Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons

Rapid dopaminergic and GABAergic modulation of calcium and voltage transients in dendrites of prefrontal cortex pyramidal neurons

The physiological responses of dendrites to dopaminergic inputs are poorly understood and controversial. We applied dopamine on one dendritic branch while simultaneously monitoring action potentials (APs) from multiple dendrites using either calcium-sensitive dye, voltage-sensitive dye or both. Dopaminergic suppression of dendritic calcium transients was rapid (<0.5 s) and restricted to the site of dopamine application. Voltage waveforms of backpropagating APs were minimally altered in the same dendrites where dopamine was confirmed to cause large suppression of calcium signals, as determined by dual voltage and calcium imaging. The dopamine effects on dendritic calcium transients were fully mimicked by D1 agonists, partially reduced by D1 antagonist and completely insensitive to protein kinase blockade; consistent with a membrane delimited mechanism. This dopamine effect was unaltered in the presence of L-, R- and T-type calcium channel blockers. The somatic excitability (i.e. AP firing) was not affected by strong dopaminergic stimulation of dendrites. Dopamine and GABA were then sequentially applied on the same dendrite. In contrast to dopamine, the pulses of GABA prohibited AP backpropagation distally from the application site, even in neurons with natural Cl− concentration (patch pipette removed). Thus, the neocortex employs at least two distinct mechanisms (dopamine and GABA) for rapid modulation of dendritic calcium influx. The spatio-temporal pattern of dendritic calcium suppression described in this paper is expected to occur during phasic dopaminergic signalling, when midbrain dopaminergic neurons generate a transient (0.5 s) burst of APs in response to a salient event.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.

Differential regulation of MeCP2 phosphorylation in the CNS by dopamine and serotonin

Systemic administration of amphetamine (AMPH) induces phosphorylation of MeCP2 at Ser421 (pMeCP2) in select populations of neurons in the mesolimbocortical brain regions. Because AMPH simultaneously activates multiple monoamine neurotransmitter systems, here we examined the ability of dopamine (DA), serotonin (5-HT), and norepinephrine (NE) to induce pMeCP2. Selective blockade of the DA transporter (DAT) or the 5-HT transporter (SERT), but not the NE transporter (NET), was sufficient to induce pMeCP2 in the CNS. DAT blockade induced pMeCP2 in the prelimbic cortex (PLC) and nucleus accumbens (NAc), whereas SERT blockade induced pMeCP2 only in the NAc. Administration of selective DA and 5-HT receptor agonists was also sufficient to induce pMeCP2; however, the specific combination of DA and 5-HT receptors activated determined the regional- and cell-type specificity of pMeCP2 induction. The D(1)-class DA receptor agonist SKF81297 induced pMeCP2 widely; however, coadministration of the D(2)-class agonist quinpirole restricted the induction of pMeCP2 to GABAergic interneurons of the NAc. Intra-striatal injection of the adenylate cyclase activator forskolin was sufficient to induce pMeCP2 in medium-spiny neurons, suggesting that the combinatorial regulation of cAMP by different classes of DA and 5-HT receptors may contribute to the cell-type specificity of pMeCP2 induction. Consistent with the regulation of pMeCP2 by multiple monoamine neurotransmitters, genetic disruption of any single monoamine transporter in DAT-, SERT-, and NET-knockout mice failed to eliminate AMPH-induced pMeCP2 in the NAc. Together, these studies indicate that combinatorial signaling through DA and 5-HT receptors can regulate the brain region- and cell-type specific pMeCP2 in the CNS.

Signaling in striatal neurons: the phosphoproteins of reward, addiction, and dyskinesia

The striatum is a deep region of the forebrain involved in action selection, control of movement, and motivation. It receives a convergent excitatory glutamate input from the cerebral cortex and the thalamus, controlled by dopamine (DA) released in response to unexpected rewards and other salient stimuli. Striatal function and its dysfunction in drug addiction or Parkinson's disease depend on the interplay between these neurotransmitters. Signaling cascades in striatal medium-sized spiny neurons (MSNs) involve multiple kinases, phosphatases, and phosphoproteins, some of which are highly enriched in these neurons. They control the properties of ion channels and the plasticity of MSNs, in part through their effects on gene transcription. This chapter summarizes signaling in MSNs and focuses on the regulation of multiple protein phosphatases through DA and glutamate receptors and the role of ERK. It is hypothesized that these pathways are particularly adapted to the specific computing properties of MSNs and the function of the basal ganglia circuits in which they participate.

Cav1.2 L-type Ca²⁺ channels mediate cocaine-induced GluA1 trafficking in the nucleus accumbens, a long-term adaptation dependent on ventral tegmental area Ca(v)1.3 channels

AMPA receptor (AMPAR) plasticity at glutamatergic synapses in the mesoaccumbal dopaminergic pathway has been implicated in persistent cocaine-induced behavioral responses; however, the precise mechanism underlying these changes remains unknown. Utilizing cocaine psychomotor sensitization, we have examined phosphorylation of GluA1 at key residues serine 845 (S845) and S831, as well as GluA1 cell surface levels in the nucleus accumbens (NAc) of cocaine-preexposed mice and the role of brain-specific Ca(v)1.2 and Ca(v)1.3 L-type Ca²⁺ channels (LTCCs), therein. We found higher basal levels of S845 phospho-GluA1 (P-GluA1) and cell surface GluA1 in the NAc following protracted withdrawal from cocaine exposure, changes that occur independently of LTCCs. In contrast, we found that a cocaine challenge that elicits expression of the cocaine-sensitized response increases S831 P-GluA1 that further increases surface GluA1 beyond the higher basal levels. Intra-NAc pharmacological manipulations indicate that the Ca(v)1.2-activated CaM kinase II (CaMKII) mediates cocaine-induced increase in S831 P-GluA1 and that both Ca(v)1.2-activated CaMKII and extracellular signal-regulated kinase 2 (ERK2) mediate the increase in GluA1 cell surface levels specific to the sensitized response. Experiments using adenoassociated viral vectors expressing Ca(v)1.3 and ERK2 siRNA further indicate that recruitment of the Ca(v)1.2 pathway in the NAc is dependent on ventral tegmental area Ca(v)1.3 LTCCs and ERK2. Together, these results identify candidate pathways that mediate cocaine-induced AMPAR plasticity in the NAc and provide a mechanism linking LTCCs and GluA1 plasticity to cocaine-induced persistent behavioral changes.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Voltage-gated calcium channels and Parkinson's disease

A complex interaction of environmental, genetic and epigenetic factors combine with ageing to cause the most prevalent of movement disorders Parkinson's disease. Current pharmacological treatments only tackle the symptoms and do not stop progression of the disease or reverse the neurodegenerative process. While some incidences of Parkinson's disease arise through heritable genetic defects, the cause of the majority of cases remains unknown. Likewise, why some neuronal populations are more susceptible to neurodegeneration than others is not clear, but as the molecular pathways responsible for the process of cell death are unravelled, it is increasingly apparent that disrupted cellular energy metabolism plays a central role. Precise control of cellular calcium concentrations is crucial for maintenance of energy homeostasis. Recently, differential cellular expression of neuronal voltage-gated calcium channel (Ca(V)) isoforms has been implicated in the susceptibility of vulnerable neurons to neurodegeneration in Parkinson's disease. Ca(V) channels are also involved in the synaptic plasticity response to the denervation that occurs in Parkinson's disease and following chronic treatment with anti-parkinsonian drugs. This review will examine the putative role neuronal Ca(V) channels have in the pathogenesis and treatment of Parkinson's disease.

