A crucial role for cAMP and protein kinase A in D1 dopamine receptor regulated intracellular calcium transients

It's all about making new contacts: How being metabotropic and phasicity help D1-like receptors promote LTP in the PFC

D1-like receptors have two important qualities, they are all metabotropic and they activate with phasic dopamine. After analyzing the molecular implications of each of these qualities separately and then combining them for the specific case of the prefrontal cortex, we propose a model that explains why long term potentiation in this cortical area depends on the amount of contact between D1-like receptors and dopamine. This simple model also explains why in order to promote long term potentiation, dopamine transporters should be scarce in the prefrontal cortex. Additionally, it explains why stimulants like methamphetamine could have such detrimental cognitive effects on regular substance consumers.

Optical control of neuronal activities with photoswitchable nanovesicles

Precise modulation of neuronal activity by neuroactive molecules is essential for understanding brain circuits and behavior. However, tools for highly controllable molecular release are lacking. Here, we developed a photoswitchable nanovesicle with azobenzene-containing phosphatidylcholine (azo-PC), coined 'azosome', for neuromodulation. Irradiation with 365 nm light triggers the trans-to-cis isomerization of azo-PC, resulting in a disordered lipid bilayer with decreased thickness and cargo release. Irradiation with 455 nm light induces reverse isomerization and switches the release off. Real-time fluorescence imaging shows controllable and repeatable cargo release within seconds (< 3 s). Importantly, we demonstrate that SKF-81297, a dopamine D1-receptor agonist, can be repeatedly released from the azosome to activate cultures of primary striatal neurons. Azosome shows promise for precise optical control over the molecular release and can be a valuable tool for molecular neuroscience studies.

Dopamine Receptor Activation Modulates the Integrity of the Perisynaptic Extracellular Matrix at Excitatory Synapses

In the brain, Hebbian-type and homeostatic forms of plasticity are affected by neuromodulators like dopamine (DA). Modifications of the perisynaptic extracellular matrix (ECM), which control the functions and mobility of synaptic receptors as well as the diffusion of transmitters and neuromodulators in the extracellular space, are crucial for the manifestation of plasticity. Mechanistic links between synaptic activation and ECM modifications are largely unknown. Here, we report that neuromodulation via D1-type DA receptors can induce targeted ECM proteolysis specifically at excitatory synapses of rat cortical neurons via proteases ADAMTS-4 and -5. We showed that receptor activation induces increased proteolysis of brevican (BC) and aggrecan, two major constituents of the adult ECM both in vivo and in vitro. ADAMTS immunoreactivity was detected near synapses, and shRNA-mediated knockdown reduced BC cleavage. We have outlined a molecular scenario of how synaptic activity and neuromodulation are linked to ECM rearrangements via increased cAMP levels, NMDA receptor activation, and intracellular calcium signaling.

Dopamine increases HIV entry into macrophages by increasing calcium release via an alternative signaling pathway

Dopaminergic dysfunction has long been connected to the development of HIV infection in the CNS. Our previous data showed that dopamine increases HIV infection in human macrophages by increasing the susceptibility of primary human macrophages to HIV entry through stimulation of both D1-like and D2-like receptors. These data suggest that, in macrophages, both dopamine receptor subtypes may act through a common signaling mechanism. To define better the mechanism(s) underlying this effect, this study examines the specific signaling processes activated by dopamine in primary human monocyte-derived macrophages (hMDM). In addition to confirming that the increase in entry is unique to dopamine, these studies show that dopamine increases HIV entry through a PKA insensitive, Ca²⁺ dependent pathway. Further examination demonstrated that dopamine can signal through a previously defined, non-canonical pathway in human macrophages. This pathway involves both Ca²⁺ release and PKC phosphorylation, and these data show that dopamine mediates both of these effects and that both were partially inhibited by the G_q/11 specific inhibitor YM-254890. Studies have shown that G_q/11 preferentially couples to the D1-like receptor D5, indicating an important role of the D1-like receptors in mediating these effects. These data indicate a role for Ca²⁺ flux in the HIV entry process, and suggest a distinct signaling mechanism mediating some of the effects of dopamine in macrophages. Together, the data indicate that targeting this alternative dopamine signaling pathway might provide new therapeutic options for individuals with elevated CNS dopamine suffering from NeuroHIV.

