Effects of chronic clonidine administration and withdrawal on free-running circadian activity rhythms

Propofol but not dexmedetomidine produce locomotor sensitization via nitric oxide in rats

RATIONALE

The abused potential of some anesthetics has been debated. Measurement of locomotor sensitization is a better way to detect the neurobehavioral plasticity of addiction.

OBJECTIVES

The present study aims to explore whether propofol and dexmedetomidine are capable of inducing locomotor sensitization.

METHODS

Male Wistar rats (250-300 g) were the subjects (n = 8 for each group). Propofol (20 and 40 mg/kg) and dexmedetomidine (2.5-20 μg/kg) or saline were injected to rats intraperitoneally (IP), and their locomotor activities were recorded for 15 min. Consequently, L-NAME (30 and 60 mg/kg)-a nitric oxide (NO) inhibitory agent-was injected to rats 30 min before propofol (40 mg/kg) or saline injections, and the locomotor activity was recorded. The process was carried out for 13 days, with 7 sessions applied every other day.

RESULTS

Dexmedetomidine did not produce any significant locomotor sensitization. While propofol (20 mg/kg) produced a significant locomotor sensitization in the last treatment session (day 13), at the higher dose, it prompted a significant locomotor sensitization from the 3rd treatment session. L-NAME blocked propofol-induced locomotor hyperactivity and sensitization significantly without producing any noteworthy changes on the locomotor activity during the testing period of 13 days when administered alone.

CONCLUSIONS

Our results suggest that propofol but not dexmedetomidine produced a significant locomotor sensitization via central nitrergic system. Dexmedetomidine may have a lesser psychostimulant type addictive potential than propofol. Sensitization development by propofol implies that this drug might be effective on the neuroadaptive processes associated with a stimulant type of dependence.

β-Adrenergic receptors control brown adipose UCP-1 tone and cold response without affecting its circadian rhythmicity

Brown adipose tissue (BAT) thermogenic functions are primarily mediated by uncoupling protein (UCP)-1. Ucp1 gene expression is highly induced by cold temperature, via sympathetic nervous system and β-adrenergic receptors (βARs). Ucp1 is also repressed by the clock gene Rev-erbα, contributing to its circadian rhythmicity. In this study, we investigated mice lacking βARs (β-less mice) to test the relationship between βAR signaling and the BAT molecular clock. We found that in addition to controlling the induction of Ucp1 and other key BAT genes at near freezing temperatures, βARs are essential for the basal expression of BAT Ucp1 at room temperature. Remarkably, although basal Ucp1 expression is low throughout day and night in β-less mice, the circadian rhythmicity of Ucp1 and clock genes in BAT is maintained. Thus, the requirement of βAR signaling for BAT activity is independent of the circadian rhythmicity of Ucp1 expression and circadian oscillation of the molecular clock genes. On the other hand, we found that βARs are essential for the normal circadian rhythms of locomotor activity. Our results demonstrate that in addition to controlling the BAT response to extreme cold, βAR signaling is necessary to maintain basal Ucp1 tone and to couple BAT circadian rhythmicity to the central clock.-Razzoli, M., Emmett, M. J., Lazar, M. A., Bartolomucci, A. β-Adrenergic receptors control brown adipose UCP-1 tone and cold response without affecting its circadian rhythmicity.

The noradrenaline reuptake inhibitor atomoxetine phase-shifts the circadian clock in mice

Circadian rhythms are recurring cycles in physiology and behaviour that repeat with periods of near 24 h and are driven by an endogenous circadian timekeeping system with a master circadian pacemaker located in the suprachiasmatic nucleus (SCN). Atomoxetine is a specific noradrenaline reuptake inhibitor that is used in the clinical management of attention-deficit/hyperactivity disorder (ADHD). In the current study we examined the effects of atomoxetine on circadian rhythms in mice. Atomoxetine (i.p.; 3 mg/kg) treatment of mice free-running in constant light (LL) at circadian time (CT) 6 induced large phase delays that were significantly different to saline controls. Treatment of animals with atomoxetine at CT13 or CT18 did not elicit any significant phase shifts. We also examined the effects of atomoxetine treatment of animals free-running in constant darkness (DD). Atomoxetine treatment at CT6 in these animals leads to more modest, but significant, phase advances, whereas treatment at CT18 did not elicit significant phase shifts. The effects of atomoxetine in LL were attenuated by pretreatment with the α-1 adrenoreceptor antagonist prazosin and were mimicked by another noradrenaline reuptake inhibitor, reboxetine. Further, atomoxetine treatment at CT6 induced a downregulation of c-Fos and CLOCK in the SCN, but did not alter the expression of PER2 and BMAL1. Atomoxetine during the night phase did not alter any of these factors. Atomoxetine treatment preceding a light pulse at CT15 enhanced the magnitude of the photic-phase shift, whereas it altered photic induction of the immediate early gene products c-Fos and ARC in the SCN. These data indicate that atomoxetine can reset the circadian clock and indicate that part of the therapeutic profile of atomoxetine may be through circadian rhythm modulation.

