An Adenosine-Mediated Glial-Neuronal Circuit for Homeostatic Sleep.

Sleep homeostasis reflects a centrally mediated drive for sleep, which increases during waking and resolves during subsequent sleep. Here we demonstrate that mice deficient for glial adenosine kinase (AdK), the primary metabolizing enzyme for adenosine (Ado), exhibit enhanced expression of this homeostatic drive by three independent measures: (1) increased rebound of slow-wave activity; (2) increased consolidation of slow-wave sleep; and (3) increased time constant of slow-wave activity decay during an average slow-wave sleep episode, proposed and validated here as a new index for homeostatic sleep drive. Conversely, mice deficient for the neuronal adenosine A1 receptor exhibit significantly decreased sleep drive as judged by these same indices. Neuronal knock-out of AdK did not influence homeostatic sleep need. Together, these findings implicate a glial-neuronal circuit mediated by intercellular Ado, controlling expression of homeostatic sleep drive. Because AdK is tightly regulated by glial metabolic state, our findings suggest a functional link between cellular metabolism and sleep homeostasis. SIGNIFICANCE STATEMENT: The work presented here provides evidence for an adenosine-mediated regulation of sleep in response to waking (i.e., homeostatic sleep need), requiring activation of neuronal adenosine A1 receptors and controlled by glial adenosine kinase. Adenosine kinase acts as a highly sensitive and important metabolic sensor of the glial ATP/ADP and AMP ratio directly controlling intracellular adenosine concentration. Glial equilibrative adenosine transporters reflect the intracellular concentration to the extracellular milieu to activate neuronal adenosine receptors. Thus, adenosine mediates a glial-neuronal circuit linking glial metabolic state to neural-expressed sleep homeostasis. This indicates a metabolically related function(s) for this glial-neuronal circuit in the buildup and resolution of our need to sleep and suggests potential therapeutic targets more directly related to sleep function.

Forebrain overexpression of CK1delta leads to down-regulation of dopamine receptors and altered locomotor activity reminiscent of ADHD.

Dopamine neurotransmission controls motor and perseverative behavior, is mediated by protein phosphorylation, and may be perturbed in disorders of attention and hyperactivity. To assess the role of casein kinase I (CK1) in the regulation of dopamine signaling, we generated a genetically modified mouse line that overexpresses CK1delta (CK1delta OE) specifically in the forebrain. Overexpression was confirmed both at the mRNA and at the protein levels. Under basal conditions, CK1delta OE mice exhibited horizontal and vertical hyperactivity, reduced anxiety, and nesting behavior deficiencies. The CK1delta OE mice also presented paradoxical responses to dopamine receptor stimulation, showing hypoactivity following injection of d-amphetamine or methylphenidate, indicating that CK1 activity has a profound effect on dopamine signaling in vivo. Interestingly, CK1delta overexpression led to significantly reduced D1R and D2R dopamine receptor levels. All together, under basal conditions and in response to drug stimulation, the behavioral phenotype of CK1delta OE mice is reminiscent of the symptoms and drug responses observed in attention-deficit/hyperactivity disorder and therefore the CK1delta OE mice appear to be a model for this disorder.

Dopamine D1 vs D5 receptor-dependent induction of seizures in relation to DARPP-32, ERK1/2 and GluR1-AMPA signalling.

Recent reports have shown that the selective dopamine D(1)-like agonist SKF 83822 [which stimulates adenylate cyclase, but not phospholipase C] induces prominent behavioral seizures in mice, whereas its benzazepine congener SKF 83959 [which stimulates phospholipase C, but not adenylate cyclase] does not. To investigate the relative involvement of D(1) vs D(5) receptors in mediating seizures, ethological behavioral topography and cortical EEGs were recorded in D(1), D(5) and DARPP-32 knockout mice in response to a convulsant dose of SKF 83822. SKF 83822-induced behavioral and EEG seizures were gene dose-dependently abolished in D(1) knockouts. In both heterozygous and homozygous D(5) knockouts, the latency to first seizure was significantly increased and total EEG seizures were reduced relative to wild-types. The majority (60%) of homozygous DARPP-32 knockouts did not have seizures; of those having seizures (40%), the latency to first seizure was significantly increased and the number of high amplitude, high frequency polyspike EEG events was reduced. In addition, immunoblotting was performed to investigate downstream intracellular signalling mechanisms at D(1)-like receptors following challenge with SKF 83822 and SKF 83959. In wild-types administered SKF 83822, levels of ERK1/2 and GluR1 AMPA receptor phosphorylation increased two-fold in both the striatum and hippocampus; in striatal slices DARPP-32 phosphorylation at Thr34 increased five-fold relative to vehicle-treated controls. These findings indicate that D(1), and to a lesser extent D(5), receptor coupling to DARPP-32, ERK1/2 and glutamatergic signalling is involved in mediating the convulsant effects of SKF 83822.

