Methylphenidate modulates interactions of anxiety with cognition.

While a large body of literature documents the impairing effect of anxiety on cognition, performing a demanding task was shown to be effective in reducing anxiety. Here we explored the mechanisms of this anxiolytic effect by examining how a pharmacological challenge designed to improve attentional processes influences the interplay between the neural networks engaged during anxiety and cognition. Using a double-blind between-subject design, we pharmacologically manipulated working memory (WM) using a single oral dose of 20 mg methylphenidate (MPH, cognitive enhancer) or placebo. Fifty healthy adults (25/drug group) performed two runs of a WM N-back task in a 3 T magnetic resonance imaging scanner. This task comprised a low (1-Back) and high (3-Back) WM load, which were performed in two contexts, safety or threat of shocks (induced-anxiety). Analyses revealed that (1) WM accuracy was overall improved by MPH and (2) MPH (vs. placebo) strengthened the engagement of regions within the fronto-parietal control network (FPCN) and reduced the default mode network (DMN) deactivation. These MPH effects predominated in the most difficult context, i.e., threat condition, first run (novelty of the task), and 3-Back task. The facilitation of neural activation can be interpreted as an expansion of cognitive resources, which could foster both the representation and integration of anxiety-provoking stimuli as well as the top-down regulatory processes to protect against the detrimental effect of anxiety. This mechanism might establish an optimal balance between FPCN (cognitive processing) and DMN (emotion regulation) recruitment.

Differential expression of the beta2-adrenergic receptor by Th1 and Th2 clones: implications for cytokine production and B cell help.

An important function of the sympathetic nervous system is to maintain homeostasis by modulating the level of cellular activity in many diverse organ systems. The sympathetic neurotransmitter norepinephrine modulates the level of T and B lymphocyte activity by binding to the beta2-adrenergic receptor (beta2AR). The present study was designed to elucidate the mechanism by which stimulation of the beta2AR affects both Th1/Th2 cell cytokine production and Th1/Th2 cell-dependent Ab production. Clones of murine Th1/Th2 cells were exposed to the beta2AR agonist terbutaline before activation by Ag-presenting B cells. Terbutaline exposure of Th1 cells before activation inhibited IFN-gamma production by Th1 cells and subsequent IgG2a production by B cells. IgG2a inhibition was prevented by addition of the betaAR antagonist nadolol or exogenous IFN-gamma. In contrast to Th1 cells, terbutaline did not affect either IL-4 production by Th2 cells or subsequent IgG1 production by B cells. Although baseline levels of intracellular cAMP were similar in both subsets, terbutaline induced an increase in cAMP levels in Th1 cells only. Radioligand binding studies showed that a detectable number of beta2AR binding sites were present on Th1 cells, but not on Th2 cells. Immunofluorescence analyses showed that Th1 cells expressed a higher level of the beta2AR cytoplasmic carboxyl terminus than did Th2 cells. These results show that expression of the beta2AR binding site by Th1 cells, but not by Th2 cells, establishes a physiologic mechanism for selective modulation of Th1 cell IFN-gamma production and IFN-gamma-dependent IgG2a production, provided that beta2AR stimulation occurs before cell activation by a B cell.

Immunodeficiency in RFM/(T6xRFM)F1 mouse chimaeras with lethal host-versus-graft syndrome.

Rather than central tolerance, the perinatal inoculation of related F1 hybrid spleen cells into inbred mice may result in host-versus-graft (HVG) reactions manifested as transient autoimmunity, or as a lethal immunodeficiency syndrome. RFM/(T6xRFM)F1 chimaeras with lethal disease die in 30 days with lymphosplenomegaly, immune complexes and impaired immune responses. The present studies used in vitro proliferation assays to show that the HVG reaction caused hyperplasia sufficient to account for the lymphosplenomegaly, while also causing severe impairment of splenic and nodal cell responses to concanavalin A (Con A) and to bacterial lipopolysaccharide (LPS). By 25 days, HVG mice could not distinguish between self and non-self as judged by mixed lymphocyte reactions (MLR) to RFM, (T6xRFM)F1 and third party A/J cells. There were no indications that host cells reactive to F1 donor cells had undergone clonal deletion, anergy or expansion. Flow cytometry revealed that donor T lymphocytes achieved stable engraftment, mostly in the nodes, despite the HVG reaction. Taken together with previous observations, these studies showed that HVG reactions in young parent F1/chimaeras can result in an immunodeficiency state which is characterized by an early appearing, profound and persistent impairment of both host and donor T and B cell functions. The results suggest that HVG reactions can contribute directly to immune deficits seen after clinical allogeneic bone marrow transplantation.

