A common molecular basis for exogenous and endogenous cannabinoid potentiation of glycine receptors

A millisecond coarse-grained simulation approach to decipher allosteric cannabinoid binding at the glycine receptor α1

Glycine receptors (GlyR) are regulated by small-molecule binding at several allosteric sites. Cannabinoids like tetrahydrocannabinol (THC) and N-arachidonyl-ethanol-amide (AEA) potentiate the GlyR response but their mechanism of action is not fully established. By combining millisecond coarse-grained (CG) MD simulations powered by Martini 3 with backmapping to all-atom representations, we have characterized the cannabinoid-binding site(s) at the zebrafish GlyR-α1 active state with atomic resolution. Based on hundreds of thousand ligand-binding events, we find that cannabinoids bind to the transmembrane domain of the receptor at both intrasubunit and intersubunit sites. For THC, the intrasubunit binding mode predicted in simulation is in excellent agreement with recent cryo-EM structures, while intersubunit binding recapitulates in full previous mutagenesis experiments. Intriguingly, AEA is predicted to bind at the same intersubunit site despite the strikingly different chemistry. Statistical analyses of the ligand-receptor interactions highlight potentially relevant residues for GlyR potentiation, offering experimentally testable predictions. The predictions for AEA have been validated by electrophysiology recordings of rationally designed mutants. The results highlight the existence of multiple cannabinoid-binding sites for the allosteric regulation of GlyR and put forward an effective strategy for the identification and structural characterization of allosteric binding sites.

Structural Basis for Molecular Recognition of Cannabinoids by Inhibitory Cys-Loop Channels

Cannabis sativa has a long history of medicinal use, dating back to ancient times. This plant produces cannabinoids, which are now known to interact with several human proteins, including Cys-loop receptors for glycine (GlyR) and gamma-aminobutyric acid (GABAAR). As these channels are the primary mediators of inhibitory signals, they contribute to the diverse effects of cannabinoids on the nervous system. Evidence suggests that cannabinoid binding sites are located within the transmembrane domain, although their precise location has remained undetermined for over a decade. The process of identification of the binding site and the computational approaches employed are the main subjects of this Perspective, which includes an analysis of the most recently resolved cryo-EM structures of zebrafish GlyR bound to Δ9-tetrahydrocannabinol and the THC-GlyR complex obtained through molecular dynamics simulations. With this work, we aim to contribute to guiding future studies investigating the molecular basis of cannabinoid action on inhibitory channels.

Deformable Vesicles with Edge Activators for the Transdermal Delivery of Non-Psychoactive Cannabinoids

BACKGROUND

Transdermal delivery of highly lipophilic molecules is challenging due to the strong barrier function of the skin. Vesicles with penetration enhancers are safe and efficient systems that could improve the transdermal delivery of non-psychoactive cannabinoids such as cannabidiol and desoxy-cannabidiol. In the last decades, research interest in desoxy-cannabidiol as a potent drug with anti-nociceptive properties has risen. Still, its scarce market availability poses a limit for both research and clinical applications. Therefore, it is necessary to improve the synthesis to produce sufficient amounts of desoxy-cannabidiol. Moreover, also the formulation aspects for this drug are challenging and require to be addressed to meet an efficient delivery to the patients.

OBJECTIVE

This work aimed to develop innovative phospholipid-based vesicles with propylene glycol (PG), oleic acid (OA), or limonene as edge activators, for the transdermal delivery of highly lipophilic drugs such as non-psychoactive cannabinoids. In particular, desoxy-cannabidiol was selected thanks to its anti-nociceptive activity, and its synthesis was improved enhancing the stereoselectivity of its synthon's production.

METHODS

Desoxy-cannabidiol was synthesized by Lewis acid-mediated condensation of p-mentha-2,8-dien- 1-ol and m-pentylphenol, improving the stereoselectivity of the first synthon's production. Transethosomes containing 20-50% w/w PG, 0.4-0.8% w/w OA, or 0.1-1% w/w limonene were optimized and loaded with cannabidiol or desoxy-cannabidiol (0.07-0.8% w/w, 0.6-7.0 mg/mL). Ex-vivo studies were performed to assess both the skin permeation and accumulation of the cannabinoids, as well as the penetration depth of fluorescein- loaded systems used as models.

RESULTS

An enantioselective bromination was added to the pathway, thus raising the production yield of pmentha- 2,8-dien-1-ol to 81% against 35%, and the overall yield of desoxy-cannabidiol synthesis from 12% to 48%. Optimized transethosomes containing 0.6 mg/mL cannabinoids were prepared with 1:10 PG:lipid weight ratio, 0.54 OA:lipid molar ratio, and 0.3 limonene:lipid molar ratio, showing good nanometric size (208 ± 20.8 nm - 321 ± 26.3 nm) and entrapment efficiency (> 80%). Ex-vivo tests showed both improved skin permeation rates of cannabinoids (up to 21.32 ± 4.27 μg/cm2 cannabidiol), and skin penetration (depth of fluorescein up to 240 μm, with PG).

CONCLUSION

Desoxy-cannabidiol was successfully produced at high yields, and formulated into transethosomes optimized for transdermal delivery. Loaded vesicles showed improved skin penetration of desoxy-cannabidiol, cannabidiol and a lipophilic probe. These results suggest the potential of these carriers for the transdermal delivery of highly lipophilic drugs.

