Neuropeptides: conductors of the immune orchestra

Analysis of the beta-endorphin structure-related activity on human monocyte chemotaxis: importance of the N- and C-terminal

We evaluated the chemotactic activity of beta-endorphin and beta-endorphin-related peptides on human monocytes. We tested beta-endorphin(1-31) and fragments (1-16), (1-17), (1-27) in which the N-terminal of the opioid is preserved, N-acetyl-beta-endorphin(1-31) and fragments (6-31) and (28-31) in which the C-terminal is preserved, and fragment (2-17) that lacks both the N- and C-terminal. The fragments in which the N- and C-terminal were preserved [with the exception of fragment (28-31)] showed a chemotactic effect, while the lack of both terminals deprived the peptides of any activity. Moreover, only the N-terminal-mediated effects were naloxone reversible, while the C-terminal effects were not. These results indicate that while the intact N-terminal is necessary for opioid like effects, both N- and C-terminal can mediate effects on the immune system, thus offering evidence for a nonopioid receptor-mediated effect of opioid peptides on the immune system.

Protection of endogenous enkephalin degradation from peptidases in human serum by actinonin

This paper describes a new method for detecting the endogenous enkephalin level in human serum without interference from serum peptidases and contaminating proteins. The method consists of a combination of addition of a peptidase inhibitor, actinonin to serum and dialysis. The activity of enkephalin degrading enzymes was completely abolished by actinonin. Furthermore, contaminating proteins which interfered with RIA assay of enkephalins were removed by the dialysis process. This combination process enabled precise detection of the concentration of enkephalin-like substances in human serum. The mean normal serum level of leucine-enkephalin assayed by this method in 12 healthy volunteers was 93.4 pg/ml.

Suppression of natural killer cell activity by FG 7142, a benzodiazepine receptor "inverse agonist"

A dose-dependent (5-50 mg/kg) suppression of natural killer (NK) cell activity was observed 2 h after administration of the benzodiazepine receptor "inverse agonist" FG 7142 (N-methyl-beta-carboline-3-carboxamide), and was still manifest 24 h later. Addition of FG 7142 (1-1000 nM) to the 4 h 51 Cr release assay did not affect NK cell activity. Pretreatment of mice with the benzodiazepine receptor antagonist Ro 15-1788 (10 mg/kg) blocked FG 7142-induced suppression of NK cell activity, but had no effect when administered alone. The suppression of NK cell activity by FG 7142, a compound which produces a syndrome resembling stress or anxiety in both animals and man, provides further evidence that the central nervous system pathways subserved by the benzodiazepine/GABA receptor chloride channel complex ("supramolecular complex") may play a role in the modulation of immune function.

Interleukin-2 induction of ACTH secretion: presence of an interleukin-2 receptor alpha-chain-like molecule on pituitary cells

Pituitary cells were shown to release corticotropin (ACTH) in response to interleukin-2 (IL-2) and to express a protein that is related to the alpha-chain of the IL-2 receptor (IL-2R). The alpha-chain-like molecule was bound by a rat monoclonal antibody to the murine IL-2 receptor as well as to IL-2. Sodium dodecylsulfate-polyacrylamide gel electrophoretic analysis of the affinity-purified material from pituitary cells demonstrated a protein which was similar to that which was isolated from activated splenocytes. Thus, IL-2 and its receptor may be one of several hormone-receptor pairs utilized by both the immune and neuroendocrine systems for intersystem communication.

