Circadian organization and neural mediation of hamster reproductive rhythms

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Mistimed restricted feeding disrupts circadian rhythms of male mating behavior and female preovulatory LH surges in mice

In rodents, eating at atypical circadian times, such as during the biological rest phase when feeding is normally minimal, reduces fertility. Prior findings suggest this fertility impairment is due, at least in part, to reduced mating success. However, the physiological and behavioral mechanisms underlying this reproductive suppression are not known. In the present study, we tested the hypothesis that mistimed feeding-induced infertility is due to a disruption in the normal circadian timing of mating behavior and/or the generation of pre-ovulatory luteinizing hormone (LH) surges (estrogen positive feedback). In the first experiment, male+female mouse pairs, acclimated to be food restricted to either the light (mistimed feeding) or dark (control feeding) phase, were scored for mounting frequency and ejaculations over 96 h. Male mounting behavior and ejaculations were distributed much more widely across the day in light-fed mice than in dark-fed controls and fewer light-fed males ejaculated. In the second experiment, the timing of the LH surge, a well characterized circadian event driven by estradiol (E2) and the SCN, was analyzed from serial blood samples taken from ovariectomized and E2-primed female mice that were light-, dark-, or ad-lib-fed. LH concentrations peaked 2 h after lights-off in both dark-fed and ad-lib control females, as expected, but not in light-fed females. Instead, the normally clustered LH surges were distributed widely with high inter-mouse variability in the light-fed group. These data indicate that mistimed feeding disrupts the temporal control of the neural processes underlying both ovulation and mating behavior, contributing to infertility.

Sex differences in the response to circadian disruption in diurnal sand rats

Most animal model studies on physiological functions and pathologies are conducted in males. However, diseases such as depression, type 2 diabetes (T2DM) and cardiovascular disease, all show different prevalence and characteristics in females and males. Moreover, most mammal studies are conducted in nocturnal mice and rats, while modelling diurnal humans. We therefore used male and female fat sand rats (Psammomys obesus), which are diurnal in the wild, as an animal model for T2DM, to explore the effects of mild circadian disruption on behavior, glucose tolerance, cholesterol and heart weight. We found significant differences between the sexes: on average, in response to short photoperiods (SP) acclimation, males showed higher levels of depression-like behavior, lower glucose tolerance, and increased plasma cholesterol levels compared with females, with no effect on heart/body weight ratio. Females, however did show an increase in heart/body weight ratio in response to SP acclimation. We also found that regardless of sex, arrhythmic animals showed higher blood glucose levels, cholesterol levels, heart/body weight ratio, and depressive-like behavior compared with rhythmic animals. Hence, we suggest that the expression of the Circadian Syndrome could be different between males and females. Additional work with females is required to clearly delineate the specific effects in both sexes, and promote sex-based health care, prevention measures and therapies.

Rapid Jetlag Resetting of Behavioral, Physiological, and Molecular Rhythms in Proestrous Female Mice

A gradual adaptation to a shifted light-dark (LD) cycle is a key element of the circadian clock system and believed to be controlled by the central circadian pacemaker, the suprachiasmatic nucleus (SCN). Endocrine factors have a strong influence on the regulation of the circadian clock network and alter acute photic responses of the SCN clock. In females, endocrine function depends on the stage of the ovarian cycle. So far, however, little is known about the effect of the estrous cycle on behavioral and molecular responses to shifts in the LD rhythm. Based on this, we investigated whether estrous state affects the kinetics of phase shift during jetlag in behavior, physiology, and molecular clock rhythms in the SCN and in peripheral tissues. Female mice exposed to an advanced LD phase at proestrous or metestrous showed different phase-shift kinetics, with proestrous females displaying accelerated adaptation in behavior and physiology. Constant darkness release experiments suggest that these fast phase shifts do not reflect resetting of the SCN pacemaker. Explant experiments on SCN, adrenal gland, and uterus confirmed this finding with proestrous females showing significantly faster clock phase shifts in peripheral tissues compared with the SCN. Together, these findings provide strong evidence for an accelerated adaptation of proestrous compared with metestrous females to new LD conditions that is accompanied by rapid behavioral, physiological, and molecular rhythm resetting. Not only do these findings open up a new avenue to understand the effect of estrous cycle on the clock network under changing environmental conditions but also imply a greater susceptibility in proestrous females.

