Modulation of Ca2+ channels by G-protein beta gamma subunits

Delta opioid receptors engage multiple signaling cascades to differentially modulate prefrontal GABA release with input and target specificity

Opioids regulate circuits associated with motivation and reward across the brain. Of the opioid receptor types, delta opioid receptors (DORs) appear to have a unique role in regulating the activity of circuits related to reward without liability for abuse. In neocortex, DORs are expressed primarily in interneurons, including parvalbumin- and somatostatin-expressing interneurons that inhibit somatic and dendritic compartments of excitatory pyramidal cells, respectively. But how DORs regulate transmission from these key interneuron classes is unclear. We found that DORs regulate inhibition from these interneuron classes using different G-protein signaling pathways that both converge on presynaptic calcium channels but regulate distinct aspects of calcium channel function. This imposes different temporal filtering effects, via short-term plasticity, that depend on how calcium channels are regulated. Thus, DORs engage differential signaling cascades to regulate inhibition depending on the postsynaptic target compartment, with different effects on synaptic information transfer in somatic and dendritic domains.

Complex regulation of Cav2.2 N-type Ca2+ channels by Ca2+ and G-proteins

G-protein coupled receptors inhibit Cav2.2 N-type Ca2+ channels by a fast, voltage-dependent pathway mediated by Gαi/Gβγ and a slow, voltage-independent pathway mediated by Gαq-dependent reductions in phosphatidylinositol 4,5-bisphosphate (PIP2) or increases in arachidonic acid. Studies of these forms of regulation generally employ Ba2+ as the permeant ion, despite that Ca2+ -dependent pathways may impinge upon G-protein modulation. To address this possibility, we compared tonic G-protein inhibition of currents carried by Ba2+ (IBa) and Ca2+ (ICa) in HEK293T cells transfected with Cav2.2. Both IBa and ICa exhibited voltage-dependent facilitation (VDF), consistent with Gβγ unbinding from the channel. Compared to that for IBa, VDF of ICa was less sensitive to an inhibitor of Gα proteins (GDP-β-S) and an inhibitor of Gβγ (C-terminal construct of G-protein coupled receptor kinase 2). While insensitive to high intracellular Ca2+ buffering, VDF of ICa that remained in GDP-β-S was blunted by reductions in PIP2. We propose that when G-proteins are inhibited, Ca2+ influx through Cav2.2 promotes a form of VDF that involves PIP2. Our results highlight the complexity whereby Cav2.2 channels integrate G-protein signaling pathways, which may enrich the information encoding potential of chemical synapses in the nervous system.

Muscarinic cannabinoid suppression of excitation, a novel form of coincidence detection

Δ9-tetrahydrocannabinol (THC), the chief psychoactive ingredient of cannabis, acts in the brain primarily via cannabinoid CB1 receptors. These receptors are implicated in several forms of synaptic plasticity - depolarization-induced suppression of excitation (DSE), metabotropic suppression of excitation (MSE), long term depression (LTD) and activation-dependent desensitization. Cultured autaptic hippocampal neurons express all of these, illustrating the rich functional and temporal heterogeneity of CB1 at a single set of synapses. Here we report that coincident activation of muscarinic acetylcholine receptors and elicitation of DSE in autaptic hippocampal neurons results in a substantial (∼40 %) and temporally precise inhibition of excitatory transmission lasting ∼10 minutes. Its induction is blocked by CB1 and muscarinic M3/M5 receptor antagonists and is absent in CB1 receptor knockout neurons. Notably, once it is established, inhibition is reversed by a CB1, but not a muscarinic, antagonist, suggesting that the inhibition occurs via persistent activation of CB1 receptors. We refer to this inhibition as muscarinic cannabinoid suppression of excitation (MCSE). MCSE can be mimicked by coapplication of muscarinic and cannabinoid agonists and requires Ca2+-release from internal stores. As such, MCSE represents a novel and targeted form of coincidence detection - important for many modes of learning and memory -- between cannabinoid and muscarinic signaling systems that elicits a medium-duration depression of synaptic signaling. Given the known roles of muscarinic and cannabinoid receptors in the hippocampus, MCSE may be important in the modulation of hippocampal signaling at the site of septal inputs, with potential implications for learning and memory, epilepsy and addiction.

Activation of G protein gated inwardly rectifying potassium (GIRK) channels in keratinocytes mediates peripheral kappa opioid receptor-mediated antinociception

Kappa opioid receptors (KOR) expressed by peripheral pain-sensing neurons (nociceptors) are a promising target for development of effective and safer analgesics for inflammatory pain that are devoid of central nervous system adverse effects. Here we sought to delineate the signaling pathways that underlie peripheral KOR-mediated antinociception in adult male and female Sprague-Dawley rats. In an inflammatory model of pain, local intraplantar (i.pl.) injection of pertussis toxin prevented antinociception induced by the KOR agonist, U50488, indicating that members of the Gi/o family mediate the antinociceptive response. Furthermore, i.pl. injection of the G protein-coupled inward-rectifying potassium (GIRK) channel blocker, TPNQ, as well as GIRK2 subunit-targeted siRNA abolished U50488-mediated antinociceptive behavioral responses in both male and female rats. Consistent with these data, i.pl. injection of ML297, a direct activator of GIRK1 subunit-containing channels, elicited peripheral antinociceptive behavior. It is well known that intraepidermal nerve fibers (IENF) that innervate the hindpaw propagate nociceptive signals to the spinal cord. However, recent studies suggest that keratinocytes, the major cell type in the epidermis, also play an active role in pain and sensory processing. Results from RT-qPCR, RNAscope and immunohistochemistry experiments confirmed that both KOR and GIRK are expressed in keratinocytes in the epidermal layer of the rat hindpaw. Knockdown of either KOR or GIRK2 subunits selectively in keratinocytes by i.pl. injection of shRNA plasmids, prevented the antinociceptive response to U50488. Taken together, these data suggest that KOR-mediated activation of GIRK channels in keratinocytes is required for peripherally-mediated antinociception.

Chemogenetic silencing reveals presynaptic Gi/o protein-mediated inhibition of developing hippocampal synchrony in vivo

Recent advances in understanding how neuronal activity shapes developing brain circuits increasingly rely on Gi/o-dependent inhibitory chemogenetic tools (Gi-DREADDs). However, their mechanisms of action and efficacy in neurons with immature Gi/o signaling are elusive. Here, we express the Gi-DREADD hM4Di in glutamatergic telencephalic neurons and analyze its impact on CA1 pyramidal neurons in neonatal mice. Using acousto-optic two-photon Ca2+ imaging, we report that activation of hM4Di leads to a complete arrest of spontaneous synchrony in CA1 in vitro. We demonstrate that hM4Di does not cause somatic hyperpolarization or shunting but rather mediates presynaptic silencing of glutamatergic neurotransmission. In vivo, inhibition through hM4Di potently suppresses early sharp waves (eSPWs) and discontinuous oscillatory network activity in CA1 of head-fixed mice before eye opening. Our findings provide insights into the role of Gi/o signaling in synchronized activity in the neonatal hippocampus and bear relevance for applying chemogenetic silencing at early developmental stages.

