Faculty Bibliography

Norepinephrine (NE) plays a central role in the acquisition of aversive learning via actions in the lateral nucleus of the amygdala (LA) [1, 2]. However, the function of NE in expression of aversively-conditioned responses has not been established. Given the role of the central nucleus of the amygdala (CeA) in the expression of such behaviors [3-5], and the presence of NE axons projections in this brain nucleus [6], we assessed the effects of NE activity in the CeA on behavioral expression using receptor-specific pharmacology and cell- and projection-specific chemogenetic manipulations. We found that inhibition and activation of locus coeruleus (LC) neurons decreases and increases freezing to aversively conditioned cues, respectively. We then show that locally inhibiting or activating LC terminals in CeA is sufficient to achieve this bidirectional modulation of defensive reactions. These findings support the hypothesis that LC projections to CeA are critical for the expression of defensive responses elicited by conditioned threats.

New protein synthesis is known to be required for the consolidation of memories, yet existing methods of blocking translation lack spatiotemporal precision and cell-type specificity, preventing investigation of cell-specific contributions of protein synthesis. Here we developed a combined knock-in mouse and chemogenetic approach for cell-type-specific drug-inducible protein synthesis inhibition that enables rapid and reversible phosphorylation of eukaryotic initiation factor 2α, leading to inhibition of general translation by 50% in vivo. We use cell-type-specific drug-inducible protein synthesis inhibition to show that targeted protein synthesis inhibition pan-neuronally and in excitatory neurons in the lateral amygdala (LA) impaired long-term memory. This could be recovered with artificial chemogenetic activation of LA neurons, although at the cost of stimulus generalization. Conversely, genetically reducing phosphorylation of eukaryotic initiation factor 2α in excitatory neurons in the LA enhanced memory strength but reduced memory fidelity and behavioral flexibility. Our findings provide evidence for a cell-specific translation program during consolidation of threat memories.

Local translation can support memory consolidation by supplying new proteins to synapses undergoing plasticity. Translation in adult forebrain dendrites is an established mechanism of synaptic plasticity and is regulated by learning, yet there is no evidence for learning-regulated protein synthesis in adult forebrain axons, which have traditionally been believed to be incapable of translation. Here we show that axons in the adult rat amygdala contain translation machinery, and use translating ribosome affinity purification (TRAP) with RNASeq to identify mRNAs in cortical axons projecting to the amygdala, over 1200 of which were regulated during consolidation of associative memory. Mitochondrial and translation-related genes were upregulated, whereas synaptic, cytoskeletal, and myelin-related genes were downregulated; the opposite effects were observed in the cortex. Our results demonstrate that axonal translation occurs in the adult forebrain and is altered after learning, supporting the likelihood that local translation is more a rule than an exception in neuronal processes.

While our understanding of appetitive motivation has benefited immensely from the use of selective outcome devaluation tools, the same cannot be said about aversive motivation. Findings from appetitive conditioning studies have shown that basal amygdala is required for behaviors that are sensitive to updates in outcome value, but similar results in aversive motivation are difficult to interpret due to a lack of outcome specificity. The studies reported here sought to develop procedures to isolate sensory-specific processes in aversive learning and behavior and to assess the possible contribution of the basal amygdala. Post-training changes to outcome value produced commensurate changes to subsequently tested conditioned responding in male rodents. Specifically, increases in shock intensity (i.e., inflation) augmented, while repeated exposure to (i.e., habituation of) an aversive sound (klaxon-horn) reduced freezing to conditioned stimuli previously paired with these outcomes. This was extended to a discriminative procedure, in which following revaluation of one event, but not the other, responding was found to be dependent on outcome value signaled by each cue. Chemogenetic inactivation of basal amygdala impaired this discrimination between stimuli signaling differently valued outcomes, but did not affect the revaluation process itself. These findings demonstrate a contribution of the basal amygdala to aversive outcome-dependent motivational processes.SIGNIFICANCE STATEMENT The specific content of pavlovian associative learning has been well studied in appetitive motivation, where the value of different foods can be easily manipulated. This has facilitated our understanding of the neural circuits that generate different forms of motivation (i.e., sensory specific vs general). Studies of aversive learning have not produced the same degree of understanding with regard to sensory specificity due to a lack of tools for evaluating sensory-specific processes. Here we use a variant of outcome devaluation procedures with aversive stimuli to study the role of basal amygdala in discriminating between aversive stimuli conveying different degrees of threat. These findings have implications for how we study generalized threat to identify dysregulation that can contribute to generalized anxiety.

The higher-order theory (HOT) of consciousness has often been misunderstood by critics. Here, we clarify its position on several issues, and distinguish it from other views, such as the global workspace theory (GWT) and early sensory models (e.g., first-order local recurrency theories). For example, HOT has been criticized for overintellectualizing consciousness. We show that, while higher-order states are cognitively assembled, the requirements are in fact considerably less than often presumed. In this sense, HOT may be viewed as an intermediate position between GWT and early sensory views. We also clarify that most proponents of HOT do not stipulate consciousness as equivalent to metacognition or confidence. Furthermore, compared with other existing theories, HOT can arguably account better for complex everyday experiences, such as emotions and episodic memories. This makes HOT particularly useful as a framework for conceptualizing pathological mental states.

Early life maltreatment by the caregiver constitutes a major risk factor for the development of later-life psychopathologies, including fear-related pathologies. Here, we used an animal model of early life maltreatment induced by the Scarcity-Adversity Model of low bedding (LB) where the mother is given insufficient bedding for nest building while rat pups were postnatal days (PN) 8-12. To assess effects of maltreatment on the expression of threat-elicited defensive behaviors, animals underwent odor-shock threat conditioning at three developmental stages: late infancy (PN18), adolescence (PN45) or adulthood (>PN75) and tested the next day with odor only presentations (cue test). Results showed that in typically developing rats, the response to threat increases with maturation, although experience with maltreatment in early infancy produced enhanced responding to threat in infancy and adulthood, but a decrease in maltreated adolescents. To better understand the unique features of this decreased threat responding in adolescence, c-Fos expression was assessed within the amygdala and ventromedial prefrontal cortex (vmPFC) associated with the cued expression of threat learning. Fos counts across amygdala subregions were lower in LB rats compared to controls, while enhanced c-Fos expression was observed in the vmPFC prelimbic cortex (PL). Correlational analysis between freezing behavior and Fos revealed freezing levels were correlated with CeA in controls, although more global correlations were detected in LB-reared rats, including the BA, LA, and CeA. Functional connectivity analysis between brain regions showed that LB reared rats exhibited more diffuse interconnectivity across amygdala subnuclei, compared the more heterogeneous patterns observed in controls. In addition, functional connectivity between the IL and LA switched from positive to negative in abused adolescents. Overall, these results suggest that in adolescence, the unique developmental decrease in fear expression following trauma is associated with distinct changes in regional function and long-range connectivity, reminiscent of pathological brain function. These results suggest that early life maltreatment from the caregiver perturbs the developmental trajectory of threat-elicited behavior. Indeed, it is possible that this form of trauma, where the infant's safety signal or "safe haven" (the caregiver) is actually the source of the threat, produces distinct outcomes across development.