This study was undertaken to identify the
alpha-adrenergic receptor type responsible for sympathetically evoked
mydriasis in
pentobarbital-anesthetized rabbits. Frequency-response curves of pupillary dilation were generated by stimulation of the preganglionic cervical sympathetic nerve (1-64 Hz). Evoked
mydriatic responses were inhibited by systemic administration of nonselective
alpha-adrenergic antagonists,
phentolamine (0.3-10 mg/kg) and
phenoxybenzamine (0.03-0.3 mg/kg), as well as the selective alpha(1)-adrenergic antagonist,
prazosin (0.1-1 mg/kg). The alpha(2)-adrenergic antagonist,
RS 79948 (0.3 mg/kg, i.v.) was without inhibitory effect, but potentiated the
mydriatic response. In addition, the selective alpha(1A)-adrenoceptor antagonist,
5-methylurapidil (0.1-1 mg/kg, i.v.), antagonized the elicited
mydriasis in a dose-dependent fashion. Unlike previous observations that
prazosin does not block the
adrenoceptor in rabbit iris dilator muscle, our results suggest that
prazosin is effective in inhibiting neuronally elicited
mydriasis in this species, and that alpha(1A)-adrenoceptors appear to mediate the response.