Characterization of Dorsal Hippocampal Activity during Extinction of Contextual Fear in Endocannabinoid-Deficient States

Abstract

Acquisition and extinction of associative fear memories are critical for guiding adaptive behavioral responses to environmental threats, and dysregulation of these processes is thought to represent important neurobehavioral substrates of trauma and stress-related disorders. We have previously shown that pharmacological inhibition of 2-Arachidonoylglycerol (2-AG) synthesis, a major endogenous cannabinoid regulating synaptic suppression, enhances fear learning and impairs within-session extinction of contextual fear. It is not clear, however, if neural activity of area CA1 of the dorsal hippocampus (dCA1), a canonical region for spatial learning, can also respond and/or represent aversive contingencies in an environment. To address this question, we used in-vivo calcium imaging coupled with pharmacological manipulation in an 8-day contextual fear learning and extinction paradigm. We found that dCA1 neurons exhibit responses to the aversive stimuli, with a significant increase in activity by late conditioning (day 4). Interestingly, the number of neurons recruited to the ensemble during context exposure decreases across conditioning days, while the number of shock-responsive neurons remains constant. The shock-responsive neurons have only around 8% of overlap between days, albeit displaying a higher response magnitude. During extinction training, we demonstrate that dCA1 neurons responded to both freezing and movement bouts; yet in an eCB-deficient state, neurons had less number of overall responses, coupled with an increased proportion to freezing events. We also found a strong negative correlation between the rate of calcium events in dCA1 and time spent freezing during early and late conditioning, which was lost in 2-AG depleted neurons later on each extinction day. Taken together, these results suggest a significant turnover or representational drift, with cells of the ensemble going silent or varying across time, but with a consistent and enhanced response to the aversive stimuli, which implies refinement and/or sensitization potentially occurring at the region-specific level. As such, our data advance the idea of neural ensembles as more "labile" and flexible systems than originally assumed, in alignment with current findings. In sum, we propose the dorsal hippocampus region as an important neurobiological substrate for acquisition of contextual fear and also extinction learning, possibly due to associative and non-associative learning processes.