Methyldopa produces central inhibition of parasympathetic activity in the cat |
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Authors: | M C Koss |
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Institution: | (1) Department of Pharmacology and of Ophthalmology, University of Oklahoma Health Sciences Center, and Dean McGee Eye Institute, P. O. Box 26901, 73190 Oklahoma City, Oklahoma, USA |
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Abstract: | Summary -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 -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 -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 g/kg i.v.). These results suggest that -methyldopa produces mydriasis in the cat by means of CNS inhibition of tonic outflow from the oculomotor nucleus and that an -adrenergic inhibitory mechanism may be involved. This conclusion is supported further by experiments in which direct measurements of ciliary nerve activity were made. |
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Keywords: | -Methyldopa" target="_blank">gif" alt="agr" align="BASELINE" BORDER="0">-Methyldopa Pupil diameter Yohimbine Phenoxybenzamine Adrenaline Parasympathectomized iris Ciliary nerve recordings |
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