Faculty Bibliography

Pavlovian fear conditioning is a particularly useful behavioral paradigm for exploring the molecular mechanisms of learning and memory because a well-defined response to a specific environmental stimulus is produced through associative learning processes. Synaptic plasticity in the lateral nucleus of the amygdala (LA) underlies this form of associative learning. Here, we summarize the molecular mechanisms that contribute to this synaptic plasticity in the context of auditory fear conditioning, the form of fear conditioning best understood at the molecular level. We discuss the neurotransmitter systems and signaling cascades that contribute to three phases of auditory fear conditioning: acquisition, consolidation, and reconsolidation. These studies suggest that multiple intracellular signaling pathways, including those triggered by activation of Hebbian processes and neuromodulatory receptors, interact to produce neural plasticity in the LA and behavioral fear conditioning. Collectively, this body of research illustrates the power of fear conditioning as a model system for characterizing the mechanisms of learning and memory in mammals and potentially for understanding fear-related disorders, such as PTSD and phobias

Although glutamate receptor 1 (GluR1)-containing alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (GluR1-AMPARs) are implicated in synaptic plasticity, it has yet to be demonstrated whether endogenous GluR1-AMPARs undergo activity-dependent trafficking in vivo to synapses to support short-term memory (STM) formation. The paradigm of pavlovian fear conditioning (FC) can be used to address this question, because a discrete region-the lateral amygdala (LA)-has been shown unambiguously to be necessary for the formation of the associative memory between a neutral stimulus (tone [CS]) and a noxious stimulus (foot shock [US]). Acquisition of STM for FC can occur even in the presence of protein synthesis inhibitors, indicating that redistribution of pre-existing molecules to synaptic junctions underlies STM. We employed electron microscopic immunocytochemistry to evaluate alterations in the distribution of endogenous AMPAR subunits at LA synapses during the STM phase of FC. Rats were sacrificed 40 minutes following three CS-US pairings. In the LA of paired animals, relative to naive animals, the proportion of GluR1-AMPAR-labeled synapses increased 99% at spines and 167% in shafts. In the LA of unpaired rats, for which the CS was never associated with the US, GluR1 immunoreactivity decreased 84% at excitatory shaft synapses. GluR2/3 immunoreactivity at excitatory synapses did not change detectably following paired or unpaired conditioning. Thus, the early phase of FC involves rapid redistribution specifically of the GluR1-AMPARs to the postsynaptic membranes in the LA, together with the rapid translocation of GluR1-AMPARs from remote sites into the spine head cytoplasm, yielding behavior changes that are specific to stimulus contingencies

Adaptive responding to threatening stressors is of fundamental importance for survival. Dysfunctional hyperactivation of corticotropin releasing factor type-1 (CRF(1)) receptors in stress response system pathways is linked to stress-related psychopathology and CRF(1) receptor antagonists (CRAs) have been proposed as novel therapeutic agents. CRA effects in diverse animal models of stress that detect anxiolytics and/or antidepressants are reviewed, with the goal of evaluating their potential therapeutic utility in depression, anxiety, and other stress-related disorders. CRAs have a distinct phenotype in animals that has similarities to, and differences from, those of classic antidepressants and anxiolytics. CRAs are generally behaviorally silent, indicating that CRF(1) receptors are normally in a state of low basal activation. CRAs reduce stressor-induced HPA axis activation by blocking pituitary and possibly brain CRF(1) receptors which may ameliorate chronic stress-induced pathology. In animal models sensitive to anxiolytics and/or antidepressants, CRAs are generally more active in those with high stress levels, conditions which may maximize CRF(1) receptor hyperactivation. Clinically, CRAs have demonstrated good tolerability and safety, but have thus far lacked compelling efficacy in major depressive disorder, generalized anxiety disorder, or irritable bowel syndrome. CRAs may be best suited for disorders in which stressors clearly contribute to the underlying pathology (e.g. posttraumatic stress disorder, early life trauma, withdrawal/abstinence from addictive substances), though much work is needed to explore these possibilities. An evolving literature exploring the genetic, developmental and environmental factors linking CRF(1) receptor dysfunction to stress-related psychopathology is discussed in the context of improving the translational value of current animal models.

