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 showed exclusive retrograde labeling of laminae I-III cells when the tracer filled a zone intermediate between the lateral tip of the lateral reticular nucleus and the spinal trigeminal nucleus, pars caudalis. This zone is here called VLMlat. Following noxious mechanical or thermal stimulation of the skin, double-labeled neurons, which stained retrogradely and were Fos-immunoreactive, prevailed in laminae I and IIo. Double-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 Menétrey et al., J. Neurophysiol., 52 (1984) 595-611) showing that lamina I cells projecting to the ventrolateral reticular medulla convey noxious messages. The occurrence of numerous double-labeled cells in lamina IIo suggests that this lamina is also involved in nociceptive transmission to the VLMlat.