Phenobarbital blockade of the preovulatory luteinizing hormone surge: association with phase-advanced circadian clock and altered suprachiasmatic nucleus Period1 gene expression

Food restriction during development delays puberty but does not affect adult seasonal reproductive responses to food availability in Siberian hamsters (Phodopus sungorus)

Seasonally breeding animals respond to environmental cues to determine optimal conditions for reproduction. Siberian hamsters (Phodopus sungorus) primarily rely on photoperiod as a predictive cue of future energy availability. When raised in long-day photoperiods (>14 h light), supplemental cues such as food availability typically do not trigger the seasonal reproductive response of gonadal regression, which curtails reproduction in unsuitable environments. We investigated whether recognition of food availability as a cue could be altered by a nutritional challenge during development. Specifically, we predicted that hamsters receiving restricted food during development would be sensitized to food restriction (FR) as adults and undergo gonadal regression in response. Male and female hamsters were given either ad libitum (AL) food or FR from weaning until d60. The FR treatment predictably limited growth and delayed puberty in both sexes. For 5 weeks after d60, all hamsters received an AL diet to allow FR hamsters to gain mass equal to AL hamsters. Then, adult hamsters of both juvenile groups received either AL or FR for 6 weeks. Juvenile FR had lasting impacts on adult male body mass and food intake. Adult FR females exhibited decreased estrous cycling and uterine horn mass indiscriminately of juvenile food treatment, but there was little effect on male reproductive measurements. Overall, we observed a delay in puberty in response to postweaning FR, but this delay appeared not to affect seasonal reproductive responses in the long term. These findings increase our understanding of seasonal reproductive responses in a relevant environmental context.

Optogenetic Activation of Arcuate Kisspeptin Neurons Generates a Luteinizing Hormone Surge-Like Secretion in an Estradiol-Dependent Manner

Traditionally, the anteroventral periventricular (AVPV) nucleus has been the brain area associated with luteinizing hormone (LH) surge secretion in rodents. However, the role of the other population of hypothalamic kisspeptin neurons, in the arcuate nucleus (ARC), has been less well characterized with respect to surge generation. Previous experiments have demonstrated ARC kisspeptin knockdown reduced the amplitude of LH surges, indicating that they have a role in surge amplification. The present study used an optogenetic approach to selectively stimulate ARC kisspeptin neurons and examine the effect on LH surges in mice with different hormonal administrations. LH level was monitored from 13:00 to 21:00 h, at 30-minute intervals. Intact Kiss-Cre female mice showed increased LH secretion during the stimulation period in addition to displaying a spontaneous LH surge around the time of lights off. In ovariectomized Kiss-Cre mice, optogenetic stimulation was followed by a surge-like secretion of LH immediately after the stimulation period. Ovariectomized Kiss-Cre mice with a low dose of 17β-estradiol (OVX+E) replacement displayed a surge-like increase in LH release during period of optic stimulation. No LH response to the optic stimulation was observed in OVX+E mice on the day of estradiol benzoate (EB) treatment (day 1). However, after administration of progesterone (day 2), all OVX+E+EB+P mice exhibited an LH surge during optic stimulation. A spontaneous LH surge also occurred in these mice at the expected time. Taken together, these results help to affirm the fact that ARC kisspeptin may have a novel amplificatory role in LH surge production, which is dependent on the gonadal steroid milieu.

Endotoxin rapidly desensitizes the gonads to kisspeptin-induced luteinizing hormone release in male Siberian hamsters (Phodopus sungorus)

Activation of the immune system induces rapid reductions in hypothalamic-pituitary-gonadal (HPG) axis activity, which in turn decreases secretion of sex steroids. This response is likely adaptive for survival by temporarily inhibiting reproduction to conserve energy; however, the physiological mechanisms controlling this response remain unclear. The neuropeptide kisspeptin is a candidate to mediate the decrease in sex hormones seen during sickness through its key regulation of the HPG axis. In this study, the effects of acute immune activation on the response to kisspeptin were assessed in male Siberian hamsters (Phodopus sungorus). Specifically, an immune response was induced in animals by a single treatment of lipopolysaccharide (LPS), and reproductive hormone concentrations were determined in response to subsequent injections of exogenous kisspeptin. Saline-treated controls showed a robust increase in circulating testosterone in response to kisspeptin; however, this response was blocked in LPS-treated animals. Circulating luteinizing hormone (LH) levels were elevated in response to kisspeptin in both LPS- and saline-treated groups and, thus, were unaffected by LPS treatment, suggesting gonad-level inhibition of testosterone release despite central HPG activation. In addition, blockade of glucocorticoid receptors by mifepristone did not attenuate the LPS-induced inhibition of testosterone release, suggesting that circulating glucocorticoids do not mediate this phenomenon. Collectively, these findings reveal that acute endotoxin exposure rapidly renders the gonads less sensitive to HPG stimulation, thus effectively inhibiting sex hormone release. More broadly, these results shed light on the effects of immune activation on the HPG axis and help elucidate the mechanisms controlling energy allocation and reproduction.

