Fear conditioning

Pavlovian fear conditioning is a behavioral paradigm in which organisms learn to predict aversive events. It is a form of learning in which an aversive stimulus (e.g. an electrical shock) is associated with a particular neutral context (e.g., a room) or neutral stimulus (e.g., a tone), resulting in the expression of fear responses to the originally neutral stimulus or context. This can be done by pairing the neutral stimulus with an aversive stimulus (e.g., an electric shock, loud noise, or unpleasant odor). Eventually, the neutral stimulus alone can elicit the state of fear. In the vocabulary of classical conditioning, the neutral stimulus or context is the "conditional stimulus" (CS), the aversive stimulus is the "unconditional stimulus" (US), and the fear is the "conditional response" (CR).

Fear conditioning has been studied in numerous species, from snails to humans. In humans, conditioned fear is often measured with verbal report and galvanic skin response. In other animals, conditioned fear is often measured with freezing (a period of watchful immobility) or fear potentiated startle (the augmentation of the startle reflex by a fearful stimulus). Changes in heart rate, breathing, and muscle responses via electromyography can also be used to measure conditioned fear. A number of theorists have argued that conditioned fear coincides substantially with the mechanisms, both functional and neural, of clinical anxiety disorders. Research into the acquisition, consolidation and extinction of conditioned fear promises to inform new drug based and psychotherapeutic treatments for an array of pathological conditions such as dissociation, phobias and post-traumatic stress disorder.

Scientists have discovered that there is a set of brain connections that determine how fear memories are stored and recalled. While studying rats' ability to recall fear memories, researchers found a newly identified brain circuit is involved. Initially, the pre-limbic prefrontal cortex (PL) and the basolateral amygdala (BLA) were identified in memory recall. A week later, the central amygdala (CeA) and the paraventricular nucleus of the thalamus (PVT) were identified in memory recall, which are responsible for maintaining fear memories. This study shows how there are shifting circuits between short term recall and long term recall of fear memories. There is no change in behavior or response, only change in where the memory was recalled from.

In addition to the amygdala, the hippocampus and the anterior cingulate cortex are important in fear conditioning. Fear conditioning in the rat is stored at early times in the hippocampus, with alterations in hippocampal gene expression observed at 1 hour and 24 hours after the event. In the mouse, changed gene expression is also seen in the hippocampus at one hour and 24 hours after fear conditioning. These changes are transient in the hippocampal neurons, and almost none are present in the hippocampus after four weeks. By 4 weeks after the event, the memory of the fear conditioning event is more permanently stored in the anterior cingulate cortex.

As shown in the rodent brain, neuronal gene expression is dynamically changed in response to fear conditioning. In particular, the expressions of immediate early genes (IEGs) such as Egr1, c-Fos, and Arc are rapidly and selectively up-regulated in subsets of neurons in specific brain regions associated with learning and memory formation.

A review in 2022 describes multiple steps in up-regulating the IEGs in neurons in the hippocampus during fear conditioning. IEGs are similarly up-regulated in the amygdala during fear conditioning. The multiple steps in up-regulating IEGs include activation of transcription factors, formation of chromatin loops, interaction of enhancers with promoters in chromatin loops and topoisomerase II beta-initiated temporary DNA double-strand breaks.

At least two IEGs up-regulated by fear conditioning, Egr1 and Dnmt3A2 (shown to be an IEG by Oliveira et al.) affect DNA methylation, and thus expression, of many genes. Up-regulated EGR1 proteins associate with pre-existing nuclear TET1 proteins, and the EGR1 proteins bring TET1 proteins to hundreds of genes, allowing TET1 to initiate DNA demethylation of those genes. DNMT3A2 protein is a de novo DNA methyltransferase, adding methylation to cytosines in DNA. Expression of DNMT3A2 proteins in hippocampus neurons in culture preferentially targeted the addition of new methylation to more than 200 genes involved in synaptic plasticity. Expressions of IEGs are a source of the dynamic changes in subsequent neuronal gene expression in response to fear conditioning.

Fear conditioning is thought to depend upon an area of the brain called the amygdala. The amygdala is involved in acquisition, storage, and expression of conditioned fear memory. Lesion studies have revealed that lesions drilled into the amygdala before fear conditioning prevent the acquisition of the conditioned response of fear, and lesions drilled in the amygdala after conditioning cause conditioned responses to be forgotten. Electrophysiological recordings from the amygdala have demonstrated that cells in that region undergo long-term potentiation (LTP), a form of synaptic plasticity believed to underlie learning. Pharmacological studies, synaptic studies, and human studies also implicate the amygdala as chiefly responsible for fear learning and memory. Additionally, inhibition of neurons in the amygdala disrupts fear acquisition, while stimulation of those neurons can drive fear-related behaviors, such as freezing behavior in rodents. This indicates that proper function of the amygdala is both necessary for fear conditioning and sufficient to drive fear behaviors. The amygdala is not exclusively the fear center, but also an area for responding to various environmental stimuli. Several studies have shown that when faced with unpredictable neutral stimuli, amygdala activity increases. Therefore, even in situations of uncertainty and not necessarily fear, the amygdala plays a role in alerting other brain regions that encourage safety and survival responses.