Circadian Rhythm Of Wheel-running Activity Research Articles

A ketogenic diet containing medium-chain triglycerides reduces REM sleep duration without significant influence on mouse circadian phenotypes

Ketogenic diets (KDs) affect the circadian rhythms of behavior and clock gene expression in experimental animals. However, these diets were designed to simulate a fasting state; thus, whether these effects are caused by diet-induced ketogenesis or persistent starvation is difficult to distinguish. The present study aimed to define the effects of a KD containing medium-chain triglycerides (MCT-KD) that increase blood ketone levels without inducing carbohydrate starvation, on circadian rhythms and sleep regulation. Mice were fed with a normal diet (CTRL) or MCT-KD for 2 weeks. Blood β-hydroxybutyrate levels were significantly increased up to 2 mM by the MCT-KD, whereas body weight gain and blood glucose levels were identical between the groups, suggesting that ketosis accumulated without carbohydrate starvation in the MCT-KD mice. Circadian rhythms of wheel-running activity and core body temperature were almost identical, although wheel-running was slightly reduced in the MCT-KD mice. The circadian expression of the core clock genes, Per1, Per2, Bmal1, and Dbp in the hypothalamus, heart, liver, epididymal adipose tissues, and skeletal muscle were almost identical between the CTRL and MCT-KD mice, whereas the amplitude of hepatic Per2 and adipose Per1 expression was increased in MCT-KD mice. The MCT-KD reduced the duration of rapid-eye-movement (REM) sleep without affecting the duration of non-REM sleep and the duration of wakefulness. These findings suggested that the impact of ketone bodies on circadian systems are limited, although they might reduce locomotor activity and REM sleep duration.

Deletion of Six3 in post-proliferative neurons produces weakened SCN circadian output, improved metabolic function, and dwarfism in male mice

ObjectiveThe increasing prevalence of obesity makes it important to increase the understanding of the maturation and function of the neuronal integrators and regulators of metabolic function. MethodsBehavioral, molecular, and physiological analyses of transgenic mice with Sine oculis 3 (Six3) deleted in mature neurons using the Synapsincreallele. ResultsConditional deletion of the homeodomain transcription factor Six3 in mature neurons causes dwarfism and weakens circadian wheel-running activity rhythms but increases general activity at night, and improves metabolic function, without impacting pubertal onset or fertility in males. The reduced growth in 6-week-old Six3fl/fl:Synapsincre (Six3syn) males correlates with increased somatostatin (SS) expression in the hypothalamus and reduced growth hormone (GH) in the pituitary. In contrast, 12-week-old Six3syn males have increased GH release, despite an increased number of the inhibitory SS neurons in the periventricular nucleus. GH is important in glucose metabolism, muscle function, and bone health. Interestingly, Six3syn males have improved glucose tolerance at 7, 12, and 18 weeks of age, which, in adulthood, is associated with increased % lean mass and increased metabolic rates. Further, 12-week-old Six3syn males have reduced bone mineralization and a lower bone mineral density, indicating that reduced GH levels during early life cause a long-term reduction in bone mineralization. ConclusionOur study points to the novel role of Six3 in post-proliferative neurons to regulate metabolic function through SS neuron control of GH release.

Memory dysfunction and anxiety-like behavior in a mouse model of chronic sleep disorders

Sleep disturbances can contribute to cognitive decline and neuropsychiatric disorders. However, the underlying mechanisms of these processes are poorly understood. The present study evaluated the effects of a chronic sleep disorder (CSD) on long-term memory formation and anxiety-like behavior in our originally established mouse model of psychophysiological stress-induced CSD characterized by disrupted circadian rhythms of wheel-running activity and sleep-wake cycles. Model mice are continuously exposed to mild stress imposed by perpetually staying on a running-wheel to avoid water. The findings of novel object recognition (NORT) and open field (OFT) tests showed that CSD impaired recognition memory and elicited anxiety-like behavior, respectively. These results suggested that the CSD impaired cognitive function and emotional status. Thus, this CSD model could be useful for studying the underlying mechanisms of neurobehavioral difficulties caused by sleep disorders. We then examined the hippocampal mRNA expression of genes associated with learning and memory, and anxiety and depression. The CSD increased the mRNA expression of Crhr1, Ngf and Phlpp1, and suppressed that of Ace, Egr2 and Slc6a4. Based on the functions of these genes, we inferred that the increase in Crhr1 mRNA was associated with the pathogenesis of psychiatric conditions, whereas mRNA levels of the other five genes were directed towards symptom relief. Upregulating hippocampal Crhr1 expression might contribute in part to the activation of corticotropin-releasing hormone (CRH)-CRH receptor1 signaling that mediates CSD-evoked mental disorders.

