Decussations of the descending paraventricular pathways to the brainstem and spinal cord autonomic centers

Decussations of descending fibers of the hypothalamic paraventricular nucleus (PVN) were investigated by using Phaseolus vulgaris-leucoagglutinin (PHA-L) in intact and brainstem-operated rats. Fibers descend ipsilaterally along the brainstem and spinal cord and decussate at four levels: 1) Supramamillary decussations (SM). PVN fibers reach this area through the lateral hypothalamus and along the third ventricle in the dorsal hypothalamus. In the posterior hypothalamus some fibers crossover in the SM and terminate in the supramamillary region bilaterally. 2) Pontine tegmentum. PVN fibers run in the lateral part of the tegmentum arching to the basis of the pons. Some fibers crossover under the fourth ventricle. The locus ceruleus and the Barrington's nucleus receive bilateral innervation with ipsilateral dominance. 3) Commissural part of the nucleus of the solitary tract (NTS). The major crossover of PVN fibers is found here. The decussated fibers form a dense network here, and loop rostralward to innervate the entire NTS. A midsagittal knife-cut through the NTS eliminated paraventricular-fibers on the contralateral side. Synaptic contacts between PHA-L-labeled boutons and tyrozine hydroxilase-positive neurons were verified in the NTS. The caudal ventrolateral medulla also receives bilateral innervation. 4) Lamina X of the thoracic spinal cord. Paraventricular fibers enter the lateral funiculus ipsilaterally and innervate the intermediolateral cell column (IML). Some fibers cross the midline ventral and dorsal to the central canal running to the contralateral IML, at the level of the decussation. Our results demonstrated that paraventricular projections form a continuous descending pathway on their side of origin, and provide crossover fibers which may terminate segmentally without forming long tracts after crossover.