Free-running circadian activity rhythms during long-term clonidine administration in rats

The circadian Clock mutation increases exploratory activity and escape-seeking behavior

Disturbances of circadian rhythms are associated with many types of mood disorders; however, it is unknown whether a dysfunctional circadian pacemaker can be the primary cause of altered emotional behavior. To test this hypothesis, male and female mice carrying a mutation of the circadian gene, Clock, were compared to wild-type mice in an array of behavioral tests used to measure exploratory activity, anxiety, and behavioral despair. Female Clock mutant mice exhibited significantly greater activity and rearing in an open field and a greater number of total arm entries in the elevated plus maze. In addition, female Clock mutant mice spent significantly more time swimming in the forced swim test than wild-type mice on both days of a 2-day test. Male Clock mutant mice also exhibited increased exploration of the open field and increased swimming in the forced swim test; however, behavioral changes were less robust in Clock mutant males compared to Clock mutant females. These changes in behavior were not dependent on the expression of a lengthened free-running period but were more or less striking depending on the testing conditions. These data indicate that the Clock mutation leads to increased exploratory behavior and increased escape-seeking behavior, and, conversely, does not result in increased anxiety or depressive-like behavior. These results suggest that the Clock gene is involved in regulating behavioral arousal, and that Clock may interact with sex hormones to produce these behavioral changes.

Circadian rhythms and the pharmacology of affective illness

The chronic effects of antidepressant drugs (ADs) on circadian rhythms of behavior, physiology and endocrinology are reviewed. The timekeeping properties of several classes of ADs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, serotonin agonists and antagonists, benzodiazepines, and melatonin are reviewed. Pharmacological effects on the circadian amplitude and phase, as well as effects on day-night measurements of motor activity, sleep-wake, body temperature (Tb), 3-methoxy-4-hydroxyphenylglycol, cortisol, thyroid hormone, prolactin, growth hormone and melatonin are examined. ADs often lower nocturnal Tb and affect the homeostatic regulation of sleep. ADs often advance the timing and decrease the amount of slow wave sleep, reduce rapid eye movement sleep and increase or decrease arousal. Together, AD effects on nocturnal Tb and sleep may be related to their therapeutic properties. ADs sometimes delay nocturnal cortisol timing and increase nocturnal melatonin, thyroid hormone and prolactin levels; these effects often vary with diagnosis, and clinical state. The effects of ADs on the coupling of the central circadian pacemaker to photic and nonphotic zeitgebers are discussed.

Neonatal desipramine treatment alters free-running circadian drinking rhythms in rats

Neonatal treatment with monoamine reuptake inhibitors results in a constellation of neurobehavioral alterations in adult rats that may model human depression. Since alterations in circadian rhythmicity have been reported in both depressed patients and in animal depression models, the present study examined the effects of neonatal desipramine treatment (5.0 mg/kg SC from postnatal day 7 through 22) on free-running circadian drinking rhythms. Rhythmicity was examined in constant darkness (DD), constant light (LL), and during adult desipramine treatment (0.25 mg/ml via the drinking water). Compared with saline-treated controls, neonatal desipramine lengthened free-running period in DD, blunted the period-altering effect of LL, and potentiated the period-altering effect of adult desipramine treatment. Neonatal desipramine treatment also increased circadian amplitude and spectral magnitude, but did not modify the effects of light or adult desipramine on these parameters. These results provide further evidence that behavioral depression is associated with alterations in circadian rhythmicity, and are consistent with the hypothesis that such relationships are mediated by brain monoaminergic systems.