Modulation of striatal projection systems by dopamine

The basal ganglia are a chain of subcortical nuclei that facilitate action selection. Two striatal projection systems--so-called direct and indirect pathways--form the functional backbone of the basal ganglia circuit. Twenty years ago, investigators proposed that the striatum's ability to use dopamine (DA) rise and fall to control action selection was due to the segregation of D(1) and D(2) DA receptors in direct- and indirect-pathway spiny projection neurons. Although this hypothesis sparked a debate, the evidence that has accumulated since then clearly supports this model. Recent advances in the means of marking neural circuits with optical or molecular reporters have revealed a clear-cut dichotomy between these two cell types at the molecular, anatomical, and physiological levels. The contrast provided by these studies has provided new insights into how the striatum responds to fluctuations in DA signaling and how diseases that alter this signaling change striatal function.

Endocytosis promotes rapid dopaminergic signaling

D(1) dopamine receptors are primary mediators of dopaminergic signaling in the CNS. These receptors internalize rapidly following agonist-induced activation, but the functional significance of this process is unknown. We investigated D(1) receptor endocytosis and signaling in HEK293 cells and cultured striatal neurons using real-time fluorescence imaging and cAMP biosensor technology. Agonist-induced activation of D(1) receptors promoted endocytosis of receptors with a time course overlapping that of acute cAMP accumulation. Inhibiting receptor endocytosis blunted acute D(1) receptor-mediated signaling in both dissociated cells and striatal slice preparations. Although endocytic inhibition markedly attenuated acute cAMP accumulation, inhibiting the subsequent recycling of receptors had no effect. Further, D(1) receptors localized in close proximity to endomembrane-associated trimeric G protein and adenylyl cyclase immediately after endocytosis. Together, these results suggest a previously unanticipated role of endocytosis, and the early endocytic pathway, in supporting rapid dopaminergic neurotransmission.

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

Molecular substrates of action control in cortico-striatal circuits

The purpose of this review is to describe the molecular mechanisms in the striatum that mediate reward-based learning and action control during instrumental conditioning. Experiments assessing the neural bases of instrumental conditioning have uncovered functional circuits in the striatum, including dorsal and ventral striatal sub-regions, involved in action-outcome learning, stimulus-response learning, and the motivational control of action by reward-associated cues. Integration of dopamine (DA) and glutamate neurotransmission within these striatal sub-regions is hypothesized to enable learning and action control through its role in shaping synaptic plasticity and cellular excitability. The extracellular signal regulated kinase (ERK) appears to be particularly important for reward-based learning and action control due to its sensitivity to combined DA and glutamate receptor activation and its involvement in a range of cellular functions. ERK activation in striatal neurons is proposed to have a dual role in both the learning and performance factors that contribute to instrumental conditioning through its regulation of plasticity-related transcription factors and its modulation of intrinsic cellular excitability. Furthermore, perturbation of ERK activation by drugs of abuse may give rise to behavioral disorders such as addiction.

Acute cocaine induces fast activation of D1 receptor and progressive deactivation of D2 receptor striatal neurons: in vivo optical microprobe [Ca2+]i imaging

Cocaine induces fast dopamine increases in brain striatal regions, which are recognized to underlie its rewarding effects. Both dopamine D1 and D2 receptors are involved in cocaine's reward but the dynamic downstream consequences of cocaine effects in striatum are not fully understood. Here we used transgenic mice expressing EGFP under the control of either the D1 receptor (D1R) or the D2 receptor (D2R) gene and microprobe optical imaging to assess the dynamic changes in intracellular calcium ([Ca(2+)](i)) responses (used as marker of neuronal activation) to acute cocaine in vivo separately for D1R- versus D2R-expressing neurons in striatum. Acute cocaine (8 mg/kg, i.p.) rapidly increased [Ca(2+)](i) in D1R-expressing neurons (10.6 ± 3.2%) in striatum within 8.3 ± 2.3 min after cocaine administration after which the increases plateaued; these fast [Ca(2+)](i) increases were blocked by pretreatment with a D1R antagonist (SCH23390). In contrast, cocaine induced progressive decreases in [Ca(2+)](i) in D2R-expressing neurons (10.4 ± 5.8%) continuously throughout the 30 min that followed cocaine administration; these slower [Ca(2+)](i) decreases were blocked by pretreatment with a D2R antagonist (raclopride). Since activation of striatal D1R-expressing neurons (direct-pathway) enhances cocaine reward, whereas activation of D2R-expressing neurons suppresses it (indirect-pathway) (Lobo et al., 2010), this suggests that cocaine's rewarding effects entail both its fast stimulation of D1R (resulting in abrupt activation of direct-pathway neurons) and a slower stimulation of D2R (resulting in longer-lasting deactivation of indirect-pathway neurons). We also provide direct in vivo evidence of D2R and D1R interactions in the striatal responses to acute cocaine administration.

Dopaminergic modulation of striatal neurons, circuits, and assemblies

In recent years, there has been a great deal of progress toward understanding the role of the striatum and dopamine in action selection. The advent of new animal models and the development of optical techniques for imaging and stimulating select neuronal populations have provided the means by which identified synapses, cells, and circuits can be reliably studied. This review attempts to summarize some of the key advances in this broad area, focusing on dopaminergic modulation of intrinsic excitability and synaptic plasticity in canonical microcircuits in the striatum as well as recent work suggesting that there are neuronal assemblies within the striatum devoted to particular types of computation and possibly action selection.

Control of striatal signaling by g protein regulators

Signaling via heterotrimeric G proteins plays a crucial role in modulating the responses of striatal neurons that ultimately shape core behaviors mediated by the basal ganglia circuitry, such as reward valuation, habit formation, and movement coordination. Activation of G protein-coupled receptors (GPCRs) by extracellular signals activates heterotrimeric G proteins by promoting the binding of GTP to their α subunits. G proteins exert their effects by influencing the activity of key effector proteins in this region, including ion channels, second messenger enzymes, and protein kinases. Striatal neurons express a staggering number of GPCRs whose activation results in the engagement of downstream signaling pathways and cellular responses with unique profiles but common molecular mechanisms. Studies over the last decade have revealed that the extent and duration of GPCR signaling are controlled by a conserved protein family named regulator of G protein signaling (RGS). RGS proteins accelerate GTP hydrolysis by the α subunits of G proteins, thus promoting deactivation of GPCR signaling. In this review, we discuss the progress made in understanding the roles of RGS proteins in controlling striatal G protein signaling and providing integration and selectivity of signal transmission. We review evidence on the formation of a macromolecular complex between RGS proteins and other components of striatal signaling pathways, their molecular regulatory mechanisms and impacts on GPCR signaling in the striatum obtained from biochemical studies and experiments involving genetic mouse models. Special emphasis is placed on RGS9-2, a member of the RGS family that is highly enriched in the striatum and plays critical roles in drug addiction and motor control.

