Genetic suppression of agrin reduces mania-like behavior in Na+ , K+ -ATPase α3 mutant mice.

Myshkin mice heterozygous for an inactivating mutation in the neuron-specific Na(+) ,K(+) -ATPase α3 isoform show behavior analogous to mania, including an abnormal endogenous circadian period. Agrin is a proteoglycan implicated as a regulator of synapses that has been proposed to inhibit activity of Na(+) ,K(+) -ATPase α3. We examined whether the mania-related behavior of Myshkin mice could be rescued by a reduction in the expression of agrin through genetic knockout. The suppression of agrin reduced hyperambulation and holeboard exploration, restored anxiety-like behavior (or reduced risk-taking behavior), improved prepulse inhibition and shortened the circadian period. Hence, agrin is important for regulating mania-like behavior and circadian rhythms. In Myshkin mice, the suppression of agrin increased brain Na(+) ,K(+) -ATPase activity by 11 ± 4%, whereas no effect on Na(+) ,K(+) -ATPase activity was detected when agrin was suppressed in mice without the Myshkin mutation. These results introduce agrin as a potential therapeutic target for the treatment of mania and other neurological disorders associated with reduced Na(+) ,K(+) -ATPase activity and neuronal hyperexcitability.

Memory for time of training modulates performance on a place conditioning task in marmosets.

In rodents, the expression of a reward-conditioned place preference (CPP) is regulated in a circadian pattern such that the preference is exhibited strongly at the circadian time of prior training but not at other circadian times. Because each animal is trained only at a single circadian phase, the concept of time as a context cue is derived from a rhythmic internal state rather than learned explicitly from the external cues. We now report that the same "time memory" is expressed following context conditioning in the common marmoset (Callithrix jacchus). Animals were trained at a specific time to discriminate between an unpaired context and a context paired with food reward. Marmosets were then tested for preference at circadian times that were either the same or different from the training time. Preference was expressed only when training and testing times matched. The results show that time of day learning can be generalized to this new world primate implying that a similar circadian mechanism might regulate craving for reward in diverse mammals including human beings.

Senescence, sleep, and circadian rhythms.

The goal of this review article is to summarize our knowledge and understanding of the overlapping (interdisciplinary) areas of senescence, sleep, and circadian rhythms. Our overview comprehensively (and visually wherever possible), emphasizes the organizational, dynamic, and plastic nature of both sleep and circadian timing system (CTS) during senescent processes in animals and in humans. In this review, we focus on the studies that deal with sleep and circadian rhythms in aged animals and how these studies have closely correlated to and advanced our understanding of similar processes in ageing humans. Our comprehensive summary of various aspects of the existing research on animal and human ageing, both normal and pathological, presented in this review underscores the invaluable advantage of close collaboration between clinicians and basic research scientists and the future challenges inherent in this collaboration. First, our review addresses the common age-related changes that occur in sleep and temporal organization of both animals and humans. Second, we examine the specific modifications that often accompany sleep and CTS during aging. Third, we discuss the clinical epidemiology of sleep dysfunctions during ageing and their current clinical management, both pharmacological and non-pharmacological. Finally, we predict the possible future promises for complementary and alternative medicine (CAM) that pave the way to the emergence of a "Holistic Sleep Medicine" approach to the treatment of sleep disorders in the ageing population. Further studies will provide additional valuable insights into the understanding of both sleep and circadian rhythms during senescence.

Circadian phase-shifted rats show normal acquisition but impaired long-term retention of place information in the water task.

It is thought that circadian rhythms may influence learning and memory processes. However, research supporting this view does not dissociate a mnemonic impairment from other performance deficits. Furthermore, published reports do not specify the type of memory system influenced by the circadian system. The present study assessed the effects of phase shifting on acquisition and expression of place navigation in the water maze, a task sensitive to hippocampal dysfunction. The results showed that phase-shifting circadian rhythms in rats impaired the expression of place information on a retention test but not initial acquisition or encoding of place information. These results suggest that disruption of circadian rhythms may impair consolidation of previously encoded hippocampal place information.

