Pharmacological insights into the role of P2X4 receptors in behavioural regulation: lessons from ivermectin

Activation of M4 muscarinic receptors in the striatum reduces tic-like behaviours in two distinct murine models of Tourette syndrome

BACKGROUND AND PURPOSE

Current pharmacotherapies for Tourette syndrome (TS) are often unsatisfactory and poorly tolerated, underscoring the need for novel treatments. Insufficient striatal acetylcholine has been suggested to contribute to tic ontogeny. Thus, we tested whether activating M1 and/or M4 receptors-the two most abundant muscarinic receptors in the striatum-reduced tic-related behaviours in mouse models of TS.

EXPERIMENTAL APPROACH

Studies were conducted using CIN-d and D1CT-7 mice, two TS models characterized by early-life depletion of striatal cholinergic interneurons and cortical neuropotentiation, respectively. First, we tested the effects of systemic and intrastriatal xanomeline, a selective M1/M4 receptor agonist, on tic-like and other TS-related responses. Then, we examined whether xanomeline effects were reduced by either M1 or M4 antagonists or mimicked by the M1/M3 agonist cevimeline or the M4 positive allosteric modulator (PAM) VU0467154. Finally, we measured striatal levels of M1 and M4 receptors and assessed the impact of VU0461754 on the striatal expression of the neural marker activity c-Fos.

KEY RESULTS

Systemic and intrastriatal xanomeline reduced TS-related behaviours in CIN-d and D1CT-7 mice. Most effects were blocked by M4, but not M1, receptor antagonists. VU0467154, but not cevimeline, elicited xanomeline-like ameliorative effects in both models. M4, but not M1, receptors were down-regulated in the striatum of CIN-d mice. Additionally, VU0467154 reduced striatal c-Fos levels in these animals.

CONCLUSION AND IMPLICATIONS

Activation of striatal M4, but not M1, receptors reduced tic-like manifestations in mouse models, pointing to xanomeline and M4 PAMs as novel putative therapeutic strategies for TS.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

Ivermectin increases striatal cholinergic activity to facilitate dopamine terminal function

Ivermectin (IVM) is a commonly prescribed antiparasitic treatment with pharmacological effects on invertebrate glutamate ion channels resulting in paralysis and death of invertebrates. However, it can also act as a modulator of some vertebrate ion channels and has shown promise in facilitating L-DOPA treatment in preclinical models of Parkinson's disease. The pharmacological effects of IVM on dopamine terminal function were tested, focusing on the role of two of IVM's potential targets: purinergic P2X4 and nicotinic acetylcholine receptors. Ivermectin enhanced electrochemical detection of dorsal striatum dopamine release. Although striatal P2X4 receptors were observed, IVM effects on dopamine release were not blocked by P2X4 receptor inactivation. In contrast, IVM attenuated nicotine effects on dopamine release, and antagonizing nicotinic receptors prevented IVM effects on dopamine release. IVM also enhanced striatal cholinergic interneuron firing. L-DOPA enhances dopamine release by increasing vesicular content. L-DOPA and IVM co-application further enhanced release but resulted in a reduction in the ratio between high and low frequency stimulations, suggesting that IVM is enhancing release largely through changes in terminal excitability and not vesicular content. Thus, IVM is increasing striatal dopamine release through enhanced cholinergic activity on dopamine terminals.

The neurobiological mechanisms and therapeutic prospect of extracellular ATP in depression

BACKGROUND

Depression is a prevalent psychiatric disorder with high long-term morbidities, recurrences, and mortalities. Despite extensive research efforts spanning decades, the cellular and molecular mechanisms of depression remain largely unknown. What's more, about one third of patients do not have effective anti-depressant therapies, so there is an urgent need to uncover more mechanisms to guide the development of novel therapeutic strategies. Adenosine triphosphate (ATP) plays an important role in maintaining ion gradients essential for neuronal activities, as well as in the transport and release of neurotransmitters. Additionally, ATP could also participate in signaling pathways following the activation of postsynaptic receptors. By searching the website PubMed for articles about "ATP and depression" especially focusing on the role of extracellular ATP (eATP) in depression in the last 5 years, we found that numerous studies have implied that the insufficient ATP release from astrocytes could lead to depression and exogenous supply of eATP or endogenously stimulating the release of ATP from astrocytes could alleviate depression, highlighting the potential therapeutic role of eATP in alleviating depression.

AIM

Currently, there are few reviews discussing the relationship between eATP and depression. Therefore, the aim of our review is to conclude the role of eATP in depression, especially focusing on the evidence and mechanisms of eATP in alleviating depression.

CONCLUSION

We will provide insights into the prospects of leveraging eATP as a novel avenue for the treatment of depression.

Purinergic receptors in cognitive disturbances

Purinergic receptors (Rs) of the ATP/ADP, UTP/UDP (P2X, P2Y) and adenosine (A1, A2A)-sensitive classes broadly interfere with cognitive processes both under quasi normal and disease conditions. During neurodegenerative illnesses, high concentrations of ATP are released from the damaged neuronal and non-neuronal cells of the brain; then, this ATP is enzymatically degraded to adenosine. Thus, the primary injury in neurodegenerative diseases appears to be caused by various protein aggregates on which a superimposed damage mediated by especially P2X7 and A2AR activation develops; this can be efficiently prevented by small molecular antagonists in animal models of the above diseases, or are mitigated in the respective knockout mice. Dementia is a leading symptom in Alzheimer's disease (AD), and accompanies Parkinson's disease (PD) and Huntington's disease (HD), especially in the advanced states of these illnesses. Animal experimentation suggests that P2X7 and A2ARs are also involved in a number of psychiatric diseases, such as major depressive disorder (MDD), obsessive compulsive behavior, and attention deficit hyperactivity disorder. In conclusion, small molecular antagonists of purinergic receptors are expected to supply us in the future with pharmaceuticals which are able to combat in a range of neurological/psychiatric diseases the accompanying cognitive deterioration.

