Influence of morphine and naltrexone on the intraocular pressure of conscious cynomolgus monkeys (Macaca fascicularis).

Intraocular pressure (IOP) was measured by applanation tonometry in four trained conscious monkeys (Macaca fascicularis) which were treated with morphine (5 mg/kg), naltrexone (1 mg/kg) and a mixture of both drugs (in the same doses as given separately). Morphine lowered IOP in all animals at both 20 and 40 min after drug administration. The inter-individual scatter was 1.8--11 mm Hg. Mydriasis was noted in three monkeys. Behaviour was characteristically affected. The morphine antagonist naltrexone one and the mixture of morphine and naltrexone had no effect on IOP or pupil. Behaviour, however, differed from the normal as well as the morphine-induced behaviour. The results indicate that the morphine effect on IOP must be mediated by a specific agonistic action on the morphine receptor.