Dietary Heat-Killed Lactobacillus brevis SBC8803 Attenuates Chronic Sleep Disorders Induced by Psychophysiological Stress in Mice.

We previously reported that dietary heat-killed Lactobacillus brevis SBC8803 affects sleep in mice and humans. The present study examined whether SBC8803 improves psychophysiological stress-induced chronic sleep disorders (CSD) using a mouse model characterized by disrupted circadian rhythms of wheel-running activity and sleep-wake cycles. Mice were fed with a diet supplemented with 0.5% heat-killed SBC8803 for 6 wk and imposed stress-induced CSD for last 2 wk. Dietary SBC8803 suppressed the reduction in wheel-running activity induced by CSD. Electroencephalography (EEG) revealed that SBC8803 significantly restored wakefulness and increased non-rapid eye movement (NREM) sleep during the second half of the active phase during CSD. The CSD-induced reduction in EEG slow wave activity, a marker of NREM sleep intensity, during the beginning of the inactive phase was significantly improved by SBC8803 supplementation. These findings suggest that dietary heat-killed SBC8803 confers beneficial effects on insomnia and circadian sleep disorders induced by psychophysiological stress.

Chronic sleep disorder induced by psychophysiological stress induces glucose intolerance without adipose inflammation in mice

Sleep disturbances are associated with various metabolic diseases such as hypertension and diabetes. We had previously established a mouse model of a psychophysiological stress-induced chronic sleep disorder (CSD) characterized by disrupted circadian rhythms of wheel-running activity, core body temperature, and sleep-wake cycles. To evaluate the underlying mechanisms of metabolic disorders induced by CSD, we created mice with CSD for six weeks and fed them with a high-fat diet. Glucose intolerance with hyperglycemia resulted, although plasma insulin levels and body weight increases were identical between control and CSD mice. Gluconeogenesis and glycolysis were enhanced and suppressed, respectively, in the livers of CSD mice, because the mRNA expression of Pck1 was significantly increased, whereas that of Gck and Pklr were significantly decreased in the CSD mice. Adipose inflammation induced by the high-fat diet seemed suppressed by the CSD, because the mRNA expression levels of Adgre1, Ccl2, and Tnf were significantly downregulated in the adipose tissues of CSD mice. These findings suggest that CSD impair glucose tolerance by inducing gluconeogenesis and suppressing glycolysis. Hyperphasia with hypoleptinemia, hypercorticosteronemia, and increased plasma free fatty acids might be involved in the impaired glucose metabolism under a CSD. Further studies are needed to elucidate the endocrine and molecular mechanisms underlying the associations between sleep disorders and impaired glucose homeostasis that consequently causes diabetes.

Circadian wheel-running activity rhythms are unaltered by acute cage enrichment with nests and shelters

C57BL/6J inbred male mice were assessed for the effects of cotton nesting material on circadian running-wheel assays at 60, 157 and 222 days of age. The oldest mice were also tested for effects of access to plastic igloo shelters and the combination of cotton plus igloos. All mice were assayed for mean activity level, acrophase and entrained circadian period in 12:12 LD, and mean activity level and free-running period in DD for circadian wheel-running activity. There were no significant effects, at any age, of either nesting material or shelters, or the combination of both, on any measures of circadian running-wheel activity. We conclude that circadian rhythm assessment is unlikely to be significantly altered by the inclusion of enrichment materials in running wheel cages.

Disrupted reproduction, estrous cycle, and circadian rhythms in female mice deficient in vasoactive intestinal peptide.

The female reproductive cycle is gated by the circadian timing system and may be vulnerable to disruptions in the circadian system. Prior work suggests that vasoactive intestinal peptide (VIP)-expressing neurons in the suprachiasmatic nucleus (SCN) are one pathway by which the circadian clock can influence the estrous cycle, but the impact of the loss of this peptide on reproduction has not been assessed. In the present study, we first examine the impact of the genetic loss of the neuropeptide VIP on the reproductive success of female mice. Significantly, mutant females produce about half the offspring of their wild-type sisters even when mated to the same males. We also find that VIP-deficient females exhibit a disrupted estrous cycle; that is, ovulation occurs less frequently and results in the release of fewer oocytes compared with controls. Circadian rhythms of wheel-running activity are disrupted in the female mutant mice, as is the spontaneous electrical activity of dorsal SCN neurons. On a molecular level, the VIP-deficient SCN tissue exhibits lower amplitude oscillations with altered phase relationships between the SCN and peripheral oscillators as measured by PER2-driven bioluminescence. The simplest explanation of our data is that the loss of VIP results in a weakened SCN oscillator, which reduces the synchronization of the female circadian system. These results clarify one of the mechanisms by which disruption of the circadian system reduces female reproductive success.

