The C‐Type Natriuretic Peptide Receptor is the Predominant Natriuretic Peptide Receptor mRNA Expressed in Rat Hypothalamus

The natriuretic peptide receptors (NPR) are membrane‐bound guanylate cyclases with extracellular binding domains specific for particular members of the natriuretic peptide family. NPR‐A binds atrial natriuretic peptide (ANP) with high affinity, whereas the NPR‐B appears to be specific for C‐type natriuretic peptide (CNP). Previous data indicating extensive overlap between localization of ANP and CNP in hypothalamic neuroendocrine circuits suggest the importance of determining whether specificity of natriuretic peptide action may be conferred via receptor type present on target cells. To address this issue, we used in situ hybridization histochemistry to localize NPR‐A and NPR‐B mRNA in the hypothalamus. NPR‐A mRNA was not found in substantial abundance in any hypothalamic nucleus; however, detectable NPR‐A signal was observed in other brain regions, including the subfornical organ and medial habenula. In contrast, NPR‐B mRNA was expressed throughout the hypothalamus, including neurons of the magnocellular and parvocellular paraventricular, the arcuate, and the supraoptic nuclei. Expression was also seen in other nuclei essential to neuroendocrine control, including the median preoptic, anteroventral periventricular, tuberomammillary, ventromedial and suprachiasmatic nuclei. NPR‐B mRNA was also observed in the neural lobe of the pituitary gland, suggesting expression by pituicytes. The results suggest that NPR‐B is the primary natriuretic peptide receptor in hypothalamus, and by inference indicate that CNP is the primary active natriuretic peptide in neuroendocrine regulation.