TGFα and AVP in the mouse suprachiasmatic nucleus: Anatomical relationship and daily profiles

Neurobiological studies of fatigue

Fatigue is a symptom associated with many disorders, is especially common in women and in older adults, and can have a huge negative influence on quality of life. Although most past research on fatigue uses human subjects instead of animal models, the use of appropriate animal models has recently begun to advance our understanding of the neurobiology of fatigue. In this review, results from animal models using immunological, developmental, or physical approaches to study fatigue are described and compared. Common across these animal models is that fatigue arises when a stimulus induces activation of microglia and/or increased cytokines and chemokines in the brain. Neurobiological studies implicate structures in the ascending arousal system, sleep executive control areas, and areas important in reward. In addition, the suprachiasmatic nucleus clearly plays an important role in homeostatic regulation of the neural network mediating fatigue. This nucleus responds to cytokines, shows decreased amplitude firing rate output in models of fatigue, and responds to exercise, one of our few treatments for fatigue. This is a young field but very important as the symptom of fatigue is common across many disorders and we do not have effective treatments.

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.

Light exposure induces short- and long-term changes in the excitability of retinorecipient neurons in suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the locus of a hypothalamic circadian clock that synchronizes physiological and behavioral responses to the daily light-dark cycle. The nucleus is composed of functionally and peptidergically diverse populations of cells for which distinct electrochemical properties are largely unstudied. SCN neurons containing gastrin-releasing peptide (GRP) receive direct retinal input via the retinohypothalamic tract. We targeted GRP neurons with a green fluorescent protein (GFP) marker for whole cell patch-clamping. In these neurons, we studied short (0.5-1.5 h)- and long-term (2-6 h) effects of a 1-h light pulse (LP) given 2 h after lights off [Zeitgeber time (ZT) 14:00-15:00] on membrane potential and spike firing. In brain slices taken from light-exposed animals, cells were depolarized, and spike firing rate increased between ZT 15:30 and 16:30. During a subsequent 4-h period beginning around ZT 17:00, GRP neurons from light-exposed animals were hyperpolarized by ∼15 mV. None of these effects was observed in GRP neurons from animals not exposed to light or in immediately adjacent non-GRP neurons whether or not exposed to light. Depolarization of GRP neurons was associated with a reduction in GABA(A)-dependent synaptic noise, whereas hyperpolarization was accompanied both by a loss of GABA(A) drive and suppression of a TTX-resistant leakage current carried primarily by Na. This suggests that, in the SCN, exposure to light may induce a short-term increase in GRP neuron excitability mediated by retinal neurotransmitters and neuropeptides, followed by long-term membrane hyperpolarization resulting from suppression of a leakage current, possibly resulting from genomic signals.

A TTX-sensitive local circuit is involved in the expression of PK2 and BDNF circadian rhythms in the mouse suprachiasmatic nucleus

The roles of a local circuit of electrophysiological activity were examined in the expression of circadian rhythms in the suprachiasmatic nucleus (SCN) of the adult mouse. The neuronal activity of cultured SCN was suppressed with tetrodotoxin (TTX), an Na+ channel blocker, and the circadian rhythms in mRNA level were assessed for 13 genes by in situ hybridization. SCN slices were cultured for 3 days and TTX was applied at the peak phase of Per1 expression rhythm. The SCN slices were examined at 4-h intervals up to 32 h after TTX application. The circadian rhythms in the expression of clock genes, Per1, Per2, Bmal1 and Cry1, and of clock-associated genes, Dec1, Dec2, Rev-erbalpha, Rev-erbbeta and DBP, were not affected by TTX treatment. By contrast, TTX completely abolished the circadian rhythm in the BDNF mRNA level and substantially damped the rhythm in PK2. The circadian rhythm in the AVP mRNA level was not changed significantly by TTX. These findings indicate that input through Na+-channel-dependent electrophysiological activity is not necessary for the expression of the circadian rhythms of clock and clock-associated genes, but necessary for full expression of the circadian rhythms of BDNF and PK2 in the SCN. A TTX-sensitive circuit is involved in the expression of BDNF and PK2 circadian rhythms in the mouse SCN.

