Drug-induced receptor occupancy: substantial differences in measurements made in vivo vs ex vivo

Rationale: The number of receptors occupied by a given drug is a central construct in understanding drug action in the brain. Two techniques have been commonly used to measure drug receptor occupancy. In one method, the drug and the radioligand used to measure occupancy compete in vivo while in the other method, the drug is injected into the living animal, the animal killed and the radioligand competes for available receptors ex vivo. While these methods are often used interchangeably, there has been no systematic comparison of their sensitivities and consistency. Objectives. In this study, we performed a systematic within-animal comparison of drug-induced receptor occupancy as measured by the in vivo vs the ex vivo methods. Methods: We examined the occupancy of dopamine D-2 receptors by different doses of the drug raclopride using the in vivo and and ex vivo autoradiographic methods in the same rat with C-11-raclopride and H-3-raclopride as radioligands, respectively. Results: The in vivo method showed a significantly greater sensitivity and internal consistency while the ex vivo method was less sensitive, and increasingly so as a function of longer incubation times. The lack of sensitivity was accounted for by the unidirectional, dissociation of the drug from the receptors in the incubation medium. Conclusions: Our-data suggest that these two methods are not interchangeable; the ex. vivo method is much less sensitive, lacks internal consistency and hence is best avoided.