Susan Sangha

Neuropeptide S-Mediated Control of Fear Expression and Extinction: Role of Intercalated GABAergic Neurons in the Amygdala

A deficient extinction of memory is particularly important in the regime of fear, where it limits the beneficial outcomes of treatments of anxiety disorders. Fear extinction is thought to involve inhibitory influences of the prefrontal cortex on the amygdala, although the detailed synaptic mechanisms remain unknown. Here, we report that neuropeptide S (NPS), a recently discovered transmitter of ascending brainstem neurons, evokes anxiolytic effects and facilitates extinction of conditioned fear responses when administered into the amygdala in mice. An NPS receptor antagonist exerts functionally opposing responses, indicating that endogenous NPS is involved in anxiety behavior and extinction. Cellularly, NPS increases glutamatergic transmission to intercalated GABAergic neurons in the amygdala via presynaptic NPS receptors on connected principal neurons. These results identify mechanisms of NPS in the brain, a key role of intercalated neurons in the amygdala for fear extinction, and a potential pharmacological avenue for treating anxiety disorders.

Patterns of Coupled Theta Activity in Amygdala-Hippocampal-Prefrontal Cortical Circuits during Fear Extinction

Signals related to fear memory and extinction are processed within brain pathways involving the lateral amygdala (LA) for formation of aversive stimulus associations, the CA1 area of the hippocampus for context-dependent modulation of these associations, and the infralimbic region of the medial prefrontal cortex (mPFC) for extinction processes. While many studies have addressed the contribution of each of these modules individually, little is known about their interactions and how they function as an integrated system. Here we show, by combining multiple site local field potential (LFP) and unit recordings in freely behaving mice in a fear conditioning paradigm, that theta oscillations may provide a means for temporally and functionally connecting these modules. Theta oscillations occurred with high specificity in the CA1-LA-mPFC network. Theta coupling increased between all areas during retrieval of conditioned fear, and declined during extinction learning. During extinction recall, theta coupling partly rebounded in LA-mPFC and CA1-mPFC, and remained at a low level in CA1-LA. Interfering with theta coupling through local electrical microstimulation in CA1-LA affected conditioned fear and extinction recall depending on theta phase. These results support the hypothesis that theta coupling provides a means for inter-areal coordination in conditioned behavioral responsiveness. More specifically, theta oscillations seem to contribute to a population code indicating conditioned stimuli during recall of fear memory before and after extinction.

Intermediate and long-term memories of associative learning are differentially affected by transcription Versus translation blockers in Lymnaea

SUMMARY Aerial respiratory behaviour in the pond snail, Lymnaea stagnalis, can be operantly conditioned. This associative learning then undergoes consolidation into a long-lasting memory which, depending on the training procedure used, causes intermediate-term memory (ITM; lasting 3 h) or long-term memory (LTM; lasting >6 h) to be formed. We determined the differential susceptibility of these two forms of memory to translation and transcription blockers. The injection of a translation blocker, Anisomycin, 2.5 h before training prevents the establishment of both ITM and LTM. On the other hand, injection of the transcription blocker Actinomycin D, 2.5 h before training, did not prevent the establishment of ITM, but did, however, prevent LTM formation. Thus in Lymnaea, following associative learning, both ITM and LTM are dependent on new protein synthesis. ITM appears to be dependent on protein synthesis from preexisting transcription factors, whilst LTM is dependent on protein synthesis from new transcription messages.

Critical role of the 65-kDa isoform of glutamic acid decarboxylase in consolidation and generalization of Pavlovian fear memory

Evidence suggests that plasticity of the amygdalar and hippocampal GABAergic system is critical for fear memory formation. In this study we investigated in wild-type and genetically manipulated mice the role of the activity-dependent 65-kDa isozyme of glutamic acid decarboxylase (GAD65) in the consolidation and generalization of conditioned fear. First, we demonstrate a transient reduction of GAD65 gene expression in the dorsal hippocampus (6 h post training) and in the basolateral complex of the amygdala (24 h post training) during distinct phases of fear memory consolidation. Second, we show that targeted ablation of the GAD65 gene in Gad65(-/-) mice results in a pronounced context-independent, intramodal generalization of auditory fear memory during long-term (24 h or 14 d) but not short-term (30 min) memory retrieval. The temporal specificity of both gene regulation and memory deficits in Gad65 mutant mice suggests that GAD65-mediated GABA synthesis is critical for the consolidation of stimulus-specific fear memory. This function appears to involve a modulation of neural activity patterns in the amygdalo-hippocampal pathway as indicated by a reduction in theta frequency synchronization between the amygdala and hippocampus of Gad65(-/-) mice during the expression of generalized fear memory.

