Monosynaptic reflexes, c-fos expression,and NADPH-diaphorase activity in the cat spinal cord: Changes induced by chronic muscle inflammation

Effects of long-term (9 days) experimental unilateral inflammation in the mm. gastrocnemius-soleus (GS), induced by injection of Freund’s complete adjuvant, namely modulation of posterior biceps-semitendinosus (PBSt) monosynaptic reflexes (MSRs), which was induced by chemical activation of high-threshold (groups III and I) muscle afferents, and changes in c-fos expression and NADPH-diaphorase (d) reactivity in the lumbosacral spinal cord, were studied in anemically decapitated and highly (at the C1) spinalized cats. The mean amplitude of the MSRs on the pretreated side did not differ significantly from that on the opposite side. In adjuvant-injected cats, ipsilateral stimulation of nociceptive muscle afferents by KCl injection induced the bilateral enhancement of flexor reflexes. Significant bilateral increases in the mean number of Fos-immunoreactive (Fos-IR) neurons within the L6, L7, and S1 segments (157.5 ± 12.7, 201 ± 18.5, and 205 ± ± 18.6 per section; P < 0.05) were also found in adjuvant-injected cats. A lot of Fos-IR neurons was observed in the marginal zone (lamina I) and the neck of the dorsal horn (laminae V and VI); the highest number of labelled cells was detected in lamina VII. In adjuvant-injected cats, co-distribution of Fos-IR neurons and numerous Fos-IR glia-like cells in the dorsal and ventral horns was also found. Significant increases in the mean number of NADPH-d-reactive cells in lamina VII bilaterally and also in lamina I and in an area around the central canal (lamina X) contralaterally within the L6, L7, and S1 segments (P < 0.05) were also observed. In conclusion, activation of the nociceptive input during long-term inflammation of the GS muscles is associated with differential patterns of c-fos expression and NADPH-d reactivity and also with central neuronal hyperexcitability that contributes to bilateral facilitation of the PBSt MSRs. Neirofiziologiya/Neurophysiology, Vol. 39, No. 3, pp. 222–231, May–June, 2007.