The effect of methadone on the immune status of B6C3F1 mice.

Previously, morphine has been shown to elevate corticosterone via the hypothalamic-pituitary-adrenal axis and to suppress the immune system. The present investigation sought to determine if the mu-opiate receptor agonist methadone incurred a similar immune suppression in B6C3F1 mice. Serum methadone and corticosterone levels peaked 1 hr following a single subcutaneous injection of 20 mg/kg methadone HCl. Indeed, the rise in corticosterone levels paralleled that of methadone. After a single injection with 20 mg/kg methadone a pharmacokinetic analysis revealed a serum half-life of approximately 2 hr. Following five injections of methadone over a 24-hr period (every 6 hr), methadone levels were elevated as would be expected; however, corticosterone levels did not become elevated. This suggests that the ability of methadone to elevate corticosterone becomes uncoupled following repeated dosing, indicative of either a tolerance or an increased catabolic mechanism. Moreover, dosing every 6 hr for 5 days induced an increase in the catabolism of methadone itself. Therefore, all assays were begun 1 hr after subcutaneous administration of methadone HCl, a time at which both methadone and corticosterone serum levels were elevated. The primary IgM antibody response to sheep red blood cells (sRBC) was suppressed when splenocytes were immunized in vitro. In contrast, animals immunized with sRBC and assayed for the primary IgM antibody response 4 days later were not suppressed. The activity of the resident macrophages of the liver and spleen as measured by the uptake of 51Cr-sRBC was suppressed in a dose-dependent manner. Previously, it has been demonstrated that morphine suppresses hepatic and splenic phagocytic activity through an opiate receptor-mediated pathway that involves the release of corticosterone. It would appear that methadone plays a similar role in the suppression of hepatic and splenic phagocytosis.

A mechanism of action for morphine-induced immunosuppression: corticosterone mediates morphine-induced suppression of natural killer cell activity.

Morphine is a drug of abuse with an ability to down-regulate immune responsiveness that could have potentially serious consequences in both heroin addicts and in the clinical environment. The exact mechanism of action by which morphine induces immunosuppression has yet to be clearly determined. A direct mechanism of action is suggested to operate through lymphocyte opiate receptors, but the nature of such receptors is still in question. The alternative, an indirect mechanism of action is proposed to be mediated by two possible pathways, hypothalamic-pituitary-adrenal axis activation with increased production of adrenal corticosteroids, or activation of the sympathetic nervous system and concomitant catecholamine release. Natural killer (NK) cell activity was used to determine potential indirect mechanisms of action for morphine. NK activity in the B6C3F1 mouse was suppressed between 12 and 48 hr after implantation of 75 mg timed-release morphine pellets. Morphine suppressed NK activity in a dose-responsive manner. The opiate antagonists naloxone and naltrexone completely blocked morphine-induced suppression of NK activity, whereas naloxone methiodide, a congener that crosses the blood-brain barrier much more slowly than naloxone, produced very little blockade. Implantation of the 75-mg morphine pellets produced a significant elevation in serum corticosterone levels. In vitro exposure to corticosterone is known to suppress NK activity directly, whereas in vitro morphine was unable to alter directly NK activity. The glucocorticoid receptor antagonist Roussel-Uclaf 38486 blocked morphine-induced suppression of NK activity in a dose-responsive fashion. Naltrexone (10-mg pellet) antagonized the morphine-induced elevation in serum corticosterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Gallium arsenide-induced increase in serum corticosterone is not responsible for suppression of the IgM antibody response.

