Recovery of viscerosensory innervation from the dorsal root ganglia of the adult rat following capsaicin-induced injury.

Capsaicin is a neurotoxin selective for C- and Adelta-type neurons. Systemic treatment with capsaicin is known to reduce this subpopulation in the dorsal root ganglia (DRG) of neonatal rats. To better understand the effects of capsaicin on adult afferent fibers, we examined DRG neurons retrogradely labeled by an i.p. injection of Fast Blue (FB) administered 3, 30, or 60 days after systemic capsaicin treatment (125 mg/kg i.p.). FB labeling in the 12th and 13th thoracic DRG was dramatically reduced 3 and 30 days post capsaicin (50% and 35% of control, respectively). However, the number of retrogradely labeled neurons rose to 65% of control by 60 days post capsaicin. In addition to FB labeling, we quantified the immunoreactivity of NR1, the obligatory N-methyl-D-aspartate receptor subunit, and Na(v)1.8, a DRG-specific sodium channel, in FB-labeled neurons as well as mRNA levels for both proteins in the 5th and 6th lumbar DRG. NR1 immunoreactivity and mRNA expression followed a pattern of early reduction and subsequent partial restoration similar to FB labeling. Na(v)1.8 immunoreactivity and mRNA expression dropped to approximately 50% of control at 3 days post capsaicin but completely recovered by 60 days. These data strongly support the conclusion that restoration of spinal afferent projections and signaling occurs in adult rats following capsaicin-induced damage.