Opiate antinociception is altered by immunemodification: the effect of interferon, cyclosporine and radiation-induced immune suppression upon acute and long-term morphine activity

Calcineurin-inhibitor pain syndrome

BACKGROUND

There has been increased recognition of calcineurin, a phosphoprotein serine/threonine phosphatase enzyme, in the regulation of many physiologic systems. Calcineurin mediates activation of lymphocytes, which play a role in immune response. Widely distributed in the central nervous system, calcinuerin also plays an important role in sensory neural function, via its role in the regulation of newly discovered 2-pore potassium channels, which greatly influence neuronal resting membrane potentials. Calcinuerin inhibition is the mechanism of action of immunomodulatory drugs such as cyclosporine and tacrolimus, which are widely used in transplantation medicine to prevent rejection. While important for immunosuppression, the use of calcineurin inhibitors has been associated with the development of a new pain syndrome called the calcineurin pain syndrome, which appears to be an untoward complication of the interruption of the physiologic function of calcineurin.

METHODS

This is a narrative review focusing on the epidemiology, pathophysiology, characterization of a newly recognized pain syndrome associated with the use of calcineurin inhibitors.

RESULTS

The use of immunosuppressants however is associated with several well-known toxicities to which the calcineurin pain syndrome can be added. The development of this syndrome most likely involves altered nociceptive processing due to the effect of calcineurin inhibition on neuronal firing, as well as effects of calcineurin on vascular tone. The most striking aspect of the treatment of this syndrome is the response to calcium channel blockers, which suggest that the effects of calcineurin inhibition on vascular tone play an important role in the development of the calcineurin pain syndrome.

CONCLUSION

The calcineurin syndrome is a newly recognized complication associated with the use of calcineurin inhibitors. There is no standard therapy at this time but anecdotal reports suggest the effectiveness of calcium channel blockers.

Ibudilast (AV-411)

The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.

Interferon and the central nervous system

Interferons (IFNs) were discovered as natural antiviral substances produced during viral infection and were initially characterized for their ability to "interfere" with viral replication, slow cell proliferation, and profound alteration of immunity. The IFNs are synthesized and secreted by monocytes, macrophages, T-lymphocytes, neurons, and glia cells. The different IFNs are classified into three classes: alpha, beta, and gamma. alpha-IFN produced in the brain exerts direct effects on the brain and endocrine system by activating the neurosecretory hypothalamic neurons and regulates the hypothalamic-pituitary-adrenocortical axis. IFNs modulate neurophysiological activities of many brain region involving in pain, temperature, and food intake regulation. alpha-IFN administration activates the sympathetic nerves innervating components of the immune system. IFNs may serve as regulatory mediators between the central nervous system, the immune system, and endocrine system. IFN is used as immunologic therapy to treat various hematologic malignancies and infectious ailments and autoimmune diseases.

In vitro opioid induced proliferation of peripheral blood immune cells correlates with in vivo cold pressor pain tolerance in humans: a biological marker of pain tolerance

There is substantial evidence for bidirectional communication between the immune system and the central nervous system, as the cells and signalling molecules of the immune system influence many central nervous system functions, for instance nociception. Opioids, such as morphine, produce analgesia and numerous other central and peripheral effects including sedation and euphoria, while their effects on the immune system are wide-ranging. There is considerable interindividual variability in basal nociception and response to opioids, however, the physiological and biological mechanisms underlying this are unclear. Therefore, we investigated the relationship between the immune system and basal nociceptive thresholds, using the proliferative response of isolated peripheral blood mononuclear cells and cold pressor pain tolerance. Here we show that the percent increase in proliferation of peripheral immune cells from 13 healthy subjects incubated with morphine ex vivo is highly correlated with the subjects' tolerance to noxious cold stimuli (Pearson r = 0.92, P < 0.0001). These pilot data provide evidence of a novel objective biological marker of pain tolerance in humans, which also links the immune and opioid systems with basal pain tolerance.