Delta opioid receptors engage multiple signaling cascades to differentially modulate prefrontal GABA release with input and target specificity

Opioids regulate circuits associated with motivation and reward across the brain. Of the opioid receptor types, delta opioid receptors (DORs) appear to have a unique role in regulating the activity of circuits related to reward without a liability for abuse. In neocortex, DORs are expressed primarily in interneurons, including parvalbumin- and somatostatin-expressing interneurons that inhibit somatic and dendritic compartments of excitatory pyramidal cells, respectively. But how DORs regulate transmission from these key interneuron classes is unclear. We found that DORs regulate inhibition from these interneuron classes using different G-protein signaling pathways that both converge on presynaptic calcium channels, but regulate distinct aspects of calcium channel function. This imposes different temporal filtering effects, via short-term plasticity, that depend on how calcium channels are regulated. Thus, DORs engage differential signaling cascades to regulate inhibition depending on the postsynaptic target compartment, with different effects on synaptic information transfer in somatic and dendritic domains.

Mint/X11 PDZ domains from non-bilaterian animals recognize and bind Ca V 2 calcium channel C-termini in vitro

PDZ domain mediated interactions with voltage-gated calcium (Ca V ) channel C-termini play important roles in localizing membrane Ca 2+ signaling. The first such interaction was described between the scaffolding protein Mint-1 and Ca V 2.2 in mammals. In this study, we show through various in silico analyses that Mint is an animal-specific gene with a highly divergent N-terminus but a strongly conserved C-terminus comprised of a phosphotyrosine binding domain, two tandem PDZ domains (PDZ-1 and PDZ-2), and a C-terminal auto-inhibitory element that binds and inhibits PDZ-1. In addition to Ca V 2 channels, most genes that interact with Mint are also deeply conserved including amyloid precursor proteins, presenilins, neurexin, and CASK and Veli which form a tripartite complex with Mint in bilaterians. Through yeast and bacterial 2-hybrid experiments, we show that Mint and Ca V 2 channels from cnidarians and placozoans interact in vitro , and in situ hybridization revealed co-expression in dissociated neurons from the cnidarian Nematostella vectensis . Unexpectedly, the Mint orthologue from the ctenophore Hormiphora californiensis strongly binds the divergent C-terminal ligands of cnidarian and placozoan Ca V 2 channels, despite neither the ctenophore Mint, nor the placozoan and cnidarian orthologues, binding the ctenophore Ca V 2 channel C-terminus. Altogether, our analyses suggest that the capacity of Mint to bind CaV2 channels predates pre-bilaterian animals, and that evolutionary changes in Ca V 2 channel C-terminal sequences resulted in altered binding modalities with Mint.

Adaptive Changes in Group 2 Metabotropic Glutamate Receptors Underlie the Deficit in Recognition Memory Induced by Methamphetamine in Mice

Cognitive dysfunction is associated with methamphetamine use disorder (MUD). Here, we used genetic and pharmacological approaches to examine the involvement of either Group 2 metabotropic glutamate (mGlu2) or mGlu3 receptors in memory deficit induced by methamphetamine in mice. Methamphetamine treatment (1 mg/kg, i.p., once a day for 5 d followed by 7 d of withdrawal) caused an impaired performance in the novel object recognition test in wild-type mice, but not in mGlu2-/- or mGlu3-/- mice. Memory deficit in wild-type mice challenged with methamphetamine was corrected by systemic treatment with selectively negative allosteric modulators of mGlu2 or mGlu3 receptors (compounds VU6001966 and VU0650786, respectively). Methamphetamine treatment in wild-type mice caused large increases in levels of mGlu2/3 receptors, the Type 3 activator of G-protein signaling (AGS3), Rab3A, and the vesicular glutamate transporter, vGlut1, in the prefrontal cortex (PFC). Methamphetamine did not alter mGlu2/3-mediated inhibition of cAMP formation but abolished the ability of postsynaptic mGlu3 receptors to boost mGlu5 receptor-mediated inositol phospholipid hydrolysis in PFC slices. Remarkably, activation of presynaptic mGlu2/3 receptors did not inhibit but rather amplified depolarization-induced [3H]-D-aspartate release in synaptosomes prepared from the PFC of methamphetamine-treated mice. These findings demonstrate that exposure to methamphetamine causes changes in the expression and function of mGlu2 and mGlu3 receptors, which might alter excitatory synaptic transmission in the PFC and raise the attractive possibility that selective inhibitors of mGlu2 or mGlu3 receptors (or both) may be used to improve cognitive dysfunction in individuals affected by MUD.

Signal Transduction of Transient Receptor Potential TRPM8 Channels: Role of PIP5K, Gq-Proteins, and c-Jun

Transient receptor potential melastatin-8 (TRPM8) is a cation channel that is activated by cold and "cooling agents" such as menthol and icilin, which induce a cold sensation. The stimulation of TRPM8 activates an intracellular signaling cascade that ultimately leads to a change in the gene expression pattern of the cells. Here, we investigate the TRPM8-induced signaling pathway that links TRPM8 channel activation to gene transcription. Using a pharmacological approach, we show that the inhibition of phosphatidylinositol 4-phosphate 5 kinase α (PIP5K), an enzyme essential for the biosynthesis of phosphatidylinositol 4,5-bisphosphate, attenuates TRPM8-induced gene transcription. Analyzing the link between TRPM8 and Gq proteins, we show that the pharmacological inhibition of the βγ subunits impairs TRPM8 signaling. In addition, genetic studies show that TRPM8 requires an activated Gα subunit for signaling. In the nucleus, the TRPM8-induced signaling cascade triggers the activation of the transcription factor AP-1, a complex consisting of a dimer of basic region leucine zipper (bZIP) transcription factors. Here, we identify the bZIP protein c-Jun as an essential component of AP-1 within the TRPM8-induced signaling cascade. In summary, with PIP5K, Gq subunits, and c-Jun, we identified key molecules in TRPM8-induced signaling from the plasma membrane to the nucleus.

