Effects of Monoamine Neurotoxins Injected in Different Brain Areas on Gonadotropin and Androgen Secretion in the Male

Abstract

In order to investigate the relative contribution of the serotoninergic and catecholaminergic innervation of different brain structures to the control of gonadotropin secretion in the male, adult rats were given bilateral injections of the selective neurotoxins, 5,7-dihydroxytryptamine and 6-hydroxydopamine, into the mediobasal hypothalamus, the medial preoptic area, the amygdala, and the medial forebrain bundle. Serum levels of luteinizing hormone (LH), follicle-stimulating hormone, testosterone, pituitary follicle-stimulating hormone, and LH content were measured 15 days later. The medial preoptic area 5,7-dihydroxytryptamine injections were followed by 2- to 3-fold increases of serum LH and testosterone levels and pituitary LH concentration. The 6-hydroxydopamine injections in the medial forebrain bundle resulted in a 60% decrease of pituitary LH content, associated with serum levels of LH and testosterone which were lowered, although not significantly. None of the treatments seemed to influence either serum or pituitary concentrations of follicle-stimulating hormone. These results suggest a hitherto undocumented major function of the serotoninergic innervation of the medial preoptic area in the control of LH secretion in the male and confirm previous reports indicating that some of the catecholaminergic fibers carried by the medial forebrain bundle can be modulatory for LH secretion.

Correlations between the Hypothalamic Density of ³H-Clonidine-Binding Sites and Plasma Testosterone Levels in Mice

“…LH and testosterone. The hypothesis is consistent with reports that decreased noradrenaline stores suppress the secretion of LH [38,39] and of testos terone [39] in adult male rats. This mechanism may be involved in the feedback control of gonadotropin secre tion, since subcutaneous or intrahypothalamic implanta tions of testosterone lead to a reduced turnover of nor adrenaline in the mediobasal hypothalamus as well as decreased plasma LH levels in castrated rats [40], Activa tion of the central cb-adrenergic system by testosterone has been demonstrated [41], Alternatively, cb-adrenoceptors may be negatively cor related with plasma testosterone levels in an indirect manner involving other neurons.…”

Correlative relationships between the specific binding of labeled adrenoligands in the brain and plasma testosterone in mice

“…The intraventricular injection of a smaller dose of clonidine induced only a decrease in the secretion of LH [7]. Since the administration of clonidine in addition decreases the concentration and metabolism of NE in the hypothalamic nuclei, which has been related to excitation of c~2-autoreceptors [5], and a decrease in the content of NE in the brain, as has been demonstrated in experiments involving 6-hydroxydopamine, suppresses the secretion of LH and T in adult male rats [9], the mechanism governing the negative correlation between the number of c~2-adrenoreceptors in the hypothalamus and the level of T in the blood, may hypothetically be represented in the form of the following events: the stimulation of the presynaptic c~2-adrenoreceptors is accompanied by a decrease in the level and release of NE from noradrenergic terminals, which leads to the succeeding decrease in the secretion of luliberin, LH, and T. It should, however, be noted that the c~2-adrenoreceptors in the brain may be located not only presynaptically, but postsynaptically as well. However, the radioligand method utilized does not permit the determination of which of these are associated negatively with the level of T.…”