α2‐Adrenoceptor agonists induce mydriasis in the rat by an action within the central nervous system

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Abstract
1 The effects of intravenous administration of the selective α2-adrenoceptor agonists clonidine, UK 14,304 and guanoxabenz on rat pupil diameter were investigated. 2 In rats anaesthetized with pentobarbitone, each agonist produced a marked dose-related increase in pupil diameter; the rank order of potency was: clonidine > UK 14,304 > guanoxabenz. 3 Pretreatment with the selective α2-adrenoceptor antagonist, RX 781094 (0.5 mg/kg, i.v.), produced a parallel 30–40 fold shift to the right of the dose-pupil dilator response curves for the three agonists. Yohimbine (1.5 mg/kg, i.v.) produced about a 10 fold rightward shift of the dose-response curve for guanoxabenz. In contrast, the α1-selective antagonist, prazosin (0.5 mg/kg, i.v.), failed to affect the dose-response relation for guanoxabenz. 4 Several antagonists of varying selectivities towards α1- and α2-adrenoceptors were tested for their ability to reverse the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). The rank order of potency of the antagonists producing a 50% reversal of this effect was: RX 781094 > yohimbine > piperoxan = rauwolscine > mianserin > RS 21361. Neither corynanthine nor prazosin reversed the guanoxabenz-induced mydriasis. 5 Topical application of RX 781094 (0.1 to 3% w/v solutions) onto one eye produced a slow reversal of guanoxabenz-induced mydriasis; the time course and degree of reversal were virtually the same in both eyes. 6 Intracerebroventricular administration of RX 781094 (1.25–15 μg total dose) caused a rapid dose-related reversal of the maximal mydriasis induced by guanoxabenz (0.3 mg/kg, i.v.). 7 Guanoxabenz (0.3 and 1.0 mg/kg, i.v.) did not produce any dilation of the physostigmine-constricted undamaged pupil of the pithed rat. Intravenous adrenaline was found to produce a small mydriatic effect, while atropine completely antagonized the effects of physostigmine in this preparation. 8 These results indicate that α2-adrenoceptor agonists induce mydriasis in the rat through a central α2-adrenoceptor mechanism. However, the site of action within the central nervous system remains to be determined.