The effect of adrenocorticotropin (ACTH)(1-24) and alpha-melanocyte stimulating hormone
(alpha-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 microg and the maximal effect was seen with 2 microg, 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[AcCys(11),
D-Nal(14), Cys(18), AspNH(2)(22)]-beta-MSH (11-22) (HS014), a selective melanocortin
4 receptor antagonist, injected into the same periventricular area 10 min before of
ACTH(1-24) or alpha-MSH. The results show that ACTH(1-24) and alpha-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.