Effects of zonisamide on dopaminergic system

Effects of zonisamide (ZNS) on extracellular dopamine (DA), its precursor 3,4-dihydroxyphenylalanine (DOPA), its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels in the striatum as well as hippocampus of freely moving rats were studied. Intracellular DA, DOPA, DOPAC and HVA levels, as well as DOPA accumulation as an index of tyrosine hydroxylase activity in the rat brain in vivo, DA re-uptake in the striatum and hippocampus, and monoamine oxidase (MAO) activities were also determined. Acute administrations of therapeutic ZNS doses (20 and 50 mg/kg) increased striatal extracellular DOPA levels, intracellular striatal and hippocampal DOPA levels, and stimulated DOPA accumulation in both brain regions. ZNS also increased striatal and hippocampal intracellular as well as extracellular DA and HVA levels, but decreased those of DOPAC levels. Chronic (3 weeks) administrations of therapeutic ZNS doses (20 and 50 mg/kg/day) increased intracellular DA, DOPA, DOPAC and HVA levels in striatum and hippocampus. ZNS-induced changes were greater in intracellular levels than in extracellular levels. Acute and chronic supratherapeutic ZNS dose (100 mg/kg) administration decreased intracellular levels of all substances detectable in both brain regions, and inhibited DOPA accumulation. Both subtypes of MAO (type A and type B) activities were weakly inhibited by ZNS. ZNS showed no effect on DA re-uptake in striatum nor in hippocampus. These results suggest that therapeutic ZNS doses increase DOPA accumulation as well as both intracellular and extracellular DA, DOPA and HVA levels. However, such doses also decrease extracellular and intracellular DOPAC levels by enhancing DA synthesis and/or by selectively inhibiting MAO-B activities. In addition, chronic therapeutic ZNS dose administration enhances DA synthesis, which results in increased intracellular DA, its precursor and its metabolites levels. On the other hand, both acute and chronic supratherapeutic ZNS dose administrations inhibit DA turnover. These ZNS effects on DA metabolism are at least partly involved in the mechanisms of action of ZNS.