Inhibition of gap junction currents by the abused solvent toluene

Lack of evidence for direct ligand-gated ion channel activity of GluD receptors

Delta receptors (GluD1 and GluD2), members of the large ionotropic glutamate receptor (iGluR) family, play a central role in numerous neurodevelopmental and psychiatric disorders. The amino-terminal domain (ATD) of GluD orchestrates synapse formation and maturation processes through its interaction with the Cbln family of synaptic organizers and neurexin (Nrxn). The transsynaptic triad of Nrxn-Cbln-GluD also serves as a potent regulator of synaptic plasticity, at both excitatory and inhibitory synapses. Despite these recognized functions, there is still debate as to whether GluD functions as a "canonical" ion channel, similar to other iGluRs. A recent report proposes that the ATD of GluD2 imposes conformational constraints on channel activity; removal of this constraint by binding to Cbln1 and Nrxn, or removal of the ATD, reveals channel activity in GluD2 upon administration of glycine (Gly) and d-serine (d-Ser), two GluD ligands. We were able to reproduce currents when Gly or d-Ser was administered to clusters of heterologous human embryonic kidney 293 (HEK293) cells expressing Cbln1, GluD2 (or GluD1), and Nrxn. However, Gly or d-Ser, but also l-glutamate (l-Glu), evoked similar currents in naive (i.e., untransfected) HEK293 cells and in GluD2-null Purkinje neurons. Furthermore, no current was detected in isolated HEK293 cells expressing GluD2 lacking the ATD upon administration of Gly. Taken together, these results cast doubt on the previously proposed hypothesis that extracellular ligands directly gate wild-type GluD channels.

The Effects of the Inhalant Toluene on Cognitive Function and Behavioral Flexibility: A Review of Recent Findings

Substance use disorder (SUD) is characterized, in part, by lack of control over drug seeking and taking. The prefrontal cortex (PFC) is highly involved in control of behavior and deficits in PFC structure and function have been demonstrated in clinical and preclinical studies of SUD. Of the various classes of drugs associated with the development of SUD, inhalants are among the least studied despite their widespread use among adolescents and children. In this work, we review what is currently known regarding the sites and mechanisms of action of inhalants with a focus on the volatile solvent toluene that is contained in a wide variety of legal and easily obtained products. We then describe how inhalants including toluene affect various behaviors with an emphasis on those associated with PFC function and how chronic use of inhalants alters brain structure and neuronal signaling. Findings from these studies highlight advances made in recent years that have expanded our understanding of the effects of inhalants on brain structure and reinforce the need for continued work in this field.

The last two decades on preclinical and clinical research on inhalant effects

This paper reviews the scientific evidence generated in the last two decades on the effects and mechanisms of action of most commonly misused inhalants. In the first section, we define what inhalants are, how they are used, and their prevalence worldwide. The second section presents specific characteristics that define the main groups of inhalants: (a) organic solvents; (b) aerosols, gases, and volatile anesthetics; and (c) alkyl nitrites. We include a table with the molecular formula, structure, synonyms, uses, physicochemical properties and exposure limits of representative compounds within each group. The third and fourth sections review the direct acute and chronic effects of common inhalants on health and behavior with a summary of mechanisms of action, respectively. In the fifth section, we address inhalant intoxication signs and available treatment. The sixth section examines the health effects, intoxication, and treatment of nitrites. The seventh section reviews current intervention strategies. Finally, we propose a research agenda to promote the study of (a) solvents other than toluene; (b) inhalant mixtures; (c) effects in combination with other drugs of abuse; (d) age and (e) sex differences in inhalant effects; (f) the long-lasting behavioral effects of animals exposed in utero to inhalants; (g) abstinence signs and neurochemical changes after interrupting inhalant exposure; (h) brain networks involved in inhalant effects; and finally (i) strategies to promote recovery of inhalant users.

