Rat milk stimulates pituitary growth hormone secretion of neonatal pituitaries in vitro

In vitro control of prolactin (PRL) and growth hormone secretion of neonatal rat pituitary glands: effects of ovine PRL, salmon calcitonin, endothelin-3, angiotensin II, bromocryptine and somatostatin

Bromocryptine potently decreased prolactin (PRL) secretion of pituitary glands of 2-day-old rats in vitro (up to 85% inhibition; ED50 between 0.1 and 1.0 nM) without altering the bioactivity to immunoreactivity (B/I) ratio. Bromocryptine tended to suppress growth hormone (GH) secretion although the effect did not reach statistical significance. Angiotensin-II (A-II; 1-1000 nM) stimulated PRL secretion in a dose-dependent manner without affecting secretion of GH. The B/I ratio of PRL secreted in response to A-II was increased. Somatostatin (SRIF) had no effect on PRL secretion but inhibited GH secretion in a dose-dependent manner; significant inhibition (50%) was observed at 100 nM. A 6-h exposure to ovine PRL (oPRL) in concentrations equipotent with 1.2-120 ng/ml rat PRL (rPRL) in the Nb2 bioassay had no effect on immunoreactive rPRL secretion. Salmon calcitonin (sCT) and endothelin-3 (ET-3; 0.1-100 nM) failed to inhibit secretion of PRL or GH. PRL secretion was slightly stimulated by sCT with no apparent dose-response relationship. The present findings suggest that neonatal pituitary glands do not display autoregulation of PRL secretion, and sCT and ET-3 (either endogenous or milk-derived) may not function as PRL inhibiting factors in 2-day-old pups. Thus, the receptors of PRL, sCT and ET-3 on lactotropes, or their functional coupling with inhibition of basal PRL secretion, occur at a later stage of development. The specificity of the PRL releasing factor (PRF) activity of A-II at this age is unique for established PRFs and might reflect a physiological function of PRL in osmoregulation. The increased B/I ratio of PRL secreted in response to A-II may be due to the release of specific PRL variants, and might be a sign of functional heterogeneity among lactotropes. The differential sensitivity of PRL and GH to the applied secretagogues suggests that the intracellular regulation of PRL and GH are compartmentalized in the mammosomatotrope cell.

Thyrotropin-releasing hormone mediates growth hormone release induced by milk and nursing in neonatal rats

Previous evidence from this laboratory suggested that growth hormone (GH) release induced by milk in vitro and by nursing in vivo from neonatal rat pituitary glands is mediated by an alternative GH-releasing factor(s) (GRF) distinct from GH-releasing hormone (GHRH(1-43) ). In the present experiments we tested whether thyrotropin-releasing hormone (TRH) could fulfil the criteria of this alternative GRF in neonatal rats. The water-soluble fraction of rat milk (infranatant, prepared by ultracentrifugation) and its methanol/acetic acid extract (milk-borne peptides) stimulated GH release from perifused pituitary glands obtained from 2-day-old rats. Dialysis of the infranatant (mol wt cut-off: 2,000) against 500 volumes of culture medium at 4°C eliminated its GH-releasing activity in the perifusion system, while the infranatant retained its full GRF-like activity when incubated at 4°C without dialysis. The milk-borne GRF eluted as a single peak and coeluted with TRH in a combined gel permeation chromatography (Sephadex G-10) and perifusion set-up. Prolactin secretion was also stimulated simultaneously with the release of GH induced either by milk or by TRH. In a stepwise C(18) reversed-phase chromatography, milk-borne GRF was highly hydrophilic and coeluted with synthetic TRH. The in vitro GH-releasing bioactivities of synthetic TRH and a milk extract purified in C(18) reversed-phase chromatography were abolished by proline-specific endopeptidase. Thus, TRH and milk-borne GRF displayed similar molecular weights, hydropathic characteristics and proteolytic enzyme resistance. In vivo, nursing (which has been reported as a potent stimulus of GH secretion even in the absence of milk-intake) increased serum GH levels in 2-day-old pups. A supramaximal dose of TRH (10 ng/g intraperitoneally) stimulated GH release in 2-day-old pups separated from their mothers for 6 h to a similar extent as nursing. Nursing-induced levels of serum GH were not further elevated by TRH. This failure of TRH to further increase serum GH levels was not due to a maximal GH output by the neonatal pituitary gland, since the GH release induced by the serotonin precursor 5-hydroxy-L-tryptophan was augmented either by TRH or by nursing. These data provide evidence that the milk-borne GRF-like activity in vitro is indistinguishable from TRH, and suggest that TRH (probably of hypothalamic origin) might be the mediator of the nursing-induced release of GH in vivo as a physiological GRF in neonatal rats.

