The central aromatic amino acid DOPA decarboxylase inhibitor, NSD‐1015, does not inhibit l‐DOPA‐induced circling in unilateral 6‐OHDA‐lesioned‐rats

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.