Afferent projections to the hypothalamus in ranid frogs

Hypothalamic afferents were examined following applications of wheat germ agglutinin-horseradish peroxidase in the infundibulum of bullfrogs (Rana catesbeiana). Neuronal populations were generally labelled bilaterally, with an ipsilateral predominance. In the telencephalon, most labelled cells were present in the medial pallium, ventral lateral pallium, lateral amygdala, and lateral septal nucleus. A moderate number of cells were found in the medial amygdala and fewer still in the caudal striatum, olfactory tubercle/diagonal band nucleus, medial septal nucleus, and median septum. In the diencephalon, labelled neurons were primarily found in the anterior and central thalamic nuclei, anterior preoptic area, and suprachiasmatic nucleus. Fewer cells were present in the ventral habenular nucleus, anterior division of the lateral thalamic nucleus, posterior thalamic nucleus, ventromedial thalamic nucleus, magnocellular preoptic nucleus, posterior entopeduncular nucleus, and along the medial forebrain bundle. In the midbrain, a few labelled cells were found in the nucleus of the medial longitudinal fasciculus, suprapeduncular nucleus, and posteroventral tegmental nucleus. The secondary isthmal nucleus contained a large number of labelled neurons and moderate numbers of labelled cells were present in the grey medial to nucleus isthmi and in the isthmal raphe. A few lightly labelled cells were also seen in the parabrachial nucleus. Finally, after the largest applications, a few cells were seen in the obex region ventrolateral to the solitary tract.