Faculty Bibliography

Norepinephrine (NE) is thought to play a key role in fear and anxiety, but its role in amygdala-dependent Pavlovian fear conditioning, a major model for understanding the neural basis of fear, is poorly understood. The lateral nucleus of the amygdala (LA) is a critical brain region for fear learning and regulating the effects of stress on memory. To understand better the cellular mechanisms of NE and its adrenergic receptors in the LA, we used antibodies directed against dopamine beta-hydroxylase (DbetaH), the synthetic enzyme for NE, or against two different isoforms of the beta-adrenergic receptors (betaARs), one that predominately recognizes neurons (betaAR 248) and the other astrocytes (betaAR 404), to characterize the microenvironments of DbetaH and betaAR. By electron microscopy, most DbetaH terminals did not make synapses, but when they did, they formed both asymmetric and symmetric synapses. By light microscopy, betaARs were present in both neurons and astrocytes. Confocal microscopy revealed that both excitatory and inhibitory neurons express betaAR248. By electron microscopy, betaAR 248 was present in neuronal cell bodies, dendritic shafts and spines, and some axon terminals and astrocytes. When in dendrites and spines, betaAR 248 was frequently concentrated along plasma membranes and at post-synaptic densities of asymmetric (excitatory) synapses. betaAR 404 was expressed predominately in astrocytic cell bodies and processes. These astrocytic processes were frequently interposed between unlabeled terminals or ensheathed asymmetric synapses. Our findings provide a morphological basis for understanding ways in which NE may modulate transmission by acting via synaptic or non-synaptic mechanisms in the LA

Activation of GABA(A)Rs in the lateral nucleus of the amygdala (LA), a key site of plasticity underlying fear learning, impairs fear learning. The role of GABA(C)Rs in the LA and other brain areas is poorly understood. GABA(C)Rs could be an important novel target for pharmacological treatments of anxiety-related disorders since, unlike GABA(A)Rs, GABA(C)Rs do not desensitize. To detect functional GABA(C)Rs in the LA we performed whole cell patch clamp recordings in vitro. We found that GABA(A)Rs and GABA(B)Rs blockade lead to a reduction of evoked inhibition and an increase increment of excitation, but activation of GABA(C)Rs caused elevations of evoked excitation, while blocking GABA(C)Rs reduced evoked excitation. Based on this evidence we tested whether GABA(C)Rs in LA contribute to fear learning in vivo. It is established that activation of GABA(A)Rs leads to blockage of fear learning. Application of GABA(C) drugs had a very different effect; fear learning was enhanced by activating and attenuated by blocking GABA(C)Rs in the LA. Our results suggest that GABA(C) and GABA(A)Rs play opposing roles in modulation of associative plasticity in LA neurons of rats. This novel role of GABA(C)Rs furthers our understanding of GABA receptors in fear memory acquisition and storage and suggests a possible novel target for the treatment of fear and anxiety disorders

The lateral nucleus of the amygdala (LA) has been implicated in the formation of long-term associative memory (LTM) of stimuli associated with danger through fear conditioning. The current study aims to detect genes that are expressed in LA following associative fear conditioning. Using oligonucleotide microarrays, we monitored gene expression in rats subjected to paired training where a tone co-terminates with a footshock, or unpaired training where the tone and footshock are presented in a non-overlapping manner. The paired protocol consistently leads to auditory fear conditioning memory formation, whereas the unpaired protocol does not. When the paired group was compared with the unpaired group 5 h after training, the expression of genes coding for the limbic system-associated membrane protein (Lsamp), kinesin heavy chain member 2 (Kif2), N-ethylmaleimide-sensitive fusion protein (NSF) and Hippocalcin-like 4 protein (Hpcal4) was higher in the paired group. These genes encode proteins that regulate neuronal axonal morphology (Lsamp, Kif2), presynaptic vesicle cycling and release (Hpcal4 and NSF), and AMPA receptor maintenance in synapses (NSF). Quantitative real-time PCR (qPCR) showed that Kif2 and Lsamp are expressed hours following fear conditioning but minutes after unpaired training. Hpcal4 is induced by paired stimulation only 5 h after the training. These results show that fear conditioning induces a unique temporal activation of molecular pathways involved in regulating synaptic transmission and axonal morphology in LA, which is different from non-associative stimulation

The study of memory in most behavioral paradigms, including emotional memory paradigms, has focused on the feed forward components that underlie Hebb's first postulate, associative synaptic plasticity. Hebb's second postulate argues that activated ensembles of neurons reverberate in order to provide temporal coordination of different neural signals, and thereby facilitate coincidence detection. Recent evidence from our groups has suggested that the lateral amygdala (LA) contains recurrent microcircuits and that these may reverberate. Additionally this reverberant activity is precisely timed with latencies that would facilitate coincidence detection between cortical and sub cortical afferents to the LA. Thus, recent data at the microcircuit level in the amygdala provide some physiological evidence in support of the second Hebbian postulate

The development and exacerbation of many psychiatric and neurologic conditions are associated with dysregulation of the hypothalamic pituitary adrenal (HPA) axis as measured by aberrant levels of cortisol secretion. Here we report on the relationship between the amplitude of diurnal cortisol secretion, measured across 3 typical days in 18 healthy individuals, and blood oxygen level dependant (BOLD) response in limbic fear/stress circuits, elicited by in-scanner presentation of emotionally negative stimuli, specifically, images of the World Trade Center (WTC) attack. Results indicate that subjects who secrete a greater amplitude of cortisol diurnally demonstrate less brain activation in limbic regions, including the amygdala and hippocampus/parahippocampus, and hypothalamus during exposure to traumatic WTC-related images. Such initial findings can begin to link our understanding, in humans, of the relationship between the diurnal amplitude of a hormone integral to the stress response, and those neuroanatomical regions that are implicated as both modulating and being modulated by that response

Dysregulation of the fear system is at the core of many psychiatric disorders. Much progress has been made in uncovering the neural basis of fear learning through studies in which associative emotional memories are formed by pairing an initially neutral stimulus (conditioned stimulus, CS; e.g., a tone) to an unconditioned stimulus (US; e.g., a shock). Despite recent advances, the question of how to persistently weaken aversive CS-US associations, or dampen traumatic memories in pathological cases, remains a major dilemma. Two paradigms (blockade of reconsolidation and extinction) have been used in the laboratory to reduce acquired fear. Unfortunately, their clinical efficacy is limited: Reconsolidation blockade typically requires potentially toxic drugs, and extinction is not permanent. Here, we describe a behavioral design in which a fear memory in rats is destabilized and reinterpreted as safe by presenting an isolated retrieval trial before an extinction session. This procedure permanently attenuates the fear memory without the use of drugs

Frontolimbic structures involved in fear conditioning have also been associated with hypothalamic-pituitary-adrenal (HPA)-axis modulation, including amygdaloid, hippocampal, and ventromedial prefrontal cortex regions. Although HPA-axis function and endocrine changes have been investigated in the context of stress provocation, much research has not been conducted using functional neuroimaging in the study of the HPA axis and frontolimbic function in response to emotional stimuli. Using functional magnetic resonance imaging, the association of blood-oxygen-level dependent signal with salivary cortisol in response to an emotional visual scene paradigm was investigated, with prescan and postscan salivary cortisol analyzed as a covariate of interest during specific conditions. Cortisol reactivity to the paradigm was positively associated with amygdalar and hippocampal activity and negatively associated with ventromedial prefrontal cortex activity in conditions involving emotional imagery