Connexin mRNA expression in single dopaminergic neurons of substantia nigra pars compacta

A fatal alliance: Glial connexins, myelin pathology and mental disorders

Mature oligodendrocytes are myelin forming glial cells which are responsible for myelination of neuronal axons in the white matter of the central nervous system. Myelin pathology is a major feature of severe neurological disorders. Oligodendrocyte-specific gene mutations and/or white matter alterations have also been addressed in a variety of mental disorders. Breakdown of myelin integrity and demyelination is associated with severe symptoms, including impairments in motor coordination, breathing, dysarthria, perception (vision and hearing), and cognition. Furthermore, there is evidence indicating that myelin sheath defects and white matter pathology contributes to the affective and cognitive symptoms of patients with mental disorders. Oligodendrocytes express the connexins GJC2; mCx47 [human (GJC2) and mouse (mCx47) connexin gene nomenclature according to Söhl and Willecke (2003)], GJB1; mCx32, and GJD1; mCx29 in both white and gray matter. Preclinical findings indicate that alterations in connexin expression in oligodendrocytes and astrocytes can induce myelin defects. GJC2; mCx47 is expressed at early embryonic stages in oligodendrocyte precursors cells which precedes central nervous system myelination. In adult humans and animals GJC2, respectively mCx47 expression is essential for oligodendrocyte function and ensures adequate myelination as well as myelin maintenance in the central nervous system. In the past decade, evidence has accumulated suggesting that mental disorders can be accompanied by changes in connexin expression, myelin sheath defects and corresponding white matter alterations. This dual pathology could compromise inter-neuronal information transfer, processing and communication and eventually contribute to behavioral, sensory-motor, affective and cognitive symptoms in patients with mental disorders. The induction of myelin repair and remyelination in the central nervous system of patients with mental disorders could help to restore normal neuronal information propagation and ameliorate behavioral and cognitive symptoms in individuals with mental disorders.

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.

Altered neural cell junctions and ion-channels leading to disrupted neuron communication in Parkinson's disease

Parkinson's disease (PD) is a neurological disorder that affects the movement of the human body. It is primarily characterized by reduced dopamine levels in the brain. The causative agent of PD is still unclear but it is generally accepted that α-synuclein has a central role to play. It is also known that gap-junctions and associated connexins are complicated structures that play critical roles in nervous system signaling and associated misfunctioning. Thus, our current article emphasizes how, alongside α-synuclein, ion-channels, gap-junctions, and related connexins, all play vital roles in influencing multiple metabolic activities of the brain during PD. It also highlights that ion-channel and gap-junction disruptions, which are primarily mediated by their structural-functional changes and alterations, have a role in PD. Furthermore, we discussed available drugs and advanced therapeutic interventions that target Parkinson's pathogenesis. In conclusion, it warrants creating better treatments for PD patients. Although, dopaminergic replenishment therapy is useful in treating neurological problems, such therapies are, however, unable to control the degeneration that underpins the disease, thereby declining their overall efficacy. This creates an additional challenge and an untapped scope for neurologists to adopt treatments for PD by targeting the ion-channels and gap-junctions, which is well-reviewed in the present article.

CO2-Sensitive Connexin Hemichannels in Neurons and Glia: Three Different Modes of Signalling?

Connexins can assemble into either gap junctions (between two cells) or hemichannels (from one cell to the extracellular space) and mediate cell-to-cell signalling. A subset of connexins (Cx26, Cx30, Cx32) are directly sensitive to CO₂ and fluctuations in the level within a physiological range affect their open probability, and thus, change cell conductance. These connexins are primarily found on astrocytes or oligodendrocytes, where increased CO₂ leads to ATP release, which acts on P2X and P2Y receptors of neighbouring neurons and changes excitability. CO₂-sensitive hemichannels are also found on developing cortical neurons, where they play a role in producing spontaneous neuronal activity. It is plausible that the transient opening of hemichannels allows cation influx, leading to depolarisation. Recently, we have shown that dopaminergic neurons in the substantia nigra and GABAergic neurons in the VTA also express Cx26 hemichannels. An increase in the level of CO₂ results in hemichannel opening, increasing whole-cell conductance, and decreasing neuronal excitability. We found that the expression of Cx26 in the dopaminergic neurons in the substantia nigra at P7-10 is transferred to glial cells by P17-21, displaying a shift from being inhibitory (to neuronal activity) in young mice, to potentially excitatory (via ATP release). Thus, Cx26 hemichannels could have three modes of signalling (release of ATP, excitatory flickering open and shut and inhibitory shunting) depending on where they are expressed (neurons or glia) and the stage of development.

