Actions of beta-endorphin and related peptide fragments on a pressor response induced by cholinergic stimulation.

The present study examined the antagonist potential of shorter beta-EP1-31 related peptide fragments in an opioid-sensitive model of blood pressure. In urethane-anesthetized rats, pretreated with methyl atropine, physostigmine (25-200 micrograms/kg i.v.) produced a pressor response that was stereoselectively depressed by i.c.v. levorphanol, an action reversed by naloxone. beta-EP1-31 and beta-EP1-27(H) (5-20 micrograms i.c.v.) also depressed the physostigmine-induced pressor response in a naloxone-reversible manner. However, injections of beta-EP1-27(O) or beta-EP1-26(H) failed to influence this response or to reverse the depressor action of levorphanol. These findings show that shorter fragments of beta-EP1-31 do not mimic the action of naloxone in an opioid-sensitive model of blood pressure.