The endocannabinoid 2-arachidonoylglycerol mediates D1 and D2 receptor cooperative enhancement of rat nucleus accumbens core neuron firing

Many motivated and addiction-related behaviors are sustained by activity of both dopamine D1- and D2-type receptors (D1Rs and D2Rs) as well as CB1 receptors (CB1Rs) in the nucleus accumbens (NAc). Here, we use in vitro whole-cell patch-clamp electrophysiology to describe an endocannabinoid (eCB)-dopamine receptor interaction in adult rat NAc core neurons. D1R and D2R agonists in combination enhanced firing, with no effect of a D1R or D2R agonist alone. This D1R+D2R-mediated firing increase required CB1Rs, since it was prevented by the CB1R antagonists AM251 and Rimonabant. The D1R+D2R firing increase also required phospholipase C (PLC), the major synthesis pathway for the eCB 2-arachidonoylglycerol (2-AG) and one of several pathways for anandamide. Further, inhibition of 2-AG hydrolysis with the monoglyceride lipase (MGL) inhibitor JZL184 allowed subthreshold levels of D1R+D2R receptor agonists to enhance firing, while inhibition of anandamide hydrolysis with the fatty acid amide hydrolase (FAAH) inhibitors URB597 or AM3506 did not. Filling the postsynaptic neuron with 2-AG enabled subthreshold D1R+D2R agonists to increase firing, and the 2AG+D1R+D2R increase in firing was prevented by a CB1R antagonist. Also, the metabotropic glutamate receptor 5 (mGluR5) blocker MPEP prevented the ability of JZL184 to promote subthreshold D1R+D2R enhancement of firing, while the 2-AG+D1R+D2R increase in firing was not prevented by the mGluR5 blocker, suggesting that mGluR5s acted upstream of 2-AG production. Thus, our results taken together are consistent with the hypothesis that NAc core eCBs mediate dopamine receptor (DAR) enhancement of firing, perhaps providing a cellular mechanism underlying the central role of NAc core D1Rs, D2Rs, CB1Rs, and mGluR5s during many drug-seeking behaviors.

Deciphering the Actions of Antiparkinsonian and Antipsychotic Drugs on cAMP/DARPP-32 Signaling

The basal ganglia are affected by several neuropsychiatric and neurodegenerative diseases, many of which are treated with drugs acting on the dopamine system. For instance, the loss of dopaminergic input to the striatum, which is the main pathological feature of Parkinson’s disease, is counteracted by administering the dopamine precursor, L-DOPA. Furthermore, psychotic disorders, including schizophrenia, are treated with drugs that act as antagonists at the D2-type of dopamine receptor (D2R). The use of L-DOPA and typical antipsychotic drugs, such as haloperidol, is limited by the emergence of motor side-effects, particularly after prolonged use. Striatal medium spiny neurons (MSNs) represent an ideal tool to investigate the molecular changes implicated in these conditions. MSNs receive a large glutamatergic innervation from cortex, thalamus, and limbic structures, and are controlled by dopaminergic projections originating in the midbrain. There are two large populations of striatal MSNs, which differ based on their connectivity to the output nuclei of the basal ganglia and on their ability to express dopamine D1 receptors (D1Rs) or D2Rs. Administration of L-DOPA promotes cAMP signaling and activates the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) in the D1R-expressing MSNs, which form the striatonigral, or direct pathway. Conversely, haloperidol activates the cAMP/DARPP-32 cascade in D2R-expressing MSNs, which form the striatopallidal, or indirect pathway. This review describes the effects produced on downstream effector proteins by stimulation of cAMP/DARPP-32 signaling in these two groups of MSNs. Particular emphasis is given to the regulation of the GluR1 subunit of the α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamate receptor, the extracellular signal-regulated protein kinases 1 and 2, focusing on functional role and potential pathological relevance.

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.

Dopamine-dependent ectodomain shedding and release of epidermal growth factor in developing striatum: target-derived neurotrophic signaling (Part 2)

Epidermal growth factor (EGF) and structurally related peptides promote neuronal survival and the development of midbrain dopaminergic neurons; however, the regulation of their production has not been fully elucidated. In this study, we found that the treatment of striatal cells with dopamine agonists enhances EGF release both in vivo and in vitro. We prepared neuron-enriched and non-neuronal cell-enriched cultures from the striatum of rat embryos and challenged those with various neurotransmitters or dopamine receptor agonists. Dopamine and a dopamine D(1) -like receptor agonist (SKF38393) triggered EGF release from neuron-enriched cultures in a dose-dependent manner. A D(2) -like agonist (quinpirole) increased EGF release only from non-neuronal cell-enriched cultures. The EGF release from striatal neurons and non-neuronal cells was concomitant with ErbB1 phosphorylation and/or with the activation of a disintegrin and metalloproteinase and matrix metalloproteinase. The EGF release from neurons was attenuated by an a disintegrin and metalloproteinase/matrix metalloproteinase inhibitor, GM6001, and a calcium ion chelator, BAPTA/AM. Transfection of cultured striatal neurons with alkaline phosphatase-tagged EGF precursor cDNA confirmed that dopamine D(1) -like receptor stimulation promoted both ectodomain shedding of the precursor and EGF release. Therefore, the activation of striatal dopamine receptors induces shedding and release of EGF to provide a retrograde neurotrophic signal to midbrain dopaminergic neurons.

Repeated cocaine exposure decreases dopamine D₂-like receptor modulation of Ca(2+) homeostasis in rat nucleus accumbens neurons

The nucleus accumbens (NAc) is a limbic structure in the forebrain that plays a critical role in cognitive function and addiction. Dopamine modulates activity of medium spiny neurons (MSNs) in the NAc. Both dopamine D₁-like and D₂-like receptors (including D1R or D(1,5)R and D2R or D(2,3,4)R, respectively) are thought to play critical roles in cocaine addiction. Our previous studies demonstrated that repeated cocaine exposure (which alters dopamine transmission) decreases excitability of NAc MSNs in cocaine-sensitized, withdrawn rats. This decrease is characterized by a reduction in voltage-sensitive Na(+) currents and high voltage-activated Ca(2+) currents, along with increased voltage-gated K(+) currents. These changes are associated with enhanced activity in the D1R/cAMP/PKA/protein phosphatase 1 pathway and diminished calcineurin function. Although D1R-mediated signaling is enhanced by repeated cocaine exposure, little is known whether and how the D2R is implicated in the cocaine-induced NAc dysfunction. Here, we performed a combined electrophysiological, biochemical, and neuroimaging study that reveals the cocaine-induced dysregulation of Ca(2+) homeostasis with involvement of D2R. Our novel findings reveal that D2R stimulation reduced Ca(2+) influx preferentially via the L-type Ca(2+) channels and evoked intracellular Ca(2+) release, likely via inhibiting the cAMP/PKA cascade, in the NAc MSNs of drug-free rats. However, repeated cocaine exposure abolished the D₂R effects on modulating Ca(2+) homeostasis with enhanced PKA activity and led to a decrease in whole-cell Ca(2+) influx. These adaptations, which persisted for 21 days during cocaine abstinence, may contribute to the mechanism of cocaine withdrawal.

