Activation of IGF1R by DARPP-32 promotes STAT3 signaling in gastric cancer cells

Dopamine and cAMP-regulated phosphoprotein, Mr 32000 (DARPP-32), is frequently overexpressed in early stages of gastric cancers. We utilized in vitro assays, 3D gastric gland organoid cultures, mouse models, and human tissue samples to investigate the biological and molecular impact of DARPP-32 on activation of IGF1R and STAT3 signaling and gastric tumorigenesis. DARPP-32 enhanced phosphorylation of IGF1R (Y1135), a step that was critical for STAT3 phosphorylation at Y705, nuclear localization, and transcription activation. By using proximity ligation and co-immunoprecipitation assays, we found that IGF1R and DARPP-32 co-existed in the same protein complex. Binding of DARPP-32 to IGF1R promoted IGF1R phosphorylation with subsequent activation of downstream SRC and STAT3. Analysis of gastric tissues from the TFF1 knockout (KO) mouse model of gastric neoplasia, demonstrated phosphorylation of STAT3 in the early stages of gastric tumorigenesis. By crossing the TFF1 KO mice with DARPP-32 (DP) knockout (KO) mice, that have normal stomach, we obtained double knockout (TFF1 KO/DP KO). The gastric mucosa from the double KO mice did not show phosphorylation of IGF1R or STAT3. In addition, the TFF1 KO/DP KO mice had a significant delay in developing neoplastic gastric lesions. Analysis of human gastric cancer tissue microarrays, showed high levels of DARPP-32 and positive immunostaining for nuclear STAT3 in cancer tissues, as compared to non-cancer histologically normal tissues. In summary, the DARPP-32-IGF1R signaling axis plays a key role in regulating the STAT3 signaling, a critical step in gastric tumorigenesis.

Profiling metabolite changes in the neuronal differentiation of human striatal neural stem cells using 1H-magnetic resonance spectroscopy

OBJECTIVE

Neural stem cells (NSCs) have been found to play an increasing clinical role in stroke. However, at present, it is not yet possible to noninvasively monitor their differentiation once implanted into the brain.

METHODS

Here, we describe the use of high-resolution H-magnetic resonance spectroscopy (MRS) to define a metabolite profile of undifferentiated human striatal NSCs from the STROC05 cell line and their differentiation after 3-weeks of treatment with purmorphamine.

RESULTS

The undifferentiated conditions were characterized by ~95% of cells expressing nestin and ~77% being Ki67(+)ve, indicating that these were still proliferating. Phosphophocholine+glycerophosphocholine (PC+GPC) as well as myo-Inositol (mI) were increased in these cells. PC+GPC and mI were markedly reduced upon differentiation, potentially serving as markers of the NSC state. Upon differentiation (~45% neurons, ~30% astrocytes, ~13% oligodendrocytes), the concentration of many metabolites decreased in absolute value. The decreasing trend of the N-acetyl-aspartate level was observed in differentiated cells when compared with NSCs. An increase in plasmalogen (enriched in myelin sheets) could potentially serve as a marker of oligodendrocytes.

CONCLUSION

These metabolite characteristics of undifferentiated and differentiated NSCs provide a basis for exploration of their possible use as markers of differentiation after cell transplantation.

Effects of protein kinase A inhibitor and activator on rewarding effects of SKF-82958 microinjected into nucleus accumbens shell of ad libitum fed and food-restricted rats

RATIONALE

Previous studies indicate that the rewarding effect of D-1 dopamine receptor stimulation in nucleus accumbens (NAc) shell is greater in food-restricted (FR) than in ad libitum fed (AL) rats. The D-1 receptor is positively coupled to adenylyl cyclase and activates protein kinase A (PKA).

OBJECTIVES

The purpose of this study was to determine whether PKA is involved in the rewarding effect of D-1 receptor stimulation and, if so, whether it is involved in the enhanced response of FR rats.

MATERIALS AND METHODS

Rats were stereotaxically implanted with microinjection cannulae in NAc shell and a stimulating electrode in lateral hypothalamus. The rewarding effects of SKF-82958 (1.5 or 3.0 μg, bilaterally) in the presence and absence of PKA inhibitor, Rp-cAMPS (8.9 μg), and PKA activator, Sp-cAMPS (8.9 μg), were assessed using the curve-shift method of intracranial self-stimulation (ICSS). Basal NAc levels of DARPP-32 phosphorylated on Thr34 and Thr75 were measured.

