Effects of alpha-MSH and melatonin on passive avoidance and on PA-induced defecation and plasma 11-OHCS in hypophysectomized rats.

The present study shows tha alpha-MSH facilitates the acquisition and delays the extinction of a Passive Avoidance Response (PAR) in the hypox animals. MSH exacerbates PA-induced defecation in both hypox and sham-hypox animals. Hypox and sham-hypox animals treated with MSH do not differ on PAR or on PA-induced defecation. Melatonin, on the other hand, has no significant effect on PAR in hypox rats, but retards acquisition and facilitates extinction of the PAR in sham-hypox rats. Melatonin also inhibits PA-induced defecation in sham-hypox rats. Sham-hypox and hypox rats treated with Melatonin do not differ on PAR learning, retention (Extinction) and PA-induced defecation. MSH and Melatonin also seem to have opposite effects on plasma 11-OHCS levels measured at the end of PAR extinction. MSH increases plasma 11-OHCS in hypox rats, whereas Melatonin decreases plasma 11-OHCS in sham-hypox rats. Melatonin does not lower further the very low level of plasma 11-OHCS in hypox rats.