Central and peripheral components of morphine mydriasis in mice

Mice treated with morphine (intracerebroventricularly, retrobulbarly or intraperitoneally) demonstrated dose related bilateral mydriasis. Intracerebroventricular (ICV) injection was much more potent than either retrobulbar (RB) or intraperitoneal (IP) injections. Morphine applied topically onto one eye resulted in bilateral mydriasis which was more marked in the treated eye. The mydriatic effect was antagonized by naloxone administered either IP or ICV or given on one eye. Here again, ICV naloxone was most effective. Naloxone eye drops diminished the mydriasis produced by systemic morphine bilaterally but more in the treated eye. These results suggest that in mice the mydriasis produced by morphine is mainly of central origin, but a local ocular effect also occurs.