Characterization of stanniocalcin 2, a novel target of the mammalian unfolded protein response with cytoprotective properties.

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) induces a highly conserved homeostatic response in all eukaryotic cells, termed the unfolded-protein response (UPR). Here we describe the characterization of stanniocalcin 2 (STC2), a mammalian homologue of a calcium- and phosphate-regulating hormone first identified in fish, as a novel target of the UPR. Expression of STC2 gene is rapidly upregulated in cultured cells after exposure to tunicamycin and thapsigargin, by ATF4 after activation of the ER-resident kinase PERK. In addition, STC2 expression is also activated in neuronal cells by oxidative stress and hypoxia but not by several cellular stresses unrelated to the UPR. In contrast, expression of another homologue, STC1, is only upregulated by hypoxia independent of PERK or ATF4 expression. In vivo studies revealed that rat cortical neurons rapidly upregulate STC2 after transient middle cerebral artery occlusion. Finally, siRNA-mediated inhibition of STC2 expression renders N2a neuroblastoma cells and HeLa cells significantly more vulnerable to apoptotic cell death after treatment with thapsigargin, and overexpression of STC2 attenuated thapsigargin-induced cell death. Consequently, induced STC2 expression is an essential feature of survival component of the UPR.

Cannabinoid action depends on phosphorylation of dopamine- and cAMP-regulated phosphoprotein of 32 kDa at the protein kinase A site in striatal projection neurons.

Herbal cannabis, smoked in the form of marihuana or hashish, is the most common illicit drug consumed in the Western world. In the brain, cannabinoids interact with neuronal CB1 receptors, thereby producing a marked reduction of motor activity. Here, we report that the motor depressant effect produced by the cannabinoid receptor agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]trans-4-(3-hydroxypropyl)cyclohexanol (CP55,940) is attenuated by genetic inactivation of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), which is abundantly expressed in the medium spiny neurons of the striatum. Point mutation of Thr34, the protein kinase A (PKA) phosphorylation site of DARPP-32, produces a similar reduction in the effect of the CB1 agonist. In contrast, point mutation of Thr75, a site on DARPP-32 specifically phosphorylated by cyclin-dependent kinase 5, does not affect the behavioral response to CP55,940. Activation of CB1 receptors, either by an agonist or by inhibition of reuptake of endogenous cannabinoids, stimulates phosphorylation at Thr34, thereby converting DARPP-32 into an inhibitor of protein phosphatase-1. Genetic inactivation either of dopamine D2 receptors or of adenosine A2A receptors reduces the phosphorylation of DARPP-32 at Thr34 and the motor depression produced by CP55,940. Our data indicate that a considerable proportion of the psychomotor effect of cannabinoids can be accounted for by a signaling cascade in striatal projection neurons involving PKA-dependent phosphorylation of DARPP-32, achieved via modulation of dopamine D2 and adenosine A2A transmission.

Phosphorylation of DARPP-32 at Threonine-34 is required for cocaine action.

