FOS-LIKE IMMUNOREACTIVITY IN THE SUPERFICIAL MEDULLARY DORSAL HORN INDUCED BY NOXIOUS AND INNOCUOUS THERMAL-STIMULATION OF FACIAL SKIN IN THE RAT

1. To examine further the ability of different classes of nociceptive and nonnociceptive primary afferent neurons to induce c-fos expression in central neurons, fos-like immunoreactivity was examined in the med ullary dorsal horn (laminae I-IV) of the rat after facial application of a range of warming and cooling thermal stimuli. Urethan-anesthetize d rats received 15 30-s thermal pulses (53, 50, 47, 41, 25, or 10-degr ees-C) applied to the vibrissal pad over a period of 30 min and were p erfused 2 h after the end of stimulation. 2. Stimulation of 41-degrees -C produced no significant increase in the number of fos-LI-labeled ce lls in lamina I or II compared with control (35-degrees-C) animals. 3. Stimulation of 47-degrees-C produced a significant increase in the nu mber of fos-LI-labeled cells in both laminae I and II. Stimulation of 50-degrees-C produced a significant increase in labeling, compared wit h that produced by 47-degrees-C, which was primarily in lamina II. Sti mulation of 53-degrees-C produced no further increase in the number of labeled cells, compared with that produced by 50-degrees-C, in lamina I or II. 4. In the cooling direction, 25-degrees-C produced a signifi cant increase in labeling above control levels in both lamina I and II , whereas 10-degrees-C produced a further increase compared with 25-de grees-C, which was restricted to lamina I. 5. None of the stimuli prod uced a significant increase in labeling in laminae III-IV. 6. The resu lts are interpreted as providing evidence that low-threshold cold rece ptors, high-threshold cold receptors, and nociceptors are capable of i nducing fos expression in dorsal horn neurons, whereas warm receptors are relatively ineffective. The results also provide evidence that neu rons that receive input from C poly-modal nociceptors are present in b oth laminae I and II, as are neurons that receive input from low-thres hold cold receptors. Neurons that receive input from high-threshold co ld receptors, but not from low-threshold cold receptors, appear to be located preferentially in lamina I. The shape of the curve relating fo s-LI labeling to stimulus temperature in the warming direction is cons istent with the expected pattern of recruitment of primary afferent no ciceptors.