The injection leads to the release of prolactin, a stress hormone that can be measured in the blood; higher levels of prolactin indicate greater serotonin activity.

Carbohydrate craving may be related to decreased serotonin activity, although experts are not sure if there is a link.

Independent studies have consistently found unusual dopamine and serotonin activity in various regions of the brain in individuals with OCD.

Serotonin is hypothesized to regulate other neurotransmitter systems; decreased serotonin activity may allow these systems to act in unusual and erratic ways.

That blockage seems to restore overall serotonin activity to a more normal state, which somehow helps alleviate many cases of depression.

The excess serotonin activity produces a spectrum of specific symptoms including cognitive, autonomic, and somatic effects.

Other studies of the 5HT serotonin receptor (linked to regulation of feeding, mood, and anxiety), suggest that serotonin activity is decreased at these sites.

Subjective tinnitus is also a side effect of some medications, such as aspirin, and may also result from an abnormally low level of serotonin activity.

Harm Avoidance (HA) - degree to which people are anxious, correlated with high serotonin activity.

Carb cravings seem to be related to decreases in serotonin activity, says Wurtman.