The neurobiology of stereotypy II: The role of stress

Repeated exposure to uncontrollable aversive situations can cause profound, long-term changes in brain organization – effects that Cabib argues could underlie a new, more biologically meaningful definition of ‘stress’. Such changes can occur even in adult animals whose brains are fully developed, and seem to fall into two classes. One type of change, readily seen in C57 mice, for instance, is that stressors come to elicit increasingly minimal dopamine response from the NAc. This seems caused by enhanced dopamine release by the prefrontal cortex (pFC), which inhibits striatal activation. It is also correlated with a decreased density of post-synaptic striatal D2 receptors (receptors which, as we saw in the previous chapter, act to inhibit the indirect pathway and thence enhance direct pathway activity). Behaviourally, this acts to increase learned helplessness (e.g. causes less active struggling and more passive floating in ‘forced swim’ tests); to reduce responsive-ness (including stereotypy) to dopamine agonists like amphetamine; and to re-duce cage stereotypies. The second possible stress-induced change, in contrast, causes stress-induced NAc dopamine responses to become increasingly pro-nounced with repeated exposure, due to a wane in prefrontal inhibitory control and an increase in the density of post-synaptic striatal D2 receptors (thence leading to increased direct pathway output).