Peripheral and central predictors of whisker afferent morphology in the rat brainstem

Prior studies suggest that whisker afferents have but one central projection pattern, despite their association with differing peripheral receptors that predict central morphology in other systems. Target factors in barrelettes are thought to dictate afferent projection patterns; yet, barrelettes differ in their size, shape and development. We tested the hypothesis that whisker afferents have differing morphologies that are predicted by peripheral and central factors. Branching patterns and collaterals of 78 Neurobiotin-stained afferents were compared in rats. Fibers from one whisker had precisely somatotopic projections but highly varied morphologies. For the entire sample, analysis of variance revealed significant intrafiber variance in collateral number and arbor shape that was attributed to the target subnucleus. Significant interfiber variance did not reflect response adaptation rate, direction sensitivity, whisker row origin or parent fiber bifurcation in the trigeminal root. Instead, we found the following. 1) Mandibular fibers had more elongated arbors than maxillary axons. In subnuclei interpolaris and principalis, mandibular fibers had larger arbors with more boutons/collateral than maxillary axons; in oralis and interpolaris, mandibular fibers had fewer collaterals than those of the maxillary division. 2) Upper lip whisker axons had more boutons than those from the B-D row in all subnuclei. 3) Rostral whisker arc afferents had larger arbors and more boutons than those from middle or caudal arcs due to significant arc effects in interpolaris and oralis. Thus, whisker afferents are not structurally uniform, and some morphological features are predictable. Intrafiber variance is attributed to the central target; interfiber variance reflects maxillary versus mandibular origin, upper lip origin and whisker rostrocaudal arc.