Kazuya Kannari

Role of serotonergic neurons in L-DOPA-derived extracellular dopamine in the striatum of 6-OHDA-lesioned rats.

The effect of L-dihydroxyphenylalanine (L-DOPA) on extracellular dopamine (DA) in the striatum was determined by microdialysis in 6-hydroxydopamine (6-OHDA)-lesioned rats treated with and without the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At the same time the intensity of L-DOPA-induced rotational behavior was assessed. In 6-OHDA-lesioned rats treated with 5,7-DHT, L-DOPA (50 mg/kg, i.p.) increased extracellular DA only to 20% of that measured in animals not treated with 5,7-DHT. Likewise, 6-OHDA-lesioned rats treated with 5,7-DHT exhibited a significantly lower number of L-DOPA-induced rotations. These results suggest that serotonergic terminals in the striatum can convert exogenously administered L-DOPA into DA that can be released into the extracellular space.

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation.

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‐HT1A and 5‐HT1B receptors), we applied in vivo brain microdialysis technique to 6‐hydroxydopamine‐lesioned rats and examined the effects of the selective 5‐HT1A receptor agonist 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT) and the selective 5‐HT1B 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.) significantly 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‐HT1A antagonist. In contrast, intrastriatal perfusion with the 5‐HT1B agonist CGS‐12066 A (10 nm and 100 nm) did not induce any changes in l‐DOPA‐derived extracellular DA. Thus, stimulation of 5‐HT1A but not 5‐HT1B 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.

Serotonergic hyperinnervation into the dopaminergic denervated striatum compensates for dopamine conversion from exogenously administered l-DOPA

The aim of this study was to determine whether raphe-striatal serotonergic neurons of adult rats with extensive nigro-striatal dopaminergic denervation are induced by injection of exogenous L-DOPA to contain dopamine. Double-labeling immunofluorescence study was conducted. In the lesioned striatum of rats that received L-DOPA, serotonergic hyperinnervation was observed, and dopamine was detected in serotonergic varicose fibers. These findings suggest that striatal serotonergic hyperinnervation can compensate for the lost function of dopaminergic neurons.

Reserpine pretreatment prevents increases in extracellular striatal dopamine following L-DOPA administration in rats with nigrostriatal denervation

Abstract: The influence of L‐DOPA and reserpine on extracellular dopamine (DA) levels in the striatum of intact and dopaminergic denervated rats was studied using the brain microdialysis technique. In intact rats, reserpine (5 mg/kg s.c.) reduced extracellular DA levels to 4% of basal values. L‐DOPA (50 mg/kg i.p.) had no effect on extracellular DA levels in reserpine‐pretreated rats. In rats with 6‐hydroxydopamine‐induced lesion of the nigrostriatal dopaminergic system, basal levels of extracellular DA were low but markedly increased by L‐DOPA (50 mg/kg i.p.). In 6‐hydroxydopamine‐lesioned rats, pretreatment with reserpine (5 mg/kg s.c.) diminished L‐DOPA (50 mg/kg i.p.)‐induced increases in extracellular DA levels to 16% of those obtained in denervated animals not pretreated with reserpine (p < 0.01). These results suggest that in the intact striatum, extracellular DA stems mainly from vesicular storage sites and that in the striatum with dopaminergic denervation, a large part of the L‐DOPA‐derived extracellular DA is also derived from a vesicular pool that is released by an exocytosis mechanism.

A serotonin 5-HT1A receptor agonist prevents behavioral sensitization to L-DOPA in a rodent model of Parkinson's disease.

Marked fluctuation of dopamine concentration in the striatum following long-term L-DOPA administration contributes to the development of L-DOPA-induced motor complications including L-DOPA-induced dyskinesias and wearing-off in patients with Parkinsons disease. We have shown that pretreatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A (5-hydroxytryptamine) receptor agonist, alleviates fluctuation of dopamine levels in the dopamine-denervated striatum of 6-hydroxydopamine-lesioned (hemiparkinsonian) rats after L-DOPA treatment. To determine whether co-administration of 8-OH-DPAT with L-DOPA prevents L-DOPA-induced motor complications, we examined rotation behavior and levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the striatum of 6-hydroxydopamine-lesioned rats treated with L-DOPA alone or L-DOPA + 8-OH-DPAT, twice daily, for 2 weeks. Co-administration of 8-OH-DPAT inhibited an increase of rotation behavior to L-DOPA and L-DOPA-induced increases in levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the dopamine-denervated striatum, both of which are established indices of L-DOPA-induced motor complications. These results suggest that pharmaceutical products that stimulate 5-HT1A receptors could prove useful in prevention of the development of L-DOPA-induced motor complications in patients with Parkinsons disease.

