Involvement of the central serotoninergic system in the changes of leu-enkephalin level in discrete rat brain areas

Molecular mechanisms of actions of interleukin-6 on the brain, with special reference to serotonin and the hypothalamo-pituitary-adrenocortical axis

Biological activities of the multifunctional cytokine, interleukin-6 (IL-6) include stimulation of B cell proliferation, immunoglobulin production, and initiation of the acute-phase response. IL-6 affects the CNS in that it activates the hypothalamo-pituitary-adrenocortical (HPA) axis and increases brain tryptophan and serotonin metabolism. IL-6 has been proposed as an important mediator of interaction between the neuroendocrine and immune systems. The peripheral and central effects of IL-6 are presumably mediated through its membrane receptor (IL-6R). IL-6, IL-6R and their respective mRNAs have been detected in several brain regions. Although the functions of cytokines overlap considerably, each displays its own characteristic properties. Expression of IL-6 in the brain has been observed in several CNS disorders, some of which have been associated with disorders of serotonin metabolism. It is proposed that interactions between IL-6 and brain serotonin is a complex process which involves corticotropin-releasing factor (CRF) and opioid peptides. It is likely that the molecular mechanisms underlying the actions of IL-6 on the HPA axis and its other brain functions involve the integrated effects of glutamate, Ca2+, 3',5'-cyclic AMP, protein kinase C, and other metabolic pathways.

Effects of dexfenfluramine and opioid peptides, alone or in combination, on food intake and brain serotonin turnover in rats

Dexfenfluramine (d-FF) and opiate agonists both act on food intake but in opposite ways. Serotonin is known to be involved in the pharmacological action of both d-FF and opiates, but not necessarily in the feeding effect of the latter. In order to test this hypothesis, the effects of three opioid agonists, beta-endorphin, dynorphin and D-Ser2-Leu-Enk-Thr6 (DSLET) and of an antagonist, naltrexone, were investigated individually and in combination with d-FF on food intake and brain serotonin turnover. The opioid agonist-d-FF combinations generally produced a similar anorectic effect to that of d-FF alone, with the exception of DSLET which showed a reciprocal antagonism. The serotonergic effects varied according to the opioid tested, alone or in combination with d-FF. This does not allow to highlight a general pattern of serotonin involvement in the feeding effects of these peptides. However, all the treatments which decreased feeding (d-FF, naltrexone and the combinations dynorphin-d-FF and beta-endorphin-d-FF) displayed similar trends in hypothalamic serotonergic variations. This study evidences a role of serotonin in the feeding effect of opiates, although not similar for all of them. The use of d-FF provides a tool for assessing this involvement.

Effects of opiate agonists and an antagonist on food intake and brain neurotransmitters in normophagic and obese "cafeteria" rats

The relationship between the effects of opiates on food intake and on central monoamines in various brain areas was investigated in normophagic and obese "cafeteria" rats. Three agonists, beta-endorphin, dynorphin, and D-Ser2-Leu-Enk-Thr6 (DSLET) and an antagonist, naltrexone, were used. The three agonists enhanced feeling in normophagic rats but had different dopaminergic effects. Serotonergic metabolism increased concomitantly with the enhancement of feeding by the agonists, whereas it decreased following treatment with the antagonist naltrexone. In the cafeteria rats, although the feeding effects of dynorphin and DSLET occurred earlier, there was a complete lack of monoaminergic effects. beta-Endorphin was completely devoid of effects in this model. There would, thus, appear to be a positive correlation between the behavioural effects of these opiates and serotonergic metabolism in normophagic rats, while stimulated feeding situations ("cafeteria" rats) the disruption of a monoaminergic modulation does not prohibit a direct effect on feeding.