The effects of centrally acting drugs on tremor in monkeys with mesencephalic lesions.

The effects of centrally acting drugs on tremor were investigated in monkeys with ventromedial tegmental lesions exhibiting hypokinesia or hypokinesia and tremor. In monkeys with resting tremor, the administration of DL-5-HTP (5-hydroxytryptophan) or of DL-DOPA (3,4-dihydroxyphenylalanine) relieves the tremor, but the simultaneous administration of DL-5-HTP or DL-DOPA and atropine results in a much more pronounced relief. These results point to an imbalance between the cholinergic and adrenergic-serotonergic systems in parkinsonism. In monkeys exhibiting hypokinesia, the administration of harmaline evokes a marked resting tremor of the extremities contralateral to the tegmental lesion. The production of tremor by harmaline is not abolished by lowering the striatal amine levels with specific inhibitors of amine synthesis. Administration of DL-5-HTP protects monkeys from tremors induced by harmaline, which might affect the functions of the central nervous system by interaction with receptors for serotonin. The present results further demonstrate the apparent role of biogenic amines in the extrapyramidal dysfunctions.