Previous investigations demonstrated gallium arsenide (GaAs) to be an immunosuppressive agent that alters the function of all cell types involved in the generation of a primary antibody response. In those studies, GaAs was administered as a particulate compound that remained in the lung at least 14 days after exposure. The extended presence of the particulate in the lung may induce a stress response that leads to the release of endogenous corticosteroids. In addition, some of the observed immunomodulatory effects of GaAs were similar to immunological alterations reported to be induced by corticosteroids. The present studies were designed to determine whether suppression of the immunoglobulin (Ig) M antibody-forming calls (AFC) response by GaAs was a result of a GaAs-induced increase in serum corticosterone. GaAs (50-200 mg/kg) significantly decreased the weights of both the thymus and spleen and the cellularity of the spleen. In addition, there was a GaAs-induced decrease in the CD4+/CD8+ thymocyte subpopulations and a concomitant increase in CD4+ and CD8+ cells. Within the spleen, there were no alterations in the percentages of CD4, CD8 or Ig-positive cells. However, when expressed as an absolute cell number, there was a 50% decrease in the numbers of CD4+ and CD8+ cells in the spleen. GaAs also dose-dependently suppressed (50-75%) the IgM AFC response. The GaAs-induced changes in cell populations and immune organ weights were correlated with an increase (6- to 10-fold) in serum corticosterone levels. The treatment of mice with the glucocorticoid antagonist mifepristone (also called RU 486) blocked the observed alterations in splenic and thymic cell populations induced by GaAs.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of brain-immune interactions in immunotoxicology.

Certain xenobiotics (or the metabolites) can damage immunocompetence by directly interacting with one or more of the cells of the immune system and adversely affecting its function. It has also been proposed that xenobiotics may indirectly affect immune function by affecting other organ systems that will in turn affect immunocompetence. This review surveys evidence that supports the existence of a functional link between the brain and the immune system. In addition, we review data that support the concept that a xenobiotic-induced dysfunction in the neuroendocrine system may be associated with an immune dysfunction as well. Such chemicals do not necessarily interact directly with immunocompetent cells but would instead act to disrupt regulatory brain-immune interactions. This class of indirectly acting immunotoxic xenobiotics would not be detected in the typical in vitro screening assays.

Follicular dendritic cell function and murine AIDS.

Infection of mice with LP-BM5 elicits an immunodeficiency state referred to as murine acquired immune deficiency syndrome (MAIDS). Shortly after infection, retrovirus particles become associated with follicular dendritic cells (FDC) and this study was undertaken to determine whether retroviruses alter FDC functions. The FDC functions examined included the ability to: (1) retain antigen (Ag) trapped prior to infection; (2) trap new Ag after infection; (3) maintain specific IgG responses; and (4) provide co-stimulatory signals to B cells. Mice were infected with LP-BM5 and the ability of their FDC to trap and retain 125I-Ag (HSA) was assessed. Serum anti-HSA levels were monitored and FDC co-stimulatory activity was indicated by increased B-cell proliferation. HSA trapped on FDC prior to infection began to disappear by 3 weeks and was practically gone by 6 weeks. Serum anti-HSA titres were maintained normally for about 3 weeks after infection and then declined precipitously. The ability of FDC to trap new Ag began to disappear around the second and third week of infection and was markedly depressed by the fourth week. However, FDC recovered from infected mice retained their ability to co-stimulate anti-mu- and interleukin-4 (IL-4)-activated B cells throughout a 5-week period. In short, the ability of FDC to trap and retain specific Ag and maintain specific antibody levels was markedly depressed after retrovirus infection. However, FDC from infected mice continued to provide co-stimulatory signals and these signals may contribute to the lymphadenopathy and splenomegaly characteristic of MAIDS.

Immunotoxicity of nitrobenzene in female B6C3F1 mice.