Vasopressin immunoreactivity, but not vasoactive intestinal polypeptide, correlates with expression of circadian rhythmicity in the suprachiasmatic nucleus of voles

In common voles (Microtus arvalis), natural variation in locomotor behavior can be exploited to study the mechanism of pacemaker control over circadian timing of behavior. Here we studied daily patterns in numbers of neuropeptide immunoreactive suprachiasmatic nucleus neurons in rhythmic, weakly rhythmic, and non-rhythmic voles. Circadian rhythmic voles showed circadian variation in numbers of vasoactive intestinal polypeptide and vasopressin immunoreactive suprachiasmatic nucleus neurons with a peak at zeitgeber time 0. In contrast, voles with weak or no circadian rhythmicity exhibited similar fluctuations for vasoactive intestinal polypeptide, but a continuous, non-rhythmic high profile for vasopressin. Vole suprachiasmatic nucleus neurons do not produce somatostatin or substance P. We conclude that the vasopressin system in the common vole suprachiasmatic nucleus acts as a principal correlate with expression of circadian behavior, in contrast to vasoactive intestinal polypeptide, somatostatin, and substance P. We also conclude that high levels of vasopressin immunoreactivity in the non-rhythmic vole suprachiasmatic nucleus is in line with previously demonstrated hampered release, probably resulting in vasopressin accumulation in the suprachiasmatic nucleus. Vasopressin could be a candidate in mediating output of the vole circadian clock, leading to circadian expression of locomotor behavior.

Circadian and photic regulation of clock and clock-controlled proteins in the suprachiasmatic nuclei of calorie-restricted mice

In mammals, behavioural and physiological rhythms as well as clock gene expression in the central suprachiasmatic clock (SCN) are phase-shifted by a timed calorie restriction (T-CR; animals receiving at midday 66% of their daily food intake). The molecular mechanism of SCN depends on feedback loops involving clock genes and their protein products. To understand how T-CR mediates its synchronizing effects, we examined the rhythmic expression of three clock proteins, PERIOD (PER) 1, 2 and CLOCK, and one clock-controlled protein (i.e. vasopressin; AVP) in the SCN of mice either fed ad libitum (AL) or with T-CR. Moreover, we evaluated expression of these proteins in the SCN of AL and T-CR mice following a 1-h light pulse. The results indicate that, while PER1 and AVP rhythms were phase-advanced in T-CR mice, the PER2 rhythm showed an increased amplitude. CLOCK was expressed constitutively in AL mice while in T-CR it was significantly reduced, especially after feeding time. A light pulse produced a delayed increase in PER1 and a larger increase in PER2 expression in the SCN of T-CR mice than in AL animals. In addition, light exposure triggered an increase in AVP-ir cells in both AL and T-CR mice, and also of CLOCK expression but in T-CR mice only. The circadian changes in clock and clock-controlled proteins and their acute responses to light in the SCN of T-CR mice demonstrate that metabolic cues induced by a calorie restriction modulate the translational regulation of the SCN clock.

Expression of Tgfα in the suprachiasmatic nuclei of nocturnal and diurnal rodents

Transforming growth factor alpha (TGFalpha) in the suprachiasmatic nuclei (SCN) has been proposed as an inhibitory signal involved in the control of daily locomotor activity. This assumption is based mainly on studies performed in nocturnal hamsters. To test whether the transcriptional regulation of Tgfalpha can be correlated with the timing of overt activity in other species, we compared Tgfalpha expression in the SCN of nocturnal Swiss mice and of diurnal Arvicanthis housed under a light/dark cycle (LD) or transferred to constant darkness (DD). In agreement with data on hamsters, Tgfalpha mRNA levels in the mouse SCN showed peak and trough levels around (subjective) dawn and dusk, respectively, roughly corresponding to the period of rest and activity in this species. In contrast, in Arvicanthis housed in DD, the circadian rhythm of SCN Tgfalpha was similar to that of the mice in spite of opposite phasing of locomotor activity. Furthermore, in Arvicanthis exposed to LD, Tgfalpha mRNA levels were constitutively high throughout the day. A tonic role of light in the regulation of Tgfalpha in Arvicanthis was confirmed by an increased expression of Tgfalpha in response to a 6-h exposure to light during daytime in animals otherwise kept in DD. In conclusion, this study shows that, contrary to what is observed in mice, Tgfalpha mRNA levels in the SCN of Arvicanthis do not match timing of locomotor activity and are modulated by light.

