β2* nAChR sensitivity modulates acquisition of cocaine self-administration in male rats

Signaling through nicotinic acetylcholine receptors (nAChRs) plays a role in cocaine reward and reinforcement, suggesting that the cholinergic system could be manipulated with therapeutics to modulate aspects of cocaine use disorder (CUD). We examined the interaction between nAChRs and cocaine reinforcement by expressing a hypersensitive β2 nAChR subunit (β2Leu9'Ser) in the ventral tegmental area of male Sprague Dawley rats. Compared to control rats, β2Leu9'Ser rats acquired (fixed ratio) intravenous cocaine self-administration faster and with greater likelihood. By contrast, β2Leu9'Ser rats were approximately equivalent to controls in their intake of cocaine on a progressive ratio schedule of reinforcement, suggesting differential effects of cholinergic signaling depending on experimental parameters. Like progressive ratio cocaine SA, β2Leu9'Ser rats and controls did not differ significantly in food SA assays, including acquisition on a fixed ratio schedule or in progressive ratio sessions. These results highlight the specific role of high-affinity, heteropentameric β2* (β2-containing) nAChRs in acquisition of cocaine SA, suggesting that mesolimbic acetylcholine signaling is active during this process.

β2 nAChR Activation on VTA DA Neurons Is Sufficient for Nicotine Reinforcement in Rats

Mesolimbic nicotinic acetylcholine receptor (nAChRs) activation is necessary for nicotine reinforcement behavior, but it is unknown whether selective activation of nAChRs in the dopamine (DA) reward pathway is sufficient to support nicotine reinforcement. In this study, we tested the hypothesis that activation of β2-containing (β2*) nAChRs on VTA neurons is sufficient for intravenous nicotine self-administration (SA). We expressed β2 nAChR subunits with enhanced sensitivity to nicotine (referred to as β2Leu9'Ser) in the VTA of male Sprague Dawley (SD) rats, enabling very low concentrations of nicotine to selectively activate β2* nAChRs on transduced neurons. Rats expressing β2Leu9'Ser subunits acquired nicotine SA at 1.5 μg/kg/infusion, a dose too low to support acquisition in control rats. Saline substitution extinguished responding for 1.5 μg/kg/inf, verifying that this dose was reinforcing. β2Leu9'Ser nAChRs also supported acquisition at the typical training dose in rats (30 μg/kg/inf) and reducing the dose to 1.5 μg/kg/inf caused a significant increase in the rate of nicotine SA. Viral expression of β2Leu9'Ser subunits only in VTA DA neurons (via TH-Cre rats) also enabled acquisition of nicotine SA at 1.5 μg/kg/inf, and saline substitution significantly attenuated responding. Next, we examined electrically-evoked DA release in slices from β2Leu9'Ser rats with a history of nicotine SA. Single-pulse evoked DA release and DA uptake rate were reduced in β2Leu9'Ser NAc slices, but relative increases in DA following a train of stimuli were preserved. These results are the first to report that β2* nAChR activation on VTA neurons is sufficient for nicotine reinforcement in rats.

Relapse-like behavior and nAChR sensitization following intermittent access nicotine self-administration

Many tobacco smokers consume nicotine intermittently, but the underlying mechanisms and neurobiological changes associated with intermittent nicotine intake are unclear. Understanding intermittent nicotine intake is a high priority, as it could promote therapeutic strategies to attenuate tobacco consumption. We examined nicotine intake behavior and neurobiological changes in male rats that were trained to self-administer nicotine during brief (5 min) trials interspersed with longer (15 min) drug-free periods. Rats readily adapted to intermittent access (IntA) SA following acquisition on a continuous access (ContA) schedule. Probabilistic analysis of IntA nicotine SA suggested reduced nicotine loading behavior compared to ContA, and nicotine pharmacokinetic modeling revealed that rats taking nicotine intermittently may have increased intake to maintain blood levels of nicotine that are comparable to ContA SA. After IntA nicotine SA, rats exhibited an increase in unreinforced responses for nicotine-associated cues (incubation of craving) and specific alterations in the striatal proteome after 7 days without nicotine. IntA nicotine SA also induced nAChR functional upregulation in the interpeduncular nucleus (IPN), and it enhanced nicotine binding in the brain as determined via [¹¹C]nicotine positron emission tomography. Reducing the saliency of the cue conditions during the 5 min access periods attenuated nicotine intake, but incubation of craving was preserved. Together, these results indicate that IntA conditions promote nicotine SA and nicotine seeking after a nicotine-free period.

Functional α7 nicotinic acetylcholine receptors in GABAergic neurons of the interpeduncular nucleus

The interpeduncular nucleus (IPN) plays a key role in nicotine dependence and is involved in regulation of fear responses, affective states, and novelty processing. IPN neurons express nicotinic acetylcholine receptors (nAChR) and receive strong cholinergic innervation from the ventral medial habenula. Dorsal medial habenula neurons are primarily peptidergic, releasing substance P (SP) mainly onto IPN neurons in the lateral subnucleus (IPL). IPL neurons are sensitive to SP, but it is not known if they are involved in cholinergic transmission like other IPN neurons. We examined nAChR subunit gene expression in IPL neurons, revealing that Chrna7 (α7 nAChR subunit) is expressed in a subset of GABAergic IPL neurons. In patch-clamp recordings from IPL neurons, ACh-evoked inward currents were attenuated by methyllycaconitine (α7 nAChR antagonist) and potentiated by NS1738 (α7 Type I positive allosteric modulator). We confirmed α7 functional expression in IPL neurons by also showing that ACh-evoked currents were potentiated by PNU-120596 (Type II positive allosteric modulator). Additional pharmacological experiments show that IPN neurons expressing α7 nAChRs also express α3β4 nAChRs. Finally, we used 2-photon laser scanning microscopy and nicotine uncaging to directly examine the morphology of IPL neurons that express α7 nAChRs. These results highlight a novel aspect of α7 nAChR neurobiology, adding to the complexity of cholinergic modulation by nAChRs in the IPN.