Gap junctions coordinate the propagation of glycogenolysis induced by norepinephrine in the pineal gland

Chemical and electrical synapses are the two major communication systems that permit cell-to-cell communication within the nervous system. Although most studies are focused on chemical synapses (glutamate, γ-aminobutyric acid, and other neurotransmitters), clearly both types of synapses interact and cooperate to allow the coordination of several cell functions within the nervous system. The pineal gland has limited independent axonal innervation and not every cell has access to nerve terminals. Thus, additional communication systems, such as gap junctions, have been postulated to coordinate metabolism and signaling. Using acutely isolated glands and dissociated cells, we found that gap junctions spread glycogenolytic signals from cells containing adrenoreceptors to the entire gland lacking these receptors. Our data using glycogen and lactate quantification, electrical stimulation, and high-performance liquid chromatography with electrochemical detection, demonstrate that gap junctional communication between cells of the rat pineal gland allows cell-to-cell propagation of norepinephrine-induced signal that promotes glycogenolysis throughout the entire gland. Thus, the interplay of both synapses is essential for coordinating glycogen metabolism and lactate production in the pineal gland.

A robotic multidimensional directed evolution approach applied to fluorescent voltage reporters

We developed a new way to engineer complex proteins toward multidimensional specifications using a simple, yet scalable, directed evolution strategy. By robotically picking mammalian cells that were identified, under a microscope, as expressing proteins that simultaneously exhibit several specific properties, we can screen hundreds of thousands of proteins in a library in just a few hours, evaluating each along multiple performance axes. To demonstrate the power of this approach, we created a genetically encoded fluorescent voltage indicator, simultaneously optimizing its brightness and membrane localization using our microscopy-guided cell-picking strategy. We produced the high-performance opsin-based fluorescent voltage reporter Archon1 and demonstrated its utility by imaging spiking and millivolt-scale subthreshold and synaptic activity in acute mouse brain slices and in larval zebrafish in vivo. We also measured postsynaptic responses downstream of optogenetically controlled neurons in C. elegans.

Review of toluene action: clinical evidence, animal studies and molecular targets

It has long been known that individuals will engage in voluntary inhalation of volatile solvents for their rewarding effects. However, research into the neurobiology of these agents has lagged behind that of more commonly used drugs of abuse such as psychostimulants, alcohol and nicotine. This imbalance has begun to shift in recent years as the serious effects of abused inhalants, especially among children and adolescents, on brain function and behavior have become appreciated and scientifically documented. In this review, we discuss the physicochemical and pharmacological properties of toluene, a representative member of a large class of organic solvents commonly used as inhalants. This is followed by a brief summary of the clinical and pre-clinical evidence showing that toluene and related solvents produce significant effects on brain structures and processes involved in the rewarding aspects of drugs. This is highlighted by tables highlighting toluene's effect on behaviors (reward, motor effects, learning, etc.) and cellular proteins (e.g. voltage and ligand-gated ion channels) closely associated the actions of abused substances. These sections demonstrate not only the significant progress that has been made in understanding the neurobiological basis for solvent abuse but also reveal the challenges that remain in developing a coherent understanding of this often overlooked class of drugs of abuse.

Screening fluorescent voltage indicators with spontaneously spiking HEK cells

Development of improved fluorescent voltage indicators is a key challenge in neuroscience, but progress has been hampered by the low throughput of patch-clamp characterization. We introduce a line of non-fluorescent HEK cells that stably express NaV 1.3 and KIR 2.1 and generate spontaneous electrical action potentials. These cells enable rapid, electrode-free screening of speed and sensitivity of voltage sensitive dyes or fluorescent proteins on a standard fluorescence microscope. We screened a small library of mutants of archaerhodopsin 3 (Arch) in spiking HEK cells and identified two mutants with greater voltage-sensitivity than found in previously published Arch voltage indicators.

