SUMMARY

The plasma sugar, 11-hydroxycorticosteroid, and growth hormone responses to insulin have been studied in patients with Cushing's disease. They showed an impaired or absent plasma 11-hydroxycorticosteroid and growth hormone rise during the test, as compared with control subjects, despite the injection of amounts of insulin which produced a similar degree of hypoglycaemia. This test proved of value in differentiating between these patients and those with 'simple ' obesity since the latter usually showed a normal growth hormone and adrenal response provided an adequate amount of insulin was administered.

The patients with Cushing's disease also had an impaired adrenal response to pyrogen and to dexamethasone administration and failed to show a normal plasma 11-hydroxycorticosteroid circadian rhythm. Their response to corticotrophin, lysine vasopressin, and metyrapone, however, was normal or enhanced. It is suggested that these findings imply an abnormality of hypothalamic or cerebral control and not a primary defect of pituitary function as proposed originally by Harvey Cushing.

The growth hormone response to insulin remained impaired in four out of six patients totally adrenalectomized for Cushing's disease but was normal in three patients adrenalectomized for other reasons. It is suggested that the defect which impairs the adrenal response to insulin may, on occasions, also impair the mechanism normally operative for growth hormone secretion.

SUMMARY

The existence of a circadian rhythm in the sensitivity of the hypothalamus of the laying hen to stimulation by progesterone was investigated by injecting 0·5 mg progesterone subcutaneously during the proposed period of maximum insensitivity. Following this treatment increases in plasma concentrations of both LH and progesterone were observed which were comparable to the spontaneous preovulatory rises in the plasma levels of the hormones.

The ability of either progesterone or luteinizing hormone releasing hormone (LH-RH) to induce premature ovulation varied according to the stage of follicular development. Neither hormone was more than 28% effective when injected within 6·5 h of the previous ovulation, whereas both hormones were 100% effective approximately 27 h after the terminal ovulation of a clutch sequence. Failure to ovulate in response to LH-RH given 6·5 h after ovulation was associated with a lack of progesterone secretion.

Both LH and progesterone were secreted when ovulation was induced by injections of either LH-RH or progesterone, and LH was secreted in response to progesterone given 6·5 h after ovulation. These results demonstrate that progesterone stimulates the secretion of LH and LH stimulates the secretion of progesterone. The precise physiological role of these two hormones, however, was not established.

ABSTRACT

To investigate the role of suprachiasmatic efferent connections in the expression of diurnal hormone rhythms, the efferent pathway from the suprachiasmatic nucleus (the putative circadian generator in the rat) to the sub-paraventricular zone (the main terminal area of suprachiasmatic efferents) was disrupted using bilateral horizontal knife cuts in ovariectomized oestrogen-treated rats. The position of the knife cut was assessed by observing its effect on vasoactive intestinal polypeptide immunoreactivity (a marker for suprachiasmatic efferents into the sub-paraventricular zone). The size of both the diurnal plasma LH and prolactin surges was markedly and consistently reduced over the 3-week period following the lesion in animals with a total deafferentation of the sub-paraventricular zone, compared with sham-operated animals or lesioned animals with an intact sub-paraventricular zone. When lesioned animals were grouped according to the presence or absence of damage to the preoptic area, no significant differences were found in the sizes of the plasma hormone surges. When similar knife cuts were given to animals whose activity cycles were observed, no significant effects were noted in the ability of the animals to synchronize to a light/dark regime or to free-run in constant light conditions. These results suggest that the suprachiasmatic nucleus influences the diurnal surges of plasma LH and prolactin in oestrogen-treated ovariectomized rats, initially by an interaction with the sub-paraventricular zone, and not by a direct influence on gonadotrophin-releasing hormone neurones or other more rostral structures.

Journal of Endocrinology (1989) 122, 593–604

SUMMARY

The concentration of ovine placental lactogen (oPL) was measured by radioimmunoassay in plasma samples from chronically catheterized ewes and their fetuses from day 110 of gestation to term (about day 145).

Concentrations of oPL in the plasma of the mother and fetus were raised after surgery, and remained raised for 3–5 days after the operation. Concentrations of oPL were greatest in the fetus at days 120–124 of gestation, and then declined until delivery. Mean concentrations of oPL in the fetus in late pregnancy for single, twin and triplet pregnancies were 101±6 (s.e.m.), 100±11 and 117±59 ng/ml respectively and were not significantly different.

