A light microscopic HRP study of limbic projections to the vasopressin-containing nuclear groups of the hypothalamus.

The anterograde HRP technique has been used to define the efferent projections from the lateral septum, amygdala and ventral subiculum to the anterior hypothalamus (AH) with particular attention to the paraventricular nucleus (PVN). Each limbic region was found to project to the PVN in a perinuclear fashion leaving the nucleus itself virtually devoid of HRP-labeled terminals. This projection pattern was also characteristic of the limbic innervation of the supraoptic (SON) and suprachiasmatic (SCN) nuclei. HRP injections into limbic sites has also enabled the description of both efferent and afferent projections to the remainder of the diencephalon. These results extend the observations made previously on the projections from limbic structures to the diencephalon in the rat, particularly in regard to the distinctive relationship of efferents to the PVN and SON, the major components of the hypothalamo-hypophyseal neurosecretory system. At the light microscopic level it cannot be stated with certainty whether or not such limbic afferents synapse with the dendrites of cells in the PVN, SON or SCN which extend beyond the cellular boundaries of each nucleus.