Mice lacking DARPP-32, a striatal-enriched phosphoprotein, show abnormal behavioral and biochemical responses to cocaine, but the role of individual phosphorylation sites in DARPP-32 in these responses is unknown. We show here that mutation of Thr-34 in DARPP-32 mimicked the behavioral phenotype of the constitutive DARPP-32 knockout in cocaine-induced place conditioning, locomotor activity, and sensitization paradigms. In contrast, mutations of Thr75 did not affect conditioned place preference or the acute locomotor response to cocaine, but DARPP-32 Thr-75 mutants showed no locomotor sensitization in response to repeated cocaine administration. Consistent with these behavioral findings, we found that cocaine regulation of gene expression in striatum, including the acute induction of the immediate early genes c-fos and arc (activity-regulated cytoskeletal-associated gene), was abolished in DARPP-32 Thr-34 mutants, but not in Thr-75 mutants. Similarly, induction of the transcription factor DeltaFosB in the ventral striatum (nucleus accumbens) by chronic cocaine was diminished by the Thr-34, but not the Thr-75, mutation. These findings highlight distinct roles of the Thr-34 and Thr-75 phosphorylation sites of DARPP-32 in mediating short- and long-term behavioral and biochemical actions of cocaine.

Disruption of orofacial movement topographies in congenic mutants with dopamine D5 but not D4 receptor or DARPP-32 transduction 'knockout'.

The role of D(1)-like [D(1), D(5)] and D(2)-like [D(2), D(3), D(4)] dopamine receptors and dopamine transduction via DARPP-32 in topographies of orofacial movement was assessed in restrained mice with congenic D(4) vs. D(5) receptor vs. DARPP-32 'knockout'. D(4) and DARPP-32 mutants evidenced no material phenotype; also, there were no alterations in topographical responsivity to either the selective D(2)-like agonist RU 24213 or the selective D(1)-like agonist SK and F 83959. In contrast, D(5) mutants evidenced an increase in spontaneous vertical jaw movements, which habituated more slowly than in wildtypes, and a decrease in horizontal jaw movements; topographical responsivity to SK and F 83959 and RU 24213 was unaltered. D(5) receptors regulate distinct topographies of vertical and horizontal jaw movement in an opposite manner. In assuming that the well-recognised role of the D(1)-like family in regulating orofacial movements involves primarily D(1) receptors, a role for their D(5) counterparts may have been overlooked.

DARPP-32 phosphorylation opposes the behavioral effects of nicotine.

BACKGROUND: The addictive properties of nicotine are mediated via dopaminergic pathways and their post-synaptic neurons in the striatum. Because post-synaptic neurons within the striatum contain high levels of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), we hypothesized that DARPP-32 may functionally contribute to the behavioral effects of nicotine. METHODS: We examined the behavioral effects of nicotine and the phosphorylation state of DARPP-32 in wild-type (WT) and DARPP-32 knockout (KO) mice. In one experiment, we assessed voluntary nicotine intake (0-50 microg/ml) of WT and KO mice in a two-bottle choice paradigm. In a separate experiment, the motor-depressant effects of acute and repeated nicotine injections (0-.8 mg/kg, subcutaneously [SC]) were assessed. The phosphorylation of DARPP-32 at threonine34 and threonine75 were examined using Western blotting. RESULTS: A heightened responsiveness to nicotine was seen in KO mice when compared with WT mice in oral intake and motor depression. The enhanced responsiveness in KO mice was not due to alterations in taste sensations, fluid intake, or blood nicotine or cotinine levels. Systemic injections of nicotine resulted in increased striatal DARPP-32 phosphorylation at threonine34 and threonine75. CONCLUSIONS: DARPP-32 opposes the behavioral effects of nicotine possibly via concurrent phosphorylation at the two threonine sites.

Reduction of cocaine place preference in mice lacking the protein phosphatase 1 inhibitors DARPP 32 or Inhibitor 1.

BACKGROUND: Modulation of protein phosphorylation by dopamine is thought to play an important role in drug reward. Protein phosphatase-1 (PP-1) is known to mediate some of the changes in neuronal signaling that occur following activation of the dopaminergic system. METHODS: Two endogenous inhibitors of PP-1 are dopamine and cyclic 3', 5' adenosine monophosphate-regulated phosphoprotein (DARPP-32) and Inhibitor-1 (I-1). Knockout mice lacking one or both of these PP-1 inhibitors were tested for responses to cocaine using in vivo amperometry and conditioned place preference. RESULTS: Presynaptic dopaminergic function appears to be unaffected by these mutations because stimulation-evoked changes in extracellular dopamine levels were unchanged between wild type mice and mice lacking one or both of these PP-1 inhibitors. In contrast, conditioned place preference to cocaine is reduced in mice lacking DARPP-32, I-1, or both phosphoproteins. This does not appear to be due to a learning deficit because mice lacking both DARPP-32 and I-1 show normal passive avoidance learning. CONCLUSIONS: These data imply that increased PP-1 function as a result of deficits in DARPP-32 or I-1 is sufficient to decrease the rewarding properties of cocaine. Furthermore, the mechanism for this altered cocaine place preference does not involve alteration of dopamine release or reuptake.