Nicotine Self-Administration Induces Plastic Changes to Nicotinic Receptors in Medial Habenula

Chronic nicotine upregulates nicotinic acetylcholine receptors (nAChRs) throughout the brain, and reducing their activity may promote somatic and affective states that lead to nicotine seeking. nAChRs are functionally upregulated in animal models using passive nicotine administration, but whether/how it occurs in response to volitional nicotine intake is unknown. The distinction is critical, as drug self-administration (SA) can induce neurotransmission and cellular excitability changes that passive drug administration does not. In this study, we probed the question of whether medial habenula (MHb) nAChRs are functionally augmented by nicotine SA. Male rats were implanted with an indwelling jugular catheter and trained to nose poke for nicotine infusions. A saline SA group controlled for non-specific responding and nicotine-associated visual cues. Using patch-clamp whole-cell recordings and local application of acetylcholine, we observed robust functional enhancement of nAChRs in MHb neurons from rats with a history of nicotine SA. To determine whether upregulated receptors are generally enhanced or directed to specific cellular compartments, we imaged neurons during recordings using two-photon laser scanning microscopy (2PLSM). nAChR activity at the cell soma and on proximal and distal dendrites was examined by local nicotine uncaging using a photoactivatable nicotine (PA-Nic) probe and focal laser flash photolysis. Results from this experiment revealed strong nAChR enhancement at all examined cellular locations. Our study demonstrates nAChR functional enhancement by nicotine SA, confirming that volitional nicotine intake sensitizes cholinergic systems in the brain. This may be a critical plasticity change supporting nicotine addiction.

Differential Nicotinic Modulation of Glutamatergic and GABAergic VTA Microcircuits

Ventral tegmental area (VTA) neurons receive glutamatergic and/or GABAergic input from other local neurons within the VTA. Nicotinic acetylcholine receptor (nAChR) activity is capable of modulating such intra-VTA transmission, but the mechanisms are unclear. Here, we isolated monosynaptic glutamate or GABA transmission from mouse medial VTA (mVTA) to lateral VTA (latVTA) using pharmacology and optogenetics, and we studied the ability of nicotine to modulate these modes of transmission. The action of nicotine on mVTA to latVTA glutamate transmission was bidirectional; nicotine enhanced glutamate release in half of the recorded latVTA cells and inhibited release in the other half. Nicotine-mediated reduction in glutamate release was reversed by blockade of GABAA receptors. This, coupled with expression data demonstrating coexpression of vesicular glutamate transporter 2 (VGluT2) and glutamate decarboxylase 2 (Gad2) in mVTA neurons, suggests that nicotine is able to stimulate GABA corelease from mVTA VGluT2+ neurons. Nicotine had an altogether different effect on mVTA to latVTA GABA release from Gad2+ cells; nicotine suppressed GABA release from mVTA Gad2+ terminals in nearly all cells tested. Together, these data uncover a complex system of local circuitry in the VTA that is modulated by nAChR activity. These actions of nicotine, which occurred at concentrations of nicotine found in the artificial CSF of cigarette smokers, may play a role in the adaptive response of the reward system to repeated nicotine exposure.

Nicotinic Cholinergic Receptors in VTA Glutamate Neurons Modulate Excitatory Transmission

Ventral tegmental area (VTA) glutamate neurons are important components of reward circuitry, but whether they are subject to cholinergic modulation is unknown. To study this, we used molecular, physiological, and photostimulation techniques to examine nicotinic acetylcholine receptors (nAChRs) in VTA glutamate neurons. Cells in the medial VTA, where glutamate neurons are enriched, are responsive to acetylcholine (ACh) released from cholinergic axons. VTA VGLUT2⁺ neurons express mRNA and protein subunits known to comprise heteromeric nAChRs. Electrophysiology, coupled with two-photon microscopy and laser flash photolysis of photoactivatable nicotine, was used to demonstrate nAChR functional activity in the somatodendritic subcellular compartment of VTA VGLUT2⁺ neurons. Finally, optogenetic isolation of intrinsic VTA glutamatergic microcircuits along with gene-editing techniques demonstrated that nicotine potently modulates excitatory transmission within the VTA via heteromeric nAChRs. These results indicate that VTA glutamate neurons are modulated by cholinergic mechanisms and participate in the cascade of physiological responses to nicotine exposure.

Selectivity of (±)-citalopram at nicotinic acetylcholine receptors and different inhibitory mechanisms between habenular α3β4* and α9α10 subtypes

The inhibitory activity of (±)-citalopram on human (h) α3β4, α4β2, and α7 nicotinic acetylcholine receptors (AChRs) was determined by Ca²⁺ influx assays, whereas its effect on rat α9α10 and mouse habenular α3β4* AChRs by electrophysiological recordings. The Ca²⁺ influx results clearly establish that (±)-citalopram inhibits (IC₅₀'s in μM) hα3β4 AChRs (5.1 ± 1.3) with higher potency than that for hα7 (18.8 ± 1.1) and hα4β2 (19.1 ± 4.2) AChRs. This is in agreement with the [³H]imipramine competition binding results indicating that (±)-citalopram binds to imipramine sites at desensitized hα3β4 with >2-fold higher affinity than that for hα4β2. The electrophysiological, molecular docking, and in silico mutation results indicate that (±)-citalopram competitively inhibits rα9α10 AChRs (7.5 ± 0.9) in a voltage-independent manner by interacting mainly with orthosteric sites, whereas it inhibits a homogeneous population of α3β4* AChRs at MHb (VI) neurons (7.6 ± 1.0) in a voltage-dependent manner by interacting mainly with a luminal site located in the middle of the ion channel, overlapping the imipramine site, which suggests an ion channel blocking mechanism. In conclusion, (±)-citalopram inhibits α3β4 and α9α10 AChRs with higher potency compared to other AChRs but by different mechanisms. (±)-Citalopram also inhibits habenular α3β4*AChRs, supporting the notion that these receptors are important endogenous targets related to their anti-addictive activities.

Chronic Nicotine Exposure Alters the Neurophysiology of Habenulo-Interpeduncular Circuitry

