AROMATIC L-AMINO-ACID DECARBOXYLASE IMMUNOREACTIVE CELLS IN THE RAT STRIATUM - A POSSIBLE SITE FOR THE CONVERSION OF EXOGENOUS L-DOPA TO DOPAMINE

The efficacy of L-dihydroxyphenylalanine (L-DOPA) in ameliorating the symptoms of Parkinson's disease (PD) is attributed to its conversion t o dopamine (DA) by the enzyme aromatic L-amino-acid decarboxylase (AAD C) in the striatum. Although the site of this conversion in the DA-den ervated striatum has yet to be identified, it has been proposed that L -DOPA could be converted to DA at non-dopaminergic sites containing AA DC. In the present study, we used immunocytochemical techniques to exa mine the localization of AADC and DA in the striatum of rats with a un ilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopami nergic projection. In the DA-denervated striatum, we observed AADC-imm unoreactive (-IR) cells with morphological characteristics similar to a class of small aspiny interneuron. Although usually obscured by a de nse plexus of AADC-IR fibers, these cells could also occasionally be d etected in the intact striatum. Acute administration of L-DOPA to DA-d enervated animals elicited contralateral rotational behavior as well a s a pronounced c-fos protein immunoreactivity in the striatum ipsilate ral to the lesion. Following acute administration of L-DOPA, but not a fter acute saline, DA-IR cells were detected in the denervated striatu m. These DA-IR cells are similar in morphology and were found in the s ame location as the AADC-IR cells. These results strongly suggest the existence of a class of AADC-containing striatal cells that can form D A from exogenous L-DOPA in the rat. In the DA deafferented striatum, D A produced by these cells from exogenous L-DOPA could be released to e xert physiological effects on DA receptive tissue. It is possible that similar cells could contribute to the efficacy of L-DOPA in the treat ment of Parkinson's disease.