Hypothalamopontine projections in the rat: anterograde axonal transport studies utilizing light and electron...

Projections to the basilar pontine nuclei (BPN) from a variety of hypothalamic nuclei were traced in the rat utilizing the anterograde transport of biotinylated dextran amine. Light microscopy revealed that the lateral hypothalamic area (LH), the posterior hypothalamic area (PH), and the medial and lateral mammillary nuclei (MMN and LMN) are the four major hypothalamic nuclei that give rise to labeled fibers and terminals reaching the rostral medial and dorsomedial BPN subdivisions. Hypothalamopontine fibers extended caudally through the pontine tegmentum dorsal to the nucleus reticularis tegmenti pontis and then coursed ventrally from the main descending bundle toward the ipsilateral basilar pontine gray. Some hypothalamopontine fibers crossed the midline in the tegmental area just dorsal to the pontine gray to terminate in the contralateral BPN. Electron microscopy revealed that the ultrastructural features of synaptic boutons formed by axons arising in the LH, PH, MMN, and LMN are similar to one another. All labeled hypothalamopontine axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions with dendritic shafts as well as dendritic appendages, and occasionally with neuronal somata. Some labeled boutons formed the central axon terminal in a glomerular synaptic complex. In summary, the present findings indicate that the hypothalamus projects predominantly to the rostral medial and dorsomedial portions of the BPN which, in turn, provide input to the paraflocculus and vermis of the cerebellum. Since the hypothalamic projection zones in the BPN also receive cerebral cortical input, including limbic-related cortex, the hypothalamopontine system might serve to integrate autonomic or limbic-related functions with movement or somatic motor-related activity. Alternatively, since the cerebellum also receives direct input from the hypothalamus, the BPN may function to provide additional somatic and visceral inputs that are used by the cerebellum to perform the integrative function.