Nitrobenzene (NBZ) is primarily employed as an oxidizing agent in the synthesis of analine and benzene compounds. It produces myelotoxic effects and effects on erythrocytes in both animal models and man. Reported hepatosplenomegaly and effects on the bone marrow are indicators that NBZ may be immunotoxic. In these studies, female B6C3F1 mice were exposed to 30, 100 and 300 mg/kg of NBZ in corn oil by gavage for 14 consecutive days. To assess the immunotoxic potential of NBZ, body and organ weights were determined and selected immunologic and host resistance responses were studied. In these studies, the liver and spleen appeared to be the primary target organs. Both liver and spleen weights were dose dependently increased. Gross histopathologic examinations revealed significant changes in the spleen, consisting of severe congestion of the red pulp areas with erythrocytes and reticulocytes. Serum chemistry profiles showed increases in alanine aminotransferase and aspartate aminotransferase activities, indicating liver toxicity. Hematologic studies showed a decrease in erythrocyte number and a concomitant increase in mean corpuscular hemoglobin and mean corpuscular volume. A dose-dependent increase in peripheral reticulocytes was also seen. DNA synthesis was enhanced, as was the number of formed elements and the number of monocyte/granulocyte stem cells in the bone marrow of treated mice. IgM responses were decreased and the phagocytic activity of macrophages in the liver was dose dependently increased with a concomitant decrease in the activities in the spleen and lung. Other immunological parameters examined were unchanged. Host resistance to microbial or viral infection was not markedly altered by NBZ; however, there were trends towards increased susceptibility where T-cell function contributes to host defense. These data indicate that NBZ-induced hemolysis and liver injury are linked to the observed alterations in bone marrow activity.

Immunotoxicity of mono-nitrotoluenes in female B6C3F1 mice: I. Para-nitrotoluene.

para-Nitrotoluene (p-nitrotoluene) is used primarily as an intermediate in the production of various dyes, explosives, pharmaceuticals, and in the production of rubber and agricultural products. Previous investigations indicated that p-nitrotoluene was mutagenic in the Ames Test and that other mono-substituted nitrotoluenes bound covalently to hepatic macromolecules. The objective of these studies was to evaluate the potential immunotoxicity of p-nitrotoluene in mice exposed by the oral route. Mice exposed to p-nitrotoluene (200-600 mg/kg) daily for 14 days showed modest dose-dependent increases in liver and spleen weights. The livers of mice exposed subchronically to 400 and 600 mg/kg showed a mild to moderate swelling of the hepatocytes adjacent to the central veins; this swelling appeared to be reversible and there was no evidence of necrosis. The proportion of monocytes in blood was decreased in mice treated with p-nitrotoluene or toluene. Serum chemistries, bone marrow cellularity and the number of CFU-M and CFU-GM were unaffected. Immunologic investigations showed p-nitrotoluene suppressed the IgM response to sRBC and the DHR response to KLH. There was a 24% decrease in the percentage of CD4+ T lymphocytes in the spleen. There was no dose-dependent alteration of peritoneal macrophage numbers or differential count, unstimulated natural killer cell activity, response to B cell mitogen LPS, C3 activity or interferon levels. Exposure of mice to p-nitrotoluene decreased resistance to Listeria monocytogenes but not to Streptococcus pneumoniae, Plasmodium yoelii or the B16F10 melanoma, and increased resistance to the PYB6 tumor. These studies indicated that the immune system is an important target for toxicity of p-nitrotoluene. The decreased host resistance to L. monocytogenes can be attributed to the decrease in T lymphocytes and to a decreased delayed hypersensitivity response to KLH.

Immunotoxicity of 2,4-diaminotoluene in female B6C3F1 mice.

