Rhythms in expression of PER1 protein in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

Circadian rhythms-related disorders in diurnal fat sand rats under modern lifestyle conditions: A review

Modern lifestyle reduces environmental rhythmicity and may lead to circadian desynchrony. We are exposed to poor day-time lighting indoors and excessive night-time artificial light. We use air-conditioning to reduce ambient temperature cycle, and food is regularly available at all times. These disruptions of daily rhythms may lead to type 2 diabetes mellitus (T2DM), obesity, cardiometabolic diseases (CMD), depression and anxiety, all of which impose major public health and economic burden on societies. Therefore, we need appropriate animal models to gain a better understanding of their etiologic mechanisms, prevention, and management.We argue that the fat sand rat (Psammomys obesus), a diurnal animal model, is most suitable for studying the effects of modern-life conditions. Numerous attributes make it an excellent model to study human health disorders including T2DM, CMD, depression and anxiety. Here we review a comprehensive series of studies we and others conducted, utilizing the fat sand rat to study the underlying interactions between biological rhythms and health. Understanding these interactions will help deciphering the biological basis of these diseases, which often occur concurrently. We found that when kept in the laboratory (compared with natural and semi-wild outdoors conditions where they are diurnal), fat sand rats show low amplitude, nocturnal or arrhythmic activity patterns, dampened daily glucose rhythm, glucose intolerance, obesity and decreased survival rates. Short photoperiod acclimation exacerbates these pathologies and further dampens behavioral and molecular daily rhythms, resulting in CMD, T2DM, obesity, adipocyte dysfunction, cataracts, depression and anxiety. Increasing environmental rhythmicity by morning bright light exposure or by access to running wheels strengthens daily rhythms, and results in higher peak-to-trough difference in activity, better rhythmicity in clock genes expression, lower blood glucose and insulin levels, improved glucose tolerance, lower body and heart weight, and lower anxiety and depression. In summary, we have demonstrated that fat sand rats living under the correspondent of “human modern lifestyle” conditions exhibit dampened behavioral and biological rhythms and develop circadian desynchrony, which leads to what we have named “The Circadian Syndrome”. Environmental manipulations that increase rhythmicity result in improvement or prevention of these pathologies. Similar interventions in human subjects could have the same positive results and further research on this should be undertaken.

Memory and the circadian system: Identifying candidate mechanisms by which local clocks in the brain may regulate synaptic plasticity

The circadian system is an endogenous biological network responsible for coordinating near-24-h cycles in behavior and physiology with daily timing cues from the external environment. In this review, we explore how the circadian system regulates memory formation, retention, and recall. Circadian rhythms in these memory processes may arise through several endogenous pathways, and recent work highlights the importance of genetic timekeepers found locally within tissues, called local clocks. We evaluate the circadian memory literature for evidence of local clock involvement in memory, identifying potential nodes for direct interactions between local clock components and mechanisms of synaptic plasticity. Our discussion illustrates how local clocks may pervasively modulate neuronal plastic capacity, a phenomenon that we designate here as circadian metaplasticity. We suggest that this function of local clocks supports the temporal optimization of memory processes, illuminating the potential for circadian therapeutic strategies in the prevention and treatment of memory impairment.

Circadian and photic modulation of daily rhythms in diurnal mammals

The temporal niche that an animal occupies includes a coordinated suite of behavioral and physiological processes that set diurnal and nocturnal animals apart. The daily rhythms of the two chronotypes are regulated by both the circadian system and direct responses to light, a process called masking. Here we review the literature on circadian regulations and masking responses in diurnal mammals, focusing on our work using the diurnal Nile grass rat (Arvicanthis niloticus) and comparing our findings with those derived from other diurnal and nocturnal models. There are certainly similarities between the circadian systems of diurnal and nocturnal mammals, especially in the phase and functioning of the principal circadian oscillator within the hypothalamic suprachiasmatic nucleus (SCN). However, the downstream pathways, direct or indirect from the SCN, lead to drastic differences in the phase of extra-SCN oscillators, with most showing a complete reversal from the phase seen in nocturnal species. This reversal, however, is not universal and in some cases the phases of extra-SCN oscillators are only a few hours apart between diurnal and nocturnal species. The behavioral masking responses in general are opposite between diurnal and nocturnal species, and are matched by differential responses to light and dark in several retinorecipient sites in their brain. The available anatomical and functional data suggest that diurnal brains are not simply a phase-reversed version of nocturnal ones, and work with diurnal models contribute significantly to a better understanding of the circadian and photic modulation of daily rhythms in our own diurnal species.

