DIFFERENTIAL DEPRESSION BY APOMORPHINE OF THE DORSAL-ROOT POTENTIALS SPREADING CAUDALLY AND CRANIALLY IN THE CAT SPINAL-CORD

The effect of apomorphine on bilateral dorsal root potentials (DRPs) spreading caudally and cranially along three segments of the lower spinal cord was studied in spinal cats anaesthetized with thiamylal sodium. The DRPs were produced by single impulses at 4 times the threshold strength. Apomorphine in a dose of 5 mg/kg, i.v., depressed ipsilateral DRPs spreading caudally to 77% of their initial amplitude and the period of depression was 50 min. Depression of DRPs spreading cranially was significantly greater (maximum reduction to 59%) and lasted about 60 min. Contralateral DRPs spreading cranially were also more depressed than those spreading caudally. These effects were not due to differences in the level of the spinal cord segments investigated. Increase in strength of stimulation to 40 times threshold did not significantly change the depression of DRPs produced by apomorphine. Depression of DRPs depended on the dose of apomorphine. Pimozide and haloperidol blocked the apomorphine-induced depression of the DRPs. It is suggested that differences in the depression of the DRPs spreading caudally and cranially result from different numbers of dopaminergic neurons terminating on interneurons transmitting depolarization cranially or caudally, respectively. © 1979 Springer-Verlag.