Methyldopa produces central inhibition of parasympathetic activity in the cat.
alpha-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 2-3 h after injection. This alpha-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 alpha-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 microgram/kg i.v.). These results suggest that alpha-methyldopa produces mydriasis in the cat by means of CNS inhibition of tonic outflow from the oculomotor nucleus and that an alpha-adrenergic inhibitory mechanism may be involved. This conclusion is supported further by experiments in which direct measurements of ciliary nerve activity were made.
D000818:Animals; D002415:Cats; D002924:Ciliary Body / Q000187:drug effects; D004837:Epinephrine / Q000494:pharmacology; D005260:Female; D007498:Iris / Q000502:physiology; D008297:Male; D008750:Methyldopa / Q000494:pharmacology*; D009184:Mydriatics; D010275:Parasympathetic Nervous System / Q000187:drug effects*; D010643:Phenoxybenzamine / Q000494:pharmacology; D011680:Pupil / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D015016:Yohimbine / Q000494:pharmacology
D009184:Mydriatics; D011942:Receptors, Adrenergic, alpha; D010643:Phenoxybenzamine; D015016:Yohimbine; D008750:Methyldopa; D004837:Epinephrine