Comprehensive Analysis of Differentially Expressed Profiles of mRNA, lncRNA, and circRNA in the Uterus of Seasonal Reproduction Sheep

Photoperiod is one of the important factors leading to seasonal reproduction of sheep. However, the molecular mechanisms underlying the photoperiod regulation of seasonal reproduction remain poorly understood. In this study, we compared the expression profiles of mRNAs, lncRNAs, and circRNAs in uterine tissues from Sunite sheep during three different photoperiods, namely, the short photoperiod (SP), short transfer to long photoperiod (SLP), and long photoperiod (LP). The results showed that 298, 403, and 378 differentially expressed (DE) mRNAs, 171, 491, and 499 DE lncRNAs, and 124, 270, and 400 DE circRNAs were identified between SP and LP, between SP and SLP, and between LP and SLP, respectively. Furthermore, functional enrichment analysis showed that the differentially expressed RNAs were mainly involved in the GnRH signaling pathway, thyroid hormone synthesis, and thyroid hormone signaling pathway. In addition, co-expression networks of lncRNA–mRNA were constructed based on the correlation analysis between the differentially expressed RNAs. Our study provides new insights into the expression changes of RNAs in different photoperiods, which might contribute to understanding the molecular mechanisms of seasonal reproduction in sheep.

Circadian disruption of food availability significantly reduces reproductive success in mice

Circadian disruptions impair reproductive health in human populations and in animal models. We tested the hypothesis that mistimed food, a common disruptive feature of shift work, impairs reproductive success in mice. Male and female mPer2^Luc mice on a C57BL/6 background were fed during the light or dark phase in two experiments. Food-induced internal misalignment of the liver clock was verified by in vivo bioluminescence in anesthetized mice in both experiments. In Experiment 1, food-restricted pairs were monitored for litters for 18 weeks. In the light-fed group, birth of the first litter was significantly delayed, and total reproductive output was significantly reduced by 38%. In Experiment 2, estrous cycling was monitored for 3 weeks, and then after pairing, copulatory plugs, pregnancy, litter sizes, and uterine implantation sites were measured. Fewer light-fed females birthed litters (25% versus 73%). This was attributable to a difference in behavior as mating success was significantly reduced in light-fed mice: 42% were observed with a copulatory plug compared to 82% for dark-fed mice. The proportion of mice displaying uterine implantation sites was the same as the proportion observed with copulatory plugs, suggesting no deficit in initiating pregnancy after mating. Estrous cycling and pregnancy maintenance did not differ between the groups. We conclude that mistimed feeding inhibits reproduction in mice by reducing successful mating behavior.

Effects of Stimulated Physical Activity on the Circadian Pacemaker of Vertebrates 1

A dogma in the field of circadian rhythms is that in order to keep accurate time, pacemakers that generate such rhythms must be relatively independent of changes in the external and internal environment. While it is true that the period of circadian oscillators is conserved within a narrow range, regardless of alterations in the external and internal envi ronment, numerous perturbations have now been found that can change the period and/or induce a phase shift in circadian pacemakers. Many of these perturbations also alter the overall level of activity and/or metabolic state of the organism. In 1960, Aschoff suggested that alterations in the "level of excitement" may induce changes in circadian clocks. Although little attention has been given to this hypothesis over the past three decades, recent findings support its validity and open new avenues for studying the function and organization of circadian clock systems.

Molecular basis for regulating seasonal reproduction in vertebrates

Animals that inhabit mid- to high-latitude regions exhibit various adaptive behaviors, such as migration, reproduction, molting and hibernation in response to seasonal cues. These adaptive behaviors are tightly regulated by seasonal changes in photoperiod, the relative day length vs night length. Recently, the regulatory pathway of seasonal reproduction has been elucidated using quail. In birds, deep brain photoreceptors receive and transmit light information to the pars tuberalis in the pituitary gland, which induces the secretion of thyroid-stimulating hormone. Thyroid-stimulating hormone locally activates thyroid hormone via induction of type 2 deiodinase in the mediobasal hypothalamus. Thyroid hormone then induces morphological changes in the terminals of neurons that express gonadotropin-releasing hormone and facilitates gonadotropin secretion from the pituitary gland. In mammals, light information is received by photoreceptors in the retina and neurally transmitted to the pineal gland, where it inhibits the synthesis and secretion of melatonin, which is crucial for seasonal reproduction. Importantly, the signaling pathway downstream of light detection and signaling is fully conserved between mammals and birds. In fish, the regulatory components of seasonal reproduction are integrated, from light detection to neuroendocrine output, in a fish-specific organ called the saccus vasculosus. Various physiological processes in humans are also influenced by seasonal environmental changes. The findings discussed herein may provide clues to addressing human diseases, such as seasonal affective disorder.

Suprachiasmatic nucleus as the site of androgen action on circadian rhythms

Androgens act widely in the body in both central and peripheral sites. Prior studies indicate that in the mouse, suprachiasmatic nucleus (SCN) cells bear androgen receptors (ARs). The SCN of the hypothalamus in mammals is the locus of a brain clock that regulates circadian rhythms in physiology and behavior. Gonadectomy results in reduced AR expression in the SCN and in marked lengthening of the period of free-running activity rhythms. Both responses are restored by systemic administration of androgens, but the site of action remains unknown. Our goal was to determine whether intracranial androgen implants targeted to the SCN are sufficient to restore the characteristic free-running period in gonadectomized male mice. The results indicate that hypothalamic implants of testosterone propionate in or very near the SCN produce both anatomical and behavioral effects, namely increased AR expression in the SCN and restored period of free-running locomotor activity. The effect of the implant on the period of the free-running locomotor rhythm is positively correlated with the amount of AR expression in the SCN. There is no such correlation of period change with amount of AR expression in other brain regions examined, namely the preoptic area, bed nucleus of the stria terminalis and premammillary nucleus. We conclude that the SCN is the site of action of androgen effects on the period of circadian activity rhythmicity.

