Amphetamine withdrawal produces region-specific and time-dependent changes in connexin36 expression in rat brain

A new path to mental disorders: Through gap junction channels and hemichannels

Behavioral disturbances related to emotional regulation, reward processing, cognition, sleep-wake regulation and activity/movement represent core symptoms of most common mental disorders. Increasing empirical and theoretical evidence suggests that normal functioning of these behavioral domains relies on fine graded coordination of neural and glial networks which are maintained and modulated by intercellular gap junction channels and unapposed pannexin or connexin hemichannels. Dysfunctions in these networks might contribute to the development and maintenance of psychopathological and neurobiological features associated with mental disorders. Here we review and discuss the evidence indicating a prominent role of gap junction channel and hemichannel dysfunction in core symptoms of mental disorders. We further discuss how the increasing knowledge on intercellular gap junction channels and unapposed pannexin or connexin hemichannels in the brain might lead to deeper mechanistic insight in common mental disorders and to the development of novel treatment approaches. We further attempt to exemplify what type of future research on this topic could be integrated into multidimensional approaches to understand and cure mental disorders.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Rapid upregulation of the hippocampal connexins 36 and 45 mRNA levels during memory consolidation

Gap junction channels are implicated in learning and memory process. However, their role on each of the particular stages of memory formation has been studied less. In this study, the time profile of the expression levels of hippocampal connexins 36 and 45 (Cx36 and Cx45) mRNAs was measured during memory consolidation, in a passive avoidance paradigm. Totally 30 adult male rats were distributed into 5 groups of each 6. At different times profiles (30min, 3, 6 and 24h) following training, rats were decapitated and their hippocampi were immediately removed and frozen in liquid nitrogen. Total RNA was extracted and cDNA was synthesized, using oligo-dt primers. A quantitative real-time PCR was used to measure the levels of each of Cx36 and Cx45 mRNAs. Both connexins showed a rapid upregulation (30min) at the transcriptional level, which declined in later times and reached to the control level at 24h. The rapid up-regulation of Cx36 and Cx45 mRNAs might be accompanied with increasing intercellular coupling via gap junction channels and neuronal oscillatory activities required for memory consolidation. The results highlight the role of gap junctional coupling between hippocampal neurons during memory consolidation in the physiological conditions.

Methamphetamine compromises gap junctional communication in astrocytes and neurons

Methamphetamine (meth) is a central nervous system (CNS) stimulant that results in psychological and physical dependency. The long-term effects of meth within the CNS include neuronal plasticity changes, blood-brain barrier compromise, inflammation, electrical dysfunction, neuronal/glial toxicity, and an increased risk to infectious diseases including HIV. Most of the reported meth effects in the CNS are related to dysregulation of chemical synapses by altering the release and uptake of neurotransmitters, especially dopamine, norepinephrine, and epinephrine. However, little is known about the effects of meth on connexin (Cx) containing channels, such as gap junctions (GJ) and hemichannels (HC). We examined the effects of meth on Cx expression, function, and its role in NeuroAIDS. We found that meth altered Cx expression and localization, decreased GJ communication between neurons and astrocytes, and induced the opening of Cx43/Cx36 HC. Furthermore, we found that these changes in GJ and HC induced by meth treatment were mediated by activation of dopamine receptors, suggesting that dysregulation of dopamine signaling induced by meth is essential for GJ and HC compromise. Meth-induced changes in GJ and HC contributed to amplified CNS toxicity by dysregulating glutamate metabolism and increasing the susceptibility of neurons and astrocytes to bystander apoptosis induced by HIV. Together, our results indicate that connexin containing channels, GJ and HC, are essential in the pathogenesis of meth and increase the sensitivity of the CNS to HIV CNS disease. Methamphetamine (meth) is an extremely addictive central nervous system stimulant. Meth reduced gap junctional (GJ) communication by inducing internalization of connexin-43 (Cx43) in astrocytes and reducing expression of Cx36 in neurons by a mechanism involving activation of dopamine receptors (see cartoon). Meth-induced changes in Cx containing channels increased extracellular levels of glutamate and resulted in higher sensitivity of neurons and astrocytes to apoptosis in response to HIV infection.

