Separate neural mechanisms mediate sufentanil-induced pupillary responses in the cat

The pharmacologic characteristics of a highly selective mu receptor agonist, sufentanil, were studied on the cat's pupillary responses (size, light reflex and fluctuations) measured with an infrared video pupillometer. The pupillary effects of sufentanil were also compared with those of morphine and clonidine, known mydriatics in the cat. Sufentanil (0.3-10 micrograms/kg i.v.) dose-dependently increased pupillary size and decreased light reflex and fluctuations. Naltrexone (10 micrograms/kg i.v.) pretreatment shifted the dose-response curve to the right by a factor of 26 for pupillary size, 9.5 for light reflex and 7.2 for fluctuations (nonvalid bioassay). Equivalent mydriatic doses of sufentanil (1 micrograms/kg), morphine (0.5 mg/kg) and clonidine (10 micrograms/kg) produced divergent effects on the light reflex and fluctuations. At these doses, morphine was more effective than sufentanil in inhibiting fluctuations. Clonidine was a more potent inhibitor of fluctuations but significantly enhanced the light reflex. Sufentanil (compared with morphine in a previous study) was 298 times more potent than morphine as a mydriatic, 100 times more potent in inhibiting the light reflex, and only slightly more potent in inhibiting fluctuations. These results indicate that separate neural mechanisms control the three pupillary components and that mu opioid receptors are more involved in mediating opiate-induced mydriasis than in inhibiting the light reflex and fluctuations in the cat.