CNS connections with the median raphe nucleus: Retrograde tracing with WGA‐apoHRP‐gold complex in the rat

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Abstract
In this work we examined the neuronal input to one of the serotoninergic centers in the brain, median raphe nucleus (MR). Special consideration is given to projections of the hypothalamus. To describe the afferents to MR, a retrograde transport technique was used after microinjection of WGA‐apoHRP‐Gold complex under pressure and subsequent gold‐silver intensification on formaldehyde‐fixed rat brain sections. Optimal conditions were obtained when the coordinates of the injection site were A ± 1.5, L ± 0.15, and H ± 2.7 according to Paxinos and Watson (The Rat Brain in Stereotaxic Coordinates. New York: Academic Press, '82). Results obtained under these conditions show a heterogeneous distribution of labeled neurons throughout the brain, including a large proportion (±65%) of hypothalamic neurons. Extrahypothalamic neurons projecting to MR were from the prefrontal cortex, lateral and medial habenular nuclei, the pontine area of the central grey, interpeduncular nucleus, dorsal raphe nucleus, oculomotor and trochlear nuclei, dorsal and laterodorsal tegmental nuclei, parabrachial nuclei, and lateral and interpositus cerebellar nuclei. Hypothalamic neurons connected to MR were found to be from medial and lateral preoptic areas, lateral hypothalamus, dorsomedian nucleus, the perifornical area, and the complex of mammillary bodies. Many other discrete regions contained different densities of labeled perikarya: the medial preoptic nucleus, paraventricular nucleus, retrochiasmatic area, arcuate nucleus, lateral magnocellular nucleus, and the posterior area. The MR appears as an integrative center receiving many neuroanatomically and functionally heterogeneous inputs from the whole brain.