Learning is often an emotional process. Emotional stimuli with different valences, such as threat and reward, can transform an otherwise neutral sensory input into one that can trigger distinct behavioral outcomes, such as defensive versus reward-seeking behaviors. The amygdala, a structure deep in the brain, plays an essential role in such emotional learning. By integrating sensory information with signals of negative or positive valences, the neuronal circuits within the amygdala are modified during emotional learning. However, how individual emotional stimuli differentially modify the amygdala to mediate the defined, sometimes opposite behavioral outcomes is largely unknown. The overarching goal of our collaborative project is to elucidate the logics and mechanisms underlying differential emotional learning at the level of neuronal circuits. A crucial step forward is to determine the activities of individual neurons within discrete amygdala circuits before, during, and after learning tasks. However, this goal is challenging to achieve. First, the amygdala is buried deep within the brain, making it difficult to access by imaging methods, such as calcium imaging, which is the technique of choice for interrogating neuronal activities with cellular resolution over large neuronal populations. Second, stress and reward signals are in part encoded as neuromodulatory activities, which have been difficult to measure especially in behaving animals. Adding to the difficulty, the identity of individual amygdala circuits, as well as where each circuit receives inputs and where it sends outputs, are only partially understood.
In on-going progress, the team of three co-PIs meets these challenges by integrating the cutting-edge, complementary technological advances from each laboratory. In defined emotional learning behavioral paradigms we can now image calcium as a proxy for neuronal activity in the amygdala by performing two-photon imaging via a tiny GRIN lens (Φ~0.5 mm), which offers optical access to deep brain structures even in behaving animals. To establish a readout for stress/reward-induced neuromodulatory signals, we have successfully imaged the activity dynamics of the cAMP/protein kinase A (PKA) signaling pathway, which is a common downstream pathway for many neuromodulators, including norepinephrine and dopamine, by combining novel genetically-encoded sensors with two-photon fluorescence lifetime imaging microscopy and GRIN lenses. In conjunction, we are performing whole-brain high-resolution imaging and computation-based anatomical circuitry analyses to dissect novel functional subdivisions of the amygdala and identify the input-output of each subdivision with cell-type specificity. Looking forward, we will integrate the advances in these three aspects to systematically map circuits and determine how neurons in each amygdala circuit are recruited by and contribute to the generation of valence-specific behaviors.
1. Introducing the important roles of amygdala in emotional learning and the challenges in understanding it.
2. Presenting the ongoing progresses in meeting the challenges towards dissecting the circuit logics of amygdala-based emotional learning.