RESULTS

Rp-cAMPS increased the rewarding effect of SKF-82958 in AL but not FR rats, doubling the ICSS threshold-lowering effect of the 3.0-μg dose. Sp-cAMPS decreased the rewarding effect of SKF-82958 in FR but not AL rats. Levels of phospho-DARPP-32 (Thr75), which inhibits PKA, were higher in FR than AL rats.

CONCLUSIONS

Results indicate that inhibition of PKA enhances the unconditioned rewarding effect of D-1 receptor stimulation and that decreased PKA may be involved in the effect of FR on drug reward. Evidence for involvement of D-2 receptor-expressing neurons in the enhancing effect of PKA inhibition is discussed.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Csnk1e is a genetic regulator of sensitivity to psychostimulants and opioids

Csnk1e, the gene encoding casein kinase 1-epsilon, has been implicated in sensitivity to amphetamines. Additionally, a polymorphism in CSNK1E was associated with heroin addiction, suggesting that this gene may also affect opioid sensitivity. In this study, we first conducted genome-wide quantitative trait locus (QTL) mapping of methamphetamine (MA)-induced locomotor activity in C57BL/6J (B6) × DBA/2J (D2)-F(2) mice and a more highly recombinant F(8) advanced intercross line. We identified a QTL on chromosome 15 that contained Csnk1e (63-86 Mb; Csnk1e=79.25 Mb). We replicated this result and further narrowed the locus using B6.D2(Csnk1e) and D2.B6(Csnk1e) reciprocal congenic lines (78-86.8 and 78.7-81.6 Mb, respectively). This locus also affected sensitivity to the μ-opioid receptor agonist fentanyl. Next, we directly tested the hypothesis that Csnk1e is a genetic regulator of sensitivity to psychostimulants and opioids. Mice harboring a null allele of Csnk1e showed an increase in locomotor activity following MA administration. Consistent with this result, coadministration of a selective pharmacological inhibitor of Csnk1e (PF-4800567) increased the locomotor stimulant response to both MA and fentanyl. These results show that a narrow genetic locus that contains Csnk1e is associated with differences in sensitivity to MA and fentanyl. Furthermore, gene knockout and selective pharmacological inhibition of Csnk1e define its role as a negative regulator of sensitivity to psychostimulants and opioids.

Phosphodiesterase 4 inhibition enhances the dopamine D1 receptor/PKA/DARPP-32 signaling cascade in frontal cortex

RATIONALE

Alteration of dopamine neurotransmission in the prefrontal cortex, especially hypofunction of dopamine D1 receptors, contributes to psychotic symptoms and cognitive deficit in schizophrenia. D1 receptors signal through the cAMP/PKA second messenger cascade, which is modulated by phosphodiesterase (PDE) enzymes that hydrolyze and inactivate cyclic nucleotides. Though several PDEs are expressed in cortical neurons, the PDE4 enzyme family (PDE4A-D) has been implicated in the control of cognitive function. The best studied isoform, PDE4B, interacts with a schizophrenia susceptibility factor, disrupted in schizophrenia 1 (DISC1).

OBJECTIVES

We explore the control of mouse frontal cortex dopamine D1 receptor signaling and associated behavior by PDE4.

RESULTS

Inhibition of PDE4 by rolipram induced activation of cAMP/PKA signaling in cortical slices and in vivo, leading to the phosphorylation of DARPP-32 and other postsynaptic and presynaptic PKA-substrates. Rolipram also enhanced DARPP-32 phosphorylation invoked by D1 receptor activation. Immunohistochemical studies demonstrated PDE4A, PDE4B, and PDE4D expression in DARPP-32-positive neurons in layer VI of frontal cortex, most likely in D1 receptor-positive, glutamatergic corticothalamic pyramidal neurons. Furthermore, the ability of rolipram treatment to improve the performance of mice in a sensorimotor gating test was DARPP-32-dependent.

