Barbara M. Bayer

Morphine inhibition of lymphocyte activity is mediated by an opioid dependent mechanism

The effects of acutely-administered morphine on mitogen stimulated lymphocyte proliferation and natural killer cell cytolytic activity were investigated. Two hours after a subcutaneous injection of morphine (25 mg/kg), blood lymphocyte proliferation was found to be 70% depressed, compared to saline-injected controls. This effect was partially antagonized in animals pretreated with naltrexone (10 mg/kg) and was present only in blood lymphocytes, since proliferative responses of splenic lymphocytes were not significantly altered. The administration of morphine, however, did result in a 30-40% inhibition of cytolytic activity of natural killer cells, which was completely antagonized in naltrexone-pretreated animals. Naltrexone alone was found to have no effect on either proliferation of blood and splenic lymphocytes or the cytolytic activity of splenic lymphocytes. Although naltrexone had no effect on the activity of lymphocytes, animals treated with either naltrexone or morphine alone, or their combination, had 4- to 8-fold increases in corticosterone in plasma. These results demonstrate that the effect of morphine on immune cells was dependent on the tissue source of lymphocytes. Furthermore, the suppression of blood lymphocyte proliferation and splenic cytolytic activity of natural killer cells by morphine was opiate receptor-mediated, as indicated by the reversibility by naltrexone of the observed effects of morphine. Finally, the accompanying increase in circulating levels of corticosterone most likely did not contribute to these effects.

Distinction between the in vitro and in vivo inhibitory effects of morphine on lymphocyte proliferation based on agonist sensitivity and naltrexone reversibility

We have previously reported that administration of a single dose of morphine (25 mg/kg) to rats results in a naltrexone-sensitive suppression of mitogen-stimulated lymphocyte proliferation. To further delineate the site of action of this inhibitory effect, the in vitro and in vivo effects of morphine on mitogen-stimulated lymphocyte proliferation were examined. In vitro, concentrations of morphine exceeding 0.1 mM exhibited a dose-dependent inhibition of Concanavalin A-induced proliferation of both whole blood and splenic lymphocytes. This inhibitory effect of morphine on lymphocyte proliferation was not attenuated by co-incubation with the opioid antagonist naltrexone (0.25 mM). These data indicate that the in vitro inhibitory effects of morphine occur at only high concentrations and are not opioid receptor mediated. In vivo, a dose-dependent inhibition of blood lymphocyte proliferation was also observed 2 h following the subcutaneous injection of morphine. In contrast to these effects, proliferation of splenic lymphocyte cultures was not significantly inhibited by morphine at doses of up to 40 mg/kg. However, following morphine administration, a greater than 90% inhibition of proliferation was obtained in cultures containing either whole blood or Ficoll-separated lymphocytes, indicating that plasma was not a contributory factor in the differential sensitivity of blood and splenic lymphocyte responses to morphine. Moreover, in these experiments, significant inhibition of lymphocyte proliferation occurred at plasma concentrations that were two orders of magnitude less than those required to produce inhibition in vitro. The in vivo inhibition of lymphocyte proliferation by morphine (10 mg/kg) was completely antagonized by pretreatment with naltrexone (5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids, immunology, and host defenses of intravenous drug abusers.

Overall, it is apparent that opioids do affect host defense mechanisms. Heroin users present with an altered and functionally impaired immune system and have a higher prevalence of infectious diseases than do nonaddicts. Individuals exposed to opioid treatment for pain management during surgical procedures or maintained on oral methadone for treatment of drug addiction show either no effect or a suppressed immune system, depending on dosage and, in the case of methadone-maintained patients, duration of drug treatment. Confounding factors in these studies undermine definitive conclusions about the mechanisms by which opioids induce their immunomodulatory effects. Animal models have provided the means by which investigators can study the effects of opioids in a complex, biologic system that is easily manipulated and controlled. Findings from these studies have confirmed human data associating a pathogenic susceptibility with opioid use. Animal models have shown the complexity of this association. Interaction of the CNS, the autonomic nervous system, and the HPA axis is required for the varied effects of opioids on the immune system. By implication, exogenous opioids may be mimicking pathways by which endogenous opioids are involved in regulating immune defenses. To minimize the increased incidence of infectious diseases in heroin users and individuals clinically exposed to opioids, it will be important to determine the individual and collective effects of the opioid-induced activation of these pathways and the consequences of that activation to the immune system.

Modulation of Immune Cell Function Following Fluoxetine Administration in Rats

Fluoxetine (FLX) and other specific serotonin uptake inhibitors (SSRIs) have become the drugs of choice for treating depression. However, only a limited number of studies have addressed the effects of FLX on immune cell function. Our lab has measured the effects of both acute and chronic FLX administration on two functions of cell-mediated immunity, mitogen-induced lymphocyte proliferation (MILP) and natural killer cell cytolytic activity (NKCA). In this article we report that acute FLX administration (10 mg/kg) resulted in a dose- and time-dependent decrease in MILP and NKCA. MILP was more sensitive than NKCA to FLX, requiring lower doses for inhibition; however, the effects were more transient. Following chronic FLX administration, these effects were no longer observed, suggesting that an apparent tolerance to these particular measures of cell-mediated immunity had developed. Finally, a single microinjection of FLX directly into the lateral ventricle produced similar suppressive effects on MILP and NKCA, suggesting that the immunomodulatory mechanism may have a central component.

