Effect of Intraventricular Injection of 5,7‐dihydroxytryptamine on Serum Gonadotropins and Prolactin*

A rich innervation of limbic and hypothalamic areas with monoaminergic terminals has been demonstrated by means of the histochemical fluorescence technique.1-3 There is little doubt that mesencephalic noradrenergic fibers that enter the hypothalamus play a key role in stimulating the preovulatory luteinizing hormone (LH) surge.4-6The function of the tuberoinfundibular dopaminergic neurons is less clear.Because this system is the primary source of intrahypothalamic dopamine (DA) content,' the extrahypothalamic DA systems do not play an important role, if any, in controlling anterior pituitary hormone secretion.The spectrum of experimental findings ranges from inhibitory to stimulatory effects of DA on gonadotropin ~e c t i o n .~. Dopamine, however, has a precise role in that it inhibits prolactin secretion at the pituitary.':',l 4 The literature on the function of serotonin is even more confusing.Intraventricular injection of 5-hydroxytryptamine (5-HT) was shown to inhibit gonadotropin release lo and to stimulate prolactin release in male rats.15Inhibition of 5-HT synthesis with p-chlorophenylalanine (p-CPA) gave inconclusive results, because in some authors hands, such treatment inhibited prolactin or LH release,16 and in others it did not.lSSystemic injection of 5hydroxytryptophan, the immediate precursor of 5-HT, was able to block the preovulatory LH discharge in intact rats, indicating an inhibitory function on LH release.lYThe LH surges induced in castrated rats by estrogen treatment, which are believed to have the same underlying neural mechanisms as those responsible for the preovulatory LH peaks, could be blocked by inhibition of 5-HT synthesis, suggesting a stimulatory function of the neurotransmitter on LH release."Until recently, relatively consistent data were available on the role of 5-HT in prolactin release.There was good evidence that this monoamine stimulates pituitary prolactin release.lg*20-22 However, Gallo et al.,lh who used different approaches, could not confirm these results; we also demonstrated recently that the stress-induced release of prolactin may be mediated via the noradrenergic system.8These highly confusing results may indicate that 5-HT has dual functions, depending on the animal used for experimentation.It was therefore interesting to investigate the endocrine effects of reducing brain 5-HT content with the serotonin neurotoxin 5,7-dihydroxytryptamine 23-2 p (5,7-DHT).