Estrous correlated modulations of circadian and ultradian wheel-running activity rhythms in LEW/Ztm rats

Estrogen treatment alters the expression of ultradian activity rhythms in male and female LEW/Ztm rats. This finding raises the possibility that the expression of ultradian rhythms may vary on different days of the estrous cycle. To test this hypothesis, we recorded the circadian and ultradian wheel-running activity rhythms of entrained (LD 12:12) and free-running sexually mature LEW/Ztm females during their 4- or 5-day estrous cycle. The mean daily activity, the duration of activity, the circadian period of activity, and the occurrence of ultradian rhythms differed significantly among the days of the estrous cycle. In LD 12:12, the phase angle difference between the beginning of activity and light offset varied reliably in 5-day cycling animals. The highest daily mean of activity, the longest duration, and the shortest circadian period length were observed on the day of estrus in both entrained and free-running animals. The day of estrus was characterized by a constant high level of activity throughout the activity phase, while the days following ovulation showed a bi- or trimodal activity pattern. Power spectrum analysis revealed significant ultradian components for the days of metestrus and diestrus, but only circadian components for the days of proestrus and estrus. These results were interpreted as indicating that endogenous changes in circulating hormone levels can induce changes in the ultradian and circadian patterns of wheel-running activity in LEW/Ztm rats.

Pharmacological manipulation of the mammalian circadian clock: implications for the control of seasonal reproductive cycles

Although artificial control of the light cycle can be used to regulate the seasonal reproductive cycle of many animals under laboratory conditions, such regulation is often not possible in a standard agricultural environment. An alternative strategy for regulating the seasonal reproductive cycle of photoperiodic animals is to use drugs that either mimic the effects of light on reproductive function or that induce an alteration in the way the circadian clock system is entrained by the light cycle. The ability to use drugs to achieve these objectives has been demonstrated in the laboratory, but it remains to be determined if such an approach can be used to regulate the breeding season of farm animals under normal agricultural conditions.

Entrainment of the circadian activity rhythm to the light-dark cycle can be altered by a short-acting benzodiazepine, triazolam

The circadian rhythm of locomotor activity in hamsters maintained in either constant darkness or constant light can be phase-shifted by a single injection of the short-acting benzodiazepine, triazolam. These results suggest that treatment with triazolam may also alter the entrainment pattern of circadian rhythms in animals that are synchronized to a light-dark (LD) cycle. To test this hypothesis, hamsters maintained on an LD 6:18 light cycle received daily injections of triazolam (or vehicle) for 10-12 days, and any subsequent effects on the phase relationship between the onset of activity and the LD cycle were determined. Daily injections of triazolam (but not vehicle) induced pronounced advances or delays in the phase relationship between the entrained activity rhythm and the LD cycle; the direction of the shift was dependent on the time of the injection. Taken together with data from previous studies, these results suggest that triazolam, and perhaps other short-acting benzodiazepines, can be used to manipulate the mammalian circadian clock under a variety of experimental conditions.

Phase shifts in the circadian activity rhythm induced by triazolam are not mediated by the eyes or the pineal gland in the hamster

A single injection of the benzodiazepine triazolam, which is though to act by potentiating the effects of the neurotransmitter gamma-aminobutyric acid (GABA), can induce permanent phase shifts in the circadian rhythm of locomotor activity of hamsters. Occurrence of GABA immunoreactivity and benzodiazepine receptors in the retina, which contains photoreceptors that relay synchronizing light-dark information to the mammalian circadian system, raises the possibility that triazolam may influence circadian rhythmicity via an action on the retina. However, the phase shifting effects of triazolam on the activity rhythm were unaffected by blinding: the direction and the magnitude of the phase shifts were similar in blind hamsters and in sighted hamsters maintained in constant darkness. Furthermore, no change in response to triazolam was observed in hamsters studied through 84 days after blinding. In addition, benzodiazepine binding sites have been found in the mammalian pineal gland, which has also been implicated in circadian rhythmicity. Therefore, its possible involvement in mediating the phase advancing effects of triazolam on the circadian clock has also been tested: the response was similar in blind and blind-pinealectomized animals. These results indicate that the effects of triazolam on the circadian clock are not mediated by the eyes or the pineal gland.