Brain Disorders and Chemical Pollutants: A Gap Junction Link?

The incidence of brain pathologies has increased during last decades. Better diagnosis (autism spectrum disorders) and longer life expectancy (Parkinson's disease, Alzheimer's disease) partly explain this increase, while emerging data suggest pollutant exposures as a possible but still underestimated cause of major brain disorders. Taking into account that the brain parenchyma is rich in gap junctions and that most pollutants inhibit their function; brain disorders might be the consequence of gap-junctional alterations due to long-term exposures to pollutants. In this article, this hypothesis is addressed through three complementary aspects: (1) the gap-junctional organization and connexin expression in brain parenchyma and their function; (2) the effect of major pollutants (pesticides, bisphenol A, phthalates, heavy metals, airborne particles, etc.) on gap-junctional and connexin functions; (3) a description of the major brain disorders categorized as neurodevelopmental (autism spectrum disorders, attention deficit hyperactivity disorders, epilepsy), neurobehavioral (migraines, major depressive disorders), neurodegenerative (Parkinson's and Alzheimer's diseases) and cancers (glioma), in which both connexin dysfunction and pollutant involvement have been described. Based on these different aspects, the possible involvement of pollutant-inhibited gap junctions in brain disorders is discussed for prenatal and postnatal exposures.

Detecting CO2-Sensitive Hemichannels in Neurons in Acute Brain Slices

This protocol provides two independent methods to functionally detect the neuronal expression of CO₂-sensitive hemichannels. These hemichannels (consisting of connexins 26 or 30) are directly gated by CO₂, independent of pH changes and until recently were thought to be only expressed by glia. This protocol outlines a method to change the concentration of CO₂ without changing pH, using isohydric solutions and then utilizing this to detect opening and closing of functional hemichannels using whole-cell patch clamp recording and dye loading.

For complete details on the use and execution of this protocol, please refer to Hill et al. (2020).

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Protocols for detecting CO₂-sensitive hemichannels in neurons of acute brain slices

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Use of electrophysiology to look for changes in neuronal firing and input resistance

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Use of dye loading to confirm neuronal CO₂-sensitive hemichannel expression

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Use of immunohistochemistry to confirm connexin expression in neurons of interest

Channels to consciousness: a possible role of gap junctions in consciousness

The neurophysiological basis of consciousness is still unknown and one of the most challenging questions in the field of neuroscience and related disciplines. We propose that consciousness is characterized by the maintenance of mental representations of internal and external stimuli for the execution of cognitive operations. Consciousness cannot exist without working memory, and it is likely that consciousness and working memory share the same neural substrates. Here, we present a novel psychological and neurophysiological framework that explains the role of consciousness for cognition, adaptive behavior, and everyday life. A hypothetical architecture of consciousness is presented that is organized as a system of operation and storage units named platforms that are controlled by a consciousness center (central executive/online platform). Platforms maintain mental representations or contents, are entrusted with different executive functions, and operate at different levels of consciousness. The model includes conscious-mode central executive/online and mental time travel platforms and semiconscious steady-state and preconscious standby platforms. Mental representations or contents are represented by neural circuits and their support cells (astrocytes, oligodendrocytes, etc.) and become conscious when neural circuits reverberate, that is, fire sequentially and continuously with relative synchronicity. Reverberatory activity in neural circuits may be initiated and maintained by pacemaker cells/neural circuit pulsars, enhanced electronic coupling via gap junctions, and unapposed hemichannel opening. The central executive/online platform controls which mental representations or contents should become conscious by recruiting pacemaker cells/neural network pulsars, the opening of hemichannels, and promoting enhanced neural circuit coupling via gap junctions.