Volatile solvents as drugs of abuse: focus on the cortico-mesolimbic circuitry

Volatile solvents such as those found in fuels, paints, and thinners are found throughout the world and are used in a variety of industrial applications. However, these compounds are also often intentionally inhaled at high concentrations to produce intoxication. While solvent use has been recognized as a potential drug problem for many years, research on the sites and mechanisms of action of these compounds lags behind that of other drugs of abuse. In this review, we first discuss the epidemiology of voluntary solvent use throughout the world and then consider what is known about their basic pharmacology and how this may explain their use as drugs of abuse. We next present data from preclinical and clinical studies indicating that these substances induce common addiction sequelae such as dependence, withdrawal, and cognitive impairments. We describe how toluene, the most commonly studied psychoactive volatile solvent, alters synaptic transmission in key brain circuits such as the mesolimbic dopamine system and medial prefrontal cortex (mPFC) that are thought to underlie addiction pathology. Finally, we make the case that activity in mPFC circuits is a critical regulator of the mesolimbic dopamine system's ability to respond to volatile solvents like toluene. Overall, this review provides evidence that volatile solvents have high abuse liability because of their selective effects on critical nodes of the addiction neurocircuitry, and underscores the need for more research into how these compounds induce adaptations in neural circuits that underlie addiction pathology.

Human concentrative nucleoside transporter 3 transfection with ultrasound and microbubbles in nucleoside transport deficient HEK293 cells greatly increases gemcitabine uptake

Gemcitabine is a hydrophilic clinical anticancer drug that requires nucleoside transporters to cross plasma membranes and enter cells. Pancreatic adenocarcinomas with low levels of nucleoside transporters are generally resistant to gemcitabine and are currently a clinical problem. We tested whether transfection of human concentrative nucleoside transporter 3 (hCNT3) using ultrasound and lipid stabilized microbubbles could increase gemcitabine uptake and sensitivity in HEK293 cells made nucleoside transport deficient by pharmacologic treatment with dilazep. To our knowledge, no published data exists regarding the utility of using hCNT3 as a therapeutic gene to reverse gemcitabine resistance. Our ultrasound transfection system - capable of transfection of cell cultures, mouse muscle and xenograft CEM/araC tumors - increased hCNT3 mRNA and ³H-gemcitabine uptake by >2,000– and 3,400–fold, respectively, in dilazep-treated HEK293 cells. Interestingly, HEK293 cells with both functional human equilibrative nucleoside transporters and hCNT3 displayed 5% of ³H-gemcitabine uptake observed in cells with only functional hCNT3, suggesting that equilibrative nucleoside transporters caused significant efflux of ³H-gemcitabine. Efflux assays confirmed that dilazep could inhibit the majority of ³H-gemcitabine efflux from HEK293 cells, suggesting that hENTs were responsible for the majority of efflux from the tested cells. Oocyte uptake transport assays were also performed and provided support for our hypothesis. Gemcitabine uptake and efflux assays were also performed on pancreatic cancer AsPC-1 and MIA PaCa-2 cells with similar results to that of HEK293 cells. Using the MTS proliferation assay, dilazep-treated HEK293 cells demonstrated 13-fold greater resistance to gemcitabine compared to dilazep-untreated HEK293 cells and this resistance could be reversed by transfection of hCNT3 cDNA. We propose that transfection of hCNT3 cDNA using ultrasound and microbubbles may be a method to reverse gemcitabine resistance in pancreatic tumors that have little nucleoside transport activity which are resistant to almost all current anticancer therapies.

A combined gene and cell therapy approach for restoration of conduction

BACKGROUND

Abnormal conduction underlies both bradyarrhythmias and re-entrant tachyarrhythmias. However, no practical way exists for restoring or improving conduction in areas of conduction slowing or block.

OBJECTIVE

This study sought to test the feasibility of a novel strategy for conduction repair using genetically engineered cells designed to form biological "conducting cables."

METHODS

An in vitro model of conduction block was established using spatially separated, spontaneously contracting, nonsynchronized human embryonic stem cell-derived cardiomyocytes clusters. Immunostaining, dye transfer, intracellular recordings, and multielectrode array (MEA) studies were performed to evaluate the ability of genetically engineered HEK293 cells, expressing the SCN5A-encoded Na(+) channel, to couple with cultured cardiomyocytes and to synchronize their electrical activity.

