Capsaicin-induced neuronal degeneration: silver impregnation of cell bodies, axons, and terminals in the central nervous system of the adult rat.

Capsaicin is a neurotoxic substance valued in neurobiological research because of its ability to selectively damage small unmyelinated primary sensory neurons. Previous work has indicated that systemic capsaicin administration causes permanent neuronal degeneration in neonatal rats, but evidence that capsaicin has a similar effect in adults is equivocal and incomplete. Therefore, we used silver impregnation, a method that labels degenerating neurons, to examine the central nervous system of adult rats after systemic capsaicin treatment. Adult rats were injected with a single intraperitoneal dose of capsaicin (50 or 90 mg/kg) or vehicle solution and killed 6, 12, 18, 24, 48, 96, or 240 hours later. Sections of brain and spinal cord were stained with the Carlsen-de Olmos cupric silver method. As reported previously, stained sections revealed degeneration in areas known to be innervated by small-diameter primary sensory fibers: the substantia gelatinosa of the spinal cord dorsal horn and spinal trigeminal nucleus, the solitary nucleus and tract, and the lateral borders of the area postrema. In addition, axon and terminal degeneration was observed in several discrete forebrain and hindbrain areas not previously associated with capsaicin-induced degeneration in either adult or neonatal rats: the inferior olive, the olivary pretectal nucleus, the interpeduncular nucleus, the suprachiasmatic nucleus, and the lateral septum/medial accumbens region. Furthermore, degenerating cell bodies were observed in the intrafascicular nucleus of the ventromedial midbrain tegmentum, in the supramammillary nucleus, and in the posterior hypothalamic area. Unilateral nodose ganglionectomy produced terminal staining on the denervated side very similar to that induced bilaterally by capsaicin. In addition, unilateral nodose ganglionectomy 1 month prior to capsaicin injection greatly attenuated staining in the ipsilateral nucleus of the solitary tract, confirming the hypothesis that capsaicin damages vagal sensory neurons innervating this nucleus. Capsaicin-induced damage in adult rats was long-lasting, since the second of two capsaicin treatments spaced 4.5 months apart produced no additional degeneration.