Beta-endorphin and related peptides suppress phorbol myristate acetate-induced respiratory burst in human polymorphonuclear leukocytes

In the present study, the immunomodulatory effect of beta-endorphin (beta-E) and shorter pro-opiomelanocortin (POMC) fragments was evaluated by assessing their influence on respiratory burst in human polymorphonuclear leukocytes (PMN). The effect of the peptides (10(-17)M - 10(-10)M) on phorbol myristate acetate (PMA)-stimulated production of reactive oxygen metabolites was measured in a lucigenin-enhanced chemiluminescence (CL) assay. Both POMC peptides with opiate-like activity (i.e. alpha-endorphin (alpha-E), beta-E and gamma-endorphin (gamma-E] and their non-opioid derivatives (i.e. des-TYR1-beta-endorphin (dT beta E), des-TYR1-gamma-endorphin (dT gamma E), and des-ENK-gamma-endorphin (dE gamma E] were tested. With the exception of alpha-E, PMA-stimulated respiratory burst was suppressed by all POMC fragments tested. A U-shaped dose-response relation was observed. Doses lower than 10(-17)M and higher than 10(-8)M were without effect. beta-E and dT beta E both suppressed PMA-induced oxidative burst in human PMN at physiological concentrations (10(-16)M - 10(-10)M). gamma-E and dT gamma E proved to be less potent inhibitors, reaching maximal effect at higher concentrations (10(-12)M - 10(-10)M). DE gamma E exerted an even less pronounced but still significant suppressive effect at the concentration of 10(-10)M. None of the endorphins tested was shown to affect resting oxidative metabolism in the PMN. The modulatory effects of the opioid peptides could not be blocked by the opioid antagonist naloxone (10(-8)M). These data show that fragments derived from the POMC-precursor molecule modulate the activation of PMN by suppressing PMA-stimulated oxidative metabolism and that this activity does not involve a classical opiate-like receptor.

Mitogenic activation of the human lymphocytes induce the release of proenkephalin derived peptides

Several reports indicate that enkephalins participate in lymphocyte proliferation and several events of the immune response. It has been proposed that peptides involved in these processes may originate in the nervous system or endocrine glands. We have found that human peripheral blood lymphocytes (PBL) activated with a mitogenic agent contain and release proenkephalin derived peptides. The kinetics of met-enkephalin and cryptic products of proenkephalin in PBL activated with phytohemaglutinin (PHA) were studied. Peptides were released to the supernatant of stimulated PBL, reaching the highest values after 18 to 24 hours. The material secreted corresponds to high, intermediate and low molecular weight peptides derived from proenkephalin, displaying met-enkephalin and synenkephalin (proenkephalin 1-70) immunoreactivity. Therefore, an intrinsic lymphocytic proenkephalin system is induced by PHA and may play an important role in the regulation of the immune response.

A stereoselective blockade by naloxone of opioid and non-opioid-induced granulocyte activation

Naloxone was found to prevent both opioid and non-opioid-induced migration of human granulocytes in a stereoselective way. Indeed, besides being able to inhibit morphine-induced migration, (-) but not (+), naloxone isomer proved to abolish either casein, serum of fMLP-induced chemotaxis. It is concluded that opioid-induced modulation of granulocyte migration is likely to be mediated through specific receptors, possibly of the mu type. Moreover, the antichemotactic effect of naloxone suggests an involvement of opioid receptors and/or endogenously released opioids in the mechanism of granulocyte activation by different chemoattractants.

Psychoneuroimmunology for the psychoneuroendocrinologist: a review of animal studies of nervous system-immune system interactions

Evidence for interactions between the nervous and immune systems arises from a number of experimental observations: the behavioral conditioning of immune responses, the effects of stimulation or lesion of brain sites on immune system function, the effects of stressors on immune responses and tumor growth, and physiological and neurochemical changes in the brain during immune responses. The links between the nervous and immune systems probably include glucocorticoids secreted from the adrenal gland, catecholamines and neuropeptides secreted by sympathetic terminals and the adrenal medulla, certain pituitary and gonadal hormones, and polypeptides produced by cells of the immune system. The effect of glucocorticoids is not exclusively immunosuppressive, nor is it adequate to explain all the effects of stress. The effects of opiates on immune function are complex; in vitro, endogenous opiates most often facilitate immune activity, but in vivo, opiates appear to inhibit immune responses and impair tumor rejection. The in vitro effects are rarely prevented by naloxone pretreatment and appear to require the integrity of the C- rather than the N-terminal of beta-endorphin, suggesting a nonopiate character. Infections or the administration of antigens increase circulating concentrations of glucocorticoids and activate cerebral catecholaminergic metabolism, especially in the hypothalamus. These responses suggest that challenges to the immune system are physiologic stressors. Interleukin-1 (IL-1) produced by immune cells may be the mediator of these effects, thus acting as an "immunoneurotransmitter". The cerebral responses suggest that the brain can monitor the progress of immune responses. IL-1 and the glucocorticoids together may form a regulatory feedback mechanism for immune responses.

