The organization of the neonatal rat's brainstem trigeminal complex and its role in the formation of central trigeminal patterns.

The present study delimits the relationship of primary trigeminal afferents to their targets, the brainstem trigeminal nuclei of the neonatal rat. Previously, the brainstem trigeminal complex of the rat has been subdivided on the basis of either cytoarchitectonics or patterns of succinic dehydrogenase activity into the principal sensory nucleus and the three subnuclei of the spinal trigeminal nucleus, oralis, interpolaris, and caudalis. In this paper, we demonstrate that each of these subdivisions can also be identified by its pattern of primary trigeminal afferents. In addition, we demonstrate that the terminations of these afferents are distributed in a punctate fashion which correlates with vibrissae-related patterns of histochemical staining. Further, vibrissae removal in the neonatal rat at any age studied results in a corresponding deafferentation of both the principal sensory nucleus and all subnuclei of the spinal trigeminal nucleus. This same procedure has a graded, age-dependent effect on the vibrissae-related pattern of cytochrome oxidase staining in somatosensory cortex. On this basis, we conclude that vibrissae-related pattern formation in the central trigeminal system can be best understood in terms of a single "sensitive" period for the entire system. We hypothesize that this is the period during which an interaction normally occurs between primary trigeminal afferents and target neurons of the principal sensory nucleus.