Spinal section and opioid receptor blockade induce the appearance of Fos-like immunoreactivity in the spinal cord of the decerebrated rabbit

The expression of Fos-like immunoreactivity has been studied in spinal segm ents L5-S1 of decerebrated, unanaesthetized, but otherwise unstimulated rab bits. The aim of the study was to establish baseline levels of Fos in such preparations, and to examine how these might change after spinalization and opioid receptor blockade. In animals with an intact spinal cord, approxima tely 30 Fos-positive profiles per section were found in the superficial dor sal horns (i.e. laminae I and II) of each 40-mu m section, while about 20 p rofiles per section were found immediately adjacent to the central canal (l amina X). Fos-like immunoreactive profiles were rare elsewhere in the gray matter. When the spinal cord was sectioned at L1 (after blockade with local anaesthetic), significantly more Fos-like immunoreactivity was found in su perficial and central regions of the gray matter (approximately 90 profiles per section) in animals perfused 4 h after decerebration, but not when per fusion was performed 2 or 8 h after decerebration. The opioid antagonist na loxone (0.25 mg/kg/h) had little effect on expression of Fos-like immunorea ctivity in spinalized preparations, but significantly increased the numbers of Fos-positive profiles in all but the ventral areas of the spinal gray m atter in non-spinalized preparations. The present data show that spinal section induces a transient increase in e xpression of Fos in the superficial and central parts of the spinal gray ma tter. II appears that spinalization induces spontaneous activity in some ne urons in these regions of the cord, presumably as a result of relief of des cending inhibition. The effects of naloxone indicate that endogenous opioid s exert tonic inhibition over Foe-expressing spinal neurons in non-spinaliz ed rabbits. (C) 1999 IBRO. Published by Elsevier Science Ltd.