ESR1 and ESR2 differentially regulate daily and circadian activity rhythms in female mice

Estrogenic signaling shapes and modifies daily and circadian rhythms, the disruption of which has been implicated in psychiatric, neurologic, cardiovascular, and metabolic disease, among others. However, the activational mechanisms contributing to these effects remain poorly characterized. To determine the activational impact of estrogen on daily behavior patterns and differentiate between the contributions of the estrogen receptors ESR1 and ESR2, ovariectomized adult female mice were administered estradiol, the ESR1 agonist propylpyrazole triol, the ESR2 agonist diarylpropionitrile, or cholesterol (control). Animals were singly housed with running wheels in a 12-hour light, 12-hour dark cycle or total darkness. Estradiol increased total activity and amplitude, consolidated activity to the dark phase, delayed the time of peak activity (acrophase of wheel running), advanced the time of activity onset, and shortened the free running period (τ), but did not alter the duration of activity (α). Importantly, activation of ESR1 or ESR2 differentially impacted daily and circadian rhythms. ESR1 stimulation increased total wheel running and amplitude and reduced the proportion of activity in the light vs the dark. Conversely, ESR2 activation modified the distribution of activity across the day, delayed acrophase of wheel running, and advanced the time of activity onset. Interestingly, τ was shortened by estradiol or either estrogen receptor agonist. Finally, estradiol-treated animals administered a light pulse in the early subjective night, but no other time, had an attenuated response compared with controls. This decreased phase response was mirrored by animals treated with diarylpropionitrile, but not propylpyrazole triol. To conclude, estradiol has strong activational effects on the temporal patterning and expression of daily and circadian behavior, and these effects are due to distinct mechanisms elicited by ESR1 and ESR2 activation.

Pet-1 deficiency alters the circadian clock and its temporal organization of behavior

The serotonin and circadian systems are two important interactive regulatory networks in the mammalian brain that regulate behavior and physiology in ways that are known to impact human mental health. Previous work on the interaction between these two systems suggests that serotonin modulates photic input to the central circadian clock (the suprachiasmatic nuclei; SCN) from the retina and serves as a signal for locomotor activity, novelty, and arousal to shift the SCN clock, but effects of disruption of serotonergic signaling from the raphe nuclei on circadian behavior and on SCN function are not fully characterized. In this study, we examined the effects on diurnal and circadian behavior, and on ex vivo molecular rhythms of the SCN, of genetic deficiency in Pet-1, an ETS transcription factor that is necessary to establish and maintain the serotonergic phenotype of raphe neurons. Pet-1^−/− mice exhibit loss of rhythmic behavioral coherence and an extended daily activity duration, as well as changes in the molecular rhythms expressed by the clock, such that ex vivo SCN from Pet-1^−/− mice exhibit period lengthening and sex-dependent changes in rhythmic amplitude. Together, our results indicate that Pet-1 regulation of raphe neuron serotonin phenotype contributes to the period, precision and light/dark partitioning of locomotor behavioral rhythms by the circadian clock through direct actions on the SCN clock itself, as well as through non-clock effects.

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.

Shift work, jet lag, and female reproduction

Circadian rhythms and "clock gene" expression are involved in successful reproductive cycles, mating, and pregnancy. Alterations or disruptions of biological rhythms, as commonly occurs in shift work, jet lag, sleep deprivation, or clock gene knock out models, are linked to significant disruptions in reproductive function. These impairments include altered hormonal secretion patterns, reduced conception rates, increased miscarriage rates and an increased risk of breast cancer. Female health may be particularly susceptible to the impact of desynchronizing work schedules as perturbed hormonal rhythms can further influence the expression patterns of clock genes. Estrogen modifies clock gene expression in the uterus, ovaries, and suprachiasmatic nucleus, the site of the primary circadian clock mechanism. Further work investigating clock genes, light exposure, ovarian hormones, and reproductive function will be critical for indentifying how these factors interact to impact health and susceptibility to disease.

Circadian rhythms and photoperiodism

The circadian timing system plays a critical role in the regulation of seasonal modifications in reproductive function. By detecting and transducing changes in the day-length (photoperiod), the neural substrates of the circadian system, including the supra-chiasmatic nuclei of the hypothalamus, trigger reproductive activity or quiescence at the appropriate seasons of the year in photoperiodic species. The circadian system also plays a role in the expression of endocrine changes that occur with seasonal breeding. Surges in luteinizing hormone secretion in female hamsters, for example, are either expressed daily during reproductive quiescence or suppressed on three out of the four days of the cycle during the breeding season. By such mechanisms a daily timer can be used in the regulation of cyclic events of much longer period.

