Age-dependent effect of Freund's adjuvant on 24-hour rhythms in plasma prolactin, growth hormone, thyrotropin, insulin, follicle-stimulating hormone, luteinizing hormone and testosterone in rats

The effect of Freund's adjuvant administration on 24-hour changes of plasma prolactin, growth hormone (GH), thyrotropin (TSH), insulin, follicle-stimulating hormone (FSH), luteinizing hormone (LH) and testosterone were studied in young (2 months) and aged (18 months) male Wistar rats. Rats were injected s.c. with Freund's adjuvant or adjuvant's vehicle and, 18 days later, they were killed at 6 different time intervals throughout a 24-hour cycle to measure circulating hormone levels by specific RIAs. Young rats receiving adjuvant's vehicle exhibited significant time-of-day-dependent variations in plasma TSH, LH and testosterone, with maximal levels at 1300 h, 0100 h and 1700 h, respectively. Prolactin and insulin levels, analyzed globally in a factorial ANOVA, showed significant time-of-day changes with maximal levels at 1300 - 1700 h and 2100 h, respectively. The daily rhythms in plasma LH and testosterone found in young rats were not longer observed in Freund's adjuvant-injected rats, while as far as TSH, a second peak was observed at 0100 h after Freund's adjuvant administration. Twenty-four hour rhythms in circulating TSH, LH and testosterone were blunted in old rats receiving either Freund's adjuvant or its vehicle. Aged rats exhibited significantly higher circulating levels of prolactin, and lower levels of GH, TSH, FSH and testosterone. The results indicate that secretion of prolactin, GH, TSH, FSH and testosterone are age-dependent, as are the responses of TSH, LH and testosterone to Freund's adjuvant administration.     

Effect of social isolation on 24-h pattern of stress hormones and leptin in rats

This work analyzes the effect of social isolation of growing male rats on 24-h changes of plasma prolactin, growth hormone, ACTH and leptin, and on plasma and adrenal corticosterone concentrations. At 35 days of life, rats were either individually caged or kept in groups (6-8 animals per cage) under a 12:12 h light/dark schedule (lights on at 08:00 h). A significant arrest of body weight gain regardless of unchanged daily food intake was found in isolated rats after 2 weeks of isolation. On the 4th week, rats were killed at 6 time intervals during a 24-h cycle, beginning at 09:00 h. In isolated rats the 24-h pattern of all parameters tested became distorted, as assessed by Cosinor analysis. When analyzed as a main factor in a factorial analysis of variance, isolation decreased plasma prolactin and growth hormone, increased plasma leptin and corticosterone while decreased adrenal corticosterone. Plasma corticosterone levels correlated significantly with plasma ACTH and with adrenal corticosterone levels in group-caged rats only. These changes can be attributed to an effect of mild stress on the endogenous clock that modulates the circadian hormone release.     

Circadian Neuroendocrine Role in Age-Related Changes in Body Fat Stores and Insulin Sensitivity of the Male Sprague-Dawley Rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e. corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats). insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%). plasma insulin concentration (40%'). and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.     

Circadian Neuroendocrine Role in Age-Related Changes in Body Fat Stores and Insulin Sensitivity of the Male Sprague-Dawley Rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e. corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats). insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%). plasma insulin concentration (40%″). and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.     

24-hour changes in ACTH, corticosterone, growth hormone, and leptin levels in young male rats subjected to calorie restriction

