Anatomical and functional characterization of clock gene expression in neuroendocrine dopaminergic neurons

Hypothalamic A11 Nuclei Regulate the Circadian Rhythm of Spinal Mechanonociception through Dopamine Receptors and Clock Gene Expression

Several types of sensory perception have circadian rhythms. The spinal cord can be considered a center for controlling circadian rhythms by changing clock gene expression. However, to date, it is not known if mechanonociception itself has a circadian rhythm. The hypothalamic A11 area represents the primary source of dopamine (DA) in the spinal cord and has been found to be involved in clock gene expression and circadian rhythmicity. Here, we investigate if the paw withdrawal threshold (PWT) has a circadian rhythm, as well as the role of the dopaminergic A11 nucleus, DA, and DA receptors (DR) in the PWT circadian rhythm and if they modify clock gene expression in the lumbar spinal cord. Naïve rats showed a circadian rhythm of the PWT of almost 24 h, beginning during the night–day interphase and peaking at 14.63 h. Similarly, DA and DOPAC’s spinal contents increased at dusk and reached their maximum contents at noon. The injection of 6-hydroxydopamine (6-OHDA) into the A11 nucleus completely abolished the circadian rhythm of the PWT, reduced DA tissue content in the lumbar spinal cord, and induced tactile allodynia. Likewise, the repeated intrathecal administration of D1-like and D2-like DA receptor antagonists blunted the circadian rhythm of PWT. 6-OHDA reduced the expression of Clock and Per1 and increased Per2 gene expression during the day. In contrast, 6-OHDA diminished Clock, Bmal, Per1, Per2, Per3, Cry1, and Cry2 at night. The repeated intrathecal administration of the D1-like antagonist (SCH-23390) reduced clock genes throughout the day (Clock and Per2) and throughout the night (Clock, Per2 and Cry1), whereas it increased Bmal and Per1 throughout the day. In contrast, the intrathecal injection of the D2 receptor antagonists (L-741,626) increased the clock genes Bmal, Per2, and Per3 and decreased Per1 throughout the day. This study provides evidence that the circadian rhythm of the PWT results from the descending dopaminergic modulation of spinal clock genes induced by the differential activation of spinal DR.

Activation of dopamine D2 receptors in the medial shell region of the nucleus accumbens increases Per1 expression to enhance alcohol consumption

Circadian genes, including Per1, in the medial shell region of nucleus accumbens (mNAcSh), regulate binge alcohol consumption. However, the upstream mechanism regulating circadian genes-induced alcohol consumption is not known. Since activation of dopamine D2 receptors (D2R) increases Per1 gene expression, we hypothesised that local infusion of quinpirole, a D2R agonist, by increasing Per1 gene expression in the mNAcSh, will increase binge alcohol consumption in mice. We performed two experiments on male C57BL/6J mice, instrumented with bilateral guide cannulas above the mNAcSh, and exposed to a 4-day drinking-in-dark (DID) paradigm. The first experiment determined the effects of bilateral infusion of quinpirole (100 ng/300 nl/site) or DMSO (Vehicle group) in the mNAcSh on Per1 gene expression and alcohol consumption. The second experiment determined the effect of antisense-induced downregulation of Per1 in the mNAcSh on the quinpirole-induced increase in alcohol consumption. Control experiments were performed by exposing the animals to sucrose (10% w/v). After the experiment, animals were euthanised, brains removed and processed for localisation of injection sites and analysis of Per1 gene expression in the mNAcSh. As compared with the DMSO, local bilateral infusion of quinpirole significantly increased the expression of Per1 in the mNAcSh along with an increase in the amount of alcohol consumed in mice exposed to DID paradigm. In addition, local antisense-induced downregulation of Per1 significantly attenuated the effects of intro-accumbal infusion of quinpirole on alcohol consumption. Our results suggest that Per1 in the mNAcSh mediates D2R activation-induced increase in alcohol consumption.

Antisense-induced downregulation of major circadian genes modulates the expression of histone deacetylase-2 (HDAC-2) and CREB-binding protein (CBP) in the medial shell region of nucleus accumbens of mice exposed to chronic excessive alcohol consumption

Circadian genes in the medial accumbal shell (mNAcSh) region regulate binge alcohol consumption. Here, we investigated if antisense-induced knockdown of major circadian genes (Per1, Per2, and NPAS2) in the mNAcSh of mice exposed to intermittent access two-bottle choice (IA2BC) paradigm modulates the expression of histone deacetylase-2 (HDAC-2) and CREB-binding protein (CBP), key epigenetic modifiers associated with withdrawal-associated behaviors such as anxiety. Adult male C57BL/6J mice (N = 28), surgically implanted with bilateral guide cannulas above the mNAcSh, were chronically (4 weeks) exposed to alcohol (20% v/v) or saccharin (0.03%) via IA2BC paradigm. In the fourth week, a mixture of antisense (AS-ODNs; N = 14/group) or nonsense (NS-ODNs; N = 14/group) oligodeoxynucleotides against circadian genes were bilaterally infused into the mNAcSh. Subsequently, alcohol/saccharin consumption and preference were measured followed by euthanization of animals and verification of microinjection sites by visual inspection and the expression of HDAC-2 and CBP by using RT-PCR along with the verification of antisense-induced downregulation of circadian genes in the mNAcSh. As compared with NS-ODNs, AS-ODNs infusion significantly attenuated the alcohol-induced increase in HDAC-2 and reduction in CBP expression in the mNAcSh along with a significant reduction in alcohol consumption and preference. No significant effect was observed on either saccharin consumption or preference. Our results suggest that circadian genes in the mNAcSh may have a causal to play in mediating epigenetic changes observed after chronic alcohol consumption.

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

Antisense-Induced Downregulation of Clock Genes in the Shell Region of the Nucleus Accumbens Reduces Binge Drinking in Mice

INTRODUCTIONS

Binge drinking is a deadly pattern of alcohol consumption. Evidence suggests that genetic variation in clock genes is strongly associated with alcohol misuse; however, the neuroanatomical basis for such a relationship is unknown. The shell region of the nucleus accumbens (NAcSh) is well known to play a role in binge drinking. Hence, we examined whether clock genes in the NAcSh regulate binge drinking.