G protein βγ regulation of KCNQ-encoded voltage-dependent K channels

The KCNQ family is comprised of five genes and the expression products form voltage-gated potassium channels (Kv7.1-7.5) that have a major impact upon cellular physiology in many cell types. Each functional Kv7 channel forms as a tetramer that often associates with proteins encoded by the KCNE gene family (KCNE1-5) and is critically reliant upon binding of phosphatidylinositol bisphosphate (PIP2) and calmodulin. Other modulators like A-kinase anchoring proteins, ubiquitin ligases and Ca-calmodulin kinase II alter Kv7 channel function and trafficking in an isoform specific manner. It has now been identified that for Kv7.4, G protein βγ subunits (Gβγ) can be added to the list of key regulators and is paramount for channel activity. This article provides an overview of this nascent field of research, highlighting themes and directions for future study.

Caged Zn2+ Photolysis in Zebrafish Whole Brains Reveals Subsecond Modulation of Dopamine Uptake

Free, ionic zinc (Zn2+) modulates neurotransmitter dynamics in the brain. However, the sub-s effects of transient concentration changes of Zn2+ on neurotransmitter release and uptake are not well understood. To address this lack of knowledge, we have combined the photolysis of the novel caged Zn2+ compound [Zn(DPAdeCageOMe)]+ with fast scan cyclic voltammetry (FSCV) at carbon fiber microelectrodes in live, whole brain preparations from zebrafish (Danio rerio). After treating the brain with [Zn(DPAdeCageOMe)]+, Zn2+ was released by application of light that was gated through a computer-controlled shutter synchronized with the FSCV measurements and delivered through a 1 mm fiber optic cable. We systematically optimized the photocage concentration and light application parameters, including the total duration and light-to-electrical stimulation delay time. While sub-s Zn2+ application with this method inhibited DA reuptake, assessed by the first-order rate constant (k) and half-life (t1/2), it had no effect on the electrically stimulated DA overflow ([DA]STIM). Increasing the photocage concentration and light duration progressively inhibited uptake, with maximal effects occurring at 100 μM and 800 ms, respectively. Furthermore, uptake was inhibited 200 ms after Zn2+ photorelease, but no measurable effect occurred after 800 ms. We expect that application of this method to the zebrafish whole brain and other preparations will help expand the current knowledge of how Zn2+ affects neurotransmitter release/uptake in select neurological disease states.

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

A sign-inverted receptive field of inhibitory interneurons provides a pathway for ON-OFF interactions in the retina

A fundamental organizing plan of the retina is that visual information is divided into ON and OFF streams that are processed in separate layers. This functional dichotomy originates in the ON and OFF bipolar cells, which then make excitatory glutamatergic synapses onto amacrine and ganglion cells in the inner plexiform layer. We have identified an amacrine cell (AC), the sign-inverting (SI) AC, that challenges this fundamental plan. The glycinergic, ON-stratifying SI-AC has OFF light responses. In opposition to the classical wiring diagrams, it receives inhibitory inputs from glutamatergic ON bipolar cells at mGluR8 synapses, and excitatory inputs from an OFF wide-field AC at electrical synapses. This "inhibitory ON center - excitatory OFF surround" receptive-field of the SI-AC allows it to use monostratified dendrites to conduct crossover inhibition and push-pull activation to enhance light detection by ACs and RGCs in the dark and feature discrimination in the light.

G-protein tonic inhibition of calcium channels in pancreatic β-cells

Voltage-gated calcium channels (CaV) conduct Ca2+ influx promoting neurotransmitters and hormone release. CaV are finely regulated by voltage-dependent and independent pathways either by G-protein-coupled receptors (GPCRs) or intramembrane lipids, respectively, in neurons and glands. Interestingly, pancreatic β-cells are abundantly innervated by both sympathetic and parasympathetic neurons, while a variety of high-voltage activated (HVA) Ca2+ channels are present in these cells. Thus, autonomic system seems to exert a tonic inhibition on HVA Ca2+ channels throughout GPCRs, constitutively preventing hormone secretion. Therefore, this work aimed to investigate noradrenergic and cholinergic inhibition of HVA Ca2+ channels in pancreatic β-cells. Experiments were conducted in pancreatic β-cells of rat by using patch-clamping methods, immunocytochemistry, pharmacological probes, and biochemical reagents. A voltage-clamp protocol with a strong depolarizing prepulse was used to unmask tonic inhibition. Herein, we consistently find a basal tonic inhibition of HVA Ca2+ channels according to a GPCRs regulation. Facilitation ratio is enhanced by noradrenaline (NA) according to a voltage-dependent regulation and a membrane-delimited mechanism, while no facilitation changes are observed with carbachol or phosphatidylinositol 4,5-bisphosphate (PIP2). Furthermore, carbachol or intramembrane lipids, such as PIP2, do not change facilitation ratio according to a voltage-independent regulation. Together, HVA Ca2+ channels of pancreatic β-cells are constitutively inhibited by GPCRs, suggesting a natural brake preventing cells from exhaustive insulin secretion.NEW & NOTEWORTHY Our results support the hypothesis that GPCRs tonically inhibit HVA Ca2+ channels in pancreatic β-cells. A voltage-clamp protocol with a strong depolarizing prepulse was used to unmask voltage-dependent inhibition of Ca2+ channels. The novelty of these results strengthens the critical role of Gβγ's in Ca2+ channel regulation, highlighting kinetic slowing and increased facilitation ratio. Together, HVA Ca2+ channels of pancreatic β-cells are constitutively inhibited by GPCRs underlying fine-tuning modulation of insulin secretion.

GABABR silencing of nerve terminals

Control of neurotransmission efficacy is central to theories of how the brain computes and stores information. Presynaptic G-protein coupled receptors (GPCRs) are critical in this problem as they locally influence synaptic strength and can operate on a wide range of time scales. Among the mechanisms by which GPCRs impact neurotransmission is by inhibiting voltage-gated calcium (Ca2+) influx in the active zone. Here, using quantitative analysis of both single bouton Ca2+ influx and exocytosis, we uncovered an unexpected non-linear relationship between the magnitude of action potential driven Ca2+ influx and the concentration of external Ca2+ ([Ca2+]e). We find that this unexpected relationship is leveraged by GPCR signaling when operating at the nominal physiological set point for [Ca2+]e, 1.2 mM, to achieve complete silencing of nerve terminals. These data imply that the information throughput in neural circuits can be readily modulated in an all-or-none fashion at the single synapse level when operating at the physiological set point.

