Regulation of gephyrin and GABAA receptor binding within the amygdala after fear acquisition and extinction

Genes and neurons: molecular insights to fear and anxiety

Experimental animal models provide an important tool for the identification of inheritable components of fear and anxiety. 'Pavlovian' fear conditioning has been tremendously successful to characterize the neuronal circuitry and cellular mechanisms of the formation, consolidation and extinction of fear memories. Here we summarize recent progress that has led to the identification of gene products contributing to such experience-dependent changes in fear and anxiety and may guide the search for genetic factors involved in the development and treatment of human anxiety disorders.

Dynamic amino acid increases in the basolateral amygdala during acquisition and expression of conditioned fear

Glutamate and gamma-aminobutyric acid (GABA) release in the amygdala are thought to be crucial for the acquisition and expression of fear memories, but the time course of amino acid changes during conditioning is unknown. We used rapid-sampling microdialysis with 14 s temporal resolution to address this issue. During auditory fear conditioning, large, rapid and transient increases in glutamate and GABA were detected, but only during the first noise-shock pairing. In contrast, rats receiving unsignaled shocks during contextual fear conditioning showed no changes in GABA and less glutamate release for the initial shock, but increased glutamate release during later shocks. Expression of conditioned fear to either a white noise or the context previously paired with shock produced similar rapid and transient increases in many amino acids in the amygdala. These experiments demonstrate glutamate and GABA levels in the amygdala are differentially modulated during auditory and contextual fear learning, and are transiently increased during the expression of fear memories.

Pharmacological treatments that facilitate extinction of fear: relevance to psychotherapy

SUMMARY

A great deal is now known about the mechanisms of conditioned fear acquisition and expression. More recently, the mechanisms of inhibition of conditioned fear have become the subject of intensive study. The major model system for the study of fear inhibition in the laboratory is extinction, in which a previously fear conditioned organism is exposed repeatedly to the fear-eliciting cue in the absence of any aversive event and the fear conditioned response declines. It is well established that extinction is a form of new learning as opposed to forgetting or "unlearning" of conditioned fear, and it is hypothesized that extinction develops when sensory pathways conveying sensory information to the amygdala come to engage GABAergic interneurons through forms of experience-dependent plasticity such as long-term potentiation. Several laboratories currently are investigating methods of facilitating fear extinction in animals with the hope that such treatments might ultimately prove to be useful in facilitating exposure-based therapy for anxiety disorders in clinical populations. This review discusses the advances that have been made in this field and presents the findings of the first major clinical study to examine the therapeutic utility of a drug that facilitates extinction in animals. It is concluded that extinction is an excellent model system for the study of fear inhibition and an indispensable tool for the screening of putative pharmacotherapies for clinical use.

Ethanol potentiates GABAergic synaptic transmission in a postsynaptic neuron/synaptic bouton preparation from basolateral amygdala

Interactions between ethanol and synaptic transmission mediated by gamma -amino-N-butyric acid (GABA) have been suggested to contribute to alcohol intoxication. Ethanol effects on postsynaptic GABAA receptors have been the major focus of this line of research. There is increasing evidence that ethanol potentiation of GABAergic transmission involves increased GABA release from presynaptic terminals. In the present study, a mechanically isolated neuron/bouton preparation from the basolateral amygdala was used to examine the effects of ethanol on spontaneous GABAergic synaptic currents elicited by GABA release from the presynaptic terminals. We found that ethanol application produced a rapid increase in the frequency of spontaneous GABAergic synaptic currents. An acute tolerance to ethanol was also observed, and this tolerance involved GABAB receptor activation. The ethanol-induced potentiation did not involve alterations in the function of postsynaptic GABAA receptors and was independent of presynaptic action potential firing. These findings indicate that ethanol potentiates GABA release, most likely via a direct action on presynaptic boutons.

Lesions of the basal amygdala block expression of conditioned fear but not extinction

Although the role of the amygdala in acquisition of conditioned fear is well established, there is debate concerning the intra-amygdala circuits involved. The lateral nucleus of the amygdala (LA) is thought to be an essential site of plasticity in fear conditioning. The LA has both direct and indirect [via the basal nuclei; basal amygdala (BA)] projections to the central nucleus (Ce) of the amygdala, an essential output for fear behaviors. Lesions of the LA or Ce prevent acquisition of conditioned freezing to a conditioned stimulus, but BA lesions do not, suggesting that the BA is not normally involved in fear conditioning. If true, posttraining BA lesions should also have no effect. Replicating previous studies, we found that rats given electrolytic BA lesions before training acquired conditioned fear normally. They also showed normal long-term retention and extinction of conditioned fear. Unexpectedly, BA lesions made after training completely blocked expression of conditioned fear. Despite this deficit, lesioned rats were able to learn a new tone-shock association. Thus, although the LA-Ce system is sufficient for fear acquisition in the absence of the BA, it is not sufficient when the BA is present, suggesting that the BA is an important site of plasticity in fear conditioning. The pattern of lesion deficits we observed (after but not before training) might be explained by homeostatic mechanisms that balance plasticity over multiple inputs, regulating the influence of the BA and LA onto Ce output neurons.