METHODS

To address this question, 2 experiments were performed on male C57BL/6J mice. In the first experiment, mice exposed to alcohol or sucrose under the 4-day drinking-in-the-dark (DID) paradigm were euthanized at 2 different time points on day 4 [7 hours after light (pre-binge drinking) or dark (post-binge drinking) onset]. The brains were processed for RT-PCR to examine the expression of circadian clock genes (Clock, Per1, and Per2) in the NAcSh and suprachiasmatic nucleus (SCN). In the second experiment, mice were exposed to alcohol, sucrose, or water as described above. On day 4, 1 hour prior to the onset of alcohol exposure, mice were bilaterally infused with either a mixture of circadian clock gene antisense oligodeoxynucleotides (AS-ODNs; antisense group) or nonsense/random ODNs (R-ODNs; control group) through surgically implanted cannulas above the NAcSh. Alcohol/sucrose/water consumption was measured for 4 hours. Blood alcohol concentration was measured to confirm binge drinking. Microinfusion sites were histologically verified using cresyl violet staining.

RESULTS

As compared to sucrose, mice euthanized post-binge drinking (not pre-binge drinking) on day 4 displayed a greater expression of circadian genes in the NAcSh but not in the SCN. Knockdown of clock genes in the NAcSh caused a significantly lower volume of alcohol to be consumed on day 4 than in the control treatment. No differences were found in sucrose or water consumption.

CONCLUSIONS

Our results suggest that clock genes in the NAcSh play a crucial role in binge drinking.

Vasopressinergic Activity of the Suprachiasmatic Nucleus and mRNA Expression of Clock Genes in the Hypothalamus-Pituitary-Gonadal Axis in Female Aging

The important involvement of the suprachiasmatic nucleus (SCN) and the activity of vasopressinergic neurons in maintaining the rhythmicity of the female reproductive system depends on the mRNA transcription-translation feedback loops. Therefore, circadian clock function, like most physiological processes, is involved in the events that determine reproductive aging. This study describes the change of mRNA expression of clock genes, Per2, Bmal1, and Rev-erbα, in the hypothalamus-pituitary-gonadal axis (HPG) of female rats with regular cycle (RC) and irregular cycle (IC), and the vasopressinergic neurons activity in the SCN and kisspeptin neurons in the arcuate nucleus (ARC) of these animals. Results for gonadotropins and the cFos/AVP-ir neurons in the SCN of IC were higher, but kisspeptin-ir was minor. Change in the temporal synchrony of the clock system in the HPG axis, during the period prior to the cessation of ovulatory cycles, was identified. The analysis of mRNA for Per2, Bmal1, and Rev-erbα in the reproductive axis of adult female rodents shows that the regularity of the estrous cycle is guaranteed by alternation in the amount of expression of Bmal1 and Per2, and Rev-erbα and Bmal1 between light and dark phases, which ceases to occur and contributes to determining reproductive senescence. These results showed that the desynchronization between the central and peripheral circadian clocks contributes to the irregularity of reproductive events. We suggest that the feedback loops of clock genes on the HPG axis modulate the spontaneous transition from regular to irregular cycle and to acyclicity in female rodents.

Regulation and function of extra-SCN circadian oscillators in the brain

Most organisms evolved endogenous, so called circadian clocks as internal timekeeping mechanisms allowing them to adapt to recurring changes in environmental demands brought about by 24-hour rhythms such as the light-dark cycle, temperature variations or changes in humidity. The mammalian circadian clock system is based on cellular oscillators found in all tissues of the body that are organized in a hierarchical fashion. A master pacemaker located in the suprachiasmatic nucleus (SCN) synchronizes peripheral tissue clocks and extra-SCN oscillators in the brain with each other and with external time. Different time cues (so called Zeitgebers) such as light, food intake, activity and hormonal signals reset the clock system through the SCN or by direct action at the tissue clock level. While most studies on non-SCN clocks so far have focused on peripheral tissues, several extra-SCN central oscillators were characterized in terms of circadian rhythm regulation and output. Some of them are directly innervated by the SCN pacemaker, while others receive indirect input from the SCN via other neural circuits or extra-brain structures. The specific physiological function of these non-SCN brain oscillators as well as their role in the regulation of the circadian clock network remains understudied. In this review we summarize our current knowledge about the regulation and function of extra-SCN circadian oscillators in different brain regions and devise experimental approaches enabling us to unravel the organization of the circadian clock network in the central nervous system.

Rhythmic Release of Corticosterone Induces Circadian Clock Gene Expression in the Cerebellum

Neurons of the cerebellar cortex contain a circadian oscillator, with circadian expression of clock genes being controlled by the master clock of the suprachiasmatic nucleus (SCN). However, the signaling pathway connecting the SCN to the cerebellum is unknown. Glucocorticoids exhibit a prominent SCN-dependent circadian rhythm, and high levels of the glucocorticoid receptor have been reported in the cerebellar cortex; we therefore hypothesized that glucocorticoids may control the rhythmic expression of clock genes in the cerebellar cortex. We here applied a novel methodology by combining the electrolytic lesion of the SCN with implantation of a micropump programmed to release corticosterone in a circadian manner mimicking the endogenous hormone profile. By use of this approach, we were able to restore the corticosterone rhythm in SCN-lesioned male rats. Clock gene expression in the cerebellum was abolished in rats with a lesioned SCN, but exogenous corticosterone restored the daily rhythm in clock gene expression in the cerebellar cortex, as revealed by quantitative real-time PCR and radiochemical in situ hybridization for the detection of the core clock genes Per1, Per2, and Arntl. On the contrary, exogenous hormone did not restore circadian rhythms in body temperature and running activity. RNAscope in situ hybridization further revealed that the glucocorticoid receptor colocalizes with clock gene products in cells of the cerebellar cortex, suggesting that corticosterone exerts its actions by binding directly to receptors in neurons of the cerebellum. However, rhythmic clock gene expression in the cerebellum was also detectable in adrenalectomized rats, indicating that additional control mechanisms exist. These data show that the cerebellar circadian oscillator is influenced by SCN-dependent rhythmic release of corticosterone.

Circadian regulation of membrane physiology in neural oscillators throughout the brain

Twenty-four-hour rhythmicity in physiology and behavior are driven by changes in neurophysiological activity that vary across the light-dark and rest-activity cycle. Although this neural code is most prominent in neurons of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are many other regions in the brain where region-specific function and behavioral rhythmicity may be encoded by changes in electrical properties of those neurons. In this review, we explore the existing evidence for molecular clocks and/or neurophysiological rhythms (i.e., 24 hr) in brain regions outside the SCN. In addition, we highlight the brain regions that are ripe for future investigation into the critical role of circadian rhythmicity for local oscillators. For example, the cerebellum expresses rhythmicity in over 2,000 gene transcripts, and yet we know very little about how circadian regulation drives 24-hr changes in the neural coding responsible for motor coordination. Finally, we conclude with a discussion of how our understanding of circadian regulation of electrical properties may yield insight into disease mechanisms which may lead to novel chronotherapeutic strategies in the future.

