Unilateral activation of caudate tyrosine hydroxylase during voluntary circling behavior

We trained rats to circle for a sucrose water reward and found that this behavior is associated with a unilateral increase in the activity of caudate tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis. The increase in tyrosine hydroxylase activity occurs in caudate contralateral to the circling direction and the change is transient, increasing during the first 20 min of circling but then plateauing and falling as turning slows. Enhanced synthetic capacity is followed by increases in the contents of dopamine and dihydroxyphenylacetic acid in the contralateral caudate nucleus. These observations are the first evidence for specific activation of a neurotransmitter synthetic enzyme during voluntary motor behavior.