Stress-induced release of anterior pituitary hormones: effect of H3 receptor-mediated inhibition of histaminergic activity or posterior hypothalamic lesion

The effect of stress- or lipopolysaccharide (LPS) endotoxin-induced release of ACTH, beta-endorphin (beta-END) and prolactin (PRL) was investigated in two groups of conscious male rats: (1) Rats pretreated with different H3 receptor agonists, which inhibit neuronal histamine (HA) synthesis and release, and (2) rats with bilateral posterior hypothalamic lesion, which destroys the histaminergic perikarya exclusively localized in the mammillary nuclei. The H3 receptor agonists R(alpha)methyl-HA, BP 2-94 or imetit injected intraperitoneally (ip) had no effect on basal secretion of ACTH or PRL but inhibited the ACTH and PRL responses to restraint stress and the ACTH response to LPS endotoxin. LPS had no effect on PRL secretion. The inhibitory effect of the agonists was prevented by prior ip administration of the H3 receptor antagonist thioperamide. Bilateral lesion of the posterior hypothalamus inhibited the ACTH, beta-END and PRL responses to restraint stress, ether stress and LPS endotoxin, whereas sham operation had no effect compared to nonoperated control rats. In addition, posterior hypothalamic lesion inhibited the PRL response but not the ACTH and beta-END responses to activation of serotonergic neurons induced by ip administration of the 5-HT precusor 5-hydroxytryptophan in combination with the 5-HT re-uptake inhibitor fluoxetine. Thus, serotonergic pathways were not damaged by the lesions. The present results support our previous findings that inhibition of neuronal HA synthesis by alpha-fluoromethylhistidine as well as blockade of H1 or H2 receptors inhibit the ACTH, beta-END and PRL responses to stress and LPS endotoxin and further substantiate an important role of histaminergic neurons in the mediation of the stress-induced release of pituitary stress hormones. Furthermore, in accordance with our previous findings, the lesion experiments indicated the existence of an interaction between the histaminergic and serotonergic system in regulation of the stress- and LPS-induced PRL release.