Glucocorticoid effects on the diurnal rhythm of circulating leptin levels

It is known that circulating leptin shows diurnal variation with a nocturnal rise; however, the mechanisms generating this rhythm have not been fully elucidated. Glucocorticoids are a potent stimulator of leptin secretion, and there is a reciprocal relationship between circulating leptin and glucocorticoid levels. We hypothesized that glucocorticoids could modulate the diurnal rhythm of circulating leptin. We therefore explored the diurnal variation of leptin under situations in which subjects showed no or some shift of glucocorticoid diurnal rhythm, such as prednisolone-administered humans, and adrenalectomized and corticosterone-replaced (ADX+B) rats. The peak level of plasma cortisol immunoreactivity was shifted from early morning to noon by prednisolone administration. The nocturnal increment of plasma leptin in prednisolone-administered patients (71.2 +/- 14.2% from 08:00 h value) was significantly greater than that in normal volunteers (12.2 +/- 7.5% from 08:00 h value), but the timing of nadir and the peak of plasma leptin was not shifted. In normal rats, the plasma concentration of leptin showed the diurnal rhythm with the bottom at 16:00 h and the top between midnight and early morning. The amplitude of leptin diurnal rhythm was significantly reduced in ADX+B rats (08:00 h: 3.0 +/- 0.2, 16:00 h: 2.7 +/- 0.2, 00:00 h; 3.7 +/- 0.2 ng/ml) compared with sham operated rats (08:00 h: 3.0 +/- 0.2, 16:00 h 2.2 +/- 0.2, 00:00 h: 4.7 +/- 0.4 ng/ml); but ADX+B rats still retained similar timing of nadir and the peak of plasma leptin as observed in sham rats. These results indicate that glucocorticoids enhance the amplitude of leptin diurnal rhythm, and are consistent with previous findings showing that glucocorticoids increase leptin secretion. Glucocorticoids appear to play modulatory, but not essential roles in generating leptin diurnal rhythm.     

Impact of meal timing and frequency on the twenty-four-hour leptin rhythm

OBJECTIVE: To study the influence of changes in meal timing and frequency on the diurnal rhythm of leptin and on the 24-hour profile of insulin and glucose. PATIENTS AND METHODS: Five obese women were studied twice during a weight-maintaining diet in either 3 daily or 8 day and night equal portions. Blood was sampled for 24-hour profiles of leptin and insulin. RESULTS: During the 8-meal intervention, the 24-hour rhythm of leptin changed significantly: the amplitude decreased (p = 0.0089) and the acrophase was delayed by 168 min (p = 0.021). Also, 8 small insulin secretion peaks occurred instead of the 3 postprandial high insulin peaks. CONCLUSION: The dispersion of food intake over 24 h affects the diurnal leptin rhythm. These changes could not be attributed to changes in circadian timing or energy balance. Instead, changes in daily insulin secretion profiles might play a role.     

2-bromo-alpha-ergocryptine mesylate (CB-154) inhibits prolactin and luteinizing hormone secretion in the prepubertal female rat

Treatment of immature female rats with 100 micrograms 2-bromo-alpha-ergocryptine mesylate (CB-154) per ml drinking water beginning on Day 30 of age until vaginal opening delayed puberty by 6 days. Rats treated with CB-154 exhibited vaginal opening at 43.3 +/- 0.6 days whereas controls exhibited vaginal opening at 37.9 +/- 0.8 days. Most interestingly, serum levels of luteinizing hormone (LH) and prolactin (PRL) on Days 31-35, determined by a homologous radioimmunoassay were significantly lower in treated rats than in controls. The ovarian concentrations of progesterone (P) and androstenedione (A) were lower in rats treated with CB-154 than in controls; ovarian estradiol (E2) concentrations were low in both groups. Serum levels of P (but not A and E2) were reduced on Days 31-35 of the treatment period. Cessation of the CB-154 treatment on the morning of Day 35 returned the onset of puberty to normal values; steroid and gonadotropin levels also returned to normal values within 2 days after removal of the CB-154 from the drinking water. Near the time of onset of puberty, serum levels of LH in rats treated with CB-154 returned to control values. These data indicate that in the female rat the delay in puberty induced by CB-154 might be due to a reduction in the secretion of LH, especially since the onset of delayed puberty in rats treated with CB-154 correlates with an increase in the serum level of LH. Further studies are needed to elucidate the specific effects of hypoprolactinemia on ovarian function and the onset of puberty in the rat.     

