Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus

The role of the adenosine system in epilepsy and its comorbidities

Epilepsy is one of the most serious and common chronic neurological conditions, characterised by recurrent hypersynchronous electrical activity in the brain that lead to seizures. Despite over 50 million people being affected worldwide, only ~70% of people with epilepsy have their seizures successfully controlled with current pharmacotherapy, and many experience significant psychiatric and physical comorbidities. Adenosine, a ubiquitous purine metabolite, is a potent endogenous anti-epileptic substance that can abolish seizure activity via the adenosine A1 G protein-coupled receptor. Activation of A1 receptors decreases seizure activity in animal models, including models of drug-resistant epilepsy. Recent advances have increased our understanding of epilepsy comorbidities, highlighting the potential for adenosine receptors to modulate epilepsy-associated comorbidities, including cardiovascular dysfunction, sleep and cognition. This review provides an accessible resource of the current advances in understanding the adenosine system as a therapeutic target for epilepsy and epilepsy-associated comorbidities. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Spreading depolarizations pose critical energy challenges in acute brain injury

Spreading depolarization (SD) is an electrochemical wave of neuronal depolarization mediated by extracellular K+ and glutamate, interacting with voltage-gated and ligand-gated ion channels. SD is increasingly recognized as a major cause of injury progression in stroke and brain trauma, where the mechanisms of SD-induced neuronal injury are intimately linked to energetic status and metabolic impairment. Here, I review the established working model of SD initiation and propagation. Then, I summarize the historical and recent evidence for the metabolic impact of SD, transitioning from a descriptive to a mechanistic working model of metabolic signaling and its potential to promote neuronal survival and resilience. I quantify the energetic cost of restoring ionic gradients eroded during SD, and the extent to which ion pumping impacts high-energy phosphate pools and the energy charge of affected tissue. I link energy deficits to adaptive increases in the utilization of glucose and O2, and the resulting accumulation of lactic acid and CO2 downstream of catabolic metabolic activity. Finally, I discuss the neuromodulatory and vasoactive paracrine signaling mediated by adenosine and acidosis, highlighting these metabolites' potential to protect vulnerable tissue in the context of high-frequency SD clusters.

Bipolar mania and epilepsy pathophysiology and treatment may converge in purine metabolism: A new perspective on available evidence

Decreased ATPergic signaling is an increasingly recognized pathophysiology in bipolar mania disease models. In parallel, adenosine deficit is increasingly recognized in epilepsy pathophysiology. Under-recognized ATP and/or adenosine-increasing mechanisms of several antimanic and antiseizure therapies including lithium, valproate, carbamazepine, and ECT suggest a fundamental pathogenic role of adenosine deficit in bipolar mania to match the established role of adenosine deficit in epilepsy. The depletion of adenosine-derivatives within the purine cycle is expected to result in a compensatory increase in oxopurines (uric acid precursors) and secondarily increased uric acid, observed in both bipolar mania and epilepsy. Cortisol-based inhibition of purine conversion to adenosine-derivatives may be reflected in observed uric acid increases and the well-established contribution of cortisol to both bipolar mania and epilepsy pathology. Cortisol-inhibited conversion from IMP to AMP as precursor of both ATP and adenosine may represent a mechanism for treatment resistance common in both bipolar mania and epilepsy. Anti-cortisol therapies may therefore augment other treatments both in bipolar mania and epilepsy. Evidence linking (i) adenosine deficit with a decreased need for sleep, (ii) IMP/cGMP excess with compulsive hypersexuality, and (iii) guanosine excess with grandiose delusions may converge to suggest a novel theory of bipolar mania as a condition characterized by disrupted purine metabolism. The potential for disease-modification and prevention related to adenosine-mediated epigenetic changes in epilepsy may be mirrored in mania. Evaluating the purinergic effects of existing agents and validating purine dysregulation may improve diagnosis and treatment in bipolar mania and epilepsy and provide specific targets for drug development.

Glycolysis regulates neuronal excitability via lactate receptor, HCA1R

Repetitively firing neurons during seizures accelerate glycolysis to meet energy demand, which leads to the accumulation of extracellular glycolytic by-product lactate. Here, we demonstrate that lactate rapidly modulates neuronal excitability in times of metabolic stress via the hydroxycarboxylic acid receptor type 1 (HCA1R) to modify seizure activity. The extracellular lactate concentration, measured by a biosensor, rose quickly during brief and prolonged seizures. In two epilepsy models, mice lacking HCA1R (lactate receptor) were more susceptible to developing seizures. Moreover, HCA1R deficient (knockout) mice developed longer and more severe seizures than wild-type littermates. Lactate perfusion decreased tonic and phasic activity of CA1 pyramidal neurons in genetically encoded calcium indicator 7 imaging experiments. HCA1R agonist 3-chloro-5-hydroxybenzoic acid (3CL-HBA) reduced the activity of CA1 neurons in HCA1R WT but not in knockout mice. In patch-clamp recordings, both lactate and 3CL-HBA hyperpolarized CA1 pyramidal neurons. HCA1R activation reduced the spontaneous excitatory postsynaptic current frequency and altered the paired-pulse ratio of evoked excitatory postsynaptic currents in HCA1R wild-type but not in knockout mice, suggesting it diminished presynaptic release of excitatory neurotransmitters. Overall, our studies demonstrate that excessive neuronal activity accelerates glycolysis to generate lactate, which translocates to the extracellular space to slow neuronal firing and inhibit excitatory transmission via HCA1R. These studies may identify novel anticonvulsant target and seizure termination mechanisms.

The complement inhibitor CD59 is required for GABAergic synaptic transmission in the dentate gyrus

Complement-dependent microglia pruning of excitatory synapses has been widely reported in physiological and pathological conditions, with few reports concerning pruning of inhibitory synapses or direct regulation of synaptic transmission by complement components. Here, we report that loss of CD59, an important endogenous inhibitor of the complement system, leads to compromised spatial memory performance. Furthermore, CD59 deficiency impairs GABAergic synaptic transmission in the hippocampal dentate gyrus (DG). This depends on regulation of GABA release triggered by Ca2+ influx through voltage-gated calcium channels (VGCCs) rather than inhibitory synaptic pruning by microglia. Notably, CD59 colocalizes with inhibitory pre-synaptic terminals and regulates SNARE complex assembly. Together, these results demonstrate that the complement regulator CD59 plays an important role in normal hippocampal function.

Roles and Sources of Calcium in Synaptic Exocytosis

Calcium ions (Ca2+) play a critical role in triggering neurotransmitter release. The rate of release is directly related to the concentration of Ca2+ at the presynaptic site, with a supralinear relationship. There are two main sources of Ca2+ that trigger synaptic vesicle fusion: influx through voltage-gated Ca2+ channels in the plasma membrane and release from the endoplasmic reticulum via ryanodine receptors. This chapter will cover the sources of Ca2+ at the presynaptic nerve terminal, the relationship between neurotransmitter release rate and Ca2+ concentration, and the mechanisms that achieve the necessary Ca2+ concentrations for triggering synaptic exocytosis at the presynaptic site.

Nck1 activity in lateral amygdala regulates long-term fear memory formation

Fear conditioning leads to long-term fear memory formation and is a model for studying fear-related psychopathological conditions such as phobias and post-traumatic stress disorder. Long-term fear memory formation is believed to involve alterations of synaptic efficacy mediated by changes in synaptic transmission and morphology in lateral amygdala (LA). Nck1 is a key neuronal adaptor protein involved in the regulation of the actin cytoskeleton and the neuronal processes believed to be involved in memory formation. However, the role of Nck1 in memory formation is not known. Here we explored the role of Nck1 in fear memory formation in lateral amygdala (LA). Reduction of Nck1 in excitatory neurons in LA enhanced long-term, but not short-term, auditory fear conditioning memory. Activation of Nck1, by using a photoactivatable Nck1 (PA-Nck1), during auditory fear conditioning in excitatory neurons in LA impaired long-term, but not short-term, fear memory. Activation of Nck1 immediately or a day after fear conditioning did not affect fear memory. The hippocampal-mediated contextual fear memory was not affected by the reduction or activation of Nck1 in LA. We show that Nck1 is localized to the presynapses in LA. Nck1 activation in LA excitatory neurons decreased the frequency of AMPA receptors-mediated miniature excitatory synaptic currents (mEPSCs). Nck1 activation did not affect GABA receptor-mediated inhibitory synaptic currents (mIPSCs). These results show that Nck1 activity in excitatory neurons in LA regulates glutamate release and sets the threshold for fear memory formation. Moreover, our research shows that Nck1 may serve as a target for pharmacological treatment of fear and anxiety disorders.

