Differential effect of ACTH1–24 and α-MSH induced grooming in the paraventricular nucleus of the hypothalamus

Injection of ACTH_1–24 as well as α-MSH in the paraventricular nucleus of the hypothalamus (PVH) induces intense grooming in the rat. While comparing the details of MSH, ACTH and control grooming, we found that the induction of grooming was highly site specific. Even injection of saline in that specific area produced some grooming, possibly due to the release of endogenous substances. To distinguish between effects caused by the peptides and the effects caused by the injection procedure, we compared the behavioural effects of saline and peptide injections in sites with exactly the same location in the PVH, in a post-hoc matched pairs design. Using this design we found that the grooming response induced by saline is of a limited. ACTH_1–24 and α-MSH prolong grooming beyond that period. Interestingly, rats receiving α-MSH continued to groom, while rats receiving ACTH_1–24 changed to scratching. This confirms earlier findings suggesting that grooming and scratching have a differential organization at the level of the PVH. Whether the peptides also have a role in the initiation of the grooming response, or just prolong a response caused by other local factors requires another experimental approach.