Ontogenic studies of the neural control of adenohypophyseal hormones in the rat. II. prolactin

1. Serum prolactin levels are low during the first 20 days of life and gradually increase toward puberty, in both male and female rats. 2. There is an age-related increase in the cell population engaged in prolactin secretion, as well as an increase in the synthesis of prolactin and of the amount of prolactin secreted from individual lactotropes. 3. The gradual increase in prolactin levels in the third week of life is not related to a decrease in dopaminergic inhibition but to an increase in the efficiency of prolactin releasing factors such as estrogen, serotonin, opiates, and posterior pituitary extracts. 4. Prolactin release induced by physiological factors, such as stress, cervical stimulation, or the expression of spontaneous diurnal and nocturnal surges, requires maturational events within the hypothalamic-pituitary axis which are evident at the end of the third week of life. 5. In the female rat the steadily increasing levels of prolactin are involved in the timing of puberty eclosion acting at the ovary and at the brain. 6. In the prepubertal male rat increasing titers of prolactin may be involved in testicular and accessory organ development and may facilitate the actions of luteinizing hormone, follicle stimulating hormone, and testosterone on male sexual organs.     

Delay of puberty and impairment of growth in female rats given a non competitive antagonist of NMDA receptors

Reportedly, excitatory amino acids are involved in the control of gonadotropin secretion of rats and non-human primates. The aim of this study was to investigate the effect of chronic blockade of NMDA (N-methyl-D-aspartic acid) receptors by the non competitive receptor antagonist MK-801 on gonadotropin secretion and the onset of puberty in female rats. Moreover, since in humans alterations of the timing of puberty frequently coexist with disturbances of body growth, suggesting a common etiology for both events, we evaluated the effect of MK-801 also on the neural mechanisms controlling growth hormone (GH) secretion. Twenty-one-day-old female rats were treated with MK-801 (0.2 mg/kg ip, bid) or placebo for 10 days and were killed after 7 days of withdrawal. Administration of MK-801 induced a significant impairment of growth rate without altering food intake, and a delay in vaginal opening. Pituitaries from rats treated with MK-801 had a reduced luteinizing hormone (LH) content, and secreted in vitro lower amounts of LH both under basal and LHRH-stimulated conditions. MK-801 treated rats had a lower pituitary GH content and basal and GHRH-stimulated GH release and reduced plasma insulin-like growth factor-I levels. These data indicate that blockade of NMDA receptors in a critical period of the female rat life-span: 1) delays puberty by reducing gonadotropin secretion; 2) impairs growth rate by reducing GH secretion, with a mechanism still to be clarified.     

The effects of nocturnal life on endocrine circadian patterns in healthy adults

We observed the 24-hour patterns of endocrine in medical students who lived either a diurnal life or nocturnal life. Nocturnal life was designed by skipping their breakfast but consuming much (>50% of their daily food intake) in the evening and at night with the sleep from 0130 h to 0830 h the next morning. After 3 weeks in the experimental life, the 24-hour plasma concentrations of melatonin, leptin, glucose and insulin were measured every three hours. Both plasma melatonin and leptin showed peaks at 0300 h in the diurnal lifestyle group, and the night peaks decreased in the nocturnal lifestyle group. The changes in the patterns of melatonin and leptin were highly consistent with that of night-eating syndrome (NES). Plasma glucose increased after all meals in both groups. Its concentration maintained a high level in the nocturnal lifestyle group between midnight and early morning while insulin secretion decreased markedly during this period. Furthermore, the strong association between glucose and insulin in the diurnal lifestyle group after meals was damaged in the nocturnal lifestyle group. It was suggested that nocturnal life leads to the impairment of insulin response to glucose. Taking these results together, nocturnal life is likely to be one of the risk factors to health of modern people, including NES, obesity and diabetes.     

