Quaternary naltrexone: its immunomodulatory activity and interaction with brain delta and kappa opioid receptors

Minor pathological changes are induced by naltrexone-poly(DL-lactide) implants in pregnant rats

Oral naltrexone is used to treat alcohol and heroin dependence but is associated with poor patient compliance. Sustained-release preparations have been developed to overcome noncompliance. Many sustained-release preparations are composed of polymers combined with naltrexone. Limited data indicate that polymers induce variable levels of tissue reactivity and that naltrexone may increase this effect. A slow-release subcutaneous naltrexone-poly (DL-lactide) implant is currently being trialed to treat heroin dependence in Western Australia. A minority of women fall pregnant and, although tissue reactivity in nonpregnant humans is relatively minor, detailed chronological data during pregnancy are lacking. Histological changes in pregnant rats were assessed; a single active tablet containing poly[trans-3,6-dimethyl-1,4-dioxyane-2,5-dione] (DL-lactide) loaded with 25 mg of naltrexone was implanted subcutaneously, and tissue response was compared with inactive polymer implantation. Rats were timed mated at 13-26 days postimplant. Tissue assessment up to 75 days by a pathologist showed that naltrexone induced chronic inflammatory response in a dose-dependent manner, although still at a low level. Furthermore, for inactive implants, minimal foreign body reaction and fibrosis, together with low-level inflammation, suggested good long-term biocompatibility. We conclude that the Australian naltrexone-poly(DL-lactide) implant is tolerated in pregnant rats, reinforcing its potential role for managing alcohol and heroin dependence in pregnant humans.

Modulation of humoral immune responses in the rat by centrally applied Met-Enk and opioid receptor antagonists: functional interactions of brain OP1, OP2 and OP3 receptors

We have previously demonstrated that central application of leucine-enkephalin (Leu-Enk) elicits potentiation and suppression of humoral immune responses through OP(1) (delta) and OP(2) (kappa) receptors, respectively. Interestingly, both effects were found to be additionally dependent on OP(3) (mu) receptor function. In the present study, we have further investigated whether opioid receptor interactions underlie the immunomodulatory effects of endogenous opioids as well as exogenously applied methionine-enkephalin (Met-Enk). For that purpose, the plaque-forming cell (PFC) response was determined in rats injected intracerebroventricularly (i.c.v.) with opioid receptor-selective antagonists and Met-Enk. Application of the OP(1) antagonist ICI 174864, but not naltrindole, resulted in suppression of the PFC response. In contrast, i.c.v. injection of the OP(2) selective antagonist nor-binaltorphimine (nor-BNI) significantly potentiated the PFC response. Both effects, presumably mediated by endogenous opioid peptides, were antagonized by the OP(3) receptor antagonist beta-funaltrexamine (beta-FNA) at a dose that was devoid of immunomodulatory activity. The immunopotentiation of the PFC response induced by Met-Enk was reversed by OP(1) receptor antagonists, naltrindole and ICI 174864, but not by beta-FNA or nor-BNI. On the basis of these and previous findings, it may be concluded that central OP(3) receptors are permissive for the central immunomodulatory action of endogenous opioid peptides and Leu-Enk. In contrast, the central immunoenhancing effect of Met-Enk appears to be mediated through OP(3)-independent OP(1) receptors.

Heroin modulates the expression of inducible nitric oxide synthase

The use of heroin (diacetylmorphine) is associated with a high incidence of infectious disease, and the immunologic alterations responsible for heroin-induced changes in resistance to infection have not been well characterized. The present study tests the hypothesis that expression of inducible nitric oxide synthase (iNOS) is modulated by the administration of heroin. The initial study using rats showed that heroin administration (0, 0.01, 0.1, or 1.0 mg/kg s.c.) results in a pronounced reduction in lipopolysaccharide (LPS)-induced expression of iNOS mRNA in spleen, lung, and liver tissue as measured by RT-PCR. Heroin also produced a reduction in the level of plasma nitrite/nitrate, the more stable end-product of nitric oxide degradation. In a subsequent study, administration of the opioid receptor antagonist, naltrexone (0.1 mg/kg) prior to the injection of heroin (1.0 mg/kg) blocked the heroin-induced reduction of iNOS expression and plasma nitrite/nitrate levels indicating that the effect is mediated via the opioid-receptor. This study provides the first evidence that heroin induces an alteration of iNOS expression, and suggests that a reduction in nitric oxide production may be involved in the increased incidence of infectious diseases amongst heroin users.