RESULTS

Connexin-43 immunostaining and calcein dye-transfer experiments confirmed the formation of functional gap junctions between the engineered cells and neighboring cardiomyocytes. MEA and intracellular recordings were performed to assess the ability of the engineered cells to restore conduction in the co-cultures. Synchronization was defined by establishment of fixed local activation time differences between the cardiomyocytes clusters and convergence of their activation cycle lengths. Nontransfected control cells were able to induce synchronization between cardiomyocytes clusters separated by distances up to 300 μm (n = 21). In contrast, the Na(+) channel-expressing cells synchronized contractions between clusters separated by up to 1,050 μm, the longest distance studied (n = 23). Finally, engineered cells expressing the voltage-sensitive K(v)1.3 potassium channel prevented synchronization at any distance.

CONCLUSION

Genetically engineered cells, transfected to express Na(+) channels, can form biological conducting cables bridging and coupling spatially separated cardiomyocytes. This novel cell therapy approach might be useful for the development of therapeutic strategies for both bradyarrhythmias and tachyarrhythmias.

Distinct detergent-resistant membrane microdomains (lipid rafts) respectively harvest K(+) and water transport systems in brain astroglia

The detergent-resistant microdomains (DRM) of cell membranes scaffold different molecules and regulate cell functions by orchestrating various signaling pathways including G-proteins and tyrosine kinase. Here we report a novel role for DRM in astroglial cells. K(+)-buffering inwardly rectifying Kir4.1 channels and the water channel AQP4 are expressed in astrocytes and they may be functionally coupled to maintain ionic and osmotic homeostasis in the brain. Kir4.1 and AQP4 channel proteins were abundant in noncaveloar DRM in the brain and also in HEK293T cells when exogenously expressed. In HEK293T cells, depletion of membrane cholesterol by methyl-beta-cyclodextrin (MbetaCD) resulted in loss of Kir4.1 association with DRM and its channel activity but did not affect either the distribution or the function of AQP4. Immunolabeling showed that Kir4.1 and AQP4 were occasionally distributed in close proximity but in distinct compartments of the astroglial cell membrane. Astroglial noncaveolar DRM may therefore be made up of at least two distinct compartments, MbetaCD-sensitive and MbetaCD-resistant microdomains, which control localization and function of, respectively, Kir and AQP4 channels on the cell membrane.

Activation of Nuclear Factor κB by Different Agents

The transcription factor nuclear factor kappaB (NF-kappaB) or other components of this pathway have been identified as possible therapeutic targets in inflammatory processes, cancer, and autoimmune diseases. In order to clarify the role of NF-kappaB in epithelial cells in response to different stresses, a cell-based screening assay for activation of NF-kappaB-dependent gene transcription in human embryonic kidney cells (HEK/293) was developed. This assay allows detection of NF-kappaB activation by measurement of the fluorescence of the reporter protein destabilized enhanced green fluorescent protein (d2EGFP). For characterization of the cell-based assay, activation of the pathway by several agents, for example, tumor necrosis factor alpha (TNF-alpha), interleukin-1beta (IL-1beta), lipopolysaccharide (LPS), camptothecin and phorbol ester (PMA), and the influence of the culture conditions on NF-kappaB activation by TNF-alpha were examined. NF-kappaB was activated by TNF-alpha, IL-1beta, PMA, and camptothecin in a dose-dependent manner, but not by LPS. TNF-alpha results in the strongest induction of NF-kappaB-dependent gene expression. However, this response fluctuated from 30 to 90% of the cell population showing d2EGFP expression. This variation can be explained by differences in growth duration and cell density at the time of treatment. With increasing confluence of the cells, the activation potential decreased. In a confluent cell layer, only 20-35% of the cell population showed d2EGFP expression. The underlying mechanism of this phenomenon can be the production of soluble factors by the cells inhibiting the NF-kappaB activation or direct communication via gap junctions in the cell layer diminishing the TNF-alpha response.