CONCLUSIONS

PDE4, which is co-expressed with DARPP-32 in D1 receptor-positive cortical pyramidal neurons in layer VI, modulates the level of D1 receptor signaling and DARPP-32 phosphorylation in the frontal cortex, likely influencing cognitive function. These biochemical and behavioral actions of PDE4 inhibitors may contribute to the hypothesized antipsychotic actions of this class of compounds.

Casein kinase 1 enables nucleus accumbens amphetamine-induced locomotion by regulating AMPA receptor phosphorylation

The closely related δ and ε isoforms of the serine/threonine protein kinase casein kinase 1 (Csnk1) have been implicated in the generation of psychostimulant-induced behaviors. In this study, we show that Csnk1δ/ε produces its effects on behavior by acting on the Darpp-32-PP1 signaling pathway to regulate AMPA receptor phosphorylation in the nucleus accumbens (NAcc). Inhibiting Csnk1δ/ε in the NAcc with the selective inhibitor PF-670462 blocks amphetamine induced locomotion and its ability to increase phosphorylation of Darpp-32 at S137 and T34, decrease PP1 activity and increase phosphorylation of the AMPA receptor subunit at S845. Consistent with these findings, preventing GluR1 phosphorylation with the alanine mutant GluR1(S845A) reduces glutamate-evoked currents in cultured medium spiny neurons and blocks the locomotor activity produced by NAcc amphetamine. Thus, Csnk1 enables the locomotor and likely the incentive motivational effects of amphetamine by regulating Darrp-32-PP1-GlurR1(S845) signaling in the NAcc. As such, Csnk1 may be a critical target for intervention in the treatment of drug use disorders.

Histone H3 phosphorylation is under the opposite tonic control of dopamine D2 and adenosine A2A receptors in striatopallidal neurons

The antipsychotic agent haloperidol regulates gene transcription in striatal medium spiny neurons (MSNs) by blocking dopamine D2 receptors (D2Rs). We examined the mechanisms by which haloperidol increases the phosphorylation of histone H3, a key step in the nucleosomal response. Using bacterial artificial chromosome (BAC)-transgenic mice that express EGFP under the control of the promoter of the dopamine D1 receptor (D1R) or the D2R, we found that haloperidol induced a rapid and sustained increase in the phosphorylation of histone H3 in the striatopallidal MSNs of the dorsal striatum, with no change in its acetylation. This effect was mimicked by raclopride, a selective D2R antagonist, and prevented by the blockade of adenosine A2A receptors (A2ARs), or genetic attenuation of the A2AR-associated G protein, Galpha(olf). Mutation of the cAMP-dependent phosphorylation site (Thr34) of the 32-kDa dopamine and cAMP-regulated phosphoprotein (DARPP-32) decreased the haloperidol-induced H3 phosphorylation, supporting the role of cAMP in H3 phosphorylation. Haloperidol also induced extracellular signal-regulated kinase (ERK) phosphorylation in striatopallidal MSNs, but this effect was not implicated in H3 phosphorylation. The levels of mitogen- and stress-activated kinase 1 (MSK1), which has been reported to mediate ERK-induced H3 phosphorylation, were lower in striatopallidal than in striatonigral MSNs. Moreover, haloperidol-induced H3 phosphorylation was unaltered in MSK1-knockout mice. These data indicate that, in striatopallidal MSNs, H3 phosphorylation is controlled by the opposing actions of D2Rs and A2ARs. Thus, blockade of D2Rs promotes histone H3 phosphorylation through the A2AR-mediated activation of Galpha(olf) and inhibition of protein phosphatase-1 (PP-1) through the PKA-dependent phosphorylation of DARPP-32.

A role for casein kinase 1 epsilon in the locomotor stimulant response to methamphetamine

RATIONALE

We previously colocalized a quantitative trait locus (QTL) for sensitivity to the locomotor stimulant effects of methamphetamine (MA) with a QTL for expression of casein kinase 1 epsilon (Csnk1-epsilon) in the nucleus accumbens (NAc). Subsequently, we identified a single nucleotide polymorphism in CSNK1E (rs135745) that was associated with increased sensitivity to the subjective effects of d-amphetamine in healthy human subjects. Based on these results, we hypothesized that differential expression of Csnk1-epsilon causes differential sensitivity to MA-induced locomotor activity in mice.