Pharmacological modulation of beta-endorphin in rat peritoneal macrophages

The neuropeptide beta-endorphin is present in cells of the immune system, i.e., lymphocytes and monocytes, and its expression can be induced by immunological stimuli. In the present study, we showed that the increase of the serotoninergic availability induces an increase of beta-endorphin concentrations in rat peritoneal macrophages that is blunted by the administration of serotonin receptor antagonists. A significant increase of beta-endorphin concentrations is also evident after blocking the dopaminergic receptors, whereas a dopaminergic agonist decreases the concentrations of the peptide. Our data are consistent with a similar modulation of beta-endorphin concentrations in central nervous system and in immune cells, e.g., rat peritoneal macrophages.

Evidence for a delta-class opioid receptor on cells of the immune system

Opioid receptors have been reported to be present on cells of the immune system. Both opioid receptor-ligand interactions which are reversible by naloxone (classical) and those interactions which are irreversible by naloxone (nonclassical) appear to alter various immunologic parameters through the ligand-receptor interactions. In an effort to characterize opioid receptors (which operate with classical interactions) on cells of the immune system, we employed the delta-class opioid receptor-specific ligand, cis-(+)-3-methylfentanylisothiocyanate (SUPERFIT). We herein report the specific labeling of a protein with a molecular weight of 70,000 Da under reducing conditions (58,000 Da under nonreducing conditions) from both B and T cell-enriched murine splenocytes and peripheral blood lymphocytes. Similar results are demonstrated by human peripheral blood lymphocytes and plastic-adherent (monocyte-enriched) leukocytes.

Aging decreases beta-endorphin enhancement of T-cell mitogenesis in mice

Numerous studies have demonstrated the role of the central nervous system in immunomodulation. beta-Endorphin, a neuropeptide that is released along with adrenocorticotropin by the pituitary in response to stress, has been shown to have various effects on immune function, although these effects are dependent on dose, animal model, and immune cell tested. Since the increased risk for infection and tumor that is observed in the elderly is thought to be in part secondary to waning cell-mediated immunity, we investigated the effect of age on beta-endorphin immunomodulation of T-cell proliferation in a murine model. Spleen cells obtained from young and old BALB/c mice were cultured in vitro with various mitogens with and without beta-endorphin. beta-Endorphin at 10(-8) M on day 3 of culture significantly enhanced concanavalin A (2.0 micrograms/10(6) cells per ml) mitogenesis but not phytohemagglutinin or lipopolysaccharide mitogenesis. Moreover, this enhancement was shown only in spleen cells from young mice and was not blocked by the opiate receptor antagonist naloxone, which suggests that enhancement of mitogenesis by beta-endorphin was mediated by a non-opiate receptor. Finally, our results support an altered response to neuroimmunomodulation with age.

