, demonstrating the existence of drug-induced chromatin remodeling in vivo. In MSK1 knock-out (KO) mice CREB and H3 phosphorylation in response to cocaine (10 mg/kg) were blocked, and induction of c-Fos and dynorphin was prevented, whereas the induction of Egr-1 (early growth response-1)/zif268/Krox24 was unaltered. MSK1-KO mice had no obvious neurological defect but displayed a contrasted behavioral phenotype in response to cocaine. Acute effects of cocaine and dopamine D 1 or D 2 agonists were unaltered. Sensitivity to low doses, but not high doses, of cocaine was increased in the conditioned place preference paradigm, whereas locomotor sensitization to repeated injections of cocaine was decreased markedly. Our results show that MSK1 is a major striatal kinase, downstream from ERK, responsible for the phosphorylation of CREB and H3 and is required specifically for the induction of c-Fos and dynorphin as well as for locomotor sensitization.

SummaryDopamine orchestrates motor behavior and reward-driven learning. Perturbations of dopamine signaling have been implicated in several neurological and psychiatric disorders, and in drug addiction. The actions of dopamine are mediated in part by the regulation of gene expression in the striatum, through mechanisms that are not fully understood. Here, we show that drugs of abuse, as well as natural reinforcement learning, promote the nuclear accumulation of dopamine-and cAMPregulated phosphoprotein Mr=32,000 . This accumulation is mediated through a signaling cascade involving dopamine D1 receptors, cAMP-dependent activation of protein phosphatase-2A, dephosphorylation of DARPP-32 at Ser-97 and inhibition of its nuclear export. The nuclear accumulation of DARPP-32, a potent inhibitor of protein phosphatase-1, increases phosphorylation of histone H3, an important component of nucleosomal response. Mutation of Ser-97 profoundly alters behavioral effects of drugs of abuse, and decreases motivation for food, underlining the functional importance of this signaling cascade.Midbrain dopamine (DA) neurons, activated following unexpected rewarding stimuli, are essential in reinforcement learning 1 . Drugs of abuse mimic the physiological action of DA neurons by increasing their firing rate or preventing DA uptake. Thus, they enhance extracellular DA levels in the forebrain, especially in the nucleus accumbens (NAc), a key structure required for the reinforcing effects of addictive drugs [2][3][4] . To understand how DA mediates reward-controlled learning, it is necessary to identify the intracellular events that trigger gene transcription alterations supporting long-lasting synaptic changes [5][6][7] . DARPP-32 (dopamine-and cAMP-regulated phosphoprotein, Mr=32,000) 8 is a prominent mediator of DA NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript signaling in the striatum 9 . DARPP-32 is highly enriched in striatal GABAergic mediumsize spiny neurons (MSN) 10 . Following activation of DA D1 receptors (D1R), DARPP-32 is phosphorylated by cAMP-dependent protein kinase (PKA) at Thr-34 and converted into a potent inhibitor of the multifunctional serine/threonine protein phosphatase-1 (PP1) 11 . DARPP-32-mediated inhibition of PP1 increases the phosphorylation of neurotransmitter receptors and ion channels crucial for synaptic function and plasticity 9 . DARPP-32 also regulates nuclear events, as demonstrated by alterations of drug-induced gene expression in mice lacking DARPP-32 or bearing a point mutation of 13 . Part of the control exerted by DARPP-32 on transcription is mediated by activation of the ERK pathway, dependent on the concomitant stimulation of D1R and glutamate NMDA receptors 13,14 . However, the precise mechanisms of information transfer from the cytoplasm to the nucleus of striatal neurons are still poorly characterized. Drugs of abuse and reinforcement learning trigger nuclear accumulation of DARPP-32 in striatal neuronsDARPP-32 has been extensively characterized as a cytoplasmic...

Akt is classically described as a prosurvival serine/threonine kinase activated in response to trophic factors. After activation by phosphoinositide 3-kinase (PI3-kinase), it can translocate to the nucleus where it promotes specific genetic programs by catalyzing phosphorylation of transcription factors. We report here that both dopamine (DA) D1 (SKF38393) and D2 (quinpirole) agonist treatments rapidly increase, in primary striatal neurons in culture, phosphorylation levels of Akt on Thr 308 , a residue that is critically involved in its kinase activity. These treatments also activate the extracellular signal-regulated kinase (ERK) pathway in the same population of striatal neurons. Induction of active, phospho-Thr 308 Akt by dopamine D1 and D2 agonists is insensitive to wortmannin and thus PI3-kinase independent, in contrast to growth factor-induced Akt activity. D1-and D2-induced phospho-Thr 308 Akt is decreased by the mitogen-activated protein kinase kinase (MEK) inhibitor, U0126, as well as by overexpression of a dominant-negative version of MEK, thus implicating the Ras/ERK signaling cascade in this process. Furthermore, overexpression of a mutant form of Akt that cannot be activated impaired cAMP response elementbinding protein (CREB) phosphorylation induced by SKF38393 and quinpirole treatments. Activation of Akt on Thr 308 was also found in vivo in striatal neurons after acute administration of cocaine, a psychostimulant that strongly increases DA transmission. Thus, multiple intracellular pathways can transduce signals from dopamine receptors to CREB in striatal neurons, one of these being Akt. We propose that this signaling pathway plays a pivotal role in DA-induced regulation of gene expression and long-term neuronal adaptation in the striatum.