Under construction: building the macromolecular superstructure and signaling components of an electrical synapse

A great deal is now known about the protein components of tight junctions and adherens junctions, as well as how these are assembled. Less is known about the molecular framework of gap junctions, but these also have membrane specializations and are subject to regulation of their assembly and turnover. Thus, it is reasonable to consider that these three types of junctions may share macromolecular commonalities. Indeed, the tight junction scaffolding protein zonula occluden-1 (ZO-1) is also present at adherens and gap junctions, including neuronal gap junctions. On the basis of these earlier observations, we more recently found that two additional proteins, AF6 and MUPP1, known to be associated with ZO-1 at tight and adherens junctions, are also components of neuronal gap junctions in rodent brain and directly interact with connexin36 (Cx36) that forms these junctions. Here, we show by immunofluorescence labeling that the cytoskeletal-associated protein cingulin, commonly found at tight junctions, is also localized at neuronal gap junctions throughout the central nervous system. In consideration of known functions related to ZO-1, AF6, MUPP1, and cingulin, our results provide a context in which to examine functional relationships between these proteins at Cx36-containing electrical synapses in brain--specifically, how they may contribute to regulation of transmission at these synapses, and how they may govern gap junction channel assembly and/or disassembly.

The role of connexin-36 gap junctions in alcohol intoxication and consumption

Ventral tegmental area (VTA) GABA neurons appear to be critical substrates underlying the acute and chronic effects of ethanol on dopamine (DA) neurotransmission in the mesocorticolimbic system implicated in alcohol reward. The aim of this study was to examine the role of midbrain connexin-36 (Cx36) gap junctions (GJs) in ethanol intoxication and consumption. Using behavioral, molecular, and electrophysiological methods, we compared the effects of ethanol in mature Cx36 knockout (KO) mice and age-matched wild-type (WT) controls. Compared to WT mice, Cx36 KO mice exhibited significantly more ethanol-induced motor impairment in the open field test, but less disruption in motor coordination in the rotarod paradigm. Cx36 KO mice, and WT mice treated with the Cx36 antagonist mefloquine (MFQ), consumed significantly less ethanol than their WT controls in the drink-in-the-dark procedure. The firing rate of VTA GABA neurons in WT mice was inhibited by ethanol with an IC₅₀ of 0.25 g/kg, while VTA GABA neurons in KO mice were significantly less sensitive to ethanol. Dopamine neuron GABA-mediated sIPSC frequency was reduced by ethanol (30 mM) in WT mice, but not affected in KO mice. Cx36 KO mice evinced a significant up-regulation in DAT and D2 receptors in the VTA, as assessed by quantitative RT-PCR. These findings demonstrate the behavioral relevance of Cx36 GJ-mediated electrical coupling between GABA neurons in mature animals, and suggest that loss of coupling between VTA GABA neurons results in disinhibition of DA neurons, a hyper-DAergic state and lowered hedonic valence for ethanol consumption.

The role of gap junctions in the brain in health and disease

Gap junctions connect the cytosolic compartments of adjacent cells for direct electrotonic and metabolic cell-to-cell communication. Gap junctions between glial cells or neurons are ubiquitously expressed in the brain and play a role in brain development including cell differentiation, cell migration and survival, tissue homeostasis, as well as in human diseases including hearing loss, skin disease, neuropathies, epilepsy, brain trauma, and cardiovascular disease. Furthermore, gap junctions are involved in the synchronization and rhythmic oscillation of hippocampal and neocotical neuronal ensembles which might be important for memory formation and consolidation. In this review the accumulated evidence from mouse mutant and pharmacological studies using gap junction blockers is summarized and the progress made in dissecting the physiological, pathophysiological and behavioral roles of gap junction mediated intercellular communication in the brain is discussed.

Therapeutic approaches for spinal cord injury induced spasticity

Spasticity is evident in both humans and animals following spinal cord injury (SCI) and can contribute to significant functional limitation and disruption in quality of life of patients with this disorder. This mini-review describes a number of preclinical and clinical studies that promise to improve outcomes for, especially in terms of spasticity and hyper-reflexia, patients with SCI. A gold standard for the quantification of spasticity has proved elusive, but the combination of H-reflex frequency dependent depression and a novel stretch reflex (SR) windup protocol have the potential to provide new insights. As the pathophysiology of hyper-reflexia and spasticity continue to be investigated, the documented onset in the animal model of SCI provides critical time points for further study into these complex mechanisms. The positive effects of a passive exercise protocol and several potential pharmacological interventions are reviewed as well as a novel potential mechanism of action. Further work is needed to determine additional mechanisms that are involved in SCI, and how to optimize multiple therapies to overcome some of the deficits induced by SCI.