Topographical Assessment of Ethological and Dopamine Receptor Agonist-Induced Behavioral Phenotype in Mutants with Congenic DARPP-32 'Knockout'.

Congenic (10 backcrosses into C57BL/6J) mutants with targeted gene deletion of DARPP-32, a neuronal phosphoprotein regarded as an essential mediator of the biological effects of dopamine (DA), were assessed phenotypically using an ethologically based approach that resolves all topographies of behavior in the mouse repertoire. Over initial exploration, female, but not male, DARPP-32 mutants evidenced increased locomotion and decreased grooming, while a decrease in rearing seated was evident in mutants of both genders; continuing assessment over several hours did not reveal additional phenotypic effects. Following challenge with the nonselective DA receptor agonist apomorphine, low doses were associated with reduced levels of sniffing, grooming, total rearing, and rearing seated in DARPP-32 mutants relative to wildtypes; this would suggest some role for DARPP-32 in mediating the biological effects of presynaptic D(2)-like autoreceptor or inhibitory postsynaptic D(2)-like receptor activation. Following challenge with higher doses, while stereotyped sniffing and locomotion with chewing was largely unaltered, the additional murine response of Straub tail was essentially abolished in DARPP-32 mutants, indicating some specific involvement of DARPP-32 in mediating this topography of behavior; additionally, there were overall reductions in levels of sniffing, total rearing, rearing seated, and grooming in DARPP-32 mutants that were unrelated to the dose of apomorphine administered, indicating broader topographical effects following the stress of the injection procedure relative to more naturalistic conditions. The developmental absence of DARPP-32 following targeted gene deletion appears to be associated with compensatory processes that maintain certain topographies of spontaneous and agonist-induced DAergic function, while other topographies remain impaired.

Regulation of AMPA receptor dephosphorylation by glutamate receptor agonists.

Phosphorylation of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 at Ser(845) enhances AMPA channel activity. This study demonstrates that Ser(845) is rapidly dephosphorylated upon AMPA receptor activation in nucleus accumbens slices. AMPA-induced dephosphorylation at Ser(845) was blocked by CNQX, an AMPA receptor antagonist, by nifedipine, an L-type Ca(2+) channel antagonist, or by cyclosporin A, a calcineurin inhibitor. N-methyl-D-aspartate (NMDA) treatment also decreased phosphorylation of Ser(845), an effect that was blocked by MK-801, an NMDA receptor antagonist, but not by nifedipine. Accumbens neurons are enriched for dopamine- and cyclic AMP (cAMP)-regulated phosphoprotein, Mr 32,000 (DARPP-32), a potent inhibitor of protein phosphatase 1 (PP1) when phosphorylated by PKA (at Thr(34)). We tested the hypothesis that the AMPA/KA or NMDA-stimulated dephosphorylation of DARPP-32 via calcineurin, leading to increased PP1 activity and dephosphorylation of GluR1. AMPA or NMDA treatment decreased phospho-Thr(34)-DARPP-32 levels, effects that were blocked by receptor antagonists, or cyclosporin A. However, dephosphorylation of Ser(845) mediated by AMPA or NMDA receptors was unaffected in DARPP-32/inhibitor-1 knockout mice. These data suggest that AMPA- or NMDA-induced dephosphorylation of GluR1 at Ser(845) occurs by a mechanism that is independent of DARPP-32 and PP1, but involves activation of calcineurin. Thus, Ca(2+)-dependent dephosphorylation of GluR1 may serve as a negative feedback mechanism for the regulation of AMPA receptor activity in neurons.

Methylphenidate restores novel object recognition in DARPP-32 knockout mice.

