Role of sympathetic system in morphine-induced mydriasis in rat

The effects of three doses (5, 30, and 60 mg/kg) of morphine on pupillary diameter (PD) were studied in rats that were either normal, neurotransmitter antagonist pretreated, guanethidine pretreated, cervical sympathectomized, or adrenalectomized. Morphine produced a dose-dependent increase in PD and exophthalmos. In addition, the PD concurrently underwent fluctuations. alpha-Adrenergic antagonists blocked both mydriasis and exophthalmos, whereas a beta-adrenergic antagonist had no significant effect. This suggested a role for the sympathetic nervous system. However, cervical sympathectomy or guanethidine pretreatment only partially blocked the increase in PD or exophthalmos. Adrenalectomy also partially blocked the mydriasis and exophthalmos. Adrenalectomy combined with guanethidine pretreatment completely blocked morphine-induced mydriasis but no exophthalmos. Thus, the results suggest that both direct sympathetic innervation and hormonal action from the adrenals are involved in morphine-induced mydriasis in the rat.