THC and CBD: Similarities and differences between siblings

Δ9-tetrahydrocannabinol (THC) and its sibling, cannabidiol (CBD), are produced by the same Cannabis plant and have similar chemical structures but differ dramatically in their mechanisms of action and effects on brain functions. Both THC and CBD exhibit promising therapeutic properties; however, impairments and increased incidence of mental health diseases are associated with acute and chronic THC use, respectively, and significant side effects are associated with chronic use of high-dose CBD. This review covers recent molecular and preclinical discoveries concerning the distinct mechanisms of action and bioactivities of THC and CBD and their impact on human behavior and diseases. These discoveries provide a foundation for the development of cannabinoid-based therapeutics for multiple devastating diseases and to assure their safe use in the growing legal market of Cannabis-based products.

Cannabidiol as a modulator of α7 nicotinic receptors

Cannabidiol (CBD), an important terpenoid compound from marijuana with no psychoactive effects, has become of great pharmaceutical interest for several health conditions. As CBD is a multitarget drug, there is a need to establish the molecular mechanisms by which CBD may exert therapeutic as well as adverse effects. The α7 nicotinic acetylcholine receptor (α7 nAChR) is a cation-permeable ACh-gated channel present in the nervous system and in non-neuronal cells. It is involved in different pathological conditions, including neurological and neurodegenerative disorders, inflammation, and cancer. By high-resolution single-channel recordings and confocal microscopy, we here reveal how CBD modulates α7 nAChR ionotropic and metabotropic functions. CBD leads to a profound concentration-dependent decrease of α7 nAChR single-channel activity with an IC50 in the sub-micromolar range. The inhibition of α7 nAChR activity, which takes place through a membrane pathway, is neither mediated by receptor phosphorylation nor overcome by positive allosteric modulators and is compatible with CBD stabilization of resting or desensitized α7 nAChR conformational states. CBD modulation is complex as it also leads to the later appearance of atypical, low-frequency α7 nAChR channel openings. At the cellular level, CBD inhibits the increase in intracellular calcium triggered by α7 nAChR activation, thus decreasing cell calcium responses. The modulation of α7 nAChR is of pharmacological relevance and should be considered in the evaluation of CBD potential therapeutic uses. Thus, our study provides novel molecular information of CBD multiple actions and targets, which is required to set the basis for prospective applications in human health.

Effects of cannabinoids on ligand-gated ion channels

Phytocannabinoids such as Δ⁹-tetrahydrocannabinol and cannabidiol, endocannabinoids such as N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, and synthetic cannabinoids such as CP47,497 and JWH-018 constitute major groups of structurally diverse cannabinoids. Along with these cannabinoids, CB1 and CB2 cannabinoid receptors and enzymes involved in synthesis and degradation of endocannabinoids comprise the major components of the cannabinoid system. Although, cannabinoid receptors are known to be involved in anti-convulsant, anti-nociceptive, anti-psychotic, anti-emetic, and anti-oxidant effects of cannabinoids, in recent years, an increasing number of studies suggest that, at pharmacologically relevant concentrations, these compounds interact with several molecular targets including G-protein coupled receptors, ion channels, and enzymes in a cannabinoid-receptor independent manner. In this report, the direct actions of endo-, phyto-, and synthetic cannabinoids on the functional properties of ligand-gated ion channels and the plausible mechanisms mediating these effects were reviewed and discussed.

Positive Allosteric Modulators of Glycine Receptors and Their Potential Use in Pain Therapies

Glycine receptors are ligand-gated ion channels that mediate synaptic inhibition throughout the mammalian spinal cord, brainstem, and higher brain regions. They have recently emerged as promising targets for novel pain therapies due to their ability to produce antinociception by inhibiting nociceptive signals within the dorsal horn of the spinal cord. This has greatly enhanced the interest in developing positive allosteric modulators of glycine receptors. Several pharmaceutical companies and research facilities have attempted to identify new therapeutic leads by conducting large-scale screens of compound libraries, screening new derivatives from natural sources, or synthesizing novel compounds that mimic endogenous compounds with antinociceptive activity. Advances in structural techniques have also led to the publication of multiple high-resolution structures of the receptor, highlighting novel allosteric binding sites and providing additional information for previously identified binding sites. This has greatly enhanced our understanding of the functional properties of glycine receptors and expanded the structure activity relationships of novel pharmacophores. Despite this, glycine receptors are yet to be used as drug targets due to the difficulties in obtaining potent, selective modulators with favorable pharmacokinetic profiles that are devoid of side effects. This review presents a summary of the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. SIGNIFICANCE STATEMENT: Chronic pain is a major cause of disability, and in Western societies, this will only increase as the population ages. Despite the high level of prevalence and enormous socioeconomic burden incurred, treatment of chronic pain remains limited as it is often refractory to current analgesics, such as opioids. The National Institute for Drug Abuse has set finding effective, safe, nonaddictive strategies to manage chronic pain as their top priority. Positive allosteric modulators of glycine receptors may provide a therapeutic option.

Combined alcohol and cannabinoid exposure leads to synergistic toxicity by affecting cerebellar Purkinje cells

Combined use of cannabis and alcohol results in greater psychoactive toxicity than either substance alone, but the underlying central mechanisms behind this worsened outcome remain unclear. Here we show that the synergistic effect of Δ⁹-tetrahydrocannabinol (THC) and ethanol on motor incoordination in mice is achieved by activating presynaptic type 1 cannabinoid receptors (CB₁R) and potentiating extrasynaptic glycine receptors (GlyR) within cerebellar Purkinje cells (PCs). The combination of ethanol and THC significantly reduces miniature excitatory postsynaptic current frequency in a CB₁R-dependent manner, while increasing the extrasynaptic GlyR-mediated chronic chloride current, both leading to decreased PC activity. Ethanol enhances THC actions by boosting the blood-brain-barrier permeability of THC and enriching THC in the cell membrane. Di-desoxy-THC, a designed compound that specifically disrupts THC-GlyR interaction without affecting the basic functions of CB₁R and GlyR, is able to restore PC function and motor coordination in mice. Our findings provide potential therapeutic strategies for overcoming the synergistic toxicity caused by combining cannabis and alcohol use.