Positive allosteric modulators at GABAB receptors exert intrinsic actions and enhance the influence of baclofen on light-induced phase shifts of hamster circadian activity rhythms

Light-induced phase shifts of hamster circadian activity rhythms are modulated by GABAB receptors. Recently, positive allosteric modulators (PAM)s at GABAB receptors were described, but it is not known whether they affect light-induced entrainment of circadian rhythms. Therefore, we studied the effects of two GABAB PAMs, GS39783 and RacBHFF, upon light-induced phase advances and delays of hamster circadian wheel-running activity rhythms. Wheel running activity was recorded for Syrian hamsters maintained in constant darkness. Drugs administered intraperitoneally were evaluated for their ability to modulate a light-induced shift of the circadian activity rhythm. Baclofen (3.75–15mg/kg) dose-dependently inhibited both light-induced phase advances and delays of hamster wheel running rhythms, and its actions were blocked by the selective GABAB antagonist, SCH50911 (5mg/kg). Neither GS39783 (3–30mg/kg) nor RacBHFF (0.63–10mg/kg) affected phase advances when injected alone, but both GS39783 (3mg/kg) and RacBHFF (10mg/kg) augmented the inhibitory effect of baclofen (5mg/kg). At doses above 3mg/kg, GS39783 and RacBHFF significantly inhibited phase delays alone, consistent with the notion of “agonist-allosteric” properties. GS39783 (0.5mg/kg), but not RacBHFF (10mg/kg), augmented the inhibitory action of baclofen on phase delays. These data are consistent with the possibility that GS39783 and RacBHFF act as PAMs at GABAB receptors inhibiting light-induced phase advances, yet that they also posses “allosteric agonist” actions at the (presumably separate) population of GABAB receptors modulating light-induced phase delays. GABAB receptors clearly warrant further investigation as agents for modulation of circadian dysfunction associated with CNS disorders such as depression.

Behavioral and Genetic Dissection of a Mouse Model for Advanced Sleep Phase Syndrome

The adaptive value of the endogenous circadian clock arises from its ability to synchronize (i.e., entrain) to external light-dark (LD) cycles at an appropriate phase. Studies have suggested that advanced circadian phase alignment might result from shortening of the period length of the clock. Here we explore mechanisms that contribute to an early activity phase in CAST/EiJ (CAST) mice. We investigated circadian rhythms of wheel-running activity in C57BL/6J (B6), CAST and 2 strains of B6.CAST congenic mice, which carry CAST segments introgressed in a B6 genome. When entrained, all CAST mice initiate daily activity several hours earlier than normal mice. This difference could not be explained by alterations in the endogenous period, as activity onset did not correlate with period length. However, the photic phase-shifting responses in these mice were phase-lagged by 3 hours relative to their activity. Attenuated light masking responses were also found in CAST mice, which allow for activity normally inhibited by light. A previously identified quantitative trait locus (QTL), Era1, which contributes to the early activity trait, was confirmed and refined here using two B6.CAST congenic strains. Surprisingly, these B6.CAST mice exhibited longer rather than shorter endogenous periods, further demonstrating that the advanced phase in these mice is not due to alterations in period. CAST mice have an advanced activity phase similar to human advanced sleep phase syndrome. This advanced phase is not due to its shorter period length or smaller light-induced phase shifts, but appears to be related to both light masking and altered coupling of the circadian pacemaker with various outputs. Lastly, a QTL influencing this trait was confirmed and narrowed using congenic mice as a first step toward gene identification.