Moderate Changes in CO2 Modulate the Firing of Neurons in the VTA and Substantia Nigra

The substantia nigra (SN) and ventral tegmental area (VTA) are vital for the control of movement, goal-directed behavior, and encoding reward. Here we show that the firing of specific neuronal subtypes in these nuclei can be modulated by physiological changes in the partial pressure of carbon dioxide (PCO2). The resting conductance of substantia nigra dopaminergic neurons in young animals (postnatal days 7-10) and GABAergic neurons in the VTA is modulated by changes in the level of CO2. We provide several lines of evidence that this CO2-sensitive conductance results from connexin 26 (Cx26) hemichannel expression. Since the levels of PCO2 in the blood will vary depending on physiological activity and pathology, this suggests that changes in PCO2 could potentially modulate motor activity, reward behavior, and wakefulness.

Binding of α-synuclein oligomers to Cx32 facilitates protein uptake and transfer in neurons and oligodendrocytes

The intercellular transfer of alpha-synuclein (α-syn) has been implicated in the progression of Parkinson's disease (PD) and multiple system atrophy (MSA). The cellular mechanisms underlying this process are now beginning to be elucidated. In this study, we demonstrate that the gap junction protein connexin-32 (Cx32) is centrally involved in the preferential uptake of α-syn oligomeric assemblies (oα-syn) in neurons and oligodendrocytes. In vitro, we demonstrate a clear correlation between Cx32 expression and oα-syn uptake. Pharmacological and genetic strategies targeting Cx32 successfully blocked oα-syn uptake. In cellular and transgenic mice modeling PD and MSA, we observed significant upregulation of Cx32 which correlates with α-syn accumulation. Notably, we could also demonstrate a direct interaction between α-syn and Cx32 in two out of four human PD cases that was absent in all four age-matched controls. These data are suggestive of a link between Cx32 and PD pathophysiology. Collectively, our results provide compelling evidence for Cx32 as a novel target for therapeutic intervention in PD and related α-synucleinopathies.

Connexins and pannexins: At the junction of neuro-glial homeostasis & disease

In the central nervous system (CNS), connexin (Cx)s and pannexin (Panx)s are an integral component of homeostatic neuronal excitability and synaptic plasticity. Neuronal Cx gap junctions form electrical synapses across biochemically similar GABAergic networks, allowing rapid and extensive inhibition in response to principle neuron excitation. Glial Cx gap junctions link astrocytes and oligodendrocytes in the pan-glial network that is responsible for removing excitotoxic ions and metabolites. In addition, glial gap junctions help constrain excessive excitatory activity in neurons and facilitate astrocyte Ca²⁺ slow wave propagation. Panxs do not form gap junctions in vivo, but Panx hemichannels participate in autocrine and paracrine gliotransmission, alongside Cx hemichannels. ATP and other gliotransmitters released by Cx and Panx hemichannels maintain physiologic glutamatergic tone by strengthening synapses and mitigating aberrant high frequency bursting. Under pathological depolarizing and inflammatory conditions, gap junctions and hemichannels become dysregulated, resulting in excessive neuronal firing and seizure. In this review, we present known contributions of Cxs and Panxs to physiologic neuronal excitation and explore how the disruption of gap junctions and hemichannels lead to abnormal glutamatergic transmission, purinergic signaling, and seizures.

GAP junctional communication in brain secondary organizers

Gap junctions (GJs) are integral membrane proteins that enable the direct cytoplasmic exchange of ions and low molecular weight metabolites between adjacent cells. They are formed by the apposition of two connexons belonging to adjacent cells. Each connexon is formed by six proteins, named connexins (Cxs). Current evidence suggests that gap junctions play an important part in ensuring normal embryo development. Mutations in connexin genes have been linked to a variety of human diseases, although the precise role and the cell biological mechanisms of their action remain almost unknown. Among the big family of Cxs, several are expressed in nervous tissue but just a few are expressed in the anterior neural tube of vertebrates. Many efforts have been made to elucidate the molecular bases of Cxs cell biology and how they influence the morphogenetic signal activity produced by brain signaling centers. These centers, orchestrated by transcription factors and morphogenes determine the axial patterning of the mammalian brain during its specification and regionalization. The present review revisits the findings of GJ composed by Cx43 and Cx36 in neural tube patterning and discuss Cx43 putative enrollment in the control of Fgf8 signal activity coming from the well known secondary organizer, the isthmic organizer.

