Coupling of c-fos expression in the spinal cord and amygdala induced by dorsal neck muscles fatigue.

c-fos gene expression in the cervical spinal cord and amygdala was examined in anaesthetized rats following muscle fatigue caused by intermittent high-rate (100 s(-1)) electrical stimulation of the dorsal neck muscles (m. trapezius and m. splenius). Fatigue-related increases in c-fos expression were observed on the stimulated muscle side in the cervical C2-C4 (layers 1, 3-5, 7 and 10) spinal segments, bilaterally in the lumbar L4-L6 (layer 1) segments and in contralateral central (Ce), medial (Me), and basomedial (BM) amygdaloid nuclei. A scarce number of staining cells were found within lateral and basolateral nuclei. The rostro-caudal extent of c-fos expression in the spinal cord supports functional coupling of the cervical and lumbar regions during the neck muscle fatigue development. The distinct c-fos expression in the Ce and Me amygdaloid nuclei suggests that they may contribute to mediating the neck muscle fatigue-related nociception, autonomic and behavioural responses.