Effects of exogenous leptin on seasonal reproductive responses to interacting environmental cues in female Siberian hamsters

Animals living in temperate climates respond to environmental cues that signal current and future resource availability to ensure that energy resources are available to support reproduction. Siberian hamsters (Phodopus sungorus) undergo robust gonadal regression in short, winter-like photoperiods as well as in response to mild food restriction in intermediate photoperiods. The goal of the present study was to investigate whether leptin is a relevant metabolic signal in regulating gonadal regression in response to diminishing food availability. Adult female hamsters housed in short-day (winter-like) or intermediate (fall-like) photoperiods received either ad libitum access to food or mild food restriction (90% of ad libitum intake) and were treated with either leptin or a vehicle for five weeks in order to determine the ability of leptin to inhibit gonadal regression. At the end of five weeks, vehicle-treated hamsters showed physiological signs associated with ongoing gonadal regression, such as decreases in body mass and food intake, cessation of estrous cycling, and small decreases in reproductive tissue mass. Leptin did not modify changes in body mass, food intake, hormone concentration, or tissue mass, but showed a tendency to support estrous cycling, particularly in response to food restriction in the intermediate photoperiod treatment. Overall, leptin appears to play a minor role in coordinating reproductive responses to multiple environmental cues, at least in the early stages of gonadal regression.

Effect of Day and Night Desflurane Anaesthesia on Melatonin Levels in Rats

OBJECTIVE

The aim of this study is to investigate the effect of day and night administration of desflurane anaesthesia on melatonin levels in rats.

METHODS

Twenty-four 15-day-old rats were included in the study and were divided into four groups. The rats were anaesthetised between 19:00-01:00 (night group) and 07:00-13:00 (day group) with 5.7% desflurane concentration in 6 L min^-1 100% oxygen. 6 L min^-1 oxygen was administered to the control groups. At the end of 6 h of anaesthesia, blood samples were taken, and rats were sacrificed. Blood samples were centrifuged and melatonin levels from plasma samples were measured with radioimmunoassay.

RESULTS

There was a statistically significant difference between the groups (p=0.007). Between group day control and group night control there was a statistically significant difference (p=0.042). Further, there was a significant difference between group day control and night desfluran as well (p=0.024). We could not find any difference between other groups.

CONCLUSION

This study showed that 6 hours of 5.7% desflurane anaesthesia during day and night hours did not significantly change melatonin levels.

Effect of arousing stimuli on circulating corticosterone and the circadian rhythms of luteinizing hormone (LH) surges and locomotor activity in estradiol-treated ovariectomized (ovx+EB) Syrian hamsters

In most proestrous hamsters, novel wheel exposure phase advances activity rhythms and blocks the preovulatory LH surge, which occurs 2h earlier the next day. Because wheel immobilization does not prevent these effects we hypothesized that arousal alone blocks and phase advances the LH surge. Ovariectomized (ovx) hamsters received a jugular vein cannula and estradiol benzoate (EB) or vehicle was injected sc. The next day (Day 1), at zeitgeber time (ZT) 4-5 (ZT 12 = lights off), after obtaining a blood sample, each hamster was exposed to constant darkness (DD), and either remained in her home cage or was transferred to a new cage and exposed to a running wheel or a 2-hour arousal paradigm. Blood samples were obtained in dim red light and activity was recorded hourly until ~ZT 10-11 on Days 1 and 2. For the next 1-2 weeks, activity was monitored in DD. Plasma LH and corticosterone were assessed by RIA. Novel wheel exposure or arousal at ZT 4 greatly attenuated the Day 1 LH surge in ovx+EB hamsters, and phase advanced the Day 2 LH surge by about 2h. In proestrous hamsters, novel wheel exposure led to a prolonged (>2h) increase in corticosterone levels only when LH surges were blocked. Phase advances in activity rhythms were enhanced by estradiol and arousal. The results suggest that estradiol modulates the effectiveness of non-photic stimuli. The role of the increased activity of the hypothalamic-pituitary-adrenal axis associated with novel wheel-induced attenuation of LH surges in ovx+EB hamsters remains to be determined.

