Amygdala ensembles encode behavioral states

INTRODUCTION Affective or metabolic states, such as anxiety, stress, or thirst, enable adaptations of perception and the selection of appropriate behaviors to achieve safety or homeostasis. Classically, changes in brain states are associated with thalamocortical circuitry and sensory coding. Yet homeostatic and affective states are associated with complex behavioral, autonomic, and hormonal responses, suggesting that state representations involve brain-wide networks, including subcortical structures such as the amygdala. Previously, amygdala function has been studied mainly in the framework of Pavlovian conditioning, leading to the identification of specific circuit elements that underlie associative plasticity at the single-cell and neural-ensemble levels. However, how internal states engage neuronal ensembles in the basal amygdala, a hub for regulating affective, homeostatic, foraging, and social behaviors via widespread connections with many other brain areas, remains unknown. RATIONALE The encoding of states governing self-paced behaviors, including foraging or place avoidance, should engage large neuronal populations, evolve on longer time scales (seconds to minutes), generalize across contexts, and lead to differences in sensory processing and action selection. We therefore used a miniature microscope and longitudinal imaging of amygdala neural activity in freely moving mice performing a series of behavioral paradigms in different contexts across multiple days. We thereby tracked neuronal population activity across distinct behavioral paradigms in which mice exhibited distinct modes of behavior manifesting different internal states. RESULTS We tracked amygdala neuronal activity across the open-field test, the elevated plus maze test, and a classical Pavlovian fear-conditioning paradigm. During open-field exploration, two large ensembles of basal amygdala neurons antagonistically conveyed information about an animal’s corner or center location. This population signature of opposing ensemble activity occurred on a slow time scale (seconds), was evident across consecutive days and paradigms, and predicted transitions from exploratory to nonexploratory, defensive states and vice versa. Notably, amygdala ensemble coding did not align with spatial areas generally thought to correspond to global anxiety states (e.g., the open-field corners and the closed arms of the elevated plus maze) but instead reflected moment-to-moment changes in the exploratory or defensive state of the animal. During fear conditioning, sensory responses of amygdala neuronal populations to conditioned (tone) and unconditioned (shock) stimuli were orthogonal to state encoding, demonstrating that fast sensory responses and slow exploratory state dynamics were separately encoded by amygdala networks. Correlations of neural responses to state transitions were largely conserved across major amygdala output pathways to the hippocampus, nucleus accumbens, and prefrontal cortex. CONCLUSION Our study reveals two large, nonoverlapping functional neuronal ensembles of the basal amygdala representing internal states. The ensembles are anatomically intermingled and encode opposing moment-to-moment states changes, especially regarding exploratory and defensive behaviors, but do not provide a scalar measure of global anxiety levels. The amygdala broadcasts state signals to a wider brain network, including cortical and subcortical areas. These signals are likely correlated with diverse aspects of brain state, including anxiety, arousal, sensory processing, and action selection. This extends the current concept of thalamocortical brain-state coding to include affective and exploratory state representations in the amygdala, which have the potential to control state-dependent regulation of behavioral output and internal drives. Our findings provide a low-dimensional amygdala population signature as a trackable measure for the state dependency of brain function and behavior in defined neuronal circuits. It remains to be tested whether a maladaptive bias in neuronal state coding in the basolateral amygdala contributes to behavioral and physiological alterations in animal disease models.