1 The effects of
intravenous administration of the selective alpha 2-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 greater than
UK 14,304 greater than
guanoxabenz. 3 Pretreatment with the selective alpha 2-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 alpha 1-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 alpha 1- and alpha 2-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 greater than
yohimbine greater than
piperoxan =
rauwolscine greater than
mianserin greater than 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 micrograms 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 alpha 2-adrenoceptor agonists induce
mydriasis in the rat through a central alpha 2-adrenoceptor mechanism. However, the site of action within the central nervous system remains to be determined.