Complex Membrane Channel Blockade: A Unifying Hypothesis for the Prodromal and Acute Neuropsychiatric Sequelae Resulting from Exposure to the Antimalarial Drug Mefloquine

The alkaloid toxin quinine and its derivative compounds have been used for many centuries as effective medications for the prevention and treatment of malaria. More recently, synthetic derivatives, such as the quinoline derivative mefloquine (bis(trifluoromethyl)-(2-piperidyl)-4-quinolinemethanol), have been widely used to combat disease caused by chloroquine-resistant strains of the malaria parasite, Plasmodium falciparum. However, the parent compound quinine, as well as its more recent counterparts, suffers from an incidence of adverse neuropsychiatric side effects ranging from mild mood disturbances and anxiety to hallucinations, seizures, and psychosis. This review considers how the pharmacology, cellular neurobiology, and membrane channel kinetics of mefloquine could lead to the significant and sometimes life-threatening neurotoxicity associated with mefloquine exposure. A key role for mefloquine blockade of ATP-sensitive potassium channels and connexins in the substantia nigra is considered as a unifying hypothesis for the pathogenesis of severe neuropsychiatric events after mefloquine exposure in humans.

Connexin 35b expression in the spinal cord of Danio rerio embryos and larvae

Electrical synapses are expressed prominently in the developing and mature nervous systems. Unlike chemical synapses, little is known about the developmental role of electrical synapses, reflecting the limitations imposed by the lack of selective pharmacological blockers. At a molecular level, the building blocks of electrical synapses are connexin proteins. In this study, we report the expression pattern for neuronally expressed connexin 35b (cx35b), the zebrafish orthologue of mammalian connexin (Cx) 36. We find that cx35b is expressed at the time of neural induction, indicating a possible early role in neural progenitor cells. Additionally, cx35b localizes to the ventral spinal cord during embryonic and early larval stages. We detect cx35b mRNA in secondary motor neurons (SMNs) and interneurons. We identified the premotor circumferential descending (CiD) interneuron as one interneuron subtype expressing cx35b. In addition, cx35b is present in other ventral interneurons of unknown subtype(s). This early expression of cx35b in SMNs and CiDs suggests a possible role in motor network function during embryonic and larval stages.

Engram formation in psychiatric disorders

Environmental factors substantially influence beginning and progression of mental illness, reinforcing or reducing the consequences of genetic vulnerability. Often initiated by early traumatic events, “engrams” or memories are formed that may give rise to a slow and subtle progression of psychiatric disorders. The large delay between beginning and time of onset (diagnosis) may be explained by efficient compensatory mechanisms observed in brain metabolism that use optional pathways in highly redundant molecular interactions. To this end, research has to deal with mechanisms of learning and long-term memory formation, which involves (a) epigenetic changes, (b) altered neuronal activities, and (c) changes in neuron-glia communication. On the epigenetic level, apparently DNA-methylations are more stable than histone modifications, although both closely interact. Neuronal activities basically deliver digital information, which clearly can serve as basis for memory formation (LTP). However, research in this respect has long time neglected the importance of glia. They are more actively involved in the control of neuronal activities than thought before. They can both reinforce and inhibit neuronal activities by transducing neuronal information from frequency-encoded to amplitude and frequency-modulated calcium wave patterns spreading in the glial syncytium by use of gap junctions. In this way, they serve integrative functions. In conclusion, we are dealing with two concepts of encoding information that mutually control each other and synergize: a digital (neuronal) and a wave-like (glial) computing, forming neuron-glia functional units with inbuilt feedback loops to maintain balance of excitation and inhibition. To better understand mental illness, we have to gain more insight into the dynamics of adverse environmental impact on those cellular and molecular systems. This report summarizes existing knowledge and draws some outline about further research in molecular psychiatry.

