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.
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