Circadian and Metabolic Effects of Light: Implications in Weight Homeostasis and Health

Daily interactions between the hypothalamic circadian clock at the suprachiasmatic nucleus (SCN) and peripheral circadian oscillators regulate physiology and metabolism to set temporal variations in homeostatic regulation. Phase coherence of these circadian oscillators is achieved by the entrainment of the SCN to the environmental 24-h light:dark (LD) cycle, coupled through downstream neural, neuroendocrine, and autonomic outputs. The SCN coordinate activity and feeding rhythms, thus setting the timing of food intake, energy expenditure, thermogenesis, and active and basal metabolism. In this work, we will discuss evidences exploring the impact of different photic entrainment conditions on energy metabolism. The steady-state interaction between the LD cycle and the SCN is essential for health and wellbeing, as its chronic misalignment disrupts the circadian organization at different levels. For instance, in nocturnal rodents, non-24 h protocols (i.e., LD cycles of different durations, or chronic jet-lag simulations) might generate forced desynchronization of oscillators from the behavioral to the metabolic level. Even seemingly subtle photic manipulations, as the exposure to a “dim light” scotophase, might lead to similar alterations. The daily amount of light integrated by the clock (i.e., the photophase duration) strongly regulates energy metabolism in photoperiodic species. Removing LD cycles under either constant light or darkness, which are routine protocols in chronobiology, can also affect metabolism, and the same happens with disrupted LD cycles (like shiftwork of jetlag) and artificial light at night in humans. A profound knowledge of the photic and metabolic inputs to the clock, as well as its endocrine and autonomic outputs to peripheral oscillators driving energy metabolism, will help us to understand and alleviate circadian health alterations including cardiometabolic diseases, diabetes, and obesity.

Dopamine: A Modulator of Circadian Rhythms in the Central Nervous System

Circadian rhythms are daily rhythms that regulate many biological processes – from gene transcription to behavior – and a disruption of these rhythms can lead to a myriad of health risks. Circadian rhythms are entrained by light, and their 24-h oscillation is maintained by a core molecular feedback loop composed of canonical circadian (“clock”) genes and proteins. Different modulators help to maintain the proper rhythmicity of these genes and proteins, and one emerging modulator is dopamine. Dopamine has been shown to have circadian-like activities in the retina, olfactory bulb, striatum, midbrain, and hypothalamus, where it regulates, and is regulated by, clock genes in some of these areas. Thus, it is likely that dopamine is essential to mechanisms that maintain proper rhythmicity of these five brain areas. This review discusses studies that showcase different dopaminergic mechanisms that may be involved with the regulation of these brain areas’ circadian rhythms. Mechanisms include how dopamine and dopamine receptor activity directly and indirectly influence clock genes and proteins, how dopamine’s interactions with gap junctions influence daily neuronal excitability, and how dopamine’s release and effects are gated by low- and high-pass filters. Because the dopamine neurons described in this review also release the inhibitory neurotransmitter GABA which influences clock protein expression in the retina, we discuss articles that explore how GABA may contribute to the actions of dopamine neurons on circadian rhythms. Finally, to understand how the loss of function of dopamine neurons could influence circadian rhythms, we review studies linking the neurodegenerative disease Parkinson’s Disease to disruptions of circadian rhythms in these five brain areas. The purpose of this review is to summarize growing evidence that dopamine is involved in regulating circadian rhythms, either directly or indirectly, in the brain areas discussed here. An appreciation of the growing evidence of dopamine’s influence on circadian rhythms may lead to new treatments including pharmacological agents directed at alleviating the various symptoms of circadian rhythm disruption.

Molecular interrogation of hypothalamic organization reveals distinct dopamine neuronal subtypes

The hypothalamus contains the highest diversity of neurons in the brain. Many of these neurons can co-release neurotransmitters and neuropeptides in a use-dependent manner. Investigators have hitherto relied on candidate protein-based tools to correlate behavioral, endocrine and gender traits with hypothalamic neuron identity. Here we map neuronal identities in the hypothalamus by single-cell RNA sequencing. We distinguished 62 neuronal subtypes producing glutamatergic, dopaminergic or GABAergic markers for synaptic neurotransmission and harboring the ability to engage in task-dependent neurotransmitter switching. We identified dopamine neurons that uniquely coexpress the Onecut3 and Nmur2 genes, and placed these in the periventricular nucleus with many synaptic afferents arising from neuromedin S+ neurons of the suprachiasmatic nucleus. These neuroendocrine dopamine cells may contribute to the dopaminergic inhibition of prolactin secretion diurnally, as their neuromedin S+ inputs originate from neurons expressing Per2 and Per3 and their tyrosine hydroxylase phosphorylation is regulated in a circadian fashion. Overall, our catalog of neuronal subclasses provides new understanding of hypothalamic organization and function.

Alterations in Metabolism and Diurnal Rhythms following Bilateral Surgical Removal of the Superior Cervical Ganglia in Rats

Mammalian circadian rhythms are controlled by a master pacemaker located in the suprachiasmatic nuclei (SCN), which is synchronized to the environment by photic and nonphotic stimuli. One of the main functions of the SCN is to regulate peripheral oscillators to set temporal variations in the homeostatic control of physiology and metabolism. In this sense, the SCN coordinate the activity/rest and feeding/fasting rhythms setting the timing of food intake, energy expenditure, thermogenesis, and active and basal metabolism. One of the major time cues to the periphery is the nocturnal melatonin, which is synthesized and secreted by the pineal gland. Under SCN control, arylalkylamine N-acetyltransferase (AA-NAT)-the main enzyme regulating melatonin synthesis in vertebrates-is activated at night by sympathetic innervation that includes the superior cervical ganglia (SCG). Bilateral surgical removal of the superior cervical ganglia (SCGx) is considered a reliable procedure to completely prevent the nocturnal AA-NAT activation, irreversibly suppressing melatonin rhythmicity. In the present work, we studied the effects of SCGx on rat metabolic parameters and diurnal rhythms of feeding and locomotor activity. We found a significant difference between SCGx and sham-operated rats in metabolic variables such as an increased body weight/food intake ratio, increased adipose tissue, and decreased glycemia with a normal glucose tolerance. An analysis of locomotor activity and feeding rhythms showed an increased daytime (lights on) activity (including food consumption) in the SCGx group. These alterations suggest that superior cervical ganglia-related feedback mechanisms play a role in SCN-periphery phase coordination and that SCGx is a valid model without brain-invasive surgery to explore how sympathetic innervation affects daily (24 h) patterns of activity, food consumption and, ultimately, its role in metabolism homeostasis.