Central application of IGF-1 postpones time of vaginal opening in normally fed, but not in food-restricted rats

BACKGROUND/AIMS: Central but also peripheral IGF-1 is suggested to play a role in the initiation of puberty as it directly affects GnRH synthesis and release. A possible intermediate in the effects of IGF-1 on puberty might be the adiposity-signaling hormone leptin, whose plasma levels are decreased in food-restricted (FR) rats. METHODS: IGF-1 was chronically centrally infused in 23-day-old prepubertal female rats which were either normally fed or 30% FR, and the effects on time of vaginal opening (VO) and plasma leptin levels were monitored. RESULTS: FR treatment postponed time of VO and decreased plasma leptin levels. In normally fed rats centrally infused with IGF-1, time of VO was found to be postponed to the same extent as FR treatment did. The IGF-1 infusion did not affect plasma leptin levels in normally fed animals but increased leptin levels in the FR group compared to controls. Daily food intake was equal between all groups but body weight course was lower in FR rats. IGF-1 treatment did not significantly affect food intake or body weight course. CONCLUSION: FR treatment delays the moment of vaginal opening to the same extent as observed in normally fed rats that were centrally infused with IGF-1.     

The possible role of prolactin in the circadian rhythm of leptin secretion in male rats

In humans there is a circadian rhythm of leptin concentrations in plasma with a minimum in the early morning and a maximum in the middle of the night. By taking blood samples from adult male rats every 3 hr for 24 hr, we determined that a circadian rhythm of plasma leptin concentrations also occurs in the rat with a peak at 0130h and a minimum at 0730h. To determine if this rhythm is controlled by nocturnally released hormones, we evaluated the effect of hormones known to be released at night in humans, some of which are also known to be released at night in rats. In humans, prolactin (PRL), growth hormone (GH), and melatonin are known to be released at night, and adrenocorticotropic hormone (ACTH) release is inhibited. In these experiments, conscious rats were injected intravenously with 0.5 ml diluent or the substance to be evaluated just after removal of the first blood sample (0.3 ml), and additional blood samples (0.3 ml) were drawn every 10 min thereafter for 2 hr. The injection of highly purified sheep PRL (500 microg) produced a rapid increase in plasma leptin that persisted for the duration of the experiment. Lower doses were ineffective. To determine the effect of blockade of PRL secretion on leptin secretion, alpha bromoergocryptine (1.5 mg), a dopamine-2-receptor agonist that rapidly inhibits PRL release, was injected. It produced a rapid decline in plasma leptin within 10 min, and the decline persisted for 120 min. The minimal effective dose of GH to lower plasma leptin was 1 mg/rat. Insulin-like growth factor (IGF-1) (10 microg), but not IGF-2 (10 microg), also significantly decreased plasma leptin. Melatonin, known to be nocturnally released in humans and rats, was injected at a dose of 1 mg/rat during daytime (1100h) or nighttime (2300h). It did not alter leptin release significantly. Dexamethasone (DEX), a potent glucocorticoid, was ineffective at a 0. 1-mg dose but produced a delayed, significant increase in leptin, manifest 100-120 min after injection of a 1 mg dose. Since glucocorticoids decrease at night in humans at the time of the maximum plasma concentrations of leptin, we hypothesize that this increase in leptin from a relatively high dose of DEX would mimic the response to the release of corticosterone following stress in the rat and that glucocorticoids are not responsible for the circadian rhythm of leptin concentration. Therefore, we conclude that an increase in PRL secretion during the night may be responsible, at least in part, for the nocturnal elevation of leptin concentrations observed in rats and humans.     

