Inhibition of Connexin43 Hemichannels Impairs Spatial Short-Term Memory without Affecting Spatial Working Memory

Insights into conscious cognitive information processing

For over a century, the neuro- and pathophysiological, behavioral, and cognitive correlates of consciousness have been an active field of theoretical considerations and empirical research in a wide range of modern disciplines. Conscious cognitive processing of information cannot be observed directly, but might be inferred from step-like discontinuities in learning performance or sudden insight-based improvements in problem solving behavior. It is assumed that a sudden step of knowledge associated with insight requires a creative reorganization of mental representations of task- or problem-relevant information and the restructuration of the task, respectively problem to overcome an cognitive dead-end or impasse. Discontinuities in learning performance or problem solving after an insight event can be used as time-tags to capture the time window in which conscious cognitive information processing must have taken place. According to the platform theory of conscious cognitive information processing, the reorganization and restructuration processes, require the maintenance of task- or problem-relevant information in working memory for the operation of executive functions on these mental representations. Electrophysiological evidence suggests that the reorganization and restructuration processes in working memory, that precede insight-based problem solutions are accompanied by an increase in the power of gamma oscillations in cortical areas including the prefrontal cortex. Empirical evidence and theoretical assumptions argue for an involvement of gap junction channels and connexin hemichannels in cortical gamma-oscillations and working memory processes. Discontinuities in learning or problem solving performance might be used as time-tags to investigate the implication of gap junction channels and hemichannels in conscious cognitive processing.

Inhibition of astroglial hemichannels prevents synaptic transmission decline during spreading depression

BACKGROUND

Spreading depression (SD) is an intriguing phenomenon characterized by massive slow brain depolarizations that affect neurons and glial cells. This phenomenon is repetitive and produces a metabolic overload that increases secondary damage. However, the mechanisms associated with the initiation and propagation of SD are unknown. Multiple lines of evidence indicate that persistent and uncontrolled opening of hemichannels could participate in the pathogenesis and progression of several neurological disorders including acute brain injuries. Here, we explored the contribution of astroglial hemichannels composed of connexin-43 (Cx43) or pannexin-1 (Panx1) to SD evoked by high-K+ stimulation in brain slices.

RESULTS

Focal high-K+ stimulation rapidly evoked a wave of SD linked to increased activity of the Cx43 and Panx1 hemichannels in the brain cortex, as measured by light transmittance and dye uptake analysis, respectively. The activation of these channels occurs mainly in astrocytes but also in neurons. More importantly, the inhibition of both the Cx43 and Panx1 hemichannels completely prevented high K+-induced SD in the brain cortex. Electrophysiological recordings also revealed that Cx43 and Panx1 hemichannels critically contribute to the SD-induced decrease in synaptic transmission in the brain cortex and hippocampus.

CONCLUSIONS

Targeting Cx43 and Panx1 hemichannels could serve as a new therapeutic strategy to prevent the initiation and propagation of SD in several acute brain injuries.

Cx43 hemichannels and panx1 channels contribute to ethanol-induced astrocyte dysfunction and damage

BACKGROUND

Alcohol, a widely abused drug, significantly diminishes life quality, causing chronic diseases and psychiatric issues, with severe health, societal, and economic repercussions. Previously, we demonstrated that non-voluntary alcohol consumption increases the opening of Cx43 hemichannels and Panx1 channels in astrocytes from adolescent rats. However, whether ethanol directly affects astroglial hemichannels and, if so, how this impacts the function and survival of astrocytes remains to be elucidated.

RESULTS

Clinically relevant concentrations of ethanol boost the opening of Cx43 hemichannels and Panx1 channels in mouse cortical astrocytes, resulting in the release of ATP and glutamate. The activation of these large-pore channels is dependent on Toll-like receptor 4, P2X7 receptors, IL-1β and TNF-α signaling, p38 mitogen-activated protein kinase, and inducible nitric oxide (NO) synthase. Notably, the ethanol-induced opening of Cx43 hemichannels and Panx1 channels leads to alterations in cytokine secretion, NO production, gliotransmitter release, and astrocyte reactivity, ultimately impacting survival.

CONCLUSION

Our study reveals a new mechanism by which ethanol impairs astrocyte function, involving the sequential stimulation of inflammatory pathways that further increase the opening of Cx43 hemichannels and Panx1 channels. We hypothesize that targeting astroglial hemichannels could be a promising pharmacological approach to preserve astrocyte function and synaptic plasticity during the progression of various alcohol use disorders.

Assessment of Connexin43 Hemichannel Functionality Based on Cytosolic Uptake of Yo-Pro1

Connexin proteins are the building blocks of gap junctions and connexin hemichannels. Both provide a pathway for cellular communication. Gap junctions support intercellular communication mechanisms and regulate homeostasis. In contrast, open connexin hemichannels connect the intracellular compartment and the extracellular environment, and their activation fuels inflammation and cell death. The development of clinically applicable connexin hemichannel blockers for therapeutic purposes is therefore gaining momentum. This chapter describes a well-established protocol optimized for assessing connexin hemichannel activity by using the reporter dye Yo-Pro1.

SARS-CoV-2 spike protein S1 activates Cx43 hemichannels and disturbs intracellular Ca2+ dynamics

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the ongoing coronavirus disease 2019 (COVID-19). An aspect of high uncertainty is whether the SARS-CoV-2 per se or the systemic inflammation induced by viral infection directly affects cellular function and survival in different tissues. It has been postulated that tissue dysfunction and damage observed in COVID-19 patients may rely on the direct effects of SARS-CoV-2 viral proteins. Previous evidence indicates that the human immunodeficiency virus and its envelope protein gp120 increase the activity of connexin 43 (Cx43) hemichannels with negative repercussions for cellular function and survival. Here, we evaluated whether the spike protein S1 of SARS-CoV-2 could impact the activity of Cx43 hemichannels.

RESULTS

We found that spike S1 time and dose-dependently increased the activity of Cx43 hemichannels in HeLa-Cx43 cells, as measured by dye uptake experiments. These responses were potentiated when the angiotensin-converting enzyme 2 (ACE2) was expressed in HeLa-Cx43 cells. Patch clamp experiments revealed that spike S1 increased unitary current events with conductances compatible with Cx43 hemichannels. In addition, Cx43 hemichannel opening evoked by spike S1 triggered the release of ATP and increased the [Ca2+]i dynamics elicited by ATP.

CONCLUSIONS

We hypothesize that Cx43 hemichannels could represent potential pharmacological targets for developing therapies to counteract SARS-CoV-2 infection and their long-term consequences.

