Monitoring Neural Activity During Exposure to Drugs of Abuse With In Vivo Fiber Photometry

Abstract

Drugs of abuse are known to alter activity in areas of brain associated with reward, cognition and decision making. Changes in neural activity in these regions that follow repeated exposures to abused substances may underlie the development of addictive behaviors and contribute to the high rates of relapse associated with drug use. Measuring real-time changes in neural activity during drug seeking and taking is important for correlating changes in behavior with alterations in neuronal signaling typically measured using ex vivo electrophysiological recordings. In this study, C57BL/6J mice or Sprague-Dawley rats were injected in different brain areas with adeno-associated viruses (AAV) encoding the calcium sensor GCaMP6f along with an optical fiber. Calcium-dependent fluorescence was monitored in the nucleus accumbens core or mPFC during and following exposure to toluene vapor and in the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) during ethanol drinking. Toluene vapor, at concentrations previously shown to induce conditioned place preference, produced a rapid decrease in the frequency of calcium transients in the NAc core of rats that recovered following washout of the toluene vapor. In a probabilistic risk task, GCaMP6 signals in rat mPFC increased just prior to lever pressing and showed decreases during the reward phase that were proportional to reward size. Toluene pretreatment elevated the signal during the decision-making period while post-lever responses were independent of reward size. Using the drinking in the dark (DID) protocol in mice, we observed a consistent increase in GCaMP6 fluorescence during the period leading up to an ethanol drinking bout, a decrease during consumption and a rebound increase following the bout. The initial increase in signal prior to consumption was greater for ethanol and sucrose than water. GCaMP6 signals in the lateral OFC also decreased during ethanol consumption and increased following bout completion while no increase in activity was noted prior to bout initiation. Following repeated cycles of chronic intermittent ethanol (CIE) exposure that enhanced ethanol consumption, OFC calcium signals during and after ethanol drinking were similar to those in air-treated animals. Addition of quinine to the ethanol solution augmented the decrease in signal during consumption in both air and CIE mice while having no effect on the magnitude of the rebound in activity. Conversely, when sucrose was added to the ethanol solution, air exposed mice showed blunted changes in GCaMP6 signals while those in CIE mice were enhanced. Overall, the results from these experiments complement and extend data from prior behavioral and electrophysiological studies and support the use of in vivo fiber photometry in the study of effects of abused substances on brain function.