Wildtype epidermal growth factor receptor (Egfr) is not required for daily locomotor or masking behavior in mice

Background

Recent studies have implicated the epidermal growth factor receptor (EGFR) within the subparaventricular zone as being a major mediator of locomotor and masking behaviors in mice. The results were based on small cohorts of mice homozygous for the hypomorphic Egfr^wa2 allele on a mixed, genetically uncontrolled background, and on intraventricular infusion of exogenous EGFR ligands. Subsequenlty, a larger study using the same genetically mixed background failed to replicate the original findings. Since both previous approaches were susceptible to experimental artifacts related to an uncontrolled genetic background, we analyzed the locomotor behaviors in Egfr^wa2 mutant mice on genetically defined, congenic backgrounds.

Methods

Mice carrying the Egfr^wa2 hypomorphic allele were bred to congenicity by backcrossing greater than ten generations onto C57BL/6J and 129S1/SvImJ genetic backgrounds. Homozygous Egfr^wa2 mutant and wildtype littermates were evaluated for defects in locomotor and masking behaviors.

Results

Mice homozygous for Egfr^wa2 showed normal daily locomotor activity and masking indistinguishable from wildtype littermates at two light intensities (200–300 lux and 400–500 lux).

Conclusion

Our results demonstrate that reduced EGFR activity alone is insufficient to perturb locomotor and masking behaviors in mice. Our results also suggest that other uncontrolled genetic or environmental parameters confounded previous experiments linking EGFR activity to daily locomotor activity and provide a cautionary tale for genetically uncontrolled studies.

Systems analysis of circadian time-dependent neuronal epidermal growth factor receptor signaling

BACKGROUND

Identifying the gene regulatory networks governing physiological signal integration remains an important challenge in circadian biology. Epidermal growth factor receptor (EGFR) has been implicated in circadian function and is expressed in the suprachiasmatic nuclei (SCN), the core circadian pacemaker. The transcription networks downstream of EGFR in the SCN are unknown but, by analogy to other SCN inputs, we expect the response to EGFR activation to depend on circadian timing.

RESULTS

We have undertaken a systems-level analysis of EGFR circadian time-dependent signaling in the SCN. We collected gene-expression profiles to study how the SCN response to EGFR activation depends on circadian timing. Mixed-model analysis of variance (ANOVA) was employed to identify genes with circadian time-dependent EGFR regulation. The expression data were integrated with transcription-factor binding predictions through gene group enrichment analyses to generate robust hypotheses about transcription-factors responsible for the circadian phase-dependent EGFR responses.

CONCLUSION

The analysis results suggest that the transcriptional response to EGFR signaling in the SCN may be partly mediated by established transcription-factors regulated via EGFR transcription-factors (AP1, Ets1, C/EBP), transcription-factors involved in circadian clock entrainment (CREB), and by core clock transcription-factors (Ror alpha). Quantitative real-time PCR measurements of several transcription-factor expression levels support a model in which circadian time-dependent EGFR responses are partly achieved by circadian regulation of upstream signaling components. Our study suggests an important role for EGFR signaling in SCN function and provides an example for gaining physiological insights through systems-level analysis.

Epidermal growth factor receptor induced Erk phosphorylation in the suprachiasmatic nucleus

A significant functional role for epidermal growth factor receptor (EGFR) in the suprachiasmatic nucleus (SCN) is suggested by the confluence of several recent findings: (1) EGFR is highly expressed in the SCN, (2) the EGFR ligand TGFalpha is expressed and apparently locally released in the SCN, and (3) EGFR activation local to the 3rd ventricle suppresses circadian locomotor behavior and drinking/feeding activities. However, it has not yet been shown that binding of EGFR in the SCN can activate the key signaling pathways associated with its function. Here, we report that EGF treatment induced Erk1/2 phosphorylation in the rat SCN cells. The observation indicates functional EGFR in the SCN and motivates further investigation of the functional role of these receptors.

Masking in waved-2 mice: EGF receptor control of locomotion questioned

It has been suggested that epidermal growth factors (EGF) are responsible for the inhibition of locomotion by light (i.e., masking) in nocturnal rodents (Kramer et al., 2001). The poor masking response of waved-2 (Egfr(wa2)) mutant mice, with reduced EGF receptor activity, was adduced in support of this idea. In the present work, we studied the responses to light over a large range in illumination levels, in a variety of tests, with pulses of light and with ultradian light-dark cycles in Egfr(wa2) mutant mice. No evidence suggested that normal functioning of epidermal growth factor receptors was required, or even involved, in masking.

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.