Increased tone-offset response in the lateral nucleus of the amygdala underlies trace fear conditioning.

Accumulating evidence suggests that the lateral nucleus of the amygdala (LA) stores associative memory in the form of enhanced neural response to the sensory input following classical fear conditioning in which the conditioned stimulus (CS) and the unconditioned stimulus (US) are presented in a temporally continuous manner. However, little is known about the role of the LA in trace fear conditioning where the CS and the US are separated by a temporal gap. Single-unit recordings of LA neurons before and after trace fear conditioning revealed that the short-latency activity to the CS offset, but not that to the onset, increased significantly and accompanied the conditioned fear response. The increased short-latency activity was evident in two aspects: the number of offset-responsive neurons was increased and the latency of the neuronal response to the CS offset was shortened. On the contrary, changes in the firing rate to either the onset or the offset were negligible following unpaired presentations of the CS and US. In sum, our results suggest that increased synaptic efficacy in the CS offset pathway in the LA might underlie the association between temporally distant stimuli in trace fear conditioning.

Valence and Salience Encoding in the Central Amygdala.

The central amygdala (CeA) has emerged as an important brain region for regulating both negative (fear and anxiety) and positive (reward) affective behaviors. The CeA has been proposed to encode affective information in the form of valence (whether the stimulus is good or bad) or salience (how significant is the stimulus), but the extent to which these two types of stimulus representation occur in the CeA is not known. Here, we used single cell calcium imaging in mice during appetitive and aversive conditioning and found that majority of CeA neurons (~65%) encode the valence of the unconditioned stimulus (US) with a smaller subset of cells (~15%) encoding the salience of the US. Valence and salience encoding of the conditioned stimulus (CS) was also observed, albeit to a lesser extent. These findings show that the CeA is a site of convergence for encoding oppositely valenced US information.

A dopamine-dependent mechanism for Reward-induced modification of Fear memories.

A positive mental state has been shown to modulate fear-related emotions associated with the recall of fear memories. These, and other observations suggest the presence of central brain mechanisms for affective states to interact. The neurotransmitter dopamine is important for both Reward- and fear-related processes, but it is unclear whether dopamine contributes to such affective interactions. Here, we show that precisely timed Reward-induced activation of dopamine neurons in mice potently modifies fear memories and enhances their extinction. This Reward-based switch in fear states is associated with changes in dopamine release and dopamine-dependent regulation of fear encoding in the central amygdala (CeA). These data provide a central mechanism for Reward-induced modification of fear states that have broad implications for treating generalized fear disorders.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats.

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory-defensive functioning.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats.

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory defensive functioning.

Differential Encoding of Trace and Delay Fear Memory in the Entorhinal Cortex.

Trace fear conditioning is characterized by a stimulus-free trace interval (TI) between the conditioned stimulus (CS) and the unconditioned stimulus (US), which requires an array of brain structures to support the formation and storage of associative memory. The entorhinal cortex (EC) has been proposed to provide essential neural code for resolving temporal discontinuity in conjunction with the hippocampus. However, how the CS and TI are encoded at the neuronal level in the EC is not clear. In Exp. 1, we tested the effect of bilateral pre-training electrolytic lesions of EC on trace vs. delay fear conditioning using rats as subjects. We found that the lesions impaired the acquisition of trace but not delay fear conditioning confirming that EC is a critical brain area for trace fear memory formation. In Exp. 2, single-unit activities from EC were recorded during the pre-training baseline and post-training retention sessions following trace or delay conditioning. The recording results showed that a significant proportion of the EC neurons modulated their firing during TI after the trace conditioning, but not after the delay fear conditioning. Further analysis revealed that the majority of modulated units decreased the firing rate during the TI or the CS. Taken together, these results suggest that EC critically contributes to trace fear conditioning by modulating neuronal activity during the TI to facilitate the association between the CS and US across a temporal gap.

Temporal scaling of dopamine neuron firing and dopamine release by distinct ion channels shape behavior.

Dopamine is broadly implicated in reinforcement learning, but how patterns of dopamine activity are generated is poorly resolved. Here, we demonstrate that two ion channels, Kv4.3 and BKCa1.1, regulate the pattern of dopamine neuron firing and dopamine release on different time scales to influence separate phases of reinforced behavior in mice. Inactivation of Kv4.3 in VTA dopamine neurons increases ex vivo pacemaker activity and excitability that is associated with increased in vivo firing rate and ramping dynamics before lever press in a learned instrumental paradigm. Loss of Kv4.3 enhances performance of the learned response and facilitates extinction. In contrast, loss of BKCa1.1 increases burst firing and phasic dopamine release that enhances learning of an instrumental response and enhances extinction burst lever pressing in early extinction that is associated with a greater change in activity between reinforced and unreinforced actions. These data demonstrate that disruption of intrinsic regulators of neuronal activity differentially affects dopamine dynamics during reinforcement and extinction learning.

