Role of Endothelin Type B Receptor in NO/cGMP Signaling Pathway in Rat Median Eminence

We studied the effect of endothelins (ETs) on receptor-mediated NO/cGMP signaling in rat arcuate nucleus–median eminence (AN-ME) fragments, an hypothalamic structure known to contain a rich plexus of nitric oxide synthase (NOS)-containing neurons and fibers together with densely arranged ETB-receptor-like immunoreactive fibers. NOS activity was determined measuring the conversion of [3H] arginine to [3H] citrulline, as an index of NO produced. cGMP production was determined by radio immunoassay. ET-1, ET-3, and the selective ETB receptor agonist, IRL1620, significantly increased cGMP formation and NOS activity. Preincubation of AN-ME fragment with L-arginine analog, N-nitro-L-arginine (L-NAME), inhibited ET-1 or IRL1620-stimulated cGMP formation. The addition of the selective ETB receptor antagonist, BQ788, blocked ET-1-, ET-3-, or IRL1620-induced increase in NOS activity and cGMP generation, while BQ123, a selective ETA receptor antagonist, was ineffective. Our results demonstrate that in whole rat AN-ME fragments, ETs stimulate NO/cGMP signaling pathway through the interaction with the ETB receptor subtype, supporting the concept that ETs may represent an important regulator of reproductive and neuroendocrine function.