Elsevier

Neuroscience

Volume 137, Issue 4, 2006, Pages 1285-1297
Neuroscience

Neuroanatomy
Complex organization of mouse and rat suprachiasmatic nucleus

https://doi.org/10.1016/j.neuroscience.2005.10.030Get rights and content

Abstract

The suprachiasmatic nucleus, site of the dominant mammalian circadian clock, contains a variety of different neurons that tend to form groups within the nucleus. The present investigation used single and multiple label tract tracing and immunofluorescence methods to evaluate the relative locations of the neuron groups and to compare them with the distributions of the three major afferent projections, the retinohypothalamic tract, geniculohypothalamic tract and the serotonergic pathway from the median raphe nucleus. The suprachiasmatic nucleus has a complex order characterized by peptidergic cell groups (vasopressin, gastrin releasing peptide, vasoactive intestinal polypeptide, calbindin, calretinin, corticotrophin releasing factor and enkephalin) that, in most cases, substantially overlap. The retinohypothalamic tract projects bilaterally to virtually all the suprachiasmatic nucleus in both rat (predominantly contralateral) and mouse (symmetric) and its terminal field overlaps that for the geniculohypothalamic tract, but with distinctions visible according to density criteria; neither provides more than sparse innervation of the dorsomedial suprachiasmatic nucleus. In the mouse, the serotonergic terminal field is densest medially and ventrally, but is also distributed elsewhere with varying density. The serotonergic terminal plexus in the rat is densest centromedially and largely, but not completely, overlaps the complete distribution of retinal terminals with density much reduced in the lateral suprachiasmatic nucleus. The locations of vasopressin neurons, retinohypothalamic tract terminals and serotonergic (mouse, rat) or geniculohypothalamic tract (rat) provide evidence for three clear, but not exclusionary, sectors of the suprachiasmatic nucleus. The data, in conjunction with emerging knowledge concerning rhythmically dynamic changes in the size of regions of neuropeptide gene expression in suprachiasmatic nucleus cells, support the view that suprachiasmatic nucleus organization is more complex than a simple “core” and “shell” arrangement. While generalizations about suprachiasmatic nucleus organization can be made with respect to location of cell phenotypes or terminal fields, oversimplification may hinder, rather than facilitate, understanding of suprachiasmatic nucleus structure–function relationships.

Section snippets

Experimental procedures

Adult male albino rats (N=21; Sprague–Dawley, Taconic Farms, Germantown, NY, USA) and male inbred C57BL/6J mice (N=23; Jackson Laboratory, Bar Harbor, ME, USA) housed under a 12-h light/dark photoperiod, were anesthetized using ketamine (100mg/kg; Wyeth Pharmaceuticals, Madison, NJ, USA) and xylazine (10mg/kg; Ben Venue Laboratories, Bedford, OH, USA). Animals were placed in a stereotaxic instrument and cholera toxin subunit B (CT-B) conjugated to Alexa 594 (item C-34777, Molecular Probes,

Mouse

The SCN is easily defined by either of two criteria, ventral periventricular cells staining more intensely for Nissl substance than those in the surrounding hypothalamus (Fig. 1A) or the location of intensely GABA-IR neurons (Fig. 2A) which are abundant and clearly distinguishable throughout most of the SCN, although less so in the periventricular dorsomedial part of the nucleus. As indicated in Fig. 1A, cells staining darkly for Nissl substance tend to be rounder than those in adjacent

Discussion

The SCN can be satisfactorily distinguished from adjacent hypothalamus by the presence of a variety of neural indicators. Nissl substance stains darkly in SCN neurons and is an easily used characteristic identifying neurons throughout the entire SCN (Card and Moore 1991, Abrahamson and Moore 2001). Presence of GABA-IR in most, if not all, SCN neurons also renders this phenotype a good cellular marker of the nucleus (Abrahamson and Moore 2001, Moore and Speh 1993, Morin and Blanchard 2001).

Conclusion

In summary, investigators using a variety of methods have provided data suggesting two, three or even four divisions within the rodent SCN, depending upon the indicators used to define divisions (see (Morin and Allen, 2006) for a review). The present analysis of SCN cell types and terminal fields in the mouse and rat demonstrates an organizational complexity that does not readily fit a simple descriptive scheme. Moreover, rhythmically dynamic changes in apparent organization that have been

Acknowledgments

Supported by NIH grants MH64471 and NS22168 to L.P.M.

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    1

    Present address: Graduate School and University Center, City University of New York, Hunter College, 695 Park Avenue, New York, NY 10021.

    2

    Present address: Department of Psychiatry, Millhauser Laboratories, Room HN607, New York University School of Medicine, 550 First Avenue, New York, NY.

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