Afferent projections to the preoptic area and hypothalamic regions in the rat brain

Abstract

Afferent projections to the preoptic area and hypothalamus were studied in the rat by using horseradish peroxidase as a retrograde tracer. Microiontophoretic injections were made stereotaxically into one of six regions: medial preoptic area, paraventricular hypothalamic nucleus, dorsomedial hypothalamic nucleus, ventromedial hypothalamic nucleus, posterior hypothalamic area and lateral hypothalamic area. The present study deals with a description of labeled cells in extrahypothalamic nuclear groups which project to these six regions. The results indicate that several of the labeled cell groups send fibers to all injection regions. In the telencephalon, the lateral septum contains labeled cells after injections in all six regions. With the exception of posterior hypothalamic injections, all injections result in labeled cells in the amygdala; different subdivisions are labeled after lateral and ventromedial hypothalamic injections. Most injections, except those in the posterior and lateral hypothalamus, produce labeled cells in the ventral subiculum. Labeled cells in the nucleus of the diagonal band of Broca are limited to cases of injections into the posterior hypothalamus. Within the diencephalon, labeled cells are observed in the medial part of the lateral habenular nucleus after horseradish peroxidase injections into the dorsomedial, posterior and lateral hypothalamus only. In the midbrain, labeled cells are seen in the periaqueductal gray and raphe nuclei after all injections, but the raphe nuclei project most heavily to the lateral hypothalamus. Labeled cells are seen in the peripeduncular nucleus after all injections except posterior hypothalamus; the largest number of labeled peripeduncular cells are seen with ventromedial hypothalamic injections. In the brainstem, labeled cells are observed in the lateral parabrachial nucleus, locus coeruleus, dorsolateral tegmental area, nucleus incertus, nucleus solitarius, nucleus commissuralis, the lateral reticular nucleus and cells adjacent to the medial longitudinal fasciculus after any of the six hypothalamic injections; relative numbers of labeled cells varied among different injection sites. Only injections in the dorsomedial and posterior hypothalamus result in labeled cells in the ventral and dorsal tegmental nuclei. Labeled cells of raphe magnus are limited to cases of dorsomedial and lateral hypothalamic injections. Although overlap is found, certain patterns of afferent connections emerge. Injections into the medial preoptic area and paraventricular nucleus result in qualitatively and quantitatively similar results. The dorsomedial hypothalamic nucleus has some afferent connections in common with both the ventromedial hypothalamic nucleus and the posterior hypothalamus. In comparison with other hypothalamic injection sites, posterior hypothalamic injections label relatively fewer telencephalic regions and more pontine regions. The lateral hypothalamus shares many ascending connections with other hypothalamic areas, but telencephalic connections to the lateral hypothalamus originate from fewer regions.