Fluoxetine shortens circadian period for wheel running activity in mice

Circadian Rhythms in Mood Disorders

Altered behavioral rhythms are a fundamental diagnostic feature of mood disorders. Patients report worse subjective sleep and objective measures confirm this, implicating a role for circadian rhythm disruptions in mood disorder pathophysiology. Molecular clock gene mutations are associated with increased risk of mood disorder diagnosis and/or severity of symptoms, and mouse models of clock gene mutations have abnormal mood-related behaviors. The mechanism by which circadian rhythms contribute to mood disorders remains unknown, however, circadian rhythms regulate and are regulated by various biological systems that are abnormal in mood disorders and this interaction is theorized to be a key component of mood disorder pathophysiology. A growing body of evidence has begun defining how the interaction of circadian and neurotransmitter systems influences mood and behavior, including the role of current antidepressants and mood stabilizers. Additionally, the hypothalamus-pituitary-adrenal (HPA) axis interacts with both circadian and monoaminergic systems and may facilitate the contribution of environmental stressors to mood disorder pathophysiology. The central role of circadian rhythms in mood disorders has led to the development of chronotherapeutics, which are treatments designed specifically to target circadian rhythm regulators, such as sleep, light, and melatonin, to produce an antidepressant response.

Pharmacological Manipulation of the Circadian Clock: A Possible Approach to the Management of Bipolar Disorder

Bipolar disorder (BD) is a mood disorder with genetic and neurobiological underpinnings, characterized primarily by recurrent episodes of mania and depression, with notable disruptions in rhythmic behaviors such as sleep, energy, appetite and attention. The chronobiological links to BD are further supported by the effectiveness of various treatment modalities such as bright light, circadian phase advance, and mood-stabilizing drugs such as lithium that have effects on the circadian clock. Over the past 30 years, the neurobiology of the circadian clock has been exquisitely described and there now exists a detailed knowledge of key signaling pathways, neurotransmitters and signaling mechanisms that regulate various dimensions of circadian clock function. With this new wealth of information, it is becoming increasingly plausible that new drugs for BD could be made by targeting molecular elements of the circadian clock. However, circadian rhythms are multidimensional and complex, involving unique, time-dependent factors that are not typically considered in drug development. We review the organization of the circadian clock in the central nervous system and briefly summarize data implicating the circadian clock in BD. We then consider some of the unique aspects of the circadian clock as a drug target in BD, discuss key methodological considerations and evaluate some of the candidate clock pathways and systems that could serve as potential targets for novel mood stabilizers. We expect this work will serve as a roadmap to facilitate the development of compounds acting on the circadian clock for the treatment of BD.

Circadian Rhythm Disturbances in Mood Disorders: Insights into the Role of the Suprachiasmatic Nucleus

Circadian rhythm disturbances are a common symptom among individuals with mood disorders. The suprachiasmatic nucleus (SCN), in the ventral part of the anterior hypothalamus, orchestrates physiological and behavioral circadian rhythms. The SCN consists of self-sustaining oscillators and receives photic and nonphotic cues, which entrain the SCN to the external environment. In turn, through synaptic and hormonal mechanisms, the SCN can drive and synchronize circadian rhythms in extra-SCN brain regions and peripheral tissues. Thus, genetic or environmental perturbations of SCN rhythms could disrupt brain regions more closely related to mood regulation and cause mood disturbances. Here, we review clinical and preclinical studies that provide evidence both for and against a causal role for the SCN in mood disorders.

Clock-Enhancing Small Molecules and Potential Applications in Chronic Diseases and Aging

Normal physiological functions require a robust biological timer called the circadian clock. When clocks are dysregulated, misaligned, or dampened, pathological consequences ensue, leading to chronic diseases and accelerated aging. An emerging research area is the development of clock-targeting compounds that may serve as drug candidates to correct dysregulated rhythms and hence mitigate disease symptoms and age-related decline. In this review, we first present a concise view of the circadian oscillator, physiological networks, and regulatory mechanisms of circadian amplitude. Given a close association of circadian amplitude dampening and disease progression, clock-enhancing small molecules (CEMs) are of particular interest as candidate chronotherapeutics. A recent proof-of-principle study illustrated that the natural polymethoxylated flavonoid nobiletin directly targets the circadian oscillator and elicits robust metabolic improvements in mice. We describe mood disorders and aging as potential therapeutic targets of CEMs. Future studies of CEMs will shed important insight into the regulation and disease relevance of circadian clocks.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light-dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6 hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

