Effects of baclofen on striatal 5-HT mediated by the nigrostriatal dopamine system

In rats, baclofen injected unilaterally into the substantia nigra increased dopamine (DA) and serotonin (5-HT) levels in the ipsilateral striata. Homovanillic acid levels first decreased and then increased to a maximum after 4 h. The content of 3,4-dihydroxyphenylacetic acid also increased after a latency of about 1 h. The effects of baclofen on striatal DA metabolism are consistent with the assumption that impulse flow in these DA neurones was diminshed by the drug. DA metabolites were also found in greater quantities in the contralateral striata, probably owing to compensatory activation of these DA neurones. Baclofen injected intranigrally did not increase the striatal 5-HT content in rats with a 6-hydroxydopamine-induced lesion of the nigrostriatal DA pathway. Our results suggest the presence of DA receptors on striatal 5-HT terminals or on interneurones synapsing axo-axonally with them.