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ABSTRACT
In the mediobasal hypothalamus (MBH) of prooestrous rats or acutely ovariectomized oestrogentreated adults a marked but short-lived increase in adrenergic activity occurs at 16.00 h, 2 h before the oestrogen-dependent surge of gonadotrophins at 18.00 h. In this study oestrogen-stimulated (noon on day 1) 22-day-old female rats were used which are known to produce surge levels of prolactin at 18.00 h on day 2 and surges of both prolactin and LH at 18.00 h on day 3; although similar treatment of 18-day-old animals or oil-treated 22-day-old rats failed to produce these effects. Radioenzymatic assays of adrenaline concentrations and of the activity of its synthesizing enzyme (phenylethanolamine-N-methyl transferase; PNMT, EC 2.1.1.28) in the MBH of oestrogen-treated 22-day-old rats showed significant (P< 0·05–0·01) increases in both parameters at 16.00 h (i.e. 2 h before surge levels of gonadotrophins) on days 2 and 3 when compared with other times of day. Such effects were not seen in oil-treated 22-day-old animals or in oestrogen-treated 16-day-old rats. Noradrenaline and dopamine concentrations in the MBH of oestrogen-treated 22-day-old rats remained at baseline levels on days 2 and 3 with the exception of noradrenaline at 17.00 h on day 3 when levels appeared higher (P<0·05) than at either 15.00 or 16.00 h. Subsequent measurements of PNMT activity in oestrogen-treated 22-day-old rats at 4-hourly intervals throughout days 2 and 3 showed the presence of a clear circadian rhythm with peak levels occurring at 16.00 h. In conclusion, a temporal relationship (not necessarily specific) exists between increased adrenergic activity in the MBH of oestrogen-treated 22-day-old rats and a surge of gonadotrophins (LH and/or prolactin) 2 h later. This relationship apparently depends on an oestrogen-stimulated circadian rhythm of PNMT activity.
J. Endocr. (1986) 109, 45–51
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ABSTRACT
This study investigated the relationship of two overt circadian rhythms, locomotor activity and melatonin synthesis in the pineal gland, by comparing their responses to asymmetrical reductions in photoperiod. Transfer of male Syrian hamsters from long to short daylengths led to an increase in the duration of both locomotor activity and the period of melatonin synthesis. Over the course of re-entrainment, the two rhythms were held in a stable phase relationship, and the direction of the switch did not influence the rate of decompression or the final phase relationships established after 8 weeks in short daylengths. Decompression of the activity rhythm was not influenced by pinealectomy. Exposure to short photoperiods caused gonadal regression and a consequent decline in serum testosterone levels from 10 to <1 nmol/l. The direction of the photoperiodic switch did not affect the time-course of gonadal regression. These data demonstrate the important influence of photoperiod upon the duration of the nocturnal peak of melatonin production by the pineal and also demonstrate that this effect is one example of a more widespread response of the circadian system. A qualitatively similar signal controls both locomotor activity and melatonin synthesis, although the neural basis of this common mechanism is unclear.
J. Endocr. (1987) 114, 221–229
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ABSTRACT
Flow cytometric analysis of nuclei stained with propidium iodide (PI) has been used to study the distribution of cells throughout the different phases of the cell cycle in the anterior pituitary gland of adult male Sprague–Dawley rats at different times of the day. According to PI fluorescence intensity the relative numbers of cells in S phase (cells with a DNA content between that of somatic cells in interphase (2n) and that of somatic cells after duplication of the DNA prior to cell division (4n)) and G2/M phase (4n) were calculated. A significant circadian rhythm was found for cells in both the S phase (P < 0·05) and the G2/M phase (P < 0·01). The wave of cells in S phase with a peak at the middle of the light period (14.00 h) precedes by about 6 h the wave of cells in G2/M phase (peak at 20.00 h). Most of the DNA-replicating cells were found during the early S phase at 11.00 h, advancing further up to the middle of this phase at 14.00 h. Cells were distributed homogeneously throughout the S phase at 17.00 h. These data strongly suggest that the beginning of the light period triggers a wave of cells to leave G0/G1 into S phase.
