Effect of neonatal capsaicin and infraorbital nerve section on whisker-related patterns in the rat trigeminal nucleus

Abstract

In the present study, we investigated the effect of neonatally administered capsaicin on whisker-related pattern formation in the rat trigeminal complex. Both normal whisker-related patterns of barrelettes and the modified patterns seen after neonatal section of the infraorbital nerve were assessed. Capsaicin caused no change in the pattern or size of cytochrome oxidase (CO) barrelettes in the principal trigeminal nucleus (Vp) or trigeminal nucleus interpolaris (Vi) or caudalis (Vc). Injections of horseradish peroxidase (HRP) or wheatgerm agglutinin conjugated to HRP (WGA-HRP) into the posteroorbital (PO) whisker follicle in vehicle-treated animals showed that WGA labelled a larger number of trigeminal ganglion cells than HRP (203 +/- 23; cf. 158 +/- 19), with an increased labelling of small-diameter neurons (HRP: 25.9 +/- 7.7 microm; WGA: 23.2 +/- 7.2 pm). Capsaicin caused a loss of smaller diameter cells but had no effect on the location, cross-sectional area, or rostrocaudal extent of the transganglionically labelled HRP terminations in Vp, Vi, Vc, and cervical dorsal horn. WGA-HRP labelling revealed similar, but less dense, central terminal areas as HRP and an additional area of superficial terminals in the caudal medulla; these were also unaffected by capsaicin treatment. After infraorbital nerve section, CO patches and transganglionically labelled afferent terminations, corresponding to innervated nonmystacial whiskers, were approximately doubled in size. Capsaicin had no effect on the increased size of these spared whisker patches or their afferent terminal areas. These results suggest that barrelette formation is not dependent on unmyelinated afferents and that the changes in response properties seen after capsaicin, such as increased receptive fields, reflect functional changes rather than anatomical expansion of afferent terminal areas.