The effects of the peripheral aromatic amino acid decarboxylase (AADC) inhibitors, carbidopa and benserazide, and the central AADC inhibitor, 3-hydroxybenzylhydrazine (NSD-1015) on peripheral and brain monoamine oxidase (MAO) A and B activity were investigated in the rat. In vitro, carbidopa, benserazide and NSD-1015 all potently inhibited hepatic MAO A and B activity (IC(50) 10-50 micro M). In ex vivo studies following systemic drug administration, NSD-1015 (100 mg/kg ip) produced 88% and 96% inhibition of hepatic and striatal MAO A and B activity respectively. Carbidopa (12.5 mg/kg i.p.) and benserazide (50 mg/kg i.p.) had no effect on striatal MAO A activity or hepatic MAO B activity. However, they inhibited striatal MAO B activity by 45 +/- 10% and 36 +/- 10% respectively. In conclusion, carbidopa and benserazide may not only protect L-DOPA from peripheral decarboxylation, but also increase striatal dopamine content through MAO inhibition. NSD-1015 should not be used to investigate the neuromodulatory role of L-DOPA as it potently inhibits rat striatal MAO.

1 Endogenous L-DOPA may act as a neuromodulator contributing to the production of motor activity. We now investigate the e ects of the centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor NSD-1015 (3-hydroxybenzyl hydrazine) on the motor actions of L-DOPA and dopamine agonist drugs in MPTP treated common marmosets. 5 These results do not support a role for endogenous L-DOPA in spontaneous or drug induced locomotor activity. Rather, they strengthen the argument for the importance of endogenous dopaminergic tone in the motor actions of dopamine agonists.

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The centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor, 3-hydroxybenzyl hydrazine (NSD-1015), is widely used to study the neurotransmitter-like actions of L-DOPA. However, the effects of NSD-1015 on L-DOPA-induced motor activity are unclear as both increases and decreases have been reported. We now investigate the effects of NSD-1015 on L-DOPA-induced contralateral circling behaviour in 6-OHDA-lesioned rats and on striatal levels of L-DOPA, 3-O-methyl-DOPA (3-OMD), dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) using microdialysis techniques. NSD-1015 (50-200 mg/kg i.p.) inhibited AADC activity both in the liver and striatum of normal rats. Administration of NSD-1015 (50-200 mg/kg i.p.), delayed the onset of circling produced by administration of L-DOPA (25 mg/kg i.p.) and carbidopa (12.5 mg/kg i. p.), suggesting blockade of central AADC activity. However, the duration of the L-DOPA-induced circling was prolonged and overall no inhibition of circling behaviour occurred. L-DOPA (25 mg/kg i.p.) plus carbidopa (12.5 mg/kg i.p.) increased extracellular levels of L-DOPA, 3-OMD, dopamine, DOPAC and HVA in the 6-OHDA-lesioned striatum. Pretreatment of rats with the central AADC inhibitor, NSD-1015 (100 mg/kg i.p.), potentiated the increase in dialysate levels of L-DOPA and 3-OMD. However, it did not reduce striatal dopamine levels in the 6-OHDA-lesioned hemisphere, which were elevated following L-DOPA administration. The increases in DOPAC and HVA levels were abolished by NSD-1015 pretreatment. These results suggest that, while NSD-1015 blocks central AADC activity, it also acts as a monoamine oxidase inhibitor so maintaining striatal dopamine concentration by reducing dopamine metabolism. NSD-1015, therefore, may not be an appropriate tool for the study of brain AADC activity and for assessing the neuromodulatory role of L-DOPA.