Investigation of rhodamine derivative on behavioral impairment in a double neurotoxin lesion of substantia nigra and locus coeruleus dysfunctional mice

Although multiple mechanisms have been studied, there is still a lack of effective treatment on non-motor symptoms in Parkinson's disease (PD) patients. Therapeutic effects of 5-(4-hydroxy-3-dimethoxybenzylidene)-thiazolidinone (RD-1), one of rhodamine derivatives, on motor recovery have been previously demonstrated, but its effects on non-motor symptoms remain unclear. Herein, we explored the beneficial effects of RD-1 on PD-related non-motor symptoms and changes in synaptic plasticity in the mesencephalon. To investigate its therapeutic effects in the non-motor symptoms of Parkinsonian model, we employed male C57BL/6N mice and double injection with noradrenergic specific neurotoxin N-2-Chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride, followed 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Next, we performed behavioral tests, histological analyses and immunoblotting. Our findings showed that RD-1 significantly alleviated locomotor abnormality, motor disturbance, anxiety/depression-like behavior and memory deficit. It rescued the levels of tyrosine hydroxylase in substantia nigra, and striatum. Moreover, RD-1 upregulated expression levels of α-synuclein, synapsin II, postsynaptic density 95 and vesicle-associated membrane protein 2. The restoration of synaptic function may underlie the neuroprotective effects of RD-1 in double lesioned mice, confirming its protective effect for dopaminergic neurodegeneration.

Astrocytic A2A receptors silencing negatively impacts hippocampal synaptic plasticity and memory of adult mice

Astrocytes are wired to bidirectionally communicate with neurons namely with synapses, thus shaping synaptic plasticity, which in the hippocampus is considered to underlie learning and memory. Adenosine A_2A receptors (A_2A R) are a potential candidate to modulate this bidirectional communication, since A_2A R regulate synaptic plasticity and memory and also control key astrocytic functions. Nonetheless, little is known about the role of astrocytic A_2A R in synaptic plasticity and hippocampal-dependent memory. Here, we investigated the impact of genetic silencing astrocytic A_2A R on hippocampal synaptic plasticity and memory of adult mice. The genetic A_2A R silencing in astrocytes was accomplished by a bilateral injection into the CA1 hippocampal area of a viral construct (AAV5-GFAP-GFP-Cre) that inactivate A_2A R expression in astrocytes of male adult mice carrying "floxed" A_2A R gene, as confirmed by A_2A R binding assays. Astrocytic A_2A R silencing alters astrocytic morphology, typified by an increment of astrocytic arbor complexity, and led to deficits in spatial reference memory and compromised hippocampal synaptic plasticity, typified by a reduction of LTP magnitude and a shift of synaptic long-term depression (LTD) toward LTP. These data indicate that astrocytic A_2A R control astrocytic morphology and influence hippocampal synaptic plasticity and memory of adult mice in a manner different from neuronal A_2A R.

Astrocytes in memory formation and maintenance

Learning and memory are fundamental but highly complex functions of the brain. They rely on multiple mechanisms including the processing of sensory information, memory formation, maintenance of short- and long-term memory, memory retrieval and memory extinction. Recent experiments provide strong evidence that, besides neurons, astrocytes crucially contribute to these higher brain functions. However, the complex interplay of astrocytes and neurons in local neuron-glia assemblies is far from being understood. Although important basic cellular principles that govern and link neuronal and astrocytic cellular functions have been established, additional mechanisms clearly continue to emerge. In this short essay, we first review current technologies allowing the experimenter to explore the role of astrocytes in behaving animals, with focus on spatial memory. We then discuss astrocytic signaling mechanisms and their role in learning and memory. We also reveal gaps in our knowledge that currently prevent a comprehensive understanding of how astrocytes contribute to acquisition, storage and retrieval of memory by modulating neuronal signaling in local circuits.

Ketogenic Diet Modulates Neuroinflammation via Metabolites from Lactobacillus reuteri After Repetitive Mild Traumatic Brain Injury in Adolescent Mice

Repetitive mild traumatic brain injury (rmTBI) is associated with a range of neural changes which is characterized by axonal injury and neuroinflammation. Ketogenic diet (KD) is regarded as a potential therapy for facilitating recovery after moderate-severe traumatic brain injury (TBI). However, its effect on rmTBI has not been fully studied. In this study, we evaluated the anti-neuroinflammation effects of KD after rmTBI in adolescent mice and explored the potential mechanisms. Experimentally, specific pathogen-free (SPF) adolescent male C57BL/6 mice received a sham surgery or repetitive mild controlled cortical impacts consecutively for 7 days. The uninjured mice received the standard diet, and the mice with rmTBI were fed either the standard diet or KD for 7 days. One week later, all mice were subjected to behavioral tests and experimental analysis. Results suggest that KD significantly increased blood beta-hydroxybutyrate (β-HB) levels and improved neurological function. KD also reduced white matter damage, microgliosis, and astrogliosis induced by rmTBI. Aryl hydrocarbon receptor (AHR) signaling pathway, which was mediated by indole-3-acetic acid (3-IAA) from Lactobacillus reuteri (L. reuteri) in gut and activated in microglia and astrocytes after rmTBI, was inhibited by KD. The expression level of the toll-like receptor 4 (TLR4)/myeloid differentiation primary response 88 (MyD88) in inflammatory cells, which mediates the NF-κB pathway, was also attenuated by KD. Taken together, our results indicated that KD can promote recovery following rmTBI in adolescent mice. KD may modulate neuroinflammation by altering L. reuteri in gut and its metabolites. The inhibition of indole/AHR pathway and the downregulation of TLR4/MyD88 may play a role in the beneficial effect of KD against neuroinflammation in rmTBI mice.

Biological Evaluation of Valeriana Extracts from Argentina with Potent Cholinesterase Inhibition for the Treatment of Neurodegenerative Disorders and Their Comorbidities-The Case of Valeriana carnosa Sm. (Caprifoliaceae) Studied in Mice

Alzheimer's disease (AD) is a neurodegenerative disorder whose pathophysiology includes the abnormal accumulation of proteins (e.g., β-amyloid), oxidative stress, and alterations in neurotransmitter levels, mainly acetylcholine. Here we present a comparative study of the effect of extracts obtained from endemic Argentinian species of valerians, namely V. carnosa Sm., V. clarionifolia Phil. and V. macrorhiza Poepp. ex DC from Patagonia and V. ferax (Griseb.) Höck and V. effusa Griseb., on different AD-related biological targets. Of these anxiolytic, sedative and sleep-inducing valerians, V. carnosa proved the most promising and was assayed in vivo. All valerians inhibited acetylcholinesterase (IC₅₀ between 1.08-12.69 mg/mL) and butyrylcholinesterase (IC₅₀ between 0.0019-1.46 mg/mL). They also inhibited the aggregation of β-amyloid peptide, were able to chelate Fe²⁺ ions, and exhibited a direct relationship between antioxidant capacity and phenolic content. Moreover, V. carnosa was able to inhibit human monoamine oxidase A (IC₅₀: 0.286 mg/mL (0.213-0.384)). A daily intake of aqueous V. carnosa extract by male Swiss mice (50 and 150 mg/kg/day) resulted in anxiolytic and antidepressant-like behavior and improved spatial memory. In addition, decreased AChE activity and oxidative stress markers were observed in treated mouse brains. Our studies contribute to the development of indigenous herbal medicines as therapeutic agents for AD.