Dopaminergic modulation of corticostriatal responses in medium spiny projection neurons from direct and indirect pathways

Suprathreshold corticostriatal responses recorded from medium spiny neurons (MSNs) from the direct and indirect pathways of the basal ganglia are different. Their differences readily distinguish D₁- and D₂-type receptor expressing MSNs in both bacterial artificial chromosome-transgenic mice and their control littermates as well as in rats: indirect pathway neurons are more excitable than direct pathway neurons revealing autoregenerative spikes underlying their spike trains, whereas direct pathway neurons exhibit more prolonged plateau potentials and spike trains. SFK 81297, a selective agonist for D₁-class receptors enhanced corticostriatal responses in direct pathway neurons, while quinelorane, a selective agonist for D₂-class receptors reduced orthodromic and autoregenerative responses in indirect pathway neurons thus making both neuron classes similarly excitable. Because dopaminergic postsynaptic actions target Ca_V1 (L) class voltage-gated calcium channels in MSNs, we hypothesized that these channels are involved and can explain a part of the dopaminergic actions on corticostriatal integration. Both 2.5 μM nicardipine and 400 nM calciseptine, selective Ca_V1 channel blockers, reduced corticostriatal responses in both D₁- and D₂-receptor expressing neurons, respectively. A previous blockade of Ca_V1 channels occluded the actions of dopamine agonists in both neuronal classes. In contrast, a Ca_V1 (L) channel activator, 2.5 μM Bay K 8644, enhanced corticostriatal responses in neurons from both pathways. It is concluded that Ca_V1 intrinsic currents mediate a part of the dopaminergic modulation during orthodromic synaptic integration of cortical inputs in both classes of MSNs.

Neuromodulatory control of striatal plasticity and behavior

Excitatory synapses onto projection neurons in the striatum, the input nucleus of the basal ganglia, play a key role in regulating basal ganglia circuit function and are a major site of long-term synaptic plasticity. Here, we review the mechanisms and regulation of both long-term potentiation and long-term depression at these synapses. In particular, we highlight the role that neuromodulators play in determining the strength and direction of plasticity, which ultimately shapes the balance of activity in basal ganglia circuits and regulates motor behavior.

Molecular switch from L-type Ca v 1.3 to Ca v 1.2 Ca2+ channel signaling underlies long-term psychostimulant-induced behavioral and molecular plasticity

L-type Ca(2+) channel (LTCC)-activated signaling cascades contribute significantly to psychostimulant-induced locomotor sensitization; however, the precise contribution of the two brain-specific subunits Ca(v)1.2 and Ca(v)1.3 remains mostly unknown. In this study, by using amphetamine and cocaine locomotor sensitization in mutant mice expressing dihydropyridine (DHP)-insensitive Ca(v)1.2 LTCCs (Ca(v)1.2DHP(-/-)), we find that, as opposed to a previously identified role of the Ca(v)1.3 subunit of LTCCs in development of sensitization, the Ca(v)1.2 subunit mediates expression of amphetamine and cocaine sensitization when examined after a 14 d drug-free period. Molecular studies to further elucidate the role of Ca(v)1.2 versus Ca(v)1.3 LTCCs in activating signaling pathways in the nucleus accumbens (NAc) of drug-naive versus drug-preexposed mice examined 14 d later revealed that an acute amphetamine and cocaine challenge in drug-naive mice increases Ser133 cAMP response element-binding protein (CREB) phosphorylation in the NAc via Ca(v)1.3 channels and via a dopamine D(1)-dependent mechanism, independent of the extracellular signal-regulated kinase (ERK) pathway, an important mediator of psychostimulant-induced plasticity. In contrast, in amphetamine- and cocaine-preexposed mice, an amphetamine or cocaine challenge no longer activates CREB unless Ca(v)1.2 LTCCs are blocked. This Ca(v)1.2-dependent blunting of CREB activation that underlies expression of locomotor sensitization occurs only after extended drug-free periods and involves recruitment of D(1) receptors and the ERK pathway. Thus, our results demonstrate that specific LTCC subunits are required for the development (Ca(v)1.3) versus expression (Ca(v)1.2) of psychostimulant sensitization and that subunit-specific signaling pathways recruited by psychostimulants underlies long-term drug-induced behavioral responses.

Altered profile and D2-dopamine receptor modulation of high voltage-activated calcium current in striatal medium spiny neurons from animal models of Parkinson's disease

In the present work we analyzed the profile of high voltage-activated (HVA) calcium (Ca2+) currents in freshly isolated striatal medium spiny neurons (MSNs) from rodent models of both idiopathic and familial forms of Parkinson's disease (PD). MSNs were recorded from reserpine-treated and 6-hydroxydopamine (6-OHDA)-lesioned rats, and from DJ-1 and PINK1 (PTEN induced kinase 1) knockout (-/-) mice. Our analysis showed no significant changes in total HVA Ca2+ current. However, we recorded a net increase in the L-type fraction of HVA Ca2+ current in dopamine-depleted rats, and of both N- and P-type components in DJ-1-/- mice, whereas no significant change in Ca2+ current profile was observed in PINK1-/- mice. Dopamine modulates HVA Ca2+ channels in MSNs, thus we also analyzed the effect of D1 and D2 receptor activation. The effect of the D1 receptor agonist SKF 83822 on Ca2+ current was not significantly different among MSNs from control animals or PD models. However, in both dopamine-depleted rats and DJ-1-/- mice the D2 receptor agonist quinpirole inhibited a greater fraction of HVA Ca2+ current than in the respective controls. Conversely, in MSNs from PINK1-/- mice we did not observe alterations in the effect of D2 receptor activation. Additionally, in both reserpine-treated and 6-OHDA-lesioned rats, the effect of quinpirole was occluded by the selective L-type Ca2+ channel blocker nifedipine, while in DJ-1-/- mice it was mostly occluded by ω-conotoxin GVIA, blocker of N-type channels. These results demonstrate that both dopamine depletion and DJ-1 deletion induce a rearrangement in the HVA Ca2+ channel profile, specifically involving those channels that are selectively modulated by D2 receptors.

Contributions of ERK signaling in the striatum to instrumental learning and performance

The striatum is critical for learning and decision making; however, the molecular mechanisms that govern striatum function are not fully understood. The extracellular signal regulated kinase (ERK) cascade is an important signaling pathway that underlies synaptic plasticity, cellular excitability, learning and arousal. This review focuses on the role of ERK signaling in striatum function. ERK is activated in the striatum by coordinated dopamine and glutamate receptor signaling, where it underlies corticostriatal synaptic plasticity and influences striatal cell excitability. ERK activation in the dorsal striatum is necessary for action-outcome learning and performance of goal-directed actions. In the ventral striatum, ERK is necessary for the motivating effects of reward-associated stimuli on instrumental performance. Dysregulation of ERK signaling in the striatum by repeated drug exposure contributes to the development of addictive behavior. These results highlight the importance of ERK signaling in the striatum as a critical substrate for learning and as a regulator of ongoing behavior. Furthermore, they suggest that ERK may be a suitable target for therapeutics to treat disorders of learning and decision making that arise from compromised striatum function.

