Synthesis of catecholamines from [3H]tyrosine in brain during the performance of operant behavior.

Changes in the synthesis rates of dopamine (DA) and norepinephrine (NE) in brain were found to accompany the performance of positively reinforced operant behavior in rats. Catecholamine synthesis rates were estimated at short time intervals after the intraventricular injection of [3H]tyrosine with use of a conversion index which expressed the levels of [3H]DA or [3H]NE accumulated in various brain regions as a function of the specific activity of [3H]tyrosine. In rats lever pressing on a fixed ratio 5 schedule of water reinforcement, the DA conversion index in the caudate putamen was 66% higher than in control rats but was not different from control values in the mesolimbic area and hypothalamus. The NE conversion index in operant-performing animals was 48% higher in the hypothalamus than in controls but was unchanged in the mesolimbic area, telencephalon and brainstem. The changes in the CA conversion indices seen during operant performance were associated with increases in the brain levels of [3H]DA or [3H]NE, but were not accompanied by alterations in either the specific activity of [3H]tyrosine or the endogenous levels of catecholamines. The relative increment in the DA conversion index seen in the caudate putamen of operant-performing rats was directly proportional to the number of lever presses emitted following [3H]tyrosine administration. These findings indicate that the performance of operant behavior is associated with increases in rate of catecholamine synthesis within select populations of central neurons.