A cytokine-based neuroimmunologic mechanism of cancer-related symptoms

While many of the multiple symptoms that cancer patients have are due to the disease, it is increasingly recognized that pain, fatigue, sleep disturbance, cognitive dysfunction and affective symptoms are treatment related, and may lead to treatment delays or premature treatment termination. This symptom burden, a subjective counterpart of tumor burden, causes significant distress. Progress in understanding the mechanisms that underlie these symptoms may lead to new therapies for symptom control. Recently, some of these symptoms have been related to the actions of certain cytokines that produce a constellation of symptoms and behavioral signs when given exogenously to both humans and animals. The cytokine-induced sickness behavior that occurs in animals after the administration of infectious or inflammatory agents or certain proinflammatory cytokines has much in common with the symptoms experienced by cancer patients. Accordingly, we propose that cancer-related symptom clusters share common cytokine-based neuroimmunologic mechanisms. In this review, we provide evidence from clinical and animal studies that correlate the altered cytokine profile with cancer-related symptoms. We also propose that the expression of coexisting symptoms is linked to the deregulated activity of nuclear factor-kappa B, the transcription factor responsible for the production of cytokines and mediators of the inflammatory responses due to cancer and/or cancer treatment. These concepts open exciting new avenues for translational research in the pathophysiology and treatment of cancer-related symptoms.

Are the symptoms of cancer and cancer treatment due to a shared biologic mechanism? A cytokine-immunologic model of cancer symptoms

BACKGROUND

Cancers and cancer treatments produce multiple symptoms that collectively cause a symptom burden for patients. These symptoms include pain, wasting, fatigue, cognitive impairment, anxiety, and depression, many of which co-occur. There is growing recognition that at least some of these symptoms may share common biologic mechanisms.

METHODS

In November 2001, basic and clinical scientists met to consider evidence for a cytokine-immunologic model of symptom expression along with directions for future research.

RESULTS

The characteristics of cytokine-induced sickness behavior in animal models have much in common with those of symptomatic cancer patients. Sickness behavior refers to a set of physiologic and behavioral responses observed in animals after the administration of infectious or inflammatory agents or certain proinflammatory cytokines. In some cases, these responses can be prevented by cytokine antagonists. A combination of animal and human research suggests that several cancer-related symptoms may involve the actions of proinflammatory cytokines.

CONCLUSIONS

Based on the similarities between cancer symptoms and sickness behavior, the authors discussed approaches to further test the implications of the relationship between inflammatory cytokines and symptoms for both symptom treatment and symptom prevention.

Inhibition by immunophilin ligands of morphine-induced tolerance and dependence in guinea pig ileum

Immunophilin ligands, cyclosporine A and FK506 (tacrolimus), besides their immunosuppressive action, have several effects on different neural functions, such as modulation of the release of many neurotransmitters, the reduction of nitric oxide (NO) production by the inhibition of dephosphorylation of neuronal nitric oxide synthase (nNOS) and the alteration of the expression of certain genes. Many of these actions apparently occur through the inhibition of calcineurin, a calcium-calmodulin-dependent phosphatase. On the other hand, several studies have shown that NO has a critical role in opioid-induced tolerance and dependence in both in vivo and in vitro models. In the present study, the effect of cyclosporine A and FK506 on the development of tolerance to and dependence on morphine in the guinea pig ileum was assessed. Morphine inhibited electrically stimulated twitch of ileum in a concentration-dependent manner (pD(2)=7.45+/-0.07). Tolerance to this effect was induced by incubation of ileum with 2 x IC(50) or 4 x IC(50) of morphine for 2 h that induced a degree of tolerance of 6.81 and 18.10, respectively. The co-incubation of ileum with morphine along with either cyclosporine A or FK506 reduced the degree of tolerance significantly (P<0.05) and restored the sensitivity of ileum to the morphine inhibitory effect. Dependence was induced by incubation with 4 x IC(50) of morphine for 2 h and was assessed based on naloxone-induced contractions (10(-5) M). Cyclosporine A (10(-9) M) and FK506 (10(-9) M) can attenuate the development of dependence to morphine as shown by the significant decrease in naloxone-induced contractions (P<0.05). These results suggest that immunophilin ligands at very low concentrations (nanomolar) can reduce the induction of acute tolerance to and dependence on morphine in the myenteric plexus of guinea pig ileum.