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.

Targeting gliovascular connexins prevents inflammatory blood-brain barrier leakage and astrogliosis

The blood-brain barrier is formed by capillary endothelial cells expressing Cx37, Cx40 and Cx43, and is joined by closely apposed astrocytes expressing Cx43 and Cx30. We investigated whether connexin-targeting peptides could limit barrier leakage triggered by LPS-induced systemic inflammation in mice. Intraperitoneal LPS increased endothelial and astrocytic Cx43 expression, elevated TNFα, IL1β, IFNγ and IL6 in plasma and IL6 in the brain, and induced barrier leakage recorded over 24h. Barrier leakage was largely prevented by global Cx43 knockdown and Cx43/Cx30 double-knockout in astrocytes, slightly diminished by endothelial Cx43 knockout and not protected by global Cx30 knockout. Intravenous administration of Gap27 or Tat-Gap19 just before LPS also prevented barrier leakage, and intravenous BAPTA-AM to chelate intracellular calcium was equally effective. Patch-clamp experiments demonstrated LPS-induced Cx43 hemichannel opening in endothelial cells, which was suppressed by Gap27, Gap19 and BAPTA. LPS additionally triggered astrogliosis that was prevented by intravenous Tat-Gap19 or BAPTA-AM. Cortically applied Tat-Gap19 or BAPTA-AM to primarily target astrocytes, also strongly diminished barrier leakage. In vivo dye uptake and in vitro patch-clamp showed Cx43 hemichannel opening in astrocytes that was induced by IL6 in a calcium-dependent manner. We conclude that targeting endothelial and astrocytic connexins is a powerful approach to limit barrier failure and astrogliosis.

Neurodegeneration in Multiple Sclerosis: The Role of Nrf2-Dependent Pathways

Multiple sclerosis (MS) encompasses a chronic, irreversible, and predominantly immune-mediated disease of the central nervous system that leads to axonal degeneration, neuronal death, and several neurological symptoms. Although various immune therapies have reduced relapse rates and the severity of symptoms in relapsing-remitting MS, there is still no cure for this devastating disease. In this brief review, we discuss the role of mitochondria dysfunction in the progression of MS, focused on the possible role of Nrf2 signaling in orchestrating the impairment of critical cellular and molecular aspects such as reactive oxygen species (ROS) management, under neuroinflammation and neurodegeneration in MS. In this scenario, we propose a new potential downstream signaling of Nrf2 pathway, namely the opening of hemichannels and pannexons. These large-pore channels are known to modulate glial/neuronal function and ROS production as they are permeable to extracellular Ca²⁺ and release potentially harmful transmitters to the synaptic cleft. In this way, the Nrf2 dysfunction impairs not only the bioenergetics and metabolic properties of glial cells but also the proper antioxidant defense and energy supply that they provide to neurons.

Dietary Supplement of Anoectochilus roxburghii (Wall.) Lindl. Polysaccharides Ameliorates Cognitive Dysfunction Induced by High Fat Diet via “Gut-Brain” Axis

Purpose

Anoectochilus roxburghii (Wall.) Lindl. polysaccharides (ARPs) have been reported to exhibit multiple pharmacological activities including anti-inflammatory and anti-hyperglycemia. This study aims to investigate the effect of ARPs on cognitive dysfunction induced by high fat diet (HFD).

Methods

Six-week-old male mice were treated with ARPs by dietary supplementation for 14 weeks. The effect of ARPs on cognitive function was determined by assessing the changes in spatial learning and memory ability, neurotrophic factors in hippocampus, inflammatory parameters, intestinal barrier integrity, and gut microbiota.

Results

ARPs supplementation can effectively ameliorate cognitive dysfunction, decrease the phosphorylation levels of Tau protein in hippocampus. Meanwhile, the increased body weight, plasma glucose, total cholesterol, inflammatory factors induced by HFD were abolished by ARPs treatment. Furthermore, ARPs treatment restored the intestinal epithelial barrier as evidenced by upregulation of intestinal tight junction proteins. Additionally, ARPs supplementation significantly decreased the relative abundance of several bacteria genus such as Parabacteroides, which may play regulatory roles in cognitive function.

Conclusion

These results suggest that ARPs might be a promising strategy for the treatment of cognitive dysfunction induced by HFD. Mechanistically, alleviation of cognitive dysfunction by ARPs might be associated with the “gut-brain” axis.

Aquaporin-4, Connexin-30, and Connexin-43 as Biomarkers for Decreased Objective Sleep Quality and/or Cognition Dysfunction in Patients With Chronic Insomnia Disorder

OBJECTIVES

To examine serum concentrations of aquaporin-4 (AQP4), connexin-30 (CX30), connexin-43 (CX43), and their correlations with cognitive function in the patients with chronic insomnia disorder (CID).

METHODS

We enrolled 76 subjects with CID and 32 healthy controls (HCs). Serum levels of AQP4, CX30, and CX43 were measured by enzyme-linked immunosorbent assays. Sleep quality was assessed with the Pittsburgh Sleep Quality Index (PSQI) and polysomnography, and mood was evaluated with 17-item Hamilton Depression Rating Scale and 14-item Hamilton Anxiety Rating Scale. Cognitive function was evaluated by the Chinese-Beijing Version of Montreal Cognitive Assessment (MoCA-C) and Nine Box Maze Test.

RESULTS

The serum levels of AQP4, CX43, and CX30 were significantly reduced in the CID group compared to the HCs. Partial correlation analysis showed that the biomarkers showed no significant correlations with PSQI score, AHI, ODI and TS90, but AQP4, CX43, and CX30 were positively associated with the percentage and total time of slow wave sleep in the CID group. Serum concentrations of AQP4 and CX30 were positively associated with MoCA-C score in the CID group, and AQP4 level negatively correlated with spatial working memory errors.

CONCLUSIONS

Subjects with CID patients have decreased serum levels of AQP4, CX30, and CX43 indicating astrocyte dysfunction, which could be related to poor objective sleep quality and/or cognition dysfunction.

