In order to study whether the membrane hyperpolarization and firing inhibition 
caused by dopamine and levodopa on rat midbrain dopamine cells are affected by 
the inhibition of brain catechol-O-methyl-transferase (COMT), intracellular 
electrophysiological recordings were made from these neurons maintained in 
vitro. Here we report that a treatment of the cerebral tissue with tolcapone, a 
central and peripheral inhibitor of COMT, does not change the membrane responses 
of midbrain dopamine neurons to dopamine and levodopa. The lack of modification 
of the dopaminergic effects by tolcapone suggests that the pharmacological 
inhibition of intracerebral COMT does not have detectable action on dopamine 
neurotransmission. Therefore, the therapeutic action of tolcapone in Parkinson's 
disease, might be dependent on the reduction of COMT activity in the 
extracerebral tissue.