Integrated levels of plasma LH and prolaction following hypothalamic deafferentation in the rat

Localization of Neuroendocrine Functions Within the Hypothalamus

The results of lesion, stimulation, deafferentation, implantatión and transplantation studies employed in the identification of hypophysiotrophic control areas in the hypothalamus to date suggest the following probable locations: corticotropic releasing factor (CRF) is formed in a diffuse area along the base of the median eminence, if not the base of the entire hypothalamus. Follicle stimulating hormone releasing factor (FSHRF) is elaborated in the paraventricular-suprachiasmatic areas but its cyclic control may reside in the anterior hypothalamic area. Luteinizing hormone (LH) is controlled by luteinizing hormone releasing factor (LHRF) formed in the suprachiasmatic area: its cyclic control may be in the preoptic area. Prolactin is controlled by prolactin inhibiting factor (PIF) localized in a diffuse area comprising the ventromedial, dorsomedial, arcuate and paraventricular nuclei. The hypothalamic area involved in thyroid control is also rather large, since thyrotropin stimulating hormone releasing factor (TSHRF) has been found in an area including the supraoptic and chiasmatic nuclei. Growth hormone releasing factor (GHRF) is elaborated in a rather narrow zone, the ventromedial hypothalamic nuclei.

Effect of hypothalamic deafferentation on the pulsatile secretion of luteinizing hormone in ovariectomized lactating rats*

Abstract The pulsatile luteinizing hormone (LH) secretion in ovariectomized lactating rats bearing complete (CD), anterior (AD), anterolateral (ALD), posterior (PD), or roof (RD) deafferentation of the hypothalamus was determined. All lactating rats were ovariectomized on Day 2 of lactation (Day 0, day of parturition). The deafferentation of nerve fibres to the mediobasal hypothalamus was performed on Day 6 or 7 of lactation. Twenty-four h after the surgery, blood samples were taken through the indwelling atrial catheter every 6 min for 3 h. Plasma concentrations of LH and prolactin (PRL) were measured by radioimmunoassay. The loss of LH pulses associated with lactation was still apparent following AD, PD and sham-deafferentation (SD); pulsatile LH secretion was, however, present in rats with CD, ALD and RD despite continued suckling. The only significant difference in plasma PRL concentrations among the various groups was a reduction in the PRL level in rats with RD in comparison to those with SD. We conclude that the neural signal responsible for the inhibition of pulsatile LH release by suckling is conveyed through the dorsal part of the hypothalamus and PRL does not mediate the suppression of LH pulses in mid-lactation.

Prolactin release following electrical stimulation of the brain in female turkeys (Meleagris gallopavo)

Different regions of the hypothalamus were electrically stimulated in anesthetized adult female turkeys. Plasma levels of prolactin (Prl) were measured before (-20, 0 min), during (5, 10, 20, 30 min), and after (5, 15, 30, 60, 90 min) a 30-min period of electrical stimulation. In turkeys with electrode placements in the regions of the preoptic-anterior hypothalamus, ventromedial hypothalamus, inferior hypothalamus, and the median eminence, the onset of stimulation increased plasma Prl within 5 min (P less than 0.05) and the maximum level was reached by the end of the 30-min stimulation period. The termination of hypothalamic stimulation was followed by decreased plasma Prl to prestimulation levels. In a subsequent study plasma Prl was measured in hens at different stages of the reproductive cycle, following electrical stimulation in the ventromedial hypothalamus. Ventromedial stimulation induced increases in plasma Prl in laying, photorefractory, and incubating turkeys; however, the relative change in the maximal Prl level was greater in incubating (21-fold) than in either laying (3.5-fold) or photorefractory (5-fold) turkeys. We conclude that Prl release in the female turkey can be affected by electrical stimulation of selected hypothalamic areas and that stimulation-induced Prl release in the ventromedial hypothalamus is enhanced in incubating turkeys.