Antagonism of nicotinic acetylcholine receptors (nAChRs) in the medial habenula (MHb) or interpeduncular nucleus (IPN) triggers withdrawal-like behaviors in mice chronically exposed to nicotine, implying that nicotine dependence involves the sensitization of nicotinic signaling. Identification of receptor and/or neurophysiological mechanisms underlying this sensitization is important, as it could promote novel therapeutic strategies to reduce tobacco use. Using an approach involving photoactivatable nicotine, we previously demonstrated that chronic nicotine (cNIC) potently enhances nAChR function in dendrites of MHb neurons. However, whether cNIC modulates downstream components of the habenulo-interpeduncular (Hb-IP) circuit is unknown. In this study, cNIC-mediated changes to Hb-IP nAChR function were examined in mouse (male and female) brain slices using molecular, electrophysiological, and optical techniques. cNIC enhanced action potential firing and modified spike waveform characteristics in MHb neurons. Nicotine uncaging revealed nAChR functional enhancement by cNIC on proximal axonal membranes. Similarly, nAChR-driven glutamate release from MHb axons was enhanced by cNIC. In IPN, the target structure of MHb axons, neuronal morphology, and nAChR expression is complex, with stronger nAChR function in the rostral subnucleus [rostral IPN (IPR)]. As in MHb, cNIC induced strong upregulation of nAChR function in IPN neurons. This, coupled with cNIC-enhanced nicotine-stimulated glutamate release, was associated with stronger depolarization responses to brief (1 ms) nicotine uncaging adjacent to IPR neurons. Together, these results indicate that chronic exposure to nicotine dramatically alters nicotinic cholinergic signaling and cell excitability in Hb-IP circuits, a key pathway involved in nicotine dependence.SIGNIFICANCE STATEMENT This study uncovers several neuropharmacological alterations following chronic exposure to nicotine in a key brain circuit involved in nicotine dependence. These results suggest that smokers or regular users of electronic nicotine delivery systems (i.e., "e-cigarettes") likely undergo sensitization of cholinergic circuitry in the Hb-IP system. Reducing the activity of Hb-IP nAChRs, either volitionally during smoking cessation or inadvertently via receptor desensitization during nicotine intake, may be a key trigger of withdrawal in nicotine dependence. Escalation of nicotine intake in smokers, or tolerance, may involve stimulation of these sensitized cholinergic pathways. Smoking cessation therapeutics are only marginally effective, and by identifying cellular/receptor mechanisms of nicotine dependence, our results take a step toward improved therapeutic approaches for this disorder.

Probing Nicotinic Acetylcholine Receptor Function in Mouse Brain Slices via Laser Flash Photolysis of Photoactivatable Nicotine

Acetylcholine (ACh) acts through receptors to modulate a variety of neuronal processes, but it has been challenging to link ACh receptor function with subcellular location within cells where this function is carried out. To study the subcellular location of nicotinic ACh receptors (nAChRs) in native brain tissue, an optical method was developed for precise release of nicotine at discrete locations near neuronal membranes during electrophysiological recordings. Patch-clamped neurons in brain slices are filled with dye to visualize their morphology during 2-photon laser scanning microscopy, and nicotine uncaging is executed with a light flash by focusing a 405 nm laser beam near one or more cellular membranes. Cellular current deflections are measured, and a high-resolution three-dimensional (3D) image of the recorded neuron is made to allow reconciliation of nAChR responses with cellular morphology. This method allows for detailed analysis of nAChR functional distribution in complex tissue preparations, promising to enhance the understanding of cholinergic neurotransmission.

Gene editing vectors for studying nicotinic acetylcholine receptors in cholinergic transmission

Nicotinic acetylcholine receptors (nAChRs), prototype members of the cys-loop ligand-gated ion channel family, are key mediators of cholinergic transmission in the central nervous system. Despite their importance, technical gaps exist in our ability to dissect the function of individual subunits in the brain. To overcome these barriers, we designed CRISPR/Cas9 small guide RNA sequences (sgRNAs) for the production of loss-of-function alleles in mouse nAChR genes. These sgRNAs were validated in vitro via deep sequencing. We subsequently targeted candidate nAChR genes in vivo by creating herpes simplex virus (HSV) vectors delivering sgRNAs and Cas9 expression to mouse brain. The production of loss-of-function insertions or deletions (indels) by these 'all-in-one' HSV vectors was confirmed using brain slice patch clamp electrophysiology coupled with pharmacological analysis. Next, we developed a scheme for cell type-specific gene editing in mouse brain. Knockin mice expressing Cas9 in a Cre-dependent manner were validated using viral microinjections and genetic crosses to common Cre-driver mouse lines. We subsequently confirmed functional Cas9 activity by targeting the ubiquitous neuronal protein, NeuN, using adeno-associated virus (AAV) delivery of sgRNAs. Finally, the mouse β2 nAChR gene was successfully targeted in dopamine transporter (DAT)-positive neurons via CRISPR/Cas9. The sgRNA sequences and viral vectors, including our scheme for Cre-dependent gene editing, should be generally useful to the scientific research community. These tools could lead to new discoveries related to the function of nAChRs in neurotransmission and behavioral processes.

Photoactivatable drugs for nicotinic optopharmacology

Photoactivatable pharmacological agents have revolutionized neuroscience, but the palette of available compounds is limited. We describe a general method for caging tertiary amines by using a stable quaternary ammonium linkage that elicits a red shift in the activation wavelength. We prepared a photoactivatable nicotine (PA-Nic), uncageable via one- or two-photon excitation, that is useful to study nicotinic acetylcholine receptors (nAChRs) in different experimental preparations and spatiotemporal scales.

Altered nicotine reward-associated behavior following α4 nAChR subunit deletion in ventral midbrain

Nicotinic acetylcholine receptors containing α4 subunits (α4β2* nAChRs) are critical for nicotinic cholinergic transmission and the addictive action of nicotine. To identify specific activities of these receptors in the adult mouse brain, we coupled targeted deletion of α4 nAChR subunits with behavioral and and electrophysiological measures of nicotine sensitivity. A viral-mediated Cre/lox approach allowed us to delete α4 from ventral midbrain (vMB) neurons. We used two behavioral assays commonly used to assess the motivational effects of drugs of abuse: home-cage oral self-administration, and place conditioning. Mice lacking α4 subunits in vMB consumed significantly more nicotine at the highest offered nicotine concentration (200 μg/mL) compared to control mice. Deletion of α4 subunits in vMB blocked nicotine-induced conditioned place preference (CPP) without affecting locomotor activity. Acetylcholine-evoked currents as well as nicotine-mediated increases in synaptic potentiation were reduced in mice lacking α4 in vMB. Immunostaining verified that α4 subunits were deleted from both dopamine and non-dopamine neurons in the ventral tegmental area (VTA). These results reveal that attenuation of α4* nAChR function in reward-related brain circuitry of adult animals may increase nicotine intake by enhancing the rewarding effects and/or reducing the aversive effects of nicotine.