Enhancement of rat lymphocyte proliferation after prolonged exposure to stress

Rats exposed to 5 weeks of isolation, water scheduling (daily consumption restricted to 30 min), and their combination demonstrated significantly enhanced splenic lymphocyte proliferative responses to phytohemagglutinin (PHA) compared to those of group-housed animals (five per cage) maintained under standard vivarium conditions. The increased response found with the combination treatment was of the same magnitude as that found with each stressor alone. Blood lymphocyte responses were similarly increased after 5 weeks of isolation. When animals were exposed to the combination treatment for 12 weeks, both blood and splenic lymphocyte responses were found to be enhanced by greater than 2- and 3-fold, respectively. At the time of sacrifice no changes were found in total white blood cell numbers or plasma corticosterone levels with any of these treatments. These data suggest that prolonged exposure to a stressor results in an enhancement of immune cell activity.

In vitro effects of cocaine, lidocaine and monoamine uptake inhibitors on lymphocyte proliferative responses

Cocaine was found to inhibit in vitro mitogen-stimulated rat B and T lymphocyte proliferation in a dose-dependent manner. The IC50 for B lymphocytes (70 microM) was 2 to 4 fold lower than that obtained with T lymphocytes. To determine whether ion channel blockade or inhibition of monoamine uptake produced a similar suppression of lymphocyte proliferation, the effects of pharmacological agents sharing each of these properties with cocaine were examined. Lidocaine (0.5 mM), a sodium channel blocker, had no significant effect on B and T cell proliferation. By comparison, cocaine inhibited lymphocyte responses by greater than 80 percent at this concentration. Monoamine uptake inhibitors were also found to suppress lymphocyte proliferation in a dose-dependent manner similar to that obtained with cocaine. Of those tested, desipramine and fluoxetine were considerably more potent than cocaine, nomifensine and nisoxetine. These data demonstrated the addition of cocaine directly to lymphocyte cultures resulted in a dose-dependent inhibition of proliferation which was not due to Na+ channel blockade. Instead, the resemblance of monoamine uptake inhibitors to the action of cocaine suggests that lymphocytes may be intrinsically sensitive to these agents.

Time-dependent effects of isolation on lymphocyte and adrenocortical activity

Individual housing of previously group-housed animals was found to result in time-dependent changes in mitogen-induced lymphocyte proliferation and plasma corticosterone concentrations. During the first week of isolation, both B and T lymphocyte responses were depressed. However, within 2 weeks at least a 2-fold enhancement was observed. The increased lymphocyte response was still present after 35 days of isolation. Within the first day of isolation and continuing for 2 weeks, plasma corticosterone levels were found to be significantly decreased in isolated animals compared to those of continuously housed animals. No significant differences in steroid levels were observed by 35 days. These data demonstrate that isolation is accompanied by a depressed adrenocortical activity and suggest that the immunosuppression associated with short-term exposure is mediated by factors other than corticosterone.

Changes in T and B lymphocyte proliferative responses in adjuvant-arthritic rats: antagonism by indomethacin

The effects of a potent non-steroidal antiinflammatory drug, indomethacin, on the inflammatory response and lymphocyte proliferation were investigated in adjuvant-arthritic rats. As shown by others, adjuvant produced a time-dependent swelling of the contralateral paw which was maximal within 14 days after administration. Upon the intraperitoneal injection of either 1 or 2 mg/kg of indomethacin, twice daily for 3 days, the swelling of the contralateral paw was completely reduced. Selective changes in proliferative responses of splenic T and B lymphocytes to mitogens were found to accompany these effects. Adjuvant arthritic rats were found to have increased T cell proliferation when stimulated with phytohemagglutinin, whereas proliferation of B lymphocytes in the presence of lipopolysaccharide was completely suppressed. Indomethacin treatment at either the 1 or 2 mg/kg dose was found to depress the proliferative responses of T cells by 60 and 95%, respectively. In contrast, the hyporesponsiveness of B lymphocytes in arthritic animals was partially reversed with 2 mg/kg indomethacin. These results suggest that indomethacin may be exerting an antiinflammatory effect through a selective alteration of T and B lymphocyte activities in lymphoid tissue.

In vitro and in vivo effects of indomethacin on phytohemagglutinin-stimulated lymphocyte mitogenesis

The in vitro and in vivo effects of indomethacin on blastogenesis of rat splenic lymphocytes in response to phytohemagglutinin (PHA) were investigated. Direct addition of indomethacin to lymphocyte cultures was found to have both a stimulatory effect at low concentrations (less than or equal to 0.003 microM) and an inhibitory effect at higher concentrations (greater than or equal to 0.01 microM). The in vitro stimulation was significant only at submaximal and supramaximal concentrations of PHA, whereas the inhibitory response was observed with a wide range of mitogen concentrations. When indomethacin was administered to animals twice daily for 3 days, similar dose-dependent stimulatory and inhibitory effects were observed. Again the stimulatory effects were associated with lower doses (0.1 microgram/kg) and were found to be significant only with submaximal PHA concentrations. The inhibition with higher doses of indomethacin (IC50 = 0.20 mg/kg) was accompanied by a dose-dependent decrease in the maximal response and an increase in the EC50 to PHA in indomethacin-treated animals. These inhibitory effects of indomethacin administration on lymphocyte proliferation were found to occur at doses which closely approximate those required for the anti-inflammatory effects of the drug.