Function of serotonin in physiologic secretion of growth hormone and prolactin: action of 5,7-dihydroxytryptamine, fenfluramine and p-chlorophenylalanine

Growth hormone (GH) and prolactin secretory patterns in male rats chronically implanted with venous cannulae were studied in response to treatments known to acutely or chronically deplete central serotonin (5-HT) or to permanently destroy a high proportion of 5-HT neural structures. Spontaneous prolactin secretion was little affected by chronic 5-HT depletion produced by 5,7-dihydroxytryptamine (5,7-DHT) or parachlorophenylalanine (PCPA). Acute 5-HT release induced by fenfluramine produced sustained prolactin secretion in intact and 5-HT-depleted rats. GH secretion was inhibited by PCPA, and for 3 days following 5,7-DHT, but was normal by 7 and 21 days. Acute 5-HT release stimulated by fenfluramine unexpectedly inhibited GH in intact and 5-HT-depleted rats. It is concluded that 5-HT is a potent facilitatory neurotransmitter for prolactin; and it appears to have both facilitatory and inhibitory effects on GH. Recovery of GH secretory patterns within 7 days of 5,7-DHT treatment indicates that mechanisms act rapidly to compensate for depletion of 5-HT, and that it is difficult to achieve functional depletion of 5-HT for more than a few days.

Effects of intraventricular 6-hydroxydopamine injections on serum prolactin and LH levels: absence of stress-induced pituitary prolactin release

The drug 6-hydroxydopamine (6-OHDA) has been reported to reduce hypothalamic norepinephrine (NE) content after administration into the lateral ventricle without altering the dopamine content of tubero-infundibular neurons. Serum prolactin levels in male rats injected with 2 X 250 mug 6-OHDA were significantly higher than in untreated control rats. Intraventricular injection of male rats with artificial cerebrospinal fluid resulted in elevated mean prolactin levels similar to those observed in 6-OHDA-treated animals. Further experimentation on animals decapitated at different times after removal from the animal quarters, indicates that prolcatin levels in 6-OHDA-treated rats are continuously elevated whereas they rise from basal levels to extremely high levels in CSF-treated rats, thus resulting in similar mean values. The CSF-treated controls ate hypersensitive to the stress of being removed from their normal environment. Such an effect was not observed in 6-OHDA-treated nor in untreated, and thus stress-inexperienced rats. In a long term study, serum prolactin and luteinizing hormone (LH) levels were followed over a period of 71 days after 6-OHDA treatment. Prolactin levels increased within one day after treatment and stayed at a high level for 15 days. Subnormal prolactin values were measured 37 days after 6-OHDA treatment. Serum LH levels were below normal 3 h and one day and were increased 37 and 71 days after 6-OHDA treatment. These results suggest that NE is important in the transmission of stressful stimuli to hypothalamic prolactin regulating centers. They further suggest functional recovery of LH and prolactin regulating mechanisms after 6-OHDA treatment.

The neuroendocrine control of gonadotrophin release in the Japanese quail. I. The role of the tuberal hypothalamus

Reproduction in the Japanese quail ( Coturnix coturnix japonica ), as in many other bird species, is controlled by the daily photoperiod. The present experiments have investigated the role of the posterior or tuberal hypothalamus in the neuroendocrine regulation of this photoperiodic response. By using various types of small knives the tuberal hypothalamus was isolated surgically in male quail maintained on short daylengths. The birds were then transferred to long daily photoperiods and after fourteen days the effects on gonadotrophin secretion assessed by measuring testicular growth and the circulating level of luteinizing hormone (LH). Complete deafferentation of the tuberal hypothalamus blocked testicular growth and the plasma level of LH remained low. Partial cuts that severed the afferents entering from anterior and antero-lateral directions also blocked gonadotrophin secretion but cuts severing posterior connections were without effect. Following deafferentation of sexually mature males the LH level fell rapidly to reach a basal concentration within two days. The normal rise in LH that follows the castration of quail on long days was blocked by complete tuberal deafferentation. Electrolytic lesions were placed medially in the tuberal hypothalamus. Those in the postero-dorsal part of the infundibular nuclear complex (p.d.-i.n.c.) completely blocked testicular growth, while those in the ventral portion of the nucleus (v.-i.n.c.) only partially disrupted gonadotrophin secretion.