The effects of combining serotonin reuptake inhibition and 5-HT7 receptor blockade on circadian rhythm regulation in rodents

Disruption of circadian rhythms may lead to mood disorders. The present study investigated the potential therapeutic utility of combining a 5-HT7 antagonist with a selective serotonin (5-HT) reuptake inhibitor (SSRI), the standard of care in depression, on circadian rhythm regulation. In tissue explants of the suprachiasmatic nucleus (SCN) from PER2::LUC mice genetically modified to report changes in the expression of a key clock protein, the period length of PER2 bioluminescence was shortened in the presence of AS19, a 5-HT7 partial agonist. This reduction was blocked by SB269970, a selective 5-HT7 antagonist. The SSRI, escitalopram, had no effect alone on period length, but a combination with SB269970, yielded significant increases. Dosed in vivo, escitalopram had little impact on the occurrence of activity onsets in rats given access to running wheels, whether the drug was given acutely or sub-chronically. However, preceding the escitalopram treatment with a single acute dose of SB269970 produced robust phase delays, in keeping with the in vitro explant data. Taken together, these findings suggest that the combination of an SSRI and a 5-HT7 receptor antagonist has a greater impact on circadian rhythms than that observed with either agent alone, and that such a multimodal approach may be of therapeutic value in treating patients with poor clock function.

A mutation in CLOCK leads to altered dopamine receptor function

Mice with a mutation in the Clock gene (ClockΔ19) have a number of behavioral phenotypes that suggest alterations in dopaminergic transmission. These include hyperactivity, increased exploratory behavior, and increased reward value for drugs of abuse. However, the complex changes in dopaminergic transmission that underlie the behavioral abnormalities in these mice remain unclear. Here we find that a loss of CLOCK function increases dopamine release and turnover in striatum as indicated by increased levels of metabolites HVA and DOPAC, and enhances sensitivity to dopamine receptor antagonists. Interestingly, this enlarged dopaminergic tone results in downstream changes in dopamine receptor (DR) levels with a surprising augmentation of both D1- and D2-type DR protein, but a significant shift in the ratio of D1 : D2 receptors in favor of D2 receptor signaling. These effects have functional consequences for both behavior and intracellular signaling, with alterations in locomotor responses to both D1-type and D2-type specific agonists and a blunted response to cAMP activation in the ClockΔ19 mutants. Taken together, these studies further elucidate the abnormalities in dopaminergic transmission that underlie mood, activity, and addictive behaviors.

Evaluation of serotonin, noradrenaline and dopamine reuptake inhibitors on light-induced phase advances in hamster circadian activity rhythms

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed for the treatment of anxiodepressive states that are often associated with perturbed circadian rhythms including, in certain patients, phase advances. Surprisingly, the influence of SSRIs upon circadian activity rhythms has been little studied in experimental models.

OBJECTIVES

Accordingly, this study examined the ability of SSRIs to modulate the phase-setting properties of light on circadian activity rhythms in hamsters. Their actions were compared to those of the mixed serotonin/noradrenaline reuptake inhibitor (SNRI), venlafaxine, the selective noradrenaline reuptake inhibitor, reboxetine, and the dopamine reuptake inhibitor, bupropion.

MATERIALS AND METHODS

Wheel-running activity rhythms were recorded in male Syrian hamsters. Drugs were administered systemically before a light stimulus that was used to advance the timing of the hamster running rhythms.

RESULTS

Four chemically diverse SSRIs, citalopram (1-10 mg/kg, intraperitoneally), fluvoxamine (1-10), paroxetine (1-10), and fluoxetine (10 and 20), all robustly and significantly inhibited the ability of light to phase advance hamster circadian wheel-running activity rhythms. Their actions were mimicked by venlafaxine (1-10) that likewise elicited a marked reduction in phase advances. Conversely, reboxetine (1-20) and bupropion (1-20) did not exert significant effects.

CONCLUSIONS

These data suggest that suppression of serotonin (but not noradrenaline or dopamine) reuptake by SSRIs and SNRIs modifies circadian locomotor activity rhythms in hamsters. Further, they support the notion that an inhibitory influence upon the early-morning light-induced advance in circadian activity contributes to the therapeutic effects of serotonin uptake inhibitors in certain depressed patients.