From a glial syncytium to a more restricted and specific glial networking

In the brain, glia represents the cell population that expresses the highest level of connexins, the membrane protein constituents of gap junction channels and hemichannels. This statement has initially led to propose the existence of a glial syncytium. Since then, functional studies have established that connexin channel-mediated communication between glial cells was more restricted and plastic that primarily thought. In particular, this is the case for astrocytes that form functional networks of communicating cells. Altogether these findings lead to reconsider the interaction between neurons and glia that should not be solely studied at the single cell level but also at a more integrated level as the interplay between neuronal circuits and glial networks.

Spike frequency adaptation is developmentally regulated in substantia nigra pars compacta dopaminergic neurons

Dopaminergic neurons of the substantia nigra pars compacta play a key role in the modulation of basal ganglia and provide a reward-related teaching signal essential for adaptative motor control. They are generally considered as a homogenous population despite several chemical and electrophysiological heterogeneities, which could underlie different preferential patterns of activity and/or different roles. Using whole-cell patch-clamp recordings in juvenile rat brain slices, we observed that the evoked activity of dopaminergic neurons displays variable spike frequency adaptation patterns. The intensity of spike frequency adaptation decreased during post-natal development. The adaptation was associated with an increase in the initial firing frequency due to faster kinetics of the afterhyperpolarization component of the spike. Adaptation was enhanced when small conductance calcium-activated potassium (SK) channels were blocked with bath application of apamine. Lastly, spike frequency adaptation of the evoked discharge was associated with more irregularity in the spontaneous firing pattern. Altogether these results show a developmental heterogeneity and electrophysiological maturation of substantia nigra dopaminergic neurons.

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.

Deletion of connexin45 in mouse neurons disrupts one-trial object recognition and alters kainate-induced gamma-oscillations in the hippocampus

Neuronal gap junctions, allowing fast intercellular electrotonic signal transfer, have been implicated in mechanisms governing learning and memory processes. We have examined conditional neuron-directed (Cx45fl/fl:Nestin-Cre) connexin45 deficient mice in terms of behavioral and electrophysiological correlates of learning and memory. Behavioral habituation to a novel environment and motor learning were not changed in these mice. Novel object recognition after delays of up to 60min was impaired in neuronal Cx45 deficient mice. However, object-place recognition was not significantly different from controls. Analysis of enhanced green fluorescent reporter protein expression controlled by the endogenous mouse Cx45 promoter in the brain of neuronal Cx45 deficient mice suggested that Cx45 is expressed in the perirhinal cortex and the CA3 subregion of the hippocampus. The neuronal Cx45 deficient mice were also examined for aberrations in the generation and synchronization of network oscillations in the hippocampus. General excitability, synaptic short time plasticity, and spontaneous high-frequency oscillations (sharp-wave ripples) in the hippocampus were not different from controls. However, bath stimulation of hippocampal slices with kainate induced significantly lower gamma-oscillation amplitudes in the CA3, but not in the CA1 subfield of the neuronal Cx45 deficient mice. Additionally, they exhibited a significantly larger full width half maximum of the frequency distribution in the CA1 subfield as compared to the controls. In conclusion, the neuron-directed deletion of Cx45 impaired one-trial novel object recognition and altered kainate-induced gamma-oscillations possibly via the disruption of inter-neuronal gap junctional communication in the hippocampus or perirhinal cortex.

The extracellular matrix controls gap junction protein expression and function in postnatal hippocampal neural progenitor cells

Background

Gap junction protein and extracellular matrix signalling systems act in concert to influence developmental specification of neural stem and progenitor cells. It is not known how these two signalling systems interact. Here, we examined the role of ECM components in regulating connexin expression and function in postnatal hippocampal progenitor cells.

