Up-regulation of dopamine D? receptors : in vitro and in vivo studies
Abstract: In the central nervous system (CNS) dopaminergic and dopaminoceptive neurons have the ability to respond to variations in dopamine levels by for example adjusting their dopamine receptor levels. One of the most well known phenomena in this respect is that long-term blockade of D2 receptors with antipsychotic drugs leads to an increase in striatal D2 receptor density of experimental animals as well as of schizophrenic patients as measured in both post-mortem studies and in vivo positron emission tomography studies of brain. Whether such a response underlies schizophrenic symptoms, is an effect of drug treatment, or both has yet to be clarified.Knowledge of underlying molecular mechanisms of D2 receptor up-regulation is sparse due to the difficulty in examining such effects both in vivo and in vitro. This work mainly describes a way to overcome these obstacles by: (1) presenting a cellular system where increased D2 receptor levels and a supersensitive D2 coupled functional response can be evoked by persistent "indirect" blockade of the receptor (2) identifying some of the molecular aspects underlying this up-regulation and (3) validating parts of this cellular model in vivo in the rat striatum.By persistently stimulating the adenylyl cyclase enzyme with forskolin in mouse Ltk- fibroblast cells stably transfected with the human long D2 receptor (D2L) (and thereby mimicking long-term D2 receptor block at the level of intracellular cAMP), D2 receptors could be up-regulated by 43%. A supersensitive response to dopamine was found in parallel. The increase in receptor number was even larger (105%) for the short D2 receptor variant. The forskolin-induced up-regulation of D2L receptors was found to be dependent on: (1) increases in intracellular cAMP levels (2) de novo protein synthesis (3) activation of protein kinase A (PKA) and (4) at least partially on an intact Gi/o protein pool. Forskolin infusion into rat lateral ventricles produced D2 receptor increases of 19% and D1 receptor decreases of 27%. In vivo effects in the rats following the forskolin infusion, most likely a function of the D2 receptor up-regulation, were shown in both behavioral and biochemical experiments. The relevance of studying D2 receptor up-regulation was strengthened by findings where subchronic treatment of rats with the atypical antipsychotic remoxipride resulted in D2 receptor up-regulation in brain regions that have receptor levels that are 38-fold lower than in the striatum. Parallel control treatments with the typical antipsychotic haloperidol gave similar results.In conclusion, the work presents an in vitro possibility for studying molecular mechanisms of D2 receptor up-regulation. The cellular model has been characterized in several respects and partly validated in an in vivo system. There seems to be good reason to pursue these issues further as the new generation of atypical antipsychotics are also likely to target and affect D2 receptor numbers in the brain.
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