Circadian rhythms, aging and memory.

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Circadian rhythms, aging and memory.

In human beings and animal models, cognitive performance is often impaired in natural and experimental situations where circadian rhythms are disrupted. This includes a general decline in cognitive ability and fragmentation of behavioural rhythms in the aging population of numerous species. There is some evidence that rhythm disruption may lead directly to cognitive impairment; however, this causal link has not been made for effects due to aging. We have tested this link by examining rhythms and performance on contextual conditioning with the conditioned place preference task, in elderly, age-matched hamsters. Young healthy hamsters developed a preference for a context that is paired with the opportunity to engage in wheel-running (experiment 1). Aged animals with consolidated locomotor rhythms developed similar degrees of preference, whereas the age-matched hamsters with fragmented rhythms did not (experiment 2). The degree of preference was also correlated with activity amplitude. These results support the notion that age-related rhythm fragmentation contributes to the age-related memory decline.

Positional syntenic cloning and functional characterization of the mammalian circadian mutation tau.

The tau mutation is a semidominant autosomal allele that dramatically shortens period length of circadian rhythms in Syrian hamsters. We report the molecular identification of the tau locus using genetically directed representational difference analysis to define a region of conserved synteny in hamsters with both the mouse and human genomes. The tau locus is encoded by casein kinase I epsilon (CKIepsilon), a homolog of the Drosophila circadian gene double-time. In vitro expression and functional studies of wild-type and tau mutant CKIepsilon enzyme reveal that the mutant enzyme has a markedly reduced maximal velocity and autophosphorylation state. In addition, in vitro CKIepsilon can interact with mammalian PERIOD proteins, and the mutant enzyme is deficient in its ability to phosphorylate PERIOD. We conclude that tau is an allele of hamster CKIepsilon and propose a mechanism by which the mutation leads to the observed aberrant circadian phenotype in mutant animals.

The effect of amphetamine on locomotion depends on the motor device utilized. The open field vs. the running wheel.

The effect of amphetamine on the level of locomotion exhibited on two different motor devices was examined in the Golden hamster. Increasing concentrations of the psychostimulant from 4 to 10 mg/kg significantly enhanced locomotor activity in hamsters exposed to an open field. A further increase to 25 mg/kg inhibited ambulatory activity to levels below the control baseline, while augmenting the occurrence of stereotypic behaviors. The activating effect of amphetamine on ambulatory activity was observed regardless of the time of testing (day or night) or lighting condition, with no apparent modulation by the circadian system. On the other hand, home-cage wheel-running activity was maximally inhibited by 10 mg/kg amphetamine, whereas a smaller dosage (1.5 mg/kg) had no effect over the wheel-running activity baseline of saline controls. Although both the running wheel and the open field quantify locomotion, the dissociation obtained shows that they measure different components of it. The results are interpreted within Lyon and Randrup's hypothesis on the actions of amphetamine (16).

Brain-stimulation reward thresholds raised by an antisense oligonucleotide for the M5 muscarinic receptor infused near dopamine cells.

Oligonucleotides targeting M5 muscarinic receptor mRNA were infused for 6 d into the ventral tegmental area of freely behaving rats trained to bar-press for lateral hypothalamic stimulation. The bar-pressing rate was determined at a range of frequencies each day to evaluate the effects of infusions on reward. M5 antisense oligonucleotide (oligo) infusions increased the frequency required for bar pressing by 48% over baseline levels, with the largest increases occurring after 4-6 d of infusion. Two control oligos had only slight effects (means of 5 and 11% for missense and sense oligos, respectively). After the infusion, the required frequency shifted back to baseline levels gradually over 1-5 d. Antisense oligo infusions decreased M5 receptors on the ipsilateral, but not the contralateral, side of the ventral tegmentum, as compared with a missense oligo. Therefore, M5 muscarinic receptors associated with mesolimbic dopamine neurons seem to be important in brain-stimulation reward.

Retinal GABA(A) receptors participate in the regulation of circadian responses to light.

