The induction of Fos-like immunoreactivity by noxious thermal, mechanical and chemical stimuli in the lumbar spinal cord of infant rats.

Abstract

The present study examined the maturation of nociceptive primary afferents using expression of Fos-like immunoreactivity in the second-order spinal cord neurons as an anatomical and functional marker. Pinch, immersion in hot water, or formalin injection applied to the hindpaw was used as the peripheral noxious stimulus in awake 0-, 1-, 2-, 3-, and 14-day-old rat pups. On the day of birth, all 3 stimuli elicited expression of the Fos protein in dorsal horn cells indicating that nociceptive primary afferents are functional at this age. The expression of the Fos protein was related to the intensity of stimulation since greater injection volumes of formalin or prolonged application of the thermal stimulus increased the number of stained nuclei. The number of stained nuclei was age dependent and older pups exhibited a greater number of stained nuclei. The results of this study are consistent with electrophysiological studies that have demonstrated that the primary nociceptive afferents continue to mature during the rats' postnatal life. Furthermore, the number of Fos immunoreactive neurons in immature rats are age and stimulus-intensity dependent.