Connexin43 hemichannels contribute to working memory and excitatory synaptic transmission of pyramidal neurons in the prefrontal cortex of rats

The gap junction is essential for the communication between astrocytes and neurons by various connexins. Connexin43 hemichannels (Cx43 HCs), one of important subunits of gap junction protein, is highly expressed in astrocytes. It has been demonstrated that Cx43 HCs is involved in synaptic plasticity and learning and memory. However, whether the role of Cx43 HCs in the prefrontal cortex (PFC), a key brain region mediating cognitive and executive functions including working memory, still remains unclear. Here, we investigate that the role of Cx43 HCs in working memory through pharmacological inhibition of Cx43 HCs in the PFC. Gap26, a specific hemichannels blocker for Cx43 HCs, was bilaterally infused into the prelimbic (PrL) area of the PFC and then spatial working memory was examined in delayed alternation task in T-maze. Furthermore, the effect of Gap26 on synaptic transmission of prefrontal pyramidal neurons was examined using whole-cell patch recording in slice containing PFC. The demonstrate that inhibition of prefrontal cortex Cx43 HCs impairs the working memory and excitatory synaptic transmission of PFC neurons, suggesting that Cx43 HCs in the PFC contributes to working memory and excitatory synaptic transmission of neurons in rats.

Exploring Refinement Strategies for Single Housing of Male C57BL/6JRj Mice: Effect of Cage Divider on Stress-Related Behavior and Hypothalamic-Pituitary-Adrenal-Axis Activity

Introduction: Single housing of laboratory mice is a common practice to meet experimental needs, or to avoid intermale aggression. However, single housing is considered to negatively affect animal welfare and may compromise the scientific validity of experiments. The aim of this study was to investigate whether the use of a cage with a cage divider, which avoids physical contact between mice while maintaining sensory contact, may be a potential refinement strategy for experiments in which group housing of mice is not possible.

Methods: Eight-week-old male C57BL/6JRj mice were single housed, pair housed or pair housed with a cage divider for four (experiment 1) or ten (experiment 2) weeks, after which we performed an open field test, Y-maze spontaneous alternation test, elevated plus maze test, an auditory fear conditioning task, and assessed responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis.

Results: Housing conditions did not affect body weight, exploratory activity, anxiety, working memory, fear memory processing or markers for HPA-axis functioning in either experiment 1 or experiment 2. There was an increased distance traveled in mice housed with a cage divider compared to pair housed mice after 4 weeks, and after 10 weeks mice housed with a cage divider made significantly more arm entries in the Y-maze spontaneous alternation test.

Conclusion: Taken together, our study did not provide evidence for robust differences in exploratory activity, anxiety, working memory and fear memory processing in male C57BL/6JRj mice that were single housed, pair housed or pair housed with a cage divider.

Astroglial Hemichannels and Pannexons: The Hidden Link between Maternal Inflammation and Neurological Disorders

Maternal inflammation during pregnancy causes later-in-life alterations of the offspring’s brain structure and function. These abnormalities increase the risk of developing several psychiatric and neurological disorders, including schizophrenia, intellectual disability, bipolar disorder, autism spectrum disorder, microcephaly, and cerebral palsy. Here, we discuss how astrocytes might contribute to postnatal brain dysfunction following maternal inflammation, focusing on the signaling mediated by two families of plasma membrane channels: hemi-channels and pannexons. [Ca²⁺]_i imbalance linked to the opening of astrocytic hemichannels and pannexons could disturb essential functions that sustain astrocytic survival and astrocyte-to-neuron support, including energy and redox homeostasis, uptake of K⁺ and glutamate, and the delivery of neurotrophic factors and energy-rich metabolites. Both phenomena could make neurons more susceptible to the harmful effect of prenatal inflammation and the experience of a second immune challenge during adulthood. On the other hand, maternal inflammation could cause excitotoxicity by producing the release of high amounts of gliotransmitters via astrocytic hemichannels/pannexons, eliciting further neuronal damage. Understanding how hemichannels and pannexons participate in maternal inflammation-induced brain abnormalities could be critical for developing pharmacological therapies against neurological disorders observed in the offspring.

Antiamnesic and Neuroprotective Effects of an Aqueous Extract of Ziziphus jujuba Mill. (Rhamnaceae) on Scopolamine-Induced Cognitive Impairments in Rats

Background

Alzheimer's disease is a neurological condition that affects about 44 million people worldwide. The available treatments target symptoms rather than the underlying causes. Ziziphus jujuba (Rhamnaceae) is widely used in traditional Cameroonian medicine to treat diabetes, pain, infections, and dementia. Previous studies reported that Z. jujuba aqueous macerate improves working memory impairment, but no study on the antiamnesic effect of a concoction of Z. jujuba in rats has been performed. Therefore, this study aimed to assess the antiamnesic and neuroprotective effects of an aqueous extract of Z. jujuba on scopolamine-induced cognitive impairments in rats.

Methods

Learning and memory impairments were induced in rats by administering scopolamine (1 mg/kg, i.p.) to 58 rats for 15 days. Rats that developed learning and memory impairments in Morris water maze and Y-maze paradigms were divided into 7 groups (8 rats each) and treated daily for 15 days as follows: the normal control group received distilled water (10 ml/kg, p.o.), the negative control group received distilled water (10 ml/kg, p.o.), positive control groups either received donepezil (1.2 mg/kg, p.o.) or tacrine (10 mg/kg, p.o.), and the three test groups were given the extract (29, 57, and 114 mg/kg, p.o.). At the end of treatments, learning and memory impairments were determined using the same paradigms. Animals were then euthanized, and biochemical parameters of oxidative stress, inflammation, and apoptosis were analyzed in the hippocampus and prefrontal cortex.

Results

On the 4th day of the acquisition phase in the Morris water maze, Z. jujuba (29 and 114 mg/kg) reduced (p < 0.001) the latency to reach the platform, while in the retention phase, Z. jujuba (57 and 114 mg/kg) decreased (p < 0.001) the time to reach the platform and increased the time in the target quadrant (p < 0.05) compared to control. Surprisingly, the extract failed to affect spontaneous alternations in the Y-maze. Furthermore, the extract (29, 57, and 114 mg/kg) reversed (p < 0.001) scopolamine-induced oxidative stress, inflammation, and apoptosis. This was supported by the reduction of neuronal alterations in the hippocampus and prefrontal cortex.

Conclusions

Compared to donepezil, a standard drug against Alzheimer's disease, these findings suggest that Z. jujuba extract possesses antiamnesic and neuroprotective effects, and these effects are mediated in part through antioxidant, anti-inflammatory, and antiapoptotic activities. These findings help to explain its use in treating psychiatric disorders in Cameroon's folk medicine.