Conditional knockout of CRMP2 in neurons, but not astrocytes, disrupts spinal nociceptive neurotransmission to control the initiation and maintenance of chronic neuropathic pain

ABSTRACT

Mechanistic studies principally focusing on primary afferent nociceptive neurons uncovered the upregulation of collapsin response mediator protein 2 (CRMP2)-a dual trafficking regulator of N-type voltage-gated calcium (Cav2.2) as well as Nav1.7 voltage-gated sodium channels-as a potential determinant of neuropathic pain. Whether CRMP2 contributes to aberrant excitatory synaptic transmission underlying neuropathic pain processing after peripheral nerve injury is unknown. Here, we interrogated CRMP2's role in synaptic transmission and in the initiation or maintenance of chronic pain. In rats, short-interfering RNA-mediated knockdown of CRMP2 in the spinal cord reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not spontaneous inhibitory postsynaptic currents, recorded from superficial dorsal horn neurons in acute spinal cord slices. No effect was observed on miniature excitatory postsynaptic currents and inhibitory postsynaptic currents. In a complementary targeted approach, conditional knockout of CRMP2 from mouse neurons using a calcium/calmodulin-dependent protein kinase II alpha promoter to drive Cre recombinase expression reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not miniature excitatory SCss. Conditional knockout of CRMP2 from mouse astrocytes using a glial fibrillary acidic protein promoter had no effect on synaptic transmission. Conditional knockout of CRMP2 in neurons reversed established mechanical allodynia induced by a spared nerve injury in both male and female mice. In addition, the development of spared nerve injury-induced allodynia was also prevented in these mice. Our data strongly suggest that CRMP2 is a key regulator of glutamatergic neurotransmission driving pain signaling and that it contributes to the transition of physiological pain into pathological pain.

Presynaptic mechanisms underlying GABAB-receptor-mediated inhibition of spontaneous neurotransmitter release

Inhibition of neurotransmitter release by neurotransmitter substances constitutes a fundamental means of neuromodulation. In contrast to well-delineated mechanisms that underlie inhibition of evoked release via suppression of voltage-gated Ca²⁺ channels, processes that underlie neuromodulatory inhibition of spontaneous release remain unclear. Here, we interrogated inhibition of spontaneous glutamate and GABA release by presynaptic metabotropic GABA_B receptors. Our findings show that this inhibition relies on Gβγ subunit action at the membrane, and it is largely independent of presynaptic Ca²⁺ signaling for both forms of release. In the case of spontaneous glutamate release, inhibition requires Gβγ interaction with the C terminus of the key fusion machinery component SNAP25, and it is modulated by synaptotagmin-1. Inhibition of spontaneous GABA release, on the other hand, is independent of these pathways and likely requires alternative Gβγ targets at the presynaptic terminal.

Altered neocortical oscillations and cellular excitability in an in vitro Wwox knockout mouse model of epileptic encephalopathy

Loss of function mutations of the WW domain-containing oxidoreductase (WWOX) gene are associated with severe and fatal drug-resistant pediatric epileptic encephalopathy. Epileptic seizures are typically characterized by neuronal hyperexcitability; however, the specific contribution of WWOX to that hyperexcitability has yet to be investigated. Using a mouse model of neuronal Wwox-deletion that exhibit spontaneous seizures, in vitro whole-cell and field potential electrophysiological characterization identified spontaneous bursting activity in the neocortex, a marker of the underlying network hyperexcitability. Spectral analysis of the neocortical bursting events highlighted increased phase-amplitude coupling, and a propagation from layer II/III to layer V. These bursts were NMDAR and gap junction dependent. In layer II/III pyramidal neurons, Wwox knockout mice demonstrated elevated amplitude of excitatory post-synaptic currents, whereas the frequency and amplitude of inhibitory post-synaptic currents were reduced, as compared to heterozygote and wild-type littermate controls. Furthermore, these neurons were depolarized and demonstrated increased action potential frequency, sag current, and post-inhibitory rebound. These findings suggest WWOX plays an essential role in balancing neocortical excitability and provide insight towards developing therapeutics for those suffering from WWOX disorders.

Calcium and Spike Timing-Dependent Plasticity

Since its discovery, spike timing-dependent synaptic plasticity (STDP) has been thought to be a primary mechanism underlying the brain's ability to learn and to form new memories. However, despite the enormous interest in both the experimental and theoretical neuroscience communities in activity-dependent plasticity, it is still unclear whether plasticity rules inferred from in vitro experiments apply to in vivo conditions. Among the multiple reasons why plasticity rules in vivo might differ significantly from in vitro studies is that extracellular calcium concentration use in most studies is higher than concentrations estimated in vivo. STDP, like many forms of long-term synaptic plasticity, strongly depends on intracellular calcium influx for its induction. Here, we discuss the importance of considering physiological levels of extracellular calcium concentration to study functional plasticity.

Efficient optogenetic silencing of neurotransmitter release with a mosquito rhodopsin

Information is carried between brain regions through neurotransmitter release from axonal presynaptic terminals. Understanding the functional roles of defined neuronal projection pathways requires temporally precise manipulation of their activity. However, existing inhibitory optogenetic tools have low efficacy and off-target effects when applied to presynaptic terminals, while chemogenetic tools are difficult to control in space and time. Here, we show that a targeting-enhanced mosquito homolog of the vertebrate encephalopsin (eOPN3) can effectively suppress synaptic transmission through the G_i/o signaling pathway. Brief illumination of presynaptic terminals expressing eOPN3 triggers a lasting suppression of synaptic output that recovers spontaneously within minutes in vitro and in vivo. In freely moving mice, eOPN3-mediated suppression of dopaminergic nigrostriatal afferents induces a reversible ipsiversive rotational bias. We conclude that eOPN3 can be used to selectively suppress neurotransmitter release at presynaptic terminals with high spatiotemporal precision, opening new avenues for functional interrogation of long-range neuronal circuits in vivo.

Neurexins regulate presynaptic GABAB-receptors at central synapses

Diverse signaling complexes are precisely assembled at the presynaptic active zone for dynamic modulation of synaptic transmission and synaptic plasticity. Presynaptic GABA_B-receptors nucleate critical signaling complexes regulating neurotransmitter release at most synapses. However, the molecular mechanisms underlying assembly of GABA_B-receptor signaling complexes remain unclear. Here we show that neurexins are required for the localization and function of presynaptic GABA_B-receptor signaling complexes. At four model synapses, excitatory calyx of Held synapses in the brainstem, excitatory and inhibitory synapses on hippocampal CA1-region pyramidal neurons, and inhibitory basket cell synapses in the cerebellum, deletion of neurexins rendered neurotransmitter release significantly less sensitive to GABA_B-receptor activation. Moreover, deletion of neurexins caused a loss of GABA_B-receptors from the presynaptic active zone of the calyx synapse. These findings extend the role of neurexins at the presynaptic active zone to enabling GABA_B-receptor signaling, supporting the notion that neurexins function as central organizers of active zone signaling complexes.

Neurexins are evolutionarily conserved cell adhesion molecules that tune synapse formation and specification. Here the authors show that neurexins play similar roles in regulating presynaptic GABA_B receptors at multiple CNS synapses.