Minireview: The neuroendocrinology of the suprachiasmatic nucleus as a conductor of body time in mammals

Circadian rhythms in physiology and behavior are regulated by a master clock resident in the suprachiasmatic nucleus (SCN) of the hypothalamus, and dysfunctions in the circadian system can lead to serious health effects. This paper reviews the organization of the SCN as the brain clock, how it regulates gonadal hormone secretion, and how androgens modulate aspects of circadian behavior known to be regulated by the SCN. We show that androgen receptors are restricted to a core SCN region that receives photic input as well as afferents from arousal systems in the brain. We suggest that androgens modulate circadian behavior directly via actions on the SCN and that both androgens and estrogens modulate circadian rhythms through an indirect route, by affecting overall activity and arousal levels. Thus, this system has multiple levels of regulation; the SCN regulates circadian rhythms in gonadal hormone secretion, and hormones feed back to influence SCN functions.

Estradiol enhances light-induced expression of transcription factors in the SCN

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master clock that regulates circadian and seasonal rhythms. Among these, the SCN regulates the phasic release of hormones and provides for the timing of the preovulatory luteinizing hormone (LH) surge necessary for ovulation in females. There is little evidence, however, of sex hormone effects on mechanisms underlying SCN function. This study examined the effects of exogenous administration of estradiol on the light-induced expression of transcription factors in the SCN of female rats. Ovariectomized (OVX) female rats were given estradiol or cholesterol implants and perfused 48 h later. Half of the animals were sacrificed 1 h after the regular onset of light within the colony. The rest had the lights go on 2 h prior to the regular time and perfused 1 h later. Collected brains were sliced and sets of SCN sections were processed for immunoreactivity (ir) detecting the Fos, pCREB, egr-1, CREB binding protein (CBP), and calbindin-D (28K) proteins. Following quantification, statistical analyses demonstrated that estradiol enhanced Fos and p-CREB-ir in the SCN of females that experienced a 2-h phase advance. The phase advance also enhanced calbindin and egr-1-ir, but the expression of these proteins was not affected by estradiol. These results demonstrate that estradiol enhances the levels of transcription factors that precede the expression of clock gene proteins in the SCN in response to advances in the onset of environmental light. These data support the hypothesis that steroid hormones play an important role in the fine tuning of the clock in the face of environmental changes in daylight.

Projections of the suprachiasmatic nuclei, subparaventricular zone and retrochiasmatic area in the golden hamster

The patterns of projections from the hamster suprachiasmatic nucleus, retrochiasmatic area and subpraventricular hypothalamic zone were examined using anterograde tracing with the plant lectin, Phaseolus vulgaris leucoagglutinin. Suprachiasmatic nucleus efferents comprise four major fiber groups: (i) an anterior projection to the ventral lateral septum, the bed nucleus of the stria terminalis and anterior paraventricular thalmus; (ii) a periventricular hypothalamic projection extending from the preoptic region to the premammillary area; (iii) a lateral thalamic projection to the intergeniculate leaflet and ventral lateral geniculate; and (iv) a posterior projection to the posterior paraventricular thalamus, precommissural nucleus and olivary pretectal nucleus. The retrochiasmatic area showed a similar projection pattern with several major exceptions. There are projections to endopiriform cortex, fundus striati, ventral pallidum, horizontal limb of the nucleus of the diagonal band and three separate routes to the amygdala. There are also projections laterally with fibers of the supraoptic commissures, which enter the superior thalamic radiation and innervate the caudal dorsomedial thalamic nuclei. Other fibers traveling with the commissures terminate in the ventral zona incerta. The subparaventricular zone projects to most targets of the suprachiasmatic nucleus, but not to the intergeniculate leaflet. There is a substantial input to both the subparaventricular zone and retrochiasmatic area from the suprachiasmatic nucleus, but little apparent reciprocity. There is extensive overlap of suprachiasmatic nuclei and retrochiasmatic efferents, and between retrochiasmatic and known medial amygdaloid efferents. The anatomical information is discussed in the context of circadian rhythm regulation, photoperiodism and chemosensory pathways controlling male hamster reproductive behavior.