Circadian influences on dopamine circuits of the brain: regulation of striatal rhythms of clock gene expression and implications for psychopathology and disease

Circadian clock proteins form an autoregulatory feedback loop that is central to the endogenous generation and transmission of daily rhythms in behavior and physiology. Increasingly, circadian rhythms in clock gene expression are being reported in diverse tissues and brain regions that lie outside of the suprachiasmatic nucleus (SCN), the master circadian clock in mammals. For many of these extra-SCN rhythms, however, the region-specific implications are still emerging. In order to gain important insights into the potential behavioral, physiological, and psychological relevance of these daily oscillations, researchers have begun to focus on describing the neurochemical, hormonal, metabolic, and epigenetic contributions to the regulation of these rhythms. This review will highlight important sites and sources of circadian control within dopaminergic and striatal circuitries of the brain and will discuss potential implications for psychopathology and disease . For example, rhythms in clock gene expression in the dorsal striatum are sensitive to changes in dopamine release, which has potential implications for Parkinson’s disease and drug addiction. Rhythms in the ventral striatum and limbic forebrain are sensitive to psychological and physical stressors, which may have implications for major depressive disorder. Collectively, a rich circadian tapestry has emerged that forces us to expand traditional views and to reconsider the psychopathological, behavioral, and physiological importance of these region-specific rhythms in brain areas that are not immediately linked with the regulation of circadian rhythms.

Pregnancy-induced adaptations of the central circadian clock and maternal glucocorticoids

Maternal physiological adaptations, such as changes to the hypothalamic-pituitary-adrenal (HPA) axis, are central to pregnancy success. Circadian variation of the HPA axis is dependent on clock gene rhythms in the hypothalamus, but it is not known whether pregnancy-induced changes in maternal glucocorticoid levels are mediated via this central clock. We hypothesized that hypothalamic expression of clock genes changes across mouse pregnancy and this is linked to altered HPA activity. The anterior hypothalamus and maternal plasma were collected from C57Bl/6J mice prior to pregnancy and on days 6, 10, 14 and 18 of gestation (term=d19), across a 24-h period (0800, 1200, 1600, 2000, 0000, 0400 h). Hypothalamic expression of clock genes and Crh was determined by qPCR, plasma ACTH concentration measured by Milliplex assay and plasma corticosterone concentration by LC-MS/MS. Expression of all clock genes varied markedly across gestation, most notably at mid-gestation when levels of each gene were elevated. The pregnancy-induced increase in maternal corticosterone levels (by up to 14-fold on day 14) was not accompanied by a parallel shift in plasma ACTH (28% lower on day 14 compared with non-pregnant levels). Moreover, while circadian rhythmicity in corticosterone was maintained up to day 14 of gestation, this was effectively lost by day 18. Overall, our data show that the central circadian clock undergoes marked adaptations throughout mouse pregnancy, changes that are likely to contribute to maternal physiological adaptations. Importantly, however, neither hypothalamic clock genes nor plasma ACTH levels appear to drive the marked increase in maternal corticosterone after mid-gestation.

Circadian Tick-Talking Across the Neuroendocrine System and Suprachiasmatic Nuclei Circuits: The Enigmatic Communication Between the Molecular and Electrical Membrane Clocks

As with many processes in nature, appropriate timing in biological systems is of paramount importance. In the neuroendocrine system, the efficacy of hormonal influence on major bodily functions, such as reproduction, metabolism and growth, relies on timely communication within and across many of the brain's homeostatic systems. The activity of these circuits is tightly orchestrated with the animal's internal physiological demands and external solar cycle by a master circadian clock. In mammals, this master clock is located in the hypothalamic suprachiasmatic nucleus (SCN), where the ensemble activity of thousands of clock neurones generates and communicates circadian time cues to the rest of the brain and body. Many regions of the brain, including areas with neuroendocrine function, also contain local daily clocks that can provide feedback signals to the SCN. Although much is known about the molecular processes underpinning endogenous circadian rhythm generation in SCN neurones and, to a lesser extent, extra-SCN cells, the electrical membrane clock that acts in partnership with the molecular clockwork to communicate circadian timing across the brain is poorly understood. The present review focuses on some circadian aspects of reproductive neuroendocrinology and processes involved in circadian rhythm communication in the SCN, aiming to identify key gaps in our knowledge of cross-talk between our daily master clock and neuroendocrine function. The intention is to highlight our surprisingly limited understanding of their interaction in the hope that this will stimulate future work in these areas.

Circadian insights into dopamine mechanisms

Almost every physiological or behavioral process in mammals follows rhythmic patterns, which depend mainly on a master circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). The dopaminergic (DAergic) system in the brain is principally implicated in motor functions, motivation and drug intake. Interestingly, DA-related parameters and behaviors linked to the motivational and arousal states, show daily rhythms that could be regulated by the SCN or by extra-SCN circadian oscillator(s) modulating DAergic systems. Here we examine what is currently understood about the anatomical and functional central multi-oscillatory circadian system, highlighting how the main SCN clock communicates timing information with other brain clocks to regulate the DAergic system and conversely, how DAergic cues may have feedback effects on the SCN. These studies give new insights into the role of the brain circadian system in DA-related neurologic pathologies, such as Parkinson's disease, attention deficit/hyperactive disorder and drug addiction.

Involvement of dopamine D2 receptor in the diurnal changes of tuberoinfundibular dopaminergic neuron activity and prolactin secretion in female rats

BACKGROUND

An endogenous dopaminergic (DA) tone acting on D3 receptors has been shown to inhibit tuberoinfundibular (TI) DA neuron activity and stimulate prolactin (PRL) surge in the afternoon of estrogen-primed ovariectomized (OVX+E2) rats. Whether D2 receptor (D2R) is also involved in the regulation of TIDA and PRL rhythms was determined in this study.

