Descending noradrenergic neurones tonically suppress spinal presynaptic inhibition in rats.

We investigated the tonic depressant effect of descending noradrenergic neurones on spinal presynaptic inhibition. Presynaptic inhibition was measured in terms of the decrease in the amplitude of the monosynaptic reflex (MSR) evoked by conditioning stimulation of the adjacent dorsal root. This reduction in the MSR by conditioning stimulation was accompanied by primary afferent depolarization and was inhibited by the GABAA antagonist bicuculline, but not by the glycine antagonist strychnine. This presynaptic inhibition was much greater in spinalized rats than in intact rats, and was potentiated by the depletion of noradrenaline in the spinal cord in intact rats. These results suggest that presynaptic inhibition evoked by stimulation of the adjacent dorsal root seems to be tonically suppressed by descending noradrenergic neurones.