Pharmacological evidence of a central effect of naltrexone, morphine, and β-endorphin and a peripheral effect of met- and leu-enkephalin on retention of an inhibitory response in mice

Effect of bestatin, an aminopeptidase inhibitor, on memory in inhibitory-avoidance and Y-maze discrimination tasks

“…The dose-response effects obtained with the two tasks used in this study are similar to those obtained in previous studies of the effects of morphine, ß-endorphin, and enkephalins (Introini, 1984;Introini & Baratti, 1984;Introini et al, 1985). In those studies, high doses did not affect memory.…”

Opioid Receptors Regulate the Extinction of Pavlovian Fear Conditioning.

“…Consistent with a role of opioid receptors in memory processing, posttraining administrations of opioid receptor agonists impair (e.g., Introini, McGaugh, & Baratti, 1985), whereas posttraining administrations of opioid receptor antagonists facilitate (e.g., Introini-Collison, Nagahara, & McGaugh, 1989;McGaugh, Introini-Collison, & Nagahara, 1988), retention of aversive conditioning indexed by passive avoidance. However, unlike the effects of pretraining injections of naloxone on the acquisition of conditioned freezing described above, these posttraining effects of naloxone on retention of passive avoidance frequently do not interact with US intensity.…”

The Endomorphin System and Its Evolving Neurophysiological Role

“…The following mechanisms of action were suggested: 1) inhibition of memory retrieval, which was observed for other -agonists, such as Tyr-D-ArgPhe-␤-Ala (Ukai et al, 1995a) and morphine (Saha et al, 1991); 2) modulation of normal animal response to fear or anxiety (Asakawa et al, 1998), similar to that elicited by morphine and DAMGO, which exert anxiolytic activity in rats (File and Rodgers, 1979;Motta and Brandao, 1993;Motta et al, 1995;Koks et al, 1999); and 3) inhibition of noradrenergic neurons, because an opposite effect was observed for the -receptor antagonist naloxone (Introini and Baratti, 1986). Intracerebroventricular administration of endomorphin-1 failed to inhibit the consolidation of conditioned defeat, whereas morphine was shown to impair the consolidation of newly acquired memories in rats and mice (Izquierdo, 1979;Introini et al, 1985;Castellano et al, 1994;Cestria and Castellano, 1997;Rudy et al, 1999). Again three possibilities were proposed: 1) the endomorphin-1 dose used was too low; 2) the time of exposure to endomorphin-1 was not long enough to develop consolidation; and 3) the interaction with the -receptor could result in secondary messenger cascade different from that activated by morphine.…”