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The neural connections and neurotransmitter content of the suprachiasmatic nucleus and intergeniculate leaflet have been characterized thoroughly in only a few mammalian species, primarily nocturnal rodents. Few data are available about the neural circadian timing system in diurnal mammals, particularly those for which the formal characteristics of circadian rhythms have been investigated. This paper describes the circadian timing system in the diurnal rodent Octodon degus, a species that manifests robust circadian responses to photic and non-photic (social) zeitgebers. Specifically, this report details: (i) the distribution of six neurotransmitters commonly found in the suprachiasmatic nucleus and intergeniculate leaflet; (ii) the retinohypothalamic tract; (iii) the geniculohypothalamic tract; and (iv) retinogeniculate projections in O. degus. Using immunocytochemistry, neuropeptide Y-immunoreactive, serotonin-immunoreactive and [Met]enkephalin-immunoreactive fibers and terminals were detected in and around the suprachiasmatic nucleus; vasopressin-immunoreactive cell bodies were found in the dorsomedial and ventral suprachiasmatic nucleus; vasoactive intestinal polypeptide-immunoreactive cell bodies were located in the ventral suprachiasmatic nucleus; [Met]enkephalin-immunoreactive cells were located sparsely throughout the suprachiasmatic nucleus; and substance P-immunoreactive fibers and terminals were detected in the rostral suprachiasmatic nucleus and surrounding the nucleus throughout its rostrocaudal dimension. Neuropeptide Y-immunoreactive and [Met]enkephalin-immunoreactive cells were identified in the intergeniculate leaflet and ventral lateral geniculate nucleus, as were neuropeptide Y-immunoreactive, [Met]enkephalin-immunoreactive, serotonin-immunoreactive and substance P-immunoreactive fibers and terminals. The retinohypothalamic tract innervated both suprachiasmatic nuclei equally; in contrast, retinal innervation to the lateral geniculate nucleus, including the intergeniculate leaflet, was almost exclusively contralateral. Bilateral electrolytic lesions that destroyed the intergeniculate leaflet depleted the suprachiasmatic nucleus of virtually all neuropeptide Y- and [Met]enkephalin-stained fibers and terminals, whereas unilateral lesions reduced fiber and terminal staining by approximately half. Thus, [Met]enkephalin-immunoreactive and neuropeptide Y-immunoreactive cells project equally and bilaterally from the intergeniculate leaflet to the suprachiasmatic nucleus via the geniculohypothalamic tract in degus. This is the first report examining the neural circadian system in a diurnal rodent for which formal circadian properties have been described. The data indicate that the neural organization of the circadian timing system in degus resembles that of the most commonly studied nocturnal rodents, golden hamsters and rats. Armed with such data, one can ascertain differences in the functional organization of the circadian system between diurnal and nocturnal mammals.
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