OBJECTIVE

In the present study, we used PF-670462 (PF), which is a selective inhibitor of Csnk1-epsilon, to directly evaluate the role of Csnk1-epsilon in the locomotor stimulant response to MA in male C57BL/6J mice.

METHODS

We administered vehicle, PF, MA, or MA + PF, either via intraperitoneal injections or bilateral intra-NAc microinjections. We also examined Darpp-32 phosphorylation in mice receiving intraperitoneal injections.

RESULTS

Intraperitoneal PF (20-40 mg/kg) attenuated the locomotor stimulant response to MA (2 mg/kg) without affecting baseline activity. The high dose of PF also significantly inhibited MA-induced phosphorylation of Darpp-32, providing a potential mechanism by which Csnk1-epsilon contributes to MA-induced locomotor activity. Furthermore, microinjection of PF (5 microg/side) into the NAc completely blocked the locomotor stimulant response to MA (2.5 microg/side) without affecting baseline activity.

CONCLUSIONS

These results provide direct evidence that Csnk1-epsilon is crucial for the locomotor stimulant response to a moderate dose of MA and suggest that genetic polymorphisms affecting Csnk1-epsilon expression or function could influence sensitivity to amphetamines in both mice and humans.

Basal and cocaine-induced sex differences in the DARPP-32-mediated signaling pathway

RATIONALE

Behavioral and dopamine responses to cocaine are sexually dimorphic: Female rats exhibit higher levels of locomotor and reward-associated behaviors after cocaine administration and dopamine release than do males. Activation of the dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa (DARPP-32) intracellular cascade mediates responses to cocaine.

OBJECTIVE

To examine the possibility that acute cocaine administration alters the DARPP-32 cascade in a sexually dimorphic pattern.

MATERIALS AND METHODS

Male and female rats received either saline or cocaine (30 mg/kg). Protein levels of DARPP-32, phosphorylation of DARPP-32 at the Thr34 site (P-Thr34-DARPP-32), protein phosphatase 1 (PP-1), and protein phosphatase 2B (PP-2B) in nucleus accumbens were measured via Western blot analysis.

RESULTS

Females had higher protein levels of DARPP-32, P-Thr34-DARPP-32, calcineurin A (CaN-A; catalytic subunit of PP-2B), and calcineurin B (CaN-B; regulatory subunit of PP-2B) than males 5 min after saline treatment. In females, CaN-A protein levels were also higher at 15 min and PP-1 protein levels were higher 30 min after saline administration than males. In male rats, cocaine significantly increased CaN-A protein levels at 30 min and CaN-B protein levels at 15 min. In females, cocaine administration significantly decreased protein levels of DARPP-32, P-Thr34-DARPP-32, and CaN-A at 45 min but increased PP-1 protein levels at 30 min. Overall, males had higher activation of the DARPP-32 pathway after cocaine administration than did females.

CONCLUSION

These novel results show that basal and cocaine-induced sex differences in the DARPP-32/PP-1 cascade may be responsible for the sexual dimorphism in acute cocaine-induced behavioral responses.

Detailed structural characterization of unbound protein phosphatase 1 inhibitors

Protein phosphatase 1 (PP1) is an essential and ubiquitous serine/threonine protein phosphatase that is regulated by more than 100 known inhibitor and targeting proteins. It is currently unclear how protein inhibitors distinctly and specifically regulate PP1 to enable rapid responses to cellular alterations. We demonstrate that two PP1 inhibitors, I-2 and DARPP-32, belong to the class of intrinsically unstructured proteins (IUPs). We show that both inhibitors have distinct preferences for transient local and long-range structure. These preferences are likely their structural signature for their interaction with PP1. Furthermore, we show that upon phosphorylation of Thr(34) in DARPP-32, which turns DARPP-32 into a potent inhibitor of PP1, neither local nor long-range structure of DARPP-32 is altered. Therefore, our data suggest a role for these transient three-dimensional topologies in binding mechanisms that enable extensive contacts with PP1's invariant surfaces. Together, these interactions enable potent and selective inhibition of PP1.

