Experimental models of anxiety

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In a fMRI activation study in normal volunteers it has been demonstrated that the amygdala is involved in conditioning and extinction of fear responses in a fashion similar to that previously observed in experimental animals (LaBar et al., 1998). There is evidence from studies in normal volunteers that abnormal patterns of limbic activity may result in symptoms resembling both features of temporal lobe complex partial seizures and features of panic attacks. In one study, intravenous injections of procaine resulted in a range of subjective experiences including emotional, somatic and visceral experiences often similar to those experienced in the auras of temporal lobe epilepsy as well as resulting in the development of panic attacks in four out of ten subjects. These experiences also included; euphoria, anxiety, depression, fear and derealization. Positron emission tomography (PET) scanning of the subjects during this experiment revealed that all these experiences, described as 'powerful and overwhelming', were associated with increased activity in anterior limbic and paralimbic regions (Servan-Schreiber et al., 1998). Both these authors and Ketter et al. (1996), using a similar paradigm, noted that procaine-related activation of the left amygdala was positively correlated with symptoms of fear and negatively correlated with feelings of calmness or euphoria. In this context it is interesting to note that it is left temporal more than right temporal lobe epilepsy that is more associated with the development of negative affective states in people with epilepsy. Amygdala activation (in this case bilaterally) has also been observed following cholecystokinin tetrapep-tide-induced anxiety in normal volunteers (Benkelfat et al., 1995). Hence there is good evidence that neurochemical activation of the amygdala and surrounding structures is associated with brief but extreme experiences of panic and anxiety.

Similarly, it is well established that seizure activity may originate in the amygdala in some patients with epilepsy. It has further been reported that seizures originat ing in the region of the amygdala are particularly associated with subjective emotional experiences that resemble aspects of PD.

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