Increased Excitatory Synaptic Transmission Associated with Adult Seizure Vulnerability Induced by Early-Life Inflammation in Mice

Early-life inflammatory stress increases seizure susceptibility later in life. However, possible sex- and age-specific differences and the associated mechanisms are largely unknown. C57BL/6 mice were bred in house, and female and male pups were injected with lipopolysaccharide (LPS; 100 μg/kg, i.p.) or vehicle control (saline solution) at postnatal day 14 (P14). Seizure threshold was assessed in response to pentylenetetrazol (1% solution, i.v.) in adolescence (∼P40) and adulthood (∼P60). We found that adult, but not adolescent, mice treated with LPS displayed ∼34% lower seizure threshold compared with controls. Females and males showed similar increased seizure susceptibility, suggesting that altered brain excitability was age dependent, but not sex dependent. Whole-cell recordings revealed no differences in excitatory synaptic activity onto CA1 pyramidal neurons from control or neonatally inflamed adolescent mice of either sex. However, adult mice of both sexes previously exposed to LPS displayed spontaneous EPSC frequency approximately twice that of controls, but amplitude was unchanged. Although these changes were not associated with alterations in dendritic spines or in the NMDA/AMPA receptor ratio, they were linked to an increased glutamate release probability from Schaffer collateral, but not temporoammonic pathway. This glutamate increase was associated with reduced activity of presynaptic GABA_B receptors and was independent of the endocannabinoid-mediated suppression of excitation. Our new findings demonstrate that early-life inflammation leads to long-term increased hippocampal excitability in adult female and male mice associated with changes in glutamatergic synaptic transmission. These alterations may contribute to enhanced vulnerability of the brain to subsequent pathologic challenges such as epileptic seizures.SIGNIFICANCE STATEMENT Adult physiology has been shown to be affected by early-life inflammation. Our data reveal that early-life inflammation increases excitatory synaptic transmission onto hippocampal CA1 pyramidal neurons in an age-dependent manner through disrupted presynaptic GABA_B receptor activity on Schaffer collaterals. This hyperexcitability was seen only in adult, and not in adolescent, animals of either sex. The data suggest a maturation process, independent of sex, in the priming action of early-life inflammation and highlight the importance of studying mature brains to reveal cellular changes associated with early-life interventions.

Recordings from neuron-HEK cell cocultures reveal the determinants of miniature excitatory postsynaptic currents

Spontaneous exocytosis of single synaptic vesicles generates miniature synaptic currents, which provide a window into the dynamic control of synaptic transmission. To resolve the impact of different factors on the dynamics and variability of synaptic transmission, we recorded miniature excitatory postsynaptic currents (mEPSCs) from cocultures of mouse hippocampal neurons with HEK cells expressing the postsynaptic proteins GluA2, neuroligin 1, PSD-95, and stargazin. Synapses between neurons and these heterologous cells have a molecularly defined postsynaptic apparatus, while the compact morphology of HEK cells eliminates the distorting effect of dendritic filtering. HEK cells in coculture produced mEPSCs with a higher frequency, larger amplitude, and more rapid rise and decay than neurons from the same culture. However, mEPSC area indicated that nerve terminals in synapses with both neurons and HEK cells release similar populations of vesicles. Modulation by the glutamate receptor ligand aniracetam revealed receptor contributions to mEPSC shape. Dendritic cable effects account for the slower mEPSC rise in neurons, whereas the slower decay also depends on other factors. Lastly, expression of synaptobrevin transmembrane domain mutants in neurons slowed the rise of HEK cell mEPSCs, thus revealing the impact of synaptic fusion pores. In summary, we show that cocultures of neurons with heterologous cells provide a geometrically simplified and molecularly defined system to investigate the time course of synaptic transmission and to resolve the contribution of vesicles, fusion pores, dendrites, and receptors to this process.

A Phase II Randomized, Double-Blind, Placebo-Controlled Study of the Efficacy, Safety, and Tolerability of Arbaclofen Administered for the Treatment of Social Function in Children and Adolescents With Autism Spectrum Disorders: Study Protocol for AIMS-2-TRIALS-CT1

Background: Autism Spectrum Disorder (ASD or autism) is characterized by difficulties in social communication and interaction, which negatively impact on individuals and their families' quality of life. Currently no pharmacological interventions have been shown to be effective for improving social communication in autism. Previous trials have indicated the potential of arbaclofen for improving social function among autistic children and adolescents with fluent speech. The AIMS2TRIALS-Clinical Trial 1 (AIMS-CT1) will examine whether arbaclofen is superior to placebo in improving social function and other secondary outcomes over 16 weeks, along with safety and tolerability profiles. Methods: AIMS-CT1 is an international, multi-site, double-blind, parallel group Phase II randomized clinical trial. It will include 130 males and females aged 5:0-17:11 years, with a diagnosis of ASD and fluent speech. Eligible participants will be randomized on a ratio of 1:1 for a 16-week treatment period. Medication will be titrated over 5 weeks. The primary outcome is the effect on social function from weeks 0 to 16 measured on the Socialization domain of the Vineland Adaptive Behavior Scales, 3rd edition^TM. Secondary outcome measures include the CGI-S (Clinical Global Impression-Severity), CGI-I (Clinical Global Impression-Improvement), other areas of adaptive function, social communication and other autism symptoms, co-occurring behavior problems and health-related quality of life. Genetic and electrophysiological markers will be examined as potential stratifiers for treatment response. Exploratory novel digital technologies will also be used to measure change, examining simultaneously the validity of digital biomarkers in natural environments. The safety and tolerability of the drug will also be examined. Our protocol is very closely aligned with a parallel Canadian trial of 90 participants (ARBA Study, US NCT number: NCT03887676) to allow for secondary combined analyses. Outcomes will be compared using both an Intent-to-reat and Per Protocol approach. Discussion: The outcomes of this trial, combined with the parallel Canadian trial, will contribute to the evidence base for medications used to help social difficulties among young autistic individuals; demonstrate the capabilities of the AIMS-2-TRIALS network of academic centers to deliver clinical trials; and support future drug development. Clinical Trial Registration: EudraCT number: 2018-000942-21 and ClinicalTrials.gov registry number: NCT03682978. Currently under protocol v.7.2, dated 20.11.2020.

The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

Neonatal Seizures and Purinergic Signalling

Neonatal seizures are one of the most common comorbidities of neonatal encephalopathy, with seizures aggravating acute injury and clinical outcomes. Current treatment can control early life seizures; however, a high level of pharmacoresistance remains among infants, with increasing evidence suggesting current anti-seizure medication potentiating brain damage. This emphasises the need to develop safer therapeutic strategies with a different mechanism of action. The purinergic system, characterised by the use of adenosine triphosphate and its metabolites as signalling molecules, consists of the membrane-bound P1 and P2 purinoreceptors and proteins to modulate extracellular purine nucleotides and nucleoside levels. Targeting this system is proving successful at treating many disorders and diseases of the central nervous system, including epilepsy. Mounting evidence demonstrates that drugs targeting the purinergic system provide both convulsive and anticonvulsive effects. With components of the purinergic signalling system being widely expressed during brain development, emerging evidence suggests that purinergic signalling contributes to neonatal seizures. In this review, we first provide an overview on neonatal seizure pathology and purinergic signalling during brain development. We then describe in detail recent evidence demonstrating a role for purinergic signalling during neonatal seizures and discuss possible purine-based avenues for seizure suppression in neonates.

An essential role for MEF2C in the cortical response to loss of sleep in mice

Neuronal activity and gene expression in response to the loss of sleep can provide a window into the enigma of sleep function. Sleep loss is associated with brain differential gene expression, an increase in pyramidal cell mEPSC frequency and amplitude, and a characteristic rebound and resolution of slow wave sleep-slow wave activity (SWS-SWA). However, the molecular mechanism(s) mediating the sleep-loss response are not well understood. We show that sleep-loss regulates MEF2C phosphorylation, a key mechanism regulating MEF2C transcriptional activity, and that MEF2C function in postnatal excitatory forebrain neurons is required for the biological events in response to sleep loss in C57BL/6J mice. These include altered gene expression, the increase and recovery of synaptic strength, and the rebound and resolution of SWS-SWA, which implicate MEF2C as an essential regulator of sleep function.

Effects of nitrous oxide on glycinergic transmission in rat spinal neurons

We investigated the effects of nitrous oxide (N₂O) on glycinergic inhibitory whole-cell and synaptic responses using a "synapse bouton preparation," dissociated mechanically from rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique can evaluate pure single- or multi-synaptic responses from native functional nerve endings and enable us to accurately quantify how N₂O influences pre- and postsynaptic transmission. We found that 70 % N₂O enhanced exogenous glycine-induced whole-cell currents (I_Gly) at glycine concentrations lower than 3 × 10^-5 M, but did not affect I_Gly at glycine concentrations higher than 10^-4 M. N₂O did not affect the amplitude and 1/e decay-time of both spontaneous and miniature glycinergic inhibitory postsynaptic currents recorded in the absence and presence of tetrodotoxin (sIPSCs and mIPSCs, respectively). The decrease in frequency induced by N₂O was observed in sIPSCs but not in mIPSCs, which was recorded in the presence of both tetrodotoxin and Cd²⁺, which block voltage-gated Na⁺ and Ca²⁺ channels, respectively. N₂O also decreased the amplitude and increased the failure rate and paired-pulse ratio of action potential-evoked glycinergic inhibitory postsynaptic currents. N₂O slightly decreased the Ba²⁺ currents mediated by voltage-gated Ca²⁺ channels in SDCN neurons. We found that N₂O suppresses glycinergic responses at synaptic levels with presynaptic effect having much more predominant role. The difference between glycinergic whole-cell and synaptic responses suggests that extrasynaptic responses seriously modulate whole-cell currents. Our results strongly suggest that these responses may thus in part explain analgesic effects of N₂O via marked glutamatergic inhibition by glycinergic responses in the spinal cord.

