Investigating Mechanisms of Drug Action at the Neuronal Ensemble Level in Behaving Mice: Advantages of In-Vivo Calcium Imaging in Drug Discovery

The miniature, integrated fluorescence microscope permits imaging studies of multiple neurons’ individual dynamics in deep brain structures in freely behaving animals. Here, we demonstrate the use of this technology in drug discovery by tracking somatic calcium dynamics in hundreds (200-800 per animal) of hippocampal neurons of pharmacologically manipulated behaving mice. At the same time, we monitored EEG, EMG, temperature, and locomotor activity to control for common confounding factors of drug action and reduce ambiguity in the interpretation of observed drug effects. To demonstrate the approach, we studied the effects of the sleep-inducing drug Zolpidem. While Zolpidem (GABAA agonist) is a commonly prescribed sleep medication, it has adverse memory-related side effects the mechanisms of which are poorly understood. We hypothesized that Zolpidem would lower hippocampal neuronal activity to below physiological level. To account for the drug-induced changes in locomotion and physiological states (i.e., REM/NREM ratios), we compared neuronal activity in the epochs matched by locomotor and physiological states identified by multimodal recordings. In support of our hypothesis, we found that Zolpidem strongly suppresses neuronal activity in hippocampus, which may provide a mechanistic explanation for the Zolpidem’s memory side effects. Therapeutic drugs for cognitive and psychiatric disorders are often characterized by their mechanism of action at the molecular levels. Here we provide a proof-of-principle for a new approach to understand mechanism of drug action in terms of information processing in large neuronal ensembles in relevant brain circuits. Understanding mechanisms of action through the neural coding perspective may provide explanations to differences in drug efficacy, kinetics, and side effects that traditional techniques have come short to reveal.