Endogenous dopaminergic tone and dopamine agonist action

Contrasting gene expression patterns induced by levodopa and pramipexole treatments in the rat model of Parkinson's disease

Whether the treatment of Parkinson's disease has to be initiated with levodopa or a D2 agonist like pramipexole remains debatable. Levodopa is more potent against symptoms than D2 agonists, but D2 agonists are less prone to induce motor complications and may have neuroprotective effects. Although regulation of plastic changes in striatal circuits may be the key to their different therapeutic potential, the gene expression patterns induced by de novo treatments with levodopa or D2 agonists are currently unknown. By studying the whole striatal transcriptome in a rodent model of early stage Parkinson's disease, we have identified the gene expression patterns underlying therapeutically comparable chronic treatments with levodopa or pramipexole. Despite the overall relatively small size of mRNA expression changes at the level of individual transcripts, our data show a robust and complete segregation of the transcript expression patterns induced by both treatments. Moreover, transcripts related to oxidative metabolism and mitochondrial function were enriched in levodopa-treated compared to vehicle-treated and pramipexole-treated animals, whereas transcripts related to olfactory transduction pathways were enriched in both treatment groups compared to vehicle-treated animals. Thus, our data reveal the plasticity of genetic striatal networks possibly contributing to the therapeutic effects of the most common initial treatments for Parkinson's disease, suggesting a role for oxidative stress in the long term complications induced by levodopa and identifying previously overlooked signaling cascades as potentially new therapeutic targets.

Negative feedback regulation of nigrostriatal dopamine release: mediation by striatal D1 receptors

The nigrostriatal dopamine system of the mammalian brain is necessary for normal voluntary motor activity. Dopamine exerts its effects by acting on two primary receptor subtypes: D1-like (D1 and D5) and D2-like (D2, D3, and D4) receptors. Previous research has indicated that both subtypes are involved in the negative feedback regulation of dopamine release in the brain. However, the role of D1-like receptors localized within the striatum remains controversial. Using in vivo microdialysis, we report that infusions of the D1/D5 antagonist SCH 23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] (5-200 microM) directly into the striatum increased dopamine release in a concentration-dependent manner. Systemic administration of the novel, full D1/D5 agonist A-77636 [(-)-(1R,3S)-3-adamantyl-1-(aminomethyl)-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyran] produced the opposite effect, a dose-dependent (0.75-3.0 mg/kg s.c.) decrease in striatal dopamine efflux. Infusions of SCH 23390 (5.0 microM) attenuated this decrease. These findings suggest that endogenous dopamine acts on D1-like receptors localized within the striatum to decrease nigrostriatal dopamine release. This negative feedback may be due to the activation of an inhibitory long-loop pathway. Knowledge of the circuitry underlying D1-mediated regulation of nigrostriatal neurons may have significance in current research on treatments for Parkinson's disease.

3,4-methylenedioxymethamphetamine (ecstasy) inhibits dyskinesia expression and normalizes motor activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Ecstasy [3,4-methylenedioxymethamphetamine (MDMA)] was shown to prolong the action of L-3,4-dihydroxyphenylalanine (L-DOPA) while suppressing dyskinesia in a single patient with Parkinson's disease (PD). The clinical basis of this effect of MDMA is unknown but may relate to its actions on either dopaminergic or serotoninergic systems in brain. In normal, drug-naive common marmosets, MDMA administration suppressed motor activity and exploratory behavior. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated, L-DOPA-primed common marmosets, MDMA transiently relieved motor disability but over a period of 60 min worsened motor symptoms. When given in conjunction with L-DOPA, however, MDMA markedly decreased dyskinesia by reducing chorea and to a lesser extent dystonia and decreased locomotor activity to the level observed in normal animals. MDMA similarly alleviated dyskinesia induced by the selective dopamine D2/3 agonist pramipexole. The actions of MDMA appeared to be mediated through 5-HT mechanisms because its effects were fully blocked by the selective serotonin reuptake inhibitor fluvoxamine. Furthermore, the effect of MDMA on L-DOPA-induced motor activity and dyskinesia was partially inhibited by 5-HT1a/b antagonists. The ability of MDMA to inhibit dyskinesia results from its broad spectrum of action on 5-HT systems. Serotoninergic receptors appear to play an important modulatory role in l-DOPA-induced dyskinesia, and this study may provide a framework for the use of serotoninergic agents in the treatment of L-DOPA-induced dyskinesia.

Role of cerebral dopamine but not plasma insulin, leptin and glucocorticoid in the development of tolerance to the anorectic effect of amphetamine

Repeated treatment with amphetamine (AMPH), an anorectic agent, induced a marked anorexia on day 1 followed by a gradual reversion of this anorexia to the normal level of feeding (tolerant anorexia). The mechanism for this tolerant anorexia remained unknown because it might be related with multiple parameters, such as the change of cerebral dopamine (DA) or the plasma levels of insulin, leptin and glucocorticoid. Results revealed that plasma insulin and leptin concentrations remained unchanged during repeated AMPH administration, revealing that these two factors are not involved. Also, glucocorticoids were not required for the development of tolerant anorexia, as this effect could not be prevented by adrenalectomy. However, AMPH-induced anorexia was decreased by the pretreatment of alpha-methyl-p-tyrosine, an inhibitor of central catecholamine synthesis, and was increased by the pretreatment of nomifensine, a blocker of DA transporter that increased extracellular DA content in brain, revealing that the change of DA content could modify the action of tolerant anorexia. It is suggested that the decrease of inhibitory action of DA in brain during repeated AMPH plays a functional role in the development of tolerance to the anorectic response of AMPH.