Peripheral origins and central modulation of subcutaneous formalin-induced activity of rat dorsal horn neurones

Extracellular recordings of single convergent dorsal horn neurones in the spinal cord region L1-L3 were made in rats anaesthetised with halothane in a gaseous mix of N2O and O2. Subcutaneous formalin (5%, 50 microliters) has previously been found to produce a prolonged distinct biphasic response of dorsal horn convergent neurones in the same preparation. The present study demonstrates that the second period of neuronal excitation which occurs at about 20 min and lasts for at least another 40 min, could be abolished by lignocaine (2%, 50 microliters) injected into the site of the formalin injection (n = 5). The inhibition of formalin-evoked activity lasted for about 10-20 min and was followed by complete recovery of the neuronal response. The same dose of i.v. lignocaine had no effect on the formalin-induced neuronal activity (n = 4). Profound inhibitions of the second phase were also produced by tactile segmental stimulation and noxious stimuli applied to widespread areas of the body (diffuse noxious inhibitory controls). These findings are discussed with regard to the peripheral and central consequences of prolonged noxious stimuli.