Previously, we have shown that Dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32) knockout mice required significantly more trials to reach criterion than wild-type mice in an operant reversal-learning task. The present study was conducted to examine adult male and female DARPP-32 knockout mice and wild-type controls in a novel object recognition test. Wild-type and knockout mice exhibited comparable behavior during the initial exploration trials. As expected, wild-type mice exhibited preferential exploration of the novel object during the substitution test, demonstrating recognition memory. In contrast, knockout mice did not show preferential exploration of the novel object, instead exhibiting an increase in exploration of all objects during the test trial. Given that the removal of DARPP-32 is an intracellular manipulation, it seemed possible to pharmacologically restore some cellular activity and behavior by stimulating dopamine receptors. Therefore, a second experiment was conducted examining the effect of methylphenidate. The results show that methylphenidate increased horizontal activity in both wild-type and knockout mice, though this increase was blunted in knockout mice. Pretreatment with methylphenidate significantly impaired novel object recognition in wild-type mice. In contrast, pretreatment with methylphenidate restored the behavior of DARPP-32 knockout mice to that observed in wild-type mice given saline. These results provide additional evidence for a functional role of DARPP-32 in the mediation of processes underlying learning and memory. These results also indicate that the behavioral deficits in DARPP-32 knockout mice may be restored by the administration of methylphenidate.

Dopamine modulation of membrane excitability in striatal spiny neurons is altered in DARPP-32 knockout mice.

The phosphoprotein DARPP-32 (dopamine and cAMP-regulated phosphoprotein 32 kDa) plays a central role in mediating the actions of a variety of neurotransmitters in medium spiny neurons of the striatum (Greengard, 1990; Fienberg et al., 1998). This study examines D1 and D2 dopamine (DA) agonist effects on the membrane properties of identified striatal neurons recorded in slices obtained from wild-type and DARPP-32-knockout mice. In wild-type spiny cells, DA D1 receptor activation decreased cell excitability, causing a 58.8 +/- 13.5% increase in rheobase current required to evoke spike discharge. In contrast, D1 agonist administration did not alter cell excitability when applied to spiny cells in slices prepared from the DARPP-32 knockout mice. D2 agonist administration decreased cell excitability in both wild-type and knockout mice. The response produced by combined D1 and D2 agonist stimulation was dependent on the sequence of agonist administration. Thus, the D1 agonist-induced decrease in excitability was reversed to a facilitation of spiking upon subsequent D2 agonist administration. In contrast, D2 agonist applied simultaneously with the D1 agonist only produced a reduction in excitability. This type of D1-dependent modulation was not present in slices from the DARPP-32 knockout mice.

Involvement of striatal and extrastriatal DARPP-32 in biochemical and behavioral effects of fluoxetine (Prozac).

Fluoxetine (Prozac) is the most widely prescribed medication for the treatment of depression. Nevertheless, little is known about the molecular basis of its clinical efficacy, apart from the fact that fluoxetine increases the synaptic availability of serotonin. Here we show that, in vivo, fluoxetine, given either acutely or chronically, regulates the phosphorylation state of dopamine- and cAMP-regulated phosphoprotein of M(r) 32,000 (DARPP-32) at multiple sites in prefrontal cortex, hippocampus, and striatum. Acute administration of fluoxetine increases phosphorylation of DARPP-32 at the protein kinase A site, Thr-34, and at the casein kinase-1 site, Ser-137, and decreases phosphorylation at the cyclin-dependent kinase 5 site, Thr-75. Each of these changes contributes, through distinct signaling pathways, to increased inhibition of protein phosphatase-1, a major serine/threonine protein phosphatase in the brain. Fluoxetine also increases phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluR1 at Ser-831 and Ser-845. Both the fluoxetine-mediated increase in AMPA receptor phosphorylation at Ser-845-GluR1 and the beneficial responsiveness to fluoxetine in an animal test of antidepressant efficacy were strongly reduced in DARPP-32 knockout mice, indicating a critical role for this phosphoprotein in the antidepressant actions of fluoxetine. Mice chronically treated with fluoxetine had increased levels of DARPP-32 mRNA and protein and a decreased ability to increase phospho-Ser-137-DARPP-32 and phospho-Ser-831-GluR1. These chronic changes may be relevant to the delayed onset of therapeutic efficacy of fluoxetine.

DARPP-32 mediates serotonergic neurotransmission in the forebrain.