Maternal fluoxetine infusion does not alter fetal endocrine and biophysical circadian rhythms in pregnant sheep

OBJECTIVE

Depression during pregnancy is frequently treated with the selective serotonin reuptake inhibitor (SSRI), fluoxetine (FX), commonly known as Prozac (Eli Lilly & Co, Indianapolis, IN). FX potentiates serotoninergic neurotransmission and serotonin has been implicated in the regulation of circadian rhythms. We have therefore investigated the effect of chronic administration of FX on maternal and fetal circadian rhythms in sheep.

METHODS

Following an initial bolus dose of 70 mg FX, an 8-day continuous infusion of FX (n = 11, 98.5 microg/kg x d) was performed. Controls (n = 13) were treated with sterile water vehicle only. Maternal and fetal plasma melatonin and prolactin concentrations were determined every 3 hours for 24 hours and then every 6 hours for 24 hours beginning on the fourth day of infusion.

RESULTS

FX treatment did not alter either the basal or circadian rhythms of either maternal or fetal plasma melatonin and prolactin concentrations. Fetal cardiovascular and behavioral state parameters were measured continuously. While the incidence of low-voltage (LV) electrocortical (ECOG) activity was significantly reduced in fetuses in the FX group, there was no effect of FX on the diurnal rhythms in fetal arterial pressure, heart rate, breathing movements, or behavioral state.

CONCLUSION

These results show that maternal FX treatment does not result in significant alterations in maternal and fetal hormonal and behavioral circadian rhythms.

Inhibition of the serotonin (5-hydroxytryptamine) transporter reduces bone accrual during growth

Selective serotonin-reuptake inhibitors (SSRIs) antagonize the serotonin (5-hydroxytryptamine) transporter (5-HTT), and are frequently prescribed to children and adolescents to treat depression. However, recent findings of functional serotonergic pathways in bone cells and preliminary clinical evidence demonstrating detrimental effects of SSRIs on bone growth have raised questions regarding the effects of these drugs on the growing skeleton. The current work investigated the impact of 5-HTT inhibition on the skeleton in: 1) mice with a null mutation in the gene encoding for the 5-HTT; and 2) growing mice treated with a SSRI. In both models, 5-HTT inhibition had significant detrimental effects on bone mineral accrual. 5-HTT null mutant mice had a consistent skeletal phenotype of reduced mass, altered architecture, and inferior mechanical properties, whereas bone mineral accrual was impaired in growing mice treated with a SSRI. These phenotypes resulted from a reduction in bone formation without an increase in bone resorption and were not influenced by effects on skeletal mechanosensitivity or serum biochemistries. These findings indicate a role for the 5-HTT in the regulation of bone accrual in the growing skeleton and point to a need for further research into the prescription of SSRIs to children and adolescents.

Serotonin transporter localization in the hamster suprachiasmatic nucleus

Pacemaker cells within the hamster suprachiasmatic nucleus generate circadian rhythms. The suprachiasmatic nucleus is heavily innervated by serotonin axons originating in the median raphe nuclei. Consequently, serotonergic agonists and antagonists or agents that alter levels of serotonin in the synapse following transmission can modulate many aspects of circadian rhythmicity. Examples of the latter are some antidepressants and the stimulant amphetamine that bind to the serotonin transporter and block serotonin reuptake. It has been hypothesized that circadian rhythm dysfunction may be involved in depression, and that the efficacy of certain antidepressants in treating depression may involve an alteration of serotonin levels and certain circadian rhythm parameters. However, although the hamster is the behavioral model of choice for the study of circadian rhythms, the identification of serotonin transporters in this species has not been reported. Therefore, in this report we describe the distribution of the serotonin transporter in the hamster suprachiasmatic nucleus using immunohistochemical techniques. Our results demonstrate a dense labeling of the serotonin transporter throughout the ventral and medial regions of the suprachiasmatic nucleus, a pattern that overlaps the distribution of serotonergic afferents in this nucleus. Amphetamines and certain antidepressants may serve as substrates for this transporter and elicit chronopharmacological activity by elevating serotonin levels in the suprachiasmatic nucleus.