RESULTS

Intracerebroventricular (icv) injection of PHNO, a D2R agonist, in the morning inhibited TIDA and midbrain DA neurons' activities, and stimulated PRL secretion. The effects of PHNO were significantly reversed by co-administration of raclopride, a D2R antagonist. A single injection of raclopride at 1200 h significantly reversed the lowered TIDA neuron activity and the increased serum PRL level at 1500 h. Dopamine D2R mRNA expression in medial basal hypothalamus (MBH) exhibited a diurnal rhythm, i.e., low in the morning and high in the afternoon, which was opposite to that of TIDA neuron activity. The D2R rhythm was abolished in OVX+E2 rats kept under constant lighting but not in OVX rats with regular lighting exposures. Pretreatment with an antisense oligodeoxynucleotides (AODN, 10 μg/3 μl/day, icv) against D2R mRNA for 2 days significantly reduced D2R mRNAs in central DA neurons, and reversed both lowered TIDA neuron activity and increased serum PRL level in the afternoon on day 3. A diurnal rhythm of D2R mRNA expression was also observed in midbrain DA neurons and the rhythm was significantly knocked down by the AODN pretreatment.

CONCLUSIONS

We conclude that a diurnal change of D2R mRNA expression in MBH may underlie the diurnal rhythms of TIDA neuron activity and PRL secretion in OVX+E2 rats.

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.

Clocks on top: the role of the circadian clock in the hypothalamic and pituitary regulation of endocrine physiology

Recent strides in circadian biology over the last several decades have allowed researchers new insight into how molecular circadian clocks influence the broader physiology of mammals. Elucidation of transcriptional feedback loops at the heart of endogenous circadian clocks has allowed for a deeper analysis of how timed cellular programs exert effects on multiple endocrine axes. While the full understanding of endogenous clocks is currently incomplete, recent work has re-evaluated prior findings with a new understanding of the involvement of these cellular oscillators, and how they may play a role in constructing rhythmic hormone synthesis, secretion, reception, and metabolism. This review addresses current research into how multiple circadian clocks in the hypothalamus and pituitary receive photic information from oscillators within the hypothalamic suprachiasmatic nucleus (SCN), and how resultant hypophysiotropic and pituitary hormone release is then temporally gated to produce an optimal result at the cognate target tissue. Special emphasis is placed not only on neural communication among the SCN and other hypothalamic nuclei, but also how endogenous clocks within the endocrine hypothalamus and pituitary may modulate local hormone synthesis and secretion in response to SCN cues. Through evaluation of a larger body of research into the impact of circadian biology on endocrinology, we can develop a greater appreciation into the importance of timing in endocrine systems, and how understanding of these endogenous rhythms can aid in constructing appropriate therapeutic treatments for a variety of endocrinopathies.

Circadian control of neuroendocrine circuits regulating female reproductive function

Female reproduction requires the precise temporal organization of interacting, estradiol-sensitive neural circuits that converge to optimally drive hypothalamo-pituitary-gonadal (HPG) axis functioning. In mammals, the master circadian pacemaker in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus coordinates reproductively relevant neuroendocrine events necessary to maximize reproductive success. Likewise, in species where periods of fertility are brief, circadian oversight of reproductive function ensures that estradiol-dependent increases in sexual motivation coincide with ovulation. Across species, including humans, disruptions to circadian timing (e.g., through rotating shift work, night shift work, poor sleep hygiene) lead to pronounced deficits in ovulation and fecundity. Despite the well-established roles for the circadian system in female reproductive functioning, the specific neural circuits and neurochemical mediators underlying these interactions are not fully understood. Most work to date has focused on the direct and indirect communication from the SCN to the gonadotropin-releasing hormone (GnRH) system in control of the preovulatory luteinizing hormone (LH) surge. However, the same clock genes underlying circadian rhythms at the cellular level in SCN cells are also common to target cell populations of the SCN, including the GnRH neuronal network. Exploring the means by which the master clock synergizes with subordinate clocks in GnRH cells and its upstream modulatory systems represents an exciting opportunity to further understand the role of endogenous timing systems in female reproduction. Herein we provide an overview of the state of knowledge regarding interactions between the circadian timing system and estradiol-sensitive neural circuits driving GnRH secretion and the preovulatory LH surge.

Circadian nursing induces PER1 protein in neuroendocrine tyrosine hydroxylase neurones in the rabbit doe

Rabbit does nurse their pups once a day with circadian periodicity and pups ingest up to 35% of their body weight in milk in < 5 min. In the doe, there is a massive release of prolactin. We hypothesised that periodic suckling synchronises dopaminergic populations that control prolactin secretion. We explored this by immunohistochemical colocalisation of PER1 protein, the product of the clock gene Per1 on tyrosine hydroxylase (TH) cells in three dopaminergic populations: tuberoinfundibular dopaminergic (TIDA), periventricular hypophyseal dopaminergic (PHDA) and incertohypothalamic dopaminergic (IHDA) cells. PER1/TH colocalisation was explored every 4 h through a complete 24-h cycle at postpartum day 7 in does that nursed their pups either at 10.00 h (ZT03) or at 02.00 h (ZT19; ZT0 = 07.00 h, time of lights on). Nonpregnant, nonlactating females were used as controls. In control females, there was a rhythm of PER1 that peaks at ZT15. By contrast, in nursed does, the PER1 peak shifted in parallel to scheduled nursing in TIDA and PHDA cells but not in IHDA cells, which are not related to the control of prolactin. Next, we determined that the absence of suckling for 48 h significantly decreases the number of PER1/TH colocalised cells in PHDA but not TIDA cells. Locomotor behaviour in control subjects was maximal at around the time of lights on but, in nursed females, shifted at around the time of scheduled nursing. Finally, in the suprachiasmatic nucleus, there is a maximal expression of PER1 at ZT11 in the three groups. However, this maximal expression was significantly lower in the nursed groups in relation to the control group and in the groups deprived of nursing for 48 h. We conclude that suckling synchronises dopaminergic cells related to the control of prolactin and appears to be a nonphotic stimulus for the suprachiasmatic nucleus.