Serotonin is implicated in the regulation of complex sensory, motor, affective, and cognitive functions. However, the biochemical mechanisms whereby this neurotransmitter exerts its actions remain largely unknown. DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of molecular weight 32,000) is a phosphoprotein that has primarily been characterized in relation to dopaminergic neurotransmission. Here we report that serotonin regulates DARPP-32 phosphorylation both in vitro and in vivo. Stimulation of 5-hydroxy-tryptamine (5-HT4 and 5-HT6 receptors causes an increased phosphorylation state at Thr34-DARPP-32, the protein kinase A site, and a decreased phosphorylation state at Thr75-DARPP-32, the cyclin-dependent kinase 5 site. Furthermore, stimulation of 5-HT2 receptors increases the phosphorylation state of Ser137-DARPP-32, the casein kinase-1 site. Behavioral and gene transcriptional effects induced by compounds that selectively release serotonin were greatly reduced in DARPP-32 knockout mice. Our data indicate that DARPP-32 is essential not only for dopaminergic but also for serotonergic neurotransmission.

Ethologically based resolution of D2-like dopamine receptor agonist-versus antagonist-induced behavioral topography in dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein of 32 kDa "knockout" mutants congenic on the C57BL/6 genetic background.

Given the critical role of dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein of 32 kDa (DARPP-32) in the regulation of dopaminergic function, DARPP-32-null mutant mice congenic on the inbred C57BL/6 strain for 10 generations were examined phenotypically for their ethogram of responsivity to the selective D2-like receptor agonist RU 24213 (N-n-propyl-N-phenylethyl-p-3-hydroxyphenylethylamine) and the selective D2-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide), using procedures that resolve all topographies of behavior in the natural repertoire. After vehicle challenge, levels of sniffing and rearing seated were reduced in DARPP-32 mutants; the injection procedure seems to constitute a "stressor" that reveals phenotypic effects of DARPP-32 deletion not apparent under natural conditions. Topographical effects of 0.3 to 10.0 mg/kg RU 24213, primarily induction of sniffing and ponderous locomotion with accompanying reductions in rearing, grooming, sifting and chewing, were not altered to any material extent in DARPP-32-null mice. However, topographical effects of 0.005 to 0.625 mg/kg YM 09151-2, namely, reduction in sniffing, locomotion, rearing, grooming, and chewing but not sifting, were essentially absent in DARPP-32 mutants. Thus, the D2-like receptor agonist-mediated ethogram was essentially conserved, whereas major elements of the corresponding D2-like receptor antagonist-mediated ethogram were essentially absent in DARPP-32-null mice. This suggests some relationship between 1) extent of tonic dopaminergic activation of DARPP-32 mechanisms and 2) compensatory mechanisms consequent to the developmental absence of DARPP-32, which may emerge to act differentially on individual elements of the DARPP-32 system. Critically, the present data indicate that phenotypic effects of a given gene deletion using an agonist acting on the system disrupted cannot be generalized to a corresponding antagonist, and vice versa.

Dopamine D1-dependent trafficking of striatal N-methyl-D-aspartate glutamate receptors requires Fyn protein tyrosine kinase but not DARPP-32.

Interactions between dopaminergic and glutamatergic systems in the striatum are thought to underlie both the symptoms and adverse effects of treatment of Parkinson's disease. We have previously reported that activation of the dopamine D1 receptor triggers a rapid redistribution of striatal N-methyl-d-aspartate (NMDA) receptors between intracellular and postsynaptic sub-cellular compartments. To unravel the signaling pathways underlying this trafficking, we studied mice with targeted disruptions of either the gene that encodes the dopamine- and cAMP-regulated phosphoprotein (DARPP-32), a potent and selective inhibitor of protein phosphatase-1, or the protein tyrosine kinase Fyn. In striatal tissue from DARPP-32-depleted mice, basal tyrosine and serine phosphorylation of striatal NMDA receptor subunits NR1, NR2A, and NR2B was normal, and activation of dopamine D1 receptors with the agonist SKF-82958 [(+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-benzazepine] produced redistribution of NMDA receptors from vesicular compartments (P3 and LP2) to synaptosomal membranes (LP1). In the Fyn knockout mice, basal tyrosine phosphorylation of NR2A and NR2B was drastically reduced, whereas serine phosphorylation of these NMDA subunits was unchanged. In the Fyn knockout mice, the dopamine D1 receptor agonist failed to induce subcellular redistribution of NMDA receptors. In addition, Fyn-depleted mice lesioned with 6-hydroxydopamine also failed to exhibit l-DOPA-induced behavioral sensitization, but this may be caused, at least in part, by resistance of these mice to the neurotoxic lesion. These findings suggest a novel mechanism for the trafficking of striatal NMDA receptors by signaling pathways that are independent of DARPP-32 but require Fyn protein tyrosine kinase. Strategies that prevent NMDA receptor subcellular redistribution through inhibition of Fyn kinase may prove useful in the treatment of Parkinson's disease.

