ACTH- and α-MSH-induced grooming, stretching, yawning and penile erection in male rats: Site of action in the brain and role of melanocortin receptors

The effect of adrenocorticotropin (ACTH)(1-24) and α-melanocyte stimulating hormone (α-MSH) on grooming, stretching, yawning and penile erection was studied after injection into different brain areas. Both peptides induce the above responses when injected into the hypothalamic periventricular region of the third ventricle. This region includes the paraventricular nucleus, the dorsomedial nucleus, the ventromedial nucleus and the anterior hypothalamic area. The minimal effective dose of both peptides was 0.5 μg and the maximal effect was seen with 2 μg, the highest dose tested. Irrespective of the injection site, grooming started 5–7 min after injection of either peptide, while stretching, yawning and penile erection started only after 15–35 min and lasted for 90–120 min. In contrast both peptides were ineffective when injected into the preoptic area, the caudate nucleus or the CA1 field of the hippocampus. Grooming, stretching and yawning, but not penile erection, were prevented by cyclic[AcCys¹¹, D-Nal¹⁴, Cys¹⁸, AspNH₂²²]-β-MSH (11-22) (HS014), a selective melanocortin 4 receptor antagonist, injected into the same periventricular area 10 min before of ACTH(1-24) or α-MSH. The results show that ACTH(1-24) and α-MSH act in the hypothalamic periventricular region to induce the above responses and that grooming, stretching and yawning, but not penile erection, are mediated by melanocortin 4 receptors.