Results

We found that Cx26, Cx29, Cx30, Cx37, Cx40, Cx43, Cx45, and Cx47 mRNA and protein but only Cx32 and Cx36 mRNA are detected in distinct neural progenitor cell populations cultured in the absence of exogenous ECM. Multipotential Type 1 cells express Cx26, Cx30, and Cx43 protein. Their Type 2a progeny but not Type 2b and 3 neuronally committed progenitor cells additionally express Cx37, Cx40, and Cx45. Cx29 and Cx47 protein is detected in early oligodendrocyte progenitors and mature oligodendrocytes respectively. Engagement with a laminin substrate markedly increases Cx26 protein expression, decreases Cx40, Cx43, Cx45, and Cx47 protein expression, and alters subcellular localization of Cx30. These changes are associated with decreased neurogenesis. Further, laminin elicits the appearance of Cx32 protein in early oligodendrocyte progenitors and Cx36 protein in immature neurons. These changes impact upon functional connexin-mediated hemichannel activity but not gap junctional intercellular communication.

Conclusion

Together, these findings demonstrate a new role for extracellular matrix-cell interaction, specifically laminin, in the regulation of intrinsic connexin expression and function in postnatal neural progenitor cells.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Connexin31.1 deficiency in the mouse impairs object memory and modulates open-field exploration, acetylcholine esterase levels in the striatum, and cAMP response element-binding protein levels in the striatum and piriform cortex

Neuronal gap junctions in the brain, providing intercellular electrotonic signal transfer, have been implicated in physiological and behavioral correlates of learning and memory. In connexin31.1 (Cx31.1) knockout (KO) mice the coding region of the Cx31.1 gene was replaced by a LacZ reporter gene. We investigated the impact of Cx31.1 deficiency on open-field exploration, the behavioral response to an odor, non-selective attention, learning and memory performance, and the levels of memory-related proteins in the hippocampus, striatum and the piriform cortex. In terms of behavior, the deletion of the Cx31.1 coding DNA in the mouse led to increased exploratory behaviors in a novel environment, and impaired one-trial object recognition at all delays tested. Despite strong Cx31.1 expression in the peripheral and central olfactory system, Cx31.1 KO mice exhibited normal behavioral responses to an odor. We found increased levels of acetylcholine esterase (AChE) and cAMP response element-binding protein (CREB) in the striatum of Cx31.1 KO mice. In the piriform cortex the Cx31.1 KO mice had an increased heterogeneity of CREB expression among neurons. In conclusion, gap-junctions featuring the Cx31.1 protein might be involved in open-field exploration as well as object memory and modulate levels of AChE and CREB in the striatum and piriform cortex.

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.

Connexin-dependent transcellular transcriptomic networks in mouse brain

Microarray experiments have generally focused on magnitude of gene expression changes in pathological conditions, thereby using the method as a high throughput screen to identify candidate marker genes and/or to validate phenotypic differences. We have used novel strategies to extract additional information from array studies, including expression variability and coordination, from which organizational principles of transcriptomes are emerging. We have reported that the expression level, variability and coordination of numerous genes are regulated in brains of connexin43 null (Gja1(-/-)) mouse with respect to wildtype. Moreover, expression coordination with Gja1 in wildtype largely predicted the expression regulation in Gja1(-/-) tissues. We now report a remarkable overlap between regulations in Gja1(-/-) and connexin32 null (Gjb1(-/-)) brains, and that both differ markedly from those in connexin36 null (Gja9(-/-)) brain. Since in brain these three connexins are expressed in different cell types (Cx43 in astrocytes, ependymal and vascular cells, Gjb1 in oligodendrocytes, and Cx36 in neurons and microglia), and because astrocytes and oligodendrocytes (and possibly neurons and microglia) may form syncytia coupled by gap junction channels, these observations suggest the existence of distinct connexin-dependent panglial and neuronal transcriptomic networks. Such networks, where linkage partners are rearranged and strengths modified in brains of knockouts, may explain downstream and parallel "ripples" of phenotypic change resulting from single gene manipulations as illustrated by alterations in transcription factor networks resulting from deletion of Gja1 or Gjb1. The transcription factors also formed network hubs with genes from other functional categories, thus allowing regulation of one functional pathway through manipulation of another.