Antidepressant treatment during social challenge prior to 1 year of age affects immune and endocrine responses in adult macaques

Antidepressants are widely used in treating depression and other behavioral problems in children and adolescents. Little is known about the long-term effects of these agents, particularly on physiological systems. The effects of previous antidepressant treatment during a social challenge in 9-month-old rhesus monkeys (Macaca mulatta) on their adult immune and endocrine responses were studied. Prior to the social challenge, the monkeys were reared either by their mother or in a peer group. Monkeys were treated with either a serotonergic agonist (fluoxetine), a noradrenergic agonist (desipramine), or saline during social separation. Non-separated, saline-treated monkeys served as control monkeys. In order to evaluate immune effects of early antidepressant treatment, adult monkeys were immunized with a novel antigen, tetanus toxoid. Blood samples were collected prior to and at 4-5-day intervals for 28 days after immunization. Plasma total immunoglobulins (IgG and IgM), complement levels (C3 and C4), tetanus antibody titers, and cortisol were assessed. Antibody levels were lowest in monkeys treated with antidepressants regardless of specific drug treatment or early rearing condition. Drug-treated subjects had elevated plasma immunoglobulins and complement protein levels. Cortisol was also highest in drug-treated subjects. These results should be considered when prescribing commonly used antidepressants for treatment of childhood disorders.

Localization of serotonin(5A) receptors in discrete regions of the circadian timing system in the Syrian hamster

Endogenous serotonin and serotonergic drugs influence many aspects of circadian rhythms, including phase shifts, onset of locomotor activity, and period length and integrity of rhythms during exposure to constant light. The receptor subtype(s) mediating all of these circadian effects of serotonin has (have) not been identified. Immunoreactivity for the serotonin(5A) (5-HT(5A)) receptor has recently been identified in the rat suprachiasmatic nucleus (SCN). In this study, we investigated the distribution of the 5-HT(5A) receptors in four neural components of the circadian timing system (the SCN, the intergeniculate leaflet, and the median and dorsal raphe nuclei), in the Syrian hamster. Single and dual immunohistochemistry were conducted using an affinity-purified rabbit antibody generated against a peptide sequence unique to the 5-HT(5A) receptor, guinea pig anti-5-HT antisera and guinea pig anti-GABA antisera. For single labeling, immunoreactivity was visualized using DAB-nickel as the chromagen. All four regions showed strong, yet distinct, immunoreactivity for the 5-HT(5A) receptor. No specific labeling was present in the absorption or omission controls. For double labeling, immunoreactivity was visualized using immunofluorescence with Cy5- and FITC-labeled second antibodies followed by confocal microscopy. In the raphe nuclei, 5-HT-immunoreactivity and 5-HT(5A)-immunoreactivity were co-localized in cell bodies and axons. GABA-immunoreactive fibers surrounded some of the 5-HT(5A) receptor-immunoreactive cell bodies in the raphe nuclei. In conclusion, the 5-HT(5A) receptors are localized within several important neuroanatomical substrates of the circadian timekeeping system, and within the raphe nuclei, appear to be present on serotonin neurons. These findings suggest that some of the circadian effects of 5-HT may be mediated by the 5-HT(5A) receptor, which may function as a presynaptic autoreceptor.

Serotonin and the regulation of mammalian circadian rhythmicity

The suprachiasmatic nucleus (SCN), the site of the primary mammalian circadian clock, contains one of the densest serotonergic terminal plexes in the brain. Although this fact has been appreciated for some time, only in the last decade has there been substantial approach toward the understanding of the function of serotonin in the circadian rhythm system. The intergeniculate leaflet, which projects to the SCN via the geniculohypothalamic tract, receives serotonergic innervation from the dorsal raphe nucleus, and the SCN receives its serotonergic input from the median raphe nucleus. This separation of serotonergic origins provides the opportunity to investigate the function of the two projections. Loss of serotonergic neurones of the median raphe yields earlier onset and later offset of the nocturnal activity phase, longer duration of the activity phase, and increased sensitivity of circadian rhythm response to light. Despite the simplicity of the origins of serotonergic anatomy with respect to the circadian rhythm system, the actual involvement of serotonin in rhythm modulation is not so obvious. A variety of pharmacological studies have clearly implicated serotonin as a direct regulator of circadian rhythm phase, but others employing different methods suggest that simple elevation of SCN serotonin concentrations does not modify rhythm phase. The most convincing role of serotonin is its apparent ability to modulate sensitivity of the circadian rhythm to light. The putative method for such modulation is via a presynaptic 5-HT1B receptor on the retinohypothalamic tract, the activation of which attenuates photic input to the SCN thereby reducing phase response to light. Serotonin may modulate phase response to benzodiazepines, but does not appear to modify such response to environmentally induced locomotor activity. Current interest in serotonergic modulation of circadian rhythmicity is strong and the research is vigorous. There is an abundance of information about serotonin and circadian rhythm function that lacks a satisfactory framework for its interpretation. The next decade is likely to see the gradual evolution of this framework as the role of serotonin in circadian rhythm regulation is further elucidated.