Developmental increase in D1-like dopamine receptor-mediated inhibition of glutamatergic transmission through P/Q-type channel regulation in the basal forebrain of rats

Whole-cell patch-clamp recordings of non-N-methyl-d-aspartate glutamatergic excitatory postsynaptic currents (EPSCs) were carried out from cholinergic neurons in slices of basal forebrain (BF) of developing rats aged 21-42 postnatal days to elucidate postnatal developmental change in Ca(2+) channel subtypes involved in the transmission as well as that in dopamine D(1)-like receptor-mediated presynaptic inhibition. The amplitude of EPSCs was inhibited by bath application of omega-conotoxin GVIA (omega-CgTX; 3 microM) or omega-agatoxin-TK (omega-Aga-TK; 200 nM) throughout the age range examined, suggesting that multiple types of Ca(2+) channel are involved in the transmission. The EPSC fraction reduced by omega-CgTX decreased with age, whereas that reduced by omega-Aga-TK increased. Inhibition of the EPSCs by a D(1)-like receptor agonist, SKF 81297 (SKF; 30 microM) increased with age in parallel with the increase in omega-Aga-TK-induced inhibition. An activator of the adenylyl cyclase (AC) pathway, forskolin (FK; 10 microM) inhibited the EPSCs, and FK-induced inhibition also increased with age in parallel with the increase in SKF-induced inhibition. Throughout the age range examined, SKF showed no further inhibitory effect on the EPSCs after omega-Aga-TK- or FK-induced effect had reached steady-state. These findings suggest that D(1)-like receptor-mediated presynaptic inhibition of glutamate release onto cholinergic BF neurons increases with age, and that the change is coupled with a developmental increase in the contribution of P/Q-type Ca(2+) channels as well as a developmental increase in AC pathway contribution.

Amphioxus expresses both vertebrate-type and invertebrate-type dopamine D(1) receptors

The cephalochordate amphioxus (Branchiostoma floridae) has recently been placed as the most basal of all the chordates, which makes it an ideal organism for studying the molecular basis of the evolutionary transition from invertebrates to vertebrates. The biogenic amine, dopamine regulates many aspects of motor control in both vertebrates and invertebrates, and in both cases, its receptors can be divided into two main groups (D1 and D2) based on sequence similarity, ligand affinity and effector coupling. A bioinformatic study shows that amphioxus has at least three dopamine D1-like receptor sequences. We have recently characterized one of these receptors, AmphiD1/β, which was found to have high levels of sequence similarity to both vertebrate D1 receptors and to β-adrenergic receptors, but functionally appeared to be a vertebrate-type dopamine D(1) receptor. Here, we report on the cloning of two further dopamine D(1) receptors (AmphiAmR1 and AmphiAmR2) from adult amphioxus cDNA libraries and their pharmacological characterisation subsequent to their expression in cell lines. AmphiAmR1 shows closer structural similarities to vertebrate D(1)-like receptors but shows some pharmacological similarities to invertebrate "DOP1" dopamine D(1)-like receptors. In contrast, AmphiAmR2 shows closer structural and pharmacological similarities to invertebrate "INDR"-like dopamine D(1)-like receptors.

Paired-pulse inhibition in the auditory cortex in Parkinson's disease and its dependence on clinical characteristics of the patients

We aimed to determine the value of the paired-pulse inhibition (PPI) in the auditory cortex in patients with Parkinson's disease (PD) and analyze its dependence on clinical characteristics of the patients. The central (Cz) auditory evoked potentials were recorded in 58 patients with PD and 22 age-matched healthy subjects. PPI of the N1/P2 component was significantly (P < .001) reduced for interstimulus intervals 500, 700, and 900 ms in patients with PD compared to control subjects. The value of PPI correlated negatively with the age of the PD patients (P < .05), age of disease onset (P < .05), body bradykinesia score (P < .01), and positively with the Mini Mental State Examination (MMSE) cognitive score (P < .01). Negative correlation between value of PPI and the age of the healthy subjects (P < .05) was also observed. Thus, results show that cortical inhibitory processes are deficient in PD patients and that the brain's ability to carry out the postexcitatory inhibition is age-dependent.

A single spiking neuron that can represent interval timing: analysis, plasticity and multi-stability

The ability to represent interval timing is crucial for many common behaviors, such as knowing whether to stop when the light turns from green to yellow. Neural representations of interval timing have been reported in the rat primary visual cortex and we have previously presented a computational framework describing how they can be learned by a network of neurons. Recent experimental and theoretical results in entorhinal cortex have shown that single neurons can exhibit persistent activity, previously thought to be generated by a network of neurons. Motivated by these single neuron results, we propose a single spiking neuron model that can learn to compute and represent interval timing. We show that a simple model, reduced analytically to a single dynamical equation, captures the average behavior of the complete high dimensional spiking model very well. Variants of this model can be used to produce bi-stable or multi-stable persistent activity. We also propose a plasticity rule by which this model can learn to represent different intervals and different levels of persistent activity.

Activation of dopamine D1/D5 receptors facilitates the induction of presynaptic long-term potentiation at hippocampal output synapses

Encoding of novel information has been proposed to rely on the time-locked release of dopamine in the hippocampal formation during novelty detection. However, the site of novelty detection in the hippocampus remains a matter of debate. According to current models, the CA1 and the subiculum act as detectors and distributors of novel sensory information. Although most CA1 pyramidal neurons exhibit regular-spiking behavior, the majority of subicular pyramidal neurons fire high-frequency bursts of action potentials. The present study investigates the efficacy of dopamine D1/D5 receptor activation to facilitate the induction of activity-dependent long-term potentiation (LTP) in rat CA1 regular-spiking and subicular burst-spiking pyramidal cells. Using a weak stimulation protocol, set at a level subthreshold for the induction of LTP, we show that activation of D1/D5 receptors for 5-10 min facilitates LTP in subicular burst-spiking neurons but not in CA1 neurons. The results demonstrate that D1/D5 receptor-facilitated LTP is NMDA receptor-dependent, and requires the activation of protein kinase A. In addition, the D1/D5 receptor-facilitated LTP is shown to be presynaptically expressed and relies on presynaptic Ca(2+) signaling. The phenomenon of dopamine-induced facilitation of presynaptic NMDA receptor-dependent LTP in subicular burst-spiking pyramidal cells is in accordance with observations of the time-locked release of dopamine during novelty detection in this brain region, and reveals an intriguing mechanism for the encoding of hippocampal output information.

