Conditioned immunomodulation: investigations of the role of endogenous activity at mu, kappa, and delta opioid receptor subtypes

Neuroimmune mechanisms of opioid-mediated conditioned immunomodulation

Morphine administration elicits pronounced effects on the immune system, including decreases in natural killer (NK) cell activity and lymphocyte mitogenic responsiveness. These immune alterations can become conditioned to environmental stimuli that predict morphine as a result of Pavlovian conditioning processes. Prior work in our laboratory has shown that acute morphine exposure produces dopamine-dependent reductions of NK cell activity that are mediated peripherally by neuropeptide Y Y1 receptors. The present study examined the involvement of dopamine D1 and neuropeptide Y Y1 receptors in the conditioned immunomodulatory effects of morphine. Rats received two conditioning sessions during which an injection of morphine was paired with a distinctive environment which served as the conditioned stimulus (CS). The results show that systemic administration of the D1 antagonist SCH-23390 prior to CS re-exposure prevented the conditioned suppression of splenic NK activity but did not alter conditioned decreases in mitogen-induced lymphocyte proliferation. Furthermore, bilateral microinjections of SCH-23390 directly into the nucleus accumbens shell fully blocked conditioned changes in NK activity. In a subsequent manipulation, subcutaneous injection of the Y1 receptor antagonist BIBP3226 prior to CS re-exposure was also shown to prevent conditioned effects on NK activity. Collectively, these findings provide evidence that the nucleus accumbens shell plays an important role in conditioned immunomodulation and further suggest that the conditioned and unconditioned immunomodulatory effects of opioids involve similar receptor mechanisms.

Buprenorphine produces naltrexone reversible alterations of immune status

Substantial evidence demonstrates that administration of high efficacy mu opioid agonists such as morphine modulate the immune response in a dose-dependent and pharmacologically specific manner, indicating functional interactions between the opioid and immune systems. In contrast to the well-characterized immunomodulatory effects of high efficacy mu opioids, little is known about how these effects generalize to other clinically employed opioids and agonists of varying degrees of mu opioid receptor stimulation. Buprenorphine is a mu opioid agonist of intermediate efficacy that is used clinically for pain management and has recently been approved for the treatment of opioid dependence. Recent evidence indicates pharmacological and mechanistic differences between buprenorphine and morphine. Therefore, the aim of the present study was to investigate whether buprenorphine also possesses immunomodulatory properties. The results demonstrate that buprenorphine dose-dependently suppresses splenic natural killer cell activity, lymphocyte proliferation and IFN-gamma production in rats in a naltrexone reversible manner, demonstrating pharmacological specificity of buprenorphine-induced immune alterations.

Involvement of the rat caudate nucleus in the immunostimulatory effect of DAGO

The involvement of the caudate nucleus, i.e., the terminal zone of the nigrostriatal dopaminergic system, in neuroimmunostimulation during the activation of mu opioid receptors by the highly specific agonist DAGO. Single doses of DAGO (100 microg/kg) in sham-operated control Wistar rats induced significant increases in the numbers of direct IgM-antibody-forming and total rosette-forming cells at the peak of the immune response after immunization with sheet red blood cells. The experiments showed that bilateral electrolytic lesioning of the caudate nucleus in rats suppressed the immune response, demonstrating its involvement in neuroimmunomodulation. Since the effect of immunostimulation induced by DAGO disappeared when given to animals with caudate nucleus lesions, it was concluded that this structure is involved in activatory immunogenesis via mu opioid mechanisms.

Morphine upregulates mu opioid receptors of human and monkey lymphocytes

Opioid receptors of subtypes delta, kappa, and mu similar to those found in brain cells have been identified in immune cells. The current study demonstrates by competitive polymerase chain reaction the treatment of human lymphocytic cells with morphine resulting in an increased amount of gene expression of mu opioid receptors. Antibodies against the MOR-1, the neuronal mu opioid receptor, were used in Western blot analysis of mu proteins and the results revealed a single band of approximately 50 kDa, the intensity of which was increased by morphine treatment. Similar results of mu opioid receptor activation were observed when monkey lymphocytes were treated with morphine. These studies suggest that in addition to causing an immune effect through communication with the neuroendocrine system, the psychoactive drug morphine may modulate immune functions by acting directly on the mu opioid receptors expressed on lymphocytes.

Pavlovian conditioning of immune function: animal investigation and the challenge of human application

Pavlovian conditioning of immune functions provided early impetus to the rapidly expanding knowledge of bi-directional communication among the immune, endocrine, and central nervous systems. Since these early investigations, the phenomenology of this response has been well characterized. However the neural mechanisms and biological relevance of conditioned immunomodulation remain unclear. To this end, we present here data from our laboratories that have: (1) revealed some of the neural mechanisms and biological relevance of an animal model of conditioned immunomodulation; (2) demonstrated the conditionability and potential mechanisms of conditioned immune responses in healthy humans, and (3) investigated conditioned immunomodulation in a clinical sample. Together, these data demonstrate that animal models provide a basis for investigating mechanisms whereby conditioned changes in immune function may modulate health status in a clinical realm.

In vivo effects of chronic treatment with [MET5]-enkephalin on hematological values and natural killer cell activity in athymic mice

The role of endogenous opioids in immunological mechanisms was examined by subjecting athymic (nu/nu) mice to chronic injections of the opioid agonist [Met5]-enkephalin (MET) or continuous opioid receptor blockade with naltrexone (NTX). After 8 days of treatment, neither excess peptide nor deprivation of opioids from receptors had any effect on body weight, spleen index (spleen to body weight ratio), total and differential white blood cell counts, and natural killer (NK) cell activity in peripheral blood or splenic lymphocytes. At 28 days, chronic treatment with MET or NTX had no effect on any of these parameters with the exception of an elevation from controls in NK cell activity in peripheral blood in mice receiving NTX, and subnormal NK cell activity related to splenic lymphocytes in the MET group. These results suggest that chronic exposure to an opioid agonist, or persistent opioid receptor blockade, have little influence on a variety of immunological properties in athymic mice, suggesting that native opioids such as MET do not play a marked role in defense mechanisms in the athymic mouse.

Conditioned immunosuppression makes subtherapeutic cyclosporin effective via splenic innervation

The present study investigated the mechanisms by which conditioned immunosuppression enhances the effectiveness of cyclosporin A (CsA) treatment in prolonging heart allograft survival. Dark Agouti rats that were administered subtherapeutic CsA (7 x 2 mg/kg on alternate days) rejected heart allografts at the same time as non-CsA-treated rats. The addition of a behavioral conditioning regimen (conditioned stimulus, saccharin; unconditioned stimulus, 20 mg/kg CsA) to the subtherapeutic CsA protocol produced a significant prolongation of graft survival, including long-term survival (>100 days) in 20% of the animals. Prior sympathetic denervation of the spleen completely blocked this effect. In nontransplanted rats both conditioning and CsA treatment reduce interleukin-2 and interferon (IFN)-gamma in the supernatant of proliferating splenocytes. Additionally, therapeutic CsA treatment decreased the number of IFN-gamma-producing CD4(+) naive and memory T cells in the spleen. In contrast, behavioral conditioning increased that number. These data indicate that behavioral conditioning prolongs heart allograft survival by inhibiting the release of these cytokines in the spleen via sympathetic innervation, supplementing the inhibited cytokine production induced by CsA treatment.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.