Circadian rhythm disruption by a novel running wheel: roles of exercise and arousal in blockade of the luteinizing hormone surge

Exposure of proestrous Syrian hamsters to a new room, cage, and novel running wheel blocks the luteinizing hormone (LH) surge until the next day in ~75% of hamsters [1]. The studies described here tested the hypotheses that 1) exercise and/or 2) orexinergic neurotransmission mediate novel wheel blockade of the LH surge and circadian phase advances. Female hamsters were exposed to a 14L:10D photoperiod and activity rhythms were monitored with infra-red detectors. In Expt. 1, to test the effect of exercise, hamsters received jugular cannulae and on the next day, proestrus (Day 1), shortly before zeitgeber time 5 (ZT 5, 7h before lights-off) the hamsters were transported to the laboratory. After obtaining a blood sample at ZT 5, the hamsters were transferred to a new cage with a novel wheel that was either freely rotating (unlocked), or locked until ZT 9, and exposed to constant darkness (DD). Blood samples were collected hourly for 2days from ZT 5-11 under red light for determination of plasma LH levels by radioimmunoassay. Running rhythms were monitored continuously for the next 10-14days. The locked wheels were as effective as unlocked wheels in blocking LH surges (no Day 1 LH surge in 6/9 versus 8/8 hamsters, P>0.05) and phase advances in the activity rhythms did not differ between the groups (P=0.28), suggesting that intense exercise is not essential for novel wheel blockade and phase advance of the proestrous LH surge. Expt. 2 tested whether orexin neurotransmission is essential for these effects. Hamsters were treated the same as those in Expt. 1 except that they were injected (i.p.) at ZT 4.5 and 5 with either the orexin 1 receptor antagonist SB334867 (15mg/kg per injection) or vehicle (25% DMSO in 2-hydroxypropyl-beta-cyclodextrin (HCD)). SB-334867 inhibited novel wheel blockade of the LH surge (surges blocked in 2/6 SB334867-injected animals versus 16/18 vehicle-injected animals, P<0.02) and also inhibited wheel running and circadian phase shifts, indicating that activation of orexin 1 receptors is necessary for these effects. Expt. 3 tested the hypothesis that novel wheel exposure activates orexin neurons. Proestrous hamsters were transferred at ZT 5 to a nearby room within the animal facility and were exposed to a new cage with a locked or unlocked novel wheel or left in their home cages. At ZT 8, the hamsters were anesthetized, blood was withdrawn, they were perfused with fixative and brains were removed for immunohistochemical localization of Fos, GnRH, and orexin. Exposure to a wheel, whether locked or unlocked, suppressed circulating LH concentrations at ZT 8, decreased the proportion of Fos-activated GnRH neurons, and increased Fos-immunoreactive orexin cells. Unlocked wheels had greater effects than locked wheels on all three endpoints. Thus in a familiar environment, exercise potentiated the effect of the novel wheel on Fos expression because a locked wheel was not a sufficient stimulus to block the LH surge. In conclusion, these studies indicate that novel wheel exposure activates orexin neurons and that blockade of orexin 1 receptors prevents novel wheel blockade of the LH surge. These findings are consistent with a role for both exercise and arousal in mediating novel wheel blockade of the LH surge.

Sex differences in behavioral circadian rhythms in laboratory rodents

There is a strong bias in basic research on circadian rhythms toward the use of only male animals in studies. Furthermore, of the studies that use female subjects, many use only females and do not compare results between males and females. This review focuses on behavioral aspects of circadian rhythms that differ between the sexes. Differences exist in the timing of daily onset of activity, responses to both photic and non-photic stimuli, and in changes across the lifespan. These differences may reflect biologically important traits that are ecologically relevant and impact on a variety of responses to behavioral and physiological challenges. Overall, more work needs to be done to investigate differences between males and females as well as differences that are the result of hormonal changes across the lifespan.

