Influence of a chronic myositis on rat spinal field potentials evoked by TTX-resistant unmyelinated skin and muscle afferents

A recent study of our group has shown that in the segments L4 and L5 of the rat, the synaptic field potentials (SFPs) evoked by tetrodotoxin-resistant (TTX-r, presumably nociceptive) muscle afferents differ in size and peak location from those of cutaneous afferents from the same body region [Lambertz D, Hoheisel U, Mense S. Distribution of synaptic field potentials induced by TTX-resistant skin and muscle afferents in rat segment L4 and L5. Neurosci Lett 2006;409:14-8]. Here, we investigated the influence of a muscle inflammation on the distribution of SFPs of TTX-r afferent fibres from muscle and skin in the thoracic and lumbar spinal cord. During a TTX block of the dorsal roots L3-L6, a skin nerve (sural, SU) or a muscle nerve (gastrocnemius-soleus, GS) were electrically stimulated at an intensity supramaximal for unmyelinated afferents and the SFPs recorded with tungsten microelectrodes. In control (non-inflamed) rats, the largest SFPs evoked by TTX-r GS afferents were recorded in laminae IV-VI with a maximum in segment L4, whereas the largest SU-induced SFPs were more superficially located with a maximum in L3. In chronic myositis animals, SFPs induced by GS TTX-r fibres exhibited significant decreases in lamina IV-VI of Th 12 and L5 as well as in lamina VII of L5. In contrast, SFPs evoked by SU TTX-r afferents showed significant increases in lamina IV-VI in L1 and in lamina VII in L4. The results demonstrate that a chronic myositis has a strong influence also on the synaptic effects of nociceptive afferents from the skin, which may explain the subjective cutaneous sensations during a pathological alteration of muscle.