Dopamine receptor modulation of hypoxic-ischemic neuronal injury in striatum of newborn piglets

Dopamine receptors regulate glutamatergic neurotransmission and Na(+),K(+)-ATPase via protein kinase A (PKA) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32)-dependent signaling. Consequently, dopamine receptor activation may modulate neonatal hypoxic-ischemic (H-I) neuronal damage in the selectively vulnerable putamen enriched with dopaminergic receptors. Piglets subjected to two durations of hypoxia followed by asphyxic cardiac arrest were treated with a D1-like (SCH23390) or D2-like (sulpiride) receptor antagonist. At 4 days of recovery from less severe H-I, the remaining viable neurons in putamen were 60% of control, but nearly completely salvaged by pretreatment with SCH23390 or sulpiride. After more severe H-I in which only 18% of neurons were viable, partial neuroprotection was seen with SCH23390 pretreatment (50%) and posttreatment (39%) and with sulpiride pretreatment (35%), but not with sulpiride posttreatment (24%). Dopamine was significantly elevated in microdialysis samples from putamen during asphyxia and the first 15 mins of reoxygenation. Pretreatment with SCH23390 or sulpiride largely attenuated the increased nitrotyrosine and the decreased Na(+),K(+)-ATPase activity that occurred at 3 h after severe H-I. Pretreatment with SCH23390, but not sulpiride, also attenuated H-I-induced increases in PKA-dependent phosphorylation of Thr34 on DARPP-32, Ser943 on the alpha subunit of Na(+),K(+)-ATPase, and Ser897 of the N-methyl-D-aspartate (NMDA) receptor NR1 subunit. These findings indicate that D1 and D2 dopamine receptor activation contribute to neuronal death in newborn putamen after H-I in association with increased protein nitration and decreased Na(+),K(+)-ATPase activity. Furthermore, mechanisms of D1 receptor toxicity may involve DARPP-32-dependent phosphorylation of NMDA receptor NR1 and Na(+),K(+)-ATPase.

Protein kinase A activates protein phosphatase 2A by phosphorylation of the B56delta subunit

Our previous studies of DARPP-32 in striatal slices have shown that activation of D1 receptors leads to cAMP-dependent dephosphorylation of Thr-75, the Cdk5 site in DARPP-32. In the current study, we have elucidated a mechanism whereby protein phosphatase 2A (PP2A) is activated by a cAMP/PKA-dependent pathway, leading to dephosphorylation of Thr-75. PP2A consists of a catalytic C subunit that associates with the scaffolding A subunit and a variety of B subunits. We have found that the A/C subunits of PP2A, in association with the B56delta (or PPP2R5D) regulatory subunit, is an active DARPP-32 phosphatase. The B56delta subunit expressed in HEK293 cells forms a heterotrimeric assembly that catalyzes PKA-mediated dephosphorylation at Thr-75 in DARPP-32 (also cotransfected into HEK293 cells). The B56delta subunit is phosphorylated by PKA, and this increases the overall activity of PP2A in vitro and in vivo. Among four PKA-phosphorylation sites identified in B56delta in vitro, Ser-566 was found to be critical for the regulation of PP2A activity. Moreover, Ser-566 was phosphorylated by PKA in response to activation of D1 receptors in striatal slices. Based on these studies, we propose that the B56delta/A/C PP2A complex regulates the dephosphorylation of DARPP-32 at Thr-75, thereby helping coordinate the efficacy of dopaminergic neurotransmission in striatal neurons. Moreover, stimulation of protein phosphatase activity by this mechanism may represent an important signaling pathway regulated by cAMP in neurons and other types of cell.

Phosphodiesterase 1B knock-out mice exhibit exaggerated locomotor hyperactivity and DARPP-32 phosphorylation in response to dopamine agonists and display impaired spatial learning

Using homologous recombination, we generated mice lacking phosphodiesterase-mediated (PDE1B) cyclic nucleotide-hydrolyzing activity. PDE1B(-/-) mice showed exaggerated hyperactivity after acute D-methamphetamine administration. Striatal slices from PDE1B(-/-) mice exhibited increased levels of phospho-Thr34 DARPP-32 and phospho-Ser845 GluR1 after dopamine D1 receptor agonist or forskolin stimulation. PDE1B(-/-) and PDE1B(+/-) mice demonstrated Morris maze spatial-learning deficits. These results indicate that enhancement of cyclic nucleotide signaling by inactivation of PDE1B-mediated cyclic nucleotide hydrolysis plays a significant role in dopaminergic function through the DARPP-32 and related transduction pathways.

