Photic regulation of circadian rhythms and the expression of p75 neurotrophin receptor immunoreactivity in the suprachiasmatic nucleus in rats

In utero Exposure to Valproic-Acid Alters Circadian Organisation and Clock-Gene Expression: Implications for Autism Spectrum Disorders

Autism Spectrum Disorder (ASD) is a pervasive neurodevelopmental disorder characterised by restrictive patterns of behaviour and alterations in social interaction and communication. Up to 80% of children with ASD exhibit sleep-wake cycle disturbances, emphasising the pressing need for novel approaches in the treatment of ASD-associated comorbidities. While sleep disturbances have been identified in ASD individuals, little has been done to assess the contribution of the circadian system to these findings. The objective of this study is to characterise circadian behaviour and clock-gene expression in a valproic acid (VPA)-induced animal model of autism to highlight perturbations potentially contributing to these disturbances. Male and female VPA-exposed offspring underwent circadian challenges, including baseline light-dark cycles, constant dark/light and light pulse protocols. Baseline analysis showed that VPA-exposed males, but not females, had a greater distribution of wheel-running behaviour across light-dark phases and a later activity offset (p < 0.0001), while controls showed greater activity confinement to the dark phase (p = 0.0256). Constant light analysis indicated an attenuated masking response and an increase in the number of days to reach arrhythmicity (p < 0.0001). A 1-h light pulse (150 lux) at CT 15 after 6 days of constant dark showed that both sexes exposed to VPA exhibited a lesser phase-shift when compared to controls (p = 0.0043). Immunohistochemical and western-blot assays reveal no alterations in retinal organisation or function. However, immunohistochemical assay of the SCN revealed altered expression of BMAL1 expression in VPA-exposed males (p = 0.0016), and in females (p = 0.0053). These findings suggest alterations within the core clockwork of the SCN and reduced photic-entrainment capacity, independent of retinal dysfunction. The results of this study shed light on the nature of circadian dysregulation in VPA-exposed animals and highlights the urgent need for novel perspectives in the treatment of ASD-associated comorbidities.

p75 neurotrophin receptor is a clock gene that regulates oscillatory components of circadian and metabolic networks

The p75 neurotrophin receptor (p75(NTR)) is a member of the tumor necrosis factor receptor superfamily with a widespread pattern of expression in tissues such as the brain, liver, lung, and muscle. The mechanisms that regulate p75(NTR) transcription in the nervous system and its expression in other tissues remain largely unknown. Here we show that p75(NTR) is an oscillating gene regulated by the helix-loop-helix transcription factors CLOCK and BMAL1. The p75(NTR) promoter contains evolutionarily conserved noncanonical E-box enhancers. Deletion mutagenesis of the p75(NTR)-luciferase reporter identified the -1039 conserved E-box necessary for the regulation of p75(NTR) by CLOCK and BMAL1. Accordingly, gel-shift assays confirmed the binding of CLOCK and BMAL1 to the p75(NTR-)1039 E-box. Studies in mice revealed that p75(NTR) transcription oscillates during dark and light cycles not only in the suprachiasmatic nucleus (SCN), but also in peripheral tissues including the liver. Oscillation of p75(NTR) is disrupted in Clock-deficient and mutant mice, is E-box dependent, and is in phase with clock genes, such as Per1 and Per2. Intriguingly, p75(NTR) is required for circadian clock oscillation, since loss of p75(NTR) alters the circadian oscillation of clock genes in the SCN, liver, and fibroblasts. Consistent with this, Per2::Luc/p75(NTR-/-) liver explants showed reduced circadian oscillation amplitude compared with those of Per2::Luc/p75(NTR+/+). Moreover, deletion of p75(NTR) also alters the circadian oscillation of glucose and lipid homeostasis genes. Overall, our findings reveal that the transcriptional activation of p75(NTR) is under circadian regulation in the nervous system and peripheral tissues, and plays an important role in the maintenance of clock and metabolic gene oscillation.

Cognitive performance as a zeitgeber: cognitive oscillators and cholinergic modulation of the SCN entrain circadian rhythms

The suprachiasmatic nucleus (SCN) is the primary circadian pacemaker in mammals that can synchronize or entrain to environmental cues. Although light exerts powerful influences on SCN output, other non-photic stimuli can modulate the SCN as well. We recently demonstrated that daily performance of a cognitive task requiring sustained periods of attentional effort that relies upon basal forebrain (BF) cholinergic activity dramatically alters circadian rhythms in rats. In particular, normally nocturnal rats adopt a robust diurnal activity pattern that persists for several days in the absence of cognitive training. Although anatomical and pharmacological data from non-performing animals support a relationship between cholinergic signaling and circadian rhythms, little is known about how endogenous cholinergic signaling influences SCN function in behaving animals. Here we report that BF cholinergic projections to the SCN provide the principal signal allowing for the expression of cognitive entrainment in light-phase trained animals. We also reveal that oscillator(s) outside of the SCN drive cognitive entrainment as daily timed cognitive training robustly entrains SCN-lesioned arrhythmic animals. Ablation of the SCN, however, resulted in significant impairments in task acquisition, indicating that SCN-mediated timekeeping benefits new learning and cognitive performance. Taken together, we conclude that cognition entrains non-photic oscillators, and cholinergic signaling to the SCN serves as a temporal timestamp attenuating SCN photic-driven rhythms, thereby permitting cognitive demands to modulate behavior.