2,4-Diaminotoluene (DAT) has been demonstrated to be a potent carcinogen. The present studies were carried out to determine the toxic and immunotoxic potential of DAT. Mice exposed to DAT at 25-100 mg/kg per day for 14 days by gavage showed a 42% increase in liver weight and a slight decrease in spleen weight. Histopathologic evaluation of selected organs showed the liver to be the major target with morphological changes which were dose dependent. The high dose (100 mg/kg) was associated with moderate centrilobular necrosis. No abnormal structure was noted in the spleen, lungs, thymus, kidney or mesenteric lymph nodes. The liver toxicity was associated with an elevation in alanine aminotransferase activity. The only change noted in selected hematologic parameters was a 64% increase in peripheral blood leukocytes. Mice exposed to DAT showed a decreased IgM and IgG response to sheep erythrocytes. The decrease was not a function of a decreased number of B cells because the number of B cells increased dose dependently. Proliferative capacity of immunocompetent cells was not impaired by exposure to DAT as measured by the response to several mitogens. The delayed hypersensitivity response to keyhole limpet hemocyanin in mice exposed to DAT was increased. Natural killer cell activity was decreased dose dependently and may represent a spleen cell pool shift because the number of B cells increased in the presence of a decreasing spleen size. Serum C3 was suppressed at the high dose of DAT. Phagocytosis by splenic macrophages, but not peritoneal macrophages, was inhibited by DAT exposure. DAT exposure for 14 days decreased host resistance to the bacteria, Streptococcus pneumoniae and Listeria monocytogenes, while host resistance to the pulmonary tumor model, B16F10, and the PYB6 fibrosarcoma was unaffected by DAT exposure. These data indicate that DAT is hepatotoxic and perturbs the differentiation and maturation of leukocytes.

Immunotoxicity of mono-nitrotoluenes in female B6C3F1 mice: II. Meta-nitrotoluene.

The nitrotoluenes are chemicals used in dyes, agricultural products, pharmaceuticals and explosives. In the present studies, the toxicology and immunotoxicity of meta-nitrotoluene (m-nitrotoluene) were evaluated. Mice, exposed to m-nitrotoluene at dose levels of 200, 400 and 600 mg/kg/body weight for 2 weeks by gastric gavage, gained body weight over the treatment period to a slightly greater extent than the control groups. Of the selected organs weighed, the liver and kidney of mice exposed to m-nitrotoluene were increased in weight while the thymus weight was decreased. The liver of mice exposed to m-nitrotoluene, but not ortho-nitrotoluene, showed slight to moderate swelling of the hepatocytes adjacent to the central veins. The hepatocyte swelling appeared to be reversible and there was no evidence of necrosis. The hematology and serum chemistries examined were unaffected by m-nitrotoluene exposure although there were modest decreases in the percentage of polymorphonuclear leukocytes and eosinophils in differential blood counts. Bone marrow cellularity and the number of CFU/M and CFU/GM were unaffected by m-nitrotoluene exposure. m-Nitrotoluene suppressed the IgM response to sRBC and the DHR response to KLH. There was a slight (8%) decrease in the percentage of B lymphocytes in the spleen. The response to the T cell mitogens was suppressed by as much as 39%. Fc-mediated adherence and phagocytosis of chicken erythrocytes and NK cell activity were increased dose dependently in mice exposed to m-nitrotoluene. Several immune parameters were unaffected by exposure to m-nitrotoluene, including the IgG response to sRBC, responses to the B cell mitogen LPS and to allogeneic cells, and serum interferon levels. Resistance to Streptococcus pneumoniae and Plasmodium yoelii were unaffected also. Resistance to the tumor model PYB6 was increased. Exposure of mice to m-nitrotoluene decreased resistance to Listeria monocytogenes. The decreased resistance to L. monocytogenes may be related to an effect on T cells, evidenced by a decrease in T cell numbers and in the DHR.

Hepatic and splenic phagocytosis in female B6C3F1 mice implanted with morphine sulfate pellets.