Calorie restriction of young male rats increases plasma prolactin, decreases luteinizing hormone (LH) and testosterone, and disrupts their 24 h secretory pattern. To study whether this could be the consequence of stress, we examined the 24 h variations of plasma adrenocorticotropic hormone (ACTH) corticosterone, growth hormone (GH), leptin, and adrenal corticosterone. Rats were submitted to a calorie restriction equivalent to a 66% of usual intake for 4 weeks, starting on day 35 of life. Controls were kept in individual cages and allowed to eat a normal calorie regimen. Significantly lower ACTH levels were detected in calorie-restricted rats. Plasma corticosterone levels during the light phase of the daily cycle were significantly higher in calorie-restricted rats. Time-of-day variation in plasma ACTH and corticosterone levels attained significance in calorie-restricted rats only, with a maximum toward the end of the resting phase. The daily pattern of adrenal gland corticosterone mirrored that of circulating corticosterone; however, calorie restriction reduced its levels. Plasma ACTH and corticosterone correlated significantly in controls only. Calorie restriction decreased plasma GH and leptin, and it distorted 24 h rhythmicity. In a second study, plasma ACTH and corticosterone levels were measured in group-caged rats, isolated control rats, and calorie-restricted rats during the light phase of the daily cycle. Plasma ACTH of calorie-restricted rats was lower, and plasma corticosterone was higher, compared with isolated or group-caged controls. The changes in the secretory pattern of hormones hereby reported may be part of the neuroendocrine and metabolic mechanisms evolved to maximize survival during periods of food shortage.     

High-performance liquid chromatographic separation of corticosteroids in plasma of rats. Its application to the determination of the circadian rhythm of 18-hydroxycorticosterone related to aldosterone and corticosterone

An efficient separation of corticosteroids in plasma of rats was obtained by reversed-phase high-performance liquid chromatography (HPLC). Plasma corticosteroid assays with HPLC separation were used to determine the circadian rhythm of 18-hydroxycorticosterone (18-OHB) and its possible relationship to aldosterone or corticosterone in conscious rats under standard conditions (regular diet; 12-hour light and 12-hour dark cycle). Significant circadian rhythms of plasma corticosterone, 18-OHB and aldosterone were observed with peak values at 20.00 h and nadir values at 08.00 h. The mean ratio of plasma 18-OHB to aldosterone during 24 h was 2.4. The circadian rhythm of 18-OHB was also correlated with that of plasma aldosterone or corticosterone.     

Circadian rhythms of food and 1% NaCl intake, urine and electrolyte excretion, plasma renin activity and insulin concentration in adrenalectomized rats

The circadian rhythms of food and 1% NaCl intake, and urine, Na+, Cl- and K+ excretion were followed up in male Wistar rats before and one week after bilateral adrenalectomy at 4-hour intervals during two consecutive days. The circadian rhythms of plasma renin activity (PRA) and plasma immunoreactive insulin (IRI) were evaluated after decapitation of both intact and adrenalectomized rats at 08, 16 and 24 h. To all rats 1% NaCl was offered instead of drinking water. Adrenalectomy did not cause any significant phase shift in the cosine curves approximating the data collected at 4-hour intervals. The circadian rhythms showed the same relationships before and after the operation: the rhythms of food intake, K+ excretion and saline intake preceded significantly the rhythms of urine, Na+ and Cl- excretion. Adrenalectomy induced an increase in mean PRA and shifted its minimal value from 08 to 24 h. After the operation mean IRI decreased and the minimal value shifted from 16 to 24 h. It was concluded that adrenal glands do not play an important role in the synchronization of the circadian rhythms of food and 1% NaCl intake, urine and synchronization of the circadian rhythms of food and 1% NaCl intake, urine and electrolyte excretion with the illumination cycle, but play a relevant role in the synchronization of the circadian rhythms of PRA and IRI in the rat.     

CIRCADIAN RHYTHMS IN THE RENIN-ANGIOTENSIN SYSTEM AND ADRENAL STEROIDS MAY CONTRIBUTE TO THE INVERSE BLOOD PRESSURE RHYTHM IN HYPERTENSIVE TGR(mREN-2)27 RATS