Photoperiodic response in the male laboratory rat

Normally photoperiodic laboratory rats can be induced to respond reproductively to a change in the length of the day by various experimental manipulations. One such paradigm that results in significant gonadal regression involves the treatment of rats with exogenous testosterone during exposure to short days. Studies were undertaken to assess various aspects of this model system including 1) the testicular response of testosterone-treated rats exposed to various photoperiods, 2) the time course for testicular regression under a short photoperiod, and 3) the role of the pineal gland as a mediator of the effects of day length on the neuroendocrine-gonadal axis. Photoperiods ranging in length from 2 to 22 h/24 h had no effect on testicular size in untreated rats. In contrast, while near normal testicular weights were maintained in laboratory rats treated with testosterone and exposed to 10 or more h of light per day, testicular regression occurred in rats implanted with testosterone-filled capsules and exposed to photoperiods of 8 or fewer h of light per day. Maximal testicular regression was reached in about 9 wk in testosterone-treated rats exposed to 6L:18D. Removal of the pineal gland totally blocked the inhibitory effects of exposure to short day lengths in testosterone-treated rats. These studies define some of the characteristics of an extant, but dormant, system for photoperiodic time measurement in the common laboratory rat and implicate a role for the pineal gland in this system. These experiments offer evidence that neuroendocrine factors that regulate continuous vs. seasonal reproductive patterns are malleable. Such flexibility in the photoperiodic response may also contribute to the evolution of seasonal to non-seasonal species and vice versa.

Adaptation of circadian rhythmicity to shift in light-dark cycle accelerated by a benzodiazepine

The light-dark cycle is the major synchronizing agent for circadian rhythms in animals. After an abrupt shift in the light-dark cycle, it usually takes many days for circadian rhythms to resynchronize. A single injection of the short-acting benzodiazepine, triazolam, to hamsters subjected to an 8-h advance of the light-dark cycle resulted in an approximately 50% reduction in the time taken for the circadian locomotor activity rhythm to be resynchronized to the new lighting schedule. These results suggest that it may be possible to use drugs to facilitate the resynchronization of human circadian rhythms following an abrupt change in environmental time.

Dose response curve for the phase-shifting effect of triazolam on the mammalian circadian clock

A dose response curve for the phase shifting effect of triazolam, a short-acting benzodiazepine commonly prescribed for the treatment of insomnia, on the circadian rhythm of locomotor activity was measured for the golden hamster. A single intraperitoneal injection of triazolam six hours before the onset of wheel-running activity induced a dose-dependent phase advance in the rhythm. A maximum phase advance, which averaged about 100 minutes, was observed in animals injected with 0.5 to 5.0 mg of triazolam. The use of drugs which promote sleep, and induce phase shifts in a central circadian clock, could be important in the treatment of sleep disorders associated with disrupted schedules and of mental and physical disorders associated with abnormal circadian rhythmicity.

Anisomycin, an inhibitor of protein synthesis, perturbs the phase of a mammalian circadian pacemaker

Administration of the protein synthesis inhibitor, anisomycin, induced both advance and delay phase-shifts in the circadian rhythm of locomotor activity in hamsters maintained in constant darkness. The magnitude and direction of the phase shift was dependent on the circadian time of drug treatment. A second protein synthesis inhibitor, cycloheximide, also induced phase-shifts in the circadian rhythm of activity, while two drugs which mimic some of the side effects of protein synthesis inhibitors had no significant effect on the activity rhythm. The phase-response curve generated by anisomycin injections in the hamster is similar to the response curves that have been measured for protein synthesis inhibitors in micro-organisms and invertebrates. Thus, the biochemical mechanisms generating circadian oscillations in mammals may share common features with those found in very distantly related phylogenetic groups.

Reproductive responsiveness to short photoperiod develops postnatally in male golden hamsters

The development of the reproductive response to short photoperiod in golden hamsters was examined by monitoring testis size in hamsters that were either: raised on 14L:10D from birth; raised on 6L:18D from birth; or born on 14L:10D and transferred to 6L:18D at either 1, 2, 3, 4, 5, 6, 7, 9, or 12 weeks of age. Testis growth occurred between 3 and 7 weeks of age at similar rates in all groups. Testicular regression occurred in all groups exposed to short days; the time of regression depended on the age of the animals when they were first transferred to 6L:18D. Testicular regression began at about 10 weeks of age in all groups transferred to 6L:18D between birth and 5 weeks of age. In contrast, regression began at 11.2, 13.3, 15.6, and 18.2 weeks of age in hamsters transferred to 6L:18D at 6, 7, 9, and 12 weeks of age, respectively. These results suggest that the reproductive system of male hamsters is refractory to the inhibitory effects of short days on reproductive function until 5 to 6 weeks of age. Thereafter, exposure to short days initiates testicular regression in hamsters within 5 to 7 weeks.

