Evidence for D1 Dopamine Receptor-mediated Modulation of the Synaptic Transmission from Motor Axon Collaterals to Renshaw Cells in the Rat Spinal Cord

The possible modulatory role of D1 dopamine receptors on the excitability of lumbar spinal Renshaw cells was studied in anesthetized rats spinalized at T4 level. Burst responses elicited by single electrical shocks to ipsilateral ventral roots L6 (frequency 0.5 Hz, stimulus width 0.1 ms) and spontaneous activity were recorded extracellularly using conventional 3 M KCl filled glass micropipettes. The specific D1 agonist SKF 38393 (0.5-1 mg/kg i.v.) enhanced Renshaw cell burst responses by 20-60% (n = 7) and increased their spontaneous discharge rate (n = 3). This effect was clearly antagonized by the specific D1 antagonist SCH 23390 (1 mg/kg i.v.) although SCH 23390 proved ineffective per se. We conclude that SKF 38393 induced facilitation was due to activation of the specific D1 receptors which could be the functional counterpart of the presynaptic D2 receptors described earlier by us in the same synapse.