Morphine sulfate has previously been shown to produce a dose-dependent decrease in hepatic phagocytosis when administered as 8-, 25- and 75-mg pellets implanted subcutaneously. This study was undertaken to determine the time course of suppression of hepatic and splenic phagocytosis after subcutaneous implantation of morphine sulfate pellets. Mice were implanted with either 75 mg of morphine sulfate or placebo pellets. The uptake of chromated sheep red blood cells by the liver and spleen was taken as an index of phagocytosis by resident Kupffer cells or macrophages, respectively. The results indicate that maximum suppression of hepatic phagocytosis by 67% occurred 18 hr after implantation of 75 mg of morphine sulfate. Hepatic phagocytic capacity returned to control levels within 48 hr of implantation. The initial time course of suppression of splenic phagocytosis by 41% was similar to that of the liver (maximum at 12 hr). However, splenic phagocytic capacity returned toward placebo levels over a longer period of time reaching control after 4 days after implantation. The opiate receptor antagonist, naltrexone (30-mg pellet), completely blocked the ability of morphine to suppress either hepatic or splenic phagocytosis. Corticosterone is known to increase in parallel with plasma morphine levels presumably through a hypothalamic-pituitary-adrenal axis. The glucocorticoid receptor antagonist RU 486 was used to block the actions of corticosterone and investigate its possible role in morphine sulfate-induced suppression of phagocytosis. RU 486 (200 mg/kg) completely blocked morphine sulfate's ability to suppress splenic phagocytosis. In contrast, RU 486 only partially blocked morphine-induced suppression of hepatic phagocytosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphine induces apoptosis in murine thymocytes in vivo but not in vitro: involvement of both opiate and glucocorticoid receptors.

We tested the hypothesis that the previously observed loss of thymic lymphocytes in mice after treatment with time-release morphine pellets was occurring through the process of apoptosis. Apoptosis is a form of cell death, distinct from necrosis, which involves a specific endonuclease that fragments the cell's own DNA. Forty-eight hours after implantation of a time-release morphine pellet in B6C3F1 mice, thymus weight and cellularity was reduced to 30% of that observed in placebo-treated mice. Thymocytes from morphine pellet-treated mice were found to have a significantly greater percentage of their DNA fragmented than did thymocytes from either placebo pellet-implanted or naive control mice. The peak level of DNA fragmentation was found to occur approximately 12 hr postpellet implant. When separated on agarose gels, the sizes of the DNA fragments observed corresponded to the multiples of 180 base pairs which are characteristic of apoptosis. In vivo, the use of either the opiate receptor antagonist naloxone, or the glucocorticoid receptor antagonist RU-38486, was able to block completely the morphine mediated increase in thymocyte apoptosis. In vitro experiments in which thymocytes were cultured with morphine concentrations as high as 10(-4) M showed no evidence of an increased rate of DNA fragmentation. These data indicate that both opiate and glucocorticoid receptors are involved in morphine-induced apoptosis and that the opiate receptor responsible is not located on the thymic lymphocytes.

Morphine-induced alterations in thymocyte subpopulations of B6C3F1 mice.

Morphine has been reported to possess immunosuppressive actions in both in vitro as well as in vivo assays of immune function. Our work in female B6C3F1 mice, surgically implanted with a 75-mg time release morphine pellet, has confirmed previous reports of a rapid loss in the cellularity of the spleen and thymus. To evaluate the effect of morphine on the subpopulations of cells in the thymus, two color fluorescence flow cytometry studies were performed. Fluorescently conjugated monoclonal antibodies specific for the murine cell surface CD4 and CD8 markers were used to identify the four major subpopulations of thymocytes. These studies indicated that morphine pellet-implanted mice suffered a loss in each of the four thymocyte subpopulations in comparison to placebo-implanted mice. However, the loss (> 90%) in the important CD4+/CD8+ subpopulation of immature thymocytes greatly exceeded that which was observed for any other subpopulation. Kinetic studies of morphine's effect on the thymocyte subpopulations revealed that the maximal depletion of the CD4+/CD8+ cells occurs approximately 4 days after pellet implantation. Thymocyte cell populations recovered by 14 days, with an increase above placebo for the double positive cells. Naltrexone administration blocked thymic alterations, suggesting that these immunologic consequences of morphine may be mediated through an opiate receptor. Measurements in thymocytes from morphine pellet-implanted mice showed an increased level of DNA fragmentation, whereas in vitro exposure to morphine (1-100 microM) produced no such increases. This suggests morphine may be working indirectly to induce apoptosis of immature thymocytes.

Assessment of cholinergic influences on a primary humoral immune response.