Temporal phasing of locomotor activity, heart rate rhythmicity, and core body temperature is disrupted in VIP receptor 2-deficient mice

Neurons of the brain's biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) generate circadian rhythms of physiology (core body temperature, hormone secretion, locomotor activity, sleep/wake, and heart rate) with distinct temporal phasing when entrained by the light/dark (LD) cycle. The neuropeptide vasoactive intestinal polypetide (VIP) and its receptor (VPAC2) are highly expressed in the SCN. Recent studies indicate that VIPergic signaling plays an essential role in the maintenance of ongoing circadian rhythmicity by synchronizing SCN cells and by maintaining rhythmicity within individual neurons. To further increase the understanding of the role of VPAC2 signaling in circadian regulation, we implanted telemetric devices and simultaneously measured core body temperature, spontaneous activity, and heart rate in a strain of VPAC2-deficient mice and compared these observations with observations made from mice examined by wheel-running activity. The study demonstrates that VPAC2 signaling is necessary for a functional circadian clock driving locomotor activity, core body temperature, and heart rate rhythmicity, since VPAC2-deficient mice lose the rhythms in all three parameters when placed under constant conditions (of either light or darkness). Furthermore, although 24-h rhythms for three parameters are retained in VPAC2-deficient mice during the LD cycle, the temperature rhythm displays markedly altered time course and profile, rising earlier and peaking ∼4-6 h prior to that of wild-type mice. The use of telemetric devices to measure circadian locomotor activity, temperature, and heart rate, together with the classical determination of circadian rhythms of wheel-running activity, raises questions about how representative wheel-running activity may be of other behavioral parameters, especially when animals have altered circadian phenotype.

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

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. 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. 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. 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. 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.

Entrainment to Feeding but Not to Light: Circadian Phenotype of VPAC2Receptor-Null Mice

The master clock driving mammalian circadian rhythms is located in the suprachiasmatic nuclei (SCN) of the hypothalamus and entrained by daily light/dark cycles. SCN lesions abolish circadian rhythms of behavior and result in a loss of synchronized circadian rhythms of clock gene expression in peripheral organs (e.g., the liver) and of hormone secretion (e.g., corticosterone). We examined rhythms of behavior, hepatic clock gene expression, and corticosterone secretion in VPAC2 receptor-null (Vipr2-/-) mice, which lack a functional SCN clock. Unexpectedly, although Vipr2-/- mice lacked robust circadian rhythms of wheel-running activity and corticosterone secretion, hepatic clock gene expression was strongly rhythmic, but advanced in phase compared with that in wild-type mice. The timing of food availability is thought to be an important entrainment signal for circadian clocks outside the SCN. Vipr2-/- mice consumed food significantly earlier in the 24 h cycle than wild-type mice, consistent with the observed timing of peripheral rhythms of circadian gene expression. When restricted to feeding only during the daytime (RF), mice develop rhythms of activity and of corticosterone secretion in anticipation of feeding time, thought to be driven by a food-entrainable circadian oscillator, located outside the SCN. Under RF, mice of both genotypes developed food-anticipatory rhythms of activity and corticosterone secretion, and hepatic gene expression rhythms also became synchronized to the RF stimulus. Thus, food intake is an effective zeitgeber capable of coordinating circadian rhythms of behavior, peripheral clock gene expression, and hormone secretion, even in the absence of a functional SCN clock.

Behavioral characterization and modulation of circadian rhythms by light and melatonin in C3H/HeN mice homozygous for the RORβ knockout

This study reports for the first time the effects of retinoid-related orphan receptors [RORbeta; receptor gene deletion RORbeta(C3H)(-/-)] in C3H/HeN mice on behavioral and circadian phenotypes. Pineal melatonin levels showed a robust diurnal rhythm with high levels at night in wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The RORbeta(C3H)(-/-) mice displayed motor ("duck gait," hind paw clasping reflex) and olfactory deficits, and reduced anxiety and learned helplessness-related behaviors. Circadian rhythms of wheel-running activity in all genotypes showed entrainment to the light-dark (LD) cycle, and free running in constant dark, with RORbeta(C3H)(-/-) mice showing a significant increase in circadian period (tau). Melatonin administration (90 microg/mouse sc for 3 days) at circadian time (CT) 10 induced phase advances, while exposure to a light pulse (300 lux) at CT 14 induced phase delays of circadian activity rhythms of the same magnitude in all genotypes. In RORbeta(C3H)(-/-) mice a light pulse at CT 22 elicited a larger phase advance in activity rhythms and a slower rate of reentrainment after a 6-h advance in the LD cycle compared with (+/+) mice. Yet, the rate of reentrainment was significantly advanced by melatonin administration at the new dark onset in both (+/+) and (-/-) mice. We conclude that the RORbeta nuclear receptor is not involved in either the rhythmic production of pineal melatonin or in mediating phase shifts of circadian rhythms by melatonin, but it may regulate clock responses to photic stimuli at certain time domains.

Dissecting the functions of the mammalian clock protein BMAL1 by tissue-specific rescue in mice.

