Structure‐Function relationships in rat brainstem subnucleus interpolaris: VII. Primary afferent central terminal arbors in adults subjected to infraorbital nerve section at birth

Prior studies in this series have clarified the normal organization of subnucleus interpolaris and the response of higher‐order neurons to neonatal deafferentation. The present report describes the response of individual rat trigeminal primary afferents to transection of the infraorbital (IO) nerve on the day of birth. Physiologically characterized afferents in adult animals were labeled by intraaxonal injection of horseradish peroxidase (HRP). Qualitative and quantitative examination of the interpolaris collaterals of 62 recovered neurons revealed: (1) an increase in the transverse area of vibrissa afferent terminal arbors, (2) a decrease in the number of boutons per collateral of vibrissa afferents, (3) a decrease in the bouton density of both vibrissa and guard hair primary afferents, (4) a decrease in the circularity of guard hair afferent arbors, (5) an increase in the number of collaterals given off by nociceptive fibers, and (6) abnormal primary afferent topography. The data support the hypothesis that vibrissa afferents respond to neonatal axotomy by central arbor expansion, but not by sprouting. Arbor expansion provides a morphological substrate for the abnormal histochemical staining patterns seen in animals subjected to IO damage in the early postnatal period.