Key differences in regulation of opioid receptors localized to presynaptic terminals compared to somas: Relevance for novel therapeutics

Opioid receptors are G protein-coupled receptors (GPCRs) that regulate activity within peripheral, subcortical and cortical circuits involved in pain, reward, and aversion processing. Opioid receptors are expressed in both presynaptic terminals where they inhibit neurotransmitter release and postsynaptic locations where they act to hyperpolarize neurons and reduce activity. Agonist activation of postsynaptic receptors at the plasma membrane signal via ion channels or cytoplasmic second messengers. Agonist binding initiates regulatory processes that include phosphorylation by G protein receptor kinases (GRKs) and recruitment of beta-arrestins that desensitize and internalize the receptors. Opioid receptors also couple to effectors from endosomes activating intracellular enzymes and kinases. In contrast to postsynaptic opioid receptors, receptors localized to presynaptic terminals are resistant to desensitization such that there is no loss of signaling in the continuous presence of opioids over the same time scale. Thus, the balance of opioid signaling in circuits expressing pre- and postsynaptic opioid receptors is shifted toward inhibition of presynaptic neurotransmitter release during continuous opioid exposure. The functional implication of this shift is not often acknowledged in behavioral studies. This review covers what is currently understood about regulation of opioid/nociceptin receptors, with an emphasis on opioid receptor signaling in pain and reward circuits. Importantly, the review covers regulation of presynaptic receptors and the critical gaps in understanding this area, as well as the opportunities to further understand opioid signaling in brain circuits. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Cholinergic deficits selectively boost cortical intratelencephalic control of striatum in male Huntington's disease model mice

Huntington's disease (HD) is a progressive, neurodegenerative disease caused by a CAG triplet expansion in huntingtin. Although corticostriatal dysfunction has long been implicated in HD, the determinants and pathway specificity of this pathophysiology are not fully understood. Here, using a male zQ175^+/- knock-in mouse model of HD we carry out optogenetic interrogation of intratelencephalic and pyramidal tract synapses with principal striatal spiny projection neurons (SPNs). These studies reveal that the connectivity of intratelencephalic, but not pyramidal tract, neurons with direct and indirect pathway SPNs increased in early symptomatic zQ175^+/- HD mice. This enhancement was attributable to reduced pre-synaptic inhibitory control of intratelencephalic terminals by striatal cholinergic interneurons. Lowering mutant huntingtin selectively in striatal cholinergic interneurons with a virally-delivered zinc finger repressor protein normalized striatal acetylcholine release and intratelencephalic functional connectivity, revealing a node in the network underlying corticostriatal pathophysiology in a HD mouse model.

N-type calcium channel blockers: a new approach towards the treatment of chronic neuropathic pain

Neuropathic pain (NP) remains maltreated for a wide number of patients by the currently available treatments and little research has been done in finding new drugs for treating NP. Ziconotide (PrialtTM) had been developed as the new drug, which belongs to the class of ω-conotoxin MVIIA. It inhibits N-type calcium channels. Ziconotide is under the last phase of the clinical trial, a new non-narcotic drug for the management of NP. Synthetically it has shown the similarities with ω-conotoxin MVIIA, a constituent of poison found in fish hunting snails (Conus magus). Ziconotide acts by selectively blocking neural N-type voltage-sensitized Ca2+ channels (NVSCCs). Certain herbal drugs also have been studied but no clinical result is there and the study is only limited to preclinical data. This review emphasizes the N-type calcium channel inhibitors, and their mechanisms for blocking calcium channels with their remedial prospects for treating chronic NP.

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

Regulation of N-type calcium channels by nociceptin receptors and its possible role in neurological disorders

Activation of nociceptin opioid peptide receptors (NOP, a.k.a. opioid-like receptor-1, ORL-1) by the ligand nociceptin/orphanin FQ, leads to G protein-dependent regulation of Cav2.2 (N-type) voltage-gated calcium channels (VGCCs). This typically causes a reduction in calcium currents, triggering changes in presynaptic calcium levels and thus neurotransmission. Because of the widespread expression patterns of NOP and VGCCs across multiple brain regions, the dorsal horn of the spinal cord, and the dorsal root ganglia, this results in the alteration of numerous neurophysiological features. Here we review the regulation of N-type calcium channels by the NOP-nociceptin system in the context of neurological conditions such as anxiety, addiction, and pain.

Functional Insights into Protein Kinase A (PKA) Signaling from C. elegans

Protein kinase A (PKA), which regulates a diverse set of biological functions downstream of cyclic AMP (cAMP), is a tetramer consisting of two catalytic subunits (PKA-C) and two regulatory subunits (PKA-R). When cAMP binds the PKA-R subunits, the PKA-C subunits are released and interact with downstream effectors. In Caenorhabditis elegans (C. elegans), PKA-C and PKA-R are encoded by kin-1 and kin-2, respectively. This review focuses on the contributions of work in C. elegans to our understanding of the many roles of PKA, including contractility and oocyte maturation in the reproductive system, lipid metabolism, physiology, mitochondrial function and lifespan, and a wide variety of behaviors. C. elegans provides a powerful genetic platform for understanding how this kinase can regulate an astounding variety of physiological responses.

Involvement of Ca2+ in Signaling Mechanisms Mediating Muscarinic Inhibition of M Currents in Sympathetic Neurons

Acetylcholine can excite neurons by suppressing M-type (KCNQ) potassium channels. This effect is mediated by M1 muscarinic receptors coupled to the Gq protein. Although PIP2 depletion and PKC activation have been strongly suggested to contribute to muscarinic inhibition of M currents (IM), direct evidence is lacking. We investigated the mechanism involved in muscarinic inhibition of IM with Ca2+ measurement and electrophysiological studies in both neuronal (rat sympathetic neurons) and heterologous (HEK cells expressing KCNQ2/KCNQ3) preparations. We found that muscarinic inhibition of IM was not blocked either by PIP2 or by calphostin C, a PKC inhibitor. We then examined whether muscarinic inhibition of IM uses multiple signaling pathways by blocking both PIP2 depletion and PKC activation. This maneuver, however, did not block muscarinic inhibition of IM. Additionally, muscarinic inhibition of IM was not prevented either by sequestering of G-protein βγ subunits from Gα-transducin or anti-Gβγ antibody or by preventing intracellular trafficking of channel proteins with blebbistatin, a class-II myosin inhibitor. Finally, we re-examined the role of Ca2+ signals in muscarinic inhibition of IM. Ca2+ measurements showed that muscarinic stimulation increased intracellular Ca2+ and was comparable to the Ca2+ mobilizing effect of bradykinin. Accordingly, 20-mM of BAPTA significantly suppressed muscarinic inhibition of IM. In contrast, muscarinic inhibition of IM was completely insensitive to 20-mM EGTA. Taken together, these data suggest a role of Ca2+ signaling in muscarinic modulation of IM. The differential effects of EGTA and BAPTA imply that Ca2+ microdomains or spatially local Ca2+ signals contribute to inhibition of IM.