The effects of FK506 on the development and expression of morphine tolerance and dependence in mice

FK506 is an immunophilin-binding ligand that inhibits calcineurin and decreases nitric oxide (NO) production in the nervous tissues. We examined the effects in mice of systemic treatment with FK506 on the induction and expression of morphine (s.c.) tolerance and dependence and compared them with the effects of the non-specific NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), and specific inducible NO synthase inhibitor, aminoguanidine. FK506 (0.5-10 mg/kg, s.c.) exerted inhibitory effects on both development and expression of tolerance to morphine-induced antinociception. FK506 also significantly decreased the expression of morphine dependence, as assessed by naloxone-precipitated (2 mg/kg, i.p.) withdrawal syndrome, but a similar effect was not found for the development of morphine dependence. A similar pattern of effects was observed with L-NAME (3-20 mg/kg, i.p.), while aminoguanidine (50-100 mg/kg, i.p.) did not alter tolerance or dependence. Examining the possible interaction between their inhibitory effects on tolerance and dependence, we combined the subeffective doses of FK506 (0.5 or 1 mg/kg) with L-NAME (3 mg/kg) or aminoguanidine (100 mg/kg). The combination of FK506 with L-NAME, but not with aminoguanidine, significantly decreased the development and expression of tolerance and expression of dependence. These data show the effectiveness of FK506 on morphine tolerance and dependence and suggest an additive effect between FK506 and the inhibition of constitutive NO synthesis in this regard.

The effect of cyclosporin A on morphine tolerance and dependence: involvement of L-arginine/nitric oxide pathway

Cyclosporin A is known to decrease nitric oxide (NO) production in nervous tissues. The effects of systemic cyclosporine A on the induction and expression of morphine tolerance and dependence, acute morphine-induced antinociception, and the probable involvement of the L-arginine/nitric oxide pathway in these effects were assessed in mice. Cyclosporin A (20 mg/kg), N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg) and a combination of the two at lower and per se non-effective doses (5 and 3 mg/kg, respectively) showed a similar pattern of action, inhibiting the induction of tolerance to morphine-induced antinociception and increasing the antinociception threshold in the expression phase of morphine tolerance. These agents also inhibited the expression of morphine dependence as assessed by naloxone-precipitated withdrawal signs, while having no effect on the induction of morphine dependence. L-Arginine, at a per se non-effective dose (60 mg/kg), inhibited the effects of Cyclosporin A. Moreover, acute administration of Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) enhanced the antinociception induced by acute administration of morphine (5 mg/kg), while chronic pretreatment with Cyclosporin A (20 mg/kg) or L-NAME (10 mg/kg) for 2 days (twice daily) did not affect morphine-induced antinociception. The inducible nitric oxide synthase inhibitor, aminoguanidine (100 mg/kg), did not alter morphine antinociception, tolerance or dependence. In conclusion, decreasing NO production through constitutive nitric oxide synthase may be a mechanism through which cyclosporin A differentially modulates morphine tolerance, dependence and antinociception.

NMDA receptor antagonist disrupts acute and chronic effects of methylphenidate

Methylphenidate (MPD) is a drug widely used for treating attention-deficit/hyperactivity disorder in children. Because of its extensive consumption and because it has pharmacological stimulant properties similar to amphetamine and cocaine, MPD has the potential of abuse. N-methyl-D-aspartate (NMDA) receptors are suggested to be involved in CNS effects of stimulants, and antagonists of the NMDA receptor can potentially alter the stimulants' effects. Dizocilpine (MK-801), a non-competitive antagonist of the NMDA receptor, has been reported to prevent sensitization elicited by repeated administration of amphetamine and cocaine. The objective of the present study was to use the tail-flick latency assay, rectal temperature, and body weight gain to assess effects of repetitive treatment of MPD and whether MK-801 treatment would alter these effects in Sprague-Dawley rats. It was found that: (Ia) Acute administration of MPD or MK-801 did not alter the tail-flick latency, (Ib) Repeated administration of MPD decreased tail-flick latency, while repeated administration of MK-801 had no significant effect on tail-flick latency, (Ic) MK-801 given prior to or with MPD reversed the chronic effect on tail-flick latency produced by MPD; (IIa) When both drugs were independently given, MPD elicited a decrease in rectal temperature, while MK-801 alone produced an increase in temperature, (IIb) When given together, MK-801 had a transient effect in blocking the sensitization to MPD but failed to reverse the sensitization of MPD once it had developed; and (III) Both MK-801 and MPD caused an unstable pattern of body weight gain. Hence, the results of this study in rats suggest that MK-801 can modulate non-motor effects of MPD.