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

Role of Adenosine in Epilepsy and Seizures

Adenosine is an endogenous anticonvulsant and neuroprotectant of the brain. Seizure activity produces large quantities of adenosine, and it is this seizure-induced adenosine surge that normally stops a seizure. However, within the context of epilepsy, adenosine plays a wide spectrum of different roles. It not only controls seizures (ictogenesis), but also plays a major role in processes that turn a normal brain into an epileptic brain (epileptogenesis). It is involved in the control of abnormal synaptic plasticity and neurodegeneration and plays a major role in the expression of comorbid symptoms and complications of epilepsy, such as sudden unexpected death in epilepsy (SUDEP). Given the important role of adenosine in epilepsy, therapeutic strategies are in development with the goal to utilize adenosine augmentation not only for the suppression of seizures but also for disease modification and epilepsy prevention, as well as strategies to block adenosine A_2A receptor overfunction associated with neurodegeneration. This review provides a comprehensive overview of the role of adenosine in epilepsy.

Simultaneous Release of Multiple Vesicles from Rods Involves Synaptic Ribbons and Syntaxin 3B

First proposed as a specialized mode of release at sensory neurons possessing ribbon synapses, multivesicular release has since been described throughout the central nervous system. Many aspects of multivesicular release remain poorly understood. We explored mechanisms underlying simultaneous multivesicular release at ribbon synapses in salamander retinal rod photoreceptors. We assessed spontaneous release presynaptically by recording glutamate transporter anion currents (I_A(glu)) in rods. Spontaneous I_A(glu) events were correlated in amplitude and kinetics with simultaneously measured miniature excitatory postsynaptic currents in horizontal cells. Both measures indicated that a significant fraction of events is multiquantal, with an analysis of I_A(glu) revealing that multivesicular release constitutes ∼30% of spontaneous release events. I_A(glu) charge transfer increased linearly with event amplitude showing that larger events involve greater glutamate release. The kinetics of large and small I_A(glu) events were identical as were rise times of large and small miniature excitatory postsynaptic currents, indicating that the release of multiple vesicles during large events is highly synchronized. Effects of exogenous Ca²⁺ buffers suggested that multiquantal, but not uniquantal, release occurs preferentially near Ca²⁺ channels clustered beneath synaptic ribbons. Photoinactivation of ribbons reduced the frequency of spontaneous multiquantal events without affecting uniquantal release frequency, showing that spontaneous multiquantal release requires functional ribbons. Although both occur at ribbon-style active zones, the absence of cross-depletion indicates that evoked and spontaneous multiquantal release from ribbons involve different vesicle pools. Introducing an inhibitory peptide into rods to interfere with the SNARE protein, syntaxin 3B, selectively reduced multiquantal event frequency. These results support the hypothesis that simultaneous multiquantal release from rods arises from homotypic fusion among neighboring vesicles on ribbons and involves syntaxin 3B.

The Purinome and the preBötzinger Complex - A Ménage of Unexplored Mechanisms That May Modulate/Shape the Hypoxic Ventilatory Response

Exploration of purinergic signaling in brainstem homeostatic control processes is challenging the traditional view that the biphasic hypoxic ventilatory response, which comprises a rapid initial increase in breathing followed by a slower secondary depression, reflects the interaction between peripheral chemoreceptor-mediated excitation and central inhibition. While controversial, accumulating evidence supports that in addition to peripheral excitation, interactions between central excitatory and inhibitory purinergic mechanisms shape this key homeostatic reflex. The objective of this review is to present our working model of how purinergic signaling modulates the glutamatergic inspiratory synapse in the preBötzinger Complex (key site of inspiratory rhythm generation) to shape the hypoxic ventilatory response. It is based on the perspective that has emerged from decades of analysis of glutamatergic synapses in the hippocampus, where the actions of extracellular ATP are determined by a complex signaling system, the purinome. The purinome involves not only the actions of ATP and adenosine at P2 and P1 receptors, respectively, but diverse families of enzymes and transporters that collectively determine the rate of ATP degradation, adenosine accumulation and adenosine clearance. We summarize current knowledge of the roles played by these different purinergic elements in the hypoxic ventilatory response, often drawing on examples from other brain regions, and look ahead to many unanswered questions and remaining challenges.

Secreted amyloid-β precursor protein functions as a GABABR1a ligand to modulate synaptic transmission

Amyloid-β precursor protein (APP) is central to the pathogenesis of Alzheimer's disease, yet its physiological function remains unresolved. Accumulating evidence suggests that APP has a synaptic function mediated by an unidentified receptor for secreted APP (sAPP). Here we show that the sAPP extension domain directly bound the sushi 1 domain specific to the γ-aminobutyric acid type B receptor subunit 1a (GABA_BR1a). sAPP-GABA_BR1a binding suppressed synaptic transmission and enhanced short-term facilitation in mouse hippocampal synapses via inhibition of synaptic vesicle release. A 17-amino acid peptide corresponding to the GABA_BR1a binding region within APP suppressed in vivo spontaneous neuronal activity in the hippocampus of anesthetized Thy1-GCaMP6s mice. Our findings identify GABA_BR1a as a synaptic receptor for sAPP and reveal a physiological role for sAPP in regulating GABA_BR1a function to modulate synaptic transmission.

Autaptic cultures of human induced neurons as a versatile platform for studying synaptic function and neuronal morphology

Recently developed technology to differentiate induced pluripotent stem cells (iPSCs) into human induced neurons (iNs) provides an exciting opportunity to study the function of human neurons. However, functional characterisations of iNs have been hampered by the reliance on mass culturing protocols which do not allow assessment of synaptic release characteristics and neuronal morphology at the individual cell level with quantitative precision. Here, we have developed for the first time a protocol to generate autaptic cultures of iPSC-derived iNs. We show that our method efficiently generates mature, autaptic iNs with robust spontaneous and action potential-driven synaptic transmission. The synaptic responses are sensitive to modulation by metabotropic receptor agonists as well as potentiation by acute phorbol ester application. Finally, we demonstrate loss of evoked and spontaneous release by Unc13A knockdown. This culture system provides a versatile platform allowing for quantitative and integrative assessment of morphophysiological and molecular parameters underlying human synaptic transmission.

Disabling the Gβγ-SNARE interaction disrupts GPCR-mediated presynaptic inhibition, leading to physiological and behavioral phenotypes

G protein-coupled receptors (GPCRs) that couple to G_i/o proteins modulate neurotransmission presynaptically by inhibiting exocytosis. Release of Gβγ subunits from activated G proteins decreases the activity of voltage-gated Ca²⁺ channels (VGCCs), decreasing excitability. A less understood Gβγ-mediated mechanism downstream of Ca²⁺ entry is the binding of Gβγ to SNARE complexes, which facilitate the fusion of vesicles with the cell plasma membrane in exocytosis. Here, we generated mice expressing a form of the SNARE protein SNAP25 with premature truncation of the C terminus and that were therefore partially deficient in this interaction. SNAP25Δ3 homozygote mice exhibited normal presynaptic inhibition by GABA_B receptors, which inhibit VGCCs, but defective presynaptic inhibition by receptors that work directly on the SNARE complex, such as 5-hydroxytryptamine (serotonin) 5-HT_1b receptors and adrenergic α_2a receptors. Simultaneously stimulating receptors that act through both mechanisms showed synergistic inhibitory effects. SNAP25Δ3 homozygote mice had various behavioral phenotypes, including increased stress-induced hyperthermia, defective spatial learning, impaired gait, and supraspinal nociception. These data suggest that the inhibition of exocytosis by G_i/o-coupled GPCRs through the Gβγ-SNARE interaction is a crucial component of numerous physiological and behavioral processes.

