The effect of phenytoin and metamphetamine on spinal motor activity

Summary

The investigation was initiated by the idea that reserpine might exert its action on spinal motor activity through facilitatory impulses in descending pathways to the spinal motoneurones. Therefore, the effect of conditioning by repetitive stimulation of descending and segmental pathways on α andγ reflex discharges was studied in the rat. Conditioning by stimulation of the dorsolateral funiculus increased a and decreasedγ reflex discharge. Conditioning by stimulation of the dorsal funiculus increased α reflex discharge without changingγ reflex activity. In partially or totally spinalized preparations, conditioning by repetitive stimulation of the same dorsal root to which the test stimulus was applied, produced an increase in a reflex activity which was associated with a decrease inγ motor activity. In the preparations with an intact neuraxis, repetitive stimulation of the dorsal root facilitating α reflex discharge did not changeγ motor activity. α motor activity also remained unchanged when α reflex discharge was inhibited by repetitive stimulation of the dorsal root in the intact or totally spinalized preparation.

Phenytoin 50 mg/kg increased the unconditioned α response in all types of preparations, apart from that in which the dorsal funiculus was isolated. Facilitation of α reflex discharge elicited by repetitive stimulation of segmental and descending pathways was depressed by phenytoin, whereas inhibition remained unchanged.

Metamphetamine 2 mg/kg increased the unconditioned α reflex discharge in all types of preparations, and decreased theγ response in the preparations with an intact neuraxis and also in those in which the dorsal funiculus was isolated. Metamphetamine did not depress the facilitatory or inhibitory effects produced by repetitive stimulation of descending or segmental pathways.

From the results obtained it is concluded that reserpine rigidity in the rat may be mediated by an increased facilitatory impulse input reaching the a motoneurones by way of descending pathways in the dorsolateral column of the spinal cord, and that phenytoin may antagonize, at least in part, the effect of reserpine by a depressant action on facilitatory processes at the spinal level, whereas abolition of reserpine rigidity produced by metamphetamine must be due to a supraspinal site of action of this drug.