Nicotine Dependence Reveals Distinct Responses from Neurons and Their Resident Nicotinic Receptors in Medial Habenula

Nicotinic acetylcholine receptors (nAChRs) are the molecular target of nicotine. nAChRs in the medial habenula (MHb) have recently been shown to play a role in nicotine dependence, but it is not clear which nAChR subtypes or MHb neuron types are most important. To identify MHb nAChRs and/or cell types that play a role in nicotine dependence, we studied these receptors and cells with brain slice electrophysiology using both acute and chronic nicotine application. Cells in the ventroinferior (MHbVI) and ventrolateral MHb (MHbVL) subregions expressed functional nAChRs with different pharmacology. Further, application of nicotine to cells in these subregions led to different action potential firing patterns. The latter result was correlated with a differing ability of nicotine to induce nAChR desensitization. Chronic nicotine caused functional upregulation of nAChRs selectively in MHbVI cells, but did not change nAChR function in MHbVL. Importantly, firing responses were also differentially altered in these subregions following chronic nicotine. MHbVI neurons treated chronically with nicotine exhibited enhanced basal pacemaker firing but a blunted nicotine-induced firing response. MHbVL neurons did not change their firing properties in response to chronic nicotine. Together, these results suggest that acute and chronic nicotine differentially affect nAChR function and output of cells in MHb subregions. Because the MHb extensively innervates the interpeduncular nucleus, an area critical for both affective and somatic signs of withdrawal, these results could reflect some of the neurophysiological changes thought to occur in the MHb to the interpeduncular nucleus circuit in human smokers.

α6-Containing nicotinic acetylcholine receptors in midbrain dopamine neurons are poised to govern dopamine-mediated behaviors and synaptic plasticity

Acetylcholine (ACh) acts through nicotinic and muscarinic ACh receptors in the ventral midbrain and striatal areas to influence dopamine (DA) transmission. This cholinergic control of DA transmission is important for processes such as attention and motivated behavior, and is manipulated by nicotine in tobacco products. Identifying and characterizing the key ACh receptors involved in cholinergic control of DA transmission could lead to small molecule therapeutics for treating disorders involving attention, addiction, Parkinson's disease, and schizophrenia. α6-Containing nicotinic acetylcholine receptors (nAChRs) are highly and specifically expressed in midbrain DA neurons, making them an attractive drug target. Here, we used genetic, pharmacological, behavioral, and biophysical approaches to study this nAChR subtype. For many experiments, we used mice expressing mutant α6 nAChRs ("α6L9S" mice) that increase the sensitivity of these receptors to agonists such as ACh and nicotine. Taking advantage of a simple behavioral phenotype exhibited by α6L9S mice, we compared the ability of full versus partial α6(∗) nAChR agonists to activate α6(∗) nAChRs in vivo. Using local infusions of both agonists and antagonists into the brain, we demonstrate that neurons and nAChRs in the midbrain are sufficient to account for this behavioral response. To complement these behavioral studies, we studied the ability of in vivo α6(∗) nAChR activation to support plasticity changes in midbrain DA neurons that are relevant to behavioral sensitization and addiction. By coupling local infusion of drugs and brain slice patch-clamp electrophysiology, we show that activating α6(∗) nAChRs in midbrain DA areas is sufficient to enhance glutamatergic transmission in ventral tegmental area (VTA) DA neurons. Together, these results from in vivo studies strongly suggest that α6(∗) nAChRs expressed by VTA DA neurons are positioned to strongly influence both DA-mediated behaviors and the induction of synaptic plasticity by nicotine.

The nicotinic α6 subunit gene determines variability in chronic pain sensitivity via cross-inhibition of P2X2/3 receptors

Chronic pain is a highly prevalent and poorly managed human health problem. We used microarray-based expression genomics in 25 inbred mouse strains to identify dorsal root ganglion (DRG)-expressed genetic contributors to mechanical allodynia, a prominent symptom of chronic pain. We identified expression levels of Chrna6, which encodes the α6 subunit of the nicotinic acetylcholine receptor (nAChR), as highly associated with allodynia. We confirmed the importance of α6* (α6-containing) nAChRs by analyzing both gain- and loss-of-function mutants. We find that mechanical allodynia associated with neuropathic and inflammatory injuries is significantly altered in α6* mutants, and that α6* but not α4* nicotinic receptors are absolutely required for peripheral and/or spinal nicotine analgesia. Furthermore, we show that Chrna6's role in analgesia is at least partially due to direct interaction and cross-inhibition of α6* nAChRs with P2X2/3 receptors in DRG nociceptors. Finally, we establish the relevance of our results to humans by the observation of genetic association in patients suffering from chronic postsurgical and temporomandibular pain.

Evidence for a role for α6(∗) nAChRs in l-dopa-induced dyskinesias using Parkinsonian α6(∗) nAChR gain-of-function mice

l-Dopa-induced dyskinesias (LIDs) are a serious side effect of dopamine replacement therapy for Parkinson's disease. The mechanisms that underlie LIDs are currently unclear. However, preclinical studies indicate that nicotinic acetylcholine receptors (nAChRs) play a role, suggesting that drugs targeting these receptors may be of therapeutic benefit. To further understand the involvement of α6β2(∗) nAChRs in LIDs, we used gain-of-function α6(∗) nAChR (α6L9S) mice that exhibit a 20-fold enhanced sensitivity to nAChR agonists. Wildtype (WT) and α6L9S mice were lesioned by unilateral injection of 6-hydroxydopamine (6-OHDA, 3μg/ml) into the medial forebrain bundle. Three to 4wk later, they were administered l-dopa (3mg/kg) plus benserazide (15mg/kg) until stably dyskinetic. l-dopa-induced abnormal involuntary movements (AIMs) were similar in α6L9S and WT mice. WT mice were then given nicotine in the drinking water in gradually increasing doses to a final 300μg/ml, which resulted in a 40% decline AIMs. By contrast, there was no decrease in AIMs in α6L9S mice at a maximally tolerated nicotine dose of 20μg/ml. However, the nAChR antagonist mecamylamine (1mg/kg ip 30min before l-dopa) reduced l-dopa-induced AIMs in both α6L9S and WT mice. Thus, both a nAChR agonist and antagonist decreased AIMs in WT mice, but only the antagonist was effective in α6L9S mice. Since nicotine appears to reduce LIDs via desensitization, hypersensitive α6β2(∗) nAChRs may desensitize less readily. The present data show that α6β2(∗) nAChRs are key regulators of LIDs, and may be useful therapeutic targets for their management in Parkinson's disease.