Neurochemical consequences of dysphoric state during amphetamine withdrawal in animal models: a review

Chronic abuse of amphetamines, such as d-amphetamine (AMPH) and d-methamphetamine, results in psychological dependence, a condition in which the drug produces a feeling of satisfaction and a drive that requires periodic or continuous administration of the drug to produce overwhelming pleasure or to avoid discomfort such as dysphoria. The dysphoric state of AMPH withdrawal has been recognized as depressive syndromes, such as anhedonia, depression, anxiety, and social inhibition, in early drug abstinence. Medication for treatment of the dysphoric state is important for AMPH abusers to avoid impulsive self-injurious behavior or acts that are committed with unconscious or uncontrolled suicidal ideation. However, successful treatments for AMPH withdrawal remain elusive, since the exact molecular basis of the expression of dysphoria has not been fully elucidated. This review focuses on the molecular aspects of AMPH withdrawal as indexed by neurochemical parameters under a variety of injection regimens (for example, levels of brain monoamines and their metabolites, and gamma-aminobutyric acid, expression of genes and proteins involved in neuronal activity, and monoamine metabolism and availability) in rodent models which exhibit significant phenotypic features relevant to the syndromes of AMPH withdrawal in humans.

Electrical synapses in basal ganglia

The basal ganglia (BG) provide a major integrative system of the forebrain involved in the organization of goal-directed behaviour. Pathological alteration of BG function leads to major motor and cognitive impairments such as observed in Parkinson's disease. Recent advances in BG research stress the role of neural oscillations and synchronization in the normal and pathological function of BG. As demonstrated in several brain structures, these patterns of neural activity can emerge from electrically coupled neuronal networks. This review aims at addressing the presence, functionality and putative role of electrical synapses in BG, with a particular emphasis on the striatum and the substantia nigra pars compacta (SNc), two main BG nuclei in which the existence and functional properties of neuronal coupling are best documented.

Behavioral synergism between D(1) and D(2) dopamine receptors in mice does not depend on gap junctions

Activation of the D(1) and D(2) classes of dopamine receptor in the striatum synergistically promotes motor stereotypy. The mechanism of D(1)/D(2) receptor interaction remains unclear. To investigate the involvement of electrical synaptic transmission in this phenomenon, genetic inactivation of the neuronal gap junction (GJ) protein connexin 36 and pharmacological blockade of GJs were utilized. Stereotyped motor behavior was quantified after selective activation of D(1) receptors, D(2) receptors, or both receptors. These patterns of activation were produced by injection of the agonist apomorphine (3.0 mg/kg) 30 min after either the D(2) antagonist eticlopride (0.3 mg/kg), the D(1) antagonist SCH 23390 (0.1 mg/kg) or vehicle, respectively. Mixed background C57/BL6-129SvEv mice homozygous or heterozygous for the connexin 36 "knockout" allele displayed potent synergistic interaction between D(1) and D(2) receptor activation, and did not differ significantly from wild-type mice on any measure. All genotypes demonstrated long-lasting stereotypic sniffing, chewing, and/or licking after simultaneous activation of D(1) and D(2) receptors, effects that were absent following selective D(1) or D(2) activation. Swiss-Webster mice treated with the GJ blockers carbenoxolone (35 mg/kg), octanol (350 mg/kg) or mefloquine (50 mg/kg) also demonstrated the normal synergistic interaction between D(1) and D(2) receptors, although these drugs did block the grooming stimulated by selective D(1) receptor activation, independently of D(2) receptors. While D(1) receptor-stimulated grooming depends on GJs composed of connexins or possibly pannexins, the synergistic interaction of D(1) and D(2) receptors in control of stereotypy does not involve GJs.

Hyperbolic discounting may be reduced to electrical coupling in dopaminergic neural circuits

Loss of self-control in drug addicts (e.g. cocaine and amphetamine dependent patients) has been associated with hyperbolic discounting of delayed rewards (i.e., inconsistency in intertemporal choice). Neurobiophysical mechanisms underlying hyperbolic discounting are still unknown in spite of recent extensive work in neuroeconomics. Understanding of neuronal mechanisms of hyperbolic discounting is important for establishing neuropharmacological treatment of addiction. At the cognitive level, previous studies have indicated that psychophysics of time-estimation (i.e., Weber-Fechner law and Steven's power law of time-perception) may explain inconsistency in intertemporal choice. Regarding neuronal substrates of time-estimation, drugs of abuse dramatically change time-estimation, indicating that dopaminergic activities may mediate time-estimation. With respect to neuronal changes induced by drugs of abuse, recent studies have revealed that gap junction proteins (e.g., connexin 36) in dopamine neurons are increased by an self-administration of dopaminergic drugs such as cocaine and amphetamine. However, it has been yet to be examined how the enhanced electrical coupling due to substance administration induces addiction. Furthermore, a recent biophysical modelling study has demonstrated that the effect of the psychophysical laws are potentiated by non-synaptic electrical coupling between neurons via gap junctions. Based on these current findings, we hypothesized that hyperbolic discounting may be reduced to biophysical characteristics of dopamine neural circuits, that is, electrical coupling between time-encoding dopaminergic neurons via gap junctions. The present hypothesis states that drugs of abuse may induce addiction by exacerbating subject's impulsivity (a discount rate) and inconsistency (hyperbolicity) in intertemporal choice partly due to enhanced expression of gap junction proteins in dopaminergic neurons. Possible psychopharmacological treatments of impulsivity in drug addicts implied by our present hypothesis, e.g., administration of a gap junction blocker, are discussed.

