Serum melanocyte-stimulating hormone levels and pituitary melanocyte-stimulating hormone content during pseudopregnancy in the rat.

Pseudopregnancy (PSP) was induced in rats by vaginal stimulation and the levels of MSH in serum and pituitary were examined. One hour after vaginal stimulation, the serum MSH level was increased, and a cyclical variation was observed by the second day of PSP. There were two surges of about 50 pg/ml, one in the morning and the other at night. On the sixth day of PSP, the pattern of serum levels still showed two peaks, but while the nocturnal peak remained at 50 pg/ml, the diurnal value was halved. The pituitary MSH content also underwent cyclical variations; the lowest levels coincided with the peak levels in serum. These observations raise the possibility that MSH is involved in PSP.     

Ability of various factors to oppose the stimulatory effect of dibutyrylcyclic AMP on the release of melanocyte-stimulating hormone by the rat pituitary in vitro.

Various agents were tested for their ability to oppose the stimulatory effect of dibutyryl cyclic AMP on the release of the melanocyte-stimulating hormone from the rat neuro intermediate lobe in vitro. Only dopamine exhibited an inhibitory effect; serotonin, gamma-amino-butyric acid, tocinoic acid, tocinamide, the tripeptide Pro-Leu-Gly-NH2 and dibutyryl cyclic GMP were all ineffective.     

Calcium control of growth hormone release from chicken pituitary glands in vitro

The influence of calcium on the basal and stimulated release of growth hormone (GH) from chicken pituitary glands has been determined in vitro. Basal GH release occurred in Ca2+ deficient media, although it was increased in proportion to the medium Ca2+ concentration. Growth hormone release was stimulated by 10(-7)-10(-9) M thyrotrophin-releasing hormone (TRH), maximal stimulation being observed in the presence of 10(-8) M TRH and 1.5 mM Ca2+. Decreases in the Ca2+ concentration (to 0.75, 0.375, or 0 mM) suppressed the GH response to 10(-8) M TRH, as did increases (to 3.0 and 6.0 mM) in the Ca2+ concentration. These results suggest that GH release in chickens is regulated by Ca2+-dependent mechanisms.     

Oestradiol receptors in the rat anterior pituitary gland during the oestrous cycle: quantitation of receptor activity in relation to gonadotrophin releasing hormone-mediated luteinizing hormone release.

Specific oestradiol binding to a receptor in nuclear and cytosol fractions of the rat anterior pituitary gland and pituitary responsiveness to gonadotrophin releasing horomone (GnRH) during the oestrous cycle have been studied. To accomplish this, both unoccupied and occupied oestradiol-binding sites in the nucleus and total cell were measured during the oestrous cycle. The concentration of unoccupied and occupied sites and total oestradiol binding in the cytosol fluctuated during the cycle. At pro-oestrus, the concentration of cytosol receptor was diminished by about 40% and replenishment occurred during oestrus. On the other hand, a profound increase is concentrations of cellular and nuclear receptors occurred at pro-oestrus. Administration of GnRH significantly LH release at all stages of the cycle. The maximum stimulation of LH release by GnRH was observed at 13.00 h of pro-oestrus. From these studies, it is concluded that pituitary responsiveness to exogenous GnRH during pro-oestrus parallels the changes in the content of oestrogen receptors in the cytosol and nucleus.     

Age-related changes in prolactin and growth hormone release from pituitary glands in vitro

The basal release of prolactin from cockerel anterior pituitary glands in vitro declined between 1 and 7 weeks of age, to a level less than that released by pituitary glands from 18 week old (adult) cockerels and hens. Basal growth hormone (GH) release increased between 1 and 7 weeks of age but had declined in adults to a level similar to that released from 4 weeks old cockerels. The responsiveness of the pituitary gland to hypothalamic stimulation, using hypothalami from 8 week old broiler fowl, was also age-related. Prolactin release was considerably higher from pituitaries of 1 week old cockerels compared to the other age groups. Stimulation of GH release by the hypothalamus was higher from pituitaries of both 1 and 7 week old cockerels compared to the other groups of birds. The increase in release of prolactin following incubation with thyrotrophin releasing hormone (TRH) declined between 1 and 7 weeks, but increased slightly in adult birds, whereas the increase in release of GH following TRH was higher from pituitaries of both 1 and 7 week old cockerels. Hypothalamic prolactin (Prl) releasing activity, measured as the ability of the hypothalamus to stimulate hormone release from 8 week old broiler fowl anterior pituitary glands, declined with the age of the donor cockerels. The hypothalami from adult hens secreted significantly more Prl releasing activity than did adult cockerel hypothalami. The secretion of GH releasing activity decreased markedly with the age of the donor bird. These results suggest that maturational patterns of hormone secretion in fowl are partly due to changes in autonomous hormone release, to changing patterns of hypothalamic activity and to differences in pituitary responsiveness to provocative stimuli.     