Diverse psychotomimetics act through a common signaling pathway.

Three distinct classes of drugs: dopaminergic agonists (such as D-amphetamine), serotonergic agonists (such as LSD), and glutamatergic antagonists (such as PCP) all induce psychotomimetic states in experimental animals that closely resemble schizophrenia symptoms in humans. Here we implicate a common signaling pathway in mediating these effects. In this pathway, dopamine- and an adenosine 3',5'-monophosphate (cAMP)-regulated phospho-protein of 32 kilodaltons (DARPP-32) is phosphorylated or dephosphorylated at three sites, in a pattern predicted to cause a synergistic inhibition of protein phosphatase-1 and concomitant regulation of its downstream effector proteins glycogen synthesis kinase-3 (GSK-3), cAMP response element-binding protein (CREB), and c-Fos. In mice with a genetic deletion of DARPP-32 or with point mutations in phosphorylation sites of DARPP-32, the effects of D-amphetamine, LSD, and PCP on two behavioral parameters-sensorimotor gating and repetitive movements-were strongly attenuated.

Molecular characterization of recombinant mouse adenosine kinase and evaluation as a target for protein phosphorylation.

The regulation of adenosine kinase (AK) activity has the potential to control intracellular and interstitial adenosine (Ado) concentrations. In an effort to study the role of AK in Ado homeostasis in the central nervous system, two isoforms of the enzyme were cloned from a mouse brain cDNA library. Following overexpression in bacterial cells, the corresponding proteins were purified to homogeneity. Both isoforms were enzymatically active and found to possess K(m) and V(max) values in agreement with kinetic parameters described for other forms of AK. The distribution of AK in discrete brain regions and various peripheral tissues was defined. To investigate the possibility that AK activity is regulated by protein phosphorylation, a panel of protein kinases was screened for ability to phosphorylate recombinant mouse AK. Data from these in vitro phosphorylation studies suggest that AK is most likely not an efficient substrate for PKA, PKG, CaMKII, CK1, CK2, MAPK, Cdk1, or Cdk5. PKC was found to phosphorylate recombinant AK efficiently in vitro. Further analysis revealed, however, that this PKC-dependent phosphorylation occurred at one or more serine residues associated with the N-terminal affinity tag used for protein purification.

Involvement of DARPP-32 phosphorylation in the stimulant action of caffeine.

Caffeine has been imbibed since ancient times in tea and coffee, and more recently in colas. Caffeine owes its psychostimulant action to a blockade of adenosine A(2A) receptors, but little is known about its intracellular mechanism of action. Here we show that the stimulatory effect of caffeine on motor activity in mice was greatly reduced following genetic deletion of DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein of relative molecular mass 32,000). Results virtually identical to those seen with caffeine were obtained with the selective A(2A) antagonist SCH 58261. The depressant effect of the A(2A) receptor agonist, CGS 21680, on motor activity was also greatly attenuated in DARPP-32 knockout mice. In support of a role for DARPP-32 in the action of caffeine, we found that, in striata of intact mice, caffeine increased the state of phosphorylation of DARPP-32 at Thr 75. Caffeine increased Thr 75 phosphorylation through inhibition of PP-2A-catalysed dephosphorylation, rather than through stimulation of cyclin-dependent kinase 5 (Cdk5)-catalysed phosphorylation, of this residue. Together, these studies demonstrate the involvement of DARPP-32 and its phosphorylation/dephosphorylation in the stimulant action of caffeine.