Deficiency of the 65 kDa Isoform of Glutamic Acid Decarboxylase Impairs Extinction of Cued But Not Contextual Fear Memory

Extinction procedures are clinically relevant for reducing pathological fear, and the mechanisms of fear regulation are a subject of intense research. The amygdala, hippocampus, and prefrontal cortex (PFC) have all been suggested to be key brain areas in extinction of conditioned fear. GABA has particularly been implicated in extinction learning, and the 65 kDa isoform of glutamic acid decarboxylase (GAD65) may be important in elevating GABA levels in response to environmental signals. Extinction of conditioned fear was examined in Gad65−/− mice while recording local field potentials from the amygdala, hippocampus, and PFC simultaneously while monitoring behavior. Gad65−/− mice showed generalization of cued fear, as reported previously, and impaired extinction of cued fear, such that fear remained high across extinction training. This endurance in cued fear was associated with theta frequency synchronization between the amygdala and hippocampus. Extinction of contextual fear, however, was unaltered in Gad65−/− mice when compared with wild-type littermates. The data imply that GAD65 plays a critical role in regulating cued fear responses during extinction learning and that, during this process, GABAergic signaling is involved in modulating synchronized activity between the amygdala and hippocampus. In view of the more pronounced effect on cued versus contextual fear extinction, these influences may rely more on GABAergic mechanisms in the amygdala.

Inhibition of Fear by Learned Safety Signals: A Mini-Symposium Review

Safety signals are learned cues that predict the nonoccurrence of an aversive event. As such, safety signals are potent inhibitors of fear and stress responses. Investigations of safety signal learning have increased over the last few years due in part to the finding that traumatized persons are unable to use safety cues to inhibit fear, making it a clinically relevant phenotype. The goal of this review is to present recent advances relating to the neural and behavioral mechanisms of safety learning, and expression in rodents, nonhuman primates, and humans.

Safety Encoding in the Basal Amygdala

Learning to fear and avoid life-threatening stimuli are critical survival skills but are maladaptive when they persist in the absence of a direct threat. Thus, it is important to detect when a situation is safe and to increase behaviors leading to naturally rewarding actions, such as feeding and mating. It is unclear how the brain distinguishes between dangerous and safe situations. Here, we present a novel protocol designed to investigate the processing of cues that predict danger, safety, or reward (sucrose). In vivo single unit recordings were obtained in the basal amygdala of freely behaving rats undergoing simultaneous reward, fear, and safety conditioning. We observed a population of neurons that did not respond to a Fear Cue but did change their firing rate during the combined presentation of a fear cue simultaneous with a second, safety, cue; this combination of Fear + Safety Cues signified “no shock.” This neural population consisted of two subpopulations: neurons that responded to the Fear + Safety Cue but not the Fear or Reward Cue (“safety” neurons), and neurons that responded to the Fear + Safety and Reward Cue but not the Fear Cue (“safety + reward” neurons). These data demonstrate the presence of neurons in the basal amygdala that are selectively responsive to Safety Cues. Furthermore, these data suggest that safety and reward learning use overlapping mechanisms in the basal amygdala.

Impairing Forgetting by Preventing New Learning and Memory

Two causes of forgetting have been promulgated: memory trace decay and retroactive interference. The authors show that forgetting is an active process requiring both new learning and memory. In the present (1)Lymnaea model system, prevention of new learning of a conflicting association, inhibition of memory consolidation, or Right Pedal Dorsal 1 soma ablation, which blocks LTM formation, are all potent means to prevent forgetting. Thus procedures that alter the ability to learn or form memory of a new conflicting aerial respiratory association prevent forgetting of a learned associative behavior. These results are the 1st demonstration in any model system that forgetting requires the soma of a single neuron.

Theta resynchronization during reconsolidation of remote contextual fear memory.

We have recently demonstrated high theta-phase synchronization between the lateral amygdala and CA1 area of the hippocampus during retrieval of long-term (1 day) fear memory, and not during short-term (2 h) or remote memory retrieval (30 days). These results indicated that the amygdalo-hippocampal interaction reflects a dynamic change of ensemble activities related to various stages of fear memory storage. In this study, we investigated theta activity during the reconsolidation of a remote contextual fear memory by re-exposing animals to the shock context 30 days after training. Consistent with our previous results, high theta synchronization was no longer apparent during re-exposure to the shock context, but was significantly higher 1 day after context re-exposure. These data indicate that the reconsolidation of remote contextual fear memory includes changes in ensemble activities between the lateral amygdala and CA1.

Decreased Sensory Stimulation Reduces Behavioral Responding, Retards Development, and Alters Neuronal Connectivity in Caenorhabditis elegans

Activity-dependent plasticity is a critical component of nervous systems. We show that in Caenorhabditis elegans, worms raised in isolation made smaller responses to mechanosensory stimulation and were smaller and slower to begin laying eggs than age-matched group-raised worms. The glutamate receptor gene GLR-1 was critical for the observed alterations in behavior but not in size, whereas the cGMP-dependent protein kinase gene EGL-4 was critical for the observed changes in size but not the changes in behavior. Mechanosensory stimulation during development reversed the effects of isolation on behavior and began to reduce the effects of isolation on size. In C. elegans, the six mechanosensory touch neurons synapse onto the four pair of command interneurons for forward and backward movement. Touch (mechanosensory) neurons of worms raised in isolation expressed lower levels of green fluorescent protein (GFP)-tagged synaptobrevin than touch neurons of worms raised in colonies. Command interneurons of worms raised in isolation expressed lower levels of GFP-tagged glutamate receptors than command interneurons of worms raised in groups. Brief mechanical stimulation during larval development rescued the expression of GFP-tagged glutamate receptors but not GFP-tagged synaptobrevin. Together, these results indicate that the level of stimulation experienced by C. elegans during development profoundly affects the development of neuronal connectivity and has widespread cellular and behavioral consequences.