Reduced risk aversion and impaired short-term memory in juvenile rats with malformation of cortical development

Malformation of cortical developments (MCDs) is currently an incurable disease and is associated with significant neuropsychological problems, such as intellectual disability, epilepsy, and anxiety disorders from a young age. Development of a suitable animal model and pathophysiological study is therefore necessary to better understand and treat MCDs from being an incurable disease. The Y-maze, open field, and fear conditioning studies were performed at postnatal days 40-44 to validate the behavioral phenotypes of the existing rat model of MCD with prenatal methylazoxymethanol exposure at their developmental period. The study results show that juvenile rats with MCD spent significantly less time inside the novel arms in Y-maze and less time in the peripheral zones of the open field. Additionally, the rats with MCDs showed attenuated freezing behavior to sound and light cues as well as to context after fear conditioning. This comprehensive behavioral analysis of rats with MCDs at the juvenile period indicate a lack of spatial memory, decreased anxiety, and learning disability in these rats, which is compatible with the human behavioral phenotype of MCDs and can be used as the behavioral biomarkers for future translational research.

The many ways astroglial connexins regulate neurotransmission and behavior

Astrocytes have emerged as major players in the brain, contributing to many functions such as energy supply, neurotransmission, and behavior. They accomplish these functions in part via their capacity to form widespread intercellular networks and to release neuroactive factors, which can modulate neurotransmission at different levels, from individual synapses to neuronal networks. The extensive network communication of astrocytes is primarily mediated by gap junction channels composed of two connexins, Cx30 and Cx43, which present distinct temporal and spatial expression patterns. Yet, astroglial connexins are also involved in direct exchange with the extracellular space via hemichannels, as well as in adhesion and signaling processes via unconventional nonchannel functions. Accumulating evidence indicate that astrocytes modulate neurotransmission and behavior through these diverse connexin functions. We here review the many ways astroglial connexins regulate neuronal activity from the molecular level to behavior.

Astroglial connexins in epileptogenesis

The astroglial network connected through gap junctions assembling from connexins physiologically balances the concentrations of ions and neurotransmitters around neurons. Astrocytic dysfunction has been associated with many neurological disorders including epilepsy. Dissociated gap junctions result in the increased activity of connexin hemichannels which triggers brain pathophysiological changes. Previous studies in patients and animal models of epilepsy indicate that the reduced gap junction coupling from assembled connexin hemichannels in the astrocytes may play an important role in epileptogenesis. This abnormal cell-to-cell communication is now emerging as an important feature of brain pathologies and being considered as a novel therapeutic target for controlling epileptogenesis. In particular, candidate drugs with ability of inhibition of connexin hemichannel activity and enhancement of gap junction formation in astrocytes should be explored to prevent epileptogenesis and control epilepsy.

Astrocytic Connexin43 Channels as Candidate Targets in Epilepsy Treatment

In epilepsy research, emphasis is put on exploring non-neuronal targets such as astrocytic proteins, since many patients remain pharmacoresistant to current treatments, which almost all target neuronal mechanisms. This paper reviews available data on astrocytic connexin43 (Cx43) signaling in seizures and epilepsy. Cx43 is a widely expressed transmembrane protein and the constituent of gap junctions (GJs) and hemichannels (HCs), allowing intercellular and extracellular communication, respectively. A plethora of research papers show altered Cx43 mRNA levels, protein expression, phosphorylation state, distribution and/or functional coupling in human epileptic tissue and experimental models. Human Cx43 mutations are linked to seizures as well, as 30% of patients with oculodentodigital dysplasia (ODDD), a rare genetic condition caused by mutations in the GJA1 gene coding for Cx43 protein, exhibit neurological symptoms including seizures. Cx30/Cx43 double knock-out mice show increased susceptibility to evoked epileptiform events in brain slices due to impaired GJ-mediated redistribution of K⁺ and glutamate and display a higher frequency of spontaneous generalized chronic seizures in an epilepsy model. Contradictory, Cx30/Cx43 GJs can traffic nutrients to high-energy demanding neurons and initiate astrocytic Ca²⁺ waves and hyper synchronization, thereby supporting proconvulsant effects. The general connexin channel blocker carbenoxolone and blockers from the fenamate family diminish epileptiform activity in vitro and improve seizure outcome in vivo. In addition, interventions with more selective peptide inhibitors of HCs display anticonvulsant actions. To conclude, further studies aiming to disentangle distinct roles of HCs and GJs are necessary and tools specifically targeting Cx43 HCs may facilitate the search for novel epilepsy treatments.

Local externalization of phosphatidylserine mediates developmental synaptic pruning by microglia

Neuronal circuit assembly requires the fine balance between synapse formation and elimination. Microglia, through the elimination of supernumerary synapses, have an established role in this process. While the microglial receptor TREM2 and the soluble complement proteins C1q and C3 are recognized as key players, the neuronal molecular components that specify synapses to be eliminated are still undefined. Here, we show that exposed phosphatidylserine (PS) represents a neuronal "eat-me" signal involved in microglial-mediated pruning. In hippocampal neuron and microglia co-cultures, synapse elimination can be partially prevented by blocking accessibility of exposed PS using Annexin V or through microglial loss of TREM2. In vivo, PS exposure at both hippocampal and retinogeniculate synapses and engulfment of PS-labeled material by microglia occurs during established developmental periods of microglial-mediated synapse elimination. Mice deficient in C1q, which fail to properly refine retinogeniculate connections, have elevated presynaptic PS exposure and reduced PS engulfment by microglia. These data provide mechanistic insight into microglial-mediated synapse pruning and identify a novel role of developmentally regulated neuronal PS exposure that is common among developing brain structures.

Permeation of Molecules through Astroglial Connexin 43 Hemichannels Is Modulated by Cytokines with Parameters Depending on the Permeant Species

Recent studies indicate that connexin hemichannels do not act as freely permeable non-selective pores, but they select permeants in an isoform-specific manner with cooperative, competitive and saturable kinetics. The aim of this study was to investigate whether the treatment with a mixture of IL-1β plus TNF-α, a well-known pro-inflammatory condition that activates astroglial connexin 43 (Cx43) hemichannels, could alter their permeability to molecules. We found that IL-1β plus TNF-α left-shifted the dye uptake rate vs. dye concentration relationship for Etd and 2-NBDG, but the opposite took place for DAPI or YO-PRO-1, whereas no alterations were observed for Prd. The latter modifications were accompanied of changes in K_d (Etd, DAPI, YO-PRO-1 or 2-NBDG) and Hill coefficients (Etd and YO-PRO-1), but not in alterations of V_max. We speculate that IL-1β plus TNF-α may distinctively affect the binding sites to permeants in astroglial Cx43 hemichannels rather than their number in the cell surface. Alternatively, IL-1β plus TNF-α could induce the production of endogenous permeants that may favor or compete for in the pore-lining residues of Cx43 hemichannels. Future studies shall elucidate whether the differential ionic/molecule permeation of Cx43 hemichannels in astrocytes could impact their communication with neurons in the normal and inflamed nervous system.