Ornithine decarboxylase activity and polyamines in the anterior pituitary gland during the rat oestrous cycle

The biosynthesis of polyamines, an ubiquitous group of amines shown to be essential for normal cellular growth and differentiation, was studied in the rat anterior pituitary gland during the different stages of the oestrous cycle. The activity of ornithine decarboxylase (ODC), which catalyses the rate-limiting step in the biosynthesis of polyamines, was low during oestrus, metoestrus and dioestrus. However, a marked transitory rise in ODC activity was found in the pituitary gland on the evening of pro-oestrus. The rise in ODC activity was accompanied by an increase in the pituitary content of the polyamines putrescine and spermidine. Ovariectomy did not significantly change the basal ODC activity in the pituitary gland. Oestrogen treatment of ovariectomized rats resulted in a marked stimulation of pituitary polyamine biosynthesis. The largest effects were observed when oestrogen was given as two injections 72 h apart, which gave rise to levels of ODC activity comparable to those observed on the evening of pro-oestrus. The increase in polyamine synthesis in the anterior pituitary gland during pro-oestrus appeared not to be related to the preovulatory secretion of LH or prolactin, since neither LH-releasing hormone nor thyrotrophin-releasing hormone (which induces a secretion of prolactin) affected pituitary ODC activity. The observed biosynthesis of polyamines may be associated with the cellular proliferation which occurs in the anterior pituitary gland at oestrus.     

Inhibition by testosterone of prolactin and growth hormone release from chicken anterior pituitary glands in vitro

Pituitary glands and hypothalami from broiler fowl were incubated in medium containing testosterone, and prolactin and GH release were determined. Pituitary glands were also preincubated for 20 h in medium containing testosterone, and then in medium containing various secretagogues. Testosterone inhibited the release of prolactin directly from the pituitary gland in a concentration-related manner. The hypothalamus stimulated the release of prolactin, but by a lesser amount in the presence of testosterone. When pituitary glands were preincubated with testosterone, subsequent release of prolactin was inhibited, except with the highest concentration which stimulated prolactin release. Hypothalamic extract (HE) markedly stimulated prolactin release from control pituitary glands although testosterone-primed glands were less responsive. The stimulation of prolactin release by thyrotrophin releasing hormone (TRH) and prostaglandin E2 (PGE2) was also reduced by preincubation of the pituitary glands with testosterone. Priming with testosterone did not affect the release of GH from pituitary glands alone, but reduced the TRH-, HE- and PGE2-stimulated release of GH. These results demonstrate that testosterone directly inhibits prolactin secretion and reduces the sensitivity of pituitary lactotrophs and somatotrophs to provocative stimuli.     

Specific binding of LH-RH to the anterior pituitary gland during the oestrous cycle in the rat.

Specific binding of luteinizing hormone-releasing hormone (LHRH) to the anterior pituitary gland during the estrous cycle was investigated in the rat. The amount of iodine-125-LHRH bound was 150 fmole/,g protein at proestrus, 20 fmole/mg at estrus, and 16 fmole/mg at metestrus. Specific binding of LHRH to the pituitary was not detectable in rats ovariectomized during the afternoon of diestrous 1 or morning of estrus but it was restored by treatment with estradiol-17-beta along or with progesterone. Peripheral plasma LHRH levels were 5 pg/ml at diestrous and increased to 20 pg/ml at proestrus (p greater than .05). Plasma LH levels were 16.7 ng/m1 at 4-5 p.m. of proestrus, 4 ng/m1 at 1-2 p.m. of preoestrus and 1-2 ng/m1 at other times. Preovulatory LH surge was not observed in rats ovariectomized at diestrus or sacrificed at the expected time of proestrus, however, estradiol treatment elevated LH levels and progesterone augmented this effect. These results suggest that in the rat the ovulatory LH surge at proestrus involves the hypothalmic release of LHRH as well as increased LHRH binding in the pituitary gland, both of which may be stimualted by the prevolulatory rise in estrogen and/or progesterone.