The role of GRK6 in animal models of Parkinson's Disease and L-DOPA treatment

G protein-coupled Receptor Kinase 6 (GRK6) belongs to a family of kinases that phosphorylate GPCRs. GRK6 levels were found to be altered in Parkinson's Disease (PD) and D2 dopamine receptors are supersensitive in mice lacking GRK6 (GRK6-KO mice). To understand how GRK6 modulates the behavioral manifestations of dopamine deficiency and responses to L-DOPA, we used three approaches to model PD in GRK6-KO mice: 1) the cataleptic response to haloperidol; 2) introducing GRK6 mutation to an acute model of absolute dopamine deficiency, DDD mice; 3) hemiparkinsonian 6-OHDA model. Furthermore, dopamine-related striatal signaling was analyzed by assessing the phosphorylation of AKT/GSK3β and ERK1/2. GRK6 deficiency reduced cataleptic behavior, potentiated the acute effect of L-DOPA in DDD mice, reduced rotational behavior in hemi-parkinsonian mice, and reduced abnormal involuntary movements induced by chronic L-DOPA. These data indicate that approaches to regulate GRK6 activity could be useful in modulating both therapeutic and side-effects of L-DOPA

Dopamine D-1 Receptor-Mediated Intracellular Responses in the Hypothalamus after Co-Administration of Caffeine with MDMA

Markers of dopamine D1 receptor activation were determined to elucidate intracellular mechanisms associated with the combined effects of caffeine and 3,4 methylenedioxymethamphetamine (MDMA), reported previously to produce increased toxicity, when compared with either drug alone. Caffeine (10 mg/kg) and MDMA (15 mg/kg) were administered to male Sprague Dawley rats alone and in combination. One hour after drug administration, core body temperature and phosphorylation of the dopamine D1‐related intracellular markers, cAMP response element binding protein (CREB), the dopamine and c‐AMP‐regulated phosphoprotein of 32 kDa (DARPP‐32) and expression of the immediate early gene and cellular activation marker c‐fos were determined in the hypothalamus. Co‐administration of caffeine with MDMA increased core body temperature when compared with MDMA or caffeine treatment alone. Pre‐treatment with the dopamine D1 receptor antagonist SCH 23390 (1 mg/kg, i.p.), 30 min. prior to caffeine and MDMA administration, produced a hypothermic response to MDMA that was unaffected by caffeine. Co‐administration of caffeine with MDMA increased p‐CREB, p‐DARPP‐32 and c‐fos expression when compared with either treatment alone. Pre‐treatment with SCH‐23390 attenuated the changes in p‐CREB, p‐DARPP and c‐fos. The results show an enhanced intracellular response when caffeine is combined with MDMA but not with either agent alone suggestive of synergistic intracellular actions convergent on a dopamine D1 receptor signalling pathway. A dopamine‐related synergy associated with the combined administration of caffeine and MDMA may have important use and safety implications for recreational drug users

Importance of ERK activation in behavioral and biochemical effects induced by MDMA in mice

1. Little is known about the cellular effects induced by 3,4-methylenedioxymethamphetamine (MDMA, ecstasy), although changes in gene expression have been observed following treatments with other psychostimulants. Thus, the aim of this study was to investigate in mice, the relationships between the ras-dependent protein kinase ERK and MDMA-induced reinforcement using the conditioned place preference (CPP) and locomotor activity measurements. This was completed using real-time quantitative PCR method by a study of immediate early-genes (IEGs) transcription known to be involved in neuronal plasticity. 2. A significant CPP was observed after repeated MDMA treatment in CD-1 mice at a dose of 9 mg kg(−1) i.p. but not at 3 and 6 mg kg(−1). This rewarding effect was abolished by the selective inhibitor of ERK activation, SL327 (50 mg kg(−1); i.p.). Similar results were obtained on MDMA-induced locomotor activity, clearly suggesting a role of ERK pathway in these behavioral responses. 3. Following acute i.p. injection, MDMA induced a strong c-fos transcription in brain structures, such as caudate putamen, nucleus accumbens and hippocampus, whereas egr-1 and egr-3 transcripts were only increased in the caudate putamen. MDMA-induced IEGs transcription was selectively suppressed by SL327 in the caudate putamen, suggesting a role for other signaling pathways in regulation of IEGs transcription in the other brain structures. In agreement with these results, MDMA-induced c-fos protein expression was blocked by SL327 in the caudate putamen. 4. This study confirms and extends to mice the reported role of ERK pathway in the development of addiction-like properties of MDMA. This could facilitate studies about the molecular mechanism of this process by using mutant mice