The amygdala is a collection of functionally discrete though intricately interconnected nuclei that is critical for the acquisition and storage of emotional memory. Also known as associative memory, this higher-order cognitive function involves the pairing of environmentally derived sensory information to produce learned behavioural responses. Previous investigation into associative learning is primarily centred on the behavioural learning paradigm fear conditioning, or its translational electrophysiological in vitro models. However, our present understanding of this structure is restricted by the limitations of conventional electrophysiological techniques. Consequently, the physiological mechanisms underlying this higher-order cognitive process are yet to be fully elucidated. Here, we utilized the light-sensitive cation channel, Channelrhodopsin-2 (ChR2), to investigate the auditory sensory inputs involved in auditory fear conditioning, a subtype within this behavioural paradigm.
Channelrhodopsin is a temporally precise, optogenetic expression system with the capacity for robust, spatially restricted neuronal activation. Therefore, this method represents a particularly powerful tool for the investigation of neuronal circuit mapping. Combining the stereotaxic delivery and viral transduction of this light-sensitive protein within GAD67-GFP transgenic mice, this study uses acute slice electrophysiology to map the functional connectivity of thalamic and cortical auditory inputs within the amygdala. The application of this input-specific stimulation method revealed an asymmetry between the GABAergic circuits recruited by thalamic and cortical afferents, and the subsequent downstream inhibition of local principal neurons. Using ChR2, we also present evidence to suggest that electrical stimulation of the so-called cortical and thalamic input sites is not input-specific, and can instead produce the undetectable co-activation of these reciprocally connected afferents. This study identifies excitatory and inhibitory cell populations that are recruited during auditory innervation and processing within the amygdala, and highlights their cell-type specific contribution within the associative learning circuitry.