Suppression of adjuvant arthritis by kappa-opioid receptor agonist: effect of route of administration and strain differences

It is well established that kappa-opioid receptor agonists exert antiinflammatory and antihyperalgesic effects during nonspecific inflammation as well as suppressive effects on the development of humoral and cell-mediated immune responses to foreign antigens. The aim of this study was to investigate the ability of the kappa-opioid receptor agonist MR 2034 to modulate adjuvant arthritis in the rat. In the first series of experiments, treatments of Wistar rats were performed using several routes of drug administration: intraperitoneal (ip), intracaudal (ic), intracerebroventricular (icv) and intraplantar (ipl). MR 2034 significantly suppressed joint swelling after ip and ic treatment, slightly reduced inflammation after ipl treatment, and did not produce any effect after icv treatment. In the second series of experiments, the suppressive effect of ip injected MR 2034 was investigated using Wistar, Dark August (DA) and Lewis rats. In Wistar rats, MR 2034 significantly decreased the incidence of adjuvant arthritis, and suppressed mean joint score and aggregate joint score. Similarly, in DA rats treated with MR 2034, mean arthritic score was significantly suppressed, but other clinical parameters were not affected. In Lewis rats, however, ip treatment with MR 2034 failed to produce any suppressive effect on joint disease and even potentiated the initial development of arthritis. These data suggest that immunosuppressive and antiinflammatory action of MR 2034 markedly depend on the route of drug administration and strain susceptibility to opioids.

Modulation of humoral immune response by central administration of leucine-enkephalin: effects of mu, delta and kappa opioid receptor antagonists

The effect of leucine-enkephalin (Leu-Enk) on primary humoral immune response was investigated following intracerebroventricular (i.c.v.) administration of the peptide in the rat. Leu-Enk stimulated plaque-forming cell (PFC) response in rats i.c.v. injected with 0.1 and 1 micrograms/kg, whereas does of 20 and 50 micrograms/kg exerted immunosuppressive effects. I.c.v. treatment of rats with delta opioid receptor antagonist ICI 174,864 and kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) blocked stimulation and suppression of PFC response induced by Leu-Enk, respectively. The mu opioid receptor antagonist beta-funaltrexamine (beta-FNA) reversed both immunomodulatory effects produced by Leu-Enk. Since beta-FNA alone had no effect on PFC response (unlike ICI 174,864 and nor-BNI), these data showed that central effects of Leu-Enk on PFC response were mediated by brain mu opioid receptors, and suggested a possible involvement of delta and kappa opioid receptors.

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.

Opposing activities of brain opioid receptors in the regulation of humoral and cell-mediated immune responses in the rat

The role of brain delta- and kappa-opioid receptors in the regulation of PFC response, Arthus hypersensitivity reactions and delayed hypersensitivity reactions was studied following intracerebroventricular (i.c.v.) administration of opioid receptor agonists and antagonists. Eight-week-old male Wistar rats, with polyethylene cannulae inserted into the lateral brain ventricles, were i.c.v. treated with different doses of delta-opioid receptor agonist methionine-enkephalin (Met-Enk), delta-opioid receptor antagonist ICI 174864, kappa-opioid receptor agonist MR 2034, and kappa-opioid receptor antagonist MR 2266. In rats sensitized for plaque-forming cell (PFC) assay, the first drug injection was given 1 h prior to immunization, and then every 24 h until day 4. One h after the last treatment, rats were sacrificed and (PFC) assay performed. In rats immunized for hypersensitivity skin reactions, the first drug injection was given 1 h before immunization, and then every 48 h until day 14. Skin reactions were elicited one h after the last drug administration. Opioid receptor agonists Met-Enk and MR 2034 stimulated and suppressed PFC response, Arthus and delayed skin reactions respectively. ICI 174864 decreased the number of PFC and intensity of hypersensitivity skin reactions whereas MR 2266 increased the number of PFC, but did not affect to a greater extent hypersensitivity reactions. Stimulation of PFC produced by 1 microgram/kg of Met-Enk was completely blocked with 10 and 50 micrograms/kg of ICI 174864. MR 2034-induced suppression was partially and completely antagonized with 10 and 50 micrograms/kg of MR 2266 respectively. The present results suggest that brain opioid receptors differentially affect humoral and cell-mediated immune responses.