Fear processing in the brain

Many experiments have been done to find out how the brain interprets stimuli and how animals develop fear responses. The emotion, fear, has been hard-wired into almost every individual, due to its vital role in the survival of the individual. Researchers have found that fear is established unconsciously and that the amygdala is involved with fear conditioning.

By understanding how fear is developed within individuals, it may be possible to treat human mental disorders such as anxiety, phobia, and posttraumatic stress disorder.

In fear conditioning, the main circuits that are involved are the sensory areas that process the conditioned and unconditioned stimuli, certain regions of the amygdala that undergo plasticity (or long-term potentiation) during learning, and the regions that bear an effect on the expression of specific conditioned responses. These pathways converge in the lateral amygdala. Long-term potentiation (LTP) and synaptic plasticity that enhances the response of lateral amygdala neurons to the conditioned stimulus occurs in the lateral amygdala. As a result, the conditioned stimulus is then able to flow from the lateral amygdala to the central nucleus of the amygdala. The basal and intercalated masses of the amygdala connect the lateral amygdala with the central nucleus of the amygdala directly and indirectly. Pathways from central nucleus of the amygdala to downstream areas then control defensive behavior (freezing) and autonomic and endocrine responses. Recent studies implicate the prelimbic cortex in fear expression as well, possibly by way of its connections to the basal and then to the central nucleus of the amygdala.

It has been observed that fear can contribute to behavioral changes. One way this phenomena has been studied is on the basis of the repeated stress model done by Camp RM et al.(among others). In this particular study, it was examined that the contribution fear conditioning may play a huge role in altering an animal's (Fischer rat's) behavior in a repeated stress paradigm. Behavioral changes that are commonly referred to as depressive-like behaviors resulted from this model of testing. After setting a control and a valid experimental design, Fischer rats were exposed daily to different stressors in a complex environment. After four days of stressor exposure, both exploratory behavior and social interaction were tested on day 5 in either the same environment or a new environment. The rats showed much decreased exploration and social interaction when tested in different contexts compared to control rats. To further make a correlation to the biochemistry (as mentioned below), chronic infusion of propranolol (beta-adrenergic receptor antagonist) prevented the behavioral changes following repeated stressor exposure thus halting long term potentiation. Some physiological changes also occurred including the decrease in body weight gain and adrenal hypertrophy observed in animals exposed to stress. Overall, the conditioned fear responses can contribute to behavioral changes in a repeated stress paradigm. This can be extended to correlate to other animals as well but with varying degrees of responses.

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