Circadian rhythms

Changes in behavior which occur on a daily or circadian basis represent one of the most ubiquitous strategies by which most living organisms have adapted to their environment. Underlying the daily changes in behavior are a multitude of endocrine and metabolic rhythms which provide adaptively significant temporal organization within the organism. In mammals there appears to be a central circadian clock in the SCN which is responsible for generating and coordinating the entire 24-hour temporal organization of the animal. The circadian clock regulates the timing, duration, and characteristics of sleep, and together the circadian clock and sleep interact to control the timing of endocrine secretions. While the impact of disturbed endocrine circadian rhythms for the survival of the species has received very little attention, the almost universal presence of circadian rhythmicity within the endocrine system argues in support of the hypothesis that a disruption of the normal circadian organization within the endocrine system can have serious consequences for the health and well-being of the organism. It is particularly noteworthy that in advanced age, various alterations in circadian endocrine rhythms have been observed and these alterations may impair the ability of the animal to adapt normally to the environment. Relatively speaking, the study of circadian rhythms is a new field of biology, and as a result, much remains to be discovered about the physiological mechanisms that underlie rhythmicity, as well as the functional significance of 24-hour temporal organization for the survival of the species.

The effects of short day exposure on seasonal and circadian reproductive rhythms in male golden hamsters

Male golden hamsters were exposed to long (LD 14:10) or short (LD 10:14) photoperiods (Groups LP and SP, respectively) and tested 4 times over a 30-week period. At each test time, animals were tested twice, once in their subjective night and once in their subjective day. During each test animals were observed for approach and copulatory behaviors in response to hormonally primed receptive females. To determine gonadal condition, changes in testes size and sperm production over weeks in SP or LP condition were also measured. Results show that SP conditions induce gonadal regression (at 9 weeks) and recrudescence (at 17 weeks); gonadal function was fully restored by 21 weeks in SP. This pattern is mirrored by a decline in copulatory behaviors during regression, followed by a resumption of sexual behaviors during gonadal recrudescence. In contrast, approach measures showed an inverse pattern; males showed the highest level of approach behaviors at week 13, during gonadal quiescence. Short day conditions also induce changes in the circadian patterning of copulatory behaviors: whereas LP animals always showed more mounts, intromissions and ejaculations in the dark than in the light, between 1 to 13 weeks in short day conditions. SP animals show equal levels of copulatory behavior in the dark and in the light. At the time of gonadal recrudescence, SP animals start to show circadian patterns of sexual behaviors comparable to LP animals. These differences were not found for approach behaviors. The relationship between the different behavioral systems and physiological measures, and the effects on these of short day exposure are discussed.

Alterations in hypothalamic serotonin metabolism in male hamsters with photoperiod-induced testicular regression

Transfer of adult male golden hamsters to a short photoperiod (SD; 8L:16D) resulted in a significant decrease in testicular weight and plasma luteinizing hormone (LH) and prolactin (Prl) levels as compared to animals maintained in a long photoperiod (LD; 16L:8D). These changes were accompanied by significant increases in serotonin (5-HT) synthesis in the medial basal hypothalamus (MBH) as determined by measuring 5-hydroxytryptophan accumulation after inhibition of aromatic amino acid decarboxylase with NSD-1015. Serotonin synthesis in the median eminence, anterior hypothalamus and olfactory bulbs was not different in SD vs LD animals. Transfer of SD gonadally regressed hamsters to LD restored MBH 5-HT synthesis to control levels.

Pinealectomy prevents short photoperiod inhibition of male hamster sexual behavior

The role of the pineal gland in mediating photoperiodic influences on copulatory behavior (CB) of male hamsters (Mesocricetus auratus) was assessed in the presence and absence of testosterone (T). The results demonstrate that the pineal gland is necessary for short photoperiod exposure to alter CB. Sexually experienced males were exposed to either long (14L:10D; LP) or short (8L:16D; SP) photoperiods for 13 weeks; after the first 2 weeks of exposure, all animals were castrated and then either pinealectomized (PINX) or sham operated (SHAM PINX). CB tests over an 8-week period following surgery indicated that copulatory impairments developed in all animals, but deficits occurred more rapidly among short photoperiod males with intact pineal glands (SP-SHAM PINX), compared to pinealectomized males housed in either the long (LP-PINX) or short photoperiod (SP-PINX). LP-PINX and SP-PINX animals were not statistically different on any of the CB measures examined. Nine weeks after castration (11 weeks of photoperiod exposure), all hamsters were given a T-filled Silastic capsule to restore CB. Restoration of sexual behavior was less rapid and less complete among SP-SHAM PINX hamsters. Additionally, males in this group took longer to initiate copulation relative to the pinealectomized hamsters. These findings are compared to other reports suggesting that photoperiodic effects on the sexual behavior of female hamsters do not require an intact pineal gland.

Short photoperiods affect male hamster sociosexual behaviors in the presence and absence of testosterone

Male hamsters were exposed to long (LD 14:10) or short (LD 8:16) photoperiods (LP; SP) to evaluate the effects of these environmental conditions on sociosexual behaviors. In Experiment 1, gonadally intact males in SP exhibited deficits in sexual behavior, reflected both in performance as well as initiation measures. Some aspects of the males' chemoinvestigation of females or their odors were also significantly different between LP and SP hamsters. In Experiment 2, castration resulted in the development of copulatory impairments, but they occurred more rapidly among males in SP conditions. Subsequent testosterone (T) replacement restored mounts, intromissions and ejaculations on tests given 2 and 4 weeks after T, but this happened more quickly in the LP group. SP males were still slower than LP males to initiate mounts and intromissions on their second test. These influences of photoperiod are discussed in the context of steroid-independent and steroid-dependent effects on behavior and the role of impaired processing of chemosensory information is evaluated.

