Alterations in the ipsi- and contralateral afferent inputs of dorsal horn cells produced by capsaicin treatment of one sciatic nerve in the rat.

Dorsal horn cells in the lumbar spinal cord of decerebrate spinal rats were examined 7-21 days following local application of capsaicin to the sciatic nerve. Such local capsaicin treatment is known not to influence the size of the incoming A and C fibre afferent volley. The receptive field properties and primary afferent input of cells on both sides of the cord, that is ipsi and contralateral to the treated nerve, were examined. On the treated side, the percentage of cells excited by C fibres from the capsaicin treated nerve was 30% of normal and the number of cells responding to noxious heating of the cutaneous receptive field was reduced by 50%. A fibre input and low and high threshold mechanical input were normal. The receptive field size was larger in many cells innervated by the treated nerve. On the side opposite to the treated nerve, responses to noxious and non-noxious stimulation of the untreated limb were unaffected as was the input from the untreated sciatic nerve. Receptive fields were somewhat larger than normal. Effects were also observed from contralateral stimuli. Cells on both sides of the cord were found with excitatory contralateral receptive fields and excitatory responses to trains of high intensity stimulation to the contralateral sciatic nerve. In untreated animals the effect of such contralateral stimulation is inhibitory. The results show that peripheral nerve capsaicin treatment causes long lasting reduction of the C fibre input to dorsal horn cells on the treated side. However, it also results in changes in the inhibitory and excitatory receptive fields of cells on both sides of the cord.