Gut microbiota regulate cognitive deficits and amyloid deposition in a model of Alzheimer's disease

Gut microbiota, comprising a vast number of microorganism species with complex metagenome, are known to be associated with Alzheimer's disease (AD) and amyloid deposition. However, studies related to gut microbiota have been mostly restricted to comparisons of amyloid deposits, while investigations on neurobehavioral changes and the pathogenesis of AD are limited. Therefore, we aimed to identify the relationship between changes in the intestinal microbiome and the pathogenesis of AD. APP^swe /PS1^ΔE9 (PAP) transgenic mice and wild-type (WT) mice of different age groups were used. The composition of intestinal bacterial communities in the mice was determined by 16S ribosomal RNA sequencing (16S rRNA Seq), and the Y maze was used to measure cognitive function. Transcriptome sequencing (RNA Seq) and Gene Expression Omnibus (GEO) database (GSE 36980) were used to filter differentially expressed genes (DEGs) between specific pathogen-free (SPF) and germ-free (GF) mice. Quantitative reverse-transcriptase PCR (qRT-PCR) and western blot (WB) were used to verify the results. We found that the intestinal microbiota was significantly different between 5-month-old PAP and WT mice and the cognition of SPF PAP mice was diminished compared to GF PAP and SPF WT mice. DEGs in 5-month-old SPF and GF mice were enriched in the MAPK signalling pathway, and expression of amyloid precursor protein and amyloid deposition increased in 5-month-old SPF PAP mice. Results from this study showed that changes in intestinal microbiota were correlated with impairment of cognitive function and might promote amyloid deposition by stimulating the MAPK signalling pathway in the brain.

Glial Connexins and Pannexins in the Healthy and Diseased Brain

Over the past several decades a large amount of data have established that glial cells, the main cell population in the brain, dynamically interact with neurons and thus impact their activity and survival. One typical feature of glia is their marked expression of several connexins, the membrane proteins forming intercellular gap junction channels and hemichannels. Pannexins, which have a tetraspan membrane topology as connexins, are also detected in glial cells. Here, we review the evidence that connexin and pannexin channels are actively involved in dynamic and metabolic neuroglial interactions in physiological as well as in pathological situations. These features of neuroglial interactions open the way to identify novel non-neuronal aspects that allow for a better understanding of behavior and information processing performed by neurons. This will also complement the "neurocentric" view by facilitating the development of glia-targeted therapeutic strategies in brain disease.

Astroglial connexins and cognition: memory formation or deterioration?

Connexins are the membrane proteins that form high-conductance plasma membrane channels and are the important constituents of gap junctions and hemichannels. Among different types of connexins, connexin 43 is the most widely expressed and studied gap junction proteins in astrocytes. Due to the key involvement of astrocytes in memory impairment and abundant expression of connexins in astrocytes, astroglial connexins have been projected as key therapeutic targets for Alzheimer's disease. On the other hand, the role of connexin gap junctions and hemichannels in memory formation and consolidation has also been reported. Moreover, deletion of these proteins and loss of gap junction communication result in loss of short-term spatial memory. Accordingly, both memory formation and memory deteriorating functions of astrocytes-located connexins have been documented. Physiologically expressed connexins may be involved in the memory formation, while pathologically increased expression of connexins with consequent excessive activation of astrocytes may induce neuronal injury and cognitive decline. The present review describes the memory formation as well as memory deteriorating functions of astroglial connexins in memory disorders of different etiology with possible mechanisms.

The Opening of Connexin 43 Hemichannels Alters Hippocampal Astrocyte Function and Neuronal Survival in Prenatally LPS-Exposed Adult Offspring

Clinical evidence has revealed that children born from mothers exposed to viral and bacterial pathogens during pregnancy are more likely to suffer various neurological disorders including schizophrenia, autism bipolar disorder, major depression, epilepsy, and cerebral palsy. Despite that most research has centered on the impact of prenatal inflammation in neurons and microglia, the potential modifications of astrocytes and neuron-astrocyte communication have received less scrutiny. Here, we evaluated whether prenatally LPS-exposed offspring display alterations in the opening of astrocyte hemichannels and pannexons in the hippocampus, together with changes in neuroinflammation, intracellular Ca²⁺ and nitric oxide (NO) signaling, gliotransmitter release, cell arborization, and neuronal survival. Ethidium uptake recordings revealed that prenatal LPS exposure enhances the opening of astrocyte Cx43 hemichannels and Panx1 channels in the hippocampus of adult offspring mice. This enhanced channel activity occurred by a mechanism involving a microglia-dependent production of IL-1β/TNF-α and the stimulation of p38 MAP kinase/iNOS/[Ca²⁺]_i-mediated signaling and purinergic/glutamatergic pathways. Noteworthy, the activity of Cx43 hemichannels affected the release of glutamate, [Ca²⁺]_i handling, and morphology of astrocytes, whereas also disturbed neuronal function, including the dendritic arbor and spine density, as well as survival. We speculate that excitotoxic levels of glutamate triggered by the activation of Cx43 hemichannels may contribute to hippocampal neurotoxicity and damage in prenatally LPS-exposed offspring. Therefore, the understanding of how astrocyte-neuron crosstalk is an auspicious avenue toward the development of broad treatments for several neurological disorders observed in children born to women who had a severe infection during gestation.

Connexin30 and Connexin43 show a time-of-day dependent expression in the mouse suprachiasmatic nucleus and modulate rhythmic locomotor activity in the context of chronodisruption

Background

The astroglial connexins Cx30 and Cx43 contribute to many important CNS functions including cognitive behaviour, motoric capacity and regulation of the sleep-wake cycle. The sleep wake cycle, is controlled by the circadian system. The central circadian rhythm generator resides in the suprachiasmatic nucleus (SCN). SCN neurons are tightly coupled in order to generate a coherent circadian rhythm. The SCN receives excitatory glutamatergic input from the retina which mediates entrainment of the circadian system to the environmental light-dark cycle. Connexins play an important role in electric coupling of SCN neurons and astrocytic-neuronal signalling that regulates rhythmic SCN neuronal activity. However, little is known about the regulation of Cx30 and Cx43 expression in the SCN, and the role of these connexins in light entrainment of the circadian system and in circadian rhythm generation.

Methods

We analysed time-of-day dependent as well as circadian expression of Cx30 and Cx43 mRNA and protein in the mouse SCN by means of qPCR and immunohistochemistry. Moreover, we analysed rhythmic spontaneous locomotor activity in mice with a targeted deletion of Cx30 and astrocyte specific deletion of Cx43 (DKO) in different light regimes by means of on-cage infrared detectors.

Results

Fluctuation of Cx30 protein expression is strongly dependent on the light-dark cycle whereas fluctuation of Cx43 protein expression persisted in constant darkness. DKO mice entrained to the light-dark cycle. However, re-entrainment after a phase delay was slightly impaired in DKO mice. Surprisingly, DKO mice were more resilient to chronodisruption.