A role for retinal gamma-aminobutyric acid Type A (GABA(A)) receptors in the regulation of circadian responses to light was examined. Intraocular injections of the GABA(A) antagonist, bicuculline, were performed during the early (Circadian Time [CT] 13.5) and late subjective night (CT 20), followed by a light pulse. Bicuculline significantly decreased the magnitude of phase delays induced by light to 65%, whereas it had no effect on phase advances. To explore the nature of the inhibition elicited by bicuculline, an intensity-response curve was performed. Intraocular injections of bicuculline inhibited phase delays only when induced by high-saturating light illuminances (20 and 100 lux). No effect was observed at light intensities < or = 5 lux. These results suggest that retinal GABA(A) receptors modulate the responsivity of the circadian system to light.

Conservation of locomotor behavior in the golden hamster: effects of light cycle and a circadian period mutation.

Locomotor activity in rodents is restricted temporally by the animal' s circadian system. The relative stability of both the species-specific pattern and the amount of locomotor activity per cycle suggested that this behavior may be regulated by conservative mechanisms. In these experiments, the wheel-running behavior of golden hamsters carrying the circadian period mutation, tau, was analyzed in animals housed in a 24-h light:dark cycle (LD) and in constant dark (DD) conditions to determine which aspects of this behavior were conserved. In DD, apart from the change in period which defines the mutation, no main effects of allele combination were found in either average amount of activity, activity profile, or length of the activity phase. In LD, wild-type behavior did not differ from that in DD; however, heterozygous mutants exhibited early onsets of activity, significant fragmentation of both activity and rest, an increase in the duration of the active phase, and an overall decrease in the amount of activity. Despite these differences, the total amount of time spent on the wheel in LD or DD was the same for all environment/genotype combinations. The data show that a conservative mechanism that may influence daily patterns of locomotor behavior is related more to a drive to perform the behavior than the quantity or timing of the behavior itself.

The significance of circadian organization for longevity in the golden hamster.

While functional roles for biological clocks have been demonstrated in organisms throughout phylogeny, the adaptive advantages of circadian organization per se are largely matters of conjecture. It is generally accepted, though without direct experimental evidence, that organisms derive primary benefits from the temporal organization of their physiology and behavior, as well as from the anticipation of daily changes in their environment and their own fluctuating physiological requirements. However, the consequences of circadian dysfunction that might demonstrate a primary adaptive advantage and explain the natural origins and apparent ubiquity of circadian systems have not been documented. The authors report that longevity in hamsters is decreased with a noninvasive disruption of rhythmicity and is increased in older animals given suprachiasmatic implants that restore higher amplitude rhythms. The results substantiate the importance of the temporal organization of physiology and behavior provided by the circadian clock to the health and longevity of an organism.

Regulation of circadian photic responses by nitric oxide.

A role for nitric oxide in circadian responses to light has been indicated in previous studies. To determine the specific function of NO-, the authors manipulated NO- and nitric oxide synthase (NOS) activity prior to light pulses that would normally induce phase shifts. The NOS inhibitor, L-NAME, selectively attenuated phase advances of locomotor rhythms and had no effect on phase delays. The NO- donor, SNAP, potentiated both photic responses, and phase delays were larger than the maximum responses that could be obtained with light alone. The date suggest a model in which NO- participates in the adaptation of the system to environmental lighting conditions by regulating in a phase-dependent manner responsiveness to light.

cGMP-dependent protein kinase inhibitors block light-induced phase advances of circadian rhythms in vivo.