Group II Metabotropic Glutamate Receptors Mediate Presynaptic Inhibition of Excitatory Transmission in Pyramidal Neurons of the Human Cerebral Cortex

Group II metabotropic glutamate receptor (mGluR) ligands are potential novel drugs for neurological and psychiatric disorders, but little is known about the effects of these compounds at synapses of the human cerebral cortex. Investigating the effects of neuropsychiatric drugs in human brain tissue with preserved synaptic circuits might accelerate the development of more potent and selective pharmacological treatments. We have studied the effects of group II mGluR activation on excitatory synaptic transmission recorded from pyramidal neurons of cortical layers 2-3 in acute slices derived from surgically removed cortical tissue of people with epilepsy or tumors. The application of a selective group II mGluR agonist, LY354740 (0.1-1 μM) inhibited the amplitude and frequency of action potential-dependent spontaneous excitatory postsynaptic currents (sEPSCs). This effect was prevented by the application of a group II/III mGluR antagonist, CPPG (0.1 mM). Furthermore, LY354740 inhibited the frequency, but not the amplitude, of action potential-independent miniature EPSCs (mEPSCs) recorded in pyramidal neurons. Finally, LY354740 did slightly reduce cells' input resistance without altering the holding current of the neurons recorded in voltage clamp at -90 mV. Our results suggest that group II mGluRs are mainly auto-receptors that inhibit the release of glutamate onto pyramidal neurons in layers 2-3 in the human cerebral cortex, thereby regulating network excitability. We have demonstrated the effect of a group II mGluR ligand at human cortical synapses, revealing mechanisms by which these drugs could exert pro-cognitive effects and treat human neuropsychiatric disorders.

GABAB receptors modulate Ca2+ but not G protein-gated inwardly rectifying K+ channels in cerebrospinal-fluid contacting neurones of mouse brainstem

KEY POINTS

Medullo-spinal CSF contacting neurones (CSF-cNs) located around the central canal are conserved in all vertebrates and suggested to be a novel sensory system intrinsic to the CNS. CSF-cNs receive GABAergic inhibitory synaptic inputs involving ionotropic GABA_A receptors, but the contribution of metabotropic GABA_B receptors (GABA_B -Rs) has not yet been studied. Here, we indicate that CSF-cNs express functional GABA_B -Rs that inhibit postsynaptic calcium channels but fail to activate inhibitory potassium channel of the Kir3-type. We further show that GABA_B -Rs localise presynaptically on GABAergic and glutamatergic synaptic inputs contacting CSF-cNs, where they inhibit the release of GABA and glutamate. Our data are the first to address the function of GABA_B -Rs in CSF-cNs and show that on the presynaptic side they exert a classical synaptic modulation whereas at the postsynaptic level they have an atypical action by modulating calcium signalling without inducing potassium-dependent inhibition.

ABSTRACT

Medullo-spinal neurones that contact the cerebrospinal fluid (CSF-cNs) are a population of evolutionary conserved cells located around the central canal. CSF-cN activity has been shown to be regulated by inhibitory synaptic inputs involving ionotropic GABA_A receptors, but the contribution of the G-protein coupled GABA_B receptors has not yet been studied. Here, we used a combination of immunofluorescence, electrophysiology and calcium imaging to investigate the expression and function of GABA_B -Rs in CSF-cNs of the mouse brainstem. We found that CSF-cNs express GABA_B -Rs, but their selective activation failed to induce G protein-coupled inwardly rectifying potassium (GIRK) currents. Instead, CSF-cNs express primarily N-type voltage-gated calcium (Ca_V 2.2) channels, and GABA_B -Rs recruit Gβγ subunits to inhibit Ca_V channel activity induced by membrane voltage steps or under physiological conditions by action potentials. Moreover, using electrical stimulation, we indicate that GABAergic inhibitory (IPSCs) and excitatory glutamatergic (EPSCs) synaptic currents can be evoked in CSF-cNs showing that mammalian CSF-cNs are also under excitatory control by glutamatergic synaptic inputs. We further demonstrate that baclofen reversibly reduced the amplitudes of both IPSCs and EPSCs evoked in CSF-cNs through a presynaptic mechanism of regulation. In summary, these results are the first to demonstrate the existence of functional postsynaptic GABA_B -Rs in medullar CSF-cNs, as well as presynaptic GABA_B auto- and heteroreceptors regulating the release of GABA and glutamate. Remarkably, postsynaptic GABA_B -Rs associate with Ca_V but not GIRK channels, indicating that GABA_B -Rs function as a calcium signalling modulator without GIRK-dependent inhibition in CSF-cNs.

Direct assessment of presynaptic modulation of cortico-striatal glutamate release in a Huntington’s disease mouse model

Glutamate is the main excitatory neurotransmitter in the brain, and impairments in its signaling are associated with many neurological disorders, including Huntington’s disease (HD). Previous studies in HD mouse models demonstrate altered glutamate receptor distribution and signaling at cortico-striatal synapses, and some studies suggest that glutamate release is altered; however, traditional methods to study synaptic glutamate release are indirect or have poor temporal resolution. Here we utilize iGluSnFR, a modified green fluorescent protein reporter for real-time imaging of glutamate transmission, to study presynaptic modulation of cortical glutamate release in the striatum of the YAC128 HD mouse model. We determined that iGluSnFR can be used to accurately measure short- and long-term changes in glutamate release caused by modulation of extracellular Ca2+ levels, activation of presynaptic receptors, and high-frequency stimulation (HFS) protocols. We also confirmed a difference in the expression of HFS-induced long-term depression in YAC128. Together, this research demonstrates the utility of iGluSnFR in studying presynaptic modulation of glutamate release in healthy mice and disease models that display impairments in glutamate signaling.

NEW & NOTEWORTHY We use iGluSnFR to directly assess presynaptic modulation of cortico-striatal glutamate release in brain slice and compare changes in glutamate release between wild type and a Huntington’s disease mouse model, YAC128. We observed reductions in glutamate release after low extracellular Ca2+ and activation of various presynaptic receptors. We also demonstrate a presynaptic mechanism of reduced glutamate release in high-frequency stimulation-induced long-term depression and show this to be altered in YAC128.

Inactivation of GIRK channels weakens the pre- and postsynaptic inhibitory activity in dorsal raphe neurons

The serotonergic tone of the dorsal raphe (DR) is regulated by 5-HT_1A receptors, which negatively control serotonergic activity via the activation of G protein-coupled inwardly rectifying K⁺ (GIRK) channels. In addition, DR activity is modulated by local GABAergic transmission, which is believed to play a key role in the development of mood-related disorders. Here, we sought to characterize the role of GIRK2 subunit-containing channels on the basal electrophysiological properties of DR neurons and to investigate whether the presynaptic and postsynaptic activities of 5-HT_1A, GABA_B, and GABA_A receptors are affected by Girk2 gene deletion. Whole-cell patch-clamp recordings in brain slices from GIRK2 knockout mice revealed that the GIRK2 subunit contributes to maintenance of the resting membrane potential and to the membrane input resistance of DR neurons. 5-HT_1A and GABA_B receptor-mediated postsynaptic currents were almost absent in the mutant mice. Spontaneous and evoked GABA_A receptor-mediated transmissions were markedly reduced in GIRK2 KO mice, as the frequency and amplitude of spontaneous IPSCs were reduced, the paired-pulse ratio was increased and GABA-induced whole-cell currents were decreased. Similarly, the pharmacological blockade of GIRK channels with tertiapin-Q prevented the 5-HT_1A and GABA_B receptor-mediated postsynaptic currents and increased the paired-pulse ratio. Finally, deletion of the Girk2 gene also limited the presynaptic inhibition of GABA release exerted by 5-HT_1A and GABA_B receptors. These results indicate that the properties and inhibitory activity of DR neurons are highly regulated by GIRK2 subunit-containing channels, introducing GIRK channels as potential candidates for studying the pathophysiology and treatment of affective disorders.