Dopamine induces Ca2+ signaling in astrocytes through reactive oxygen species generated by monoamine oxidase

Dopamine is a neurotransmitter that plays a major role in a variety of brain functions, as well as in disorders such as Parkinson disease and schizophrenia. In cultured astrocytes, we have found that dopamine induces sporadic cytoplasmic calcium ([Ca(2+)](c)) signals. Importantly, we show that the dopamine-induced calcium signaling is receptor-independent in midbrain, cortical, and hippocampal astrocytes. We demonstrate that the calcium signal is initiated by the metabolism of dopamine by monoamine oxidase, which produces reactive oxygen species and induces lipid peroxidation. This stimulates the activation of phospholipase C and subsequent release of calcium from the endoplasmic reticulum via the inositol 1,4,5-trisphosphate receptor mechanism. These findings have major implications on the function of astrocytes that are exposed to dopamine and may contribute to understanding the physiological role of dopamine.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Dopamine-dependent tuning of striatal inhibitory synaptogenesis

Dopaminergic projections to the striatum, crucial for the correct functioning of this brain region in adulthood, are known to be established early in development, but their role is currently uncharacterized. We demonstrate here that dopamine, by activating D(1)- and/or D(2)-dopamine receptors, decreases the number of functional GABAergic synapses formed between the embryonic precursors of the medium spiny neurons, the principal output neurons of the striatum, with associated changes in spontaneous synaptic activity. Activation of these receptors reduces the size of postsynaptic GABA(A) receptor clusters and their overall cell-surface expression, without affecting the total number of clusters or the size or number of GABAergic nerve terminals. These changes result from an increased internalization of GABA(A) receptors, and are mediated by distinct signaling pathways converging at the level of GABA(A) receptors to cause a transient PP2A/PP1-dependent dephosphorylation. Thus, tonic D(1)- and D(2)-receptor activity limits the extent of collateral inhibitory synaptogenesis between medium spiny neurons, revealing a novel role of dopamine in controlling the development of intrinsic striatal microcircuits.

Signaling pathways controlling the phosphorylation state of WAVE1, a regulator of actin polymerization

The Wiskott-Aldrich syndrome protein (WASP)-family verprolin homologous protein 1 (WAVE1) is a key regulator of Arp (actin-related protein) 2/3 complex-mediated actin polymerization. We have established previously that the state of phosphorylation of WAVE1 at three distinct residues controls its ability to regulate actin polymerization and spine morphology. Cyclin-dependent kinase 5 phosphorylates WAVE1 at Ser310, Ser397 and Ser441 to a high basal stoichiometry, resulting in inhibition of WAVE1 activity. Our previous and current studies show that WAVE1 can be dephosphorylated at all three sites and thereby activated upon stimulation of the D1 subclass of dopamine receptors and of the NMDA subclass of glutamate receptors, acting through cAMP and Ca(2+) signaling pathways, respectively. Specifically, we have identified protein phosphatase-2A and protein phosphatase-2B as the effectors for these second messengers. These phosphatases act on different sites to mediate receptor-induced signaling pathways, which would lead to activation of WAVE1.

Transient viral-mediated overexpression of alpha-calcium/calmodulin-dependent protein kinase II in the nucleus accumbens shell leads to long-lasting functional upregulation of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors: dopamine type-1 receptor and protein kinase A dependence

Calcium/calmodulin-dependent protein kinase II (CaMKII) activity is necessary for the long-lasting expression of locomotor sensitization and enhanced drug-taking observed in rats previously exposed to psychostimulants. Exposure to these drugs also transiently increases alphaCaMKII levels in the nucleus accumbens (NAcc), an effect that, when mimicked by transient viral-mediated overexpression of alphaCaMKII in NAcc shell neurons, leads to long-lasting enhancement in locomotor responding to amphetamine and NAcc alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA). The present experiments characterized the dopamine (DA) dependence of the functional AMPA receptor upregulation observed long after transient overexpression of alphaCaMKII. Rats infected with herpes simplex virus-alphaCaMKII in the NAcc shell showed a transient increase in alphaCaMKII levels that peaked at 4 days post-infection and returned to baseline 8 days later. When challenged with AMPA (0.8 nmol/side) in the NAcc shell at 20 days post-infection, these rats showed enhanced locomotion compared with controls. This sensitized locomotor response was blocked when AMPA was coinfused with either the DA type-1 receptor antagonist SCH23390 (0.8 nmol/side) or the protein kinase A inhibitor Rp-cAMPS (80 nmol/side). Neither SCH23390 nor Rp-cAMPS produced locomotor effects when infused by itself into the NAcc shell. Furthermore, these antagonists did not block the acute non-sensitized locomotor response to AMPA observed in control rats. These findings show that transient viral-mediated overexpression of alphaCaMKII in neurons of the NAcc shell leads to long-lasting functional upregulation of AMPA receptors that is DA type-1 receptor and protein kinase A dependent. Thus, transient increases in levels of alphaCaMKII in the NAcc shell produce long-lasting changes in the way that DA and glutamate interact in this site to generate locomotor behavior.

Localization of dopamine- and cAMP-regulated phosphoprotein-32 and inhibitor-1 in area 9 of Macaca mulatta prefrontal cortex

The actions of dopamine D1 family receptors (D1R) depend upon a signal transduction cascade that modulates the phosphorylation state of important effector proteins, such as glutamate receptors and ion channels. This is accomplished both through activation of protein kinase A (PKA) and the inhibition of protein phosphatase-1 (PP1). Inhibition of PP1 occurs through PKA-mediated phosphorylation of dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP-32) or the related protein inhibitor-1 (I-1), and the availability of DARPP-32 is essential to the functional outcome of D1R activation in the basal ganglia. While D1R activation is critical for prefrontal cortex (PFC) function, especially working memory, the functional role played by DARPP-32 or I-1 is less clear. In order to examine this more thoroughly, we have utilized immunoelectron microscopy to quantitatively determine the localization of DARPP-32 and I-1 in the neuropil of the rhesus monkey PFC. Both were distributed widely in the different components of the neuropil, but were enriched in dendritic shafts. I-1 label was more frequently identified in axon terminals than was DARPP-32, and DARPP-32 label was more frequently identified in glia than was I-1. We also quantified the extent to which these proteins were found in dendritic spines. DARPP-32 and I-1 were present in small subpopulations of dendritic spines, (4.4% and 7.7% and respectively), which were substantially smaller than observed for D1R in our previous studies (20%). Double-label experiments did not find evidence for colocalization of D1R and DARPP-32 or I-1 in spines or terminals. Thus, at the least, not all prefrontal spines which contain D1R also contain I-1 or DARPP-32, suggesting important differences in D1R signaling in the PFC compared to the striatum.

Transient overexpression of alpha-Ca2+/calmodulin-dependent protein kinase II in the nucleus accumbens shell enhances behavioral responding to amphetamine

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is known to contribute to the expression of psychostimulant sensitization by regulating dopamine (DA) overflow from DA neuron terminals in the nucleus accumbens (NAcc). The present experiments explored the contribution of CaMKII in NAcc neurons postsynaptic to these terminals where it is known to participate in a number of signaling pathways that regulate responding to psychostimulant drugs. Exposure to amphetamine transiently increased alphaCaMKII levels in the shell but not the core of the NAcc. Thus, HSV (herpes simplex viral) vectors were used to transiently overexpress alphaCaMKII in NAcc neurons in drug-naive rats, and behavioral responding to amphetamine was assessed. Transiently overexpressing alphaCaMKII in the NAcc shell led to long-lasting enhancement of amphetamine-induced locomotion and self-administration manifested when alphaCaMKII levels were elevated and persisting long after they had returned to baseline. Enhanced locomotion was not observed after infection in the NAcc core or sites adjacent to the NAcc. Transient elevation of NAcc shell alphaCaMKII levels also enhanced locomotor responding to NAcc AMPA and increased phosphorylation levels of GluR1 (Ser831), a CaMKII site, both soon and long after infection. Similar increases in pGluR1 (Ser831) were observed both soon and long after exposure to amphetamine. These results indicate that the transient increase in alphaCaMKII observed in neurons of the NAcc shell after viral-mediated gene transfer and likely exposure to amphetamine leads to neuroadaptations in AMPA receptor signaling in this site that may contribute to the long-lasting maintenance of behavioral and incentive sensitization by psychostimulant drugs like amphetamine.