A role for serotonin in the circadian system revealed by the distribution of serotonin transporter and light-induced Fos immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet

Components of the circadian system, the suprachiasmatic nucleus and the intergeniculate leaflet receive serotonin input from the raphe nuclei. Manipulations of serotonin neurotransmission disrupt cellular, electrophysiological, and behavioural responses of the circadian system to light, suggesting that serotonin plays a modulatory role in photic regulation of circadian rhythms. To study the relation between serotonin afferents and light-activated cells in the suprachiasmatic nucleus and intergeniculate leaflet, we used immunostaining for the serotonin transporter and for the transcription factor, Fos. Serotonin transporter, a plasma membrane protein located on serotonin neurons, regulates the amount of serotonin available for neurotransmission by re-accumulating released serotonin into presynaptic neurons; expression of Fos in the suprachiasmatic nucleus identifies light-activated cells involved in photic resetting of circadian clock phase. In the suprachiasmatic nucleus, immunostaining for serotonin transporter revealed a dense plexus of fibres concentrated primarily in the ventrolateral region. In the intergeniculate leaflet, serotonin transporter immunostaining identified vertically-oriented columns of fibres. Serotonin transporter immunostaining was abolished by pretreatment with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Exposure to light for 30 min during the dark phase of the light cycle induced Fos expression in the ventrolateral suprachiasmatic nucleus and intergeniculate leaflet regions. In both structures the Fos-expressing cells were encircled by serotonin transporter-immunoreactive fibres often in close apposition to these cells. These results support the idea that serotonin activity plays a modulatory role in processing of photic information within the circadian system.

Serotonin-containing fibers in the suprachiasmatic hypothalamus attenuate light-induced phase delays in mice

Photic and non-photic stimuli phase shift and entrain circadian rhythms through distinct but interacting mechanisms which impinge on the suprachiasmatic nucleus (SCN), the circadian pacemaker. Our understanding of this mechanism is incomplete. Serotonin (5-HT) injected locally at the SCN reduces light-induced glutamate release and decreases the expression of c-fos, a marker of photic transduction. Furthermore, in SCN slices, 5-HT application reduces field potentials after optic nerve stimulation. We therefore predicted that 5-HT-terminal destruction restricted to the SCN would augment phase shifts of circadian rhythms induced by light exposure. To investigate this possibility, we compared photic phase delays and Fos-like immunoreactivity in mice which had previously received bilateral infusions directed at the SCN containing either the selective 5-HT neurotoxin 5,7-dihydroxytryptamine (DHT, n = 16) or vehicle (VEH, n = 12). Phase delays after a light pulse given during the mid-subjective night (30 lux, 30 min starting at circadian time (CT) 12-20) in DHT-mice were 50% greater than in VEH-mice (P = 0.017). DHT mice (n = 5) had 76% larger Fos responses to a mid-subjective night light pulse than VEH-mice (n = 5) (P = 0.029). We conclude that 5-HT at or near the SCN in mice reduces photic phase shifts and modulates the magnitude of the photic phase response in the mouse.

Lesion of the serotonergic terminals in the suprachiasmatic nuclei limits the phase advance of body temperature rhythm in food-restricted rats fed during daytime

The daily rhythm of body temperature was recorded in control rats fed ad libitum and subsequently fed during daytime 50% of ad libitum food intake. Aside from the expression of a feeding-associated component, body temperature rhythm was phase advanced (7 h) by a timed caloric restriction; the new plateau of the acrophase of the nocturnal peak was close to the light-dark transition. A lesion of serotonergic (5-HTergic) terminals in the suprachiasmatic nuclei (SCN)-the endogenous circadian clock(s)-was performed by microinjection of the 5-HT neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). During the ad libitum-fed state, the acrophase of body temperature rhythm was not modified by the 5,7-DHT treatment. In response to a timed caloric restriction, however, the phase advance of the nocturnal peak of body temperature rhythm was reduced by 2 h in rats with 5,7-DHT lesions as compared to that of sham-operated rats. Magnitude and day-night pattern of wheel-running activity between the two groups of rats also were analyzed. No intergroup difference was found in the amount of wheel-running activity prior to the time of feeding. Moreover, the phase advance of nocturnal component of locomotor activity rhythm observed toward the time of feeding in sham-operated rats was limited by 5,7-DHT treatment. It is concluded that the photic synchronization of body temperature rhythm does not depend on the 5-HTergic projection to SCN under ad libitum conditions. By contrast, the phase-advancing property of a timed caloric restriction on the daily rhythm of body temperature is mediated by a neuronal circuit involving the 5-HTergic projection to SCN. That the phase advance was not fully eliminated by 5,7-DHT treatment suggests that other pathways participate in this mediation.