Norepinephrine receptors have been studied in emotion, memory, and attention. However, the role of alpha1-adrenergic receptors in fear conditioning, a major model of emotional learning, is poorly understood. We examined the effect of terazosin, an alpha1-adrenergic receptor antagonist, on cued fear conditioning. Systemic or intra-lateral amygdala terazosin delivered before conditioning enhanced short- and long-term memory. Terazosin delivered after conditioning did not affect consolidation. In vitro, terazosin impaired lateral amygdala inhibitory postsynaptic currents leading to facilitation of excitatory postsynaptic currents and long-term potentiation. Since alpha1 blockers are prescribed for hypertension and post-traumatic stress disorder, these results may have important clinical implications

BACKGROUND: The lateral (LA) and central (CE), but not basal (B), amygdala nuclei are necessary for reactive Pavlovian fear responses such as freezing. The amygdala also plays a key role in the acquisition and expression of active instrumental defensive behaviors, but little is known about the specific roles of amygdala nuclei. Using a Sidman active avoidance (AA) task, we examined the necessity of LA, B, and CE for learning and performance. Pavlovian freezing was simultaneously assessed to examine the contributions of amygdala nuclei to the transition from reactive to active defensive responding. METHODS: Rats received electrolytic lesions of LA, CE, or B before AA training, or following overtraining. Rats that expressed low levels of AA performance during training received bilateral electrolytic lesions to CE to eliminate competing freezing reactions and rescue AA. AA performance and freezing were assessed. RESULTS: Damage to LA and B, but not CE, impaired the acquisition of AA. Performance of AA became amygdala-independent following overtraining. CE lesions abolished Pavlovian freezing and rescued instrumental AA performance in rats that expressed low levels of avoidance responses and high levels of freezing during training. CONCLUSIONS: Although the acquisition of Pavlovian fear depends on LA and CE, but not B, acquisition of instrumental AA is dependent on LA and B, but not CE. CE-dependent Pavlovian processes that control freezing can constrain avoidance behavior. Performance of well-trained AA becomes independent of all three amygdala nuclei. Thus, it appears that different output pathways of LA mediate reactive and active conditioned defensive responding

Fear learning is associated with changes in synapse strength in the lateral amygdala (LA). To examine changes in LA dendritic spine structure with learning, we used serial electron microscopy to re-construct dendrites after either fear or safety conditioning. The spine apparatus, a smooth endoplasmic reticulum (sER) specialization found in very large spines, appeared more frequently after fear conditioning. Fear conditioning was associated with larger synapses on spines that did not contain a spine apparatus, whereas safety conditioning resulted in smaller synapses on these spines. Synapses on spines with a spine apparatus were smaller after safety conditioning but unchanged with fear conditioning, suggesting a ceiling effect. There were more polyribosomes and multivesicular bodies throughout the dendrites from fear conditioned rats, indicating increases in both protein synthesis and degradation. Polyribosomes were associated with the spine apparatus under both training conditions. We conclude that LA synapse size changes bidirectionally with learning and that the spine apparatus has a central role in regulating synapse size and local translation

Using a two-way signaled active avoidance (2-AA) learning procedure, where rats were trained in a shuttle box to avoid a footshock signaled by an auditory stimulus, we tested the contributions of the lateral (LA), basal (B), and central (CE) nuclei of the amygdala to the expression of instrumental active avoidance conditioned responses (CRs). Discrete or combined lesions of the LA and B, performed after the rats had reached an asymptotic level of avoidance performance, produced deficits in the CR, whereas CE lesions had minimal effect. Fiber-sparing excitotoxic lesions of the LA/B produced by infusions of N-methyl-d-aspartate (NMDA) also impaired avoidance performance, confirming that neurons in the LA/B are involved in mediating avoidance CRs. In a final series of experiments, bilateral electrolytic lesions of the CE were performed on a subgroup of animals that failed to acquire the avoidance CR after 3 d of training. CE lesions led to an immediate rescue of avoidance learning, suggesting that activity in CE was inhibiting the instrumental CR. Taken together, these results indicate that the LA and B are essential for the performance of a 2-AA response. The CE is not required, and may in fact constrain the instrumental avoidance response by mediating the generation of competing Pavlovian responses, such as freezing

Converging lines of evidence suggest that synaptic plasticity at auditory inputs to the lateral amygdala (LA) is critical for the formation and storage of auditory fear memories. Auditory information reaches the LA from both thalamic and cortical areas, raising the question of whether they make distinct contributions to fear memory storage. Here we address this by comparing the induction of long-term potentation (LTP) at the two inputs in vivo in anesthetized rats. We first show, using field potential measurements, that different patterns and frequencies of high-frequency stimulation (HFS) consistently elicit stronger LTP at cortical inputs than at thalamic inputs. Field potential responses elicited during HFS of thalamic inputs were also smaller than responses during HFS of cortical inputs, suggesting less effective postsynaptic depolarization. Pronounced differences in the short-term plasticity profiles of the two inputs were also observed: whereas cortical inputs displayed paired-pulse facilitation, thalamic inputs displayed paired-pulse depression. These differences in short- and long-term plasticity were not due to stronger inhibition at thalamic inputs: although removal of inhibition enhanced responses to HFS, it did not enhance thalamic LTP and left paired-pulse depression unaffected. These results highlight the divergent nature of short- and long-term plasticity at thalamic and cortical sensory inputs to the LA, pointing to their different roles in the fear learning system