Synchronization of circadian rhythms is thought to be accomplished primarily through daily phase delays and advances of the endogenous circadian clock that, in mammals, is located in the hypothalamic suprachiasmatic nucleus (SCN). In the SCN, numerous second messenger pathways may participate in photic signal transduction. In these studies, the involvement of cyclic nucleotide-dependent kinases was examined in vivo using inhibitors of adenosine 3',5'-cyclic monophosphate (cAMP)- and guanosine 3',5'-cyclic monophosphate (cGMP)-dependent kinase (PKA and PKG, respectively). In constant dark, selective and nonselective inhibitors of PKG injected near the SCN of hamsters had no effect on phase delays produced by light pulses given in the early subjective night (early in the animals' active period) but significantly attenuated phase advances induced late in the subjective night. PKA inhibition had no effect at either time point. In addition, cGMP agonists had no effect on rhythmicity in the absence of light. The results suggest that PKG activity is necessary, but not sufficient, for normal photic responsiveness and that PKA activity is not required. The phase dependence of the effect of PKG inhibition supports the notion that photic entrainment is influenced by biochemical pathways that differentially regulate sensitivity in a phase-dependent manner.

The circadian system of c-fos deficient mice.

We examined the role of c-fos in the synchronization of circadian rhythms to environmental light cycles using a line of gene-targeted mice carrying a null mutation at this locus. Circadian locomotor rhythms in mutants had similar periods as wild-type controls but took significantly longer than controls to entrain to 12:12 light-dark cycles. Light-induced phase shifts of rhythms in constant dark were attenuated in mutants although the circadian timing of phase delays and advances was not changed. A functional retinohypothalamic projection was indicated from behavioral results and light-induced jun-B expression in the SCN. The results indicate that while c-fos activation is not an absolute requirement for rhythm generation nor photic responses, it is required for normal entrainment of the mammalian biological clock.

Pacemaker interactions in the mammalian circadian system.

Circadian rhythms in mammals are generated by pacemaker cells located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The identity of these cells, however, is not known, and little information exists regarding the mechanisms by which they communicate with each other and with the organism. Nonetheless, pacemaker interactions must occur to produce single, coherent rhythms of behavior and physiology. Recently it has become possible to observe the result of these interactions using circadian chimeras, animals with two clocks with distinct periods, that have been produced by SCN transplantation. Using the tau mutation in golden hamsters, chimeras expressing two circadian rhythms of behavior simultaneously were created. The two rhythms exhibited complex interactions including cases of relative coordination. This basic result indicates that pacemaker interactions are rhythmic and phase dependent. Further analysis should help to elucidate the nature of the coupling signal and the identity of the pacemaker cells.

Let there be light: signal transduction in a mammalian circadian system.

Mammalian circadian rhythms are controlled by a biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). This clock is entrained by light through a retinohypothalamic pathway that interacts with the SCN through glutamate neurotransmission. Light pulses during the subjective night induce phase shifts of behavioral rhythms, and also trigger intracellular changes such as the expression of immediate-early genes and activation of transcription factors. In this review, we present a model of the signal transduction pathway leading to photic synchronization of the circadian clock, including the activity of specific second messenger systems, gene expression, and interaction between potential agents capable of producing phase shifts.

Pacemaker interactions in the mammalian circadian system

Circadian rhythms in mammals are generated by pacemaker cells located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The identity of these cells, however, is not known, and little information exists regarding the mechanisms by which they communicate with each other and with the organism. Nonetheless, pacemaker interactions must occur to produce single, coherent rhythms of behavior and physiology. Recently it has become possible to observe the result of these interactions using circadian chimeras, animals with two clocks with distinct periods, that have been produced by SCN transplantation. Using the tau mutation in golden hamsters, chimeras expressing two circadian rhythms of behavior simultaneously were created. The two rhythms exhibited complex interactions including cases of relative coordination. This basic result indicates that pacemaker interactions are rhythmic and phase dependent. Further analysis should help to elucidate the nature of the coupling signal and the identity of the pacemaker cells.

Let there be light: signal transduction in a mammalian circadian system

Mammalian circadian rhythms are controlled by a biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). This clock is entrained by light through a retinohypothalamic pathway that interacts with the SCN through glutamate neurotransmission. Light pulses during the subjective night induce phase shifts of behavioral rhythms, and also trigger intracellular changes such as the expression of immediate-early genes and activation of transcription factors. In this review, we present a model of the signal transduction pathway leading to photic synchronization of the circadian clock, including the activity of specific second messenger systems, gene expression, and interaction between potential agents capable of producing phase shifts.