Global depletion of dopamine using intracerebroventricular 6-hydroxydopamine injection disrupts normal circadian wheel-running patterns and PERIOD2 expression in the rat forebrain

Normal circadian rhythms of behavior are disrupted in disorders involving the dopamine (DA) system, such as Parkinson's disease. We have reported previously using unilateral injections of the catecholamine toxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle that DA signaling regulates daily expression of the clock protein, PERIOD2 (PER2), in the dorsal striatum of the rat. In the present study, we made widespread lesions of DA fibers using large injections of 6-OHDA into the third ventricle to determine the involvement of DA in normal daily rhythms of wheel-running activity and PER2 patterns in the suprachiasmatic nucleus (SCN) and several regions of the limbic forebrain. Rats injected with 6-OHDA and housed in constant darkness were less active in the wheel and showed a disorganized pattern of activity in which wheel running was not confined to a specific phase over 24 h. The 6-OHDA injection had no effect on the daily PER2 pattern in the SCN, but blunted the normal rise in PER2 in the dorsal striatum. 6-OHDA also blunted PER2 expression in the periventricular nucleus of the hypothalamus, a region in which a daily PER2 pattern has not been previously reported in male rats, and in the oval nucleus of the bed nucleus of the stria terminalis, but not in the central nucleus of the amygdala. These results indicate that DA plays a prominent role in regulating circadian activity at both behavioral and molecular levels.

The spontaneous firing rates of dopamine-inhibited dorsomedial arcuate neurons exhibit a diurnal rhythm in brain slices obtained from ovariectomized plus estrogen-treated rats

The activity of tuberoinfundibular dopaminergic (TIDA) neurons exhibits a diurnal rhythm in female rats, as determined by neurochemical investigation. Whether the spontaneous firing rates of presumed TIDA neurons in the dorsomedial arcuate nucleus (dmARN) also exhibit a diurnal pattern has yet to be ascertained. Single-unit activities of 131 dmARN neurons were recorded in brain slices prepared from 83 ovariectomized plus estrogen-primed rats, and grouped according to their responses to dopamine and the time at which they were observed. In dopamine-inhibited dmARN neurons, significantly lower firing rates were observed in the afternoon compared to those recorded in the morning (2.51 ± 0.27 Hz, n=15, from 1130 to 1330 h vs. 1.08 ± 0.07 Hz, n=47, from 1430 to 1630 h). No such change was observed in dopamine-excited or nonresponsive dmARN neurons (1.83 ± 0.32 Hz, n=9 vs. 1.46 ± 0.17 Hz, n=21). Four dmARN neurons were continuously recorded from 1130 to 1600 h or even longer until 2000 h. The averaged firing rates decreased significantly between 1300 and 1600 h, two neurons were also inhibited by dopamine and a selective D(2) receptor agonist, PHNO, in both normal and low Ca(2+), high Mg(2+) perfusion mediums. This study revealed the existence of diurnal changes in the firing rates of dopamine-inhibited dmARN neurons. These results are strongly correlated with the rhythmic changes observed in TIDA neuronal activity determined through neurochemical methods.

Activity of hypothalamic dopaminergic neurones during the day of oestrus: involvement in prolactin secretion

A secretory surge of prolactin occurs on the afternoon of oestrus in cycling rats. Pituitary prolactin is inhibited by dopamine. We evaluated the activity of the neuroendocrine dopaminergic neurones during oestrus and dioestrus, as determined by dopaminergic activity in the median eminence and neurointermediate lobe of the pituitary, as well as Fos-related antigen expression in tyrosine hydroxylase (TH)-immunoreactive (ir) neurones of the arcuate nucleus (ARC) and periventricular nucleus (Pe). During oestrus, the 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence decreased at 16.00 h, coinciding with the increase in plasma prolactin levels. Similarly, the expression of Fos-related antigen in TH-ir neurones of Pe and rostral-, dorsomedial- and caudal-ARC also decreased at 16.00 h. On dioestrus, 4-dihydroxyphenylacetic acid/dopamine ratio in the median eminence and Fos-related antigen expression in TH-ir neurones of Pe and rostral-ARC decreased at 18.00 h, whereas prolactin levels were unaltered. No variation in dopaminergic activity was found in the neurointermediate lobe of the pituitary on either oestrus or dioestrus. The number of TH-ir neurones in the ARC and parameters of dopaminergic activity were found to be generally lower on oestrus compared to dioestrus. The transitory decrease in the activity of neuroendocrine dopaminergic neurones temporally associated with the prolactin surge on the afternoon of oestrus suggests a role for dopamine in the generation of the oestrous prolactin surge.

Cadmium chronotoxicity at pituitary level: effects on plasma ACTH, GH, and TSH daily pattern

Cadmium is an endocrine disruptor that has been shown to induce chronotoxic effects. The present study was designed to evaluate the possible cadmium effects on the daily secretory pattern of adrenocorticotropin hormone (ACTH), growth hormone (GH), and thyroid-stimulating hormone (TSH) in adult male Sprague-Dawley rats. For this purpose, animals were treated with cadmium at two different doses [25 and 50 mg/l cadmium chloride (CdCl(2))] in the drinking water for 30 days. Control age-matched rats received cadmium-free water. After the treatment, rats were killed at six different time intervals throughout a 24-h cycle. Cadmium exposure modified the 24-h pattern of plasma ACTH and GH levels, as the peak of ACTH content between 12:00 and 16:00 h in controls appeared at 12:00 h in the group treated with the lowest dose used, while it appeared between 16:00 and 20:00 h in rats exposed to 50 mg/l CdCl(2). In addition, the peak of GH content found at 04:00 h in controls moved to 16:00 h in rats exposed to 25 mg/l CdCl(2), and the highest dose used abolished 24-h changes of GH secretion. The metal treatment did not modify ACTH secretory pattern. Exposure to cadmium also increased ACTH and TSH medium levels around the clock with both doses used. These results suggest that cadmium modifies ACTH and TSH medium levels around the clock, as well as disrupted ACTH and GH secretory pattern, thus confirming the metal chronotoxicity at pituitary level.

Clock gene expression during chronic inflammation induced by infection with Trypanosoma brucei brucei in rats

African sleeping sickness is characterized by alterations in rhythmic functions. It is not known if the disease affects the expression of clock genes, which are the molecular basis for rhythm generation. We used a chronic rat model of experimental sleeping sickness, caused by the extracellular parasite Trypanosoma brucei brucei (Tb brucei), to study the effects on clock gene expression. In tissue explants of pituitary glands from Period1-luciferase (Per1-luc) transgenic rats infected with Tb brucei, the period of Per1-luc expression was significantly shorter. In explants containing the suprachiasmatic nuclei (SCN), the Per1-luc rhythms were flat in 21% of the tissues. We also examined the relative expression of Per1, Clock, and Bmal1 mRNA in the SCN, pineal gland, and spleen from control and infected rats using qPCR. Both Clock and Bmal1 mRNA expression was reduced in the pineal gland and spleen following Tb brucei infection. Infected rats were periodic both in core body temperature and in locomotor activity; however, early after infection, we observed a significant decline in the amplitude of the locomotor activity rhythm. In addition, both activity and body temperature rhythms exhibited decreased regularity and "robustness." In conclusion, although experimental trypanosome infection has previously been shown to cause functional disturbances in SCN neurons, only 21% of the SCN explants had disturbed Per1-luc rhythms. However, our data show that the infection overall alters molecular clock function in peripheral clocks including the pituitary gland, pineal gland, and spleen.