The opioid-cytokine connection

Opioids (exogenous opiates and endogenous opioid peptides) have a diversity of effects on the immune system. Although numerous studies have shown that opioid-induced immunosuppression can be mediated indirectly via the central nervous system (CNS) or through direct interactions with immunocytes, the precise cellular mechanisms underlying the immunomodulatory effects of opioids are largely unknown. In recent years, investigations from several laboratories have indicated that opioids can operate as cytokines, the principal communication signals of the immune system. All of the major properties of cytokines are shared by opioids, i.e., production by immune cells with paracrine, autocrine, and endocrine sites of action, functional redundancy, pleiotropy and effects that are both dose- and time-dependent. Studies of the effects of opioids on peripheral blood mononuclear cells (PBMC) or brain cells cocultured with HIV-infected cells suggest that some of the immunoregulatory actions of opioids are mediated by ultrahigh affinity receptors on PBMC and glial cells. Because the CNS is populated predominantly by astroglia and microglia which have properties of immune cells, it is possible that certain of the CNS effects of opioids involve cytokine-like interactions with glial cells. Although there is mounting evidence supporting the concept that opioids are members of the cytokine family, the relative contribution of the opioids to immunoregulation remains unclear. The importance of opiate addiction in the AIDS epidemic means that gaining a better understanding of the mechanisms of opioid-induced immunomodulation is of more than academic interest.

Is interferon-alpha a neuromodulator?

Interferons were initially characterized for their ability to 'interfere' with viral replication, slow cell proliferation, and profoundly alter immunity. They are a group of hormone-like molecules synthesized and secreted by macrophages, monocytes, T lymphocytes, glia, and neurons. These cytokines have been shown to have several regulatory roles and diverse biological activities, including control of cellular and humoral immune responses, inflammation, and tumor regression. In addition, there are many reports indicating that interferon-alpha (IFN-alpha) participates in the regulation of various cellular and humoral processes such as the endocrine system modulates behavior, brain activity, temperature, glucose sensitive neurons, feeding pattern and opiate activity. Therefore, IFN-alpha can be considered as a physiological modulator, with only one of its functions being the ability to hinder viral replication intracellularly.

Lateral hypothalamus: site involved in pain modulation

The present study is an attempt to examine the neuronal circuitry of a supraspinal site engaged in pain modulation. Five physiological measures were postulated as the criteria for defining a central nervous system site engaged in the circuitry of pain modulation. The lateral hypothalamus met these five measures: (i) 81% of the lateral hypothalamus neurons (247/304) responded to noxious stimuli using a single cell recording procedure; (ii) stimulation of the periaqueductal gray-dorsal raphe area or the habenula modulated 98% and 87% of the lateral hypothalamus noxious-evoked activity; (iii) microiontophoretically applied morphine modulated 77% of the lateral hypothalamus noxious evoked activity; (iv) electrical stimulation of the lateral hypothalamus produced behavioral analgesia proportional to the stimulus intensity as assessed by the tail flick assay; and (v) morphine application into the lateral hypothalamus produced behavioral analgesia in a dose-response manner using the tail flick assay. In conclusion, the lateral hypothalamus can be considered one of the pain modulation sites.

Norepinephrine and serotonin-induced antinociception are blocked by naloxone with different dosages

The effects of intrathecally (IT) administered naloxone (Nal) on the antinociception produced by IT norepinephrine (NE), serotonin (5-HT), or morphine (Mor) were observed and compared in rats using the tail-flick (TF) assay. The results show that: a) NE, 5-HT, and Mor in doses of 1 nmol, 240 nmol, and 0.5 nmol, respectively, produce similar increases in amplitude and time in TF latency (TFL); b) Nal treatment of 240 and 360 nmol has no effects on TFL; c) the antinociception produced by NE (1 nmol) can be blocked by Nal (240 nmol); d) antinociception produced by Mor (0.5 nmol) can also be blocked by Nal (240 nmol); e) 240 nmol of Nal does not affect the 5-HT (120 nmol)-produced antinociception, while 360 nmol of Nal show a delayed blockade to the 5-HT (120 nmol)-produced antinociception. The results suggest that endogenous opiate-like substances may be involved in both NE- or 5-HT-produced antinociception at the spinal level, and these effects may be mediated through different types of opiate receptors.