Central amygdala circuits in valence and salience processing.

Behavioral responses to environmental stimuli are dictated by the affective valence of the stimulus, good (positive valence) or bad (negative valence). These stimuli can innately elicit an affective response that promotes approach or avoidance behavior. In addition to innately valenced stimuli, valence can also be assigned to initially neutral stimuli through associative learning. A stimulus of a given valence can vary in salience depending on the strength of the stimulus, the underlying state of the animal, and the context of the stimulus presentation. Salience endows the stimulus with the ability to direct attention and elicit preparatory responses to mount an incentive-based motivated behavior. The central nucleus of the amygdala (CeA) has emerged as an early integration point for valence and salience detection to engage preparatory autonomic responses and behavioral posturing in response to both aversive and appetitive stimuli. There are numerous cell types in the CeA that are involved in valence and salience processing through a variety of connections, and we will review the recent progress that has been made in identifying these circuit elements and their roles in these processes.

'Fearful-place' coding in the amygdala-hippocampal network.

Animals seeking survival needs must be able to assess different locations of threats in their habitat. However, the neural integration of spatial and risk information essential for guiding goal-directed behavior remains poorly understood. Thus, we investigated simultaneous activities of fear-responsive basal amygdala (BA) and place-responsive dorsal hippocampus (dHPC) neurons as rats left the safe nest to search for food in an exposed space and encountered a simulated 'predator.' In this realistic situation, BA cells increased their firing rates and dHPC place cells decreased their spatial stability near the threat. Importantly, only those dHPC cells synchronized with the predator-responsive BA cells remapped significantly as a function of escalating risk location. Moreover, optogenetic stimulation of BA neurons was sufficient to cause spatial avoidance behavior and disrupt place fields. These results suggest a dynamic interaction of BA's fear signalling cells and dHPC's spatial coding cells as animals traverse safe-danger areas of their environment.

Dynamic coding of predatory information between the prelimbic cortex and lateral amygdala in foraging rats.

Predation is considered a major selective pressure in the evolution of fear, but the neurophysiology of predator-induced fear is unknown. We simultaneously recorded lateral amygdala (LA) and prelimbic (PL) area neuronal activities as rats exited a safe nest to search for food in an open space before, during, and after encountering a "predator" robot programmed to surge from afar. Distinct populations of LA neurons transiently increased spiking as rats either advanced or fled the robot, whereas PL neurons showed longer-lasting spike trains that preceded and persisted beyond LA activity. Moreover, discrete LA-PL cell pairs displayed correlated firings only when the animals either approached or fled the robot. These results suggest a general fear function of the LA-PL circuit where the PL participates in the initial detection of potential threats, the LA signals the occurrence of real threats, and the dynamic LA-PL interaction optimizes defensive readiness for action.

Alterations of hippocampal place cells in foraging rats facing a "predatory" threat.

Fear is an adaptive mechanism evolved to influence the primal decisions of foragers in "approach resource-avoid predator" conflicts. To survive and reproduce, animals must attain the basic needs (food, water, shelter, and mate) while avoiding the ultimate cost of predation. Consistent with this view, ecological studies have found that predatory threats cause animals to limit foraging to fewer places in their habitat and/or to restricted times. However, the neurophysiological basis through which animals alter their foraging boundaries when confronted with danger remains largely unknown. Here, we investigated place cells in the hippocampus, implicated in processing spatial information and memory, in male Long-Evans rats foraging for food under risky situations that would be common in nature. Specifically, place cells from dorsal cornu ammonis field 1 (CA1) were recorded while rats searched for food in a semi-naturalistic apparatus (consisting of a nest and a relatively large open area) before, during, and after encountering a "predatory" robot situated remotely from the nest. The looming robot induced remapping of place fields and increased the theta rhythm as the animals advanced toward the vicinity of threat, but not when they were around the safety of the nest. These neurophysiological effects on the hippocampus were prevented by lesioning of the amygdala. Based on these findings, we suggest that the amygdalar signaling of fear influences the stability of hippocampal place cells as a function of threat distance in rats foraging for food.