Structural basis for cannabinoid-induced potentiation of alpha1-glycine receptors in lipid nanodiscs

Nociception and motor coordination are critically governed by glycine receptor (GlyR) function at inhibitory synapses. Consequentially, GlyRs are attractive targets in the management of chronic pain and in the treatment of several neurological disorders. High-resolution mechanistic details of GlyR function and its modulation are just emerging. While it has been known that cannabinoids such as Δ9-tetrahydrocannabinol (THC), the principal psychoactive constituent in marijuana, potentiate GlyR in the therapeutically relevant concentration range, the molecular mechanism underlying this effect is still not understood. Here, we present Cryo-EM structures of full-length GlyR reconstituted into lipid nanodisc in complex with THC under varying concentrations of glycine. The GlyR-THC complexes are captured in multiple conformational states that reveal the basis for THC-mediated potentiation, manifested as different extents of opening at the level of the channel pore. Taken together, these structural findings, combined with molecular dynamics simulations and functional analysis, provide insights into the potential THC binding site and the allosteric coupling to the channel pore.

Considerations for Cannabinoids in Perioperative Care by Anesthesiologists

Increased usage of recreational and medically indicated cannabinoid compounds has been an undeniable reality for anesthesiologists in recent years. These compounds’ complicated pharmacology, composition, and biological effects result in challenging issues for anesthesiologists during different phases of perioperative care. Here, we review the existing formulation of cannabinoids and their biological activity to put them into the context of the anesthesia plan execution. Perioperative considerations should include a way to gauge the patient’s intake of cannabinoids, the ability to gain consent properly, and vigilance to the increased risk of pulmonary and airway problems. Intraoperative management in individuals with cannabinoid use is complicated by the effects cannabinoids have on general anesthetics and depth of anesthesia monitoring while simultaneously increasing the potential occurrence of intraoperative hemodynamic instability. Postoperative planning should involve higher vigilance to the risk of postoperative strokes and acute coronary syndromes. However, most of the data are not up to date, rending definite conclusions on the importance of perioperative cannabinoid intake on anesthesia management difficult.

Understanding the complex pharmacology of cannabidiol: Mounting evidence suggests a common binding site with cholesterol

Cannabidiol is claimed to bind to a large number of protein targets based on in vitro assays. This suggests opportunities for a wide range of therapeutic applications. On the other hand, the existence of phytochemical 'nuisance compounds' suggests some measure of caution - these compounds are capable of altering membrane biophysical properties and changing protein function without directly contacting a binding site. Like cannabidiol, cholesterol alters membrane properties, but it also binds directly to membrane proteins through abundant cholesterol recognition sites. We present the evidence that cannabidiol and cholesterol may bind to the same site on some proteins. As a starting point for further research, we also used blind docking to show that cannabidiol binds to a cholesterol binding site on the CB1 receptor. Elucidation of the mechanism(s) of action of cannabidiol will assist the prioritisation of in vitro hits across targets, improve the success rate of medicinal chemistry campaigns, and ultimately benefit patient populations by focusing resources on programs with the most translational potential.

Membrane cholesterol dependence of cannabinoid modulation of glycine receptor

Cannabinoids exert therapeutic effects on several diseases such as chronic pain and startle disease by targeting glycine receptors (GlyRs). Our previous studies have shown that cannabinoids target a serine residue at position 296 in the third transmembrane helix of the α1/α3 GlyR. This site is located on the outside of the ion channel protein at the lipid interface where the cholesterol concentrates. However, whether membrane cholesterol regulates cannabinoid-GlyR interaction remains unknown. Here, we show that GlyRs are closely associated with cholesterol/caveolin-rich domains at subcellular levels. Membrane cholesterol reduction significantly inhibits cannabinoid potentiation of glycine-activated currents in cultured spinal neurons and in HEK 293T cells expressing α1/α3 GlyRs. Such inhibition is fully rescued by cholesterol replenishment in a concentration-dependent manner. Molecular docking calculations further reveal that cholesterol regulates cannabinoid enhancement of GlyR function through both direct and indirect mechanisms. Taken together, these findings suggest that cholesterol is critical for the cannabinoid-GlyR interaction in the cell membrane.

The synthetic cannabinoid dehydroxylcannabidiol restores the function of a major GABAA receptor isoform in a cell model of hyperekplexia

The functions of the glycine receptor (GlyR) and GABAA receptor (GABAAR) are both impaired in hyperekplexia, a neurological disorder usually caused by GlyR mutations. Although emerging evidence indicates that cannabinoids can directly restore normal GlyR function, whether they affect GABAAR in hyperekplexia remains unknown. Here we show that dehydroxylcannabidiol (DH-CBD), a synthetic nonpsychoactive cannabinoid, restores the GABA- and glycine-activated currents (IGABA and IGly , respectively) in HEK293 cells coexpressing a major GABAAR isoform (α1β2γ2) and GlyRα1 carrying a human hyperekplexia-associated mutation (GlyRα1R271Q). Using coimmunoprecipitation and FRET assays, we found that DH-CBD disrupts the protein interaction between GABAAR and GlyRα1R271Q Furthermore, a point mutation of GlyRα1, changing Ser-296 to Ala-296, which is critical for cannabinoid binding on GlyR, significantly blocked DH-CBD-induced restoration of IGABA and IGly currents. This S296A substitution also considerably attenuated DH-CBD-induced disruption of the interaction between GlyRα1R271Q and GABAAR. These findings suggest that, because it restores the functions of both GlyRα1 and GABAAR, DH-CBD may represent a potentially valuable candidate drug to manage hyperekplexia.