Response of circadian locomotor activity and the proestrous luteinizing hormone surge to phase shifts of the light-dark cycle in the hamster

Female hamsters with regular 4 day estrous cycles were exposed to either a 3 hour phase advance or delay of the 14:10 light-dark (LD) cycle on the first, second or third day before proestrus. Blood samples were taken on proestrus to characterize the LH surge, and locomotor activity onset was recorded. Both the LH surge and activity onset phase delayed more quickly than they advanced, which can be explained by the free-running period of the hamster (longer than 24 hours). Higher estradiol levels were correlated with more rapid advances of activity onset.

Seasonal differences in testicular receptors and steroidogenesis

Gonadal function in most animal species exhibits considerable annual fluctuations, with gametogenesis and fertility often being confined to a short and rigidly controlled breeding season. In males, production of androgenic steroids by the testis is usually maximal immediately before and during the breeding season. In the golden hamster, seasonal regression of the testes is associated with decrease in the total content of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) receptors, and similar findings have been reported for other mammalian species. However, the concentration of LH and FSH receptors per unit of testis weight is typically elevated rather than suppressed during testicular regression. Reduction in the number of testicular LH and PRL receptors in adult golden hamsters exposed to short photoperiod is due primarily to suppression of pituitary PRL release under these circumstances. Regulation of seasonal changes in testicular FSH binding, as well as regulation of the levels of LH, PRL and FSH receptors in other seasonally breeding species remain to be elucidated. Reduction in the content of LH receptors in the testes is accompanied by reduced capacity to produce androgens in response to LH stimulation. Although these events are likely to be causally related, other mechanisms are also involved. In particular, seasonal regression is accompanied by reduced capacity of the testes to convert C21 steroid precursors into biologically active androgens. Seasonal loss of FSH receptors was reported to be accompanied by increased rather than reduced responsiveness of the Sertoli cells to FSH, thus resembling the situation in immature animals. It can be concluded that alterations in the ability of the testes to bind pituitary gonadotropins and to respond to gonadotropic stimulation are among the mechanisms responsible for seasonal shifts between gonadal activity and quiescence.

The effect of photoperiod on development of sexual behavior and fertility in golden hamsters

Male hamsters were raised on short (SD) or long (LD) photoperiods from birth, and the expression of sexual behavior at 6, 7, 10, and 14 weeks was compared. Neither mounting, intromissions, ejaculations, nor ultrasonic vocalizations differed in the two groups until 14 weeks of age, when these behaviors were performed more often by LD males. Sperm appeared in daily penile smears at an earlier age in SD than LD males, but LD males had larger flank glands and testes beginning at 6 and 7 weeks, respectively. Female siblings of the males in this study were mated around 7 weeks of age to adult males. Almost all females on both photoperiods conceived normally, and greater than 90% of the offspring delivered survived to weaning in both groups. Together, these results show that hamsters reared on SD are behaviorally and physiologically capable of reproducing for a period of time after puberty. The possibility that social influences or environmental factors other than photoperiod act on the juvenile hamster to retard reproductive development is discussed.

Role of prolactin in the regulation of sensitivity of the hypothalamic-pituitary system to steroid feedback

During sexual maturation, pituitary gonadotropins stimulate the gonads to produce increasing amounts of biologically active steroids and yet gonadotropin release does not become suppressed until concentrations of sex hormones, LH and FSH, in peripheral circulation stabilizes at a higher adult level. There is a substantial amount of evidence that in many mammals, this transition from prepubertal to adult level of activity of the pituitary-gonadal axis is associated with a reduction in the sensitivity of the hypothalamic-adenohypophyseal system to negative feedback of gonadal steroids. In the female, these changes are accompanied by the appearance of positive estrogen feedback on gonadotropin release. In seasonal breeders, annual transitions between the periods of gonadal activity and quiescence are associated with corresponding shifts in the sensitivity to steroid feedback. Peripheral levels of pituitary prolactin (PRL) typically increase during sexual maturation and exhibit large seasonal fluctuations in response to changes in photoperiod and ambient temperature. We propose that PRL is one of the factors which regulate the sensitivity of gonadotropin release to gonadal steroid feedback. In hyperprolactinemic women, responsiveness to negative estrogen feedback increases, while LH response to positive estrogen feedback is reduced or absent. In hyperprolactinemic men, both LH and testosterone levels are reduced, implying increased sensitivity of LH release to negative testosterone feedback. In the male rat, both physiological amounts of PRL and experimentally-induced hyperprolactinemia increase the ability of exogenous testosterone to suppress LH and FSH release. Different regulatory mechanisms appear to operate in the seasonally breeding male golden hamster, in which short photoperiod causes concomitant suppression of PRL, LH, FSH and testosterone release. In this species, pharmacologic suppression of PRL release leads to increased responsiveness of plasma gonadotropin levels to negative feedback effects of testosterone, while PRL-secreting ectopic pituitary transplants exert an opposite effect. We have examined some of the suspected mechanisms of PRL modulation of testosterone feedback in male golden hamsters. In immature animals, the amount of cytoplasmic androgen receptors in the anterior pituitary was decreased by mild hyperprolactinemia and increased by treatment with bromocriptine, an inhibitor of PRL release. Bromocriptine increased pituitary androgen binding also in adult hamsters. These findings would imply that PRL modulates the responsiveness to negative steroid feedback at the pituitary level.(ABSTRACT TRUNCATED AT 400 WORDS)

