Pharmacological modulation of endogenous dopamine and DOPAC outflow from nucleus accumbens

Monitoring dopamine in vivo by microdialysis sampling and on-line CE-laser-induced fluorescence

Microdialysis sampling was coupled on-line to micellar electrokinetic chromatography (MEKC) to monitor extracellular dopamine concentration in the brains of rats. Microdialysis probes were perfused at 0.3 microL/min and the dialysate mixed on-line with 6 mM naphthalene-2,3-dicarboxaldehye and 10 mM potassium cyanide pumped at 0.12 microL/min each into a reaction capillary. The reaction mixture was delivered into a flow-gated interface and separated at 90-s intervals. The MEKC separation buffer consisted of 30 mM phosphate, 6.5 mM SDS, and 2 mM HP-beta-CD at pH 7.4, and the electric field was 850 V/cm applied across a 14-cm separation distance. Analytes were detected by laser-induced fluorescence excited using the 413-nm line of a 14-mW diode-pumped laser. The detection limit for dopamine was 2 nM when sampling by dialysis. The basal dopamine concentration in dialysates collected from the striatum of anesthetized rats was 18 +/- 3 nM (n = 12). The identity of the putative dopamine peak was confirmed by showing that dopamine uptake inhibitors increased the peak and dopamine synthesis inhibitors eliminated the peak. The utility of this method for behavioral studies was demonstrated by correlating dopamine concentrations in vivo and with psychomotor behavior in freely moving rats following the intravenous administration of cocaine. Over 60 additional peaks were detected in the electropherograms, suggesting the potential for monitoring many other substances in vivo by this method.

Dopamine and GABA increases produced by activation of glutamate receptors in the nucleus accumbens are decreased during aging

The aim of the present study was to investigate the effects of aging on the increases of dopamine and GABA induced by activation of ionotropic and metabotropic glutamate receptors in the nucleus accumbens of the freely moving rat. The effects of local perfusion of the agonists NMDA (10, 100 and 500 microM), AMPA (1, 20 and 100 microM) and ACPD (100, 500 and 1000 microM) on extracellular concentration of dopamine and GABA in the nucleus accumbens of young (2-4 months), middle-aged (10-14 months) and aged (24-32 months) male Wistar rats were studied using microdialysis. In young rats, perfusion of the agonists NMDA and AMPA, but not ACPD, produced an increase of dialysate concentrations of dopamine. Perfusion of the three glutamate agonists (NMDA, AMPA and ACPD) produced an increase of dialysate GABA. This increase was delayed in time compared with the increase of dopamine. In the nucleus accumbens of middle-aged and aged rats, the increases of dopamine induced by NMDA were significantly lower than those in young rats. Also the increases of dopamine induced by AMPA were lower in aged rats than those in young rats. The effects of AMPA, NMDA and ACPD on dialysate GABA were significantly lower in aged rats than in young rats. These findings suggest that aging changes the interaction between the neurotransmitters glutamate and dopamine and glutamate and GABA in the nucleus accumbens of the freely moving rat.

GBR-12909 effect on dopamine outflow depends on phosphorylation in the caudate nucleus of the rat

Modulation of the dopamine (DA) transporter inhibitor GBR-12909 effect on DA release by protein kinases and protein phosphatases was studied in slices of the rat caudate nucleus measuring DA outflow in the superfusate of static chambers. Activation of protein kinase A and C markedly enhanced the effect of GBR-12909, whereas protein kinase inhibition by H7 reduced the GBR-12909 effect. Inhibition of protein phosphatases (PPP) 1 and 2A by okadaic acid did not modify basal outflow of DA. However, after the addition of okadaic acid a dramatic and biphasic effect was found when DA outflow was enhanced by GBR-12909. Inhibition of PPP 2A enhanced extracellular DA levels, while inhibition of PPP 1 and 2A completely abolished the effect of GBR-12909. In contrast to tetrodotoxin, the voltage-activated calcium channel blocker omega-conotoxin MVIIC inhibited GBR-12909 effects on DA outflow. Additionally, in aCSF devoid of calcium GBR-12909 did not increase DA liberation. These results suggest a complex and strong influence of phosphorylation on GBR-12909 effects on calcium channel-dependent DA outflow at low-affinity piperazine binding sites in slices of the rat caudate nucleus in vitro.

Synthesis and release of dopamine in rat striatal slices: requirement for exogenous tyrosine in the medium

When incubated in a tyrosine-free medium, the tissue dopamine (DA) level of rat striatal slices increased by about 921 +/- 15 pmol/mg protein during 90 min of preincubation. In contrast, the tissue-free tyrosine level declined only 130 pmol/mg protein in the same assay period. Depolarization of the slices with high K+ increased both DA and DOPAC outputs and depleted tissue DA level by about 75%. Although 60 min of resting after high K+ depolarization significantly restored the tissue DA levels, neither this restoration nor depolarization-induced DA release was altered by exogenous tyrosine. Similarly, failure of exogenous tyrosine was also observed during three successive depolarization periods of striatal slices. These results indicate that nigrostriatal dopaminergic neurons are able to synthesize and release the DA in the absence of exogenous tyrosine in the medium. Since the free tyrosine level in the slices does not seem to be a sufficient source, it is likely that tyrosine mobilized from its bound source(s) supports the DA synthesis under in vitro experimental conditions.

Interaction of the dopaminergic and serotonergic systems in the rat striatum: effects of selective antagonists and uptake inhibitors

The effects of some selective dopamine (DA) and 5-hydroxytryptamine (5-HT) antagonists and uptake inhibitors on the outflow of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum were studied using microdialysis in conscious rats. Eticlopride (D2 antagonist, 2.5 mg/kg) increased HVA but decreased 5-HIAA, while nomifensine (DA uptake inhibitor, 10 mg/kg) decreased both. Mianserin (5-HT2 antagonist, 2 mg/kg) and citalopram (5-HT uptake inhibitor, 11 mg/kg) also decreased both HVA and 5-HIAA. The selectivity of these drugs and the time course of their effects on HVA and 5-HIAA suggested that the dopaminergic drugs may affect the serotonergic systems primarily via changes in extraneuronal DA and conversely, serotonergic drugs affect the dopaminergic systems via changes in extraneuronal 5-HT. The results obtained are consistent with current evidence that DA and 5-HT release from striatal terminals are regulated by D2 and 5-HT1B autoreceptors, respectively. They also support the idea that, in the rat striatum, presynaptic inhibitory heteroreceptors are a major mechanism that contributes to a reciprocal interaction of the dopaminergic and serotonergic systems.