Impact of aging on diurnal expression patterns of CLOCK and BMAL1 in the mouse brain

Mammalian circadian rhythms are generated by a network of transcriptional and translational loops in the expression of a panel of clock genes in various brain and peripheral sites. Many of the output rhythms controlled by this system are significantly affected by ageing, although the mechanisms of age-related circadian dysfunction remain opaque. The aim of this study was to investigate the effect of aging on the daily oscillation of two clock gene proteins (CLOCK, BMAL1) in the mouse brain. Clock gene protein expression in the brain was measured by means of immunohistochemistry in groups of young (4 months) and older (16 months) mice sampled every 4h over a 24-h cycle. CLOCK and BMAL1 were constitutively expressed in the suprachiasmatic nucleus (SCN; the master circadian pacemaker) in young adult animals. We report novel rhythmic expression of CLOCK and BMAL1 in a number of extra-SCN sites in the young mouse brain, including the hippocampus, amygdala and the paraventricular, arcuate and dorsomedial nuclei of the hypothalamus. Aging altered the amplitude and/or phase of expression in these regions. These results indicate hitherto unreported expression patterns of CLOCK and BMAL1 in non-SCN brain circadian oscillators, and suggest that alterations of these patterns may contribute to age-related circadian dysfunction.

Circadian timekeeping and multiple timescale neuroendocrine rhythms

The hormones secreted by the anterior pituitary gland regulate major functions such as reproduction, as well as body growth and metabolism. Their efficiency of action highly depends on their temporal profile of release in the blood stream. This review summarises the recent evidence suggesting that the circadian clock genes that pace our daily rhythms may also contribute to the regulation of pituitary pulsatility, even in the non 24-h range. This inter-relation between molecular circadian oscillators and endocrine rhythmicities is discussed in light of the longstanding literature that has considered the involvement of the central circadian pacemaker located within the suprachiasmatic nuclei. Other arguments that suggest a role for circadian clock genes outside the suprachiasmatic nuclei are also presented, with a special emphasis on endocrine pituitary cells and hypothalamic neuroendocrine neurones that directly pace pituitary secretion rates.

Central clock regulates the cervically stimulated prolactin surges by modulation of dopamine and vasoactive intestinal polypeptide release in ovariectomized rats

BACKGROUND/AIMS

Cervical stimulation induces a circadian rhythm of prolactin secretion and antiphase dopamine release. The suprachiasmatic nucleus (SCN) controls this rhythm, and we propose that it does so through clock gene expression within the SCN.

METHODS

To test this hypothesis, serial blood samples were taken from animals injected with an antisense deoxyoligonucleotide cocktail for clock genes (generated against the 5' transcription start site and 3' cap site of per1, per2, and clock mRNA) or with a random-sequence deoxyoligonucleotide in the SCN. To determine whether disruption of clock genes in the SCN compromises the neural mechanism controlling prolactin secretion, we sacrificed another group of rats (under the same treatments) at 12.00 or 17.00 h. Dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured using HPLC/electrochemical detection in the median eminence as well as the intermediate and the neural lobe of the pituitary gland, and the DOPAC:dopamine ratio was used as an index of dopamine activity. Vasoactive intestinal polypeptide (VIP) content was determined in tissue punches of the SCN and paraventricular nucleus (PVN), an SCN efferent.

RESULTS

Treatment with clock gene antisense deoxyoligonucleotide cocktail abolished both the diurnal and nocturnal prolactin surges induced by cervical stimulation. This treatment abolished the antiphase relationship established by cervical stimulation between dopamine neuronal activity and prolactin secretion. Also, VIP content increased in the SCN and decreased in the PVN.

CONCLUSION

These results suggest that the SCN clock determines the circadian rhythm of prolactin secretion in cervically stimulated rats by regulating dopamine neuronal activity and VIP inputs to the PVN.

A riot of rhythms: neuronal and glial circadian oscillators in the mediobasal hypothalamus

Background

In mammals, the synchronized activity of cell autonomous clocks in the suprachiasmatic nuclei (SCN) enables this structure to function as the master circadian clock, coordinating daily rhythms in physiology and behavior. However, the dominance of this clock has been challenged by the observations that metabolic duress can over-ride SCN controlled rhythms, and that clock genes are expressed in many brain areas, including those implicated in the regulation of appetite and feeding. The recent development of mice in which clock gene/protein activity is reported by bioluminescent constructs (luciferase or luc) now enables us to track molecular oscillations in numerous tissues ex vivo. Consequently we determined both clock activities and responsiveness to metabolic perturbations of cells and tissues within the mediobasal hypothalamus (MBH), a site pivotal for optimal internal homeostatic regulation.

Results

Here we demonstrate endogenous circadian rhythms of PER2::LUC expression in discrete subdivisions of the arcuate (Arc) and dorsomedial nuclei (DMH). Rhythms resolved to single cells did not maintain long-term synchrony with one-another, leading to a damping of oscillations at both cell and tissue levels. Complementary electrophysiology recordings revealed rhythms in neuronal activity in the Arc and DMH. Further, PER2::LUC rhythms were detected in the ependymal layer of the third ventricle and in the median eminence/pars tuberalis (ME/PT). A high-fat diet had no effect on the molecular oscillations in the MBH, whereas food deprivation resulted in an altered phase in the ME/PT.

Conclusion

Our results provide the first single cell resolution of endogenous circadian rhythms in clock gene expression in any intact tissue outside the SCN, reveal the cellular basis for tissue level damping in extra-SCN oscillators and demonstrate that an oscillator in the ME/PT is responsive to changes in metabolism.