Nicotine and ethanol cooperate to enhance ventral tegmental area AMPA receptor function via α6-containing nicotinic receptors

Nicotine + ethanol co-exposure results in additive and/or synergistic effects in the ventral tegmental area (VTA) to nucleus accumbens (NAc) dopamine (DA) pathway, but the mechanisms supporting this are unclear. We tested the hypothesis that nAChRs containing α6 subunits (α6* nAChRs) are involved in the response to nicotine + ethanol co-exposure. Exposing VTA slices from C57BL/6 WT animals to drinking-relevant concentrations of ethanol causes a marked enhancement of α-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA) receptor (AMPAR) function in VTA neurons. This effect was sensitive to α-conotoxin MII (an α6β2* nAChR antagonist), suggesting that α6* nAChR function is required. In mice expressing hypersensitive α6* nAChRs (α6L9S mice), we found that lower concentrations (relative to C57BL/6 WT) of ethanol were sufficient to enhance AMPAR function in VTA neurons. Exposure of live C57BL/6 WT mice to ethanol also produced AMPAR functional enhancement in VTA neurons, and studies in α6L9S mice strongly suggest a role for α6* nAChRs in this response. We then asked whether nicotine and ethanol cooperate to enhance VTA AMPAR function. We identified low concentrations of nicotine and ethanol that were capable of strongly enhancing VTA AMPAR function when co-applied to slices, but that did not enhance AMPAR function when applied alone. This effect was sensitive to both varenicline (an α4β2* and α6β2* nAChR partial agonist) and α-conotoxin MII. Finally, nicotine + ethanol co-exposure also enhanced AMPAR function in VTA neurons from α6L9S mice. Together, these data identify α6* nAChRs as important players in the response to nicotine + ethanol co-exposure in VTA neurons.

Nicotine exploits a COPI-mediated process for chaperone-mediated up-regulation of its receptors

Chronic exposure to nicotine up-regulates high sensitivity nicotinic acetylcholine receptors (nAChRs) in the brain. This up-regulation partially underlies addiction and may also contribute to protection against Parkinson's disease. nAChRs containing the α6 subunit (α6* nAChRs) are expressed in neurons in several brain regions, but comparatively little is known about the effect of chronic nicotine on these nAChRs. We report here that nicotine up-regulates α6* nAChRs in several mouse brain regions (substantia nigra pars compacta, ventral tegmental area, medial habenula, and superior colliculus) and in neuroblastoma 2a cells. We present evidence that a coat protein complex I (COPI)-mediated process mediates this up-regulation of α6* or α4* nAChRs but does not participate in basal trafficking. We show that α6β2β3 nAChR up-regulation is prevented by mutating a putative COPI-binding motif in the β3 subunit or by inhibiting COPI. Similarly, a COPI-dependent process is required for up-regulation of α4β2 nAChRs by chronic nicotine but not for basal trafficking. Mutation of the putative COPI-binding motif or inhibition of COPI also results in reduced normalized Förster resonance energy transfer between α6β2β3 nAChRs and εCOP subunits. The discovery that nicotine exploits a COPI-dependent process to chaperone high sensitivity nAChRs is novel and suggests that this may be a common mechanism in the up-regulation of nAChRs in response to chronic nicotine.

Enhanced synthesis and release of dopamine in transgenic mice with gain-of-function α6* nAChRs

α6β2* nicotinic acetylcholine receptors (nAChRs)s in the ventral tegmental area to nucleus accumbens (NAc) pathway are implicated in the response to nicotine, and recent work suggests these receptors play a role in the rewarding action of ethanol. Here, we studied mice expressing gain-of-function α6β2* nAChRs (α6L9'S mice) that are hypersensitive to nicotine and endogenous acetylcholine. Evoked extracellular dopamine (DA) levels were enhanced in α6L9'S NAc slices compared to control, non-transgenic (non-Tg) slices. Extracellular DA levels in both non-Tg and α6L9'S slices were further enhanced in the presence of GBR12909, suggesting intact DA transporter function in both mouse strains. Ongoing α6β2* nAChR activation by acetylcholine plays a role in enhancing DA levels, as α-conotoxin MII completely abolished evoked DA release in α6L9'S slices and decreased spontaneous DA release from striatal synaptosomes. In HPLC experiments, α6L9'S NAc tissue contained significantly more DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid compared to non-Tg NAc tissue. Serotonin (5-HT), 5-hydroxyindoleacetic acid, and norepinephrine (NE) were unchanged in α6L9'S compared to non-Tg tissue. Western blot analysis revealed increased tyrosine hydroxylase expression in α6L9'S NAc. Overall, these results show that enhanced α6β2* nAChR activity in NAc can stimulate DA production and lead to increased extracellular DA levels.

α4α6β2* nicotinic acetylcholine receptor activation on ventral tegmental area dopamine neurons is sufficient to stimulate a depolarizing conductance and enhance surface AMPA receptor function

Tobacco addiction is a serious threat to public health in the United States and abroad, and development of new therapeutic approaches is a major priority. Nicotine activates and/or desensitizes nicotinic acetylcholine receptors (nAChRs) throughout the brain. nAChRs in ventral tegmental area (VTA) dopamine (DA) neurons are crucial for the rewarding and reinforcing properties of nicotine in rodents, suggesting that they may be key mediators of nicotine's action in humans. However, it is unknown which nAChR subtypes are sufficient to activate these neurons. To test the hypothesis that nAChRs containing α6 subunits are sufficient to activate VTA DA neurons, we studied mice expressing hypersensitive, gain-of-function α6 nAChRs (α6L9'S mice). In voltage-clamp recordings in brain slices from adult mice, 100 nM nicotine was sufficient to elicit inward currents in VTA DA neurons via α6β2* nAChRs. In addition, we found that low concentrations of nicotine could act selectively through α6β2* nAChRs to enhance the function of 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA) receptors on the surface of these cells. In contrast, α6β2* activation did not enhance N-methyl-D-aspartic acid receptor function. Finally, AMPA receptor (AMPAR) function was not similarly enhanced in brain slices from α6L9'S mice lacking α4 nAChR subunits, suggesting that α4α6β2* nAChRs are important for enhancing AMPAR function in VTA DA neurons. Together, these data suggest that activation of α4α6β2* nAChRs in VTA DA neurons is sufficient to support the initiation of cellular changes that play a role in addiction to nicotine. α4α6β2* nAChRs may be a promising target for future smoking cessation pharmacotherapy.

Nicotinic acetylcholine receptors containing α6 subunits contribute to alcohol reward-related behaviours

Evidence is emerging that neuronal nicotinic acetylcholine receptors (nAChRs) in the mesolimbic dopamine (DA) system are involved in mediating the reinforcing effects of alcohol. Midbrain DA neurons express high levels of α6 subunit-containing nAChRs that modulate DA transmission, implicating their involvement in reward-related behaviours. This study assessed the role of α6-containing nAChRs in modulating alcohol reward using transgenic mice expressing mutant, hypersensitive α6 nAChR subunits (α6L9'S mice). α6L9'S mice and littermate controls were tested in three well-established models of alcohol reward: 24-h two-bottle choice drinking, drinking in the dark (DID), and conditioned place preference (CPP). Confocal microscopy and patch-clamp electrophysiology were used to show the localization and function of hypersensitive α6 subunit-containing nAChRs. Results indicate that female α6L9'S mice showed significantly higher alcohol intake at low concentrations of alcohol (3% and 6%) in the two-bottle choice procedure. Both male and female α6L9'S mice drank significantly more in the DID procedure and displayed an alcohol-induced place preference using a low dose of alcohol (0.5 g/kg) that was ineffective in littermate controls. Confocal microscopy showed that α6 subunit-containing nAChRs are selectively expressed on ventral tegmental area (VTA) DAergic, but not GABAergic neurons. Patch-clamp electrophysiology showed that VTA DA neurons of α6L9'S mice are hypersensitive to ACh. Collectively, these results suggest that α6L9'S mice are more sensitive to the rewarding effects of alcohol, and suggest that VTA α6 subunit-containing nAChRs modulate alcohol reward. Thus, α6 subunit-containing nAChRs may be a promising therapeutic target for treatment of alcohol use disorders.

