Papers And Originals

Action of Thymoxamine on Mydriasis induced by Levodopa and Dopamine

Br Med J 1971; 2 doi: (Published 22 May 1971) Cite this as: Br Med J 1971;2:438
  1. A. S. D. Spiers,
  2. D. B. Calne,
  3. S. D. Vakil,
  4. T. M. French


    Eye-drops of levodopa and dopamine induce pupillary dilatation which is inhibited by thymoxamine, an alpha-adrenergic blocking drug. This indicates that the mydriatic action of levodopa and dopamine involves excitation of alpha-adrenergic receptors of the dilator pupillae muscle. Such a conclusion is in accord with the previously expressed suggestion that levodopa is rapidly converted to dopamine, which displaces noradrenaline from adrenergic nerve endings.

    The findings that dopamine exerts alpha-adrenergic effects at the periphery may be construed as evidence in support of the view that the hypotensive action of levodopa is mediated via the central nervous system.