A cholinergic antagonist, mecamylamine, blocks the phase-shifting effects of light on the circadian rhythm of locomotor activity in the golden hamster

Despite the well known role of the light-dark cycle in the entrainment of circadian rhythms, very little is known about the neurochemical events that mediate the effects of light on the mammalian circadian clock. Recent anatomical and pharmacological data support the hypothesis that acetylcholine may be involved in relaying light-dark information from the retina to, or within, the circadian clock of rodents. If acetylcholine is required for this response, it should be possible to block the phase-shifting effects of a light pulse by blocking cholinergic neurotransmission. To test this possibility, hamsters free-running in constant darkness received an intraventricular injection of the anticholinergic drug, mecamylamine (450 micrograms), 10 min before being exposed to a 5-min pulse of light known to induce sub-maximal phase shifts in the circadian rhythm of wheel-running behavior. Compared to vehicle-injected control animals, mecamylamine treatment blocked or reduced both the phase-advancing and phase-delaying effects of light. These results support the hypothesis that acetylcholine is involved in mediating the phase-shifting effects of light on the mammalian circadian clock.

Circadian and seasonal control of neuroendocrine-gonadal activity

A circadian clock(s) located in the suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in regulating the timing of pituitary gonadotropin release in many mammalian species. The results from studies involving two unusual experimental paradigms are presented to demonstrate a role for this circadian clock in the regulation of the timing of the preovulatory LH surge in the golden hamster as well as in the measurement of the seasonal change in day length in the Djungarian hamster; information which is used in the regulation of neuroendocrine-gonadal activity on a seasonal basis. The data also demonstrate that the circadian rhythm of locomotor activity can be used as a "marker" rhythm for the clock involved in both ovulatory and seasonal cycles in hamsters.

Circadian principles and design of rotating shift work schedules

Circadian rhythms may be disrupted when shift workers rotate from one work schedule to another. It has been suggested that in order to minimize the time needed to readjust circadian rhythms to a new work schedule the work time of shift workers should be rotated in a delaying rather than an advancing direction. However, delaying or advancing the work time does not imply that the sleep-wake cycle is also shifted in a similar manner. Indeed, after a complete rotation between the day, evening, and night shifts the sleep time will be advanced once, delayed once, and not shifted once, regardless of whether the workers are on a delaying or an advancing work rotation schedule. Thus circadian rhythms are likely to be perturbed in a similar manner whether the work schedule is rotated in a delaying or an advancing direction.

Circadian and photoperiodic effects of brief light pulses in male Djungarian hamsters

The effects of brief light pulses (1-60 min in duration) on the circadian rhythm of locomotor activity and/or the neuroendocrine-gonadal axis was investigated in male Djungarian hamsters. Exposure of hamsters free-running in constant darkness to a single 1-h pulse of light induced phase-dependent phase shifts in the rhythm of locomotor activity. The general shape of the "phase-response curve" was similar to that observed in other animals; phase-delays and phase-advances were induced by light pulses delivered in the early and late subjective night, respectively, while light pulses during the subjective day induced little or no phase-shift in the activity rhythm. Animals exposed for 7 days to 1-min of light during the night in animals otherwise exposed to 6L:18D resulted in increased levels of serum FSH and testicular weight. Daily exposure to two 1-h or two 10-min pulses of light (but not two 1-min pulses) for 10 days resulted in stable entrainment of the activity rhythm as well as testicular weight gains and serum FSH increases. When two 10-min pulses of light were presented 8 and 16 h apart, some animals showed a short-day entrainment pattern (i.e., locomotor activity confined to the long period of darkness) while other animals showed a long-day entrainment pattern (i.e., locomotor activity confined to the short period of darkness). Importantly, the stimulatory effects of light on neuroendocrine-gonadal activity were clearly dependent on the phase-relationship between the light pulses and the circadian rhythm of locomotor activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A benzodiazepine used in the treatment of insomnia phase-shifts the mammalian circadian clock

Between 5 and 20% of the adult population in Western countries suffer from insufficient and/or unsatisfying sleep, often associated with certain psychiatric disorders or with certain types of professional activities (for example, shift workers) and travel schedules (for example, jet lag). The benzodiazepines are at present the drug treatment of choice for the management of anxiety and stress-related conditions as well as insomnia. Benzodiazepines are thought to act by potentiating the action of the neurotransmitter gamma-aminobutyric acid (GABA), a widely distributed transmitter in the central nervous system. The circadian system has a key role in the regulation of the sleep-wake cycle, and at least some forms of insomnia may be the result of a disorder of the circadian sleep-wake rhythm. Similarly, at least some forms of depression may also involve disruption of normal circadian rhythmicity. A central pacemaker for the generation of many circadian rhythms in mammals, including the sleep-wake cycle, appears to be located in the suprachiasmatic nucleus, and recent research indicates that both cell bodies and axons containing GABA are present within the bilaterally paired suprachiasmatic nuclei. These findings raise the possibility that the benzodiazepines, commonly prescribed for sleep and mental disorders, may have an effect on the central circadian pacemaker. Here we report that the acute administration of triazolam, a short-acting benzodiazepine commonly prescribed for the treatment of insomnia, induces a phase-shift in the circadian rhythm of locomotor activity in golden hamsters. This suggests a role for GABA-containing neurones in the mammalian circadian system.

