Cellular-Resolution and Bulk-Fluorescence Recordings of Calcium Activity Yield Reciprocal Readouts of In Vivo Drug Efficacy

Genetically encoded fluorescent sensors of neural activity have become a mainstay of basic neuroscience. However, preclinical drug development has been slower to adopt these tools. Recently, we used miniature microscopes to record Ca²⁺ activity in D1 and D2 dopamine receptor-expressing spiny projection neurons (SPNs) in response to antipsychotic drugs or candidates. Despite the fact that most antipsychotics block D2 receptors, clinical efficacy was associated with the normalization of D1-SPN activity under hyperdopaminergic conditions. In this study, we re-processed these data to approximate a fiber photometry signal and asked whether the conclusions were the same. This re-evaluation is important because fiber photometry has several advantages over cellular-resolution imaging. Consistent with our previous finding that bulk and cellular-resolution imaging report distinct SPN Ca²⁺ dynamics, here the two data types suggested reciprocal effects of drug treatment on D1-SPN and D2-SPN Ca²⁺ activity. While amphetamine treatment increased D1-SPN and decreased D2-SPN Ca²⁺ event rates in cellular-resolution data, it increased the fluorescence of individual neurons but decreased their bulk fluorescence in both cell types. Analyzing detected bulk-fluorescence “events” yielded a closer correlation between the bulk and somatic Ca²⁺ fluorescence. However, it did not fully replicate the results of our previous cellular-resolution recordings following amphetamine or antipsychotic drug treatment. Our results highlight important distinctions between cellular-resolution and bulk measurements of in vivo Ca²⁺ activity. While experimenters using in vivo imaging to understand drug effects on neural activity should heed these distinctions, they should also utilize them to gain a more holistic view of drug action.