Subicular projections to the hypothalamus and brainstem: some novel aspects revealed in the rat by the anterograde Phaseolus vulgaris leukoagglutinin (PHA-L) tracing method.

The efferent projections from the subiculum to the hypothalamus were examined in the rat by using the anterograde PHA-L tract-tracing method. The density of the subicular projections to the hypothalamus increases at successively more ventral levels of the subiculum. The ventral tip of the region projects to the hypothalamus via 3 different routes: the postcommissural fornix, the medial corticohypothalamic tract (mht) and a ventral pathway running via the amygdala. The fibers in the fornix innervate the median subgroup of the mammillary bodies and, through collaterals, also the lateral hypothalamus. Axons innervating the medial preoptic area, anterior hypothalamus, ventromedial and dorsomedial nuclei as well as the median part of the mammillary nucleus run in the mht. The ventral pathway innervates the supraoptic and medial preoptic nuclei as well as the lateral hypothalamus. Taken together, efferents of the ventral subiculum reach most major areas situated along the longitudinal axis of the hypothalamus. Importantly, however, subicular efferents reach beyond the hypothalamus: PHA-L immunoreactive axons were traced into the mesencephalic central gray and medulla. These anatomical findings show that highly processed cortical information may reach hypothalamic and brainstem areas involved in the integration of endocrine and autonomic functions as well as motivated behaviors.