Dark Classics in Chemical Neuroscience: Δ9-Tetrahydrocannabinol

Cannabis ( Cannabis sativa) is the most widely used illicit drug in the world, with an estimated 192 million users globally. The main psychoactive component of cannabis is (-)- trans-Δ⁹-tetrahydrocannabinol (Δ⁹-THC), a compound with a diverse range of pharmacological actions. The unique and distinctive intoxication caused by Δ⁹-THC primarily reflects partial agonist action at central cannabinoid type 1 (CB₁) receptors. Δ⁹-THC is an approved therapeutic treatment for a range of conditions, including chronic pain, chemotherapy-induced nausea and vomiting, and multiple sclerosis, and is being investigated in indications such as anorexia nervosa, agitation in dementia, and Tourette's syndrome. It is available as a regulated pharmaceutical in products such as Marinol, Sativex, and Namisol as well as in an ever-increasing range of unregistered medicinal and recreational cannabis products. While cannabis is an ancient medicament, contemporary use is embroiled in legal, scientific, and social controversy, much of which relates to the potential hazards and benefits of Δ⁹-THC itself. Robust contemporary debate surrounds the therapeutic value of Δ⁹-THC in different diseases, its capacity to produce psychosis and cognitive impairment, and the addictive and "gateway" potential of the drug. This review will provide a profile of the chemistry, pharmacology, and therapeutic uses of Δ⁹-THC as well as the historical and societal import of this unique, distinctive, and ubiquitous psychoactive substance.

Glycine receptors and glycine transporters: targets for novel analgesics?

Glycinergic neurotransmission has long been known for its role in spinal motor control. During the last two decades, additional functions have become increasingly recognized-among them is a critical contribution to spinal pain processing. Studies in rodent pain models provide proof-of-concept evidence that enhancing inhibitory glycinergic neurotransmission reduces chronic pain symptoms. Apparent strategies for pharmacological intervention include positive allosteric modulators of glycine receptors and modulators or inhibitors of the glial and neuronal glycine transporters GlyT1 and GlyT2. These prospects have led to drug discovery efforts in academia and in industry aiming at compounds that target glycinergic neurotransmission with high specificity. Available data show promising analgesic efficacy. Less is currently known about potential unwanted effects but the presence of glycinergic innervation in CNS areas outside the nociceptive system prompts for a careful evaluation not only of motor function, but also of potential respiratory impairment and addictive properties.

Involvement of glycine receptor α1 subunits in cannabinoid-induced analgesia

Some cannabinoids have been shown to suppress chronic pain by targeting glycine receptors (GlyRs). Although cannabinoid potentiation of α3 GlyRs is thought to contribute to cannabinoid-induced analgesia, the role of cannabinoid potentiation of α1 GlyRs in cannabinoid suppression of chronic pain remains unclear. Here we report that dehydroxylcannabidiol (DH-CBD), a nonpsychoactive cannabinoid, significantly suppresses chronic inflammatory pain caused by noxious heat stimulation. This effect may involve spinal α1 GlyRs since the expression level of α1 subunits in the spinal cord is positively correlated with CFA-induced inflammatory pain and the GlyRs antagonist strychnine blocks the DH-CBD-induced analgesia. A point-mutation of S296A in TM3 of α1 GlyRs significantly inhibits DH-CBD potentiation of glycine currents (I_Gly) in HEK-293 cells and neurons in lamina I-II of spinal cord slices. To explore the in vivo consequence of DH-CBD potentiation of α1 GlyRs, we generated a GlyRα1^S296A knock-in mouse line. We observed that DH-CBD-induced potentiation of I_Gly and analgesia for inflammatory pain was absent in GlyRα1^S296A knock-in mice. These findings suggest that spinal α1 GlyR is a potential target for cannabinoid analgesia in chronic inflammatory pain.

CB1 Receptor Signaling in the Brain: Extracting Specificity from Ubiquity

Endocannabinoids (eCBs) are amongst the most ubiquitous signaling molecules in the nervous system. Over the past few decades, observations based on a large volume of work, first examining the pharmacological effects of exogenous cannabinoids, and then the physiological functions of eCBs, have directly challenged long-held and dogmatic views about communication, plasticity and behavior in the central nervous system (CNS). The eCBs and their cognate cannabinoid receptors exhibit a number of unique properties that distinguish them from the widely studied classical amino-acid transmitters, neuropeptides, and catecholamines. Although we now have a loose set of mechanistic rules based on experimental findings, new studies continue to reveal that our understanding of the eCB system (ECS) is continuously evolving and challenging long-held conventions. Here we will briefly summarize findings on the current canonical view of the 'ECS' and will address novel aspects that reveal how a nearly ubiquitous system can determine highly specific functions in the brain. In particular, we will focus on findings that push for an expansion of our ideas around long-held beliefs about eCB signaling that, while clearly true, may be contributing to an oversimplified perspective on how cannabinoid signaling at the microscopic level impacts behavior at the macroscopic level.

