Central neuropeptide B administration activates stress hormone secretion and stimulates feeding in male rats

Neuropeptide B (NPB) was identified to be an endogenous, peptide ligand for the orphan receptors GPR7 and GPR8. Because GPR7 is expressed in rat brain and, in particular, in the hypothalamus, we hypothesized that NPB might interact with neuroendocrine systems that control hormone release from the anterior pituitary gland. No significant effects of NPB were observed on the in vitro releases of prolactin, adrenocorticotropic hormone (ACTH) or growth hormone (GH) when log molar concentrations ranging from 1 pM to 100 nM NPB were incubated with dispersed anterior pituitary cells harvested from male rats. In addition NPB (100 nM) did not alter the concentration response stimulation of prolactin secretion by thyrotropin-releasing hormone, ACTH secretion by corticotropin-releasing factor (CRF) and GH secretion by GH-releasing hormone. However, NPB, when injected into the lateral cerebroventricle (i.c.v.) of conscious, unrestrained male rats, elevated prolactin and corticosterone, and lowered GH levels in circulation. The threshold dose for the effect on corticosterone and prolactin levels was 1.0 nmol, while that for the effect on GH release was 3.0 nmol NPB. Pretreatment with a polyclonal anti-CRF antiserum completely blocked the ability of NPB to stimulate ACTH release and significantly inhibited the effect of NPB on plasma corticosterone levels. NPB administration i.c.v. did not significantly alter plasma vasopressin and oxytocin levels in conscious rats. It did stimulate feeding (minimum effective dose 1.0 nmol) in sated animals in a manner similar to that of the other endogenous ligand for GPR7, neuropeptide W. We conclude that NPB can act in the brain to modulate neuroendocrine signals accessing the anterior pituitary gland, but does not itself act as a releasing or inhibiting factor in the gland, at least with regard to prolactin, ACTH and GH secretion.