What is some of the biochemistry behind anxiety? by Connie b. Dellobuono

Answer by Connie b. Dellobuono:

From Wiki:
Neural circuitry involving the amygdala (which regulates emotions like anxiety and fear, stimulating the HPA Axis and sympathetic nervous system) and hippocampus (which is implicated in emotional memory along with the amygdala) is thought to underlie anxiety.[45] People who suffer from anxiety tend to show high activity in response to emotional stimuli in the amygdala.[46] Some writers believe that excessive anxiety can lead to an overpotentiation of the limbic system (which includes the amygdala and nucleus accumbens), giving increased future anxiety, but this does not appear to have been proven.[47][48]
Research upon adolescents who as infants had been highly apprehensive, vigilant, and fearful finds that their nucleus accumbens is more sensitive than that in other people when deciding to make an action that determined whether they received a reward.[49] This suggests a link between circuits responsible for fear and also reward in anxious people. As researchers note, "a sense of 'responsibility', or self agency, in a context of uncertainty (probabilistic outcomes) drives the neural system underlying appetitive motivation (i.e., nucleus accumbens) more strongly in temperamentally inhibited than noninhibited adolescents".[49]
From http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4309892/
Depression, anxiety disorders, and metabolic syndrome in a population at risk for type 2 diabetes mellitus. Study data showed that in a high-risk group for type 2 diabetes mellitus (T2DM) , there is association between depressive disorders and metabolic syndrome (MetS), pointing to a similar role of anxiety disorders. Screening for anxiety and depression is recommended in this group at risk for T2DM.
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What is some of the biochemistry behind anxiety?