Horizontal knife cuts either ventral or dorsal to the hypothalamic paraventricular nucleus block testicular regression in golden hamsters maintained in short days

The possible involvement of efferent pathways from the suprachiasmatic nucleus (SCN) in the photoperiodic regulation of reproduction was studied by measuring the testis size of hamsters bearing a horizontal knife cut either ventral or dorsal to the hypothalamic paraventricular nucleus (PVN) that were transferred from photostimulatory long days (light:dark (L:D) 14:10 h) to non-stimulatory short days (L:D 6:18 h). Knife cuts placed either ventral or dorsal to the PVN blocked testicular regression induced by exposure to short days. These results indicate that efferent fibers running dorsally from the SCN to the PVN are involved in relaying photoperiodic information from the SCN to the PVN. Furthermore, recently-defined efferents that leave the PVN dorsally and terminate in the spinal cord appear to be responsible for relaying seasonal information about day-length to the pineal-reproductive axis of hamsters.

Circadian neural rhythms in mammals

A number of different experimental findings indicate that endogenous circadian neural rhythms are generated within the SCN region of mammals. Rhythmic neural signals from the SCN directly, or indirectly, appear to regulate many, if not all, biochemical, physiological, and behavioral circadian rhythms. The circadian system in mammals is referred to as being "multioscillatory" in nature, although this term is often used to mean different things. The SCN itself may contain many circadian neural oscillators and the coordinated output of these multioscillators may regulate circadian rhythms. The term multioscillatory is also used to indicate the presence of anatomically distinct oscillators. However, the location and properties of circadian pacemakers that may lie outside of the SCN region remain unknown. Indeed, there is no conclusive evidence that circadian rhythms can be generated over a prolonged period of time in the absence of the SCN in mammals maintained under constant environmental conditions. Whereas the SCN appears capable of generating circadian neural signals indefinitely in the absence of any 24-hr fluctuations in the external environment, other components of the circadian system may function as damped oscillators (i.e. persist for only a few cycles in the absence of a periodic input) and/or only emit diurnal rhythms in the presence of rhythmic input from either the external (e.g. light-dark cycle) and/or internal (e.g. SCN) environment. On a conceptual as well as a physiological level, our understanding of the organization of the mammalian circadian system remains limited at the present time. Unfortunately, the limitations on our understanding of the circadian system are not always fully appreciated, and quite often a hypothesis based on limited experimental data is treated as fact. The study of the circadian system is still in the early stages of development, and further progress in elucidating the physiological mechanisms underlying the generation and expression of circadian rhythms may require new ways of looking at old problems as well as new experimental approaches. One final comment should be made concerning the importance of an increased understanding of circadian rhythms for human health. Because the study of the physiology of the circadian system is still in its early stages of development, little is known about the importance of the normal functioning of the circadian system for the health and well-being of the organism. Recent studies in humans suggest that disorders within the circadian system itself may be involved in the etiology of at least some forms of mental illness.

Neurochemical basis for the photic control of circadian rhythms and seasonal reproductive cycles: role for acetylcholine

A pharmacological approach was used to examine the role of acetylcholine in the photic control of circadian rhythms and seasonal reproductive cycles. The experimental protocol was designed to determine whether the administration of carbachol, a cholinergic agonist, could mimic the effects of brief light pulses on gonadal function and/or the circadian rhythm of wheel-running activity in golden hamsters. Intraventricular injections of carbachol, administered singularly at discrete phase points throughout the circadian cycle, induced phase-dependent shifts in the free-running rhythm of activity similar to those caused by a brief light exposure. Injections of carbachol once every 23.33 hr for 9 weeks entrained the activity rhythm and stimulated the neuroendocrine-gonadal axis in a manner similar to that observed after the presentation of 1-hr light pulses at this frequency. In contrast, the administration of carbachol once every 24 hr did not consistently provide an entraining signal for the activity rhythm and did not stimulate reproductive function. Importantly, the effects of carbachol on the seasonal reproductive response were correlated with the timing of the injections relative to the activity rhythm. These findings suggest that acetylcholine may play an important role in the mechanism by which light regulates circadian rhythms and seasonal reproductive cycles.