Plasma progesterone concentrations and length of the first spontaneous oestrous cycle in pubertal rats

The length of the first spontaneous oestrous cycle in pubertal Wistar-Imamichi strain rats determined by vaginal smears varied from 5 to 18 days. The variation was ascribed to the period (3-16 days) of the stage of vaginal smears consisting of leucocytic cells (L stage). Plasma progesterone concentration and the decidual reaction in the uterus were used as indicators of the function of the corpus luteum and the L stage period was categorized as short, lasting for 3-6 days (average 4 days) with non-functional corpora lutea, or long, lasting 9-16 days (average 12 days) and with functional corpora lutea. Rats with the long L stage showed nocturnal and diurnal prolactin surges, but no daily changes in prolactin values were observed in rats with a short L stage. Daily changes in prolactin concentrations were maintained by the administration of progesterone in rats ovariectomized on Day 6 of the L stage. Plasma progesterone values on Day 6 of the L stage decreased with ergocornine treatment on Days 4 and 5 of the L stage and administration of bovine prolactin restored the level. These results indicate that the L stage observed in the first oestrous cycle is maintained by a positive feedback relation between progesterone and prolactin secretions.     

A role for aromatizable androgens in female rat puberty

The function that aromatizable androgens may have in female puberty is unclear. The present experiments were undertaken to examine, using a quantitative approach, the role that physiological levels of these androgens may play in determining the timing of vaginal opening and first ovulation in female rats. Serum androstenedione (delta 4) levels increased markedly between Postnatal Days 4 and 8, remained elevated through Day 16, and declined thereafter to remain at about 100 pg/ml throughout juvenile development (Days 20-32). Serum testosterone (T) also increased, though less prominently after Postnatal Day 4. Maximal values were found at Day 12 (about 150 pg/ml); thereafter, T levels decreased to intermediate values (about 100 pg/ml), which were maintained during juvenile days. Serum dehydroepiandrosterone (DHA) remained undetectable throughout prepubertal development. At puberty, serum delta 4 increased 2.5-fold, but only at the time of the preovulatory luteinizing hormone (LH) surge. In contrast, T levels increased significantly 2-fold on the early proestrous-2 phase of puberty, 3.5-fold on the morning of first proestrus, and 9-fold at the time of the LH surge. Serum DHA remained undetectable. Implantation of Silastic capsules containing T at 2 or 6 mg/ml oil into juvenile 28-day-old rats resulted in serum T levels similar to those found on early proestrous 2 (about 150-180 pg/ml) and at 1300 h of first proestrus (ca. 300-400 pg/ml), respectively. Both treatments induced precocious vaginal opening, but failed to advance first ovulation. About 50% of the T-implanted rats had ambiguous estrous-type vaginal cytology preceding the day of first diestrus, and failed to show corpora lutea at this time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Leptin fails to reduce ethanol intake in Marchigian Sardinian alcohol-preferring rats

Leptin, a hormone secreted by the adipocytes and involved in feeding and energy balance control, has been proposed to modulate alcohol craving in mice and humans. This study evaluated whether leptin modulates alcohol intake in Marchigian Sardinian alcohol-preferring (msP) rats. Rats were offered 10% ethanol either 2h per day at the beginning of dark period of the 12:12h light/dark cycle, or 24h per day. Leptin was injected into the lateral ventricle (LV), the third ventricle (3V), or intraperitoneally (IP) once a day, 1h before the onset of the dark period. Neither acute nor chronic (9 days) leptin injections (1 or 8microg per rat) into the LV or 3V modified ethanol intake in male msP rats, offered ethanol 2h per day. Chronic LV injection of leptin (8 or 32 microg per rat in male rats and 8 or 16 microg per rat in female rats for 7 days), or chronic IP injections of leptin (1mg/kg in male rats for 5 days) failed to modify the intake of ethanol, offered 24h per day. Finally, chronic LV leptin injections (8 or 32 microg per rat for 12 days) did not modify ethanol intake in male msP rats, adapted to ad libitum access to ethanol and then tested after a 6-day period of ethanol deprivation. In contrast, in most of these conditions leptin significantly reduced food intake. These data do not support a role for leptin in alcohol intake, preference, or craving in msP rats.     