NaV1.9 current in muscle afferent neurons is enhanced by substances released during muscle activity

Skeletal muscle contraction triggers the exercise pressor reflex (EPR) to regulate the cardiovascular system response to exercise. During muscle contraction, substances are released that generate action potential activity in group III and IV afferents that mediate the EPR. Some of these substances increase afferent activity via G-protein-coupled receptor (GPCR) activation, but the mechanisms are incompletely understood. We were interested in determining if tetrodotoxin-resistant (TTX-R) voltage-dependent sodium channels (NaV) were involved and investigated the effect of a mixture of such compounds (bradykinin, prostaglandin, norepinephrine, and ATP, called muscle metabolites). Using whole cell patch-clamp electrophysiology, we show that the muscle metabolites significantly increased TTX-R NaV currents. The rise time of this enhancement averaged ∼2 min, which suggests the involvement of a diffusible second messenger pathway. The effect of muscle metabolites on the current-voltage relationship, channel activation and inactivation kinetics support NaV1.9 channels as the target for this enhancement. When applied individually at the concentration used in the mixture, only prostaglandin and bradykinin significantly enhanced NaV current, but the sum of these enhancements was <1/3 that observed when the muscle metabolites were applied together. This suggests synergism between the activated GPCRs to enhance NaV1.9 current. When applied at a higher concentration, all four substances could enhance the current, which demonstrates that the GPCRs activated by each metabolite can enhance channel activity. The enhancement of NaV1.9 channel activity is a likely mechanism by which GPCR activation increases action potential activity in afferents generating the EPR.NEW & NOTEWORTHY G-protein-coupled receptor (GPCR) activation increases action potential activity in muscle afferents to produce the exercise pressor reflex (EPR), but the mechanisms are incompletely understood. We provide evidence that NaV1.9 current is synergistically enhanced by application of a mixture of metabolites potentially released during muscle contraction. The enhancement of NaV1.9 current is likely one mechanism by which GPCR activation generates the EPR and the inappropriate activation of the EPR in patients with cardiovascular disease.

Implications of a Neuronal Receptor Family, Metabotropic Glutamate Receptors, in Cancer Development and Progression

Cancer is the second leading cause of death, and incidences are increasing globally. Simply defined, cancer is the uncontrolled proliferation of a cell, and depending on the tissue of origin, the cancer etiology, biology, progression, prognosis, and treatment will differ. Carcinogenesis and its progression are associated with genetic factors that can either be inherited and/or acquired and are classified as an oncogene or tumor suppressor. Many of these genetic factors converge on common signaling pathway(s), such as the MAPK and PI3K/AKT pathways. In this review, we will focus on the metabotropic glutamate receptor (mGluR) family, an upstream protein that transmits extracellular signals into the cell and has been shown to regulate many aspects of tumor development and progression. We explore the involvement of members of this receptor family in various cancers that include breast cancer, colorectal cancer, glioma, kidney cancer, melanoma, oral cancer, osteosarcoma, pancreatic cancer, prostate cancer, and T-cell cancers. Intriguingly, depending on the member, mGluRs can either be classified as oncogenes or tumor suppressors, although in general most act as an oncogene. The extensive work done to elucidate the role of mGluRs in various cancers suggests that it might be a viable strategy to therapeutically target glutamatergic signaling.

Optogenetics at the presynapse

Optogenetic actuators enable highly precise spatiotemporal interrogation of biological processes at levels ranging from the subcellular to cells, circuits and behaving organisms. Although their application in neuroscience has traditionally focused on the control of spiking activity at the somatodendritic level, the scope of optogenetic modulators for direct manipulation of presynaptic functions is growing. Presynaptically localized opsins combined with light stimulation at the terminals allow light-mediated neurotransmitter release, presynaptic inhibition, induction of synaptic plasticity and specific manipulation of individual components of the presynaptic machinery. Here, we describe presynaptic applications of optogenetic tools in the context of the unique cell biology of axonal terminals, discuss their potential shortcomings and outline future directions for this rapidly developing research area.

A Molecular Landscape of Mouse Hippocampal Neuromodulation

Adaptive neuronal circuit function requires a continual adjustment of synaptic network parameters known as “neuromodulation.” This process is now understood to be based primarily on the binding of myriad secreted “modulatory” ligands such as dopamine, serotonin and the neuropeptides to G protein-coupled receptors (GPCRs) that, in turn, regulate the function of the ion channels that establish synaptic weights and membrane excitability. Many of the basic molecular mechanisms of neuromodulation are now known, but the organization of neuromodulation at a network level is still an enigma. New single-cell RNA sequencing data and transcriptomic neurotaxonomies now offer bright new lights to shine on this critical “dark matter” of neuroscience. Here we leverage these advances to explore the cell-type-specific expression of genes encoding GPCRs, modulatory ligands, ion channels and intervening signal transduction molecules in mouse hippocampus area CA1, with the goal of revealing broad outlines of this well-studied brain structure’s neuromodulatory network architecture.

Local modulation by presynaptic receptors controls neuronal communication and behaviour

Central nervous system neurons communicate via fast synaptic transmission mediated by ligand-gated ion channel (LGIC) receptors and slower neuromodulation mediated by G protein-coupled receptors (GPCRs). These receptors influence many neuronal functions, including presynaptic neurotransmitter release. Presynaptic LGIC and GPCR activation by locally released neurotransmitters influences neuronal communication in ways that modify effects of somatic action potentials. Although much is known about presynaptic receptors and their mechanisms of action, less is known about when and where these receptor actions alter release, especially in vivo. This Review focuses on emerging evidence for important local presynaptic receptor actions and ideas for future studies in this area.

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.

How changes in dopamine D2 receptor levels alter striatal circuit function and motivation

It was first posited, more than five decades ago, that the etiology of schizophrenia involves overstimulation of dopamine receptors. Since then, advanced clinical research methods, including brain imaging, have refined our understanding of the relationship between striatal dopamine and clinical phenotypes as well as disease trajectory. These studies point to striatal dopamine D2 receptors, the main target for all current antipsychotic medications, as being involved in both positive and negative symptoms. Simultaneously, animal models have been central to investigating causal relationships between striatal dopamine D2 receptors and behavioral phenotypes relevant to schizophrenia. We begin this article by reviewing the circuit, cell-type and subcellular locations of dopamine D2 receptors and their downstream signaling pathways. We then summarize results from several mouse models in which D2 receptor levels were altered in various brain regions, cell-types and developmental periods. Behavioral, electrophysiological and anatomical consequences of these D2 receptor perturbations are reviewed with a selective focus on striatal circuit function and alterations in motivated behavior, a core negative symptom of schizophrenia. These studies show that D2 receptors serve distinct physiological roles in different cell types and at different developmental time points, regulating motivated behaviors in sometimes opposing ways. We conclude by considering the clinical implications of this complex regulation of striatal circuit function by D2 receptors.