Inhibition of mitogen-stimulated T lymphocyte proliferation by calcitonin gene-related peptide

The effect of calcitonin gene-related peptide (CGRP) on mouse lymphocyte proliferation stimulated by mitogens was studied. CGRP (10(-10)-10(-7) M) dose-dependently inhibited the proliferative response of mouse lymph node cells and spleen cells stimulated by T cell mitogens concanavalin A (Con A) and phytohemagglutinin (PHA), whereas a B cell mitogen lipopolysaccharide (LPS) did not inhibit this response. The maximal inhibition by this peptide was 50% to 80% at 10(-8) and 10(-7) M. The addition of 10(-8) and 10(-7) M CGRP to lymph node cell cultures 24 hr after stimulation with Con A or PHA also had a significant inhibitory effect on the proliferative response. Furthermore, in the same concentration range (10(-10)-10(-7) M) CGRP increased intracellular cyclic AMP concentration in nylon wool nonadherent cells, but not in nylon wool adherent cells. CGRP had no significant effect on intracellular cyclic GMP concentration. In addition, specific binding of CGRP was observed in mouse spleen cells. Our present study suggests that CGRP inhibits the proliferative response of T lymphocytes to the mitogens by interacting with cell receptors coupled with adenylate cyclase. CGRP may be implicated in the regulation of T cell function.

The effect of opiates upon prostatic carcinoma cell growth

The effect of opiate receptor agonists upon cell growth of the prostatic carcinoma cell line DU145 were studied. Dynorphin-A increased growth significantly with a peak response at 10(-13) M, of 21 +/- 4% (mean +/- SEM). The dose response curve had a typical inverted-U shape. Dynorphin fragments 1-13 and 1-7 also increased growth at 10(-13) M, while the 2-13 fragment failed to increase growth. Naloxone increased growth at high concentration (10(-7) M) suggesting a stimulatory effect, while at the same time blocking the effect of dynorphin-A. This data demonstrates that agents which stimulate opiate receptors, especially the kappa receptor agonist dynorphin, increase the growth of prostatic carcinoma, and that this effect is controlled by changes at the N-terminal end of the peptide. This effect is blocked by Naloxone.

Studies of the in vitro human plasma degradation of methionine-enkephalin

1. Incubation of [3H]tyrosine methionine-enkephalin (6 x 10(-9) M final concentration) with human platelet-poor plasma (1:9 ratio to Trizma Base buffer, pH 7.4) results mostly (greater than 95%) in hydrolysis of the tyrosyl-glycine peptide bond. This enzymatic reaction is essentially completed within 90 min, showing a half-life, Km and Vmax of 12.8 +/- 2.5 min, 0.70 +/- 0.01 mM and 17.90 +/- 1.05 mumol/L/min, respectively. These values are comparable to those previously reported for the human plasma degradation of leucine-enkephalin. 2. As expected hydrolysis of the methionine-enkephalin tyrosyl-glycine peptide bond was blocked by the known aminopeptidase inhibitors bestatin and puromycin (IC50 1.2 +/- 0.4 and 4.3 +/- 2.4 microM, respectively) but not by either thiorphan or captopril. 3. Neither the storing (up to 60 days) nor the freezing and thawing (up to ten times during a 60 days periods) significantly changed the above kinetic parameters, showing the stability of the plasma methionine-enkephalin degrading aminopeptidase.

Brain peptide reverses effect of morphine on human lymphocytes

E-rosette formation by human lymphocytes incubated with sheep red blood cells (sRBC) is inhibited by morphine. We studied the ability of the opiate antagonists naloxone and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) to block this action. Active E-rosette formation by lymphocytes incubated with morphine was reduced from the control of 35.7 +/- 1.7% to 23.7 +/- 1.5% (p less than 0.001). Similarly, total E-rosette formation was reduced by morphine from the control of 65.8 +/- 1.3% to 53.2 +/- 2.9% (p less than 0.001). These effects were blocked by co-incubation of the lymphocytes with either Tyr-MIF-1 or naloxone (p less than 0.05). Tyr-MIF-1 was active (p less than 0.05) at concentrations as dilute as 10(-13) M. These results indicate that the neuropeptide Tyr-MIF-1 exerts an antiopiate effect at the human T-lymphocyte.