The transgenic TGR(mREN-2)27 rat is not only characterized by fulminant hypertension, but also by a disturbance in circadian blood pressure regulation, resulting in inverse circadian blood pressure profiles. The reasons for these alterations are not very well understood at present. We therefore investigated the circadian rhythms in several hormones participating in blood pressure regulation. From TGR and Sprague-Dawley (SPRD) control rats synchronized to 12h light and 12h dark (LD 12:12) blood was collected at different circadian times (07, 11, 15, 19, 23, 03, and 07 again, 5 rats per strain and time). The activities of plasma renin and converting enzyme, as well as plasma concentrations of corticosterone and aldosterone, were determined by radioimmunoassay (RIA). SPRD rats showed significant circadian rhythms in all variables except plasma renin activity, with maxima occurring during the day. TGR rats showed significant circadian rhythmicity in plasma renin activity and corticosterone and daily variation in aldosterone; angiotensin-converting enzyme (ACE) activity did not reach statistical significance. In TGR rats, 24h means in plasma renin activity and aldosterone were approximately sevenfold and fourfold higher, respectively, than in SPRD rats. Peak concentrations in corticosterone around 15h were more than two times higher in TGR rats than in SPRD rats, whereas no differences were observed during the night. It is concluded that, in TGR rats, the overall increase in plasma renin activity and aldosterone may contribute to the elevated blood pressure. The comparatively high levels in corticosterone and plasma renin activity during daytime may be involved in the inverse circadian blood pressure profiles in the transgenic animals. (Chronobiology International, 17(5), 645–658, 2000)     

Circadian rhythms in the renin-angiotensin system and adrenal steroids may contribute to the inverse blood pressure rhythm in hypertensive TGR(mREN-2)27 rats

The transgenic TGR(mREN-2)27 rat is not only characterized by fulminant hypertension, but also by a disturbance in circadian blood pressure regulation, resulting in inverse circadian blood pressure profiles. The reasons for these alterations are not very well understood at present. We therefore investigated the circadian rhythms in several hormones participating in blood pressure regulation. From TGR and Sprague-Dawley (SPRD) control rats synchronized to 12h light and 12h dark (LD 12:12) blood was collected at different circadian times (07, 11, 15, 19, 23, 03, and 07 again, 5 rats per strain and time). The activities of plasma renin and converting enzyme, as well as plasma concentrations of corticosterone and aldosterone, were determined by radioimmunoassay (RIA). SPRD rats showed significant circadian rhythms in all variables except plasma renin activity, with maxima occurring during the day. TGR rats showed significant circadian rhythmicity in plasma renin activity and corticosterone and daily variation in aldosterone; angiotensin-converting enzyme (ACE) activity did not reach statistical significance. In TGR rats, 24h means in plasma renin activity and aldosterone were approximately sevenfold and fourfold higher, respectively, than in SPRD rats. Peak concentrations in corticosterone around 15h were more than two times higher in TGR rats than in SPRD rats, whereas no differences were observed during the night. It is concluded that, in TGR rats, the overall increase in plasma renin activity and aldosterone may contribute to the elevated blood pressure. The comparatively high levels in corticosterone and plasma renin activity during daytime may be involved in the inverse circadian blood pressure profiles in the transgenic animals. (Chronobiology International, 17(5), 645-658, 2000)     

Short-term effects of vasoligation upon plasma follicle-stimulating hormone, luteinizing hormone, and prolactin in the adult rat: further evidence for a direct neural connection between the testes and the central nervous system

The purpose of these experiments was to determine whether bilateral vasoligation of adult male rats had any short-term effects upon plasma levels of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Adult male rats (250-300 g) were either bilaterally vasoligated or sham vasoligated, and blood samples were obtained by cardiac puncture preoperatively and at 24 h and 7 days following surgery. Plasma levels of both FSH and LH were significantly (P less than 0.01) decreased at 24 h following vasoligation compared to preoperative levels and those of sham-operated controls. However, the response was differential since, at 7 days following vasoligation, plasma FSH was still significantly decreased while LH was returning to control levels. Conversely, plasma prolactin levels were significantly (P less than 0.01) increased at 24 h compared to preoperative values and those in sham-operated controls, and at 7 days prolactin had returned to preoperative control levels. Sham vasoligation did not significantly change plasma levels of FSH, LH, or prolactin at any of the time intervals investigated. These results provide further evidence that suggests that there may be a direct connection between the testis and central nervous system that may be involved in the short-term regulation of gonadotropin and prolactin secretion.     

