSH induces hypotension and bradycardia of SHR via MC-4R signaling, which activates cAMP/PKA and iNOS.