Effects of fluoxetine and olfactory bulbectomy on mouse circadian activity rhythms

Olfactory bulbectomy (OBX) in SWR outbred male mice lengthened the free-running period and delayed the phase of a circadian rhythm for wheel-running activity. OBX also increased mean levels of activity. Two weeks of daily intraperitoneal injections of Fluoxetine (8 mg/kg), a serotonin re-uptake inhibitor, reversed the effects of bulbectomy on the mean level of activity and significantly shortened the free-running period of the activity rhythm. The phase of the activity rhythm was not significantly affected by the Fluoxetine treatment. These results are consistent with a hyposerotonergic mediation of the effects of OBX on circadian period and activity level.

Antidepressant and depressogenic drugs lack consistent effects on hamster circadian rhythms

The circadian wheel-running rhythm of golden hamsters was monitored during chronic oral treatment with four antidepressants and two potentially depressogenic agents. Desmethylimipramine shortened the circadian period (tau) by 0.1 hour. In contrast, clorgyline lengthened tau by 0.1 hour and delayed light-synchronized wheel-running rhythms by 1.4 hour. Phenelzine, fluoxetine, clonidine, and propranolol did not significantly alter light-entrained phase or free-running period over a range of doses. Other rhythm parameters were also unaffected by antidepressant or depressogenic drugs. These data suggest that mood-altering drugs do not consistently influence circadian rhythms in the hamster.

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.

Specific destruction of the serotonergic afferents to the suprachiasmatic nuclei prevents triazolam-induced phase advances of hamster activity rhythms

Administration of Triazolam (Tz)--a short acting benzodiazepine (BZ)--induces permanent phase-shifts in locomotor activity of golden hamsters (Mesocricetus auratus). However, the target area(s) as well as the mechanism involved in the Tz-induced changes are not known. Previous results indicated that raphe nuclei (RN) would appear to be a likely site for Tz-induced phase shifts. Therefore, we specifically destroyed the 5-HT fibers connecting the RN with the SCN--the site of the endogenous mammalian clock--by microinjections of the selective neurotoxin 5,7 dihydroxytryptamine (5,7-DHT) at the level of SCN. Infusion of 5,7-DHT resulted in long lasting damage of the ascending serotonergic projection from RN to the hypothalamus. Subsequently, the phase-shifting effect of Tz was investigated. Only complete or almost complete depletion of the 5-HT input to the SCN was accompanied with a pronounced reduction of the phase shift together with a significant reduction of wheel-running activity during the 6 h following Tz injection. Our present results support the view that the 5-HT innervation of the SCN represents an essential link in the phase-shifting action following peripheral Tz injections.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Monoamine depletion alters the entrainment and the response to light of the circadian activity rhythm in hamsters

Reduced amplitude, shorter free-running periods and desynchronization among a number of circadian rhythms are associated with advanced age in rodents. The response of the hamster circadian system to photic stimuli is also altered during senescence. Decreased monoamine levels, receptor sites and neuronal populations are commonly observed in the aging brain. The objective of the present study was to determine if monoamine depletion with reserpine in young hamsters induces changes in the circadian rhythm of locomotor activity similar to those that occur spontaneously with aging. Wheel-running activity of 12 young hamsters under a 14 h-light/10 h-dark cycle was continuously monitored. The total activity level, the times of activity onset, peak and offset and the duration of activity were determined during a 1-week period after vehicle treatment and for three 1-week periods after reserpine treatment (4 mg/kg). A second group of eight reserpine-treated and six vehicle-treated animals was kept in constant darkness (DD). The period of the circadian activity rhythm in DD and the phase-shifts after short light pulses at circadian time 19 (CT19) were determined in the control and reserpine-treated groups. Brain monoamines in the hypothalamus, striatum and pons/medulla after reserpine and vehicle treatment were determined by high-pressure liquid chromatography. The data were analyzed with x2 periodogram and one-way ANOVA followed by Duncan's post hoc test. Reserpine treatment significantly reduced total wheel-running activity and the monoamine levels in the hypothalamus, striatum and pons/medulla.(ABSTRACT TRUNCATED AT 250 WORDS)