Different mechanisms of fear extinction dependent on length of time since fear acquisition

Fear extinction is defined as a decline in conditioned fear responses (CRs) following nonreinforced exposure to a feared conditioned stimulus (CS). Behavioral evidence indicates that extinction is a form of inhibitory learning: Extinguished fear responses reappear with the passage of time (spontaneous recovery), a shift of context (renewal), and unsignaled presentations of the unconditioned stimulus (reinstatement). However, there also is evidence to suggest that extinction is an "unlearning" process corresponding to depotentiation of potentiated synapses within the amygdala. Because depotentiation is induced more readily at short intervals following LTP induction and is not inducible at all at a sufficient delay, it may be that extinction initiated shortly following fear acquisition preferentially engages depotentiation/"unlearning," whereas extinction initiated at longer delays recruits a different mechanism. We investigated this possibility through a series of behavioral experiments examining the recoverability of conditioned fear following extinction. Consistent with an inhibitory learning mechanism of extinction, rats extinguished 24-72 h following acquisition exhibited moderate to strong reinstatement, renewal, and spontaneous recovery. In contrast, and consistent with an erasure mechanism, rats extinguished 10 min to 1 h after acquisition exhibited little or no reinstatement, renewal, or spontaneous recovery. These data support a model in which different neural mechanisms are recruited depending on the temporal delay of fear extinction.

Enhancement of conditioned fear extinction by infusion of the GABAAagonist muscimol into the rat prefrontal cortex and amygdala

In auditory fear conditioning, repeated presentation of the tone in the absence of the shock leads to extinction of the acquired fear response. Both the infra limbic prefrontal cortex (IL) and the basolateral amygdala (BLA) are involved in extinction. In this study, we examine the involvement of these two regions in extinction by manipulating the gamma-aminobutyric acid (GABA)ergic system, in the Sprague-Dawley rat. We microinfused a low dose of the GABA(A) agonist muscimol into the IL or BLA. Muscimol infused to IL before extinction training, but not after either a short (five-trials) or long (15-trials) extinction training, resulted in long-term facilitation of extinction. Infusion of muscimol to the BLA following a short (five-trial) extinction session facilitated extinction at least 48-h post-drug infusion. The differences in the temporal parameters of the effects of muscimol in the IL or BLA, suggest differential involvement of these structures in long-term extinction of fear memory. We propose a facilitating role for GABA(A) neurotransmission in the IL in triggering the onset of fear extinction and its maintenance, whereas in the BLA, GABA(A) neurotransmission facilitates extinction consolidation. The involvement of GABA(A) receptors in fear extinction in the prefrontal cortex and amygdala is of particular interest, because of the role of these areas in emotional processes, and the role of the GABA(A) receptors in anxiety states.

Fear extinction in rodents: basic insight to clinical promise

Fear extinction, the reduction of fear by repeated exposure to the object of fear, is a crucial paradigm of inhibitory learning and the acknowledged preclinical model for behavior therapy of human anxiety. Recent insights have clarified roles for infralimbic prefrontal cortex, hippocampus and periaqueductal gray in extinction learning, while maintaining a central role for the basolateral amygdaloid nucleus in the acquisition and storage of this learning. Simultaneously, molecular insights have implicated several neurotransmitter and second messenger systems in extinction learning, and revealed that extinction is surprisingly easy to improve, yielding the promise of a novel approach to improved psychiatric treatments for a variety of human anxiety disorders.

Reinstatement of extinguished fear by β-adrenergic arousal elicited by a conditioned context

Five experiments examined the reinstatement of fear (freezing) produced by recent reexposure to a dangerous context. Rats were trained to fear a conditioned stimulus (CS) and a distinctive context with shock. The CS was then extinguished. A 2-min interval between reexposure to the dangerous context and presentation of the extinguished CS in a different context reinstated freezing when the CS was tested the next day. Propranolol (a beta-adrenergic antagonist) blocked reinstatement of extinguished fear without decreasing freezing to a nonextinguished CS. Administration of epinephrine (an adrenergic agonist) reinstated extinguished fear without reexposure to the dangerous context. The results suggest a role for beta-adrenergic activity elicited by exposure to a conditioned context in the reinstatement of extinguished fear.