Circadian variation in renal function of the obese rat.

The influence of food and water intake on renal function was assessed by comparisons between the hyperphagic Zucker obese rat and its lean littermate, which demonstrates nocturnal dominance in activity. Serum creatinine and cortisol levels, creatine kinase activities, creatinine and urine clearances, and sodium and potassium excretion rates were measured over a 24-hour period in both lean and obese rats (n = 24 each). Six rats in each group were studied every 8 h to permit characterization over a 12-hour light/dark cycle at 2-hour intervals. Urine and creatinine clearances were increased in lean rats during the dark phase coincident with onset of eating. Similarly, renal sodium and potassium excretion rates were markedly increased during the dark cycle, despite relatively constant serum potassium and sodium levels over the 24-hour period. In contrast, no circadian patterns in urine and creatinine clearances were found in the obese rat, which exhibits continuous feeding habits throughout the 24-hour period. Moreover, renal electrolyte excretion in the obese rat was modestly increased during the dark cycle, unlike the significant differences over time observed in lean rats. Serum creatinine levels were increased during the dark cycle in both rat groups. Creatine kinase activity, a measure of ambulatory activity, was constant in lean rats during the study period. Although creatine kinase activity was increased in obese rats during the dark cycle, no correlations with renal functional parameters were found. These results indicate that differences in food and water intake are significant determinants in diurnal cyclic changes in renal function.     

Stress response patterns of plasma corticosterone, prolactin, and growth hormone in the rat, following handling or exposure to novel environment.

Corticosterone, prolactin, and growth hormone responses to 5 s of handling or 3 min of novel environment were compared in rats at crest and trough of the diurnal adrenal rhythm 0, 5, 15, 30, and 60 min after stimulation. All hormones responded to stimulation, corticosterone and prolactin with a dramatic rise, and growth hormone with a precipitous fall. Resting corticosterone levels evidenced the expected diurnal variation, and prolactin but not growth hormone also showed a baseline diurnal variation of small magnitude at the times studied. Growth hormone response characteristics were unaffected by time of day or type of stimulation. Both corticosterone and prolactin response profiles differed at both times of day and following both types of stimulation. Corticosterone and prolactin levels were highly correlated and each was negatively correlated with growth hormone levels. This study confirms that hormone responses to stress are complex and depend not only on the stimulus but the context of stimulation.     

Entrainment of rhythmic melatonin secretion in man to a 12-hour phase shift in the light/dark cycle

Daily rhythms in melatonin secretion were monitored in four healthy adult males by measuring the melatonin contents of sequential 4-hour urine specimens and of plasma samples collected at 12-hour intervals, or, in one subject, continuously for 24 hours. All subjects exhibited similar diurnal rhythms, with peak urinary melatonin excretion rates and blood melatonin levels occurring during the daily period of darkness and sleep. When the daily light/dark regimen was phase-shifted by 180°, the plasma and urinary melatonin rhythms required 5–7 days (depending on the subject) to re-entrain to the new schedule. Simultaneous measurements of plasma melatonin levels and melatonin excretion rates indicate that urinary melatonin reflects, with remarkable fidelity, circulating melatonin levels.     

Effect of bromocriptine on the control of plasma aldosterone diurnal variation in normal supine man.

In order to investigate the role of prolactin in the control of the circadian rhythm of plasma aldosterone (PA), plasma renin activity (PRA), cortisol (PC), aldosterone and prolactin (PRL) levels were determined in samples at 4-hour intervals from 5 normal supine men over a period of 24 h under basal conditions and subsequently over a period of 24 h during suppression of prolactin release by bromocriptine (CB-154). After suppression of prolactin, statistically signific1nt circadian rhythms in PC and PA have been detected with a moderate decrease of PA concentration, while the PC level remained unalterated. PRA rhythmicity persisted with a significant shift of acrophase and remarkable reduction of plasma levels. Moreover, during CB administration a significant correlation was obtained between PA and PC, while no correlation was detected between PA and PRA. These data are consistent with the following concepts: (a) the prolactin does not play a significant role in the regulation of circadian rhythm and concentration of plasma aldosterone in normal supine men, and (b) bromocriptine induces a remarkable reduction of PRA and a variable decrease in plasma aldosterone, but it does not influence the secretion of cortisol in normal subjects.     

