NEURONS IN THE SUPERFICIAL DORSAL HORN OF THE RAT SPINAL-CORD PROJECTING TO THE MEDULLARY VENTROLATERAL RETICULAR-FORMATION EXPRESS C-FOS AFTER NOXIOUS-STIMULATION OF THE SKIN

The nociceptive nature of the neurons of the superficial dorsal horn ( laminae I-III) which project to the medullary ventrolateral reticular formation is studied in the rat. Medullary injections of Fluoro-Gold s howed exclusive retrograde labeling of laminae I-III cells when the tr acer filled a zone intermediate between the lateral tip of the lateral reticular nucleus and the spinal trigeminal nucleus, pars caudalis. T his zone is here called VLMlat. Following noxious mechanical or therma l stimulation of the skin, double-labeled neurons, which stained retro gradely and were Fos-immunoreactive, prevailed in laminae I and IIo. D ouble-labeled neurons were few in lamina IIi after thermal stimulation and entirely lacking in lamina III after the two kinds of stimulation . Findings in lamina I confirm previous electrophysiological data (see Menetrey et al., J. Neurophysiol., 52 (1984) 595-611) showing that la mina I cells projecting to the ventrolateral reticular medulla convey noxious messages. The occurrence of numerous double-labeled cells in l amina IIo suggests that this lamina is also involved in nociceptive tr ansmission to the VLMlat.