Three fluorescent protein voltage sensors exhibit low plasma membrane expression in mammalian cells

Three first-generation fluorescent protein voltage sensitive probes (FP-voltage sensors) were characterized in mammalian cells. Flare, a Kv1.4 variant of FlaSh [Siegel MS, Isacoff EY. Neuron 1997;19(October (4)):735-41], SPARC [Ataka K, Pieribone VA. Biophys J 2002;82(January (1 Pt 1)):509-16], and VSFP-1 [Sakai R, Repunte-Canonigo V, Raj CD, Knopfel T. Eur J Neurosci 2001;13(June (12)):2314-18] were expressed, imaged and voltage clamped in HEK 293 cells and in dissociated hippocampal neurons. We were unable to detect a signal in response to changes in membrane potential after averaging16 trials with any of the three constructs. Using the hydrophobic voltage sensitive dye, di8-ANEPPS, as a surface marker, confocal analyses demonstrated poor plasma membrane expression for Flare, SPARC and VSFP-1 in both HEK 293 cells and dissociated hippocampal neurons. Almost all of the expressed FP-voltage sensors reside in internal membranes in both cell types. This internal expression generates a background fluorescence that increases the noise in the optical measurement.

The last decade of solvent research in animal models of abuse: mechanistic and behavioral studies

The abuse of volatile organic solvents (inhalants) leads to diverse sequelae at levels ranging from the cell to the whole organism. This paper reviews findings from the last 10 years of animal models investigating the behavioral and mechanistic effects of solvent abuse. In research with animal models of inhalant abuse, NMDA, GABA(A), glycine, nicotine, and 5HT(3) receptors appear to be important targets of action for several abused solvents with emerging evidence suggesting that other receptor subtypes and nerve membrane ion channels may be involved as well. The behavioral effects vary in magnitude and duration among the solvents investigated. The behavioral effects of acute and chronic inhalant abuse include motor impairment, alterations in spontaneous motor activity, anticonvulsant effects, anxiolytic effects, sensory effects, and effects on learning, memory and operant behavior (e.g., response rates and discriminative stimulus effects). In addition, repeated exposure to these solvents may produce tolerance, dependence and/or sensitization to these effects.

Effects of the abused inhalant toluene on ethanol-sensitive potassium channels expressed in oocytes

Toluene (methylbenzene) is representative of a class of industrial solvents that are voluntarily inhaled as drugs of abuse. Previous data from this laboratory and others have shown that these compounds alter the function of a variety of ion channels including ligand-gated channels activated by ATP, acetylcholine, GABA, glutamate and serotonin, as well as voltage-dependent sodium and calcium channels. It is less clear what effects toluene may have on potassium channels that act to reduce the excitability of most cells. Previous studies have shown that ethanol potentiates the function of both the large conductance, calcium-activated potassium channel (BK) and specific members of the G-protein-coupled inwardly rectifying potassium channels (GirKs). Since toluene and other abused inhalants share many behavioral effects with ethanol, it was hypothesized that toluene would also enhance the function of these channels. This hypothesis was tested using two-electrode voltage-clamp electrophysiology to measure the activity of BK and GirK potassium channel currents expressed in Xenopus laevis oocytes. As reported previously, ethanol potentiated currents in oocytes expressing either BK or GirK2 channels. In contrast, toluene caused a concentration-dependent inhibition of BK channel currents with 3 mM producing approximately 50% inhibition of control currents. Currents in oocytes injected with GirK2 mRNA were also inhibited by toluene while those expressing GirK1/2 and 1/4 channels were minimally affected. In oocytes co-injected with mRNA for GirK2 and the mGluR1a metabotropic receptor, exposure to glutamate potentiated currents evoked by a high-potassium solution. Toluene inhibited these glutamate-activated currents to approximately the same degree as those induced under basal conditions. The results of these studies show that toluene has effects on BK and GirK channels that are opposite to those of ethanol, suggesting that these channels are unlikely to underlie behaviors that these two drugs of abuse share.