Regulation of phosphorylation of the GluR1 AMPA receptor in the neostriatum by dopamine and psychostimulants in vivo

The activation of cAMP-dependent protein kinase regulates the physiological activity of AMPA-type glutamate receptors. In this study, phosphorylation of the AMPA receptor subunit GluR1 at Ser(845) was increased in neostriatal slices by activation of D1-type dopamine receptors and by inhibitors of protein phosphatase 1/protein phosphatase 2A. In contrast, Ser(831), a residue which, when phosphorylated by protein kinase C or calcium/calmodulin-dependent kinase II, increases AMPA receptor channel conductance, was unaffected by either D1 or D2 receptor agonists in neostriatal slices. The phosphorylation of Ser(845), but not Ser(831), was strongly increased in neostriatum in vivo in response to the psychostimulants cocaine and methamphetamine. The effects of dopamine and psychostimulants on the phosphorylation of GluR1 were attenuated in dopamine and cAMP-regulated phosphoprotein M(r) 32 kDa (DARPP-32) knock-out mice. These results identify DARPP-32 and AMPA-type glutamate receptors as likely essential cellular effectors for psychostimulant actions.

Expression of the striatal DARPP-32/ARPP-21 phenotype in GABAergic neurons requires neurotrophins in vivo and in vitro

The medium spiny neuron (MSN) is the major output neuron of the caudate nucleus and uses GABA as its primary neurotransmitter. A majority of MSNs coexpress DARPP-32 and ARPP-21, two dopamine and cyclic AMP-regulated phosphoproteins, and most of the matrix neurons express calbindin. DARPP-32 is the most commonly used MSN marker, but previous attempts to express this gene in vitro have failed. In this study we found that DARPP-32 is expressed in <12% of E13- or E17-derived striatal neurons when they are grown in defined media at high or low density in serum, dopamine, or Neurobasal/N2 (Life Technologies), and ARPP-21 is expressed in <1%. The percentage increases to 25% for DARPP-32 and 10% for ARPP-21 when the same cells are grown in Neurobasal/B27 (Life Technologies) for 7 d. After growth in Neurobasal/B27 plus brain-derived neurotrophic factor (BDNF) for 7 d, E13-derived MSNs are 53.7% DARPP-32-positive and 29. 0% ARPP-21-positive; E17-derived MSNs are 66.8% DARPP-32-positive and 51.5% ARPP-21-positive. The percentage of calbindin-positive neurons also is increased under these conditions. Finally, ARPP-21 expression is reduced in mice with a targeted deletion of the BDNF gene. We conclude that BDNF is required for the maturation of a large subset of patch and matrix MSNs in vivo and in vitro. In addition, we introduce a culture system in which highly differentiated MSNs may be generated, maintained, and studied.

A dopamine/D1 receptor/protein kinase A/dopamine- and cAMP-regulated phosphoprotein (Mr 32 kDa)/protein phosphatase-1 pathway regulates dephosphorylation of the NMDA receptor

We have investigated the mechanism by which activation of dopamine (DA) receptors regulates the glutamate sensitivity of medium spiny neurons of the nucleus accumbens. Our results demonstrate that DA regulates the phosphorylation state of the NR1 subunit of NMDA-type glutamate receptors. The effect of DA was mimicked by SKF82526, a D1-type DA receptor agonist, and by forskolin, an activator of cAMP-dependent protein kinase (PKA), and was blocked by H-89, a PKA inhibitor. These data indicate that DA increases NR1 phosphorylation through a PKA-dependent pathway. DA-induced phosphorylation of NR1 was blocked in mice bearing a targeted deletion of the gene for dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa (DARPP-32), a phosphoprotein that is a potent and selective inhibitor of protein phosphatase-1, indicating that the effect of PKA is mediated, in part, by regulation of the DARPP-32/protein phosphatase-1 cascade. In support of this interpretation, NR1 phosphorylation was increased by calyculin A, a protein phosphatase-1/2A inhibitor. A model is proposed in which the ability of DA to regulate NMDA receptor sensitivity is attributable to a synergistic action involving increased phosphorylation and decreased dephosphorylation of the NR1 subunit of the NMDA receptor.