Retarded growth of the suprachiasmatic nucleus and pineal body in dw and lit dwarf mice

The suprachiasmatic nucleus (SCN) and the pineal body in 3 types of inherited hormone-deficient mice, the dw, lit and hyt mice were examined by morphological, morphometric and biochemical techniques. In the dw and lit mice the SCN was underdeveloped. In the ventral part of the SCN, where most of the retinal fibers appeared to terminate, both cell number and cell size were decreased, although the size of the SCN was unaltered. In addition, the pineal bodies of both mice were morphologically underdeveloped and showed low levels of N-acetyltransferase activity. In contrast, the hyt SCN was comparable to the normal controls in every respect. The hyt pineal was well developed and showed levels of enzyme activity comparable to the controls. However, in all the deficient mice, the optic nerve appeared to be normal in morphological and biochemical studies. These results suggest that the underdevelopment of the pineal body, the reduced levels of spontaneous locomotion and the indistinct diurnal periodicity of the dw and lit mice might be related to the retarded neuronal growth of the SCN, and that growth hormone likely is indispensable for the development of the SCN.

Effects of estrous cycles and ovarian steroids on body weight and energy expenditure in Syrian hamsters

In Experiment 1, highly significant changes were observed over the estrous cycle in body weight gain, but not in food intake, daytime resting oxygen consumption or brown fat thermogenesis in Syrian hamsters. In Experiments 2 and 3, body weight and composition, food intake, resting oxygen consumption, and brown fat thermogenesis were measured following estradiol or estradiol plus progesterone treatment in ovariectomized hamsters. The significant changes in body weight could not be explained by changes in food intake, and were not accompanied by significant alterations in daytime oxygen consumption or brown fat thermogenic activity. In Experiment 4, resting oxygen consumption and body weight were measured every 6 hours over the estrous cycle. There was a striking absence of the usual nocturnal peak in resting oxygen consumption on the night of estrus (the night of the largest body weight gain). However, brown fat thermogenic activity did not differ among groups of hamsters killed on different nights of the estrous cycle. Estradiol-induced changes in energy storage may be mediated by changes in the daily rhythm of energy expenditure which are not dependent on alterations in brown fat thermogenesis.

Effects of melatonin implants on structures and behaviors of the house finch (Carpodacus mexicanus) eye

This study aimed to determine the extraretinal effects of melatonin upon the eyes of an avian species, the House Finch (Carpodacus mexicanus). Twelve birds (full-grown, second-year males) each received a Silastic tubing intraperitoneal implant, six containing melatonin (average release = 24 micrograms/d/bird; = M birds) and six being empty (= C birds). Microscopic study of pupillary and palpebral behaviors during the final week demonstrated lesser pupillary diameters and interpalpebral distances in M birds under all test conditions. These effects could have diminished mean light levels reaching parts of the retina. Characteristics of the relative miosis and ptosis of M birds resemble signs in some CNS disorders, such as altered inhibition of the Edinger-Westphal nucleus, and especially lesions in, or lowered activity of, higher sympathetic centers (a subtype of Horner's syndrome). Weights of eyes and their parts were the same in M and C birds, contrasting with previously reported results from male Golden Hamsters, possibly due to species differences and/or preexperimental attainment of full growth in the finches. Effects of melatonin on pupillary and palpebral behaviors, demonstrated here for the first time, foster caveats for simplistic experimental designs and interpretations with melatonin when sensory-neural-behavioral interactions are affected. Quantitative changes in pupillary and palpebral behaviors may, nevertheless, provide a window for monitoring central actions of melatonin in living test subjects in chronic studies.