HIV, Tat and dopamine transmission

Human Immunodeficiency Virus (HIV) is a progressive infection that targets the immune system, affecting more than 37 million people around the world. While combinatorial antiretroviral therapy (cART) has lowered mortality rates and improved quality of life in infected individuals, the prevalence of HIV associated neurocognitive disorders is increasing and HIV associated cognitive decline remains prevalent. Recent research has suggested that HIV accessory proteins may be involved in this decline, and several studies have indicated that the HIV protein transactivator of transcription (Tat) can disrupt normal neuronal and glial function. Specifically, data indicate that Tat may directly impact dopaminergic neurotransmission, by modulating the function of the dopamine transporter and specifically damaging dopamine-rich regions of the CNS. HIV infection of the CNS has long been associated with dopaminergic dysfunction, but the mechanisms remain undefined. The specific effect(s) of Tat on dopaminergic neurotransmission may be, at least partially, a mechanism by which HIV infection directly or indirectly induces dopaminergic dysfunction. Therefore, precisely defining the specific effects of Tat on the dopaminergic system will help to elucidate the mechanisms by which HIV infection of the CNS induces neuropsychiatric, neurocognitive and neurological disorders that involve dopaminergic neurotransmission. Further, this will provide a discussion of the experiments needed to further these investigations, and may help to identify or develop new therapeutic approaches for the prevention or treatment of these disorders in HIV-infected individuals.

Chronic cocaine disrupts mesocortical learning mechanisms

The addictive power of drugs of abuse such as cocaine comes from their ability to hijack natural reward and plasticity mechanisms mediated by dopamine signaling in the brain. Reward learning involves burst firing of midbrain dopamine neurons in response to rewards and cues predictive of reward. The resulting release of dopamine in terminal regions is thought to act as a teaching signaling to areas such as the prefrontal cortex and striatum. In this review, we posit that a pool of extrasynaptic dopaminergic D1-like receptors activated in response to dopamine neuron burst firing serve to enable synaptic plasticity in the prefrontal cortex in response to rewards and their cues. We propose that disruptions in these mechanisms following chronic cocaine use contribute to addiction pathology, in part due to the unique architecture of the mesocortical pathway. By blocking dopamine reuptake in the cortex, cocaine elevates dopamine signaling at these extrasynaptic receptors, prolonging D1-receptor activation and the subsequent activation of intracellular signaling cascades, and thus inducing long-lasting maladaptive plasticity. These cellular adaptations may account for many of the changes in cortical function observed in drug addicts, including an enduring vulnerability to relapse. Therefore, understanding and targeting these neuroadaptations may provide cognitive benefits and help prevent relapse in human drug addicts.

Dopamine receptor activation increases HIV entry into primary human macrophages

Macrophages are the primary cell type infected with HIV in the central nervous system, and infection of these cells is a major component in the development of neuropathogenesis and HIV-associated neurocognitive disorders. Within the brains of drug abusers, macrophages are exposed to increased levels of dopamine, a neurotransmitter that mediates the addictive and reinforcing effects of drugs of abuse such as cocaine and methamphetamine. In this study we examined the effects of dopamine on HIV entry into primary human macrophages. Exposure to dopamine during infection increased the entry of R5 tropic HIV into macrophages, irrespective of the concentration of the viral inoculum. The entry pathway affected was CCR5 dependent, as antagonizing CCR5 with the small molecule inhibitor TAK779 completely blocked entry. The effect was dose-dependent and had a steep threshold, only occurring above 108 M dopamine. The dopamine-mediated increase in entry required dopamine receptor activation, as it was abrogated by the pan-dopamine receptor antagonist flupenthixol, and could be mediated through both subtypes of dopamine receptors. These findings indicate that the effects of dopamine on macrophages may have a significant impact on HIV pathogenesis. They also suggest that drug-induced increases in CNS dopamine may be a common mechanism by which drugs of abuse with distinct modes of action exacerbate neuroinflammation and contribute to HIV-associated neurocognitive disorders in infected drug abusers.