Cholinergic ligands can affect some lymphocyte functions, and binding of labelled cholinergic ligands to lymphocytes has been reported. However, the role of endogenous cholinergic stimulation in immunomodulation in vivo is unclear. It has been suggested that suppression of primary humoral immune responses in vivo by administration of organophosphorus compounds is caused by excessive cholinergic stimulation. If this is correct, it would demonstrate cholinergic immunomodulation in vivo and might serve as a useful model for the characterization of this phenomenon. In the present study, the organophosphorus insecticide parathion and its major metabolite, paraoxon, suppressed the primary IgM response to sheep red blood cells (SRBC) in vitro in Mishell-Dutton cultures at concentrations similar to those probably reached in vivo. In contrast, cholinergic agonists did not suppress the in vitro response, but tended to enhance it. However, antagonists also tended to enhance the response and the effects of agonists were not blocked by antagonists. Binding studies with a radiolabelled cholinergic antagonist ([3H-]QNB) did not indicate the presence of specific, saturable cholinergic receptors on lymphocytes. A membrane preparation from brain was used as a positive control, and specific, saturable binding was observed. These results suggest that suppression of primary immune responses in vivo by parathion is mediated at least in part by direct action of parathion and/or its major metabolite, paraoxon, on the immune system. The data provide no evidence that direct interaction of cholinergic ligands with the immune system contributes to parathion-induced immunosuppression. In fact, the absence of expected agonist-antagonist relationships in Mishell-Dutton cultures, the absence of saturable [3H]QNB binding, and puzzling inconsistencies in the literature on this subject cast doubt on the conclusion that lymphocytes express specific cholinergic receptors.

Morphine suppresses primary humoral immune responses by a predominantly indirect mechanism.

Morphine suppresses humoral immune responses, causes thymic hypoplasia and suppresses NK (natural killer) activity in animal models. There is evidence that thymic hypoplasia and NK suppression are predominantly mediated by indirect mechanisms. The mechanism of morphine-induced humoral immunosuppression is less certain. Recent reports suggest that morphine and other opioids can directly act on cells of the immune system to suppress the generation of antibody-forming cells (AFC) in Mishell-Dutton cultures. The present study was designed to assess the roles of direct and indirect mechanisms in morphine-induced suppression of humoral immunity. Splenocytes from mice treated with morphine by s.c. implantation of a slow-release 75 mg pellet were dysfunctional in Mishell-Dutton cultures. Exposure to morphine in vivo for 12 or 24 hr caused significant suppression of the AFC production stimulated by sheep erythrocytes in Mishell-Dutton cultures. In contrast, direct addition of morphine or the kappa selective opioid agonist U50,488H to Mishell-Dutton cultures under a variety of conditions had little or no effect on AFC generation. These results indicate that suppression of humoral responses by morphine is not primarily mediated by direct action of morphine on the immune system. Suppression of AFC responses by administration of morphine in vivo was substantially blocked by treating mice with the glucocorticoid antagonist RU 38486, suggesting that glucocorticoids may be involved in the indirect mechanism by which morphine causes splenocyte dysfunction.

The T-lymphocyte is the primary cellular target for potentiation of the in vitro T-dependent IgM antibody response by the B subunit of cholera toxin.

The B (or binding) subunit of cholera toxin (CTB) was reported previously to potentiate the in vitro T-dependent IgM antibody response by a mechanism independent of the cyclic AMP-generating capacity of the intact toxin. In the present report, experiments were designed to determine the immune cell type mediating potentiation by CTB. Firstly, CTB did not potentiate T-independent antibody responses at concentrations that effectively enhanced T-dependent responses. Secondly, separation/reconstitution studies with splenocytes from CTB- and vehicle-treated mice demonstrated potentiation of T-dependent responses by CTB treatment of either the Sephadex G10 non-adherent population or the T-lymphocyte + macrophage population of cells. Potentiation was not observed by CTB treatment of the plastic adherent population or the B-lymphocyte + macrophage population. The evidence indicates that the T-lymphocyte is the primary cellular target for CTB-induced effects on the T-dependent IgM antibody response. Monosialoganglioside GM1, the putative binding site for CTB, is most likely the site of action for CTB on T-lymphocytes. These studies provide new insight on the mechanism of immunomodulation by cholera toxin, and CTB should provide a useful tool for further understanding the role of gangliosides in cellular immune responses.