The effects of L-dopa on dorsal horn cell responses to innocuous skin stimulation

The effects of L-DOPA on responses of dorsal horn cells to innocuous mechanical skin stimulation were studied. Following intravenous administration of L-DOPA (10--40 mg/kg) to cats with intact spinal cords, dorsal horn cells, which could be activated by only innocuous or innocuous and noxious stimuli, demonstrated increased reponses manifested by an increase in the average number of spikes per stimulus, increased receptive field size and occasional changes in adequate stimuli. When cats with acute cord transections were studied, L-DOPA increased the responses of cells located in lamina 4 and those cells which responded only to innocuous stimuli; cells which responded to noxious stimuli and those located in other laminae had depressed responses following L-DOPA administration. The inhibitory effects of L-DOPA were in part abolished, in spinal cats, by pretreatment with parachlorophenylalanine, a serotonin depletor. Studies of dorsal cells in chronic spinal cats indicated that the effects of L-DOPA were largely dependent on functioning terminals of descending suprasegmental pathways. The conclusion was reached that descending noradrenergic pathways, indirectly, cause an increase in transmission from cutaneous afferents to dorsal horn cells and that some of the inhibitory effects of L-DOPA described by others are a manifestation of L-DOPA interactions with inhibitory serotonergic systems.