Diurnal expression of period 2 and urocortin 1 in neurones of the non-preganglionic Edinger-Westphal nucleus in the rat

Period 2 (Per2) is an important clock gene involved in the regulation of the major circadian clock in the mammalian central nervous system, the suprachiasmatic nucleus. In addition, Per2 is expressed in many other stress-sensitive brain structures. We have previously showed that the non-preganglionic Edinger-Westphal nucleus (npEW) is the main site of the corticotropin-releasing factor peptide family member urocortin 1 (Ucn1) and that this peptide undergoes conspicuous expression changes in response to various stressors. Here, we hypothesized that in the rat npEW both Per2 and Ucn1 would be produced in a diurnal, rhythmical fashion. This hypothesis was tested by following this expected rhythm on two days in rats killed at four time points each day (Zeitgeber times 0, 6, 12, and 18). We showed the co-existence of Per2 and Ucn1 in the npEW with double-label immunofluorescence and demonstrated with quantitative RT-PCR and semi-quantitative immunocytochemistry diurnal rhythms in Per2 mRNA expression and Per2 protein content, each on a single different day, with a minimum at lights-off and a maximum at lights-on. We furthermore revealed a diurnal rhythm in the number of Ucn1-immunopositive neurones and in their Ucn1 peptide content, with a minimum at night and at the beginning of the light period and a peak at lights-off, while the Ucn1 mRNA content paralleled the Per2 mRNA rhythm. The rhythms were accompanied by a diurnal rhythm in plasma corticosterone concentration. Our results are in line with the hypothesis that both Per2 and Ucn1 in the rat npEW are produced in a diurnal fashion, a phenomenon that may be relevant for the regulation of the diurnal rhythm in the stress response.

Daily rhythms in PER1 within and beyond the suprachiasmatic nucleus of female grass rats (Arvicanthis niloticus)

Although circadian rhythms of males and females are different in a variety of ways in many species, their mechanisms have been primarily studied in males. Furthermore, rhythms are dramatically different in diurnal and nocturnal animals but have been studied predominantly in nocturnal ones. In the present study, we examined rhythms in one element of the circadian oscillator, the PER1 protein, in a variety of cell populations in brains of diurnal female grass rats. Every 4 h five adult female grass rats kept on a 12-h light/dark (LD) cycle were perfused and their brains were processed for immunohistochemical detection of PER1. Numbers of PER1-labeled cells were rhythmic not only within the suprachiasmatic nucleus (SCN), the locus of the primary circadian clock in mammals, but also in the peri-suprachiasmatic region, the oval nucleus of the bed nucleus of the stria terminalis, the central amygdala, and the nucleus accumbens. In addition, rhythms were detected within populations of neuroendocrine cells that contain tyrosine hydroxylase. The phase of the rhythm within the SCN was advanced compared with that seen previously in male grass rats. Rhythms beyond the SCN were varied and different from those seen in most nocturnal species, suggesting that signals originating in the SCN are modified by its direct and/or indirect targets in different ways in nocturnal and diurnal species.

Knockdown of clock genes in the suprachiasmatic nucleus blocks prolactin surges and alters FRA expression in the locus coeruleus of female rats

The nature of the circadian signal from the suprachiasmatic nucleus (SCN) required for prolactin (PRL) surges is unknown. Because the SCN neuronal circadian rhythm is determined by a feedback loop of Period (Per) 1, Per2, and circadian locomotor output cycles kaput (Clock) gene expressions, we investigated the effect of SCN rhythmicity on PRL surges by disrupting this loop. Because lesion of the locus coeruleus (LC) abolishes PRL surges and these neurons receive SCN projections, we investigated the role of SCN rhythmicity in the LC neuronal circadian rhythm as a possible component of the circadian mechanism regulating PRL surges. Cycling rats on proestrous day and estradiol-treated ovariectomized rats received injections of antisense or random-sequence deoxyoligonucleotide cocktails for clock genes (Per1, Per2, and Clock) in the SCN, and blood samples were taken for PRL measurements. The percentage of tyrosine hydroxylase-positive neurons immunoreactive to Fos-related antigen (FRA) was determined in ovariectomized rats submitted to the cocktail injections and in a 12:12-h light:dark (LD) or constant dark (DD) environment. The antisense cocktail abolished both the proestrous and the estradiol-induced PRL surges observed in the afternoon and the increase of FRA expression in the LC neurons at Zeitgeber time 14 in LD and at circadian time 14 in DD. Because SCN afferents and efferents were probably preserved, the SCN rhythmicity is essential for the magnitude of daily PRL surges in female rats as well as for LC neuronal circadian rhythm. SCN neurons therefore determine PRL secretory surges, possibly by modulating LC circadian neuronal activity.

Estrogen regulation of genes important for K+ channel signaling in the arcuate nucleus

Estrogen affects the electrophysiological properties of a number of hypothalamic neurons by modulating K(+) channels via rapid membrane actions and/or changes in gene expression. The interaction between these pathways (membrane vs. transcription) ultimately determines the effects of estrogen on hypothalamic functions. Using suppression subtractive hybridization, we produced a cDNA library of estrogen-regulated, brain-specific guinea pig genes, which included subunits from three prominent K+ channels (KCNQ5, Kir2.4, Kv4.1, and Kvbeta(1)) and signaling molecules that impact channel function including phosphatidylinositol 3-kinase (PI3K), protein kinase Cepsilon (PKCepsilon), cAMP-dependent protein kinase (PKA), A-kinase anchor protein (AKAP), phospholipase C (PLC), and calmodulin. Based on these findings, we dissected the arcuate nucleus from ovariectomized guinea pigs treated with estradiol benzoate (EB) or vehicle and analyzed mRNA expression using quantitative real-time PCR. We found that EB significantly increased the expression of KCNQ5 and Kv4.1 and decreased expression of KCNQ3 and AKAP in the rostral arcuate. In the caudal arcuate, EB increased KCNQ5, Kir2.4, Kv4.1, calmodulin, PKCepsilon, PLCbeta(4), and PI3Kp55gamma expression and decreased Kvbeta(1). The effects of estrogen could be mediated by estrogen receptor-alpha, which we found to be highly expressed in the guinea pig arcuate nucleus and, in particular, proopiomelanocortin neurons. In addition, single-cell RT-PCR analysis revealed that about 50% of proopiomelanocortin and neuropeptide Y neurons expressed KCNQ5, about 40% expressed Kir2.4, and about 60% expressed Kv4.1. Therefore, it is evident that the diverse effects of estrogen on arcuate neurons are mediated in part by regulation of K(+) channel expression, which has the potential to affect profoundly neuronal excitability and homeostatic functions, especially when coupled with the rapid effects of estrogen on K(+) channel function.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.