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The present study demonstrates that: (1) activation of micro -opioid receptors by systemic administration of a highly selective agonist DAGO (100 microg/kg) results in a significant increase in the number of plaque- and rosette-forming cells in the spleen of CBA mice as well as Wistar rats on the 5th day following sheep red blood cells (5 x 10(8)) immunization, (2) the immunostimulatory effect of DAGO is mediated by central mechanisms including the hypothalamus-hypophysis complex; (3) the postsynaptic dopamine (DA) receptors of D2 type are involved in the DAGO-induced immunostimulation since the combined treatment of animals with haloperidol (2 mg/kg), a blocker of DA D2 receptors, and DAGO abolished this effect; (4) the nuclei caudatus and accumbens of the nigrostriatal and mesolimbic DAergic systems, respectively, are implicated in the immune response stimulation caused by DAGO.

Experiments on CBA mice showed that selective stimulation of 2A serotonin receptors with DOI agonist (1 mg/kg) led to suppression of the immune response and reduction of the spleen and peripheral blood CD8(+)T cell counts with the cytotoxic/suppressor function. Selective blockade of these receptors with ketanserin (1 mg/kg) had an opposite effect: immunostimulation with an increase in CD8(+)T cell count in the spleen. These data indicate the involvement of 2A serotonin receptors in immunosuppressive mechanisms of serotoninergic system.

The present experiments demonstrate that: (1) activation of delta-opioid receptors by a highly selective agonist DSLET leads to immunosuppression, whereas the blockade of these receptors by enkephalin analog ICI 174864 produces immunostimulation; (2) the immunomodulating effect of delta-opioid receptors is mediated by central mechanisms involving the hypothalamus-hypophysis complex; (3) the postsynaptic 5-HT (serotonin) receptors of 5-HT-2 type are involved in the immunoinhibitory action of DSLET. On the other hand, the immunostimulating effect of ICI 174864 is realized with the participation of D-2 postsynaptic DA (dopamine) receptors. It has also been shown that thymus is implicated in the immunostimulating influence of ICI 174864. The data obtained are explained as a result of the interaction of the neuromediator and opioid peptidergic systems in neuroimmunomodulation.