A comparison of the orexin receptor distribution in the brain between diurnal Nile grass rats (Arvicanthis niloticus) and nocturnal mice (Mus musculus)

The neuropeptide orexin/hypocretin regulates a wide range of behaviors and physiology through its receptors OX1R and OX2R, or HCRTR-1 and HCRTR-2. Although the distributions of these receptors have been established in nocturnal rodents, their distributions in the brain of diurnal species have not been studied. In the present study, we examined spatial patterns of OX1R and OX2R mRNA expression in diurnal Nile grass rats (Arvicanthis niloticus) by in situ hybridization and compared them with those in nocturnal mice (Mus musculus). Both receptors showed similar spatial patterns between species in most brain regions. However, species-specific expression was found in several regions that are mainly implicated in regulation of sleep/wakefulness, emotion and cognition. OX1R expression was detected in the caudate putamen and ventral tuberomammillary nucleus only in grass rats, while it was detected in the bed nucleus of the stria terminalis, medial division, posteromedial part only in mice. The distribution of OX2R mRNA was mostly consistent between the two species, although it was more widely expressed in the ventral tuberomammillary nucleus in grass rats compared to mice. These results suggest that neuronal pathways of the orexin system differ between chronotypes, and these differences could underlie the distinct profiles in behaviors and physiology between diurnal and nocturnal species.

Circadian modulation of memory and plasticity gene products in a diurnal species

Cognition is modulated by circadian rhythms, in both nocturnal and diurnal species. Rhythms of clock gene expression occur in brain regions that are outside the master circadian oscillator of the suprachiasmatic nucleus and that control cognitive functions, perhaps by regulating the expression neural-plasticity genes such as brain derived neurotrophic factor (BDNF) and its high affinity receptor, tyrosine kinase B (TrkB). In the diurnal grass rat (Arvicanthis niloticus), the hippocampus shows rhythms of clock genes that are 180° out of phase with those of nocturnal rodents. Here, we examined the hypothesis that this reversal extends to the optimal phase for learning a hippocampal-dependent task and to the phase of hippocampal rhythms in BDNF/TrkB expression. We used the Morris water maze (MWM) to test for time of day differences in reference memory and monitored daily patterns of hippocampal BDNF/TrkB expression in grass rats. Grass rats showed superior long-term retention of the MWM, when the training and testing occurred during the day as compared to the night, at a time when nocturnal laboratory rats show superior retention; acquisition of the MWM was not affected by time of day. BDNF/TrkB expression was rhythmic in the hippocampus of grass rats, and the phase of the rhythms was reversed compared to that of nocturnal rodents. Our findings provide correlational evidence for the claim that the circadian regulation of cognition may involve rhythms of BDNF/TrkB expression in the hippocampus and that their phase may contribute to species differences in the optimal phase for learning.

Central control of circadian phase in arousal-promoting neurons

Cells of the dorsomedial/lateral hypothalamus (DMH/LH) that produce hypocretin (HCRT) promote arousal in part by activation of cells of the locus coeruleus (LC) which express tyrosine hydroxylase (TH). The suprachiasmatic nucleus (SCN) drives endogenous daily rhythms, including those of sleep and wakefulness. These circadian oscillations are generated by a transcriptional-translational feedback loop in which the Period (Per) genes constitute critical components. This cell-autonomous molecular clock operates not only within the SCN but also in neurons of other brain regions. However, the phenotype of such neurons and the nature of the phase controlling signal from the pacemaker are largely unknown. We used dual fluorescent in situ hybridization to assess clock function in vasopressin, HCRT and TH cells of the SCN, DMH/LH and LC, respectively, of male Syrian hamsters. In the first experiment, we found that Per1 expression in HCRT and TH oscillated in animals held in constant darkness with a peak phase that lagged that in AVP cells of the SCN by several hours. In the second experiment, hamsters induced to split their locomotor rhythms by exposure to constant light had asymmetric Per1 expression within cells of the middle SCN at 6 h before activity onset (AO) and in HCRT cells 9 h before and at AO. We did not observe evidence of lateralization of Per1 expression in the LC. We conclude that the SCN communicates circadian phase to HCRT cells via lateralized neural projections, and suggests that Per1 expression in the LC may be regulated by signals of a global or bilateral nature.

Antibodies for assessing circadian clock proteins in the rodent suprachiasmatic nucleus

Research on the mechanisms underlying circadian rhythmicity and the response of brain and body clocks to environmental and physiological challenges requires assessing levels of circadian clock proteins. Too often, however, it is difficult to acquire antibodies that specifically and reliably label these proteins. Many of these antibodies also lack appropriate validation. The goal of this project was to generate and characterize antibodies against several circadian clock proteins. We examined mice and hamsters at peak and trough times of clock protein expression in the suprachiasmatic nucleus (SCN). In addition, we confirmed specificity by testing the antibodies on mice with targeted disruption of the relevant genes. Our results identify antibodies against PER1, PER2, BMAL1 and CLOCK that are useful for assessing circadian clock proteins in the SCN by immunocytochemistry.