CB1 Receptors Mediated Inhibition of ATP-Induced [Ca2+]i Increase in Cultured Rat Spinal Dorsal Horn Neurons

Spinal cannabinoid receptor 1 (CB₁R) and purinergic P2X receptors (P2XR) play a critical role in the process of pathological pain. Both CB₁R and P2XR are expressed in spinal dorsal horn (DH) neurons. It is not clear whether CB₁ receptor activation modulates the function of P2X receptor channels within dorsal horn. For this reason, we observed the effect of CP55940 (cannabinoid receptor agonist) on ATP-induced Ca²⁺ mobilization in cultured rat DH neurons. The changes of intracellular calcium concentration ([Ca²⁺]i) were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescent indicator. 100 μM ATP caused [Ca²⁺]i increase in cultured DH neurons. ATP-evoked [Ca²⁺]i increase in DH neurons was blocked by chelating extracellular Ca²⁺ and P2 purinoceptor antagonist PPADS. At the same time, ATP-γ-S (a non-hydrolyzable ATP analogue) mimicked the ATP action, while P2Y receptor agonist ADP failed to evoke [Ca²⁺]i increase in cultured DH neurons. These data suggest that ATP-induced [Ca²⁺]i elevation in cultured DH neurons is mediated by P2X receptor. Subsequently, we noticed that, in cultured rat DH neurons, ATP-induced Ca²⁺ mobilization was inhibited after pretreated with CP55940 with a concentration-dependent manner, which implies that the opening of P2X receptor channels are down-regulated by activation of cannabinoid receptor. The inhibitory effect of CP55940 on ATP-induced Ca²⁺ response was mimicked by ACEA (CB₁R agonist), but was not influenced by AM1241 (CB₂R agonist). Moreover, the inhibitory effect of CP55940 on ATP-induced Ca²⁺ mobilization was blocked by AM251 (CB₁ receptor antagonist), but was not influenced by AM630 (CB₂ receptor antagonist). In addition, we also observed that forskolin (an activator of adenylate cyclase) and 8-Br-cAMP (a cell-permeable cAMP analog) reversed the inhibitory effect of CP55940, respectively. In a summary, our observations raise a possibility that CB₁R rather than CB₂R can downregulate the opening of P2X receptor channels in DH neurons. The reduction of cAMP/PKA signaling is a key element in the inhibitory effect of CB₁R on P2X-channel-induced Ca²⁺ mobilization.

Recent advances in managing chronic prostatitis/chronic pelvic pain syndrome

Chronic prostatitis/chronic pelvic pain syndrome is a common disorder seen in men under the age of 50 and has a considerable negative impact on quality of life; it is a complex and difficult condition to treat, owing to its wide symptomatology. In order to effectively treat this condition, the UPOINT system was developed: it allows clinical profiling of a patient's symptoms into six broad categories (urinary symptoms, psychological dysfunction, organ-specific symptoms, infectious causes, neurologic dysfunction, and tenderness of the pelvic floor muscles) to allow individualized and multimodal therapy. In this review, we present the most recent advancements in the treatment of chronic prostatitis/chronic pelvic pain syndrome from the past few years.

Modulation of Glycine-Mediated Spinal Neurotransmission for the Treatment of Chronic Pain

Chronic pain constitutes a significant and expanding worldwide health crisis. Currently available analgesics poorly serve individuals suffering from chronic pain, and new therapeutic agents that are more effective, safer, and devoid of abuse liabilities are desperately needed. Among the myriad of cellular and molecular processes contributing to chronic pain, spinal disinhibition of pain signaling to higher cortical centers plays a critical role. Accumulating evidence shows that glycinergic inhibitory neurotransmission in the spinal cord dorsal horn gates nociceptive signaling, is essential in maintaining physiological pain sensitivity, and is diminished in pathological pain states. Thus, it is hypothesized that agents capable of enhancing glycinergic tone within the dorsal horn could obtund nociceptor signaling to the brain and serve as analgesics for persistent pain. This Perspective highlights the potential that pharmacotherapies capable of increasing inhibitory spinal glycinergic neurotransmission hold in providing new and transformative analgesic therapies for the treatment of chronic pain.

New approaches to target glycinergic neurotransmission for the treatment of chronic pain

Inhibitory glycinergic neurotransmission in the spinal cord dorsal horn plays an important role in regulating nociceptive signalling by inhibiting neuronal excitation. Blocking glycinergic transmission in the dorsal horn causes normally innocuous stimuli to become painful (allodynia) and increases sensitivity to noxious stimuli (hyperalgesia). Loss of inhibitory signalling is thought to contribute to the development of pathological pain. Management of neuropathic pain with current therapeutics is challenging and there is a great need for more effective treatments. Preclinical studies using drugs that increase glycinergic signalling by potentiating glycine receptor activity or inhibiting transporter activity suggest that targeting this system is a good therapeutic strategy. The spatially restricted expression of glycine receptors and transporters is an advantage for targeting specific pathologies such as pain. However, until recently there have been few pharmacological modulators identified and most of which do not specifically target glycinergic signalling. This mini-review provides an overview of recent advances in the development of therapeutics and novel approaches that aim to increase glycinergic neurotransmission for the treatment of persistent pain.

