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SUMMARY
Six rams of an ancient breed of domesticated sheep (Soay) were subjected to an artificial light régime of alternating periods of long days (16 h light:8 h darkness) and short days (8 h light: 16 h darkness) which induced seasonal development and regression of the testes during a period of 36 weeks. Over 2000 blood samples were taken, and the changes in plasma levels of FSH, LH and testosterone were related to the cycle of testicular activity. During long days plasma levels of gonadotrophins became very low and the testes regressed to about 20% of their maximum size; there was a corresponding reduction in plasma testosterone levels. When the rams were returned to short days reproductive development was again stimulated after 2–3 weeks with a progressive increase in plasma FSH and LH levels and consequent hypertrophy of the testes. It took about 16 weeks of short days for testicular activity to become maximal.
Blood samples collected at hourly intervals for 24 h on ten occasions during the study revealed transitory peaks in plasma FSH and LH levels indicative of episodic release. Changes in gonadotrophin secretion were modulated primarily by alterations in the frequency of episodic release; < 1 spike per 24 h during long days increased to a maximum of 10 spikes/24 h under short daylengths. The peaks of FSH release were of smaller amplitude than those of LH, although during periods of frequent episodic release basal levels of FSH were increased to a greater extent than those of LH.
A circadian rhythm was observed in the plasma levels of FSH, LH and testosterone, which was related to increased gonadotrophin release during the dark phase of the 24 h cycle; changes in blood haematocrit were also observed. The circadian changes appeared to be correlated with the activity cycle of the animals which in turn was dictated by daylight. A possible interrelationship between the circadian cycle and the seasonal cycle is discussed.
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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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The circadian rhythms of many night-shift workers are maladapted to their imposed behavioural schedule, and this factor may be implicated in the increased occurrence of cardiovascular disease (CVD) reported in shift workers. One way in which CVD risk could be mediated is through inappropriate hormonal and metabolic responses to meals. This study investigated the responses to standard meals at different circadian times in a group of night-shift workers on a British Antarctic Survey station at Halley Bay (75 degrees S) in Antarctica. Twelve healthy subjects (ten men and two women) were recruited. Their postprandial hormone and metabolic responses to an identical mixed test meal of 3330 kJ were measured on three occasions: (i) during daytime on a normal working day, (ii) during night-time at the beginning of a period of night-shift work, and (iii) during the daytime on return from night working to daytime working. Venous blood was taken for 9 h after the meal for the measurement of glucose, insulin, triacylglycerol (TAG) and non-esterified fatty acids. Urine was collected 4-hourly (longer during sleep) on each test day for assessment of the circadian phase via 6-sulphatoxymelatonin (aMT6s) assay. During normal daytime working, aMT6s acrophase was delayed (7.7+/-1.0 h (s.e.m.)) compared with that previously found in temperate zones in a comparable age-group. During the night shift a further delay was evident (11.8+/-1.9 h) and subjects' acrophases remained delayed 2 days after return to daytime working (12.4+/-1.8 h). Integrated postprandial glucose, insulin and TAG responses were significantly elevated during the night shift compared with normal daytime working. Two days after their return to daytime working, subjects' postprandial glucose and insulin responses had returned to pre-shift levels; however, integrated TAG levels remained significantly elevated. These results are very similar to those previously found in simulated night-shift conditions; it is the first time such changes have been reported in real shift workers in field conditions. They provide evidence that the abnormal metabolic responses to meals taken at night during unadapted night shifts are due, at least in part, to a relative insulin resistance, which could contribute to the documented cardiovascular morbidity associated with shift work. When applied to the 20% of the UK workforce currently employed on shift work, these findings have major significance from an occupational health perspective.
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rhythms of electrical activity ( Welsh et al . 1995 ). This, together with the presence of circadian clocks in unicellular organisms ( Roenneberg & Merrow 2002 , Mackey & Golden 2007 ), suggested that the oscillator works in a cell-autonomous manner
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SUMMARY
Serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were measured by radioimmunoassay in groups of Wistar rats at 6-hourly intervals during the course of a 4-day oestrous cycle. In addition to a surge at about 18.00 h on the day of pro-oestrus, a circadian rhythm in LH levels was observed which was accentuated during metoestrus. FSH levels showed a protracted periovulatory rise which reached peak levels at about 03.00 h on the day of oestrus. A daily rhythm was not observed.
The incorporation of [35S]methionine (injected subcutaneously 1 h before death) into the trichloroacetic acid-precipitable proteins of the anterior pituitary and of discrete brain areas implicated in the control of gonadotrophin release, was also measured. An increase in protein synthetic activity was observed in the anterior pituitary and the area of the median eminence on the evening of pro-oestrus with peak levels coinciding with the LH surge at 18.00 h on the day of pro-oestrus. An increase in protein synthetic activity relative to that in a 'control' area (the putamen) was observed in the preoptic area and in the amygdala 15–18 h before the LH surge. It is suggested that these changes in protein synthetic activity are associated with the train of neural and humoral events leading to ovulation.
After ovariectomy, protein synthetic activity in all areas investigated was at the low levels observed during the oestrous cycle. Daily injections of 20 μg oestradiol benzoate into intact females led to levels of protein synthesis as great as, or greater than, the maximal levels observed during the oestrous cycle.
Measurements of variations in total protein concentration/unit wet weight of pituitary or brain tissue showed that they were apparently unrelated to variations in protein synthetic activity. Protein concentration appeared to be high in all the brain samples taken at oestrus and low in those taken at metoestrus. Furthermore, superimposed upon a marked daily rhythm, there was a peak of protein concentration on the evening of dioestrus both in the preoptic area and in the area of the median eminence.
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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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ABSTRACT
Androgenized, oestrogenized and control female and male rats were used to establish possible differences in the alteration of the prolactin control system. Neonatal treatment involved administration of oestradiol benzoate or testosterone propionate (TP) on days 1 and 5 to the males and on day 5 to the females. Oil-treated animals were used as controls. Plasma prolactin levels were measured in these animals during adulthood (a) before gonadectomy, performed on day 80, and 27 days after gonadectomy and (b) on the 2 days (at 10.00 and 17.00 h) after administration of a single dose of TP to gonadectomized animals. Oestrogenized rats had the highest plasma prolactin concentrations just before and after gonadectomy. Testosterone propionate increased plasma prolactin levels in all groups. This response was more notable in the female than in the male groups, and was highest in the oestrogenized animals. Temporal rhythms of the prolactin response to TP were daily, perhaps circadian, with increased levels in the afternoon compared with those in the morning. This pattern was not present in oestrogenized females and androgenized males. Results suggest (a) that oestrogens and androgens given neonatally differ in their ability to alter the prolactin control system, and (b) that females seem to be more sensitive than males to changes in hypothalamic differentiation induced by neonatal steroid treatment.
J. Endocr. (1985) 105, 429–433
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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.
