In order to determine whether L-DOPA-derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5-HT 1A and 5-HT 1B receptors), we applied in vivo brain microdialysis technique to 6-hydroxydopamine-lesioned rats and examined the effects of the selective 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the selective 5-HT 1B receptor agonist CGS-12066 A on L-DOPA-derived extracellular DA levels. Single L-DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after L-DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8-OH-DPAT (i.p.) signi®cantly attenuated an increase in L-DOPA-derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after L-DOPA injection, respectively. These 8-OH-DPAT-induced changes in L-DOPA-derived extracellular DA were antagonized by further pretreatment with WAY-100635, a selective 5-HT 1A antagonist. In contrast, intrastriatal perfusion with the 5-HT 1B agonist CGS-12066 A (10 nM and 100 nM) did not induce any changes in L-DOPA-derived extracellular DA. Thus, stimulation of 5-HT 1A but not 5-HT 1B receptors attenuated an increase in extracellular DA derived from exogenous L-DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous L-DOPA in the absence of dopaminergic neurons.

No abstract

To determine the role of norepinephrine transporter in reuptake of L-DOPA-derived extracellular DA in the DA-denervated Parkinsonian striatum, we examined extracellular DA levels in the striatum of 6-hydroxyDA-lesioned rats that received L-DOPA (50 mg/kg with 12.5 mg/kg of benserazide) and L-DOPA plus desipramine (25 mg/kg), a selective norepinephrine reuptake inhibitor, using in vivo microdialysis. The pretreatment with desipramine increased levels of extracellular DA derived from administrated L-DOPA in the DA-denervated striatum. This study provides evidence that L-DOPA-derived DA is taken up by the norepinephrine transporter, instead of the dopamine transporter, in the striatum with dopaminergic denervation. This result suggests that the norepinephrine transporter could be a promising target in the treatment for Parkinson's disease.

No abstract

We investigated the effect of fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on L-DOPA-derived extracellular dopamine (DA) levels in the striatum of rats with nigrostriatal dopaminergic denervation using in vivo microdialysis. Treatment with fluoxetine (10 mg/kg, i.p.) induced a 41% reduction in the cumulative amount of extracellular DA during 300 min following L-DOPA administration (50 mg/kg, i.p.; p < 0.01). This effect was antagonized by pretreatment with WAY-100635, a potent 5-HT1A antagonist, indicating that this effect of fluoxetine is due to its indirect 5-HT1A agonistic property. These results suggest that SSRIs may impair motor functions in patients with Parkinson's disease by reducing efflux of exogenous L-DOPA-derived DA.