Indirect projections from the suprachiasmatic nucleus to major arousal-promoting cell groups in rat: Implications for the circadian control of behavioural state

Nov 25, 2004Neuroscience

Connections from the brain’s internal clock to key arousal areas in rats and their role in daily behavior rhythms

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Abstract

Tracer injections resulted in moderate to dense anterograde labeling in the orexin field and the tuberomammillary nucleus.

The suprachiasmatic nucleus (SCN) influences daily sleep-wake cycles through various brain regions.
The medial preoptic area (MPA), subparaventricular zone (SPVZ), and dorsomedial hypothalamic nucleus (DMH) may relay SCN output to sleep and wake-regulatory neurons.
Anterograde labeling was observed in multiple regions including the locus coeruleus and the ventral tegmental area, indicating possible connections to wake-regulatory groups.
Indirect pathways from the SCN to sleep/wake-regulatory areas could be significant for circadian control of sleep and other physiological functions.

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The circadian clock housed in the suprachiasmatic nucleus (SCN) controls various circadian rhythms including daily sleep-wake cycles. Using dual tract-tracing, we recently showed that the medial preoptic area (MPA), subparaventricular zone (SPVZ) and dorsomedial hypothalamic nucleus (DMH) are well positioned to relay SCN output to two key sleep-promoting nuclei, namely, the ventrolateral and median preoptic nuclei. The present study examined the possibility that these three nuclei may link the SCN with wake-regulatory neuronal groups. Biotinylated dextran-amine with or without cholera toxin B subunit was injected into selected main targets of SCN efferents; the retrograde labeling in the SCN was previously analyzed. Here, anterograde labeling was analyzed in immunohistochemically identified cholinergic, orexin/hypocretin-containing and aminergic cell groups. Tracer injections into the MPA, SPVZ and DMH resulted in moderate to dense anterograde labeling of varicose fibers in the orexin field and the tuberomammillary nucleus. The locus coeruleus, particularly the dendritic field, contained moderate anterograde labeling from the MPA and DMH. The ventral tegmental area, dorsal raphe nucleus, and laterodorsal tegmental nucleus all showed moderate anterograde labeling from the DMH. The substantia innominata showed moderate anterograde labeling from the MPA. These results suggest that the MPA, SPVZ and DMH are possible relay nuclei for indirect SCN projections not only to sleep-promoting preoptic nuclei as previously shown, but also to wake-regulatory cell groups throughout the brain. In the absence of major direct SCN projections to most of these sleep/wake-regulatory regions, indirect neuronal pathways probably play an important role in the circadian control of sleep-wake cycles and other physiological functions.

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Indirect projections from the suprachiasmatic nucleus to major arousal-promoting cell groups in rat: Implications for the circadian control of behavioural state

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