Thyroid and thyroxine effects on adrenoreceptors in relation to circadian activity

Experiments were conducted to ascertain if changes in central adrenergic receptors could be associated with altered circadian activity patterns induced by thyroparathyroidectomy (TPX) and thyroxine. An initial experiment used TPX and sham-operated rats that had been exposed to dim red light for 7 months. The alpha and beta receptor densities were compared in the suprachiasmatic nuclei (SCN), preoptic (PO), septum, and caudate-putamen. TPX animals showed significant reductions in beta 1 and beta 2 receptor densities in SCN and PO, and alpha 1 densities in SCN, but no other changes. A second experiment, lasting 4 months, examined the effects of thyroxine, which has been shown to reverse the period-shortening effects of TPX surgery. Thyroxine significantly increased beta 1 receptors in both the SCN and ventromedial hypothalamus (VMH), the only regions that displayed significant reductions in TPXs during the second experiment. Increases of sevenfold and threefold were observed in the SCNs of TPXs and shams, respectively, but thyroxine's action in the VMH was limited to TPX animals, an effect that mimics thyroxine's action on circadian activity rhythms.

Circadian drinking rhythms in SHR and WKY rats: effects of increasing light intensity

This study sought to define the generality of a previous finding that the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rat strains differ in free-running circadian period when maintained in running-wheel cages under constant light. Circadian drinking rhythms were monitored in SHRs and WKYs housed without access to running wheels under an increasing series of light intensities beginning with constant darkness. Strain differences in circadian period were seen only at relatively high light intensities, indicating that SHRs and WKYs differ in circadian light sensitivity. Since SHRs and WKYs differ in circadian period with or without access to running wheels, this strain difference is not likely to depend on differential locomotor activity levels. SHRs and WKYs also differed in spectral profile and circadian waveform, but only under low light intensities. At higher intensities, dissociation of rhythmicity was seen in both strains.

Circadian activity rhythms in SHR and WKY rats: strain differences and effects of clonidine

The spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) inbred rat strains have been subjected to extensive behavioral and neurochemical characterization. The present study examined free-running circadian activity rhythms in these two strains. Because previous studies indicated that free-running rhythms are altered during chronic clonidine administration, and that SHRs and WKYs may respond differentially to clonidine, the effects of this agent on rhythmicity were compared in the two strains. SHRs were hyperactive and showed shorter free-running periods than did WKYs. Clonidine administration altered free-running rhythms similarly in the two strains, but reduced activity levels only in the relatively hyperactive SHRs. These results are consistent with the hypothesis that central noradrenergic systems influence circadian locomotor activity rhythms.

Activation of α2-adrenoceptors suppresses excessive wheel running in the semistarvation-induced hyperactive rat

Male Wistar rats were housed in cages linked to running wheels and fed on a schedule designed to reduce their body weight by 20-30%. During this period of semistarvation, rats increased their daily running wheel activity (RWA) by up to 30 km/day. RWA could be kept at this level provided that body weight was kept constant. Different alpha-adrenergic agonists and antagonists were tested for their effects on RWA and it was found that RWA could be suppressed only by agonists that display high affinity for the alpha 2-receptor (clonidine and guanfacine). Neither antagonist had an effect on RWA. Clonidine's inhibiting effect on RWA was prevented by pretreatment with yohimbine, which also has high affinity for alpha 2-receptors. From these results, we conclude that semistarvation-induced hyperactivity can be blocked by alpha 2-agonists. In view of this result and those that were obtained in previous studies, a theoretical model for the development of semistarvation-induced hyperactivity will be presented.

Circadian rhythmicity and behavioral depression: II. Effects of lighting schedules

Two studies explore the relationship between rhythmicity and behavioral depression. Behavioral depression was induced using inescapable footshock, and assessed by measuring subsequent responses to escapable shock, in rats housed under different light-dark conditions. Experiment 1 compared escape performance in free-running and entrained animals following inescapable shock. Free-running and entrained animals did not exhibit differential vulnerability to the effects of inescapable shock. In addition, there were no systematic effects on phase following shock. However, several free-running animals showed increased circadian period following shock, and lengthening of period was significantly correlated with escape performance. Individual differences in baseline period or phase were not predictive of escape performance. In Experiment 2, "aftereffects" of entrainment to long or short light-dark cycles were utilized to create groups of animals with long or short free-running periods. After the administration of inescapable shock, escape performance was tested. There were no significant differences among experimental groups in escape performance. These results suggest that plasticity of circadian period, but not baseline period per se, may be associated with the ability to adapt to environmental challenges.