Hormonal and metabolic rhythms associated with the daily scheduled nursing in rabbit pups

Young rabbits are nursed every 24 h for a period of 3-5 min. As a consequence, pups are synchronized to this nursing event; this synchronization is characterized by increased locomotor activity and a peaking of core temperature and plasma corticosterone in anticipation of the daily meal. Ghrelin is a hormone suggested to play a role in meal initiation and to promote food intake. The present study explored the role of ghrelin in food-entrained conditions. Newborn rabbits were maintained in constant darkness and nursed once daily at 1000 by the lactating dam. On postnatal day 7, rabbits were killed at six different time points to complete a 24-h cycle. All pups developed locomotor rhythms entrained by mealtime and exhibited anticipatory activity. Food-entrained rhythms in plasma corticosterone and free fatty acids were observed even if two meals were omitted. In contrast, daily food-driven rhythms in stomach weight, plasma glucose, liver glycogen, and ghrelin did not persist when two meals were omitted. Peak ghrelin levels were observed at the moment in the cycle when the stomach weight was lowest, i.e., before initiation of anticipation. The present data are in agreement with previous data from rabbit pups maintained in light-dark conditions and provide evidence that 7- to 9-day-old rabbits in constant darkness can exhibit metabolic and hormonal rhythms mainly driven by the restricted daily nursing.     

Twenty-four hour rhythms of hypothalamic corticotropin-releasing hormone, thyrotropin-releasing hormone, growth hormone-releasing hormone and somatostatin in rats injected with Freund's adjuvant

The effect of Freund's adjuvant injection on 24-hour variation of hypothalamic corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), GH-releasing hormone (GRH) and somatostatin levels was examined in adult rats kept under light between 0800 and 2000 h daily. Groups of rats receiving Freund's complete adjuvant or its vehicle 3 days before sacrifice were killed at six different time intervals throughout a 24-hour cycle. In the median eminence, adjuvant vehicle-injected rats exhibited significant 24-hour variations for the four hormones examined, with maxima at noon. These 24-hour rhythms were inhibited or suppressed by Freund's adjuvant injection. In the anterior hypothalamus of adjuvant vehicle-treated rats, CRH content peaked at 1600 h, while two peaks were found for TRH and GRH levels, i.e., at 2400-0400 h and 1600 h. Freund's adjuvant injection suppressed 24-hour rhythm of anterior hypothalamic CRH, TRH and GRH content and uncovered a peak in anterior hypothalamic somatostatin levels at 0400 h. In the medial hypothalamus of adjuvant vehicle-treated rats, significant 24-hour variations were detectable for TRH (peaks at 1600 and 2400 h) and somatostatin (peak at 2400 h) which disappeared after Freund's adjuvant injection. In the posterior hypothalamus of adjuvant vehicle-treated rats, two peaks were apparent for CRH, TRH and somatostatin levels, i.e. at 1600 h and 2400-0400 h, this hormonal profile remaining unmodified after Freund's adjuvant administration. The administration of the immunosuppressant drug cyclosporine (5 mg/kg, 5 days) impaired the depressing effect of Freund's adjuvant injection on CRH, TRH and somatostatin content in median eminence, but not that on GRH. In the anterior hypothalamus, cyclosporine generally prevented the effect of immunization on hormone levels an revealed a second maximum in TRH at 0400 h. Cyclosporine also restored 24-hour variations in TRH and somatostatin levels of medial hypothalamus of Freund's adjuvant-injected rats but was unable to modify them in the posterior hypothalamus. The results further support the existence of a significant effect of immune-mediated inflammatory response at an early phase after Freund's adjuvant injection on hypothalamic levels which was partially sensitive to immunosuppression by cyclosporine.     