The Endogenous Stress Hormone CRH Modulates Excitatory Transmission and Network Physiology in Hippocampus

Memory is strongly influenced by stress but underlying mechanisms are unknown. Here, we used electrophysiology, neuroanatomy, and network simulations to probe the role of the endogenous, stress-related neuropeptide corticotropin-releasing hormone (CRH) in modulating hippocampal function. We focused on neuronal excitability and the incidence of sharp waves (SPWs), a form of intrinsic network activity associated with memory consolidation. Specifically, we blocked endogenous CRH using 2 chemically distinct antagonists of the principal hippocampal CRH receptor, CRHR1. The antagonists caused a modest reduction of spontaneous excitatory transmission onto CA3 pyramidal cells, mediated, in part by effects on I_AHP. This was accompanied by a decrease in the incidence but not amplitude of SPWs, indicating that the synaptic actions of CRH are sufficient to alter the output of a complex hippocampal network. A biophysical model of CA3 described how local actions of CRH produce macroscopic consequences including the observed changes in SPWs. Collectively, the results provide a first demonstration of the manner in which subtle synaptic effects of an endogenously released neuropeptide influence hippocampal network level operations and, in the case of CRH, may contribute to the effects of acute stress on memory.

Isoflurane disrupts excitatory neurotransmitter dynamics via inhibition of mitochondrial complex I

BACKGROUND

The mechanisms of action of volatile anaesthetics are unclear. Volatile anaesthetics selectively inhibit complex I in the mitochondrial respiratory chain. Mice in which the mitochondrial complex I subunit NDUFS4 is knocked out [Ndufs4(KO)] either globally or in glutamatergic neurons are hypersensitive to volatile anaesthetics. The volatile anaesthetic isoflurane selectively decreases the frequency of spontaneous excitatory events in hippocampal slices from Ndufs4(KO) mice.

METHODS

Complex I inhibition by isoflurane was assessed with a Clark electrode. Synaptic function was measured by stimulating Schaffer collateral fibres and recording field potentials in the hippocampus CA1 region.

RESULTS

Isoflurane specifically inhibits complex I dependent respiration at lower concentrations in mitochondria from Ndufs4(KO) than from wild-type mice. In hippocampal slices, after high frequency stimulation to increase energetic demand, short-term synaptic potentiation is less in KO compared with wild-type mice. After high frequency stimulation, both Ndufs4(KO) and wild-type hippocampal slices exhibit striking synaptic depression in isoflurane at twice the 50% effective concentrations (EC₅₀). The pattern of synaptic depression by isoflurane indicates a failure in synaptic vesicle recycling. Application of a selective A₁ adenosine receptor antagonist partially eliminates isoflurane-induced short-term depression in both wild-type and Ndufs4(KO) slices, implicating an additional mitochondria-dependent effect on exocytosis. When mitochondria are the sole energy source, isoflurane completely eliminates synaptic output in both mutant and wild-type mice at twice the (EC₅₀) for anaesthesia.

CONCLUSIONS

Volatile anaesthetics directly inhibit mitochondrial complex I as a primary target, limiting synaptic ATP production, and excitatory vesicle endocytosis and exocytosis.

Calcium dependence of spontaneous neurotransmitter release

Spontaneous release of neurotransmitters is regulated by extracellular [Ca2+ ] and intracellular [Ca2+ ]. Curiously, some of the mechanisms of Ca2+ signaling at central synapses are different at excitatory and inhibitory synapses. While the stochastic activity of voltage-activated Ca2+ channels triggers a majority of spontaneous release at inhibitory synapses, this is not the case at excitatory nerve terminals. Ca2+ release from intracellular stores regulates spontaneous release at excitatory and inhibitory terminals, as do agonists of the Ca2+ -sensing receptor. Molecular machinery triggering spontaneous vesicle fusion may differ from that underlying evoked release and may be one of the sources of heterogeneity in release mechanisms.

Impact of Coffee and Cacao Purine Metabolites on Neuroplasticity and Neurodegenerative Disease

Increasing evidence suggests that regular consumption of coffee, tea and dark chocolate (cacao) can promote brain health and may reduce the risk of age-related neurodegenerative disorders. However, the complex array of phytochemicals in coffee and cacao beans and tea leaves has hindered a clear understanding of the component(s) that affect neuronal plasticity and resilience. One class of phytochemicals present in relatively high amounts in coffee, tea and cacao are methylxanthines. Among such methylxanthines, caffeine has been the most widely studied and has clear effects on neuronal network activity, promotes sustained cognitive performance and can protect neurons against dysfunction and death in animal models of stroke, Alzheimer's disease and Parkinson's disease. Caffeine's mechanism of action relies on antagonism of various subclasses of adenosine receptors. Downstream xanthine metabolites, such as theobromine and theophylline, may also contribute to the beneficial effects of coffee, tea and cacao on brain health.

Novel Ca2+-dependent mechanisms regulate spontaneous release at excitatory synapses onto CA1 pyramidal cells

Although long thought to simply be a source of synaptic noise, spontaneous, action potential-independent release of neurotransmitter from presynaptic terminals has multiple roles in synaptic function. We explored whether and to what extent the two predominantly proposed mechanisms for explaining spontaneous release, stochastic activation of voltage-gated Ca²⁺ channels (VGCCs) or activation of Ca²⁺-sensing receptors (CaSRs) by extracellular Ca²⁺, played a role in the sensitivity of spontaneous release to the level of extracellular Ca²⁺ concentration at excitatory synapses at CA1 pyramidal cells of the adult male mouse hippocampus. Blocking VGCCs with Cd²⁺ had no effect on spontaneous release, ruling out stochastic activation of VGCCs. Although divalent cation agonists of CaSRs, Co²⁺ and Mg²⁺, dramatically enhanced miniature excitatory postsynaptic current (mEPSC) frequency, potent positive and negative allosteric modulators of CaSRs had no effect. Moreover, immunoblot analysis of hippocampal lysates failed to detect CaSR expression, ruling out the CaSR. Instead, the increase in mEPSC frequency induced by Co²⁺ and Mg²⁺ was mimicked by lowering postsynaptic Ca²⁺ levels with BAPTA. Together, our results suggest that a reduction in intracellular Ca²⁺ may trigger a homeostatic-like compensatory response that upregulates spontaneous transmission at excitatory synapses onto CA1 pyramidal cells in the adult hippocampus. NEW & NOTEWORTHY We show that the predominant theories for explaining the regulation of spontaneous, action potential-independent synaptic release do not explain the sensitivity of this type of synaptic transmission to external Ca²⁺ concentration at excitatory synapses onto hippocampal CA1 pyramidal cells. In addition, our data indicate that intracellular Ca²⁺ levels in CA1 pyramidal cells regulate spontaneous release, suggesting that excitatory synapses onto CA1 pyramidal cells may express a novel, rapid form of homeostatic plasticity.

Endomorphin-2 Decreases Excitatory Synaptic Transmission in the Spinal Ventral Horn of the Rat

Motor impairment is one of the serious side-effects of morphine, which is an exogenous agonist of the μ-opioid receptor (MOR) as well as a widely used analgesic drug in clinical practice for chronic pain treatment. Endomorphins (EMs, including EM-1 and EM-2), the most effective and specific endogenous agonists of the MOR, exert more potent analgesia in acute and neuropathic pain than other opiates, such as morphine. Although EMs had fewer side-effects comparing to other opiates, motor impairment was still one unwanted reaction which limited its clinical application. In order to prevent and treat the motor impairment, it is critical to reveal the neural mechanisms underlying such locomotion disorder. The purpose of the present study was to reveal the neural mechanisms underlying the effects of EM-2 on the activity of motoneurons in the spinal ventral horn. First, we examine the distribution of EM-2-immunoreactive (IR) primary afferent fibers and their synaptic connections with the motoneurons innervating the skeletal muscles of the lower limb revealed by sciatic nerve retrograde tracing. The results showed that EM-2-IR fibers and terminals were sparsely observed in lamina IX and they formed symmetric synaptic connections with the motoneurons within lamina IX of the spinal ventral horn. Then, whole-cell patch-clamp technique was used to observe the effects of EM-2 on the spontaneous excitatory postsynaptic current (sEPSC) of motoneurons in lamina IX. The results showed that EM-2 could decrease both the frequency and amplitude of the sEPSC of the motoneurons in lamina IX, which was reversed by the MOR antagonist CTOP. These results indicate that EM-2-IR fibers originated from primary afferent fibers form symmetric synaptic connections with motoneurons innervating skeletal muscles of the lower limbs in lamina IX of the spinal ventral horn and EM-2 might exert inhibitory effects on the activities of these motoneurons through both presynaptic and postsynaptic mechanisms.

