Impairment of neuroplasticity and altered levels of neurotrophins are involved in bipolar disorder. NGF has been found to be decreased overall in people with bipolar disorder. More specifically, while in a manic state NGF is especially low. This leads to elevated or irritable mood with increased energy and decreased need for sleep while in a manic state. This decreased NGF may serve as a biological marker when assessing a person's present bipolar disorder state. When treated with lithium, their NGF concentrations increased in the frontal cortex, limbic forebrain, hippocampus, and amygdala.