Phase preference for the display of activity is associated with the phase of extra-suprachiasmatic nucleus oscillators within and between species

Many features of the suprachiasmatic nucleus (SCN) are the same in diurnal and nocturnal animals, suggesting that differences in phase preference are determined by mechanisms downstream from the SCN. Here, we examined this hypothesis by characterizing rhythmic expression of Period 1 (PER1) and Period 2 (PER2) in several extra-SCN areas in the brains of a diurnal murid rodent, Arvicanthis niloticus (grass rats). In the shell of the nucleus accumbens, dorsal striatum, piriform cortex, and CA1 of the hippocampus, both PER1 and PER2 were rhythmic, with peak expression occurring at ZT10. PER1 in the dentate gyrus also peaked at ZT10, but PER2 was arrhythmic in this region. In general, these patterns are 180 degrees out of phase with those reported for nocturnal species. In a second study, we examined inter-individual differences in the multioscillator system of grass rats. Here, we housed grass rats in cages with running wheels, under which conditions some individuals spontaneously adopt a day active (DA) and others a night active (NA) phase preference. In the majority of the extra-SCN regions sampled, the patterns of PER1 and PER2 expression of NA grass rats resembled those of nocturnal species, while those of DA grass rats were similar to the ones seen in grass without access to running wheels. In contrast, the rhythmic expression of both PER proteins was identical in the SCN and ventral subparaventricular zone (vSPZ) of DA and NA animals. Differences in the phase of oscillators downstream from the SCN, and perhaps the vSPZ, appear to determine the phase preference of particular species, as well as that of members of a diurnal species that show voluntary phase reversals. The latter observation has important implications for the understanding of health problems associated with human shift work.

PER2 rhythms in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central pacemaker that controls circadian rhythms in mammals. In diurnal grass rats (Arvicanthis niloticus), many functional aspects of the SCN are similar to those of nocturnal rodents, making it likely that the difference in the circadian system of diurnal and nocturnal animals lies downstream from the SCN. Rhythms in clock genes expression occur in several brain regions outside the SCN that may function as extra-SCN oscillators. In male grass rats PER1 is expressed in the oval nucleus of the bed nucleus of the stria terminalis (BNST-ov) and in the central and basolateral amygdala (CEA and BLA, respectively); several features of PER1 expression in these regions of the grass rat brain differ substantially from those of nocturnal species. Here we describe PER2 rhythms in the same three brain regions of the grass rat. In the BNST-ov and CEA PER2 expression peaked early in the light period Zeitgeber time (ZT) 2 and was low during the early night, which is the reverse of the pattern of nocturnal rodents. In the BLA, PER2 expression was relatively low for most of the 24-h cycle, but showed an acute elevation late in the light period (ZT10). This pattern is also different from that of nocturnal rodents that show elevated PER2 expression in the mid to late night and into the early day. These results are consistent with the hypothesis that diurnal behavior is associated with a phase change between the SCN and extra-SCN oscillators.

Morphine withdrawal produces circadian rhythm alterations of clock genes in mesolimbic brain areas and peripheral blood mononuclear cells in rats

Previous studies have shown that clock genes are expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus, other brain regions, and peripheral tissues. Various peripheral oscillators can run independently of the SCN. However, no published studies have reported changes in the expression of clock genes in the rat central nervous system and peripheral blood mononuclear cells (PBMCs) after withdrawal from chronic morphine treatment. Rats were administered with morphine twice daily at progressively increasing doses for 7 days; spontaneous withdrawal signs were recorded 14 h after the last morphine administration. Then, brain and blood samples were collected at each of eight time points (every 3 h: ZT 9; ZT 12; ZT 15; ZT 18; ZT 21; ZT 0; ZT 3; ZT 6) to examine expression of rPER1 and rPER2 and rCLOCK. Rats presented obvious morphine withdrawal signs, such as teeth chattering, shaking, exploring, ptosis, and weight loss. In morphine-treated rats, rPER1 and rPER2 expression in the SCN, basolateral amygdala, and nucleus accumbens shell showed robust circadian rhythms that were essentially identical to those in control rats. However, robust circadian rhythm in rPER1 expression in the ventral tegmental area was completely phase-reversed in morphine-treated rats. A blunting of circadian oscillations of rPER1 expression occurred in the central amygdala, hippocampus, nucleus accumbens core, and PBMCs and rPER2 expression occurred in the central amygdala, prefrontal cortex, nucleus accumbens core, and PBMCs in morphine-treated rats compared with controls. rCLOCK expression in morphine-treated rats showed no rhythmic change, identical to control rats. These findings indicate that withdrawal from chronic morphine treatment resulted in desynchronization from the SCN rhythm, with blunting of rPER1 and rPER2 expression in reward-related neurocircuits and PBMCs.