Distinct Properties of Long-Term Potentiation in the Dentate Gyrus along the Dorsoventral Axis: Influence of Age and Inhibition

The hippocampus is important for spatial navigation, episodic memory and affective behaviour. Increasing evidence suggests that these multiple functions are accomplished by different segments along the dorsal-ventral (septal-temporal) axis. Long-term potentiation (LTP), the best-investigated cellular correlate of learning and memory, has distinct properties along this axis in the CA1 region, but so far, little is known about longitudinal differences in dentate gyrus (DG). Therefore, here we examined potential dorsoventral differences in DG-LTP using in vitro multi-electrode array recordings. In young mice, we found higher basal synaptic transmission in the dorsal DG, while the LTP magnitude markedly increased towards the ventral pole. Strikingly, these differences were greatly reduced in slices from middle-aged mice. Short-term plasticity, evaluated by paired-pulse ratios, was similar across groups. Recordings in the presence and absence of GABA_A-receptor blocker picrotoxin suggested a higher inhibitory tone in the ventral DG of young mice, confirmed by an increased frequency of miniature inhibitory postsynaptic currents. Our findings support the view that the hippocampus contains discrete functional domains along its dorsoventral axis and demonstrate that these are subject to age-dependent changes. Since these characteristics are presumably conserved in the human hippocampus, our findings have important clinical implications for hippocampus- and age-related disorders.

Gβγ directly modulates vesicle fusion by competing with synaptotagmin for binding to neuronal SNARE proteins embedded in membranes

G_i/o-coupled G protein-coupled receptors can inhibit neurotransmitter release at synapses via multiple mechanisms. In addition to Gβγ-mediated modulation of voltage-gated calcium channels (VGCC), inhibition can also be mediated through the direct interaction of Gβγ subunits with the soluble N-ethylmaleimide attachment protein receptor (SNARE) complex of the vesicle fusion apparatus. Binding studies with soluble SNARE complexes have shown that Gβγ binds to both ternary SNARE complexes, t-SNARE heterodimers, and monomeric SNAREs, competing with synaptotagmin 1(syt1) for binding sites on t-SNARE. However, in secretory cells, Gβγ, SNAREs, and synaptotagmin interact in the lipid environment of a vesicle at the plasma membrane. To approximate this environment, we show that fluorescently labeled Gβγ interacts specifically with lipid-embedded t-SNAREs consisting of full-length syntaxin 1 and SNAP-25B at the membrane, as measured by fluorescence polarization. Fluorescently labeled syt1 undergoes competition with Gβγ for SNARE-binding sites in lipid environments. Mutant Gβγ subunits that were previously shown to be more efficacious at inhibiting Ca²⁺-triggered exocytotic release than wild-type Gβγ were also shown to bind SNAREs at a higher affinity than wild type in a lipid environment. These mutant Gβγ subunits were unable to inhibit VGCC currents. Specific peptides corresponding to regions on Gβ and Gγ shown to be important for the interaction disrupt the interaction in a concentration-dependent manner. In in vitro fusion assays using full-length t- and v-SNAREs embedded in liposomes, Gβγ inhibited Ca²⁺/synaptotagmin-dependent fusion. Together, these studies demonstrate the importance of these regions for the Gβγ-SNARE interaction and show that the target of Gβγ, downstream of VGCC, is the membrane-embedded SNARE complex.

Food restriction induces synaptic incorporation of calcium-permeable AMPA receptors in nucleus accumbens

Chronic food restriction potentiates behavioral and cellular responses to drugs of abuse and D-1 dopamine receptor agonists administered systemically or locally in the nucleus accumbens (NAc). However, the alterations in NAc synaptic transmission underlying these effects are incompletely understood. AMPA receptor trafficking is a major mechanism for regulating synaptic strength, and previous studies have shown that both sucrose and d-amphetamine rapidly alter the abundance of AMPA receptor subunits in the NAc postsynaptic density (PSD) in a manner that differs between food-restricted and ad libitum fed rats. In this study we examined whether food restriction, in the absence of reward stimulus challenge, alters AMPAR subunit abundance in the NAc PSD. Food restriction was found to increase surface expression and, specifically, PSD abundance, of GluA1 but not GluA2, suggesting synaptic incorporation of GluA2-lacking Ca2+-permeable AMPARs (CP-AMPARs). Naspm, an antagonist of CP-AMPARs, decreased the amplitude of evoked EPSCs in NAc shell, and blocked the enhanced locomotor response to local microinjection of the D-1 receptor agonist, SKF-82958, in food-restricted, but not ad libitum fed, subjects. Although microinjection of the D-2 receptor agonist, quinpirole, also induced greater locomotor activation in food-restricted than ad libitum fed rats, this effect was not decreased by Naspm. Taken together, the present findings are consistent with the synaptic incorporation of CP-AMPARs in D-1 receptor-expressing medium spiny neurons in NAc as a mechanistic underpinning of the enhanced responsiveness of food-restricted rats to natural rewards and drugs of abuse.

Mechanisms of Neuronal Silencing After Cortical Spreading Depression

Cortical spreading depression (CSD) is associated with migraine, stroke, and traumatic brain injury, but its mechanisms remain poorly understood. One of the major features of CSD is an hour-long silencing of neuronal activity. Though this silencing has clear ramifications for CSD-associated disease, it has not been fully explained. We used in vivo whole-cell recordings to examine the effects of CSD on layer 2/3 pyramidal neurons in mouse somatosensory cortex and used in vitro recordings to examine their mechanism. We found that CSD caused a reduction in spontaneous synaptic activity and action potential (AP) firing that lasted over an hour. Both pre- and postsynaptic mechanisms contributed to this silencing. Reductions in frequency of postsynaptic potentials were due to a reduction in presynaptic transmitter release probability as well as reduced AP activity. Decreases in postsynaptic potential amplitude were due to an inhibitory shift in the ratio of excitatory and inhibitory postsynaptic currents. This inhibitory shift in turn contributed to the reduced frequency of APs. Thus, distinct but complementary mechanisms generate the long neuronal silence that follows CSD. These cellular changes could contribute to wider network dysfunction in CSD-associated disease, while the pre- and postsynaptic mechanisms offer separate targets for therapy.

Hippocampal GABAergic transmission: a new target for adenosine control of excitability

Physiological network functioning in the hippocampus is dependent on a balance between glutamatergic cell excitability and the activity of diverse local circuit neurons that release the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Tuners of neuronal communication such as adenosine, an endogenous modulator of synapses, control hippocampal network operations by regulating excitability. Evidence has been recently accumulating on the influence of adenosine on different aspects of GABAergic transmission to shape hippocampal function. This review addresses how adenosine, through its high-affinity A₁ (A₁ R) and A_2A receptors (A_2A R), interferes with different GABA-mediated forms of inhibition in the hippocampus to regulate neuronal excitability. Adenosine-mediated modulation of phasic/tonic inhibitory transmission, of GABA transport mechanisms and its interference with other modulatory systems are discussed together with the putative implications for neuronal function in physiological and pathological conditions. This article is part of a mini review series: 'Synaptic Function and Dysfunction in Brain Diseases'.

An Adenosine-Mediated Glial-Neuronal Circuit for Homeostatic Sleep

Sleep homeostasis reflects a centrally mediated drive for sleep, which increases during waking and resolves during subsequent sleep. Here we demonstrate that mice deficient for glial adenosine kinase (AdK), the primary metabolizing enzyme for adenosine (Ado), exhibit enhanced expression of this homeostatic drive by three independent measures: (1) increased rebound of slow-wave activity; (2) increased consolidation of slow-wave sleep; and (3) increased time constant of slow-wave activity decay during an average slow-wave sleep episode, proposed and validated here as a new index for homeostatic sleep drive. Conversely, mice deficient for the neuronal adenosine A1 receptor exhibit significantly decreased sleep drive as judged by these same indices. Neuronal knock-out of AdK did not influence homeostatic sleep need. Together, these findings implicate a glial-neuronal circuit mediated by intercellular Ado, controlling expression of homeostatic sleep drive. Because AdK is tightly regulated by glial metabolic state, our findings suggest a functional link between cellular metabolism and sleep homeostasis.