The secretogranin II gene is a signal integrator of glutamate and dopamine inputs

Cooperative gene regulation by different neurotransmitters likely underlies the long-term forms of associative learning and memory, but this mechanism largely remains to be elucidated. Following cDNA microarray analysis for genes regulated by Ca(2+) or cAMP, we found that the secretogranin II gene (Scg2) was cooperatively activated by glutamate and dopamine in primary cultured mouse hippocampal neurons. The Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) and the mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor PD98059 prevented Scg2 activation by glutamate or dopamine; thus, the Ca(2+) /MEK pathway is predicted to include a convergence point(s) of glutamatergic and dopaminergic signaling. Unexpectedly, the protein kinase A inhibitor KT5720 enhanced Scg2 activation by dopamine. The protein-synthesis inhibitor cycloheximide also enhanced Scg2 activation, and the proteasome inhibitor ZLLLH diminished the KT5720-mediated augmentation of Scg2 activation. These results are concordant with the notion that dopaminergic input leads to accumulation of a KT5720-sensitive transcriptional repressor, which is short-lived because of rapid degradation by proteasomes. This repression pathway may effectively limit the time window permissive to Scg2 activation by in-phase glutamate and dopamine inputs via the Ca(2+) /MEK pathway. We propose that the regulatory system of Scg2 expression is equipped with machinery that is refined for the signal integration of in-phase synaptic inputs. We proposed hypothetical mechanism for the regulation of the secretogranin II gene as a signal integrator of glutamate and dopamine inputs. Glutamate or dopamine activates the Ca(2+) /MEK/ERK pathway, which thus contributes to the signal integration. Concurrently, activation of the PKA inhibitor KT5720-sensitive pathway by dopamine leads to accumulation of the repressor protein X that is otherwise susceptible to proteasome degradation. This repression system may determine the time window permissive to the cooperative activation by in-phase glutamate and dopamine inputs.

Mechanism of dopamine D2 receptor-induced Ca(2+) release in PC-12 cells

Intracellular Ca(2+) levels are tightly regulated in the neuronal system. The loss of Ca(2+) homeostasis is associated with many neurological diseases and neuropsychiatric disorders such as Parkinson's, Alzheimer's, and schizophrenia. We investigated the mechanisms involved in intracellular Ca(2+) signaling in PC-12 cells. The stimulation of NGF-differentiated PC-12 cells with 3μM ATP caused an early Ca(2+) release followed by a delayed Ca(2+) release. The delayed Ca(2+) release was dependent on prior ATP priming and on dopamine secretion by PC-12 cells. Delayed Ca(2+) release was abolished in the presence of spiperone, suggesting that it is due to the activation of D2 dopamine receptors (D2R) by dopamine secreted by PC-12 cells. This was shown to be independent of PKA activation but dependent on PLC activity. An endocytosis step was required for inducing the delayed Ca(2+) release. Given the importance of calcyon in clathrin-mediated endocytosis, we verified the role of this protein in the delayed Ca(2+) release phenomenon. siRNA targeting of calcyon blocked the delayed Ca(2+) release, decreased ATP-evoked IP3R-mediated Ca(2+) release, and impaired subsequent Ca(2+) oscillations. Our results suggested that calcyon is involved in an unknown mechanism that causes a delayed IP3R-mediated Ca(2+) release in PC-12 cells. In schizophrenia, Ca(2+) dysregulation may depend on the upregulation of calcyon, which maintains elevated Ca(2+) levels as well as dopamine signaling.

