To establish the neurotransmitter role(s) of L-3,4-dihydroxyphenylalanine (DOPA) 
in its own right, we attempted to clarify whether i.p. injection of a DOPA 
antagonist, DOPA cyclohexyl ester (CHE), would antagonize the behavioral 
responses of conscious rats to DOPA in the presence of 3-hydroxybenzylhydrazine 
(NSD-1015) (100 mg/kg i.p.), a central aromatic L-amino acid decarboxylase 
(AADC) inhibitor. DOPA-CHE (40, 60 and 100 mg/kg) elicited a dose-dependent 
partial antagonism against the increase in locomotor activity induced by DOPA 
(100 mg/kg i.p.). A low dose of DOPA-CHE (10 mg/kg) elicited full antagonism 
against the potentiating effect of a non-effective dose of DOPA (20 mg/kg) on 
the increase in locomotor activity induced by a dopamine D(2) agonist quinpirole 
(0.3 mg/kg s.c.). DOPA-CHE (100 mg/kg) elicited full antagonism against licking 
behavior induced by DOPA (100 mg/kg). We confirmed that DOPA (100 mg/kg) 
increased the striatal dopamine content but elicited no effect on locomotor 
activity in the presence of benserazide (50 mg/kg i.p.), a peripheral AADC 
inhibitor. DOPA also increased the dopamine content in the presence of NSD-1015 
to a maximal degree similar to that in the presence of benserazide. Thus, we 
conclude that DOPA-CHE is a suitable DOPA antagonist that would be available 
under in vivo experimental conditions. DOPA plays a role in the neuromodulation 
of behavior.