What proportion of striatal D2 receptors are occupied by endogenous dopamine at baseline? A meta-analysis with implications for understanding antipsychotic occupancy.

Using molecular imaging techniques - positron emission tomography (PET) and single-photon emission computed tomography (SPECT) - in conjunction with an acute dopamine depletion challenge (alpha-methyl-para-tyrosine) it is possible to estimate endogenous dopamine levels occupying striatal dopamine D receptors (DR) in humans in vivo. However, it is unclear what proportion of striatal DR are occupied by endogenous dopamine under normal conditions. This is important since it has been suggested that in schizophrenia there may be a substantial proportion of striatal DR which are occupied by endogenous dopamine and not accessible by therapeutic doses of antipsychotics. In order to clarify these issues, we conducted a meta-analysis of dopamine depletion studies using substituted benzamide radiotracers in healthy persons. This meta-analysis suggests that anywhere from 8 to 21% (weighted average 11%) of striatal DR may be occupied by endogenous dopamine at baseline. Using these estimates, we propose an updated occupancy model and tentatively suggest that antipsychotics inhibit a smaller proportion of the total pool of striatal DR in vivo than previously acknowledged. This article is part of the issue entitled 'Special Issue on Antipsychotics'.