Local application of drugs to study nicotinic acetylcholine receptor function in mouse brain slices

Tobacco use leads to numerous health problems, including cancer, heart disease, emphysema, and stroke. Addiction to cigarette smoking is a prevalent neuropsychiatric disorder that stems from the biophysical and cellular actions of nicotine on nicotinic acetylcholine receptors (nAChRs) throughout the central nervous system. Understanding the various nAChR subtypes that exist in brain areas relevant to nicotine addiction is a major priority. Experiments that employ electrophysiology techniques such as whole-cell patch clamp or two-electrode voltage clamp recordings are useful for pharmacological characterization of nAChRs of interest. Cells expressing nAChRs, such as mammalian tissue culture cells or Xenopus laevis oocytes, are physically isolated and are therefore easily studied using the tools of modern pharmacology. Much progress has been made using these techniques, particularly when the target receptor was already known and ectopic expression was easily achieved. Often, however, it is necessary to study nAChRs in their native environment: in neurons within brain slices acutely harvested from laboratory mice or rats. For example, mice expressing "hypersensitive" nAChR subunits such as α4 L9'A mice (1) and α6 L9'S mice (2), allow for unambiguous identification of neurons based on their functional expression of a specific nAChR subunit. Although whole-cell patch clamp recordings from neurons in brain slices is routinely done by the skilled electrophysiologist, it is challenging to locally apply drugs such as acetylcholine or nicotine to the recorded cell within a brain slice. Dilution of drugs into the superfusate (bath application) is not rapidly reversible, and U-tube systems are not easily adapted to work with brain slices. In this paper, we describe a method for rapidly applying nAChR-activating drugs to neurons recorded in adult mouse brain slices. Standard whole-cell recordings are made from neurons in slices, and a second micropipette filled with a drug of interest is maneuvered into position near the recorded cell. An injection of pressurized air or inert nitrogen into the drug-filled pipette causes a small amount of drug solution to be ejected from the pipette onto the recorded cell. Using this method, nAChR-mediated currents are able to be resolved with millisecond accuracy. Drug application times can easily be varied, and the drug-filled pipette can be retracted and replaced with a new pipette, allowing for concentration-response curves to be created for a single neuron. Although described in the context of nAChR neurobiology, this technique should be useful for studying many types of ligand-gated ion channels or receptors in neurons from brain slices.

Insights into the neurobiology of the nicotinic cholinergic system and nicotine addiction from mice expressing nicotinic receptors harboring gain-of-function mutations

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated, cation-selective ion channels expressed throughout the brain. Although these channels have been investigated for several decades, it is still challenging 1) to identify the important nAChR subunits in cholinergic transmission and nicotine dependence and 2) to develop nAChR subtype-specific ligands. To overcome these challenges, we and others have studied mice expressing mutant, gain-of-function nAChR subunits. In this review, we discuss this research approach and the results it has yielded to date. Gain-of-function mutations, including those in nAChR subunits, provide an approach that is complementary to loss-of-function studies such as gene knockouts; the former allows one to answer questions of sufficiency and the latter addresses questions of necessity. Mutant mice expressing gain-of-function nAChR subunits are commonly produced using traditional gene targeting in embryonic stem cells, but novel approaches such as bacterial artificial chromosome transgenesis have yielded important insights as well. α7 nAChRs were the first nAChRs to be targeted with a gain-of-function mutation, followed by a pair of α4 nAChR gain-of-function mutant mice. These α4 nAChR gain-of-function mice (α4 L9'S mice, followed by α4 L9'A mice) provided an important system to probe α4 nAChR function in vivo, particularly in the dopamine reward system. α6 nAChR gain-of-function mice provided the first robust system allowing specific manipulation of this receptor subtype. Other targeted mutations in various nAChR subunits have also been produced and have yielded important insights into nicotinic cholinergic biology. As nAChR research advances and more details associated with nAChR expression and function emerge, we expect that existing and new mouse lines expressing gain-of-function nAChR subunits will continue to provide new insights.

Striatal pathology underlies prion infection-mediated hyperactivity in mice

Although prion diseases are most commonly modeled using the laboratory mouse, the diversity of prion strains, behavioral testing and neuropathological assessments hamper our collective understanding of mouse models of prion disease. Here we compared several commonly used murine strains of prions in C57BL/6J female mice in a detailed home cage behavior detection system and a systematic study of pathological markers and neurotransmitter systems. We observed that mice inoculated with RML or 139A prions develop a severe hyperactivity phenotype in the home cage. A detailed assessment of pathology markers, such as microglial marker IBA1, astroglial marker GFAP and degeneration staining indicate early striatal pathology in mice inoculated with RML or 139A but not in those inoculated with 22L prions. An assessment of neuromodulatory systems including serotonin, dopamine, noradrenalin and acetylcholine showed surprisingly little decline in neuronal cell bodies or their innervations of regions controlling locomotor behavior, except for a small decrease in dopaminergic innervations of the dorsal striatum. These results implicate the dorsal striatum in mediating the major behavioral phenotype of 139A and RML prions. Further, they suggest that measurements of activity may be a sensitive manner in which to diagnose murine prion disease. With respect to neuropathology, our results indicate that pathological stains as opposed to neurotransmitter markers are much more informative and sensitive as markers of prion disease in mouse models.

