Capsaicin-induced effects on c-fos expression and NADPH-diaphorase activity in the feline spinal cord

The distribution of c-fos expression and NADPH-diaphorase reactivity in the cervical and lumbar segments after stimulation of the vanilloid receptors in the dorsal neck muscles with capsaicin was studied in cats anaesthetized with alpha-chloralose. After the unilateral intramuscular injection of capsaicin, the mean number of Fos-immunoreactive neurons detected with an avidin-biotin-peroxidase technique was significantly increased in the superficial laminae (I), neck of the dorsal horn (V), and area around the central canal (VII) within both the cervical and lumbar spinal cord. Most Fos-immunoreactive neurons in the cervical spinal cord were giant and small cells. The widespread distribution of Fos-immunoreactive cells throughout the cervical cord within the intermediate zone (VII) coincided with the sites of localization of last-order premotor interneurons and cells of origin of inter-segmental crossed and uncrossed descending propriospinal pathways to the lumbar spinal cord. Fos-immunoreactive neurons were co-distributed with nitric oxide-generating cells at both levels of the spinal cord, although the double-labeled cells were not observed. In conclusion, the analysis of c-fos expression and NADPH-diaphorase reactivity shows that stimulation of vanilloid receptors in the neck muscles can initiate distinctive neuronal plasticity in the cervical (C1-C8) and lumbar (L1-L7) segments, and confirms the anatomical and functional coupling of both regions during processing of nociceptive signals from the dorsal neck muscles.