Generation and characterization of hD5 and C-terminal Mutant hD(5m) transgenic rats

Dopamine D1-like receptors play important roles in many brain activities such as cognition and emotion. We have generated human hD5 and mutant human hD5 (hD(5m)) transgenic rats. The C-terminal juxtamembrane domain of mutant hD5 was identical to that of hD5 pseudogenes. The transgenes were driven by the CAMKII promoter that led the expression mainly in the cerebral cortex and hippocampus. We have used different dopamine receptor agonists to compare the pharmacological profiles of the human hD5 and hD(5m) receptors. The results showed that they exhibited distinct pharmacological properties. Our results of pharmacological studies indicated that the C-terminal of D5 receptor could play important roles in agonist binding affinity. Hippocampal long-term potentiation (LTP) evoked by tetanic stimulation was significantly reduced in both transgenic rats. In addition, we found that the overexpression of dopamine hD5 and hD(5m) receptors in the rat brain resulted in memory impairments. Interestingly, an atypical D1-like receptor agonist, SKF83959, could induce anxiety in hD(5m) receptor transgenic rats but had no effect on the anxiety-like behavior in D5 receptor transgenic and wild-type rats.

Lantana macrophylla Schauer (Verbenaceae) ethanolic extract induces activation of ERK1/2 and p38 MAPKs pathway and Ca2+ imbalance in human trophoblasts derived cell lines

Lantana macrophylla Schauer (Verbenaceae) a medicinal plant used to treat menstrual and respiratory disorders was investigated. The ethanolic extract from leaves was subjected to phytochemical and biological analysis. BeWo and JEG-3 cells were used to evaluate human chorionic gonadotropin hormone (hCG) production, syncytial formation, Ca2+ uptake and Ca2+ handling protein expression. The cAMP production and the mitogen-activated protein kinases (MAPKs) phosphorylation were also investigated. Phytochemical analysis yield three triterpenes: oleanolic, ursolic and latonolic acid. Viability assay showed no significant cytotoxic effect. A significant decrease in hCG production but not a disturbance on BeWo cell fusion were observed. The cAMP pathway was not affected by L. macrophylla extract alone; although the cAMP production inducted by forskolin was diminished. Both ERK1/2 and p38 MAPKs pathways were activated. Increased intracellular Ca2+ concentration ([Ca2+]i) was observed after 24 h treatment in a time and dose dependent manner; however only L. macrophylla at 10 μg/mL induced increased [Ca2+]i after 10 min treatment. CaBP28K and PMCA1/4 were modulated at protein and mRNA levels, respectively. This study showed for the first time the effect of triterpenoids from L. macrophylla leaves on trophoblasts-like cells and indicates a potential toxic effect of this plant in the placental development and fetal growth.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

The physiology, signaling, and pharmacology of dopamine receptors

G protein-coupled dopamine receptors (D1, D2, D3, D4, and D5) mediate all of the physiological functions of the catecholaminergic neurotransmitter dopamine, ranging from voluntary movement and reward to hormonal regulation and hypertension. Pharmacological agents targeting dopaminergic neurotransmission have been clinically used in the management of several neurological and psychiatric disorders, including Parkinson's disease, schizophrenia, bipolar disorder, Huntington's disease, attention deficit hyperactivity disorder (ADHD(1)), and Tourette's syndrome. Numerous advances have occurred in understanding the general structural, biochemical, and functional properties of dopamine receptors that have led to the development of multiple pharmacologically active compounds that directly target dopamine receptors, such as antiparkinson drugs and antipsychotics. Recent progress in understanding the complex biology of dopamine receptor-related signal transduction mechanisms has revealed that, in addition to their primary action on cAMP-mediated signaling, dopamine receptors can act through diverse signaling mechanisms that involve alternative G protein coupling or through G protein-independent mechanisms via interactions with ion channels or proteins that are characteristically implicated in receptor desensitization, such as β-arrestins. One of the future directions in managing dopamine-related pathologic conditions may involve a transition from the approaches that directly affect receptor function to a precise targeting of postreceptor intracellular signaling modalities either directly or through ligand-biased signaling pharmacology. In this comprehensive review, we discuss dopamine receptor classification, their basic structural and genetic organization, their distribution and functions in the brain and the periphery, and their regulation and signal transduction mechanisms. In addition, we discuss the abnormalities of dopamine receptor expression, function, and signaling that are documented in human disorders and the current pharmacology and emerging trends in the development of novel therapeutic agents that act at dopamine receptors and/or on related signaling events.