From Phytocannabinoids to Cannabinoid Receptors and Endocannabinoids: Pleiotropic Physiological and Pathological Roles Through Complex Pharmacology

Apart from having been used and misused for at least four millennia for, among others, recreational and medicinal purposes, the cannabis plant and its most peculiar chemical components, the plant cannabinoids (phytocannabinoids), have the merit to have led humanity to discover one of the most intriguing and pleiotropic endogenous signaling systems, the endocannabinoid system (ECS). This review article aims to describe and critically discuss, in the most comprehensive possible manner, the multifaceted aspects of 1) the pharmacology and potential impact on mammalian physiology of all major phytocannabinoids, and not only of the most famous one Δ9-tetrahydrocannabinol, and 2) the adaptive pro-homeostatic physiological, or maladaptive pathological, roles of the ECS in mammalian cells, tissues, and organs. In doing so, we have respected the chronological order of the milestones of the millennial route from medicinal/recreational cannabis to the ECS and beyond, as it is now clear that some of the early steps in this long path, which were originally neglected, are becoming important again. The emerging picture is rather complex, but still supports the belief that more important discoveries on human physiology, and new therapies, might come in the future from new knowledge in this field.

The importance of botanical treatments in traditional societies and challenges in developing countries

Epilepsy is one of the most common neurological conditions worldwide, with many affected persons found in Asia, Latin America, and sub-Saharan Africa. Relatedly, the large majority found in these regions does not receive the appropriate therapy with antiepileptic drugs (AEDs), stemming from various reasons among which are lack of access to AEDs, social stigma, and negative cultural attitudes. The presence of epilepsy resistant to the available AEDs coupled with the frequent AED side effects has further fueled the widespread and growing use of botanicals as alternative therapy in several traditional societies in these developing countries since people with epilepsy (PWE) consider them as safe and effective. There have, however, been few botanicals that have been examined for their pharmacological activities related to traditional uses, and there is hardly any conclusive evidence regarding their efficacy in humans or knowledge about the exact mechanism(s) of action. This review discusses some botanical treatments that have been used for epilepsy in developing countries and the challenges faced. This article is part of a Special Issue entitled "Botanicals for Epilepsy".

Selective potentiation of alpha 1 glycine receptors by ginkgolic acid

Glycine receptors (GlyRs) belong to the superfamily of pentameric cys-loop receptor-operated channels and are involved in numerous physiological functions, including movement, vision, and pain. In search for compounds performing subunit-specific modulation of GlyRs we studied action of ginkgolic acid, an abundant Ginkgo biloba product. Using patch-clamp recordings, we analyzed the effects of ginkgolic acid in concentrations from 30 nM to 25 μM on α1–α3 and α1/β, α2/β configurations of GlyR and on GABA_ARs expressed in cultured CHO-K1 cells and mouse neuroblastoma (N2a) cells. Ginkgolic acid caused an increase in the amplitude of currents mediated by homomeric α1 and heteromeric α1/β GlyRs and provoked a left-shift of the concentration-dependent curves for glycine. Even at high concentrations (10–25 μM) ginkgolic acid was not able to augment ionic currents mediated by α2, α2/β, and α3 GlyRs, or by GABA_AR consisting of α1/β2/γ2 subunits. Mutation of three residues (T59A/A261G/A303S) in the α2 GlyR subunit to the corresponding ones from the α1 converted the action of ginkgolic acid to potentiation with a distinct decrease in EC₅₀ for glycine, suggesting an important role for these residues in modulation by ginkgolic acid. Our results suggest that ginkgolic acid is a novel selective enhancer of α1 GlyRs.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

Ethanol effects on glycinergic transmission: From molecular pharmacology to behavior responses

It is well accepted that ethanol is able to produce major health and economic problems associated to its abuse. Because of its intoxicating and addictive properties, it is necessary to analyze its effect in the central nervous system. However, we are only now learning about the mechanisms controlling the modification of important membrane proteins such as ligand-activated ion channels by ethanol. Furthermore, only recently are these effects being correlated to behavioral changes. Current studies show that the glycine receptor (GlyR) is a susceptible target for low concentrations of ethanol (5-40mM). GlyRs are relevant for the effects of ethanol because they are found in the spinal cord and brain stem where they primarily express the α1 subunit. More recently, the presence of GlyRs was described in higher regions, such as the hippocampus and nucleus accumbens, with a prevalence of α2/α3 subunits. Here, we review data on the following aspects of ethanol effects on GlyRs: (1) direct interaction of ethanol with amino acids in the extracellular or transmembrane domains, and indirect mechanisms through the activation of signal transduction pathways; (2) analysis of α2 and α3 subunits having different sensitivities to ethanol which allows the identification of structural requirements for ethanol modulation present in the intracellular domain and C-terminal region; (3) Genetically modified knock-in mice for α1 GlyRs that have an impaired interaction with G protein and demonstrate reduced ethanol sensitivity without changes in glycinergic transmission; and (4) GlyRs as potential therapeutic targets.