Neonatal corticoid administration: retardation of adrenal rhythmicity and desynchronization of puberty.

The development of the adrenal corticosterone and progesterone diurnal rhythms was retarded by a single injection of cortisol acetate, dexamethasone acetate or progesterone into 3-day-old female rats. This effect was transient with the establishment of the corticosterone rhythm by day 70. The retardation of adrenal hormone rhythmicity may alter the normal process of sexual maturation, because cortisol and dexamethasone-treated animals but not progesterone treated rats exhibited a desynchronization of the events associated with puberty. Vaginal opening was delayed, and dissociated from first ovulation in these rats. Further, neonatal cortisol administration lengthened the estrous cycle in adulthood. It can be concluded from our study that the maturation of the neural centers controlling adrenal steroid secretion may be essential for the proper timing of pubertal events in the female rat.     

Effects of maternal leptin treatment during lactation on the body weight and leptin resistance of adult offspring

We investigate whether leptin treatment to lactating rats affects food intake, body weight and leptin serum concentration and its anorectic effect on their adult offspring. Lactating rats were divided into 2 groups: Lep-single injected with recombinant rat leptin (8 microg/100 g of body weight, daily for the last 3 consecutive days of lactation) and control group (C) that received the same volume of saline. After weaning all pups had free access to the control diet, their body weight and food intake were monitored at each 4 days until 180 days of age, when they were tested for its food intake and response to either leptin (0.5 mg/kg body wt, ip) or saline vehicle. The offspring of the leptin-treated dams gained more weight and had higher food intake from day 37 onward (p<0.05), higher amount of retroperitoneal white adipose tissue (RPWAT) (37%, p<0.05) and higher leptin serum concentration (40%, p<0.05) at 180 days of age compared to control group. The food intake at 2, 4, 6 and 24 h was unaffected after acute injection of leptin in these animals, suggesting resistance to the anorectic effect of leptin. The maternal leptin treatment during lactation makes their adult offspring more susceptible to overweight with resistance to the anorectic effect of leptin.     

From daily behavior to hormonal and neurotransmitters rhythms: Comparison between diurnal and nocturnal rat species

Mammalian species can be defined as diurnal or nocturnal, depending on the temporal niche during which they are active. Even if general activity occurs during nighttime in nocturnal rodents, there is a patchwork of general activity patterns in diurnal rodents, including frequent bimodality (so-called crepuscular pattern, i.e., dawn and dusk peaks of activity) and a switch to a nocturnal pattern under certain circumstances. This raises the question of whether crepuscular species have a bimodal or diurnal - as opposed to nocturnal - physiology. To this end, we investigated several daily behavioral, hormonal and neurochemical rhythms in the diurnal Sudanian grass rat (Arvicanthis ansorgei) and the nocturnal Long-Evans rat (Rattus norvegicus). Daily rhythms of general activity, wheel-running activity and body temperature, with or without blocked wheel, were diurnal and bimodal for A. ansorgei, and nocturnal and unimodal for Long-Evans rats. Moreover, A. ansorgei and Long-Evans rats exposed to light-dark cycles were respectively more and less active, compared to conditions of constant darkness. In contrast to other diurnal rodents, wheel availability in A. ansorgei did not switch their general activity pattern. Daily, unimodal rhythm of plasma leptin was in phase-opposition between the two rodent species. In the hippocampus, a daily, unimodal rhythm of serotonin in A. ansorgei occurred 7 h earlier than that in Long-Evans rats, whereas a daily, unimodal rhythm of dopamine was unexpectedly concomitant in both species. Multiparameter analysis demonstrates that in spite of bimodal rhythms linked with locomotor activity, A. ansorgei have a diurnally oriented physiology.     

