The effect of a thoracic spinal block on fos expression in the lumbar spinal cord of the rat induced by a noxious electrical stimulus at the hindpaw.

Background: Fos expression in the lumbar spinal cord, resulting from a noxious electrical stimulus at the hindpaw, is hypothesized to originate from three sources: direct sensory input of the noxious stimulus, local interactions in the spinal cord, and input of modulating signals from supraspinal regions. Our aim in this study was to discriminate among these sources by eliminating the supraspinal input.

Methods: Therefore, a spinal block was administered in male Wistar rats by administering a local anesthetic (bupivacaine) through an intrathecal catheter at the mid-thoracic level. This thoracic spinal block completely suppressed the noxious stimulation-induced withdrawal reflex that is normally elicited by electrical stimulus. Fos immunoreactivity (Fos-IR) was quantified in all laminae of the L4 segment of the spinal cord.

Results: Noxious stimulation resulted in a general and strong increase in Fos-IR in the ipsilateral dorsal horn, mainly in Laminae I, II, and V. Thoracic spinal block caused a remarkable increase in the amount of Fos-IR in Lamina V, but had no significant effect on the Fos-IR in Laminae I and II.

Conclusions: The increase in Fos-IR in Lamina V may have resulted from the interruption of a pain-modulating descending mechanism from the brain. A known modulating descending mechanism is the serotonergic system, controlled by the periaqueductal gray. This system inhibits the neurons in the superficial laminae. Another nonserotonergic system originates in the anterior pretectal nucleus. The latter facilitates neurons in the superficial laminae, while neurons in Lamina V are inhibited. We conclude that both systems are probably involved in the observed effects of the peripheral noxious stimulation given in the present model.