TT2 controls rice thermotolerance through SCT1-dependent alteration of wax biosynthesis

Global warming threatens crop production. G proteins mediate plant responses to multiple abiotic stresses. Here we identified a natural quantitative trait locus, TT2 (THEROMOTOLERANCE 2), encoding a Gγ subunit, that confers thermotolerance in rice during both vegetative and reproductive growth without a yield penalty. A natural allele with loss of TT2 function was associated with greater retention of wax at high temperatures and increased thermotolerance. Mechanistically, we found that a transcription factor, SCT1 (Sensing Ca²⁺ Transcription factor 1), functions to decode Ca²⁺ through Ca²⁺-enhanced interaction with calmodulin and acts as a negative regulator of its target genes (for example, Wax Synthesis Regulatory 2 (OsWR2)). The calmodulin-SCT1 interaction was attenuated by reduced heat-triggered Ca²⁺ caused by disrupted TT2, thus explaining the observed heat-induced changes in wax content. Beyond establishing a bridge linking G protein, Ca²⁺ sensing and wax metabolism, our study illustrates innovative approaches for developing potentially yield-penalty-free thermotolerant crop varieties.

A Life of Biophysics

Biophysics is a way of approaching biological problems through numbers, physical laws, models, and quantitative logic. In a long scientific career, I have seen the formation and fruition of the ion channel concept through biophysical study. Marvelous discoveries were made as our instruments evolved from vacuum tubes to transistors; computers evolved from the size of an entire building to a few chips inside our instruments; and genome sequencing, gene expression, and atom-level structural biology became accessible to all laboratories. Science is rewarding and exhilarating. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Opioid Receptor Regulation of Neuronal Voltage-Gated Calcium Channels

Neuronal voltage-gated calcium channels play a pivotal role in the conversion of electrical signals into calcium entry into nerve endings that is required for the release of neurotransmitters. They are under the control of a number of cellular signaling pathways that serve to fine tune synaptic activities, including G-protein coupled receptors (GPCRs) and the opioid system. Besides modulating channel activity via activation of second messengers, GPCRs also physically associate with calcium channels to regulate their function and expression at the plasma membrane. In this mini review, we discuss the mechanisms by which calcium channels are regulated by classical opioid and nociceptin receptors. We highlight the importance of this regulation in the control of neuronal functions and their implication in the development of disease conditions. Finally, we present recent literature concerning the use of novel μ-opioid receptor/nociceptin receptor modulators and discuss their use as potential drug candidates for the treatment of pain.

A photoswitchable GPCR-based opsin for presynaptic inhibition

Optical manipulations of genetically defined cell types have generated significant insights into the dynamics of neural circuits. While optogenetic activation has been relatively straightforward, rapid and reversible synaptic inhibition has proven more elusive. Here, we leveraged the natural ability of inhibitory presynaptic GPCRs to suppress synaptic transmission and characterize parapinopsin (PPO) as a GPCR-based opsin for terminal inhibition. PPO is a photoswitchable opsin that couples to Gi/o signaling cascades and is rapidly activated by pulsed blue light, switched off with amber light, and effective for repeated, prolonged, and reversible inhibition. PPO rapidly and reversibly inhibits glutamate, GABA, and dopamine release at presynaptic terminals. Furthermore, PPO alters reward behaviors in a time-locked and reversible manner in vivo. These results demonstrate that PPO fills a significant gap in the neuroscience toolkit for rapid and reversible synaptic inhibition and has broad utility for spatiotemporal control of inhibitory GPCR signaling cascades.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Presynaptic Gαo (GOA-1) signals to depress command neuron excitability and allow stretch-dependent modulation of egg laying in Caenorhabditis elegans

Egg laying in the nematode worm Caenorhabditis elegans is a two-state behavior modulated by internal and external sensory input. We have previously shown that homeostatic feedback of embryo accumulation in the uterus regulates bursting activity of the serotonergic HSN command neurons that sustains the egg-laying active state. How sensory feedback of egg release signals to terminate the egg-laying active state is less understood. We find that Gαo, a conserved Pertussis Toxin-sensitive G protein, signals within HSN to inhibit egg-laying circuit activity and prevent entry into the active state. Gαo signaling hyperpolarizes HSN, reducing HSN Ca2+ activity and input onto the postsynaptic vulval muscles. Loss of inhibitory Gαo signaling uncouples presynaptic HSN activity from a postsynaptic, stretch-dependent homeostat, causing precocious entry into the egg-laying active state when only a few eggs are present in the uterus. Feedback of vulval opening and egg release activates the uv1 neuroendocrine cells which release NLP-7 neuropeptides which signal to inhibit egg laying through Gαo-independent mechanisms in the HSNs and Gαo-dependent mechanisms in cells other than the HSNs. Thus, neuropeptide and inhibitory Gαo signaling maintains a bi-stable state of electrical excitability that dynamically controls circuit activity in response to both external and internal sensory input to drive a two-state behavior output.

Distribution of functional opioid receptors in human dorsal root ganglion neurons

Preclinical evidence has highlighted the importance of the μ-opioid peptide (MOP) receptor on primary afferents for both the analgesic actions of MOP receptor agonists, as well as the development of tolerance, if not opioid-induced hyperalgesia. There is also growing interest in targeting other opioid peptide receptor subtypes (δ-opioid peptide [DOP], κ-opioid peptide [KOP], and nociceptin/orphanin-FQ opioid peptide [NOP]) on primary afferents, as alternatives to MOP receptors, which may not be associated with as many deleterious side effects. Nevertheless, results from several recent studies of human sensory neurons indicate that although there are many similarities between rodent and human sensory neurons, there may also be important differences. Thus, the purpose of this study was to assess the distribution of opioid receptor subtypes among human sensory neurons. A combination of pharmacology, patch-clamp electrophysiology, Ca imaging, and single-cell semiquantitative polymerase chain reaction was used. Our results suggest that functional MOP-like receptors are present in approximately 50% of human dorsal root ganglion neurons. δ-opioid peptide-like receptors were detected in a subpopulation largely overlapping that with MOP-like receptors. Furthermore, KOP-like and NOP-like receptors are detected in a large proportion (44% and 40%, respectively) of human dorsal root ganglion neurons with KOP receptors also overlapping with MOP receptors at a high rate (83%). Our data confirm that all 4 opioid receptor subtypes are present and functional in human sensory neurons, where the overlap of DOP, KOP, and NOP receptors with MOP receptors suggests that activation of these other opioid receptor subtypes may also have analgesic efficacy.