The basic helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) domain transcription factor BMAL1 is an essential component of the mammalian circadian pacemaker. Bmal1-/- mice lose circadian rhythmicity but also display tendon calcification and decreased activity, body weight, and longevity. To investigate whether these diverse functions of BMAL1 are tissue-specific, we produced transgenic mice that constitutively express Bmal1 in brain or muscle and examined the effects of rescued gene expression in Bmal1-/- mice. Circadian rhythms of wheel-running activity were restored in brain-rescued Bmal1-/- mice in a conditional manner; however, activity levels and body weight were lower than those of wild-type mice. In contrast, muscle-rescued Bmal1-/- mice exhibited normal activity levels and body weight yet remained behaviorally arrhythmic. Thus, Bmal1 has distinct tissue-specific functions that regulate integrative physiology.

Illuminating serotonergic gateways for strong resetting of the mammalian circadian clock

circadian clocks regulate animal behavior and physiology by generating daily rhythms and coordinating these with local time. If you sleep well and abstain from flying around the globe or working rotating shift schedules, this pervasive influence is easy to take for granted, a background rhythm of

The selective tachykinin neurokinin 1 (NK 1) receptor antagonist, GR 205,171, stereospecifically inhibits light-induced phase advances of hamster circadian activity rhythms

Circadian rhythms in mammals are generated by master pacemaker cells located within the suprachiasmatic nucleus of the hypothalamus. In hamsters, the suprachiasmatic nucleus contains a small collection of cells immunoreactive for substance P, the endogenous ligand of tachykinin neurokinin 1 (NK 1) receptors. In addition, two other nuclei which form part of the circadian system, the intergeniculate leaflet of the thalamus and the raphe nuclei, also contain fibers and/or cell bodies immunoreactive for substance P. In light of these observations, we evaluated the influence of the selective tachykinin NK 1 receptor antagonist, GR 205,171, upon circadian activity rhythms in the hamster. Systemic injection of GR 205,171 dose-dependently (2.5–40.0 mg/kg, i.p.) inhibited light-induced phase advances in hamster circadian wheel running activity rhythms by approximately 50%. In contrast, GR 226,206, the less active enantiomer of GR 205,171, failed to affect light-induced phase advances. In addition, we examined the potential ability of GR 205,171 to induce non-photic phase shifts in hamster wheel running rhythms when injected at mid-day to late night circadian times. However, GR 205,171 (40 mg/kg) did not elicit non-photic phase shifts at these times indicating that tachykinin NK 1 receptor antagonists are only effective when a light stimulus is applied to the pacemaker. Although GR 205,171 may, in theory, activate several sites within the circadian system, we suggest that GR 205,171 acts in the raphe nuclei to increase inhibitory serotonergic input to pacemaker cells in the suprachiasmatic nuclei, thereby suppressing photic modulation of the pacemaker. These findings have important implications for the use of tachykinin NK 1 receptor antagonists in the treatment of depression and other central nervous system disorders.

Serotonergic serotonin 1A mixed agonists/antagonists elicit large-magnitude phase shifts in hamster circadian wheel-running rhythms

The biological clock that generates circadian rhythms in mammals is located within the suprachiasmatic nuclei at the base of the hypothalamus. The circadian clock is entrained to the daily light/dark cycle by photic information from the retina. The retinal input to the clock is inhibited by exogenously applied serotonin agonists, perhaps mimicking an endogenous inhibitory serotonergic input to the clock arriving from the midbrain raphe. In the present study, a unique class of serotonergic compounds was tested for its ability to modulate retinal input to the circadian clock. The serotonergic ligands 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8-azaspiro(4.5)decane-7,9-dione dihydrochloride (BMY 7378), S 15535, and 8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride (MDL 73005 EF) can all be classified as mixed agonists/antagonists at type 1A serotonin receptors. Circadian wheel-running activity rhythms were monitored in Syrian hamsters maintained in constant darkness. Dim white-light pulses administered to the hamsters at circadian time 19 advanced the phase of their running rhythms by 1–2 h. Injection of BMY 7378, S 15535, and to a lesser degree MDL 73005 EF, prior to the light pulses resulted in phase advances from 5 to 6 h, and by as much as 8 h. Neither BMY 7378 nor S 15535 had any effect on light-induced phase delays in hamster activity rhythms at circadian time 14. Further, BMY 7378 is able to phase advance circadian rhythms by approximately 1 h at night even without light exposure. Finally, the effects of BMY 7378 on circadian rhythms is opposite to that observed with the prototypical serotonin 1A agonist (±)-8-hydroxy-2-(DI- n-propyl-amino)tetralin hydrobromide (8-OH-DPAT) (8-OH-DPAT elicits non-photic phase advances in the day and inhibits photic-induced phase advances at night). These results suggest that pharmacologically blocking raphe input to the suprachiasmatic circadian clock results in substantially larger photically induced phase advances in wheel-running rhythms. This is further evidence that raphe input to the circadian clock is probably acting to dampen the clock’s response to light under certain conditions. The large-magnitude phase shifts, and temporal-activity profile seen with BMY 7378 and S 15535, suggest that compounds with this unique pharmacological profile may be beneficial in the treatment of circadian phase delays recently reported to be a complication resulting from Alzheimer’s disease.

