Naunyn-Schmiedeberg's Archives of Pharmacology

, Volume 314, Issue 2, pp 135–139 | Cite as

Methyldopa produces central inhibition of parasympathetic activity in the cat

  • M. C. Koss


α-Methyldopa (10–100 mg/kg i.v.) produced a dose-dependent pupillary dilation in anaesthetized cats which was antagonized by subsequent administration of yohimbine hydrochloride (0.5 mg/kg i.v.). The peak effects were observed approximately 2–3h after injection. This α-methyldopa-induced mydriasis was present only when the parasympathetic innervation to the iris was intact. Prior treatment with yohimbine (0.5 mg/kg i.v.) 30 min before α-methyldopa also antagonized the mydriatic effect, whereas pretreatment with phenoxybenzamine (2.5 mg/kg i.v.) did not. In contrast, phenoxybenzamine, but not yohimbine, effectively antagonized the pupillary dilation produced by adrenaline (0.3–10.0 μg/kg i.v.). These results suggest that α-methyldopa produces mydriasis in the cat by means of CNS inhibition of tonic outflow from the oculomotor nucleus and that an α-adrenergic inhibitory mechanism may be involved. This conclusion is supported further by experiments in which direct measurements of ciliary nerve activity were made.

Key words

α-Methyldopa Pupil diameter Yohimbine Phenoxybenzamine Adrenaline Parasympathectomized iris Ciliary nerve recordings 


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Copyright information

© Springer-Verlag 1980

Authors and Affiliations

  • M. C. Koss
    • 1
  1. 1.Department of Pharmacology and of OphthalmologyUniversity of Oklahoma Health Sciences Center, and Dean McGee Eye InstituteOklahoma CityUSA

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