Structural differences determine the relative selectivity of nicotinic compounds for native alpha 4 beta 2*-, alpha 6 beta 2*-, alpha 3 beta 4*- and alpha 7-nicotine acetylcholine receptors

Mammalian brain expresses multiple nicotinic acetylcholine receptor (nAChR) subtypes that differ in subunit composition, sites of expression and pharmacological and functional properties. Among known subtypes of receptors, alpha 4 beta 2* and alpha 6 beta 2*-nAChR have the highest affinity for nicotine (where * indicates possibility of other subunits). The alpha 4 beta 2*-nAChRs are widely distributed, while alpha 6 beta 2*-nAChR are restricted to a few regions. Both subtypes modulate release of dopamine from the dopaminergic neurons of the mesoaccumbens pathway thought to be essential for reward and addiction. alpha 4 beta 2*-nAChR also modulate GABA release in these areas. Identification of selective compounds would facilitate study of nAChR subtypes. An improved understanding of the role of nAChR subtypes may help in developing more effective smoking cessation aids with fewer side effects than current therapeutics. We have screened a series of nicotinic compounds that vary in the distance between the pyridine and the cationic center, in steric bulk, and in flexibility of the molecule. These compounds were screened using membrane binding and synaptosomal function assays, or recordings from GH4C1 cells expressing h alpha 7, to determine affinity, potency and efficacy at four subtypes of nAChRs found in brain, alpha 4 beta 2*, alpha 6 beta 2*, alpha 7 and alpha 3 beta 4*. In addition, physiological assays in gain-of-function mutant mice were used to assess in vivo activity at alpha 4 beta 2* and alpha 6 beta 2*-nAChRs. This approach has identified several compounds with agonist or partial agonist activity that display improved selectivity for alpha 6 beta 2*-nAChR.

In vivo activation of midbrain dopamine neurons via sensitized, high-affinity alpha 6 nicotinic acetylcholine receptors

Alpha6-containing (alpha6*) nicotinic ACh receptors (nAChRs) are selectively expressed in dopamine (DA) neurons and participate in cholinergic transmission. We generated and studied mice with gain-of-function alpha6* nAChRs, which isolate and amplify cholinergic control of DA transmission. In contrast to gene knockouts or pharmacological blockers, which show necessity, we show that activating alpha6* nAChRs and DA neurons is sufficient to cause locomotor hyperactivity. alpha6(L9'S) mice are hyperactive in their home cage and fail to habituate to a novel environment. Selective activation of alpha6* nAChRs with low doses of nicotine, by stimulating DA but not GABA neurons, exaggerates these phenotypes and produces a hyperdopaminergic state in vivo. Experiments with additional nicotinic drugs show that altering agonist efficacy at alpha6* provides fine tuning of DA release and locomotor responses. alpha6*-specific agonists or antagonists may, by targeting endogenous cholinergic mechanisms in midbrain or striatum, provide a method for manipulating DA transmission in neural disorders.

Subcellular trafficking, pentameric assembly, and subunit stoichiometry of neuronal nicotinic acetylcholine receptors containing fluorescently labeled alpha6 and beta3 subunits

Neuronal nicotinic acetylcholine (ACh) receptors are ligand-gated, cation-selective ion channels. Nicotinic receptors containing alpha4, alpha6, beta2, and beta3 subunits are expressed in midbrain dopaminergic neurons, and they are implicated in the response to smoked nicotine. Here, we have studied the cell biological and biophysical properties of receptors containing alpha6 and beta3 subunits by using fluorescent proteins fused within the M3-M4 intracellular loop. Receptors containing fluorescently tagged beta3 subunits were fully functional compared with receptors with untagged beta3 subunits. We find that beta3- and alpha6-containing receptors are highly expressed in neurons and that they colocalize with coexpressed, fluorescent alpha4 and beta2 subunits in neuronal soma and dendrites. Förster resonance energy transfer (FRET) reveals efficient, specific assembly of beta3 and alpha6 into nicotinic receptor pentamers of various subunit compositions. Using FRET, we demonstrate directly that only a single beta3 subunit is incorporated into nicotinic acetylcholine receptors (nAChRs) containing this subunit, whereas multiple subunit stoichiometries exist for alpha4- and alpha6-containing receptors. Finally, we demonstrate that nicotinic ACh receptors are localized in distinct microdomains at or near the plasma membrane using total internal reflection fluorescence (TIRF) microscopy. We suggest that neurons contain large, intracellular pools of assembled, functional nicotinic receptors, which may provide them with the ability to rapidly up-regulate nicotinic responses to endogenous ligands such as ACh, or to exogenous agents such as nicotine. Furthermore, this report is the first to directly measure nAChR subunit stoichiometry using FRET and plasma membrane localization of alpha6- and beta3-containing receptors using TIRF.

Regulation of RGS2 and Second Messenger Signaling in Vascular Smooth Muscle Cells by cGMP-dependent Protein Kinase

RGS2, a GTPase-activating protein (GAP) for G(q)alpha, regulates vascular relaxation and blood pressure. RGS2 can be phosphorylated by type Ialpha cGMP-dependent protein kinase (cGKIalpha), increasing its GAP activity. To understand how RGS2 and cGKIalpha regulate vascular smooth muscle signaling and function, we identified signaling pathways that are controlled by cGMP in an RGS2-dependent manner and discovered new mechanisms whereby cGK activity regulates RGS2. We show that RGS2 regulates vasoconstrictor-stimulated Ca(2+) store release, capacitative Ca(2+) entry, and noncapacitative Ca(2+) entry and that RGS2 is required for cGMP-mediated inhibition of vasoconstrictor-elicited phospholipase Cbeta activation, Ca(2+) store release, and capacitative Ca(2+) entry. RGS2 is degraded in vascular smooth muscle cells via the proteasome. Inhibition of cGK activity blunts RGS2 degradation. However, inactivation of the cGKIalpha phosphorylation sites in RGS2 does not stabilize the protein, suggesting that cGK activity regulates RGS2 degradation by other mechanisms. cGK activation promotes association of RGS2 with the plasma membrane by a mechanism requiring its cGKIalpha phosphorylation sites. By regulating GAP activity, plasma membrane association, and degradation, cGKIalpha therefore may control a cycle of RGS2 activation and inactivation. By diminishing cGK activity, endothelial dysfunction may impair RGS2 activation, thereby blunting vascular relaxation and contributing to hypertension.

R7BP augments the function of RGS7*Gbeta5 complexes by a plasma membrane-targeting mechanism

The RGS7 (R7) family of G protein regulators, Gbeta5, and R7BP form heterotrimeric complexes that potently regulate the kinetics of G protein-coupled receptor signaling. Reversible palmitoylation of R7BP regulates plasma membrane/nuclear shuttling of R7*Gbeta5*R7BP heterotrimers. Here we have investigated mechanisms whereby R7BP controls the function of the R7 family. We show that unpalmitoylated R7BP undergoes nuclear/cytoplasmic shuttling and that a C-terminal polybasic motif proximal to the palmitoylation acceptor sites of R7BP mediates nuclear localization, palmitoylation, and plasma membrane targeting. These results suggest a novel mechanism whereby palmitoyltransferases and nuclear import receptors both utilize the C-terminal domain of R7BP to determine the trafficking fate of R7*Gbeta5*R7BP heterotrimers. Analogous mechanisms may regulate other signaling proteins whose distribution between the plasma membrane and nucleus is controlled by palmitoylation. Lastly, we show that cytoplasmic RGS7*Gbeta5*R7BP heterotrimers and RGS7*Gbeta5 heterodimers are equivalently inefficient regulators of G protein-coupled receptor signaling relative to plasma membrane-bound heterotrimers bearing palmitoylated R7BP. Therefore, R7BP augments the function of the complex by a palmitoylation-regulated plasma membrane-targeting mechanism.