Nerve growth factor-induced circadian phase shifts and MAP kinase activation in the hamster suprachiasmatic nuclei

Circadian rhythms are entrained by light and by several neurochemical stimuli. In hamsters housed in constant darkness, i.c.v. administration of nerve growth factor (NGF) at various times in their circadian cycle produced phase shifts of locomotor activity rhythms that were similar in direction and circadian timing to those produced by brief pulses of light. Moreover, the effect of NGF and light were not additive, indicating signalling points in common. These points include the immediate-early gene c-fos and ERK1/2, a component of the mitogen-activated protein kinases (MAPK) family. NGF activates c-FOS and ERK1/2-MAPK in the suprachiasmatic nuclei, the site of a circadian clock in mammals, when administered during the subjective night but not during the day. The effect of NGF on ERK1/2 activation was not inhibited by the administration of MK-801, a glutamate/NMDA receptor antagonist. These results suggest that NGF, acting through MAPK activation, plays a role in photic entrainment of the mammalian circadian clock.

Modulation of photic resetting in rats by lesions of projections to the suprachiasmatic nuclei expressing p75 neurotrophin receptor

The suprachiasmatic nuclei of the hypothalamus (SCN) are the site of the master circadian clock in mammals. The SCN clock is mainly entrained by the light-dark cycle. Light information is conveyed from the retina to the SCN through direct, retinohypothalamic fibres. The SCN also receive other projections, like cholinergic fibres from basal forebrain. To test whether cholinergic afferents are involved in photic resetting, lesions of cholinergic projections were performed in rats with intracerebroventricular (i.c.v.) injections or intra-SCN microinjections of 192 IgG-saporin. When injected in the SCN, this immunotoxin destroys the cholinergic projections and retinohypothalamic afferents that express p75 low-affinity nerve growth factor (p75(NGF)) receptors. The extent of lesions in the basal forebrain and SCN was assessed by acetylcholinesterase histochemistry, p75(NGF) receptor, choline acetyl-transferase, calbindin-D28K and VIP immunocytochemistry. The intra-SCN treatment reduced light-induced phase advances by 30%, and induced a complete loss of forebrain and retinal afferents expressing p75(NGF) receptors within the SCN and a decrease of forebrain cholinergic neurons, most likely those projecting to the SCN. The i.c.v. treatment reduced light-induced phase advances by 40%, increased phase delays and led to extensive damage of forebrain p75(NGF)-expressing neurons, while sparing half of the fibres expressing p75(NGF) receptors (retinal afferents?) in the SCN. Because the integrity of forebrain p75(NGF)-expressing neurons appears to be critical in mediating the effects on light-induced phase advances, we therefore suggest that anterior cholinergic projections expressing p75(NGF) receptors modulate the sensitivity of the SCN clock to the phase advancing effects of light.

The eyes suppress a circadian rhythm of FOS expression in the suprachiasmatic nucleus in the absence of light

Photic information transmitted from the eyes to the suprachiasmatic nucleus (SCN) is essential for entrainment of circadian behavioral and physiological rhythms in mammals. Under conditions of constant darkness, these rhythms are maintained by the circadian pacemaker cells of the SCN [Bioessays 22 (2000) 23]. It is possible, however, that even in the absence of light, the eyes, which also contain autonomous circadian pacemakers [Science 272 (1996) 419; Chronobiol Int 16 (1999) 229], modulate circadian rhythms in the SCN. Indeed, it was shown recently that removal of the eyes abolishes an endogenous circadian rhythm within cells of the SCN [Nat Neurosci 6 (2003) 111], a finding that led to the suggestion that specific rhythms of the SCN are driven by input from the eyes. In contrast, we show here that removal of the eyes amplifies a normally dampened endogenous circadian rhythm within the SCN, indicating that the eyes can suppress the expression of specific rhythms within the SCN while promoting others.

Effect of 192 IgG-saporin on circadian activity rhythms, expression of P75 neurotrophin receptors, calbindin-D28K, and light-induced Fos in the suprachiasmatic nucleus in rats

Photic entrainment of circadian rhythms in mammals is mediated through a direct retinal projection to the core region of the suprachiasmatic nucleus (SCN), the circadian clock. A proportion of this projection contains the low-affinity p75 neurotrophic receptor (p75NTR). Neonatal monosodium glutamate (MSG) treatment, which dramatically reduces p75NTR immunoreactivity in the SCN has no impact on photic entrainment. In order to clarify the contribution of p75NTR fibers in photic entrainment, targeted lesions of the p75NTR-immunoreactive SCN plexus were performed using intracerebroventricular (ICV) or intrahypothalamic injections of the immunotoxin 192 IgG-saporin (SAP) in rats. SAP treatment effectively abolished p75NTR immunoreactivity within the SCN core. ICV SAP treatment produced three different behavioral activity patterns: Animals became arrhythmic, displayed a shorter free-running period, or remained rhythmic following the lesion. Arrhythmic animals had large hypothalamic lesion which encompassed the entire SCN. In rhythmic rats, ICV-SAP significantly reduced immunostaining for calbindin-D28k (CaBP) in the SCN, and rats with shortened free-running periods had the lowest number of CaBP immunoreactive cells. ICV SAP also attenuated light-induced Fos expression in the SCN core. Despite lack of p75NTR and reduced CaBP and Fos expression in the SCN, SAP-treated rhythmic rats displayed normal photic entrainment. Intrahypothalamic SAP treatment reduced CaBP expression in the SCN but had no effect on light-induced Fos expression, free-running rhythms, or photic entrainment. The data show that p75NTR-immunoreactive elements in the SCN are not required for photic entrainment.