Multiple circadian oscillators regulate the timing of behavioral and endocrine rhythms in female golden hamsters

A single daily "surge" in pituitary luteinizing hormone release was observed in ovariectomized-estrogen-treated hamsters expressing an intact circadian rhythm of locomotor activity. In contrast, two luteinizing hormone surges occurred within a single 24-hour period in hamsters whose activity rhythm had dissociated or "split" into two distinct components. These observations indicate that both behavioral and endocrine circadian rhythms are regulated by the same multioscillator system, which seems to be composed of at least two distinct circadian oscillators.

Photoperiodic control of testicular size in the golden hamster: lack of a role for the deep pineal gland

Removal of the superficial pineal gland prevents testicular regression from occurring in golden hamsters exposed to a short-day photoperiod, but to date no studies have examined the role of the deep pineal gland in the photoperiodic control of gonadal function. We investigated the role of the deep pineal gland in this process by monitoring testicular size in hamsters with an electrolytic lesion of the deep pineal gland. Sexually mature hamsters that had been sham-lesioned or that had received a lesion aimed at the deep pineal gland were transferred from 14L:10D to 6L:18D or were maintained in 14L:10D for 73 d. Testicular widths were determined at 2-3-wk intervals and after 73 d the testes were removed and weighed. The mean testis widths of the seven animals with histologically confirmed deep pineal lesions and exposed to 14L:10D remained large, similar to those of the sham-operated control animals exposed to the same photoperiod. The testes regressed in the five animals with confirmed deep pineal lesions and exposed to 6L:18D in a similar manner to those of the sham-operated control animals exposed to the same photoperiod. No significant difference (P greater than .05) was found between the testicular weights of lesioned and unlesioned animals exposed to either photoperiod. The lack of a significant effect of a deep pineal lesion on testicular size demonstrates that this portion of the pineal complex is not required for normal regulation of testicular function by photoperiod. These results also suggest that nerve fibres projecting from the brain to the superficial pineal gland via the deep pineal gland are not involved in photoperiodic regulation of testicular function.

Periodic exposure to a brief light signal stimulates neuroendocrine-gonadal activity in golden hamsters

The photoperiodic effects of a periodic light pulse on neuroendocrine-gonadal activity in the male golden hamster was examined using a night interruption paradigm. Adult male hamsters that had been housed 3 to 5 per cage on LD 14:10 were placed either in individual cages, each equipped with a running wheel, or maintained in communal housing conditions, without access to a running wheel. Animals were then transferred to either LD 6:18 or to a LD 6:18 light cycle with a periodic 10-second night interruption (8 hours after lights off) occurring once every two, four, or seven days. As expected, exposure to LD 6:18 for 11 weeks induced complete regression of the testes and seminal vesicles, accompanied by low serum levels of LH and FSH, in both individually and communally-housed animals. However, in individually-housed animals receiving a 10-second night interruption every other day on LD 6:18, paired testis and seminal vesicle weights, as well as serum gonadotropin levels, were maintained at values comparable to those normally observed in hamsters exposed to photostimulatory long days. Furthermore, the presentation of a periodic 10-second night interruption once every four or seven days to individually-housed animals with access to a running wheel was sufficient to partially or totally block the inhibitory effects of short days on neuroendocrine-gonadal activity. Communally-housed hamsters receiving a 10-second light pulse once every two, four, or seven days also exhibited paired testis and seminal vesicle weights, as well as serum gonadotropin levels, that were consistently higher than the values obtained for animals exposed only to LD 6:18.(ABSTRACT TRUNCATED AT 250 WORDS)

The hypothalamic paraventricular nucleus mediates the photoperiodic control of reproduction but not the effects of light on the circadian rhythm of activity

Photoperiods of less than 12.5 h of light/24 h induce gonadal regression in the golden hamster. Photic information is relayed from the retina to the hypothalamic suprachiasmatic nucleus (SCN), a structure responsible for the generation of many circadian rhythms including the circadian rhythms in locomotor activity and pineal melatonin synthesis and release. Although pineal melatonin mediates the photoperiodic-neuroendocrine response, the complete neural circuit from the SCN to the pineal gland is unknown. Complete destruction of the hypothalamic paraventricular nucleus (PVN) prevented short-day-induced testicular regression without affecting the circadian rhythm of locomotor activity. The results indicate that the PVN plays an important role in the photoperiodic-neuroendocrine circuit and is responsible for relaying information from the SCN to the pineal. A different efferent pathway connects the SCN to structures in the brain responsible for locomotor activity.