Nocturnal variation of prolactin secretion in the Mouflon (Ovis gmelini musimon) and domestic sheep (Ovis aries): seasonal changes

Seasonal changes in nocturnal prolactin secretion and their relationship with melatonin secretion were monitored in wild (Mouflon, Ovis gmelini musimon) and domesticated sheep (breed Manchega, Ovis aries). Two groups of eleven adult females each, were maintained outdoors under natural photoperiod. Plasma concentrations of prolactin and melatonin were determined during the summer and winter solstices and the autumn and spring equinoxes. Blood samples were collected every 3h during the night hours, and 1h before and after the onset of darkness and sunrise. Maximum mean plasma concentrations of prolactin during the dark-phase in Mouflons were observed in the summer solstice, (P<0.001) and in the summer solstice and spring equinox in Manchega ewes (P<0.001). Mean plasma concentrations of prolactin were higher in the wild species (P<0.001) during the summer solstice. In contrast, during the spring equinox, mean levels of prolactin were higher in Manchega ewes than in Mouflons (P<0.05). Plasma prolactin concentrations showed a nocturnal rhythm in both breeds, with seasonal variations (P<0.001). The increase in plasma melatonin levels during the first hour after sunset was accompanied to increasing concentrations of PRL 1h after the onset of darkness, only in the autumn and spring equinox for the Mouflon, and in the summer solstice and spring equinox for the Manchega ewes. In Mouflons, the fall of plasma PRL concentrations about the middle dark-phase in all the periods studied, coincided with high levels of melatonin. A similar relation was observed in Manchega ewes only in the winter solstice and spring equinox. The current study shows that the nocturnal rhythm of prolactin secretion exhibits seasonal variation; differences in the patterns of prolactin secretion between Mouflon and Manchega sheep are taken to represent the effects of genotype.     

Chronic leptin infusion advances, and immunoneutralization of leptin postpones puberty onset in normally fed and feed restricted female rats

Does leptin play a vital role in initiating puberty in female rats and can it overrule a nutrionally imposed (i.e. a 30% feed restriction, FR) delay in puberty onset? Prepubertal female rats were chronically infused for 14 days with leptin (icv or sc) or leptin-antiserum (icv) while puberty onset was monitored by means of scoring the moment of vaginal opening (VO). Median VO age was higher (35 days versus 27 days) in FR animals but leptin levels at VO were significantly decreased (1.44 +/- 0.17 ng/ml versus 2.79 +/- 0.31 ng/ml). Centrally (icv) and peripherally (sc) infused leptin (1 microg/day) advanced VO age compared to FR controls (30 days versus 35 days and 31 days versus 41 days, respectively). Congruently, centrally (icv) administered leptin-antiserum (0.6 microg/day) delayed puberty onset. In normally fed rats median VO age was only marginally advanced (26 days versus 27 days) but only if leptin was applied centrally. The effects of FR on puberty onset are counteracted or even normalized by the infusion of leptin, whereas immunoneutralization of central leptin postpones puberty onset. We therefore conclude that central leptin is crucial for initiating puberty in female rats.     

Nocturnal surges and reflexive release of prolactin in parentally behaving virgin female and male rats

Maternally behaving virgin rats are capable of releasing prolactin reflexively in response to stimulation by pups, especially during the proestrous/estrous phase of the cycle. When such rats are chronically exposed to pups they usually undergo a state of pseudopregnancy during which prolactin is secreted in a pattern of nocturnal surges. The present series of experiments evaluated the initiation of nocturnal prolactin surges in maternally behaving virgins, the role of estrogen in the reflexive release of prolactin, and the influence of gender on these two modes of prolactin secretion. It was found that the nocturnal surges of prolactin are already present on Days 1 and 2 of pup-induced pseudopregnancy. At this stage, however, the surges are not yet autonomous, seeing that pseudopregnancy is interrupted shortly after removal of the pups on Day 2. Activation by vaginocervical stimulation of the "mnemonic" neurogenic system that controls the autonomous nocturnal prolactin surges did not interfere with the reflexive pup-induced release of prolactin in maternally behaving virgins. The capacity of reflexive prolactin release in the virgin rat was abolished by ovariectomy, restored by estrogen replacement, and persisted for only 24 hr following estrogen removal. Paternally behaving males subjected to chronic exposure to pups were incapable of secreting nocturnal surges of prolactin characteristic of the pseudopregnant female. Such males were also incapable of releasing prolactin reflexively in response to stimulation by pups, even when supplemented with exogenous estrogen. These results indicate that the two modes of prolactin secretion are sex dependent, and that the maternally behaving virgin, unlike the postpartum rat, requires concurrent estrogenic facilitation for releasing prolactin in response to stimulation by young.     

