Effects of morphine withdrawal syndrome on endo-oligopeptidase (EC 3.4.22.19) activity

Aminopeptidase activity in the postmortem brain of human heroin addicts

Several studies have reported that the chronic administration of opioids induces changes in the biosynthesis of endogenous opioid peptides or their precursors in specific brain regions of the adult central nervous system. However, little is known about the catabolic regulation of opioid peptides and its contribution to neuroadaptative changes underlying drug addiction. In the present study, we have analyzed the activity of two enkephalin-degrading enzymes (puromycin-sensitive aminopeptidase or PSA and aminopeptidase N or APN) and two functionally different, soluble aminopeptidases (aminopeptidase B and aspartyl-aminopeptidase) in postmortem samples of prefrontal cortex and caudate nucleus of eight human heroin addict brains and eight matched-controls. Enzyme activities were fluorimetrically measured using beta-naphthylamide derivatives. An increase in the activity of soluble PSA in the prefrontal cortex of heroin abusers was observed (heroin addict group: 51,452+/-3892 UAP/mg protein versus control group: 42,003+/-2597 UAP/mg protein; P<0.05), while the activity of the other peptidases in both brain regions remained unaltered. This result agrees with previous findings in morphine-tolerant rats, and indicates that soluble PSA may be involved in neurobiological processes which underlie heroin addiction.

Biochemical and pharmacological aspects of two bradykinin-potentiating peptides obtained from tryptic hydrolysis of casein

Peptides that display bradykinin-potentiating activity have been obtained from a number of distinct sources, such as snake venoms, fibrinogen, and casein. This paper describes the characterization of two new peptides generated by tryptic hydrolysis of casein. No homology was found with other known vasoactive or vasopotentiating peptides, especially by the lack of Ile-Pro-Pro motif. The peptides EMPFPK and YPVEPFTE, corresponding to the gamma casein sequence (108-113 and 114-121, respectively), displayed a selective potentiating activity on isolated guinea pig ileum for bradykinin. Besides, the octapeptide YPVEPFTE showed an in vitro competitive inhibitor effect on angiotensin-converting enzyme and thimet oligopeptidase and presented an opiate-like activity, increasing two times the latence time in the hot-plate assay. The results suggest that the isolated bioactive peptides act on conversion and/or inactivation of endogenous peptides by enzymes such as angiotensin-converting enzyme and thimet oligopeptidase by modifying several systemic responses such as blood-pressure regulation and in pain response.

Metabolism of dynorphins by peptidases of pulmonary artery endothelial cells

Degradation of several dynorphins by peptidases expressed in cultured porcine pulmonary artery endothelial cells was studied by incubation of the peptide in cell suspensions followed by electrospray ionization and tandem mass spectrometric analyses. Under the in vitro conditions applied, only the metabolism of dynorphin A1-8 occurred in a significant extent. Studies involving specific peptidase inhibitors indicated that mainly bestatin-sensitive aminopeptidases, thiorphan-sensitive endopeptidases, and cFPAAF-pAB-sensitive endopeptidases expressed by the endothelial cells were involved in the process that converted dynorphin A1-8 to dynorphin A2-8, dynorphin A1-6, and leucine enkephalin (dynorphin A1-5), respectively. These peptidases may form a metabolic barrier for the cellular penetration of intact dynorphin A1-8 and/or control effects of the circulating peptide on endothelial opioid receptors of the cells.

Effects of morphine administration and its withdrawal on rat brain aminopeptidase activities

The endogenous opioid neuropeptide system seems to be involved in the neural processes which underlie drug addiction. Several studies have reported that the administration of morphine induces changes in the levels and/or activity of endogenous opioid peptides (enkephalin, dynorphin) and their precursors in specific brain regions of the adult CNS. The aim of this work was to study the effects of chronic morphine exposure and its withdrawal on certain aminopeptidases capable of degrading opioid peptides in brain areas including the amygdala, hypothalamus, hippocampus, striatum and brain cortices. In animals treated with morphine, aminopeptidase N presented higher enzyme activity levels in the striatum, the hypothalamus and the amygdala compared to control animals, although statistically significant differences were observed only in the case of the striatum. In addition, the activity of soluble puromycin-sensitive aminopeptidase (PSA) was found to be higher in the frontal cortex of these rats. In contrast, rats experiencing withdrawal symptoms presented decreased levels of aminopeptidase activity in certain brain areas. Thus, the activity of aminopeptidase N in the hippocampus and soluble puromycin-sensitive aminopeptidase in the frontal cortex were found to be lower in rats experiencing naloxone precipitated withdrawal symptoms, compared to the corresponding controls. Finally, the activity of the three studied aminopeptidases in vitro was unaltered by incubation with morphine, suggesting that the observed effects are not due to a direct action of this opioid upon the aminopeptidases. The results of the present report indicate that aminopeptidases may play an important role in the processes of tolerance and withdrawal associated with morphine administration.

Opiate modulation of dynorphin conversion in primary cultures of rat cerebral cortex

Rat brain cortical cells in primary culture were used to investigate long-term effects of opiates on endopeptidases acting on dynorphin peptides. Enzyme activity in the soluble fraction of the cells converted dynorphin B to Leu-enkephalin-Arg6 and to a lesser extent to Leu-enkephalin. Five day treatment with 10 microM morphine increased the conversion to Leu-enkephalin-Arg6 by 370%. This effect was prevented by the presence of naloxone in the culture medium. The opiate-inducible activity was directed to the Arg-Arg bond in dynorphins with preference for dynorphin B > alpha-neoendorphin > > dynorphin A. The Km for the generation of Leu-enkephalin-Arg6 from dynorphin B was 40 microM. Enzyme activity was inhibited by dynorphin fragments, in the following order of potency: dynorphin A(1-13) > A(2-13) > A(1-17) > A(2-17) and by SH-reagents, suggesting the presence of a cysteine-protease. The opiate-stimulated dynorphin-converting enzyme (DCE)-activity affects the balance between dynorphin peptides (selective for kappa-opioid receptors) and enkephalin peptides (selective for delta-opioid receptors). Since both types of opioid peptides can influence the development of opiate tolerance, the change in the extent of this transformation may be functionally important.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.