Evidence of antagonism between prolactin and gonadotrophin secretion: effect of methalibure on perphenazine-induced prolactin secretion inovariectomized rats

Calcitonin-like peptide containing gonadotrophs are juxtaposed to cup-shaped lactotrophs

Calcitonin (CT) is known to inhibit basal and TRH-stimulated prolactin release in cultured anterior pituitary cells in vitro and pituitary CT-like peptide (pit-CT) is synthesized and released by isolated anterior pituitary cells. However, the specific cell type containing pit-CT has not been identified. To determine this, double label immunohistochemistry was performed on pituitary sections from male rats using antisera for specific marker peptides of gonadotrophs, thyrotrophs, lactotrophs, somatotrophs, corticotrophs, and folliculo-stellate cells. CT was only colocalized with gonadotroph-specific markers and the distribution of pit-CT immunoreactive (IR) cells followed the patterns of gonadotroph distribution in male and female rats. Double and triple label immunohistochemistry using antiserum for CT, FSH, and PRL showed an apposition of calcitonin-like peptide containing gonadotrophs to cup-shaped lactotrophs. To examine whether pit-CT IR was altered, similarly to gonadotrophs, with known changes in PRL serum levels, studies were extended to ovariectomized, pregnant, and lactating rats. The area covered by pit-CT immunoreactivity and the tissue content of pit-CT significantly differed between physiological states and the pit-CT level was inversely related to the known PRL status. Pit-CT containing gonadotrophs were in all cases apposed to cup-shaped lactotrophs. These results provide histological support for previous studies proposing that pit-CT serves as a paracrine inhibitor of PRL release.

TRH-induced hyperprolactinemia and pituitary-ovarian function

To elucidate the role of prolactin in the control of pituitary-ovarian function, eight healthy women were given 80 mg of synthetic thyrotropin-releasing hormone (TRH) orally on two consecutive days during the luteal phase of their menstrual cycle. TRH elevated serum prolactin to a mean concentration of 43.8 ng/ml on the first day and to 15.5 ng/ml on the second day. The reduced response to the second TRH dose was statistically significant (p < 0.05). Accompanying changes in concentrations of gonadotropins or ovarian steroids were not consistent. Short-term oral administration of TRH and/or the temporary hyperprolactinemia induced by its use do not modify the pituitary-ovarian function during the luteal phase of the menstrual cycle.

Anorexia nervosa and the secretion of prolactin

Theoretically, one would expect to find increases in prolactin secretion in patients with anorexia nervosa: the evidence for this is reviewed. However, relevant work to date seems to indicate that this is not so. These paradoxical findings are discussed, and areas of further research are outlined.

Prolactin

Prolactin exists in man as a distinct and separate anterior pituitary hormone from growth hormone. It is important in lactation and the control of gonadal function, although it may have a much wider and basic metabolic role, similar to its role in lower forms. In clinical endocrinology it is important as an index of pituitary and hypothalamic diseases; thus prolactin levels are elevated in association with these conditions and this reflects the normal tonic inhibitory hypothalamic control of prolactin by PIF; DA is the most important PIF. Hyperprolactinaemia causes hypogonadism in both men and women; it may present in women with amenorrhoea, oligomenorrhoea, polymenorrhoea, regular cycles with anovulation or a defective luteal phase, and impotence in men. In either sex galactorrhoea is reported to occur in only 30 per cent of patients. Neurotransmitter therapy, with dopamine agonists which act as functional analogues of PIF, restores prolactin levels to normal and leads to a return of normal gonadal function. The mechanism of the hypogonadism is not clear and is discussed together with the problems associated with inducing pregnancy in these patients, who may harbour microadenomata of the pituitary.

Plasma prolactin and luteinizing hormone levels in anorexia nervosa

Five emaciated patients suffering from anorexia nervosa were studied. Mean plasma prolactin levels, as measured by radioimmunoassay, were within the normal range, although mean plasma levels of luteinizing hormone were depressed. This association is of interest as it suggests that basal levels of circulating prolactin are not affected by the hypothalamic disorder found in patients with anorexia nervosa.

Galactorrhoea: successful treatment with reduction of plasma prolactin levels by brom-ergocryptine

In five patients with inappropriate lactation and amenorrhoea or impotence brom-ergocryptine was found to suppress the lactation and to diminish the raised plasma prolactin levels. The responses to treatment suggest that there may be an inverse relationship between prolactin secretion and gonadotrophin secretion in man.