Identification and characterization of a novel amphioxus dopamine D-like receptor

Dopamine receptors function to control many aspects of motor control and other forms of behaviour in both vertebrates and invertebrates. They can be divided into two main groups (D(1) and D(2)) based on sequence similarity, ligand affinity and effector coupling. However, little is known about the pharmacology and functionality of dopamine receptors in the deuterostomian invertebrates, such as the cephalochordate amphioxus (Branchiostoma floridae) which has recently been placed as the most basal of all the chordates. A bioinformatic study shows that amphioxus has at least three dopamine D(1)-like receptor sequences. One of these receptors, AmphiD(1)/beta, was found to have high levels of sequence similarity to both vertebrate D(1) receptors and to beta-adrenergic receptors. Here, we report on the cloning of AmphiD(1)/beta from an adult amphioxus cDNA library, and its pharmacological characterization subsequent to its expression in both mammalian cell lines and Xenopus oocytes. It was found that AmphiD(1)/beta has a similar pharmacology to vertebrate D(1) receptors, including responding to benzodiazepine ligands. The pharmacology of the receptor exhibits 'agonist-specific coupling' depending upon the second messenger pathway to which it is linked. Moreover, no pharmacological characteristics were observed to suggest that AmphiD(1)/beta may be an amphioxus orthologue of vertebrate beta-adrenergic receptors.

Impact of serotonin 2C receptor null mutation on physiology and behavior associated with nigrostriatal dopamine pathway function

The impact of serotonergic neurotransmission on brain dopaminergic pathways has substantial relevance to many neuropsychiatric disorders. A particularly prominent role has been ascribed to the inhibitory effects of serotonin 2C receptor (5-HT(2C)R) activation on physiology and behavior mediated by the mesolimbic dopaminergic pathway, particularly in the terminal region of the nucleus accumbens. The influence of this receptor subtype on functions mediated by the nigrostriatal dopaminergic pathway is less clear. Here we report that a null mutation eliminating expression of 5-HT(2C)Rs produces marked alterations in the activity and functional output of this pathway. 5-HT(2C)R mutant mice displayed increased activity of substantia nigra pars compacta (SNc) dopaminergic neurons, elevated baseline extracellular dopamine concentrations in the dorsal striatum (DSt), alterations in grooming behavior, and enhanced sensitivity to the stereotypic behavioral effects of d-amphetamine and GBR 12909. These psychostimulant responses occurred in the absence of phenotypic differences in drug-induced extracellular dopamine concentration, suggesting a phenotypic alteration in behavioral responses to released dopamine. This was further suggested by enhanced behavioral responses of mutant mice to the D(1) receptor agonist SKF 81297. Differences in DSt D(1) or D(2) receptor expression were not found, nor were differences in medium spiny neuron firing patterns or intrinsic membrane properties following dopamine stimulation. We conclude that 5-HT(2C)Rs regulate nigrostriatal dopaminergic activity and function both at SNc dopaminergic neurons and at a locus downstream of the DSt.

A neurocomputational model of tonic and phasic dopamine in action selection: a comparison with cognitive deficits in Parkinson's disease

The striatal dopamine signal has multiple facets; tonic level, phasic rise and fall, and variation of the phasic rise/fall depending on the expectation of reward/punishment. We have developed a network model of the striatal direct pathway using an ionic current level model of the medium spiny neuron that incorporates currents sensitive to changes in the tonic level of dopamine. The model neurons in the network learn action selection based on a novel set of mathematical rules that incorporate the phasic change in the dopamine signal. This network model is capable of learning to perform a sequence learning task that in humans is thought to be dependent on the basal ganglia. When both tonic and phasic levels of dopamine are decreased, as would be expected in unmedicated Parkinson's disease (PD), the model reproduces the deficits seen in a human PD group off medication. When the tonic level is increased to normal, but with reduced phasic increases and decreases in response to reward and punishment, respectively, as would be expected in PD medicated with L-Dopa, the model again reproduces the human data. These findings support the view that the cognitive dysfunctions seen in Parkinson's disease are not solely either due to the decreased tonic level of dopamine or to the decreased responsiveness of the phasic dopamine signal to reward and punishment, but to a combination of the two factors that varies dependent on disease stage and medication status.

Anatomically dissociable effects of dopamine D1 receptor agonists on reward and relief of withdrawal in morphine-dependent rats

RATIONALE

Chronic opiate administration induces neuroadaptations within the nucleus accumbens (NAc) and ventral tegmental area (VTA) that can contribute to dependence. We have shown that morphine dependence shifts the behavioral consequences of D1 dopamine (DA) receptor signaling: systemic administration of a D1 receptor agonist is rewarding and blocks naloxone-precipitated withdrawal signs in morphine-dependent rats, but has minimal effects in nondependent rats. These data suggest that D1 receptors acquire the ability to regulate reward and withdrawal in morphine-dependent rats. The brain regions involved in these effects are not known.

OBJECTIVE

Studies were designed to test the hypothesis that the nucleus accumbens shell (NASh) and the ventral tegmental area (VTA) are important sites for mediating the behavioral effects of D1 receptor activation in morphine-dependent rats.

MATERIALS AND METHODS

The effects of microinjecting the D1 receptor agonist SKF 82958 into the NASh or the VTA on place conditioning and somatic withdrawal signs were studied in morphine-dependent and nondependent rats.

RESULTS

Intra-NASh microinjection of SKF 82958 (1 microg/side) established conditioned place preferences in morphine-dependent but not nondependent rats, but had no effect on naloxone-induced place aversions or somatic withdrawal signs. Intra-VTA microinjection of SKF 82958 (2 microg) did not establish place preferences under any conditions, but blocked naloxone-induced place aversions without effects on somatic withdrawal signs.

CONCLUSIONS

There is an anatomical dissociation between D1 receptor-mediated reward and relief of withdrawal in morphine-dependent rats. When combined, the individual effects of D1 receptor activation in the NASh and VTA on the affective signs of precipitated morphine withdrawal resemble those seen with systemic administration.

Physiology and pharmacology of striatal neurons

The basal ganglia occupy the core of the forebrain and consist of evolutionarily conserved motor nuclei that form recurrent circuits critical for motivation and motor planning. The striatum is the main input nucleus of the basal ganglia and a key neural substrate for procedural learning and memory. The vast majority of striatal neurons are spiny GABAergic projection neurons, which exhibit slow but temporally precise spiking in vivo. Contributing to this precision are several different types of interneurons that constitute only a small fraction of total neuron number but play a critical role in regulating striatal output. This review examines the cellular physiology and modulation of striatal neurons that give rise to their unique properties and function.