Gelsemium analgesia and the spinal glycine receptor/allopregnanolone pathway

Gelsemium, a small genus of flowering plant from the family Loganiaceae, comprises five species including the popular Gelsemium sempervirens Ait. and Gelsemium elegans Benth., which are indigenous to North America and China/East Asia, respectively. Approximately 120 alkaloids have been isolated and identified from Gelsemium, with the predominant indole alkaloids including gelsemine, koumine, gelsemicine, gelsenicine, gelsedine, sempervirine, koumidine, koumicine and humantenine. Gelsemine is the principal active alkaloid in G. sempervirens Ait., and koumine and gelsemine are the most and second-most dominant alkaloids in G. elegans Benth. Gelsemium extract and its active alkaloids serve a variety of biological functions, including neurobiological, immunosuppressive and antitumor effects, and have traditionally been used to treat pain, neuralgia, anxiety, insomnia, asthma, respiratory ailments and cancers. This review focuses on animal-based studies of Gelsemium as a pain treatment and its mechanism of action. In contrast to morphine, when administered intrathecally and systemically, koumine, gelsemine and gelsenicine have marked antinociception in inflammatory, neuropathic and bone cancer pains without inducing antinociceptive tolerance. Gelsemium and its active alkaloids may produce antinociception by activating the spinal α3 glycine/allopregnanolone pathway. The results of this review support the clinical use of Gelsemium and suggest that its active alkaloids may be developed to treat intractable and other types of pain, preferably after chemical modification. However, Gelsemium is a known toxic plant, and its toxicity limits its appropriate dosage and clinical use. To avoid or decrease the side/toxic effects of Gelsemium, an individual monomer of highly potent alkaloids must be selected, or alkaloids that exhibit greater α3 glycine receptor selectivity may be discovered or modified.

Long-term potentiation of glycinergic synapses triggered by interleukin 1β

Long-term potentiation (LTP) is a persistent increase in synaptic strength required for many behavioral adaptations, including learning and memory, visual and somatosensory system functional development, and drug addiction. Recent work has suggested a role for LTP-like phenomena in the processing of nociceptive information in the dorsal horn and in the generation of central sensitization during chronic pain states. Whereas LTP of glutamatergic and GABAergic synapses has been characterized throughout the central nervous system, to our knowledge there have been no reports of LTP at mammalian glycinergic synapses. Glycine receptors (GlyRs) are structurally related to GABAA receptors and have a similar inhibitory role. Here we report that in the superficial dorsal horn of the spinal cord, glycinergic synapses on inhibitory GABAergic neurons exhibit LTP, occurring rapidly after exposure to the inflammatory cytokine interleukin-1 beta. This form of LTP (GlyR LTP) results from an increase in the number and/or change in biophysical properties of postsynaptic glycine receptors. Notably, formalin-induced peripheral inflammation in vivo potentiates glycinergic synapses on dorsal horn neurons, suggesting that GlyR LTP is triggered during inflammatory peripheral injury. Our results define a previously unidentified mechanism that could disinhibit neurons transmitting nociceptive information and may represent a useful therapeutic target for the treatment of pain.

The involvement of accumbal glycine receptors in the dopamine-elevating effects of addictive drugs

The ability of drugs of abuse to increase mesolimbic levels of dopamine is a characteristic associated with their rewarding effects. Exactly how these effects are produced by different substances is not as well characterised. Our previous work in rats has demonstrated that accumbal glycine receptors (GlyRs) are involved in mediating the dopamine-activating effects of ethanol, and in modulating ethanol intake. In this study the investigation of GlyR involvement was extended to include several different drugs of abuse. By using microdialysis and electrophysiology we compared effects of addictive drugs, with and without the GlyR antagonist strychnine, on dopamine levels and neurotransmission in nucleus accumbens. The dopamine-increasing effect of systemic ethanol and the drug-induced change in neurotransmission in vitro, as measured by microdialysis and field potential recordings, were dependent on GlyRs in nAc. Accumbal GlyRs were also involved in the actions of tetrahydrocannabinol and nicotine, but not in those of cocaine or morphine. These data indicate that accumbal GlyRs play a key role in ethanol-induced dopamine activation and contribute also to that of cannabinoids and nicotine.

Seeking structural specificity: direct modulation of pentameric ligand-gated ion channels by alcohols and general anesthetics

Alcohols and other anesthetic agents dramatically alter neurologic function in a wide range of organisms, yet their molecular sites of action remain poorly characterized. Pentameric ligand-gated ion channels, long implicated in important direct effects of alcohol and anesthetic binding, have recently been illuminated in renewed detail thanks to the determination of atomic-resolution structures of several family members from lower organisms. These structures provide valuable models for understanding and developing anesthetic agents and for allosteric modulation in general. This review surveys progress in this field from function to structure and back again, outlining early evidence for relevant modulation of pentameric ligand-gated ion channels and the development of early structural models for ion channel function and modulation. We highlight insights and challenges provided by recent crystal structures and resulting simulations, as well as opportunities for translation of these newly detailed models back to behavior and therapy.

Presynaptic glycine receptors as a potential therapeutic target for hyperekplexia disease

Although postsynaptic glycine receptors (GlyRs) as αβ heteromers attract considerable research attention, little is known about the role of presynaptic GlyRs, likely α homomers, in diseases. Here, we demonstrate that dehydroxylcannabidiol (DH-CBD), a nonpsychoactive cannabinoid, can rescue GlyR functional deficiency and exaggerated acoustic and tactile startle responses in mice bearing point mutations in α1 GlyRs that are responsible for a hereditary startle-hyperekplexia disease. The GlyRs expressed as α1 homomers either in HEK-293 cells or at presynaptic terminals of the calyceal synapses in the auditory brainstem are more vulnerable than heteromers to hyperekplexia mutation-induced impairment. Homomeric mutants are more sensitive to DH-CBD than are heteromers, suggesting presynaptic GlyRs as a primary target. Consistent with this idea, DH-CBD selectively rescues impaired presynaptic GlyR activity and diminished glycine release in the brainstem and spinal cord of hyperekplexic mutant mice. Thus, presynaptic α1 GlyRs emerge as a potential therapeutic target for dominant hyperekplexia disease and other diseases with GlyR deficiency.

