The Effect of DOPA on the Spinal Cord. 1. Influence on Transmission from Primary Aflerents

Abstract

Abstract In unanesthetized spinal cats an i.v. injection of DOPA depresses transmission of short latency effects from the flexor reflex afferents (FRA), including transmission of excitatory and inhibitory action to motoneurones and ascending pathways as well as transmission to primary afferent terminals. The depression resembles that of the decerebrate preparation. After DOPA the flexor reflex evoked by natural stimulation of the skin is increased. This increase is due to a long latency, long lasting discharge in flexor motoneurones evoked by repetitive activity in the FRA. The late discharge is accompanied by a late wave of depolarization in the dorsal roots. These events are due to long lasting activity in the spinal cord and do not depend upon a peripheral loop. The selective action of DOPA shows that its action on transmission in the spinal cord is not secondary to circulatory changes. The working hypothesis is forwarded that DOPA inhibits transmission of short latency effects from the FRA through liberation of transmitter from a descending noradrenergic pathway.