The effect of β-(Tyr⁹)melanotropin-(9-18) on the active avoidance and open-field behavior on rats pretreated with atropine or propranolol

β-(Tyr⁹)melanotropin-(9-18) inhibited the extinction of active avoidance behavior. The muscarinic cholinergic blocker atropine did not influence the peptide-induced inhibition, whereas the β-receptor blocker propranolol decreased it. Furthermore, the peptide increased the ambulation of the animals. Neither the muscarinic blocker nor the β-blocker had any action on this effect of β-(Tyr⁹)melanotropin-(9-18). Atropine markedly decreased the defecation of the animals, and this effect was not influenced by the peptide. The results suggest that the β-receptors play an important role in the inhibition of extinction induced by the peptide, however, the actions on the open-field activity are mediated by different mechanisms.