Nitric oxide directly inhibits ghrelin-activated neurons of the arcuate nucleus

The hypothalamic arcuate nucleus (Arc) is a target site for signals regulating energy homeostasis. The orexigenic hormone ghrelin directly activates neurons of the medial arcuate nucleus (ArcM) in rats. Nitric oxide (NO) is a neuromodulator implicated in the control of food intake and body weight. NO is produced by nitric oxide synthase (NOS) and induces the formation of cyclic guanosine monophosphate (cGMP) via a stimulation of soluble guanylate cyclase (sGC). Both enzymes NOS and sGC have been identified in the Arc. Using extracellular recordings we characterized the effects of NO signaling on ArcM neurons and their co-sensitivity to ghrelin. The artificial NO donor sodium nitroprusside (10(-4) M) reversibly inhibited 91% of all ArcM neurons by a direct postsynaptic mechanism. 52% of ArcM neurons were excited by ghrelin. In all but one of these neurons SNP caused inhibitory responses. The SNP-induced inhibitions were mediated by cGMP since they were blocked by the specific sGC inhibitor ODQ (1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, 10(-4) M). Furthermore, the membrane permeating cGMP analogue 8-Br-cGMP (10(-4) M) mimicked the inhibitory responses of SNP. In immunohistological in vitro studies SNP induced a cGMP formation, which could also be blocked by ODQ. The current studies demonstrate that NO/cGMP signaling inhibits a large population of ArcM neurons including ghrelin-excited cells. Since an activation of the latter neurons is regarded as a correlate of negative energy balance, NO may represent an anorectic neuromodulator in the Arc and/or restrain the action of signals promoting energy intake. NO signaling in the Arc is also induced following inflammation suggesting a possible role of Arc-intrinsic NO in disease-related anorexia.