Effects of baclofen on dopamine metabolism and interaction with neuroleptic effects.

Baclofen increased striatal levels of dopamine (DA), homovanillic (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) dose-dependently above 10 mg/kg i.p. The effect on the DA metabolites was shown to be caused only by the (-)-isomer. The HVA increase after 20 mg/kg i.p. was not antagonized by either scopolamine or picrotoxin. Repeated treatment produced a smaller increase in HVA than a single administration. Baclofen reduced both the disappearance of DA after alpha-methyl-p-tyrosine and the acceleration of the DA disappearance caused by neuroleptics in corpus striatum and in the mesolimbic area. The neuroleptic-induced increases in HVA and DOPAC and in DOPA accumulation after central decarboxylase inhibition were also reduced. Picrotoxin could not antagonize these effects of baclofen which therefore cannot be regarded as being garbergic. Baclofen effects on DA metabolism are similar to those reported for gamma-hydroxybutyric acid and are probably a consequence of inhibition of firing of DA neurons.