[Anesthesia for non-specific surgery in a post-transplantation patient]

The increase of non specific surgeries in transplanted patients may be related to the better survival achieved by the efficacy of immunosuppressive therapy and improved surgical and intensive care conditions. Therefore, the anaesthetist may be mandated to give anaesthesia in such patients, treated in hospitals which are not involved in transplantation procedures. The ignorance of the main physiologic and pharmacological changes in the new grafted organ as well as the knowledge of high risks of rejection or infection contribute to the anxiety often encountered in front of these patients. The denervated heart is unable to respond to stimulations requiring the integrity of autonomic neural mechanisms. Modulation of cardiac output depends on intrinsic activity (Frank-Starling mechanism) and therefore of end diastolic volume (preload). The denervated transplanted lung shows inability to elicit cough reflex; the latter is totally abolished in case of tracheal anastomosis. These physiologic changes have no deleterious effects on early cardiac and pulmonary functions following transplantation. In the same way, renal, liver or pancreatic functions are restored after respective replacement. However chronic rejection occurs frequently in 50% of patients in a mean time of 5 years following surgery except for liver transplanted patients which seem to be better protected. It results in a progressive decrease in organ function tests. The preoperative assessment requires primary contact with the transplant center. This communication should give precious information about the last biological and functional results as well as about the immunosuppressive therapy. Standard preoperative investigations include measurements of haemoglobin, urea, electrolyte and creatinine concentrations, liver tests, ECG, chest X-ray and coagulation pattern. Previsible difficult intubation should be detected in case of previous pancreas transplantation. Immunosuppressive therapy and other treatments should not be disrupted until surgery. Usual premedication may be used. Previsional peroperative transfusion requires specific packed red blood cells, fresh frozen plasma and platelets in order to reduce CMV contamination and GVH reactions. Locoregional or general anaesthesia may be used with respect to usual contraindications. Special attention should be given in cardiac transplanted patients in order to maintain adequate preload. As atropine is ineffective, bradycardia may be treated by isoprenaline. Patients with lung transplants require a reduction of vascular loading and of hydratation and early postoperative pulmonary physiotherapy. Pancreas transplanted patients often suffer from severe cardiac diseases (coronaropathy). The immunosuppressive therapy modifies the pharmacological behavior of many anaesthetic agents. Ciclosporine enhances mainly the effects of muscle relaxants. Peroperative invasive monitoring requires full aseptic techniques. Invasive monitoring should be discussed in terms of benefit-risk ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavioral effects of mouse interferons-alpha and -gamma and human interferon-alpha in mice

Behavioral effects of murine interferon-alpha and -gamma were explored using a well-characterized system for detecting interferon effects. In addition, the effectiveness of human interferon-alpha was determined. Mouse gamma-interferon decreased activity and food-related behavior, effects that were similar to the effects of mouse alpha-interferon. Equivalent doses of the human preparation had little effect upon these measures in mice. Some common action of alpha- and gamma-interferon is likely responsible for the similar effects seen for the two molecules. Low effectiveness of the human preparation in mice was predicted due to the differences in receptors between species.

Cyclosporine affects central nervous system opioid activity via direct and indirect means

It is demonstrated that cyclosporine (CsA), a novel fungal-derived immunosuppressive agent, attenuates naloxone-precipitated morphine withdrawal in an unusual dose-dependent manner following direct intracerebroventricular (icv) administration. However, comparison and contrast of this effect of CsA following icv versus ip administration demonstrates that although CsA does alter the severity of withdrawal by a direct effect within the CNS, the agent is most effective when given systemically. It is also demonstrated that CsA alters the electrophysiologic properties of discrete brain nuclei both when given alone and when given concomitant with morphine and naloxone.

Mu-, but not delta-, opioid receptor-mediated antinociception in mice is attenuated by gamma-irradiation

Mice were exposed to whole-body irradiation (500 rads) from a 137Cs gamma-source and tested 2 h later for antinociception (tail-flick test) produced by intracerebroventricular administration of morphine or the more delta-selective opioid peptide, [D-Pen2,L-Pen5]enkephalin (DPLPE). Irradiation significantly attenuated the antinociception produced by morphine, but not by DPLPE. These results demonstrate a differential sensitivity of mu- and delta-opioid receptors to gamma-irradiation and, in addition, may be of clinical relevance for cancer patients receiving concurrent radiation therapy and opioid analgesics.

