L. Krulich

Neurotransmitters in the Control of Anterior Pituitary Function

Earlier papers in this volume have amply documented the fact that the anterior pituitary is controlled by a family of hypothalamic releasing and inhibiting hormones which are secreted into the hypophyseal portal vessels in the median eminence (ME) to stimulate or inhibit the release of individual pituitary hormones. The distribution of the peptidergic neurons which synthesize and release the various releasing hormones within the hypothalamus brings these neurons in close contact with a variety of putative synaptic transmitters. The purpose of this chapter will be to review the evidence that some of these transmitters may be involved in controlling release of the releasing and inhibiting horaiones. We will stress results from our laboratory but will include crucial experiments from other laboratories so as to present a balanced picture of the present status of knowledge in this field.

Influence of Growth Hormone (GH) on Content of GH in the Pituitaries of Normal Rats

Summary Alterations in the content of pituitary GH as estimated by the tibial epi-physeal cartilage assay were produced in rats by the daily, subcutaneous injection of large doses (2 and 6 mg/100 g/day) of bovine GH. An elevation of pituitary GH was observed one day after the first injection of GH. This level returned towards normal after 6 days and the GH content was depressed at 11 days following the initial injection. The injection of saline was without effect on pituitary GH. Blood glucose concentration was slightly elevated at 6 days after the first injection of the higher dose of GH. The rate of weight gain of rats treated with GH was increased. It is suggested that large doses of exogenous GH may alter its secretion by the adenohypophy-sis.

The effect of median eminence (ME) lesions on plasma levels of FSH, LH, and prolactin in the rat.

Summary Lesions were located in the ME of ovariectomized rats to determine their effect on plasma levels of FSH, LH, and prolactin as determined by RIA. Initial levels of FSH and LH were elevated as a result of removal of steroid feedback from the ovaries. One week after placement of lesions plasma LH levels had fallen, 32-fold, to very low values. There was a lesser, 3-fold, fall in FSH, and a dramatic, 15-fold, rise in prolactin to levels nearly as high as those observed in lactating rats and in females in proestrus. In sham-operated controls, in which the electrodes were lowered into the hypothalamus stopping short of the ME, no significant changes were seen in the plasma levels of all three hormones. The results demonstrate that the release of LH and to a lesser extent FSH is decreased after removal of hypothalamic control, whereas the release of prolactin is greatly enhanced.

The effect of pentobarbital (Nembutal) on prolactin release in the rat.

Summary The effect of Nembutal on plasma prolactin levels of ovariectomized rats was investigated in three situations which bring about increased release of the hormone. Prolactin was determined by radioimmunoassay. Nembutal lowered the elevated plasma prolactin in etherized rats. It completely reversed the elevation induced by α-methyl-m-tyrosine, an inhibitor of catecholamine synthesis, and also reduced the elevated levels observed in estrogen-treated animals. The results support the hypothesis that Nembutal blocks the release of a prolactin-releasing factor which is released by ether stress, α-methyl-m-tyrosine and estrogen.

Plasma gonadotropins and prolactin in pseudopregnancy in the rat.

Summary 1. Cervical probing of estrous rats resulted in an elevation of plasma prolactin 15 min later. 2. Plasma prolactin was not elevated on day 4 of pseudopregnancy and was actually lower on day 9 than in diestrous controls. 3. On day 4 of pseudopregnancy plasma LH levels were lower than those observed in the controls; where FSH levels were elevated on day 4 and 9. 4. Following castration on day 4 of pseudopregnancy, plasma FSH and LH levels both rose, but plasma LH rose to a higher value in the pseudopregnant castrates than in controls on the 5th day after spaying. Prolactin levels remained unchanged following spaying. 5. It is concluded that the discharge of prolactin which initiates pseudopregnancy in the rat is not maintained throughout pseudopregnancy and is associated with a suppression of LH and an augmentation of FSH release from the pituitary.