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.

Corticotropin-releasing factor receptors induce calcium mobilization through cross-talk with Gq-coupled receptors

The cross-talk between corticotropin-releasing factor (CRF) and muscarinic receptors was investigated by measuring evoked transient increases in cytosolic calcium concentration. HEK293 cells stably expressing human CRF type 1 (hCRF(1)) and type 2(a) (hCRF(2(a))) receptors were stimulated with the muscarinic receptor agonist carbachol and shortly after by a CRF agonist. Unexpectedly, this second response was enhanced when compared to stimulating naive cells either with carbachol or CRF agonist only. Priming with 100 microM carbachol increased the maximal CRF agonist response and shifted its concentration-response curve to the left to attain almost the same potency as for stimulating the production of the natural second messenger cyclic AMP. Yet, priming did not affect CRF agonist-stimulated cyclic AMP production itself. Carbachol priming was not restricted to recombinant CRF receptors only since endogenously expressed beta(2)-adrenoceptors also started to produce a robust calcium signal. Without priming no such signal was observed. Similar findings were made in the human retinoblastoma cell line Y79 for endogenously expressed CRF(1) receptors and the type 1 pituitary adenylate cyclase-activating polypeptide receptors but not for the CRF(2(a)) receptors. This differentiation between CRF(1) and CRF(2) receptors was further supported by use of selective agonists and antagonists. The results suggest that stimulating a Gq-coupled receptor shortly before stimulating a Gs-coupled receptor may result in a parallel signaling event on top of the classical cyclic AMP pathway.

Dopamine agonist signalling in the hypothalamus of female rats is independent of calcium-dependent kinases

We have previously demonstrated that dopamine agonist, SKF38396 (SKF), can substitute for progesterone in the facilitation of female reproductive behaviour in oestradiol benzoate-primed female rats and mice. We also reported that both progesterone- and SKF-initiated signalling were mediated by the cAMP-dependent protein kinase A signal transduction cascade. As the rapid effects of progesterone are also mediated by calcium-dependent kinases, calcium- and calmodulin-dependent kinase (CaMKII) and protein kinase (PKC), we sought to determine whether SKF-initiated signalling also recruited calcium as a second messenger. We measured the changes in the activation of CaMKII and PKC in the ventromedial nucleus (VMN) of the hypothalamus and preoptic area (POA) of the rat brain, which are the two regions implicated in the regulation of female reproductive behaviour in rodents. We measured the basal activities representing the activation of the kinases by in vivo treatments, as well as the total kinase activities assayed in the presence of exogenous cofactors in vitro. We report that, in contrast to progesterone-initiated signalling, there was no recruitment of calcium by SKF in the hypothalamus, as shown by the absence of changes in CaMKII activities in the VMN and POA. Furthermore, SKF-treatment resulted in a rapid increase in calcium-independent basal PKC activity in the VMN but not the POA. These rapid changes were not the result of changes in PKC protein levels or phosphorylation status. These data indicate that progesterone- and SKF-recruit distinct signalling molecules within the same regions of the brain to activate region-specific signal transduction pathways.

Dopamine regulates the expression of the glutamate transporter GLT1 but not GLAST in developing striatal astrocytes

Dopamine and L: -glutamate are important signals which guide the development of functional neural circuits within the striatal complex. Disequilibrium of these neurotransmitter systems is believed to be etiological for the genesis of neurological and psychiatric diseases. Since dopamine plays a crucial role for the early transmitter-regulated differentiation of striatal GABAergic neurons, we emphasized that dopaminergic transmission may also be involved in the fine tuning of intra-striatal glutamate action. In this study, we report that dopamine decreases the expression of the glutamate transporter GLT1 but not GLAST in striatal astrocytes by measuring gene and protein expression. Using glutamate-uptake approaches, we demonstrate an increase in glutamate clearance of externally added glutamate in dopamine-treated cultures compared to controls. Our findings imply that dopamine regulates the availability of L: -glutamate in the developing striatum. It is also suggested that the application of dopaminergic drugs can interfere with ontogenetic processes within the striatal complex.