Conclusion

Circadian fluctuation of Cx30 and Cx43 protein expression in the SCN is differently regulated. Cx30 and astroglial Cx43 play a role in rhythm stability and re-entrainment under challenging conditions.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0370-2) contains supplementary material, which is available to authorized users.

Heavy Alcohol Exposure Activates Astroglial Hemichannels and Pannexons in the Hippocampus of Adolescent Rats: Effects on Neuroinflammation and Astrocyte Arborization

A mounting body of evidence indicates that adolescents are specially more susceptible to alcohol influence than adults. However, the mechanisms underlying this phenomenon remain poorly understood. Astrocyte-mediated gliotransmission is crucial for hippocampal plasticity and recently, the opening of hemichannels and pannexons has been found to participate in both processes. Here, we evaluated whether adolescent rats exposed to ethanol exhibit changes in the activity of astrocyte hemichannels and pannexons in the hippocampus, as well as alterations in astrocyte arborization and cytokine levels. Adolescent rats were subjected to ethanol (3.0 g/kg) for two successive days at 48-h periods over 14 days. The opening of hemichannels and pannexons was examined in hippocampal slices by dye uptake, whereas hippocampal cytokine levels and astroglial arborization were determined by ELISA and Sholl analysis, respectively. We found that adolescent ethanol exposure increased the opening of connexin 43 (Cx43) hemichannels and pannexin-1 (Panx1) channels in astrocytes. Blockade of p38 mitogen-activated protein kinase (MAPK), inducible nitric oxide synthase (iNOS) and cyclooxygenases (COXs), as well as chelation of intracellular Ca²⁺, drastically reduced the ethanol-induced channel opening in astrocytes. Importantly, ethanol-induced Cx43 hemichannel and Panx1 channel activity was correlated with increased levels of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), IL-6 in the hippocampus, as well as with profound alterations in astrocyte arbor complexity. Thus, we propose that uncontrolled opening of astrocyte hemichannels and pannexons may contribute not only to the glial dysfunction and neurotoxicity caused by adolescent alcohol consumption, but also to the pathogenesis of alcohol use disorders in the adulthood.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

Therapeutic Targeting of Connexin Channels: New Views and Challenges

Connexins, in particular connexin 43 (Cx43), function as gap junction channels (GJCs) and hemichannels (HCs). Only recently, specific tools have been developed to study their pleiotropic functions. Based on various protein interaction sites, distinct connexin-mimetic peptides have been established that enable discrimination between the function of HCs and GJCs. Although the precise mechanism of action of most of these peptides is still a matter of debate, an increasing number of studies report on important effects of those compounds in disease models. In this review, we summarize the structure, life cycle, and the most important physiological and pathological functions of both connexin GJCs and HCs. We provide a critical overview on the use of connexin-targeting peptides, in particular targeting Cx43, with a special focus on the remaining questions and hurdles to be taken in the research field of connexin channels.

Inhibition of Connexin 43 Hemichannels Alleviates Cerebral Ischemia/Reperfusion Injury via the TLR4 Signaling Pathway

Connexin 43 (Cx43) widely exists in all components of the neurovascular unit (NVU) and is a constituent of gap junctions and hemichannels. In physiological states, gap junctions are open for regular intercellular communication, and the hemichannels present low open probability in astrocytes. After cerebral ischemia, a large number of hemichannels are unusually opened, leading to cell swelling and even death. Most known hemichannel blockers also inhibit gap junctions and sequentially obstruct normal electrical cell-cell communication. In this study, we tested the hypothesis that Gap19, a selective Cx43-hemichannel inhibitor, exhibited neuroprotective effects on cerebral ischemia/reperfusion (I/R). An obvious improvement in neurological scores and infarct volume reduction were observed in Gap19-treated mice after brain ischemia induced by middle cerebral artery occlusion (MCAO). Gap19 treatment attenuated white matter damage. Moreover, Gap19 treatment suppressed the expression of Cx43 and Toll-like receptor 4 (TLR4) pathway-relevant proteins and prevented the overexpression of tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). To further explore downstream signaling, we established an in vitro model-oxygen glucose deprivation (OGD) to simulate ischemic conditions. Immunofluorescence staining showed that Cx43 co-existed with TLR4 in astrocytes. The hemichannel activity was increased after OGD and Gap19 could inhibit this effect on astrocytes. Gap19 substantially improved relative cell vitality and decreased the expression of Cx43, TLR4 and inflammatory cytokines in vitro. In addition, in the lipopolysaccharide (LPS) stimulation OGD model, Gap19 also exhibited a protective effect via inhibiting TLR4 pathway activation. In summary, our results showed that Gap19 exerted a neuroprotective effect after stroke via inhibition of the TLR4-mediated signaling pathway.

Bushen-Yizhi Formula Alleviates Neuroinflammation via Inhibiting NLRP3 Inflammasome Activation in a Mouse Model of Parkinson's Disease

Parkinson's disease (PD), the second most common neurodegenerative disease, is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. Although the molecular mechanisms underlying dopaminergic neuronal degeneration in PD remain unclear, neuroinflammation is considered as the vital mediator in the pathogenesis and progression of PD. Bushen-Yizhi Formula (BSYZ), a traditional Chinese medicine, has been demonstrated to exert antineuroinflammation in our previous studies. However, it remains unclear whether BSYZ is effective for PD. Here, we sought to assess the neuroprotective effects and explore the underlying mechanisms of BSYZ in a 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine- (MPTP-) induced mouse model of PD. Our results indicate that BSYZ significantly alleviates the motor impairments and dopaminergic neuron degeneration of MPTP-treated mice. Furthermore, BSYZ remarkably attenuates microglia activation, inhibits NLPR3 activation, and decreases the levels of inflammatory cytokines in MPTP-induced mouse brain. Also, BSYZ inhibits NLRP3 activation and interleukin-1β production of the 1-methyl-4-phenyl-pyridinium (MPP⁺) stimulated BV-2 microglia cells. Taken together, our results indicate that BSYZ alleviates MPTP-induced neuroinflammation probably via inhibiting NLRP3 inflammasome activation in microglia. Collectively, BSYZ may be a potential therapeutic agent for PD and the related neurodegeneration diseases.