2,2,2-Tribromoethanol phase-shifts the circadian rhythm of the liver clock in Per2::Luciferase knockin mice: lack of dependence on anesthetic activity

Comprehensive gene expression profiling in mice in response to the inhalation of sevoflurane has revealed that circadian clock gene expression is affected strongly in the liver, heart, lung, and kidney, in this order, but moderately in the spleen and slightly in the brain. Therefore, we examined whether the administration of general anesthetics at different times of the day induces phase shifts of the liver clock in Per2::Luciferase knockin mice. One to 4 days of intraperitoneal injection of 2,2,2-tribromoethanol (240 mg/kg, anesthetic time 60 min) or 2,2,2-trichloroethanol (240 mg/kg, 60 min), common anesthetics in veterinary surgery, caused phase delays when injected during the daytime and phase advances when injected during the nighttime. Inhalation administration of isoflurane for 30 or 60 min during the daytime did not induce a phase delay. Injection of propofol (300 mg/kg, 17 min) during the daytime induced an insignificant phase delay of the Per2 bioluminescence rhythm. Injection of 2,2,2-tribromoethanol did not induce a phase shift in the suprachiasmatic nucleus, the main oscillator, or in behavioral locomotor rhythms, suggesting that 2,2,2-tribromoethanol induced phase shifts of the liver clock independent of the main suprachiasmatic clock. The expression of clock genes, such as Bmal1 and Clock, in mouse liver was decreased strongly 1 and 4 h after a single injection of 2,2,2-tribromoethanol. These results demonstrate that 2,2,2-tribromoethanol or 2,2,2-trichloroethanol produce phase shifts of the peripheral clock, independent of anesthetic activity. These anesthetics may cause circadian rhythm disorders in peripheral organs when administered as general anesthetics several times during the day.

Clocks on top: the role of the circadian clock in the hypothalamic and pituitary regulation of endocrine physiology

Recent strides in circadian biology over the last several decades have allowed researchers new insight into how molecular circadian clocks influence the broader physiology of mammals. Elucidation of transcriptional feedback loops at the heart of endogenous circadian clocks has allowed for a deeper analysis of how timed cellular programs exert effects on multiple endocrine axes. While the full understanding of endogenous clocks is currently incomplete, recent work has re-evaluated prior findings with a new understanding of the involvement of these cellular oscillators, and how they may play a role in constructing rhythmic hormone synthesis, secretion, reception, and metabolism. This review addresses current research into how multiple circadian clocks in the hypothalamus and pituitary receive photic information from oscillators within the hypothalamic suprachiasmatic nucleus (SCN), and how resultant hypophysiotropic and pituitary hormone release is then temporally gated to produce an optimal result at the cognate target tissue. Special emphasis is placed not only on neural communication among the SCN and other hypothalamic nuclei, but also how endogenous clocks within the endocrine hypothalamus and pituitary may modulate local hormone synthesis and secretion in response to SCN cues. Through evaluation of a larger body of research into the impact of circadian biology on endocrinology, we can develop a greater appreciation into the importance of timing in endocrine systems, and how understanding of these endogenous rhythms can aid in constructing appropriate therapeutic treatments for a variety of endocrinopathies.

Novel wheel running blocks the preovulatory luteinizing hormone surge and advances the hamster circadian pacemaker

In rodents, the preovulatory luteinizing hormone (LH) surge is timed by a circadian rhythm. We recently reported that a phenobarbital-induced delay of the estrous cycle in Syrian hamsters is associated with an approximately 2-h phase advance in both the circadian locomotor activity rhythm and the timing of the LH surge. The following study tests the hypothesis that a >2-h nonpharmacological phase advance in the circadian pacemaker that delays the estrous cycle by a day will also phase advance the LH surge by approximately 2 h. Activity rhythms were continuously monitored in regularly cycling hamsters using running wheels or infrared detectors for about 10 days prior to jugular cannulation. The next day, on proestrus, hamsters were transferred to the laboratory for 1 of 3 treatments: transfer to a "new cage" (and wheel) from zeitgeber time (ZT) 4 to 8 (with ZT12 defined as time of lights-off), or exposure to a "novel wheel" at ZT5 or ZT1. All animals were then placed in constant dark (DD). Blood samples were obtained just before onset of DD and hourly for the next 6 h, on that day and the next day for determination of plasma LH concentrations. Running activity was monitored in DD for about 10 more days. Transfer to a novel wheel at either ZT5 or ZT1 delayed the LH surge to day 2 in most hamsters, whereas exposure to a new cage did not. Only the delayed LH surges were phase advanced at least 2.5 h on average in all 3 groups. However, wheel-running activity was similarly phase advanced in all 3 groups regardless of the timing of the LH surge; thus, the phase advances in circadian activity rhythms were not associated with the 1-day delay of the LH surge. Interestingly, the number of wheel revolutions was closely associated with the 1-day delay of LH surges following exposure to a novel wheel at either ZT1 or ZT5. These results suggest that the intensity of wheel running (or an associated stimulus) plays an important role in the circadian timing mechanism for the LH surge.