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.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Biased ligands at opioid receptors: Current status and future directions

The opioid crisis represents a major worldwide public health crisis that has accelerated the search for safer and more effective opioids. Over the past few years, the identification of biased opioid ligands capable of eliciting selective functional responses has provided an alternative avenue to develop novel therapeutics without the side effects of current opioid medications. However, whether biased agonism or other pharmacological properties, such as partial agonism (or low efficacy), account for the therapeutic benefits remains questionable. Here, we provide a summary of the current status of biased opioid ligands that target the μ- and κ-opioid receptors and highlight advances in preclinical and clinical trials of some of these ligands. We also discuss an example of structure-based biased ligand discovery at the μ-opioid receptor, an approach that could revolutionize drug discovery at opioid and other receptors. Last, we briefly discuss caveats and future directions for this important area of research.

Drebrin regulates cytoskeleton dynamics in migrating neurons through interaction with CXCR4

Stromal cell-derived factor-1 (SDF-1) and chemokine receptor type 4 (CXCR4) are regulators of neuronal migration (e.g., GnRH neurons, cortical neurons, and hippocampal granule cells). However, how SDF-1/CXCR4 alters cytoskeletal components remains unclear. Developmentally regulated brain protein (drebrin) stabilizes actin polymerization, interacts with microtubule plus ends, and has been proposed to directly interact with CXCR4 in T cells. The current study examined, in mice, whether CXCR4 under SDF-1 stimulation interacts with drebrin to facilitate neuronal migration. Bioinformatic prediction of protein-protein interaction highlighted binding sites between drebrin and crystallized CXCR4. In migrating GnRH neurons, drebrin, CXCR4, and the microtubule plus-end binding protein EB1 were localized close to the cell membrane. Coimmunoprecipitation (co-IP) confirmed a direct interaction between drebrin and CXCR4 using wild-type E14.5 whole head and a GnRH cell line. Analysis of drebrin knockout (DBN1 KO) mice showed delayed migration of GnRH cells into the brain. A decrease in hippocampal granule cells was also detected, and co-IP confirmed a direct interaction between drebrin and CXCR4 in PN4 hippocampi. Migration assays on primary neurons established that inhibiting drebrin (either pharmacologically or using cells from DBN1 KO mice) prevented the effects of SDF-1 on neuronal movement. Bioinformatic prediction then identified binding sites between drebrin and the microtubule plus end protein, EB1, and super-resolution microscopy revealed decreased EB1 and drebrin coexpression after drebrin inhibition. Together, these data show a mechanism by which a chemokine, via a membrane receptor, communicates with the intracellular cytoskeleton in migrating neurons during central nervous system development.

Serotonergic Modulation of Spontaneous and Evoked Transmitter Release in Layer II Pyramidal Cells of Rat Somatosensory Cortex

As axons from the raphe nuclei densely innervate the somatosensory cortex, we investigated how serotonin (5-HT) modulates transmitter release in layer II pyramidal cells of rat barrel cortex. In the presence of tetrodotoxin and gabazine, 10 μM 5-HT caused a waxing and waning in the frequency of miniature excitatory postsynaptic currents (mEPSC) with no effect on amplitude. Specifically, within 15 min of recording the mEPSC frequency initially increased by 28 ± 7%, then dropped to below control (-15 ± 3%), before resurging back to 27 ± 7% larger than control. These changes were seen in 47% of pyramidal cells (responders) and were mediated by 5-HT2C receptors (5-HT2CR). Waxing resulted from phospholipase C activation, IP3 production, and Ca2+ release from presynaptic stores. Waning was prevented if PKC was blocked. In contrast, in paired recordings, the unitary EPSC amplitude was reduced by 50 ± 3% after 5-HT exposure in almost all cases with no significant effect on paired-pulse ratio and synaptic dynamics. This sustained EPSC reduction was also caused by 5-HT2R, but was mediated by presynaptic Gβγ subunits likely limiting influx through CaV2 channels. EPSC reduction, together with enhanced spontaneous noise in a restricted subset of inputs, could temporarily diminish the signal-to-noise ratio and affect the computation in the neocortical microcircuit.

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.

Functions of Presynaptic Voltage-gated Calcium Channels

Voltage-gated calcium channels are the principal conduits for depolarization-mediated Ca²⁺ entry into excitable cells. In this review, the biophysical properties of the relevant members of this family of channels, those that are present in presynaptic terminals, will be discussed in relation to their function in mediating neurotransmitter release. Voltage-gated calcium channels have properties that ensure they are specialized for particular roles, for example, differences in their activation voltage threshold, their various kinetic properties, and their voltage-dependence of inactivation. All these attributes play into the ability of the various voltage-gated calcium channels to participate in different patterns of presynaptic vesicular release. These include synaptic transmission resulting from single action potentials, and longer-term changes mediated by bursts or trains of action potentials, as well as release resulting from graded changes in membrane potential in specialized sensory synapses.

Epigenetics and the endocannabinoid system signaling: An intricate interplay modulating neurodevelopment

The endocannabinoid (eCB) system is a complex system comprising endogenous cannabinoids (eCBs), their synthesis and degradation enzymes, and cannabinoid receptors. These elements crucially regulate several biological processes during neurodevelopment, such as proliferation, differentiation, and migration. Recently, eCBs were also reported to have an epigenetic action on genes that play key functions in the neurotransmitter signaling, consequently regulating their expression. In turn, epigenetic modifications (e.g. DNA methylation, histone modifications) may also modulate the function of eCB system's elements. For example, the expression of the cnr gene in the central nervous system may be epigenetically regulated (e.g. DNA methylation, histone modifications), thus altering the function of the cannabinoid receptor type-1 (CB₁R). Considering the importance of the eCB system during neurodevelopment, it is thus reasonable to expect that alterations in this interaction between the eCB system and epigenetic modifications may give rise to neurodevelopmental disorders. Here, we review key concepts related to the regulation of neuronal function by the eCB system and the different types of epigenetic modifications. In particular, we focus on the mechanisms involved in the intricate interplay between both signaling systems and how they control cell fate during neurodevelopment. Noteworthy, such mechanistic understanding assumes high relevance considering the implications of the dysregulation of key neurogenic processes towards the onset of neurodevelopment-related disorders. Moreover, considering the increasing popularity of cannabis and its synthetic derivatives among young adults, it becomes of utmost importance to understand how exogenous cannabinoids may epigenetically impact neurodevelopment.

