Spinal pathways mediating coeruleospinal antinociception in the rat

In a previous study, we showed in rats that axons of some locus coeruleus/subcoeruleus (LC/SC) neurons involved in coeruleospinal modulation of nociception descend through the ipsilateral side of the spinal cord and cross the midline at spinal segmental levels. The present study was designed to investigate a possible spinal pathway of these descending axons from the LC/SC. Extracellular recordings were made from the left dorsal horn with a carbon filament electrode (4-6 M(omega)). To block impulses from the LC/SC which descend through spinal pathways ipsilateral to the recording sites, a hemisection of the spinal cord ipsilateral to the recording sites was performed at the C2 level with fine forceps in all rats tested. In these rats, responses of dorsal horn neurons to noxious heat (53 degrees C) applied to receptive fields were inhibited during electrical stimulation (100 microA, 100 Hz, 0.1 ms pulses) of the LC/SC. The transection of the dorsolateral funiculus contralateral to the recording sites did not affect LC/SC stimulation-produced inhibition. Following transection of the ventrolateral funiculus (VLF) contralateral to the recording sites, LC/SC stimulation failed to inhibit heat-evoked responses. These results suggest that interruption of descending inhibition from the LC/SC produced by the VLF transections is due to the blockage of axons descending in the ventrolateral quadrant of the spinal cord, but not in the dorsolateral quadrant.