Effects of amphetamine on serotoninergic and GABAergic expression of developing brain

Roles of age and withdrawal were explored in mechanisms underlying the action of amphetamine (Amph), by monitoring the serotonergic and GABAergic expression in key brain regions of the rat. Postnatal 21 and 60 day-old male rats were intraperitoneally injected with D-Amph, 5 mg/kg, or saline, three times daily for 14 days and then withdrawn from Amph for 0 or 14 days; these animals received single injections on day 15 (W0d) or day 29 (W14d). Following Amph injections, though both age groups exhibited hyperlocomotion, stereotypy and behavioral sensitization, the juvenile showed 100-300% longer latencies to reach and 30%-42% shorter duration of maximal behavioral scores than the adult from day 2-29. Immunocytochemical analysis revealed down-regulation of 42-76% in 5-hydroxytryptamine (HT) immunoreactive processes in motor and somatosensory cortices, and hippocampus of both ages after Amph exposure at W0d. At W14d, the 5-HT resembled saline-control in the Amph-treated juvenile, whereas remained weakened in the adult. By contrast, densities of GAD67 (glutamic acid decarboxylase)-boutons were up-regulated by 35-545% in the neocortical areas, nucleus accumbens, caudate-putamen and hippocampus of all Amph-administered rats. After 14 days withdrawal, the juvenile recovered the decreased 5-HT fibers, but not the increased GABAergic, indicating unique roles of the two systems in response to Amph.

A single evoked afterdischarge produces rapid time-dependent changes in connexin36 protein expression in adult rat dorsal hippocampus

Gap junctions between neurons contribute to synchronous neuronal firing and may play a role in the pathophysiology of epilepsy. We examined the expression of a number of gap junction subunits, including the neuronal gap junction forming protein connexin36 (Cx36), in the hippocampus at various time points following an electrically stimulated afterdischarge (AD) in freely-moving animals. Once recovered from electrode implantation, animals were tested with an escalating series of stimulations until an AD was evoked. Suprathreshold stimulation produced a brief AD with no convulsion. Groups of animals were sacrificed at 3, 12, and 24h post-stimulation, and connexin expression was assessed via semiquantitative immunoblotting. Compared to implanted non-stimulated controls, a significant decrease in Cx36 expression was observed in the stimulated dorsal hippocampus at 3h post-stimulation, which returned to control levels by 24h. No changes were seen in the ventral hippocampus. As well, no changes were seen in other selected connexin proteins including Cx26, Cx32, and Cx43, thought to be expressed primarily in glia, in either dorsal or ventral hippocampus. These data suggest that a relatively brief hypersynchronous neuronal discharge can produce rapid and specific changes in Cx36 expression, which may have implications for both normal brain function and the pathophysiology of epilepsy.

Extended cocaine self-administration and deprivation produces region-specific and time-dependent changes in connexin36 expression in rat brain

Cocaine addiction is a disease that develops over time, and it is thought that drug-induced neuro-adaptations underlie the changes in behavior seen across the addictive process. While a number of alterations in synaptic transmission have been identified, little is currently known regarding cocaine's effects on gap junctional communication between neurons. Here we examine the effects of a cocaine self-administration regimen, previously shown to increase the reinforcing efficacy of cocaine, on the expression of the neuron-specific gap junction-forming protein connexin36 (C x 36). Using real-time RT-PCR and immunoblotting, we show that binge cocaine self-administration produces region-specific and time-dependent changes in C x 36 mRNA and protein expression in the nucleus accumbens, prefrontal cortex, and hippocampus. A number of changes in C x 36 were present 1 day and 7 days following self-administration, and C x 36 mRNA and protein appeared to be differentially regulated in a region-specific manner. C x 36 protein was significantly decreased in the prefrontal cortex 7 days following self-administration, a time point when behavioral sensitization to the reinforcing effects of cocaine is observed. These results suggest that changes in neuronal gap junction expression may be one mechanism by which cocaine self-administration produces enduring changes in behavior.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.