Gelsemine, a principal alkaloid from Gelsemium sempervirens Ait., exhibits potent and specific antinociception in chronic pain by acting at spinal α3 glycine receptors

The present study examined the antinociceptive effects of gelsemine, the principal alkaloid in Gelsemium sempervirens Ait. A single intrathecal injection of gelsemine produced potent and specific antinociception in formalin-induced tonic pain, bone cancer-induced mechanical allodynia, and spinal nerve ligation-induced painful neuropathy. The antinociception was dose-dependent, with maximal inhibition of 50% to 60% and ED50 values of 0.5 to 0.6 μg. Multiple daily intrathecal injections of gelsemine for 7 days induced no tolerance to antinociception in the rat model of bone cancer pain. Spinal gelsemine was not effective in altering contralateral paw withdrawal thresholds, and had only a slight inhibitory effect on formalin-induced acute nociception. The specific antinociception of gelsemine in chronic pain was blocked dose-dependently by the glycine receptor (GlyR) antagonist strychnine with an apparent ID50 value of 3.8 μg. Gelsemine concentration-dependently displaced H(3)-strychnine binding to the membrane fraction of rat spinal cord homogenates, with a 100% displacement and a Ki of 21.9μM. Gene ablation of the GlyR α3 subunit (α3 GlyR) but not α1 GlyR, by a 7-day intrathecal injection of small interfering RNA (siRNA) targeting α3 GlyR or α1 GlyR, nearly completely prevented gelsemine-induced antinociception in neuropathic pain. Our results demonstrate that gelsemine produces potent and specific antinociception in chronic pain states without induction of apparent tolerance. The results also suggest that gelsemine produces antinociception by activation of spinal α3 glycine receptors, and support the notion that spinal α3 glycine receptors are a potential therapeutic target molecule for the management of chronic pain.

Why do cannabinoid receptors have more than one endogenous ligand?

The endocannabinoid system was revealed following the understanding of the mechanism of action of marijuana's major psychotropic principle, Δ(9)-tetrahydrocannabinol, and includes two G-protein-coupled receptors (GPCRs; the cannabinoid CB1 and CB2 receptors), their endogenous ligands (the endocannabinoids, the best studied of which are anandamide and 2-arachidonoylglycerol (2-AG)), and the proteins that regulate the levels and activity of these receptors and ligands. However, other minor lipid metabolites different from, but chemically similar to, anandamide and 2-AG have also been suggested to act as endocannabinoids. Thus, unlike most other GPCRs, cannabinoid receptors appear to have more than one endogenous agonist, and it has been often wondered what could be the physiological meaning of this peculiarity. In 1999, it was proposed that anandamide might also activate other targets, and in particular the transient receptor potential of vanilloid type-1 (TRPV1) channels. Over the last decade, this interaction has been shown to occur both in peripheral tissues and brain, during both physiological and pathological conditions. TRPV1 channels can be activated also by another less abundant endocannabinoid, N-arachidonoyldopamine, but not by 2-AG, and have been proposed by some authors to act as ionotropic endocannabinoid receptors. This article will discuss the latest discoveries on this subject, and discuss, among others, how anandamide and 2-AG differential actions at TRPV1 and cannabinoid receptors contribute to making this signalling system a versatile tool available to organisms to fine-tune homeostasis.

Inhibitory glycine receptors: an update

Strychnine-sensitive glycine receptors (GlyRs) mediate synaptic inhibition in the spinal cord, brainstem, and other regions of the mammalian central nervous system. In this minireview, we summarize our current view of the structure, ligand-binding sites, and chloride channel of these receptors and discuss recently emerging functions of distinct GlyR isoforms. GlyRs not only regulate the excitability of motor and afferent sensory neurons, including pain fibers, but also are involved in the processing of visual and auditory signals. Hence, GlyRs constitute promising targets for the development of therapeutically useful compounds.

Cannabinoids suppress inflammatory and neuropathic pain by targeting α3 glycine receptors

Systemic and intrathecal administration of derivatives of a nonpsychoactive component of marijuana significantly suppresses chronic inflammatory and neuropathic pain, without causing analgesic tolerance, in several rodent models.

Certain types of nonpsychoactive cannabinoids can potentiate glycine receptors (GlyRs), an important target for nociceptive regulation at the spinal level. However, little is known about the potential and mechanism of glycinergic cannabinoids for chronic pain treatment. We report that systemic and intrathecal administration of cannabidiol (CBD), a major nonpsychoactive component of marijuana, and its modified derivatives significantly suppress chronic inflammatory and neuropathic pain without causing apparent analgesic tolerance in rodents. The cannabinoids significantly potentiate glycine currents in dorsal horn neurons in rat spinal cord slices. The analgesic potency of 11 structurally similar cannabinoids is positively correlated with cannabinoid potentiation of the α3 GlyRs. In contrast, the cannabinoid analgesia is neither correlated with their binding affinity for CB1 and CB2 receptors nor with their psychoactive side effects. NMR analysis reveals a direct interaction between CBD and S296 in the third transmembrane domain of purified α3 GlyR. The cannabinoid-induced analgesic effect is absent in mice lacking the α3 GlyRs. Our findings suggest that the α3 GlyRs mediate glycinergic cannabinoid-induced suppression of chronic pain. These cannabinoids may represent a novel class of therapeutic agents for the treatment of chronic pain and other diseases involving GlyR dysfunction.