Exogenous and endogenous opioids as biological response modifiers

Leucine enkephalin degradation in allergopathic versus normal human plasma

The enzyme hydrolysis of labelled leu-enkephalin in the presence of plasma enzymes was studied by kinetic and chromatographic techniques in a group of allergopathic patients in the acute and quiescent stage; data obtained have been compared with those obtained with normal controls. Results shown indicate that in the quiescent stage substrate degradation is reduced, and that the pattern of the hydrolysis by-products is modified with respect to the controls. In the acute as compared to the quiescent stage, enkephalin hydrolysis is further reduced, and the pattern of hydrolysis by-products is further modified. ANOVA analysis of these data indicates that the dependency of hydrolysis reduction upon the grouping of subjects (i.e., controls, quiescent and acute stage) is statistically very significant. Reduced substrate hydrolysis, and modified hydrolysis pattern, appears to be associated with decreased activity of the enzymes involved and more significantly with increased activity of the low molecular weight plasma inhibitors. The combination of these two factors appears to define a hydrolysis pattern characteristic of the allergopathic subjects, similar in the quiescent and acute phase, and different from that observed in the controls.

Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Recently, mu-, delta- and kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major opioid receptor types, more extensive studies have suggested the possible existence of other opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, beta-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative epsilon-opioid receptor. Many epsilon-opioid receptors are functionally coupled to G-proteins. The functional epsilon-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as beta-endorphin, but not by selective mu-, delta- or kappa-opioid receptor agonists. Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the epsilon-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no epsilon-selective ligand currently available, these findings provide strong evidence for the existence of the putative epsilon-opioid receptor and its unique function in the brain.

Age-induced increase of leucine enkephalin enzyme degradation in human plasma

Possible age-induced variations of the hydrolysis of leucine enkephalin in the presence of plasma enzymes were studied by kinetic and chromatographic techniques in a group of elderly individuals. Results obtained indicate that in elderly individuals the activity of enkephalin-degrading plasma enzymes is greater than in the controls; ANOVA analysis of these data indicates that the dependency of the variation of hydrolysis upon the two age groups is statistically significant. Increased substrate hydrolysis, and a modified hydrolysis pattern, appear to be associated with increased activity of the enzymes involved, and with different distribution of the individual enzymes within each class, as well as with severely reduced activity of the low molecular weight plasma inhibitors. The combination of these factors defines a characteristic hydrolysis pattern for the elderly individuals, different from that found in the controls.

Immunoregulatory effects of morphine on human lymphocytes

It is now well established that parenteral drug abuse is a significant risk factor for contracting human immunodeficiency virus type 1 (HIV-1) infection and subsequently developing AIDS. Earlier studies have shown that morphine can modulate various immune responses and therefore support the premise that morphine is a cofactor in susceptibility to and progression of HIV infection. Dysregulation of interferon (IFN) production, nonspecific apoptosis of T cells, and the immune response to soluble HIV gene products have been associated with potential mechanisms of pathogenesis in HIV disease. The present study was undertaken to examine the immunomodulatory role of morphine on HIV protein-induced lymphocyte proliferative responses, Sendai and Newcastle disease virus-induced alpha IFN (IFN-alpha) and IFN-beta production by lymphocytes and fibroblast cells, respectively, and induction of apoptosis of normal lymphocytes in vitro. Our results demonstrate that HIV protein-induced human lymphocyte proliferative responses were significantly inhibited by morphine in a dose-dependent manner. Furthermore, morphine significantly inhibited both IFN-alpha and IFN-beta production by normal lymphocytes and fibroblasts but induced apoptosis of normal lymphocytes. Inhibition of IFN-alpha production by morphine could be reversed by the opiate receptor antagonist naloxone. This suggests that the immunomodulatory effects of morphine are mediated through the opioid receptor. These studies support a role of morphine as a cofactor in the pathogenesis of HIV infection and describe some of the possible pathologic mechanisms which underlie the immunoregulatory effects of morphine.

Immunomodulation by two potent analogs of met-enkephalin

Met-enkephalin (Tyr-Gly-Gly-Phe-Met) and its more stable analogs, Tyr-D-Ala-Gly-MePhe-Met-NHC3H gamma-iso (1) and Tyr-D-Ala-Gly-MePhe-Gly-NHC3H gamma-iso (2) significantly enhanced human T-cell proliferation in vitro after 5 days of incubation in the absence of mitogen. The activity was completely inhibited by naloxone, an opioid antagonist. These peptides significantly enhanced human active T-cell rosette (CD2R) also on in vitro treatment. Furthermore, these analogs stimulated interleukin-2 production by human peripheral blood mononuclear cells in vitro which was completely inhibited by naloxone. These observations suggest that human T-cells bear receptors for Met-enkephalin on their surface. Such findings may provide a link between the central nervous system and the immune system.

kappa opioid receptors in human microglia downregulate human immunodeficiency virus 1 expression

Microglial cells, the resident macrophages of the brain, play an important role in the neuropathogenesis of human immunodeficiency virus type 1 (HIV-1), and recent studies suggest that opioid peptides regulate the function of macrophages from somatic tissues. We report herein the presence of kappa opioid receptors (KORs) in human fetal microglia and inhibition of HIV-1 expression in acutely infected microglial cell cultures treated with KOR ligands. Using reverse transcriptase-polymerase chain reaction and sequencing analyses, we found that mRNA for the KOR was constitutively expressed in microglia and determined that the nucleotide sequence of the open reading frame was identical to that of the human brain KOR gene. The expression of KOR in microglial cells was confirmed by membrane binding of [3H]U69,593, a kappa-selective ligand, and by indirect immunofluorescence. Treatment of microglial cell cultures with U50,488 or U69,593 resulted in a dose-dependent inhibition of expression of the monocytotropic HIV-1 SF162 strain. This antiviral effect of the kappa ligands was blocked by the specific KOR antagonist, nor-binaltrophimine. These findings suggest that kappa opioid agonists have immunomodulatory activity in the brain, and that these compounds could have potential in the treatment of HIV-1-associated encephalopathy.

Endogenous opiates: 1995

This article is the eighteenth installment of our annual review of research concerning the opiate system. It includes articles published during 1995 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects. 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.