Neuroanatomy of the extended circadian rhythm system

Abstract

The suprachiasmatic nucleus (SCN), site of the primary clock in the circadian rhythm system, has three major afferent connections. The most important consists of a retinohypothalamic projection through which photic information, received by classical rod/cone photoreceptors and intrinsically photoreceptive retinal ganglion cells, gains access to the clock. This information influences phase and period of circadian rhythms. The two other robust afferent projections are the median raphe serotonergic pathway and the geniculohypothalamic (GHT), NPY-containing pathway from the thalamic intergeniculate leaflet (IGL). Beyond this simple framework, the number of anatomical routes that could theoretically be involved in rhythm regulation is enormous, with the SCN projecting to 15 regions and being directly innervated by about 35. If multisynaptic afferents to the SCN are included, the number expands to approximately brain 85 areas providing input to the SCN. The IGL, a known contributor to circadian rhythm regulation, has a still greater level of complexity. This nucleus connects abundantly throughout the brain (to approximately 100 regions) by pathways that are largely bilateral and reciprocal. Few of these sites have been evaluated for their contributions to circadian rhythm regulation, although most have a theoretical possibility of doing so via the GHT. The anatomy of IGL connections suggests that one of its functions may be regulation of eye movements during sleep. Together, neural circuits of the SCN and IGL are complex and interconnected. As yet, few have been tested with respect to their involvement in rhythm regulation.

The deletion of M4 muscarinic receptors increases motor activity in females in the dark phase

“…There are areas with near proximity to SCN, such as the subparaventricular zone (SPVZ), the dorsomedial nucleus, and the posterior hypothalamic area (PHA) and the tuberomammillary nucleus (Abrahamson & Moore, 2006; Kramer et al., 2001). The striatum, the thalamus, and the intergeniculate leaflet (IGL (Hughes & Piggins, 2012; Morin, 2013)) are also areas with locomotor biorhythmic effects. The SCN is innervated by cholinergic nerves (Hut & Van der Zee, 2011), but does not need to be necessarily intrinsically cholinergic (van den Pol & Tsujimoto, 1985).…”

Clinical Significance and Pathogenesis of Visual Impairment in Parkinson’s Disease

“…The retinothalamic system transmits critical information about circadian rhythms, thanks to which biological rhythms, the wake-sleep cycle are regulated, locomotive activity and food consumption change during a day, etc. Most fibers comprised in this system are axons of RGC [10,11]. There also is the centrifugal hypothalamoretinal pathway starting from the hypothalamic neurosecreting fibers and finishing in the RGC, etc.…”

What Role do Circadian Rhythms Play in Learning and Memory?

“…However, whether the time-dependent LTP observed in hippocampus and/or the circadian modulated memory formations are SCN-dependent are questions to which answers remains largely elusive. Anatomically, the SCN has connections with many brain regions [53][54][55]. Of particular interest is its projections to the hippocampus, both directly [55] and indirectly via the locus coeruleus [56], which in turn mediates hippocampal activation [57].…”

“…Anatomically, the SCN has connections with many brain regions [53][54][55]. Of particular interest is its projections to the hippocampus, both directly [55] and indirectly via the locus coeruleus [56], which in turn mediates hippocampal activation [57]. Whether these connections are responsible for the circadian expressions of clock genes in hippocampus, or hippocampal synaptic plasticity is not known.…”