Rapid induction of serotonergic hyperinnervation in the adult rat striatum with extensive dopaminergic denervation

The aim of our study was to determine whether serotonergic hyperinnervation is rapidly induced in the striatum of adult rats with extensive dopaminergic denervation. Immunohistochemical study was performed on the brain sections obtained at 2 and 8 weeks after injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The extent of dopaminergic denervation was evaluated as a percentage loss of tyrosine hydroxylase immunopositive neurons in the substantia nigra pars compacta. The immunopositive areas for serotonin (5-HT) in the striatum were measured. In the lesioned rats 97.5+/-0.6% of dopamine neurons were lost. 5-HT immunopositive areas in the striatum were significantly increased both at 2 and 8 weeks after 6-OHDA injection (P<0.01). These results suggest that extensive dopaminergic denervation in adult rats induces rapid serotonergic hyperinnervation in the striatum as early as 2 weeks after lesioning.

Fluoxetine reduces L-DOPA-derived extracellular DA in the 6-OHDA-lesioned rat striatum.

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 Parkinsons disease by reducing efflux of exogenous L-DOPA-derived DA.

Reuptake of L-DOPA-derived extracellular dopamine in the striatum with dopaminergic denervation via serotonin transporters.

The aim of our present study was to determine whether exogenous L-DOPA-derived extracellular dopamine (DA) in the denervated striatum is taken up via serotonin (5-HT) transporters. Rats with dopaminergic denervation by 6-hydroxydopamine were injected with L-DOPA 50 mg/kg (i.p.) and extracellular DA levels in the denervated striatum were measured using in vivo microdialysis technique. During the experiments, fluoxetine, a selective serotonin reuptake inhibitor, was locally perfused into the striatum through the microdialysis probe. Cumulative amounts of L-DOPA-derived extracellular DA during 300 min were increased by fluoxetine dose-dependently (10 microM and 50 microM). These results suggest that reuptake of DA via 5-HT transporters plays an important role in the clearance of L-DOPA-derived DA from the extracellular space when dopaminergic neurons are extensively denervated.

Loss of regulation by presynaptic dopamine D2 receptors of exogenous l-DOPA-derived dopamine release in the dopaminergic denervated striatum

To determine whether dopamine release derived from exogenous l-DOPA is under inhibitory control of presynaptic dopamine D2 receptors in the dopaminergic denervated striatum, extracellular dopamine levels were measured in the striatum of 6-hydroxydopamine-lesioned rats using in vivo brain microdialysis. Quinpirole, a D2 agonist, dose-dependently (0.01-3 mg/kg s.c.) inhibited endogenous dopamine release both in the intact and dopaminergic denervated striatum. The dose-response curve obtained from the denervated striatum showed a shift to the right. Administration of l-DOPA (30 mg/kg i.p.) with carbidopa (25 mg/kg i.p.) increased dopamine release to 130% of basal levels in the intact striatum and 770% of basal levels in the denervated striatum. In the intact striatum, dopamine release was continuously inhibited by quinpirole pretreatment (1 mg/kg s.c.) even after l-DOPA administration. In the denervated striatum, l-DOPA-derived dopamine release was not affected by quinpirole pretreatment. These results suggest that, in the striatum with dopaminergic denervation, regulation by presynaptic D2 receptors is still operative on endogenous dopamine release but it does not work on dopamine release derived from exogenously administered l-DOPA.

Reuptake of L-DOPA-derived extracellular DA in the striatum of a rodent model of Parkinson's disease via norepinephrine transporter.

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 Parkinsons disease. Synapse 62:632–635, 2008.