Orthograde axonal transport studies of projections from the zona incerta and pretectum to the basilar pontine nuclei in the rat

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Abstract
This study employed orthogradely transported axonal tracers to demonstrate, in the rat, projections that reach the basilar pontine nuclei from the zona incerta or pretectal nuclei. Except for the most rostral levels, all subdivisions of the zona incerta give rise to substantive basilar pontine projections. Although some topographic differences exist among the temination patterns of various subdivisions, no clear somatotopically organized scheme is apparent. Most incertopontine axons descend to the basilar pons in association with fibers of the medial lemniscus or crus cerebri and reach ipsilateral ventral and medial pontine gray regions. A sparse number of terminals are evident in the contralateral medial pontine gray. The anterior pretectal axons also descend with the medial lemniscus and crus cerebri to enter exclusively the ipsilateral basilar pons where they terminate most densely in ventral and medial regions. Dual orthograde labeling experiments indicate that some pretectal terminal fields in the pontine gray are shared with incertopontine projections and with afferents from the dorsal column nuclei. This potential convergence of basilar pontine afferent projections is significant in light of (1) the known somatosensory input to the zona incerta and pretectum and (2), the fractured somatotopy of peripheral cutaneous inputs that arrive in the cerebellar cortex via mossy fibers. The present studies also employed electron microscopy to identify synaptic boutons formed by incerto‐ and pretectopontine axons, and they proved to be remarkably similar. Each is a medium to small‐sized bouton that contains spheroidal synaptic vesicles and forms asymmetric membrane specializations. Most incerto‐ and pretectopontine boutons participate in glomerular synaptic complexes that include a single, centrally located bouton contacted on its perimeter by several types of dendritic profiles including shafts and spine‐like appendages. A relatively small number of labeled boutons of either type contacts single, isolated dendritic elements in the neuropil. Taken together, these findings suggest that some basilar pontine neurons might receive convergent inputs from the zona incerta and pretectum as well as other somatosensory related systems such as the dorsal column nuclei and sensorimotor cortex.