Suprachiasmatic neurons in tissue slices from ovariectomized rats: electrophysiological and neuropharmacological characterization and the effects of estrogen treatment

Single-unit activity and unit responses to putative neurotransmitters were recorded from suprachiasmatic nucleus (SCN) neurons in brain tissue slices from ovariectomized rats either treated or untreated with estrogen. Altogether, 204 units were studied from estrogen-treated and untreated preparations, and at the resting state, 37% of these units fired regularly, 57% fired irregularly, and 6% were silent but evokable by electrical stimulation. Most of the irregular units fired continuously (n = 100), while the rest fired intermittently (n = 12) or phasically (n = 4). Neurons with different types of firing patterns also varied significantly in resting firing rate and in responsiveness to transmitters and to estrogen treatment. The average resting firing rate decreased significantly from regular, irregular and continuous, intermittent, to silent units. Acetylcholine (ACh) and/or serotonin (5-HT) injected directly into the perfusion chamber evoked responses from more irregular (69% of 61 units) than regular units (20% of 46 units). None of the 5 silent units tested was activated by ACh or 5-HT. Responses to ACh (predominantly inhibitory) and 5-HT (predominantly excitatory) seen here in vitro were opposite to those observed in vivo with iontophoretic application, and were not reversed or abolished by the blockade of synaptic transmission. Comparisons of data between the two types of preparations showed that only the responsiveness of the irregular units to ACh and to 5-HT were significantly different: both types of responsiveness were higher in estrogen-treated than in untreated preparations. No significant difference was found in the responsiveness of regular units, or in firing patterns or firing rate. Thus, the present in vitro studies have demonstrated that SCN contains a heterogeneous population of neurons distinguishable by their electrophysiological and neuropharmacological characteristics, and that estrogen has a specific action on specific types of SCN neurons.

Effects of constant bright illumination on reproductive processes in the female rat

Physiological and behavioral reproductive changes in the female rat which occur under constant bright illumination (LL) are examined. The development of LL-induced persistent estrus (PE) is discussed first in relation to other conditions in which PE is displayed. Next, mechanisms are reviewed which may account for the LL-induced changes. These include: (1) role of the retina, the retinohypothalamic tract and the suprachiasmatic nucleus; (2) influence of adrenal, pineal and Harderian glands; and (3) disruptions in either the 4-day endocrine rhythms or circadian neural component of the estrous cycle. Additional topics which are examined include the ontogeny of age-induced PE and the effects of LL on hormone receptor binding, puberty, sexual receptivity and mating.

Role of the preoptic brain in the regulation of preovulatory gonadotropin surges in the hamster

These studies have examined the role of the preoptic-suprachiasmatic area (POA-SC) in brains of cyclic female hamsters in regulating proestrous preovulatory gonadotropin surges. The spontaneous release of LH and FSH which normally occurs on proestrous afternoon was blocked with phenobarbital. Temporal changes in serum LH and FSH were measured in such blocked animals after delivery of direct current (100 muA/60 s) to the POA-SC or that portion of the medial basal hypothalamus (MBH) which includes the arcuate nuclei and the median eminence. Bilateral dc treatment of MBH resulted in a 30-fold increase in serum LH and a 4-fold rise in serum FSH over basal concentrations. Unilateral MBH dc treatment produced a 12-fold increase in serum LH but FSH levels remained basal. In contrast, the delivery of dc to the POA-SC did not evoke any increase in serum LH or FSH. Sham electrode placement in the MBH or POA-SC also did not alter basal LH and FSH serum concentrations. These results suggest that, unlike the rat, passage of dc with concomitant production of an irritative lesion and deposition of ferrous ion does not activate structures responsible for preovulatory gonadotropin surges in hamsters. In a second study, discrete electrochemical lesions were produced in the basal anterior portion of the preoptic brain. These lesions did not involve the medial preoptic area or the suprachiasmatic nuclei. Following such brain destruction, spontaneous ovulation, LH surges, and 4-day vaginal cyclicity ceased. When the suprachiasmatic nuclei or extensive regions of the dorsal medial preoptic area, including the anterior commissure, were destroyed, vaginal cyclicity was disrupted for only 8-12 days. Thereafter, these animals had spontaneous preovulatory gonadotropin surges and ovulated. Seemingly, input from the medial basal anterior preoptic region (anterior to the SC) is essential for preovulatory LH and FSH surges to occur.

In vivo metabolic activity of a putative circadian oscillator, the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) has been proposed as a site for an endogenous circadian oscillator in mammals, since lesions of the nucleus abolish a wide spectrum of overt-circadian rhythms. To demonstrate that a directly measurable property of the SCN itself in intact (unlesioned) animals is affected by environmental light and exhibits circadian rhythmicity, we used the autoradiographic 2-deoxy-D-[14C]glucose method to determine glucose utilization of rat SCN under a variety of lighting conditions. Our experiments indicate an important role for the SCN in circadian rhythm organization, and we believe the deoxyglucose method will prove useful as a tool for better understanding the functions and mechanisms of circadian clocks. Key words: suprachiasmatic nucleus, circadian rhythm, 2-deoxy-D-[14C]glucose.