Circadian rhythm of circulating corticosterone and prolactin levels in spontaneously hypertensive rats

Circulating levels of corticosterone and prolactin were measured by radioimmunoassay at hourly intervals during a 24 h period to establish the diurnal rhythm of these hormones in spontaneously hypertensive rats (SHR). Corticosterone levels exhibited a distinct circadian variation with concentrations reaching a zenith at 2000 h and a nadir at 1200 h in male and female SHR. Corticosterone levels in females were consistently greater than males. Circulating prolactin levels were greater during the light h than dark in the female; the opposite occurred in males. Measurement of pituitary prolactin content tended to be low when serum prolactin levels were high and vice versa. The circadian rhythm of circulating corticosterone and prolactin in the hypertensive SHR was found to be similar to the day-night patterns established for normotensive rats. However, these measurements were made under quiescent conditions. It is suggested that because SHR are hyper-responsive to stress and because corticosterone and prolactin have synchronous effects on stress-induced adrenal steroidogenesis, further investigation of prolactin and corticosterone may reveal a participatory role of these hormones in the pathogenesis of the genetically-programmed hypertension of SHR.     

Adrenalectomy affects pain behavior of rats after formalin injection

Stressful stimuli can activate the hypothalamo-pituitary-adrenal-axis and the endogenous opioid system. In addition, corticosterone and opioid release might cause analgesia. This rat study used adrenalectomy for corticosterone withdrawal and naloxone administration for opioid antagonism in order to study pain behavior and hypophyseal hormone release in the plasma after a formalin test. Twelve days before the formalin testing, male Sprague Dawley rats underwent adrenalectomy or sham-adrenalectomy, and non-operated rats were used as reference. The number of flinches and the duration of licking or biting behavior were measured during the early and late phase. In reference and sham-operated rats, injection of formalin 5% resulted in a marked pain behavior in the early and late phase with significant increases in ACTH and corticosterone plasma levels. In adrenalectomized rats, pain behavior was decreased during both phases. Naloxone, administered before the late phase, did not alter pain behavior in sham or reference rats, whereas in adrenalectomized rats pain reactivity returned to those levels observed in reference rats. Beta-endorphin plasma levels above the detection limit were more frequently found in adrenalectomized rats. Thyrotropin and prolactin levels were not different between studied groups. We speculate that the observed reduced pain behavior in adrenalectomized rats after formalin, is the result of an increased production of pro-opiomelanocortin, the pro-drug of both adrenocorticotrophic hormone and beta-endorphin.     

The circadian rhythm of the adrenal glucocorticoid function in female rats selected for the reaction of the estrous cycle to constant illumination

Studies have been made of the circadian rhythms of a glucorticoid hormone, corticosterone, in the adrenals and blood serum in female Wistar rats from two substrains selected for high (ESTH) and low (ESTL) ability to develop permanent oestrus under constant illumination. Significant changes in parameters of the circadian rhythm of the hormone were observed in animals of the 26th generation of selection. Total alleviation of corticosterone rhythm in the blood was on observed in ESTL rats, while in ESTH animals maximum level of the hormone in the blood was shifted to the dark time. Comparison of a high corticosterone content of the adrenals in ESTL rats with a low concentration in the blood plasma indicates the increase in metabolic clearance of the hormone in animals from this strain. It is suggested that the decreased corticosteroid production in the adrenals of ESTH rats facilitates the development of permanent oestrus under constant illumination.     

Circadian variation in plasma melanocyte stimulating hormone activity in the rat.

Non--stress levels of plasma melanocyte stimulating hormone (MSH) and corticosterone were determined at 3-h intervals during controlled light--dark cycles in adult, male rats. Significant 24-h periodicity was demonstrated for both plasma MSH and corticosterone. Whereas plasma MSH values were not as high during the early evening hours as during the early morning hours, plasma corticosterone levels showed a marked rise during the early afternoon and crest values occurred just before the lights off phase of the 24-h light--dark cycle. These results indicate that in the rat, the level of MSH in plasma fluctuates with a low amplitude circadian frequency which is not in phase with that observed for plasma corticosterone.