INDUCTION OF C-FOS GENE-EXPRESSION BY SPINAL-CORD TRANSECTION IN THE RAT

Sympathetic nerve activity is maintained after high spinal injury thro ugh circuits that remain in question. We evaluated patterns of c-fos g ene induction as a monitor of spinal neurons responding to high spinal cord transection in the rat. Rats were anesthetized with isofluorane. Lower cervical or upper thoracic spinal segments were exposed, immers ed in warm mineral oil and transected. Spinal cords were exposed but n ot transected in anesthetized controls. After 2.5 h, spinalized and co ntrol rats were perfused for immunocytochemistry. Cervical and thoraco lumbar spinal segments and dorsal root ganglia were sectioned coronall y. Tissues were incubated in primary, polyclonal antisera raised in ra bbit or sheep against a peptide sequence unique to the N-terminal doma in of Fos, and processed immunocytochemically. Neurons were induced to express Fos-like immunoreactivity (FLI), bilaterally, in the spinal g ray, but not in primary sensory ganglia. Spinal cord transection induc ed neurons to express FLI in thoracic laminae I, IIo (outer substantia gelatinosa), Vre (lateral reticulated division), VII (lamina intermed ia) and X, and the intermediolateral cell column. Lamina VIII was also labeled in spinal-injured but not in control animals. Immunolabeled n uclei were prominent in lumbar segments and were concentrated in the m edial third of laminae I and IIo, and in laminae VII and X. Few cells were labeled in upper cervical or sacral segments. FLI was sparse in t he spinal gray of controls and expressed mainly within the dorsal root entry zone of upper thoracic segments. Patterns of c-fos gene express ion were site-specific and correlated with laminae that respond predom inantly to noxious stimulation and that contain sympathetic interneuro ns. Laminae that are responsive to non-noxious stimuli and activated b y walking, IIi, nucleus proprius, medial V and layer VI were not induc ed to express FLI. We conclude that neurons in specific spinal laminae that process high threshold afferents and that harbor neurons with sy mpathetic nerve-related activity are activated selectively by spinal c ord transections. We hypothesize that peripheral afferents processed b y spinal-sympathetic circuit neurons may regulate sympathetic discharg e in the absence of supraspinal drive. (C) 1997 Elsevier Science B.V.