Effects on c-Fos expression in the PAG and thalamus by selective input via tetrodotoxin-resistant afferent fibres from muscle and skin

Nociceptive information from skin and muscle is differently processed at many levels of the central nervous system. However, with regard to nociceptive input from muscle to the thalamus, only few data are available. Here, we investigated the c-Fos expression in the thalamus and the periaqueductal grey matter (PAG) induced by electrical stimulation of tetrodotoxin-resistant (TTX-r), presumably nociceptive, afferent fibres. In addition, a comparison between the effects of TTX-r input from muscle and skin was made. In anaesthetised rats, a skin or a muscle nerve was stimulated electrically for 1 h at an intensity supramaximal for unmyelinated fibres. To block TTX-sensitive afferents, TTX was applied to the sciatic nerve. c-Fos was visualized using DAB immunohistochemistry. Here we report for the first time that in the PAG and medial thalamus, the main effect of TTX-r input from muscle was a reduction in c-Fos expression, and that in some thalamic nuclei (e.g. posterior, reuniens, and central medial nuclei), significant differences in the number of c-Fos-positive cells were found after muscle and cutaneous input, respectively. The thalamic regions with the strongest effects of muscle input were the VL bilaterally and the VPL contralaterally (increase in c-Fos expression) as well as the rhomboid nucleus (decrease in c-Fos expression).