Evaluation of prenatal aldrin intoxication in rats

The aim of the present study was to evaluate the impact of prenatal exposure to low doses of aldrin on physical and behavioral developments of rats (1-21 days old). To detect the possible persistent adversities produced by this exposure, the animals were also tested when adults (90 days old). Plasma determinations of both aldrin and its metabolite dieldrin and histopathological evaluations of brain slices were also performed in adult animals. Pregnant female rats were subcutaneously (s.c.) treated with aldrin (1.0 mg/kg) or with its vehicle (0.9% NaCl solution plus Tween-80) from day 1 of pregnancy until delivery. Results show that prenatal aldrin administration was able to decrease the median effective time (TE50) for incisor teeth eruption and to increase the TE50 for testes descent; other parameters indicative of physical development were not modified. Aldrin and dieldrin were not found in plasma of the adult rats; no differences were observed between control and experimental rats in the cellular and structural organization of the cerebral cortex neurones. Prenatal aldrin administration produced no impairment of adult animal's behavior in an avoidance learning test; nevertheless, the locomotor frequency of the experimental rats was higher than that of controls at 21 and 90 days old. When adults, these experimental rats had their performance in a hole-board apparatus (total number and duration of head-dips) also higher than that of the control ones. It was concluded that prenatal aldrin exposure induced not only developmental changes in the rat pups but also persistent behavioral alterations in adulthood, when the pesticide was not present in these animals.

Regulation of Basal and nursing-induced secretion of growth hormone in the neonatal rat: the involvement of serotonergic, muscarinic cholinergic, alpha-adrenergic, somatostatin and growth hormone-releasing hormone systems

Abstract Various neural factors are involved in the suckling-induced increase in serum growth hormone (GH) levels in neonatal rats, and, in the present study the serotonergic, cholinergic, somatostatin and GH-releasing hormone (GHRH) systems were investigated. The serotonin (5-HT) precursor 5-hydroxy-L-tryptophan (5-HTP) and the 5-HT receptor agonist quipazine maleate stimulated serum GH levels in 2-day-old rat pups separated from their mothers for 6 h. The increase in serum GH during suckling was further elevated by 5-HTP. The 5-HT antagonist cyproheptadine decreased serum GH levels in separated 2-day-old pups, and although it reduced the amplitude of the suckling-induced increase in serum GH concentration, it did not alter the increase in serum GH on a percentage basis. The effect of the cholinergic muscarinic antagonist atropine sulfate (ATR) was similar to that of cyproheptadine. Moreover, in separated pups, ATR prevented the increase in serum GH induced by 5-HTP. In contrast with 2-day-old pups, ATR completely eliminated the suckling-induced release of GH in 10-day-old rats. However, ATR failed to prevent GH release induced by the alpha(2)-adrenergic agonist clonidine HCI in 10-day-old male pups. While thyrotropin-releasing hormone increased serum GH levels, rat GHRH failed to alter serum GH levels either in separated or in suckled 2-day-old rat pups. Immunoneutralization for rat GHRH eliminated the increase in serum GH induced by clonidine HCI in 10-day-old pups, but (on a percentage basis) failed to prevent the GH-increasing effect of suckling in 2-day-old pups. While somatostatin failed to significantly decrease serum GH in separated 2-day-old pups, it effectively decreased serum GH levels in 2-day-old pups which were suckled. Cysteamine, which depletes hypothalamic somatostatin, increased serum GH in separated 2-day-old pups, and further increased the suckling-induced levels of serum GH. Cysteamine partially prevented the GH-decreasing effect of ATR. The present findings suggest that 1) the serotonergic and cholinergic systems are involved in the regulation of GH secretion as early as day 2 postpartum; 2) the serotonergic and cholinergic systems modulate the basal, and do not modulate the suckling-induced levels of serum GH; 3) the serotonergic system may exert its stimulatory influence on GH secretion only in the presence of a functional muscarinic cholinergic system; 4) the cholinergic system, at least in part, stimulates GH secretion via a cysteamine-sensitive system (probably by inhibiting somatostatin); 5) the cholinergic system is not functionally coupled with the alpha(2)-adrenergic system, which stimulates GH secretion via rat GHRH; 6) since in 10-day-old pups clonidine HCI was effective only in males, while suckling was effective in both sexes, the alpha(2)-adrenergic system is not involved in the suckling-induced increase of serum GH; and finally 7) neither somatostatin nor rat GHRH seem to be involved in the suckling-induced changes in serum GH. The findings are consistent with the hypothesis that the high circulating GH levels in the neonatal rat are due to alternative GH-releasing factors, perhaps thyrotropin-releasing hormone or gamma-aminobutyric acid. The neurohumoral mediator of the suckling-induced GH release in neonatal rats remains to be identified.

Prenatal versus postnatal effects on offspring weight gain of rats exposed to diphenhydramine: a critical evaluation of fostering procedures in rats

1. In order to evaluate the relative contribution of fostering procedures in the analysis of the development of rats prenatally exposed to diphenhydramine (20 mg/kg/day, sc) the weight gain of litters fostered or not by their biological mothers were examined from 2-21 days of age. 2. Maternal behavior and milk production were also assessed. 3. The results showed a decreased weight gain only in offspring fostered by mothers from different prenatal treatments and a lack of effects on maternal behavior and milk production. 4. It was concluded that the reduced weight of cross-fostered litters was mediated through an interference with postnatal mother-offspring interaction, which was not herein identified.