Common afferent projections to the dorsal raphe (DR) and locus coeruleus (LC) nuclei were analyzed in the rat by making paired injections of retrograde tracers, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold) and Fluorogold (FG), into the DR and the nuclear core of the LC. Our results demonstrate that the largest number of double-labeled neurons was located at various preoptic regions including medial preoptic area, lateral preoptic nucleus, and ventrolateral preoptic nucleus. The majority of labeled cells were also observed at the lateral hypothalamus, where the number of labeled cells was comparable to that of neurons at the medial preoptic area or lateral preoptic nucleus. A few double-labeled cells were observed at various hypothalamic regions including anterior, medial tuberal, posterior, and arcuate nuclei, as well as mesencephalic areas including substantia nigra compacta and ventrolateral/lateral periaqueductal gray matter. Cells were also observed at prelimbic/infralimbic prefrontal cortices, diagonal band of Broca, bed nucleus of stria terminalis, and pontine/medullary regions including various raphe nuclei, Barrington's nucleus, gigantocellularis, paragigantocellularis, prepositus hypoglossi, subcoeruleus, and dorsomedial tegmental area. Although electrophysiological studies need to be performed, a large number of double-labeled neurons located at preoptic regions as well as lateral hypothalamus might have their major role in simultaneous control over these monoaminergic nuclei as a means of influencing various sleep and arousal states of the animal. Double-labeled cells at the other locations might be positioned to influence a variety of other functions such as analgesia, cognition, and stress responses.
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