Some efferent connections of the rostral hypothalamus in the squirrel monkey (Saimiri sciureus) and cat.

The efferent connections of the ventromedial nucleus and of the anterior hypothalamic area have been studied autoradiographically in the squirrel monkey and cat, and some observations have also been made on the efferent connections of the periventricular, paraventricular and accessory supraoptic nuclei. As in the rat, the ventromedial nucleus has been found, in the monkey and cat, to have rather widespread ascending and descending projections. The ascending projections follow three routes: (i) pervientricularly to the periventricular nuclei of the hypothalamus and preoptic area, and to the anterior paraventricular nucleus of the thalamus; (ii) through the medial hypothalamus and preoptic area to the adjoining nuclei; and (iii) through the medial forebrain bundle to the lateral hypothalamic and preoptic areas, to the medial part of the bed nucleus of the stria terminalis, and the ventral part of the lateral septal nucleus. In addition there is a substantial projection to the amygdaloid complex partly through the stria terminalis and partly through a ventral pathway associated with the ansa peduncularis; the latter projection is bilateral. Descending projections from the nucleus also follow three main pathways: (i) in the periventricular system as far caudally as the locus coeruleus; in its course this system appears to give off fibers to central gray itself, to the dorsal parts of the central tegmental fields, to the cuneiform nucleus, the pedunculopontine nucleus and the locus coeruleus; (ii) through the medial forebrain bundle to the fibrous capsule of the mammillary body, the supramammillary region, the ventral tegmental area and the central tegmental fields; and (iii) through the ventral supraoptic commissure to the basal nucleus of Meynert, to the central tegmental fields, and to the peripeduncular nucleus of both sides. In addition the nucleus appears to have shorter, intrahypothalamic connections to the adjoining nuclei including the dorsomedial, the arcuate and the premammillary nuclei. As we have shown elsewhere (Saper et al., '78) the anterior hypothalamic area projects primarily upon the surrounding hypothalamic and preoptic cell groups, including the arcuate, the periventricular, the dorsomedial and ventromedial nuclei, and the dorsal premammillary region bilaterally. A small number of fibers ascends to the zone between the medial and lateral septal nuclei, and some descend through the capsule of the mammillary complex and the supramammillary nuclei to the ventral tegmental area, and through the periventricular system to the central gray. The paraventricular and accessory supraoptic nuclei both contribute fibers to the posterior lobe of the pituitary by way of the paraventriculo-hypophysial tract which passes through the external zone of the median eminence in the monkey and through its internal lamina, in the cat. The paraventricular nucleus also has an important descending projection that is distributed partly to the mammillary capsule, the ventral tegmental area and the central tegmental fields, and partly to the preganglionic autonomic centers of the medulla and spinal cord. The adjoining periventricular nucleus is a major component of the tubero-hypophysial system, and in the monkey has a significant projection to the central core of the median eminence.