Modulation of voltage-gated CaV2.2 Ca2+ channels by newly identified interaction partners

Voltage-gated Ca²⁺ channels are typically integrated in a complex network of protein-protein-interactions, also referred to as Ca²⁺ channel nanodomains. Amongst the neuronal Ca_V2 channel family, Ca_V2.2 is of particular importance due to its general role for signal transmission from the periphery to the central nervous system, but also due to its significance for pain perception. Thus, Ca_V2.2 is an ideal target candidate to search for pharmacological inhibitors but also for novel modulatory interactors. In this review we summarize the last years findings of our intense screenings and characterization of the six Ca_V2.2 interaction partners, tetraspanin-13 (TSPAN-13), reticulon 1 (RTN1), member 1 of solute carrier family 38 (SLC38), prostaglandin D2 synthase (PTGDS), transmembrane protein 223 (TMEM223), and transmembrane BAX inhibitor motif 3 (Grina/TMBIM3) containing protein. Each protein shows a unique way of channel modulation as shown by extensive electrophysiological studies. Amongst the newly identified interactors, Grina/TMBIM3 is most striking due to its modulatory effect which is rather comparable to G-protein regulation.

Arrestin recruitment to dopamine D2 receptor mediates locomotion but not incentive motivation

The dopamine (DA) D2 receptor (D2R) is an important target for the treatment of neuropsychiatric disorders such as schizophrenia and Parkinson's disease. However, the development of improved therapeutic strategies has been hampered by our incomplete understanding of this receptor's downstream signaling processes in vivo and how these relate to the desired and undesired effects of drugs. D2R is a G protein-coupled receptor (GPCR) that activates G protein-dependent as well as non-canonical arrestin-dependent signaling pathways. Whether these effector pathways act alone or in concert to facilitate specific D2R-dependent behaviors is unclear. Here, we report on the development of a D2R mutant that recruits arrestin but is devoid of G protein activity. When expressed virally in "indirect pathway" medium spiny neurons (iMSNs) in the ventral striatum of D2R knockout mice, this mutant restored basal locomotor activity and cocaine-induced locomotor activity in a manner indistinguishable from wild-type D2R, indicating that arrestin recruitment can drive locomotion in the absence of D2R-mediated G protein signaling. In contrast, incentive motivation was enhanced only by wild-type D2R, signifying a dissociation in the mechanisms that underlie distinct D2R-dependent behaviors, and opening the door to more targeted therapeutics.

Mechanisms and Regulation of Neuronal GABAB Receptor-Dependent Signaling

γ-Aminobutyric acid B receptors (GABA_BRs) are broadly expressed throughout the central nervous system where they play an important role in regulating neuronal excitability and synaptic transmission. GABA_BRs are G protein-coupled receptors that mediate slow and sustained inhibitory actions via modulation of several downstream effector enzymes and ion channels. GABA_BRs are obligate heterodimers that associate with diverse arrays of proteins to form modular complexes that carry out distinct physiological functions. GABA_BR-dependent signaling is fine-tuned and regulated through a multitude of mechanisms that are relevant to physiological and pathophysiological states. This review summarizes the current knowledge on GABA_BR signal transduction and discusses key factors that influence the strength and sensitivity of GABA_BR-dependent signaling in neurons.

Gα/GSA-1 works upstream of PKA/KIN-1 to regulate calcium signaling and contractility in the Caenorhabditis elegans spermatheca

Correct regulation of cell contractility is critical for the function of many biological systems. The reproductive system of the hermaphroditic nematode C. elegans contains a contractile tube of myoepithelial cells known as the spermatheca, which stores sperm and is the site of oocyte fertilization. Regulated contraction of the spermatheca pushes the embryo into the uterus. Cell contractility in the spermatheca is dependent on actin and myosin and is regulated, in part, by Ca2+ signaling through the phospholipase PLC-1, which mediates Ca2+ release from the endoplasmic reticulum. Here, we describe a novel role for GSA-1/Gαs, and protein kinase A, composed of the catalytic subunit KIN-1/PKA-C and the regulatory subunit KIN-2/PKA-R, in the regulation of Ca2+ release and contractility in the C. elegans spermatheca. Without GSA-1/Gαs or KIN-1/PKA-C, Ca2+ is not released, and oocytes become trapped in the spermatheca. Conversely, when PKA is activated through either a gain of function allele in GSA-1 (GSA-1(GF)) or by depletion of KIN-2/PKA-R, the transit times and total numbers, although not frequencies, of Ca2+ pulses are increased, and Ca2+ propagates across the spermatheca even in the absence of oocyte entry. In the spermathecal-uterine valve, loss of GSA-1/Gαs or KIN-1/PKA-C results in sustained, high levels of Ca2+ and a loss of coordination between the spermathecal bag and sp-ut valve. Additionally, we show that depleting phosphodiesterase PDE-6 levels alters contractility and Ca2+ dynamics in the spermatheca, and that the GPB-1 and GPB-2 Gβ subunits play a central role in regulating spermathecal contractility and Ca2+ signaling. This work identifies a signaling network in which Ca2+ and cAMP pathways work together to coordinate spermathecal contractions for successful ovulations.

Sexual Dimorphism in Stress-induced Hyperthermia in SNAP25Δ3 mice, a mouse model with disabled Gβγ regulation of the exocytotic fusion apparatus

Behavioral assays in the mouse can show marked differences between male and female animals of a given genotype. These differences identified in such preclinical studies may have important clinical implications. We recently made a mouse model with impaired presynaptic inhibition through Gβγ-SNARE signaling. Here, we examine the role of sexual dimorphism in the severity of the phenotypes of this model, the SNAP25Δ3 mouse. In males, we already reported that SNAP25Δ3 homozygotes demonstrated phenotypes in motor coordination, nociception, spatial memory and stress processing. We now report that while minimal sexually dimorphic effects were observed for the nociceptive, motor or memory phenotypes, large differences were observed in the stress-induced hyperthermia paradigm, with male SNAP25Δ3 homozygotes exhibiting an increase in body temperature subsequent to handling relative to wild-type littermates, while no such genotype-dependent effect was observed in females. This suggests sexually dimorphic mechanisms of Gβγ-SNARE signaling for stress processing or thermoregulation within the mouse. Second, we examined the effects of heterozygosity with respect to the SNAP25Δ3 mutation. Heterozygote SNAP25Δ3 animals were tested alongside homozygote and wild-type littermates in all of the aforementioned paradigms and displayed phenotypes similar to wild-type animals or an intermediate state. From this, we conclude that the SNAP25Δ3 mutation does not behave in an autosomal dominant manner, but rather displays incomplete dominance for many phenotypes.