Palmitoylation regulates plasma membrane-nuclear shuttling of R7BP, a novel membrane anchor for the RGS7 family

The RGS7 (R7) family of RGS proteins bound to the divergent Gβ subunit Gβ5 is a crucial regulator of G protein–coupled receptor (GPCR) signaling in the visual and nervous systems. Here, we identify R7BP, a novel neuronally expressed protein that binds R7–Gβ5 complexes and shuttles them between the plasma membrane and nucleus. Regional expression of R7BP, Gβ5, and R7 isoforms in brain is highly coincident. R7BP is palmitoylated near its COOH terminus, which targets the protein to the plasma membrane. Depalmitoylation of R7BP translocates R7BP–R7–Gβ5 complexes from the plasma membrane to the nucleus. Compared with nonpalmitoylated R7BP, palmitoylated R7BP greatly augments the ability of RGS7 to attenuate GPCR-mediated G protein–regulated inward rectifying potassium channel activation. Thus, by controlling plasma membrane nuclear–shuttling of R7BP–R7–Gβ5 complexes, reversible palmitoylation of R7BP provides a novel mechanism that regulates GPCR signaling and potentially transduces signals directly from the plasma membrane to the nucleus.

FKBP12-Rapamycin-associated Protein or Mammalian Target of Rapamycin (FRAP/mTOR) Localization in the Endoplasmic Reticulum and the Golgi Apparatus

FKBP12-rapamycin-associated protein (FRAP) or mammalian target of rapamycin (mTOR) and its effector proteins form a critical signaling pathway that regulates eukaryotic cell growth and proliferation. Although the protein components in this pathway have begun to be identified, little is known about their subcellular localization or the physiological significance of their localization. By immunofluorescence, we find that both endogenous and recombinant FRAP/mTOR proteins show localization predominantly in the endoplasmic reticulum (ER) and the Golgi apparatus. Consistent with this finding, FRAP/mTOR is cofractionated with calnexin, an ER marker protein. Biochemical characterization suggests that FRAP/mTOR is a peripheral ER/Golgi protein with tight membrane association. Finally, we have identified domains of FRAP/mTOR which may mediate its association with the ER and the Golgi apparatus.

Receptor number and caveolar co-localization determine receptor coupling efficiency to adenylyl cyclase

Recent evidence suggests that many signaling molecules localize in microdomains of the plasma membrane, particularly caveolae. In this study, overexpression of adenylyl cyclase was used as a functional probe of G protein-coupled receptor (GPCR) compartmentation. We found that three endogenous receptors in neonatal rat cardiomyocytes couple with different levels of efficiency to the activation of adenylyl cyclase type 6 (AC6), which localizes to caveolin-rich membrane fractions. Overexpression of AC6 enhanced the maximal cAMP response to beta(1)-adrenergic receptor (beta(1)AR)-selective activation 3.7-fold, to beta(2)AR-selective activation only 1.6-fold and to prostaglandin E(2) (PGE(2)) not at all. Therefore, the rank order of efficacy in coupling to AC6 is beta(1)AR > beta(2)AR > prostaglandin E(2) receptor (EP(2)R). beta(2)AR coupling efficiency was greater when we overexpressed the receptor or blocked its desensitization by expressing betaARKct, an inhibitor of G protein-coupled receptor kinase activation, but was not significantly greater when cells were treated with pertussis toxin. Assessment of receptor and AC expression indicated co-localization of AC5/6, beta(1)AR, and beta(2)AR, but not EP(2)R, in caveolin-rich membranes and caveolin-3 immunoprecipitates, likely explaining the observed activation of AC6 by betaAR subtypes but lack thereof by PGE(2). When cardiomyocytes were stimulated with a betaAR agonist, beta(2)AR were no longer found in caveolin-3 immunoprecipitates; an effect that was blocked by expression of betaARKct. Thus, agonist-induced translocation of beta(2)AR out of caveolae causes a sequestration of receptor from effector and likely contributes to the lower efficacy of beta(2)AR coupling to AC6 as compared with beta(1)AR, which do not similarly translocate. Therefore, spatial co-localization is a key determinant of efficiency of coupling by particular extracellular signals to activation of GPCR-linked effectors.

Key role for constitutive cyclooxygenase-2 of MDCK cells in basal signaling and response to released ATP

Madin-Darby canine kidney (MDCK) cells release ATP upon mechanical or biochemical activation, initiating P2Y receptor signaling that regulates basal levels of multiple second messengers, including cAMP (J Biol Chem 275: 11735--11739, 2000). Data shown here document inhibition of cAMP formation by Gd(3+) and niflumic acid, channel inhibitors that block ATP release. cAMP production is stimulated via Ca(2+)-dependent activation of cytosolic phospholipase A(2), release of arachidonic acid (AA), and cyclooxygenase (COX)-dependent production of prostaglandins, which activate prostanoid receptors coupled to G(s) and adenylyl cyclase. In the current investigation, we assessed the expression and functional role of the two known isoforms of COX, COX-1 and COX-2. Treatment of cells with either a COX-1-selective inhibitor, SC-560, or COX-2-selective inhibitors, SC-58125 or NS-398, inhibited basal and UTP-stimulated cAMP levels. COX inhibitors also decreased forskolin-stimulated cAMP formation, implying this response is in part attributable to an action of AA metabolites. These findings imply an important role for the inducible form of COX, COX-2, under basal conditions. Indeed, COX-2 expression was readily detectable by immunoblot, and treatments that induce or reduce COX-2 expression in other cells (interleukin-1beta, tumor necrosis factor-alpha, phorbol ester, or dexamethasone) had minimal or no effect on the levels of COX-2 immunoreactivity. RT-PCR using isoform-specific primers detected COX-2 mRNA. We conclude that COX-2 is constitutively expressed in MDCK-D(1) cells and participates in basal and P2Y(2)-mediated signaling, implying a key role for COX-2 in regulation of epithelial cell function.