Mean concentrations of oPL in the mother in late pregnancy for single, twin and triplet pregnancies were 718±227, 1387±160 and 1510±459 ng/ml respectively; the difference between these means was significant (P <0·05). Peak concentrations were noted at days 130–139 of gestation after which concentrations fell and were significantly lower on the day of delivery (P <0·01). Concentrations of oPL in the mother showed no circadian rhythm. The mean concentrations of oPL in maternal plasma during late pregnancy was significantly correlated to the combined fetal weight at birth (r = 0·624, P <0·01).

Insulin signaling cascade in peripheral insulin-sensitive tissues regulates whole-body glucose metabolism. Any deregulation in this pathway leads to insulin resistance, ultimately leading to metabolic diseases like type 1 diabetes, type 2 diabetes, and obesity. Insulin signaling in the brain has also been studied for many decades and associated with many primary functions like maintenance of synaptic plasticity, regulation of cognition, and circadian rhythm. Importantly, neuronal insulin signaling has also been associated with the regulation of neuronal glucose uptake. Any impairment in neuronal insulin signaling affecting neuronal glucose uptake has been associated with neurodegenerative disorders like Alzheimer’s disease, the process now being termed as type 3 diabetes. Since the criticality lies in proper signaling cascade, determining important points of deregulation is important. In this review, we have discussed some critical points of such deregulation, dividing them into two classes of enzymes: kinases and phosphatases. We have highlighted their individual roles in neuronal insulin signaling, along with their possible implications in neuronal insulin resistance. Future strategies targeting these nodes in neuronal insulin signaling might be helpful in exploring potential therapeutic opportunities to overcome neuronal insulin resistance and related neurodegenerative diseases.

ABSTRACT

Male Wistar-derived rats (200–250 g) were treated for 14 days with prednisolone 21-sodium succinate at a concentration of 1035 μmol/l in their drinking water. The drug was then replaced with normal tap water and groups of animals were killed at various times during recovery, trunk blood being collected after decapitation. At the same time, hypothalamic slices, anterior pituitary gland fragments and adrenals were removed and their responsiveness assessed by exposure to appropriate stimuli in vitro. Tissues were also extracted to measure changes in content of hormones during recovery. Treatment with prednisolone produced marked reductions in body weight gain, adrenal weight and pituitary ACTH content, but no significant change in hypothalamic corticotrophin-releasing factor (CRF) bio- or immunoreactivity. The ACTH content was restored by 5 days after withdrawal but adrenal weight remained significantly reduced after 9 days of recovery. The responsiveness of the hypothalamus to acetylcholine in vitro was markedly inhibited and was still significantly reduced 7 days after withdrawal. The responsiveness of the anterior pituitary gland to synthetic CRF or arginine vasopressin and that of the adrenal gland to ACTH added in vitro were restored simultaneously after 7 days of withdrawal. In vivo, recovery was assessed by measurement of the response to laparotomy stress. Treatment with prednisolone prevented the increase in the plasma concentrations of ACTH and corticosterone produced by stress, and these responses recovered by 5 days (corticosterone) and 7 days (ACTH) after withdrawal. The abolition of the circadian rhythms of ACTH and corticosterone by treatment was also reversed by 5 days after withdrawal. This pattern of recovery is different from that which we observed after long-term treatment with dexamethasone, where the responsiveness of the hypothalamus and adrenal gland in vitro recovered before that of the anterior pituitary gland.

J. Endocr. (1987) 113, 239–247

ABSTRACT

The aim of the present work was to study the relationship between sex hormones and plasma renin levels during the oestrous cycle in a Wistar-derived rat strain. Plasma renin activity (PRA) as well as plasma renin concentration (PRC) were increased during the day of oestrus in rats with controlled 4-day oestrous cycles. This increase in PRA and PRC was not found when rats were ovariectomized on dioestrus day 2 and samples measured on the expected day of oestrus. The increase in PRA and PRC was not found when normal cyclic rats were treated with either tamoxifen or the progesterone receptor blocker RU 38486. Treatment with progesterone at pro-oestrus after ovariectomy on dioestrus day 2 partially increased the PRA and PRC when compared with the values found during the day of oestrus in control rats. The combined treatment of ovariectomized rats on dioestrus day 2 with oestrogen and progesterone restored the normal increase in PRA and PRC values on the expected day of oestrus. We therefore postulate that the sodium diuresis promoted by progesterone may be modulated by the previous peak of oestrogen. However, stimulation of extrarenal sources of renin cannot be excluded nor can an involvement of inactive precursors of renin in the fluctuations of active renin that occur during the oestrous cycle. No important change in plasma renin substrate (PRC) was observed during the oestrous cycle. PRA, PRC and PRS were determined every 4 h during the 4-day oestrous cycle. Our results clearly show a rhythmic variation in PRA and PRC which increases during the day of oestrus with a peak at 06.00 h. No circadian variation related to the sleep-wakefulness rhythm or other regular daily changes in PRA, PRC or PRS was found.