SIGNIFICANCE STATEMENT

The work presented here provides evidence for an adenosine-mediated regulation of sleep in response to waking (i.e., homeostatic sleep need), requiring activation of neuronal adenosine A1 receptors and controlled by glial adenosine kinase. Adenosine kinase acts as a highly sensitive and important metabolic sensor of the glial ATP/ADP and AMP ratio directly controlling intracellular adenosine concentration. Glial equilibrative adenosine transporters reflect the intracellular concentration to the extracellular milieu to activate neuronal adenosine receptors. Thus, adenosine mediates a glial-neuronal circuit linking glial metabolic state to neural-expressed sleep homeostasis. This indicates a metabolically related function(s) for this glial-neuronal circuit in the buildup and resolution of our need to sleep and suggests potential therapeutic targets more directly related to sleep function.

Mechanisms, pools, and sites of spontaneous vesicle release at synapses of rod and cone photoreceptors

Photoreceptors have depolarized resting potentials that stimulate calcium-dependent release continuously from a large vesicle pool but neurons can also release vesicles without stimulation. We characterized the Ca(2+) dependence, vesicle pools, and release sites involved in spontaneous release at photoreceptor ribbon synapses. In whole-cell recordings from light-adapted horizontal cells (HCs) of tiger salamander retina, we detected miniature excitatory post-synaptic currents (mEPSCs) when no stimulation was applied to promote exocytosis. Blocking Ca(2+) influx by lowering extracellular Ca(2+) , by application of Cd(2+) and other agents reduced the frequency of mEPSCs but did not eliminate them, indicating that mEPSCs can occur independently of Ca(2+) . We also measured release presynaptically from rods and cones by examining quantal glutamate transporter anion currents. Presynaptic quantal event frequency was reduced by Cd(2+) or by increased intracellular Ca(2+) buffering in rods, but not in cones, that were voltage clamped at -70 mV. By inhibiting the vesicle cycle with bafilomycin, we found the frequency of mEPSCs declined more rapidly than the amplitude of evoked excitatory post-synaptic currents (EPSCs) suggesting a possible separation between vesicle pools in evoked and spontaneous exocytosis. We mapped sites of Ca(2+) -independent release using total internal reflectance fluorescence (TIRF) microscopy to visualize fusion of individual vesicles loaded with dextran-conjugated pHrodo. Spontaneous release in rods occurred more frequently at non-ribbon sites than evoked release events. The function of Ca(2+) -independent spontaneous release at continuously active photoreceptor synapses remains unclear, but the low frequency of spontaneous quanta limits their impact on noise.

Presynaptic G Protein-Coupled Receptors Differentially Modulate Spontaneous Glutamate Release in the Supraoptic Nucleus

Spontaneous glutamate release in the supraoptic nucleus is modulated by a number of inhibitory G protein coupled receptors (GPCR), including GABAB , adenosine A1 and group III metabotropic glutamate receptors (mGluR). It remains unclear whether they have distinct roles or are redundant mechanisms that protect from hyperexcitation. To address this question, we facilitated spontaneous glutamate release using nifedipine or forskolin, which act in a protein kinase A (PKA)-independent and -dependent manner, respectively, and tested the effects of inhibitory GPCR agonists. We found that a GABAB receptor (GABAB R) agonist specifically inhibited forskolin-induced miniature excitatory postsynaptic currents (mEPSC), in contrast to an adenosine A1 receptor (A1R) agonist, which specifically inhibited nifedipine-induced mEPSCs. This suggests that GABAB Rs and A1 Rs modulate independent mechanisms activated by forskolin and nifedipine, respectively. However, the inhibitory effects of GABAB R and A1 R agonists on basal mEPSCs occluded each other, suggesting that these receptors also have an overlapping role. Group III mGluRs appear to have a greater control over glutamate release because agonists to these receptors inhibited both nifedipine- and forskolin-induced mEPSCs. mEPSCs induced by norepinephrine had the same characteristics as those induced by forskolin [i.e. PKA-dependence and sensitivity to GABAB R and group III mGluR agonists, but not an A1 R agonist]. In summary, the present study highlights the differential effects of GABAB R, A1 R and mGluR agonists on glutamate release stimulated by different secretagogues, including the endogenous neuromodulator norepinephrine. These results suggest that the roles of these inhibitory GPCRs are not completely redundant, and also indicate the physiological implications of having different excitatory and inhibitory GPCRs on the same synapse.

Ethanol-Induced Cerebellar Ataxia: Cellular and Molecular Mechanisms

The cerebellum is an important target of ethanol toxicity given that cerebellar ataxia is the most consistent physical manifestation of acute ethanol consumption. Despite the significance of the cerebellum in ethanol-induced cerebellar ataxia (EICA), the cellular and molecular mechanisms underlying EICA are incompletely understood. However, two important findings have shed greater light on this phenomenon. First, ethanol-induced blockade of cerebellar adenosine uptake in rodent models points to a role for adenosinergic A1 modulation of EICA. Second, the consistent observation that intracerebellar administration of nicotine in mice leads to antagonism of EICA provides evidence for a critical role of cerebellar nitric oxide (NO) in EICA reversal. Based on these two important findings, this review discusses the potential molecular events at two key synaptic sites (mossy fiber-granule cell-Golgi cell (MGG synaptic site) and granule cell parallel fiber-Purkinje cell (GPP synaptic site) that lead to EICA. Specifically, ethanol-induced neuronal NOS inhibition at the MGG synaptic site acts as a critical trigger for Golgi cell activation which leads to granule cell deafferentation. Concurrently, ethanol-induced inhibition of adenosine uptake at the GPP synaptic site produces adenosine accumulation which decreases glutamate release and leads to the profound activation of Purkinje cells (PCs). These molecular events at the MGG and GPP synaptic sites are mutually reinforcing and lead to cerebellar dysfunction, decreased excitatory output of deep cerebellar nuclei, and EICA. The critical importance of PCs as the sole output of the cerebellar cortex suggests normalization of PC function could have important therapeutic implications.

Presynaptic Adenosine Receptor-Mediated Regulation of Diverse Thalamocortical Short-Term Plasticity in the Mouse Whisker Pathway

Short-term synaptic plasticity (STP) sets the sensitivity of a synapse to incoming activity and determines the temporal patterns that it best transmits. In “driver” thalamocortical (TC) synaptic populations, STP is dominated by depression during stimulation from rest. However, during ongoing stimulation, lemniscal TC connections onto layer 4 neurons in mouse barrel cortex express variable STP. Each synapse responds to input trains with a distinct pattern of depression or facilitation around its mean steady-state response. As a result, in common with other synaptic populations, lemniscal TC synapses express diverse rather than uniform dynamics, allowing for a rich representation of temporally varying stimuli. Here, we show that this STP diversity is regulated presynaptically. Presynaptic adenosine receptors of the A1R type, but not kainate receptors (KARs), modulate STP behavior. Blocking the receptors does not eliminate diversity, indicating that diversity is related to heterogeneous expression of multiple mechanisms in the pathway from presynaptic calcium influx to neurotransmitter release.

Microglia-derived purines modulate mossy fibre synaptic transmission and plasticity through P2X4 and A1 receptors

Recent data have provided evidence that microglia, the brain‐resident macrophage‐like cells, modulate neuronal activity in both physiological and pathophysiological conditions, and microglia are therefore now recognized as synaptic partners. Among different neuromodulators, purines, which are produced and released by microglia, have emerged as promising candidates to mediate interactions between microglia and synapses. The cellular effects of purines are mediated through a large family of receptors for adenosine and for ATP (P2 receptors). These receptors are present at brain synapses, but it is unknown whether they can respond to microglia‐derived purines to modulate synaptic transmission and plasticity. Here, we used a simple model of adding immune‐challenged microglia to mouse hippocampal slices to investigate their impact on synaptic transmission and plasticity at hippocampal mossy fibre (MF) synapses onto CA3 pyramidal neurons. MF–CA3 synapses show prominent forms of presynaptic plasticity that are involved in the encoding and retrieval of memory. We demonstrate that microglia‐derived ATP differentially modulates synaptic transmission and short‐term plasticity at MF–CA3 synapses by acting, respectively, on presynaptic P2X₄ receptors and on adenosine A₁ receptors after conversion of extracellular ATP to adenosine. We also report that P2X₄ receptors are densely located in the mossy fibre tract in the dentate gyrus–CA3 circuitry. In conclusion, this study reveals an interplay between microglia‐derived purines and MF–CA3 synapses, and highlights microglia as potent modulators of presynaptic plasticity.