Opiate receptors in the rabbit iris.

Area of Science:

• Pharmacology
• Ophthalmology
• Neuroscience

Background:

• Opiate receptors are known to exist in various tissues.
• Their presence and function in the iris are not fully understood.

Purpose of the Study:

• To investigate the presence of opiate receptors in the rabbit iris.
• To characterize the effects of opiates on pupillary size in vivo.

Main Methods:

• In vivo study involving intraocular injections of morphine and D-Ala-met-enkephalinamide in rabbits.
• Dose-response assessments of pupillary size changes.
• Administration of naloxone to evaluate antagonism.

Main Results:

• Morphine and D-Ala-met-enkephalinamide induced a dose-dependent decrease in pupillary size (miosis).
• Pupillary size showed appreciable fluctuations following opiate administration.
• Intravenous naloxone administration blocked the miotic effects and fluctuations.
• Parallel dose-response curves in the presence and absence of naloxone indicated competitive reversible antagonism.

Conclusions:

• The rabbit iris contains functional opiate receptors.
• These receptors mediate miotic responses to opiates.
• Naloxone acts as a competitive antagonist at these iris opiate receptors.