Specific deletion connexin43 in astrocyte ameliorates cognitive dysfunction in APP/PS1 mice

Emerging data indicate an important role for connexin43 (Cx43) in cognitive function, but there is a lack of direct evidence of the role of astroglial Cx43 in cognitive dysfunction in Alzheimer's disease (AD). Here we evaluated the expression pattern of Cx43 in AD and found progressive upregulation of the mRNA and protein levels of Cx43. Subsequently, we generated an astroglial Cx43 knockout (KO) AD mouse model by crossbreeding Gfap (glial fibrillary acidic protein)-Cx43 KO mice with APP/PS1 mice. Then we assessed the cognitive function of 12-month-old APP (amyloid precursor protein)/PS1 (presenilin 1)/Gfap-Cx43 KO mice, which demonstrated that the deletion of astroglial Cx43 significantly ameliorated cognitive dysfunction. To further investigate the underlying mechanisms, we evaluated amyloid plaque formation, astrogliosis, and synaptic function. The number and area of amyloid plaques were not altered, but GFAP expression was significantly decreased and the number of synapses was markedly upregulated. These results suggest that deletion of astroglial Cx43 in APP/PS1 mice did not affect the formation of amyloid plaques but depressed astrogliosis and upregulated synaptic function. Moreover, levels of critical modulators of astroglial activation were also notably reduced, but those of pro- and anti-inflammatory cytokines were not altered. Furthermore, Cx43 regulation of postsynaptic elements targets mainly NMDAR (N-methyl-d-aspartate). In addition, the prevention of AD pathology was reversed by Cx43 re-expression. In sum, specific deletion of astroglial Cx43 in APP/PS1 mice improved cognitive dysfunction by decreasing astrogliosis and increasing synaptic function without affecting amyloid plaque formation or the inflammatory response.

Inhibition of astroglial connexin43 hemichannels with TAT-Gap19 exerts anticonvulsant effects in rodents

Accumulating evidence shows a key function for astrocytic connexin43 (Cx43) signaling in epilepsy. However, the lack of experimental distinction between Cx43 gap junction channels (GJCs) and hemichannels (HCs) has impeded the identification of the exact contribution of either channel configurations to epilepsy. We therefore investigated whether TAT-Gap19, a Cx mimetic peptide that inhibits Cx43 HCs but not the corresponding Cx43 GJCs, influences experimentally induced seizures in rodents. Dye uptake experiments in acute hippocampal slices of mice demonstrated that astroglial Cx43 HCs open in response to the chemoconvulsant pilocarpine and this was inhibited by TAT-Gap19. In vivo, pilocarpine-induced seizures as well as the accompanying increase in D-serine microdialysate levels were suppressed by Cx43 HC inhibition. Moreover, the anticonvulsant action of TAT-Gap19 was reversed by exogenous D-serine administration, suggesting that Cx43 HC inhibition protects against seizures by lowering extracellular D-serine levels. The anticonvulsive properties of Cx43 HC inhibition were further confirmed in electrical seizure mouse models, i.e. an acute 6 Hertz (Hz) model of refractory seizures and a chronic 6 Hz corneal kindling model. Collectively, these results indicate that Cx43 HCs play a role in seizures and underscore their potential as a novel and druggable target in epilepsy treatment.

The Neuroglial Dialog Between Cannabinoids and Hemichannels

The formation of gap junctions was initially thought to be the central role of connexins, however, recent evidence had brought to light the high relevance of unopposed hemichannels as an independent mechanism for the selective release of biomolecules during physiological and pathological conditions. In the healthy brain, the physiological opening of astrocyte hemichannels modulates basal excitatory synaptic transmission. At the other end, the release of potentially neurotoxic compounds through astroglial hemichannels and pannexons has been insinuated as one of the functional alterations that negatively affect the progression of multiple brain diseases. Recent insights in this matter have suggested encannabinoids (eCBs) as molecules that could regulate the opening of these channels during diverse conditions. In this review, we discuss and hypothesize the possible interplay between the eCB system and the hemichannel/pannexon-mediated signaling in the inflamed brain and during event of synaptic plasticity. Most findings indicate that eCBs seem to counteract the activation of major neuroinflammatory pathways that lead to glia-mediated production of TNF-α and IL-1β, both well-known triggers of astroglial hemichannel opening. In contrast to the latter, in the normal brain, eCBs apparently elicit the Ca²⁺-activation of astrocyte hemichannels, which could have significant consequences on eCB-dependent synaptic plasticity.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

New Implications for the Melanocortin System in Alcohol Drinking Behavior in Adolescents: The Glial Dysfunction Hypothesis

Alcohol dependence causes physical, social, and moral harms and currently represents an important public health concern. According to the World Health Organization (WHO), alcoholism is the third leading cause of death worldwide, after tobacco consumption and hypertension. Recent epidemiologic studies have shown a growing trend in alcohol abuse among adolescents, characterized by the consumption of large doses of alcohol over a short time period. Since brain development is an ongoing process during adolescence, short- and long-term brain damage associated with drinking behavior could lead to serious consequences for health and wellbeing. Accumulating evidence indicates that alcohol impairs the function of different components of the melanocortin system, a major player involved in the consolidation of addictive behaviors during adolescence and adulthood. Here, we hypothesize the possible implications of melanocortins and glial cells in the onset and progression of alcohol addiction. In particular, we propose that alcohol-induced decrease in α-MSH levels may trigger a cascade of glial inflammatory pathways that culminate in altered gliotransmission in the ventral tegmental area and nucleus accumbens (NAc). The latter might potentiate dopaminergic drive in the NAc, contributing to increase the vulnerability to alcohol dependence and addiction in the adolescence and adulthood.

Long-term effects of interference on short-term memory performance in the rat

A distinction has always been made between long-term and short-term memory (also now called working memory, WM). The obvious difference between these two kinds of memory concerns the duration of information storage: information is supposedly transiently stored in WM while it is considered durably consolidated into long-term memory. It is well acknowledged that the content of WM is erased and reset after a short time, to prevent irrelevant information from proactively interfering with newly stored information. In the present study, we used typical WM radial maze tasks to question the brief lifespan of spatial WM content in rodents. Groups of rats were submitted to one of two different WM tasks in a radial maze: a WM task involving the repetitive presentation of a same pair of arms expected to induce a high level of proactive interference (PI) (HIWM task), or a task using a different pair in each trial expected to induce a low level of PI (LIWM task). Performance was effectively lower in the HIWM group than in LIWM in the final trial of each training session, indicative of a "within-session/short-term" PI effect. However, we also observed a different "between-session/long-term" PI effect between the two groups: while performance of LIWM trained rats remained stable over days, the performance of HIWM rats dropped after 10 days of training, and this impairment was visible from the very first trial of the day, hence not attributable to within-session PI. We also showed that a 24 hour-gap across training sessions known to allow consolidation processes to unfold, was a necessary and sufficient condition for the long-term PI effect to occur. These findings suggest that in the HIWM task, WM content was not entirely reset between training sessions and that, in specific conditions, WM content can outlast its purpose by being stored more permanently, generating a long-term deleterious effect of PI. The alternative explanation is that WM content could be transferred and stored more permanently in an intermediary form or memory between WM and long-term memory.