MODE OF ACTION OF L-DOPA ON γ-MOTOR SYSTEM IN THE RAT

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The effect of L-dopa on γ-motor system, especially on static γ-activity induced by pinna pinching, was investigated in the anesthetized rats. γ-Activity was detected indirectly as the change in the rate of muscle spindle afferent discharges. L-Dopa (25-100 mg/kg, i. v.) caused a dose-dependent inhibition in static γ-activity. The effect of L-dopa was reduced significantly by the pretreatment with α-methyl-dopa (400 mg/kg, i. p.). On the other hand, the pretreatment with disulfiram (400mmg/kg, i. p.) augmented the inhibitory effect of L-dopa. Phenoxybenzamine hydrochloride (2.5 mg/kg, i.v.) augmented the inhibitory effect of L-dopa, whereas haloperidol (0.025 mg/kg, i.v.) had no influence on the effect of the drug. Amphetamine sulfate (2.5 mg/kg, i.v.) and amantadine hydrochloride (10 mg/kg, i.v.) facilitated static γ-activity, and effects of these drugs were considerably reduced by the pretreatment with disulfiram and inhibited by phenoxybenzamine hydrochloride (10 mg/kg, i. v.). These results suggest that L-dopa exerts its inhibitory effect after being metabolized to dopamine, and that amphetamine and amantadine exhibit their facilitatory actions on the static γ-activity through the noradrenergic pathways.
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