Opioid peptides modulate immune functions. A review

In the past few years it has become evident that neuropeptides may be direct mediators in the modulation of the immune response and the unspecific defense by the brain. Lymphocytes have been thought to have opioid receptors and to respond to opioids with an increase in blastogenesis, cytotoxicity and factor release. Lymphocytes are said to release various neuropeptides. Furthermore, there are some unexplained effects of morphine on the immune system and of the immune system on morphine withdrawal. The purpose of this paper is to review what has been previously published in this field. The well established modulation of phagocyte functions by opioids will only be scanned.

Immune response products alter CNS activity: interferon modulates central opioid functions

The present report provides evidence to support the hypothesis that peptides released during an immune response alter CNS activity and thus may provide a means for the immune system to transmit afferent signals into the brain. Specifically, it is demonstrated that recombinant interferon-alpha (rIFN-alpha), a peptide associated with the immune response to viral infection, can alter opiate withdrawal severity in a dose-dependent manner upon direct injection into brain areas essential for this phenomenon. These results are compared and contrasted with the effect of systemically injected rIFN-alpha upon opiate withdrawal. In addition, an electrophysiological investigation into the basis of the interaction of opioids and rIFN-alpha in brain structures essential for the expression of opioid activities is also presented. Finally, the effects of rIFN-alpha upon the functions of both the CNS and other systems is discussed in terms of the effects reported for other peptides associated with immune responses.

Irradiation exposure modulates central opioid functions

Exposure to low doses of gamma irradiation results in the modification of both the antinociceptive properties of morphine and the severity of naloxone-precipitated withdrawal in morphine-dependent rats. To better define the interactions between gamma irradiation and these opiate-mediated phenomena, dose-response studies were undertaken of the effect of irradiation on morphine-induced antinociception, and on the naloxone-precipitated withdrawal syndrome of morphine-dependent rats. In addition, electrophysiologic studies were conducted in rats after irradiation exposure and morphine treatment correlating with the behavioral studies. The observations obtained demonstrated that the antinociceptive effects of morphine as well as naloxone-precipitated withdrawal were modified in a dose-dependent manner by irradiation exposure. In addition, irradiation-induced changes in the evoked responses obtained from four different brain regions demonstrated transient alterations in both baseline and morphine-treated responses that may reflect the alterations observed in the behavioral paradigms. These results suggest that the effects of irradiation on opiate activities resulted from physiologic alterations of central endogenous opioid systems due to alterations manifested within peripheral targets.

Evidence of an immune system to brain communication axis that affects central opioid functions: muramyl peptides attenuate opiate withdrawal

Muramyl peptides are metabolic breakdown products of bacterial cell walls formed in vivo by the reticuloendothelial system. These agents have a variety of immune modulatory and neuropharmacologic effects. It has previously been demonstrated that a variety of immune modifying agents can induce alterations in certain behaviors elicited by opiates. In the present study we investigate possible reciprocal interactions between muramyl dipeptides (MDPs) and central opioid systems using three different experimental models: (1) naloxone-precipitated withdrawal behavior in morphine-dependent rats; (2) the tail immersion assay for determination of morphine-induced antinociception and (3) rectal temperature measurement of the pyrogenic activity of MDP. It is shown that two derivatives of MDP attenuate the severity of naloxone-precipitated withdrawal and morphine-induced antinociception. In addition, it is demonstrated that the pyrogenic activity of a stearoyl derivative of MDP is altered by chronic morphine treatment. These findings suggest both novel neuropharmacologic properties of muramyl dipeptides, as well as demonstrate that yet another immune modifier interacts with centrally mediated opioid phenomena.

Participation of lymphoid cells in the withdrawal syndrome of opiate dependent rats

Treatment of rats with 500 Rads whole-body ionizing irradiation prior to chronic administration of morphine reduced the severity of the naloxone induced withdrawal signs. In contrast, adoptive transfer of 2-6 X 10(8) lymphoid cells to irradiated rats prior to chronic morphine treatment completely restored the ability to manifest the withdrawal signs precipitated by naloxone. These observations offer the possibility that the immune system participates in opiate addiction.