5-HT1B receptor knockout mice exhibit an enhanced response to constant light.

Serotonin (5-HT) can act presynaptically at 5-HT1B receptors on retinal terminals in the suprachiasmatic nucleus (SCN) to inhibit glutamate release, thereby modulating the effects of light on circadian behavior. 5-HT1B receptor agonists (1) inhibit light-induced phase shifts of circadian activity rhythms, (2) attenuate light-induced Fos expression in the SCN, and (3) reduce the amplitude of optic nerve-evoked excitatory postsynaptic currents in SCN neurons in vitro. To determine whether functional disruption of the 5-HT1B presynaptic receptors would result in an amplified response of the SCN to light, the period (tau) of the circadian rhythm of wheel-running activity was estimated under several different conditions in 5-HT1B receptor knockout (KO) mice and genetically matched wild-type animals. Under constant light (LL) conditions, the tau of 5-HT1B receptor KO mice was significantly greater than the tau of wild-type mice. A quantitative analysis of the wheel-running activity revealed no differences between wild-type and KO mice in either total activity or the temporal distribution of activity under LL conditions, suggesting that the observed increase in tau was not a function of reduced activity. Under constant dark conditions, the period of the circadian rhythm of wheel-running activity of wild-type and 5-HT1B receptor KO mice was similar. In addition, no differences were noted between wild-type and 5-HT1B receptor KO mice in the rate of reentrainment to a 6 h phase advance in the 12:12 light:dark cycle or in phase shifts in response to a 10 min light pulse presented at circadian time 16. The enhanced response of the SCN circadian clock of the 5-HT1B receptor KO mice to LL conditions is consistent with the hypothesis that the endogenous activation of 5-HT1B presynaptic receptors modulates circadian behavior by attenuating photic input to the SCN.

Circadian rhythmicity in dopamine content of mammalian retina: role of the photoreceptors.

Dopamine, the predominant retinal catecholamine, is a neurotransmitter and neuromodulator known to regulate light-adaptive retinal processes. Because dopamine influences several rhythmic events in the retina it is also a candidate for a retinal circadian signal. Using high performance liquid chromatography (HPLC), we have tested whether dopamine and its breakdown products are rhythmic in Royal College of Surgeons (RCS) rats with normal and dystrophic retinas. In both normal and mutant animals entrained to a 12-h light/12-h dark cycle, we found robust daily rhythms of dopamine and its two major metabolites. To address circadian rhythmicity of dopamine content, rats were entrained to light/dark cycles and released into constant darkness, using the circadian rhythm of wheel-running activity as a marker of each individual's circadian phase. Circadian rhythms of dopamine and metabolite content persisted in both wild type and retinally degenerate animals held for two weeks in constant darkness. Our results demonstrate for the first time clear circadian rhythms of dopamine content and turnover in a free-running mammal, and suggest that rods and cones are not required for dopamine rhythmicity.

Entrainment impaired, masking spared: an apparent genetic abnormality that prevents circadian rhythm entrainment to 24-h lighting cycles in California mice

Lighting cycles can influence the expression of daily activity rhythms in two ways: by entraining the circadian pacemaker that normally drives this rhythm, and by directly affecting the expression of activity itself, thereby ‘masking’ the influence of the pacemaker. We describe a California mouse ( Peromyscus californicus) in which these processes are dissociated. Circadian rhythms of wheel-running activity were recorded continuously while the animal was housed in a standard light/dark cycle and in constant darkness. This animal expressed a normal circadian rhythm that failed to entrain to the light/dark cycle, but was completely masked during the light phase. This animal's phenotype appears to have a genetic basis, since the progeny of selective crosses of his descendants showed similar abnormalities. These mice are the first example of animals expressing apparently normal circadian rhythms that are not entrained by light, but that still show potent masking responses to light exposure.