Journal of Endocrinology (1989) 121, 261–267

In order to determine the temporal relationships between variations in 5-hydroxy-tryptamine (5-HT, serotonin) metabolism in the suprachiasmatic nucleus (SCN) and the cyclic LH surge, and also to check whether implantation of oestradiol capsules might modulate 5-HT metabolism in the SCN, we carried out a parallel study of 5-HT content in the SCN and median eminence, and 5-HT metabolism in the SCN and supraoptic region in vitro. These experiments were performed on intact male rats, ovariectomized females and ovariectomized females implanted with oestradiol.

It was only in ovariectomized rats implanted with oestradiol, in which we have described the existence of a clear-cut circadian rhythm of LH secretion, that we found fluctuations in the content, synthesis and utilization of 5-HT. The content and synthesis were characterized by a peak between 12.00 and 15.00 h, whereas utilization was 50% higher at 09.00 and 19.00 h than at 15.00 h. These fluctuations in 5-HT content and metabolism were specific to the SCN; the median eminence and the supraoptic region did not show such variations. They were also specific to ovariectomized rats implanted with oestradiol, since the patterns of 5-HT content and metabolism in the SCN were the same in males and ovariectomized females and did not differ from those in the median eminence, the supraoptic region or the whole hypothalamus.

These results suggest that 5-HT terminals in the SCN play an important role in the control of cyclic LH secretion at a critical period. Moreover, oestradiol seems to be partly responsible for the fluctuations of 5-HT metabolism in the SCN of ovariectomized rats implanted with oestradiol.

SUMMARY

The concentration of prolactin in serum after oestrogen and progesterone injection into spayed rats was measured by radioimmunoassay.

After a single injection of 5 μg oestradiol benzoate (OB) into long-term ovariectomized rats, serum prolactin concentrations showed a circadian rhythm with high levels in the afternoon and almost no changes in the morning. Peaks of prolactin occurred 2, 3 and 4 days after the injection. Below a dose of 1 μg OB, the response was dose-dependent, but the response was then maximal.

In spayed rats primed with 5 μg OB, the injection of 2 mg progesterone 2, 3 or 4 days later resulted in a significant increase in serum prolactin. This response, in contrast to that of oestrogen, occurred in the morning and in the evening and was found to be dose-dependent. The rise in serum prolactin after injection of 1 mg progesterone also showed a close relationship to the priming dose of OB. Progesterone had no effect in spayed, untreated animals. Maximal levels of prolactin were attained 3–4 h after the s.c. injection of progesterone. The release of prolactin which can be induced either by OB or by progesterone was blocked by the administration of progesterone injected 1 day before the expected release would occur. These results indicate that progesterone exerts both facilitatory and inhibitory effects on prolactin secretion. Male rats were found to be less sensitive to the ovarian steroid treatment.

It is suggested that oestrogen could be responsible for the rise in prolactin observed at pro-oestrus and progesterone for the increase in prolactin in pseudopregnancy and pregnancy.

SUMMARY

The relationship of plasma prolactin concentration and renal electrolyte excretion has been investigated in six normal male volunteers. In two studies, 80 mg frusemide were administered at 18.00 h on Day 1 and followed by dietary sodium restriction. In study A, after 38 h of sodium depletion, a second dose of frusemide was administered at 08.00 h on Day 3. In study B, after 14 h of sodium depletion, the effect of administration of 100 mg spironolactone or 45 mg prorenoate potassium (another aldosterone antagonist) at 08.00 h on Day 2 was compared with that of a placebo.

After the first dose of frusemide in study A, the mean plasma prolactin concentration correlated negatively with the urinary Na and K excretion over 5 h. After 38 h sodium depletion, the plasma prolactin concentration correlated positively with urinary Na excretion following the second dose of frusemide. In study B, after Na depletion for 14 h the plasma prolactin concentration at 08.00 h on Day 2 had a positive correlation with the 24 h urinary log₁₀ Na:K ratio following placebo administration and had negative correlations with the true urinary log₁₀ Na:K ratio following spironolactone and prorenoate potassium administration.

Neither acute Na deprivation nor the administration of single doses of frusemide, spironolactone or prorenoate potassium appeared to affect the normal circadian rhythm of plasma prolactin concentrations which remained constant for each subject throughout the 3 months covered by the investigation.

The correlations of plasma prolactin concentration to renal excretion of electrolytes, with no evidence for a negative feedback control mechanism, suggest an indirect relationship between prolactin and renal function.