Vol. 300, Issue 2, 521-525, February 2002

_1A-Adrenoceptors Mediate Sympathetically Evoked Pupillary Dilation in Rats

Yongxin Yu and Michael C. Koss

Department of Cell Biology, University of Oklahoma College of Medicine, Oklahoma City, Oklahoma

Evidence suggests that in some species (cats, rabbits, and possibly humans) -adrenoceptors in the iris dilator muscle are "atypical" in that they cannot be readily classified by conventional criteria. This study was undertaken in an attempt to characterize the -adrenoceptor subtype(s) mediating sympathetically elicited mydriasis in rats. Frequency-response pupillary dilator curves were generated by stimulation of the preganglionic cervical sympathetic nerve (1-32 Hz) in pentobarbital-anesthetized rats. Evoked responses were inhibited by systemic administration of nonselective -adrenergic antagonists, phentolamine (0.3-10 mg/kg) and phenoxybenzamine (0.03-1 mg/kg). The selective ₁-adrenergic antagonist, prazosin (0.01-1 mg/kg), also was effective, although ₂-adrenergic antagonism with rauwolscine (0.1-1 mg/kg) was not. _1A-Adrenoceptor-selective antagonists, 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101; 0.1-1 mg/kg) and 5-methylurapidil (0.1-1 mg/kg), as well as the _1D-adrenoceptor-selective antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY-7378; 1-3 mg/kg), were used to determine the subtype(s) involved. Evoked mydriasis was significantly antagonized by both WB-4101 and 5-methylurapidil but not by BMY-7378. These results suggest that, unlike some other species, adrenoceptors in the rat iris dilator mediating neurogenic mydriasis are "typical" and, in addition, can be characterized as being primarily of the _1A-adrenoceptor subtype.