Expression of human amyloid precursor protein in rat cortical neurons inhibits calcium oscillations

Synchronous calcium oscillations are observed in primary cultures of rat cortical neurons when mature networks are formed. This spontaneous neuronal activity needs an accurate control of calcium homeostasis. Alteration of intraneuronal calcium concentration is described in many neurodegenerative disorders, including Alzheimer disease (AD). Although processing of amyloid precursor protein (APP) that generates Abeta peptide has critical implications for AD pathogenesis, the neuronal function of APP remains unclear. Here, we report that expression of human APP (hAPP) in rat cortical neurons increases L-type calcium currents, which stimulate SK channels, calcium-dependent K(+) channels responsible for medium afterhyperpolarization (mAHP). In a neuronal network, increased mAHP in some neurons expressing hAPP leads to inhibition of calcium oscillations in all the cells of the network. This inhibition is independent of production and secretion of Abeta and other APP metabolites. In a neuronal network, reduction of endogenous APP expression using shRNA increases the frequency and reduces the amplitude of calcium oscillations. Altogether, these data support a key role for APP in the control of neuronal excitability.

D1/D5 modulation of synaptic NMDA receptor currents

Converging evidence suggests that salience-associated modulation of behavior is mediated by the release of monoamines and that monoaminergic activation of D(1)/D(5) receptors is required for normal hippocampal-dependent learning and memory. However, it is not understood how D(1)/D(5) modulation of hippocampal circuits can affect salience-associated learning and memory. We have observed in CA1 pyramidal neurons that D(1)/D(5) receptor activation elicits a bidirectional long-term plasticity of NMDA receptor-mediated synaptic currents with the polarity of plasticity determined by NMDA receptor, NR2A/B subunit composition. This plasticity results in a decrease in the NR2A/NR2B ratio of subunit composition. Synaptic responses mediated by NMDA receptors that include NR2B subunits are potentiated by D(1)/D(5) receptor activation, whereas responses mediated by NMDA receptors that include NR2A subunits are depressed. Furthermore, these bidirectional, subunit-specific effects are mediated by distinctive intracellular signaling mechanisms. Because there is a predominance of NMDA receptors composed of NR2A subunits observed in entorhinal-CA1 inputs and a predominance of NMDA receptors composed of NR2B subunits in CA3-CA1 synapses, potentiation of synaptic NMDA currents predominates in the proximal CA3-CA1 synapses, whereas depression of synaptic NMDA currents predominates in the distal entorhinal-CA1 synapses. Finally, all of these effects are reproduced by the release of endogenous monoamines through activation of D(1)/D(5) receptors. Thus, endogenous D(1)/D(5) activation can (1) decrease the NR2A/NR2B ratio of NMDA receptor subunit composition at glutamatergic synapses, a rejuvenation to a composition similar to developmentally immature synapses, and, (2) in CA1, bias NMDA receptor responsiveness toward the more highly processed trisynaptic CA3-CA1 circuit and away from the direct entorhinal-CA1 input.

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.

Dopamine D2 and adenosine A2A receptors regulate NMDA-mediated excitation in accumbens neurons through A2A-D2 receptor heteromerization

Bursting activity of striatal medium spiny neurons results from membrane potential oscillations between a down- and an upstate that could be regulated by G-protein-coupled receptors. Among these, dopamine D(2) and adenosine A(2A) receptors are highly enriched in striatal neurons and exhibit strong interactions whose physiological significance and molecular mechanisms remain partially unclear. More particularly, respective involvements of common intracellular signaling cascades and A(2A)-D(2) receptor heteromerization remain unknown. Here we show, by performing perforated-patch-clamp recordings on brain slices and loading competitive peptides, that D(2) and A(2A) receptors regulate the induction by N-methyl-D-aspartate of a depolarized membrane potential plateau through mechanisms relying upon specific protein-protein interactions. Indeed, D(2) receptor activation abolished transitions between a hyperpolarized resting potential and a depolarized plateau potential by regulating the Ca(V)1.3a calcium channel activity through interactions with scaffold proteins Shank1/3. Noticeably, A(2A) receptor activation had no effect per se but fully reversed the effects of D(2) receptor activation through a mechanism in which A(2A)-D(2) receptors heteromerization is strictly mandatory, demonstrating therefore a first direct physiological relevance of these heteromers. Our results show that membrane potential transitions and firing patterns in striatal neurons are tightly controlled by D(2) and A(2A) receptors through specific protein-protein interactions including A(2A)-D(2) receptors heteromerization.

Activation of phosphatidylinositol-linked novel D1 dopamine receptors inhibits high-voltage-activated Ca2+ currents in primary cultured striatal neurons

Recent evidences indicate the existence of a putative novel phosphatidylinositol (PI)-linked D(1) dopamine receptor that mediates excellent anti-Parkinsonian but less severe dyskinesia action. To further understand the basic physiological function of this receptor in brain, the effects of a PI-linked D(1) dopamine receptor-selective agonist 6-chloro-7,8-dihydroxy-3-methyl-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF83959) on high-voltage activated (HVA) Ca(2+) currents in primary cultured striatal neurons were investigated by whole cell patch-clamp technique. The results indicated that stimulation by SKF83959 induced an inhibition of HVA Ca(2+) currents in a dose-dependent manner in substance-P (SP)-immunoreactive striatal neurons. Application of D(1) receptor, but not D(2), alpha(1) adrenergic, 5-HT receptor, or cholinoceptor antagonist prevented SKF83959-induced reduction, indicating that a D(1) receptor-mediated event assumed via PI-linked D(1) receptor. SKF83959-induced inhibitory modulation was mediated by activation of phospholipase C (PLC), mobilization of intracellular Ca(2+) stores and activation of calcineurin. Furthermore, the inhibitory effects were attenuated significantly by the L-type calcium channel antagonist nifedipine, suggesting that L-type calcium channels involved in the regulation induced by SKF83959. These findings may help to further understand the functional role of the PI-linked dopamine receptor in brain.

Differential excitability and modulation of striatal medium spiny neuron dendrites

The loss of striatal dopamine (DA) in Parkinson's disease (PD) models triggers a cell-type-specific reduction in the density of dendritic spines in D(2) receptor-expressing striatopallidal medium spiny neurons (D(2) MSNs). How the intrinsic properties of MSN dendrites, where the vast majority of DA receptors are found, contribute to this adaptation is not clear. To address this question, two-photon laser scanning microscopy (2PLSM) was performed in patch-clamped mouse MSNs identified in striatal slices by expression of green fluorescent protein (eGFP) controlled by DA receptor promoters. These studies revealed that single backpropagating action potentials (bAPs) produced more reliable elevations in cytosolic Ca(2+) concentration at distal dendritic locations in D(2) MSNs than at similar locations in D(1) receptor-expressing striatonigral MSNs (D(1) MSNs). In both cell types, the dendritic Ca(2+) entry elicited by bAPs was enhanced by pharmacological blockade of Kv4, but not Kv1 K(+) channels. Local application of DA depressed dendritic bAP-evoked Ca(2+) transients, whereas application of ACh increased these Ca(2+) transients in D(2) MSNs, but not in D(1) MSNs. After DA depletion, bAP-evoked Ca(2+) transients were enhanced in distal dendrites and spines in D(2) MSNs. Together, these results suggest that normally D(2) MSN dendrites are more excitable than those of D(1) MSNs and that DA depletion exaggerates this asymmetry, potentially contributing to adaptations in PD models.