PERIPHERAL AND CENTRAL PREDICTORS OF WHISKER AFFERENT MORPHOLOGY IN THE RAT BRAIN-STEM

Prior studies suggest that whisker afferents have but one central proj ection pattern, despite their association with differing peripheral re ceptors that predict central morphology in other systems. Target facto rs in barrelettes are thought to dictate afferent projection patterns; yet, barrelettes differ in their size, shape and development. We test ed the hypothesis that whisker afferents have differing morphologies t hat are predicted by peripheral and central factors. Branching pattern s and collaterals of 78 Neurobiotin-stained afferents were compared in rats. Fibers from one whisker had precisely somatotopic projections b ut highly varied morphologies. For the entire sample, analysis of vari ance 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, directi on 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 interpol aris and principalis, mandibular fibers had larger arbors with more bo utons/collateral than maxillary axons; in oralis and interpolaris, man dibular fibers had fewer collaterals than those of the maxillary divis ion. 2) Upper lip whisker axons had more boutons than those from the B -D row in all subnuclei. 3) Rostral whisker are afferents had larger a rbors and more boutons than those from middle or caudal arcs due to si gnificant are effects in interpolaris and oralis. Thus, whisker affere nts 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 are. (C) 1996 Wiley-Liss, Inc.