Working for food shifts nocturnal mouse activity into the day

Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few hours in daytime. Timed food access also results in reduced food intake, but the role of food intake in circadian organization per se has not been described. By simulating natural food shortage in mice that work for food we show that reduced food intake alone shifts the activity phase from the night into the day and eventually causes nocturnal torpor (natural hypothermia). Release into continuous darkness with ad libitum food, elicits immediate reversal of activity to the previous nocturnal phase, indicating that the classical circadian pacemaker maintained its phase to the light-dark cycle. This flexibility in behavioral timing would allow mice to exploit the diurnal temporal niche while minimizing energy expenditure under poor feeding conditions in nature. This study reveals an intimate link between metabolism and mammalian circadian organization.     

Circadian regulation of gonadotropin-releasing hormone neurons and the preovulatory surge in luteinizing hormone in the diurnal rodent, Arvicanthis niloticus, and in a nocturnal rodent, Rattus norvegicus

Daily rhythms in the timing of the preovulatory surge and the display of reproductive behavior are reversed in diurnal and nocturnal rodents, but little is known about the neural mechanisms underlying these differences. We examined this issue by comparing a diurnal murid rodent, Arvicanthis niloticus (the grass rat), to a nocturnal one, Rattus norvegicus (the lab rat). In the first study, we established that sequential estradiol and progesterone treatment induces a proestrous-like rise in LH secretion and in the percentage of GnRH neurons that express Fos in grass rats, as is the case in lab rats. Next, we tested the hypothesis that differences in the timing of estrus-related events in diurnal and nocturnal species are caused by differences in rhythms in responsiveness to steroid hormones. We found rhythms in GnRH neuron activity, as indicated by Fos, that were 12 hours out of phase in grass rats and lab rats. These patterns persisted in both species when animals were housed in constant darkness for 5 days, suggesting that they are driven by an endogenous circadian mechanism. These results indicate that steroid-primed grass rats and lab rats are similar with respect to the temporal relationship among estrus-related events, but that the timing of these events relative to the light-dark cycle is dramatically different and that this difference is caused by endogenous circadian mechanisms.     

Prolactin inhibition in lactating rats changes leptin transfer through the milk.

Malnutrition during lactation reduces milk production and changes pup's leptin serum levels. To test prolactin role in this nutritional state, we evaluated whether prolactin suppression during lactation changes serum leptin in dams, its transfer through the milk, and pup's serum leptin. Lactating rats were treated with bromocryptine (1 mg/twice a day, s.c.) or saline three days before sacrifice (days 2-4 or days 19-21). Food intake and body weight were measured until sacrifice (4th and 21st day). Serum prolactin and leptin were determined by radioimmunoassay. Bromocryptine injected dams had lower serum prolactin and milk production as expected. The mothers presented lower food ingestion (day 21: -25%), lower body weight (day 4: -12%; day 21: -10%), higher serum leptin (day 4: +68%), lower milk leptin on the 4th day (11 times) and higher (8 times) on the 21st day. The offspring of bromocryptine-treated mothers presented lower body weight in both periods of lactation and lower serum leptin on the 4th day (-40%) and higher on the 21st day (+37%) of lactation. We suggest that prolactin, through its effect on leptin secretion into the milk, may play an important role in signalizing maternal nutritional status to the pups.