Spinal c-fos expression is differentially induced by brief or persistent noxious stimulation

The influence of stimulus duration on spinal induction of the c-fos proto-oncogene was analysed in the rat by pinching or heating the skin for periods varying from 20 s to 2 h. At stimulation periods shorter than 20 min, c-fos activation occurred in laminae I-IIi following mechanical stimulation and I-IIo following thermal stimulation. Mechanical stimulation produced delayed activation in laminae III-IV, V and VII at 30 min, 60 min and 2 h, respectively, and thermal stimulation in lamina IIi at 50 min. It is suggested that late c-fos activation signals inflammatory pain and is due to sensitization of primary afferent neurones.