Journal of Endocrinology (1991) 129, 329–333
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SUMMARY
The physiological regulation of the plasma corticosteroid concentration, measured by competitive protein-binding, was studied in female rhesus monkeys (M. mulatta) sedated with phencyclidine hydrochloride. Morning basal levels of plasma corticosteroids were found to be in the range 8·0–25·2 μg/100 ml, which is lower than that previously reported in this species. A circadian rhythm in plasma cortisol concentration was demonstrated. Prolonged sedation with phencyclidine was associated with a gradual increase in the plasma cortisol concentration. Synthetic α1–24 adrenocorticotrophic hormone given intravenously caused a rapid rise in plasma cortisol, the minimum effective dose was between 1 and 10 ng/kg body weight and the response was maximal after 1000 ng/kg. The administration of lysinevasopressin and the induction of hypoglycaemia by insulin were both followed by an increase in the plasma corticosteroid concentration. Metyrapone caused a decline in plasma 11-hydroxycorticosteroids and a concomitant increase in total corticosteroids measured by competitive protein-binding. It is concluded that the hypothalamic-pituitary-adrenal system in the rhesus monkey functions in a manner which is qualitatively and quantitatively similar to that of man.
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SUMMARY
Ovulation was delayed for 24 h after the administration of sodium pentobarbitone (Nembutal, 35 mg/kg body weight) at 14.00 h, before the critical period on the afternoon of prooestrus. The expected preovulatory surge of serum LH at 18.00 h of pro-oestrus was also delayed until 21.00 h on the following day; however, increased levels (> 12 ng/ml) were observed in 14 out of 23 animals (killed by decapitation) at 21.00 h on the day of Nembutal administration. The serum FSH rise observed on the morning of expected oestrus was extended after Nembutal treatment, and a further rise was noted 24 h later.
Peak levels of incorporation of 35S from methionine into protein of the median eminence area (ME) and of the anterior pituitary (AP) which normally occur about the time of the preovulatory LH surge, were also delayed until 21.00 h on the day following Nembutal administration.
Neither ovulation nor the preovulatory gonadotrophin rises with their accompanying changes in incorporation in the ME and the AP, were altered by Nembutal administered after the pro-oestrous critical period.
Thus Nembutal, while blocking ovulation, inhibits the circadian rhythm of incorporation of 35S from methionine in the brain as well as the peaks of incorporation in the median eminence and the anterior pituitary which accompany the normal preovulatory surges of gonadotrophin.
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SUMMARY
Blood samples were taken every 3 h, over a 27 h period, from (1) a group of 12 intact male quail on short days (lights on 09.00–17.00 h) and during the 2nd, 15th and 36th day of photostimulation (lights on 09.00–05.00 h); (2) 12 castrated male quail on the 2nd, 20th and 43rd long day, and (3) 12 intact male quail on the 12th long day. Plasma LH was measured in all samples and FSH in the 43rd long day castrate and 12th long day intact male samples. Although there was considerable variation in the levels of LH and FSH, both between birds and between samples taken from the same bird, statistical analyses failed to reveal any diurnal (or circadian) rhythm at any time. There was a marked correlation between the LH and FSH levels in all samples.
Possible episodic LH secretion was investigated by taking blood samples every 15 min for between 3 and 6 h from six intact male quail and six laying females on long days. Samples were obtained from each bird at three time-periods which were arranged so as to overlap and cover the first 12 h of the daily photoperiod. Statistical analysis suggested that episodes of secretion occurred 6–10 times/day in males, and 4–8 times/day in females. The pulses appeared to occur at random.
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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.
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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.
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SUMMARY
The rate of [35S]methionine incorporation into protein in discrete cerebral areas was measured before and after the administration of oestradiol benzoate (OB) to chronically ovariectomized rats. The circadian rhythm of incorporation which is normally seen in the intact cyclic female rat was deleted by ovariectomy. A daily rhythm of incorporation reappeared, however, in all the brain areas studied 30 h after a single injection of OB (20 μg), and was still present 12 days later.
The release of luteinizing hormone (LH) after administration of 20 μg OB was measured in chronically ovariectomized animals and was found to be biphasic. High levels of LH after ovariectomy were initially reduced by negative feedback, but this phase was followed 52 h later by a facilitation of LH release between 15.00 and 18.00 h. The facilitation of LH release at this time of day was still detectable 12 days after the initial injection.
The evidence for a functional link between the rhythm of neural activity which is reflected by [35S]methionine incorporation, and the ability to 'time' the facilitation of LH release is discussed.
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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).
