enkephalin--ala(2)-mephe(4)-gly(5)- and amastatin

Previous in vitro studies showed that the degradation of dynorphin-(1-8) [dyn-(1-8)] by cerebral membrane preparations is almost completely prevented by a mixture of three peptidase inhibitors (PIs), amastatin, captopril and phosphoramidon. In the present investigations, effects of the three PIs on the anti-nociception induced by the intra-third-ventricular (i.t.v.) administration of dyn-(1-8) were examined. The inhibitory effect of dyn-(1-8) on the tail-flick response was increased more than 100-fold by the i.t.v. pretreatment of rats with the three PIs. The inhibition produced by dyn-(1-8) in rats pretreated with any combination of two PIs was significantly smaller than that in rats pretreated with three PIs, indicating that any residual single peptidase could inactivate significant amounts of dyn-(1-8). The antagonistic effectiveness of naloxone, a relatively selective mu-opioid antagonist, indicates that dyn-(1-8)-induced inhibition of tail-flick response in rats pretreated with three PIs is mediated by mu-opioid receptors. Furthermore, mu-receptor-mediated inhibition induced by dyn-(1-8) was significantly greater than that produced by [Met5]-enkephalin in rats pretreated with three PIs. The data obtained in the present investigations together with those obtained in previous studies strongly indicate that dyn-(1-8) not only has well-known kappa-agonist activity but also has high mu-agonist activity.

Topics: Analgesics, Opioid; Analysis of Variance; Angiotensin-Converting Enzyme Inhibitors; Animals; Anti-Bacterial Agents; Captopril; Drug Interactions; Dynorphins; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Glycopeptides; Injections, Intraventricular; Male; Naloxone; Narcotic Antagonists; Pain; Pain Measurement; Peptide Fragments; Peptides; Protease Inhibitors; Rats; Rats, Wistar; Receptors, Opioid, mu

The s.c. administration of [Met5]-enkephalin to 10-day-old rats pretreated with the mixture of 3 peptidase inhibitors, amastatin, captopril and phosphoramidon, produced the inhibition of tail-flick response and loss of righting reflex. When infant rats were pretreated with the mixture of any combination of two peptidase inhibitors, however, the change in both the response and the reflex were not produced at all by enkephalin injection, indicating that 3 kinds of enzymes, amastatin-sensitive aminopeptidase(s), captopril-sensitive peptidyl dipeptidase A and phosphoramidon-sensitive endopeptidase 24.11, played an important role in the inactivation of enkephalin after its systemic administration. Additionally, the fact that the two enkephalin-induced effects were more effectively antagonized by naloxone, a relatively selective mu-opioid antagonist, than by naltrindole, a specific delta-antagonist, or by nor-binaltorphimine, a specific kappa-antagonist, showed that these two effects were produced by the interaction of enkephalin with mu receptors. Moreover the involvement of mu receptors in the production of these two effects was shown by the fact that the s.c. administration of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a selective mu agonist, also produced these two effects which were more effectively antagonized by naloxone than by naltrindole or nor-binaltorphimine. Since the magnitude of the two effects induced by enkephalins in 15-day-old rats was significantly lower than that in 10-day-old rats, and the two enkephalin-induced effects were not produced at all in 20-day-old rats, a maturation-induced decrease in the permeability of the blood-brain barrier against opioid peptides was indicated.

Topics: Aging; Animals; Anti-Bacterial Agents; Captopril; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Methionine; Enkephalins; Female; Glycopeptides; Injections, Subcutaneous; Male; Oligopeptides; Pain; Peptides; Posture; Protease Inhibitors; Rats; Rats, Wistar; Reflex