NOCICEPTIVE C-FOS EXPRESSION IN SUPRASPINAL AREAS IN AVOIDANCE OF DESCENDING SUPPRESSION AT THE SPINAL RELAY STATION

The number and distribution of Fos-like-immunoreactive neurons in diff erent supraspinal brain areas induced by formalin injection into one h indpaw was estimated in rats with transected dorsal half of the spinal cord at the thoracic level in an attempt to avoid most of the descend ing modulatory actions. The results showed that: (i) after spinal lesi on, the peripheral noxious inputs, going up mainly through the ventral spinal cord, elicited a more widespread and densely located Fos-like- immunoreactive neurons in subcortical areas, many of them showed no Fo s expression when noxious stimulation was given in rats with intact sp inal cord; (ii) at the same time, a small number of subcortical areas, such as the lateral ventroposterior thalamic nucleus and dorsal raphe nucleus, exhibited no significant increase of nociceptive Fos-like im munoreactive neurons after spinal lesion as compared to that with inta ct spinal cord; and (iii) there appeared a prominent expansion of cort ical areas with densely located Fos-like-immunoreactive neurons in spi nal-lesioned rats as compared with the limited labelled areas in the c ontrol group with intact spinal cord. These results indicate that: (i) in avoiding the spinally descending modulatory mechanisms, more wides pread supraspinal and cortical neurons will be recruited and activated in response to the noxious stimulation; and (ii) the descending syste ms exert differential actions on the spinal targets which project noci ceptive signals to different supraspinal regions. The implication of t hese facts is discussed. (C) 1998 IBRO. Published by Elsevier Science Ltd.