Mating-related stimulation induces phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein-32 in progestin receptor-containing areas in the female rat brain

Vaginal-cervical stimulation induces a number of physiological and behavioral events, including the facilitation of mating behavior. Although the facilitation of one component of mating behavior, lordosis, by vaginal-cervical stimulation does not require the presence of progesterone, it appears to be mediated by neural progestin receptors. Abundant evidence suggests that dopamine may play a role in the neural circuitry activated by vaginal-cervical stimulation, including the mating-induced release of dopamine in progestin receptor-containing areas of the brain, changes in the activational state of progestin receptors because of dopamine D1 receptor stimulation, facilitation of lordosis by D1 receptor stimulation in estradiol-primed rats via progesterone-independent events, and D1 agonist-induced neuronal responses in progestin receptor-containing areas and cells. We tested the hypothesis that vaginal-cervical stimulation induces phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32; Mr = 32,000), a protein phosphorylated predominantly in response to the stimulation of D1 receptors. At 9 d after ovariectomy, female rats were injected subcutaneously with a behaviorally effective dose of estradiol benzoate. At 48 hr later they received vaginal-cervical or control (perineal) stimulation, and they were perfused 1 hr later. Vaginal-cervical stimulation increased the number of cells expressing pDARPP-32 immunoreactivity by 92% in the medial preoptic nucleus, 134% in the caudal ventromedial hypothalamic nucleus, 123% in the posterodorsal medial amygdala, and 103% in the bed nucleus of the stria terminalis. These results suggest that some of the neuronal effects of vaginal-cervical stimulation, and perhaps other social or environmental stimuli, are mediated by phosphorylation of DARPP-32, perhaps via stimulation of D1 receptors, within progestin receptor-containing areas.

Molecular components of striatal plasticity: the various routes of cyclic AMP pathways

Neuroplasticity serves an important role for normal striatal function and in disease states. One route to neuroplasticity involves activation of the transcription factor cyclic 3', 5'-adenosine monophosphate (cyclic AMP) response element binding protein (CREB) by phosphorylation of the amino acid 133Ser. Dopamine and glutamate, the two predominant neurotransmitters in the striatum, induce CREB phosphorylation in primary cultures of rat striatum through cyclic AMP and Ca2+ pathways. Here we present the role of N-methyl-D-aspartate receptors and Ca2+ in cyclic AMP-mediated CREB phosphorylation.

Bidirectional regulation of DARPP-32 phosphorylation by dopamine

Dopamine has been shown to stimulate phosphorylation of DARPP-32, a phosphoprotein highly enriched in medium-sized spiny neurons of the neostriatum. Here, we investigated the contribution of D1-like and D2-like dopamine receptors in the regulation of DARPP-32 phosphorylation in mouse striatal slices. D1-like and D2-like receptors had opposing effects on the state of DARPP-32 phosphorylation. The D1 receptor agonist SKF82526 increased DARPP-32 phosphorylation. In contrast, the D2 receptor agonist quinpirole decreased basal as well as D1 agonist-, forskolin-, and 8-bromo-cAMP-stimulated phosphorylation of DARPP-32. The ability of quinpirole to decrease D1-stimulated DARPP-32 phosphorylation was calcium-dependent and was blocked by the calcineurin inhibitor cyclosporin A, suggesting that the D2 effect involved an increase in intracellular calcium and activation of calcineurin. In support of this interpretation, Ca2+-free/EGTA medium induced a greater than 60-fold increase in DARPP-32 phosphorylation and abolished the ability of quinpirole to dephosphorylate DARPP-32. The antipsychotic drug raclopride, a selective D2 receptor antagonist, increased phosphorylation of DARPP-32 under basal conditions and in D2 agonist-treated slices. The results of this study demonstrate that dopamine exerts a bidirectional control on the state of phosphorylation of DARPP-32.