It has been suggested that the expression of c-fos and other immediate early genes in spinal dorsal horn neurons would trigger changes in the phenotype of nociceptive neurons which may lead to long-term changes in spinal nociception. To test this hypothesis, we have used a minimally invasive intrathecal stimulation and injection technique which can be applied to adult Sprague-Dawley rats under brief ether anesthesia to induce massive c-fos expression in spinal neurons without affecting peripheral nociceptors. Electrical intrathecal stimulation (0.5ms pulses, 15V, 3Hz for 15 rain) or intrathecal injection of N-methyl-D-aspartate (25nmol) produced massive c-Fos immunoreactivity in neurons throughout the sacral spinal cord and the dorsal horn of the lumber spinal cord. Immunoreactivity declined to control values at mid-thoracic levels. To assess effects of these intrathecal stimuli on nociception, hot-plate and tail-flick latencies and mechanical thresholds of hindlimb withdrawal reflexes were measured once every day for 14 days before and up to 14 days after conditioning stimulation. Spontaneous locomotion of each animal was video-taped daily for 5 min and analysed off-line. On the day of the intrathecal stimulation the tests were performed 1 h before and also 6 h after conditioning stimulation. Thermal and mechanical nociceptive thresholds were temporarily enhanced 6 h after intrathecal stimulation but they were not different from controls one to 14 days later. Thus, the massive expression of c-fos in spinal neurons is not, as previously suggested, a sufficient condition for the induction of long-term changes in spinal nociception. Copyright