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.

Monoamine depletion blocks triazolam-induced phase advances of the circadian clock in hamsters

Injections with the short-acting benzodiazepine, triazolam (Tz), 6 h before activity onset (CT6) produce large phase advances of the circadian pacemaker in hamsters. An increase in locomotor activity and/or the state of arousal is considered essential for the effects of Tz, suggesting the potential involvement of central monoaminergic systems in this process. The present study examines the effect of reserpine-induced monoamine depletion on the phase-shifting effects of Tz in hamsters. Wheel running activity of 16 male golden hamsters (14 weeks old) was continuously monitored in constant darkness. After a stable free-running circadian rhythm was established half of the animals received reserpine (2.5 mg/kg, s.c.) and the other half vehicle treatment. Ten days later all animals were given Tz injections (10 mg/kg i.p.) at CT6 and the circadian activity rhythm was monitored for 2 more weeks. An additional 10 animals were used to determine the effect of reserpine on the central monamine levels using high pressure liquid chromatography. A circadian rhythm of locomotor activity with reduced amplitude and longer free-running period persisted after reserpine treatment, despite the significant monoamine depletion. Triazolam injections at CT6 induced large phase-advances (93.1 +/- 14.9) in the control group that were markedly attenuated in 7 out of the 8 reserpine-treated animals (3.12 +/- 17.7 min, P < 0.01). Our results suggest that monoaminergic systems are essential for the phase-shifting effect of Tz upon the circadian system in hamsters.

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.

Ethanol and circadian rhythms in the Syrian hamster: effects on entrained phase, reentrainment rate, and period

Wheel-running rhythms were examined in male hamsters with access to 28% ethanol in lieu of water. One group was recorded in a light-dark (LD) cycle that was phase advanced by 8 h on three occasions separated by 23-27 days. On two of the three occasions, hamsters were subjected to a 2- to 3-h cage change procedure designed to stimulate wheel running, which accelerates the rate of reentrainment to 8-h advances. Ethanol and control hamsters showed no group differences in rhythm amplitude, entrained phase, or reentrainment rate. Both groups showed faster reentrainment in the cage change condition. A second group of hamsters recorded in constant dim showed a small but significant lengthening of the free-running period of their wheel-running rhythm when provided with a 28% ethanol solution. Wheel running decreased during ethanol access in this group. Voluntary ethanol consumption evidently can slow the circadian pacemaker regulating activity rhythms in hamsters but has no measurable effect on photic entrainment or pacemaker response to LD shifts or nonphotic manipulations (stimulated activity). Period lengthening may be secondary to decreased activity, but other period-activity correlations obtained did not reveal a strong association between these two variables.

Circadian rhythmicity and behavioral depression: I. Effects of stress

Rats were exposed to repeated sessions of inescapable footshock, and behavioral depression was subsequently assessed by measuring escape performance during exposure to escapable shock in a different testing environment. Free-running circadian activity rhythms were assessed using running wheels for approximately three weeks before and after administration of inescapable shock. Several animals showed lengthening of free-running period and decreases in activity level following shock. Similar effects were also seen in rats that were removed from their running wheels, placed within the shock apparatus, and not given shock, but not in nonhandled control animals. Furthermore, period lengthening in shocked and handled rats was positively correlated with escape performance, suggesting that circadian rhythm alterations occurred in those animals that were best able to cope with shock or handling-related stressors. In contrast, individual differences in circadian period and activity level during baseline conditions were not predictive of either escape performance or circadian rhythm alterations. These results suggest that successful behavioral adaptation to stress may be associated with alterations of circadian rhythmicity.

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.

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

Experimental and clinical studies indicate that the alpha-adrenergic agonist clonidine can alter mood and activity. However, the behavioral effects of this agent are complex and appear to depend on duration of treatment. Recent work from this laboratory demonstrated that clonidine systematically alters the period, amplitude, and level of free-running circadian activity rhythms in rats. The present study confirms and extends previous observations by employing a longer duration of clonidine treatment. The results show that chronic clonidine administration reversibly shortens the free-running period and reduces the amplitude of the free-running rhythm in constant light. Furthermore, clonidine treatment can increase or decrease the level of activity, depending on baseline activity level, and these effects are not consistently reversed following the termination of treatment. These observations support the hypothesis that noradrenergic systems influence both the circadian periodicity and the level of spontaneous activity, and that clonidine may influence these two parameters by acting at different neural or neuronal loci.