Altered purinergic receptor expression in the frontal cortex in schizophrenia

ATP functions as a neurotransmitter, acting on the ubiquitously expressed family of purinergic P2 receptors. In schizophrenia (SCZ), the pathways that modulate extracellular ATP and its catabolism to adenosine are dysregulated. However, the effects of altered ATP availability on P2 receptor expression in the brain in SCZ have not been assessed. We assayed P2 receptor mRNA and protein expression in the DLPFC and ACC in subjects diagnosed with SCZ and matched, non-psychiatrically ill controls (n = 20-22/group). P2RX7, P2RX4 and male P2RX5 mRNA expression were significantly increased (p < 0.05) in the DLPFC in SCZ. Expression of P2RX7 protein isoform was also significantly increased (p < 0.05) in the DLPFC in SCZ. Significant increases in P2RX4 and male P2RX5 mRNA expression may be associated with antipsychotic medication effects. We found that P2RX4 and P2RX7 mRNA are significantly correlated with the inflammatory marker SERPINA3, and may suggest an association between upregulated P2XR and neuroinflammation in SCZ. These findings lend support for brain-region dependent dysregulation of the purinergic system in SCZ.

The Purinergic System as a Target for the Development of Treatments for Bipolar Disorder

The neurobiological and neurochemical mechanisms underlying the pathophysiology of bipolar disorder are complex and not yet fully understood. From circadian disruption to neuroinflammation, many pathways and signaling molecules are important contributors to bipolar disorder development, some specific to a disease subtype or a cycling episode. Pharmacological agents for bipolar disorder have shown only partial efficacy, including mood stabilizers and antipsychotics. The purinergic hypothesis for bipolar disorder emerges in this scenario as a promising target for further research and drug development, given its role in neurotransmission and neuroinflammation that results in behavioral and mood regulation. Here, we review the basic concepts of purinergic signaling in the central nervous system and its contribution to bipolar disorder pathophysiology. Allopurinol and novel P2X7 receptor antagonists are promising candidates for treating bipolar disorder. We further explore currently available pharmacotherapies and the emerging new purinergic targets for drug development in bipolar disorder.

The role of the P2X4 receptor in trigeminal neuralgia, a common neurological disorder

Neurological disorders, which include various types of diseases with complex pathological mechanisms, are more common in the elderly and have shown increased prevalence, morbidity and mortality worldwide. Unfortunately, current therapies for these diseases are usually suboptimal or have undesirable side effects. This necessitates the development of new potential targets for disease-modifying therapies. P2X4R, a type of purinergic receptor, has multiple roles in neurological disorders. In this review, we briefly introduce a neurological disorder, trigeminal neuralgia and its' symptoms, etiology and pathology. Moreover, we focused on the role of P2X4R in neurological disorders and their related pathophysiologic mechanisms. Further studies of P2X4R are required to determine potential therapeutic effects for these pathophysiologies.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

[A Review of the State of Purinergic Signaling and Psychological Stress]

Purinergic signaling is involved in multiple physiological and pathological processes. Psychological stress, as an inharmonious state in response to stressors, is closely related to the function or dysfunction of purinergic signaling. Abnormal expression of ATP interceptors caused by stress leads to psychological stress-related diseases, such as anxiety, depression, post-traumatic stress disorder and schizophrenia. Recent studies demonstrate that a complex network of purinergic signaling (such as ATP, adenosine and P2X2R, P2X3R, P2X4R, P2X7R, A1R, A2AR) plays a key role in psychological stress, but the specific mechanism remains to be further studied. And few studies focus on the application of ATP real-time detecting to psychological stress animal models, so the specific biological role of ATP in the process of stress is still unknown. This review will summarize the relationship between purinergic signaling and psychological stress and propose to apply the duplicate ATP real-time detection technology and purinergic compounds on psychological stress research in order to provide novel potential targets for the treatment of stress-related diseases.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

The macrocyclic lactones ivermectin and moxidectin show differential effects on rotational behavior in the 6-hydroxydopamine mouse model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease characterized by motor and cognitive deficits, the result of dopamine (DA)-depletion within the basal ganglia. Currently, DA replacement therapy in the form of Sinemet (L-DOPA plus Carbidopa) provides symptomatic motor benefits and remains the "gold standard" for treatment. Several pharmacological approaches can enhance DA neurotransmission including the administration of DA receptor agonists, the inhibition of DA metabolism, and enhancing pre-synaptic DA release. DA neurotransmission is regulated by several receptor subtypes including signaling through the purinergic system. P2 × 4 receptors (P2 × 4Rs) are a class of cation-permeable ligand-gated ion channels activated by the synaptic release of extracellular adenosine 5'-triphosphate (ATP). P2 × 4Rs are expressed throughout the central nervous system including the dopaminergic circuitry of the substantia nigra, basal ganglia, and related reward networks. Previous studies have demonstrated that P2 × 4Rs can modulate several DA-dependent characteristics including motor, cognitive, and reward behaviors. Ivermectin (IVM) and moxidectin (MOX) are two macrocyclic lactones that can potentiate P2 × 4Rs. In this study, we sought to investigate the role of P2 × 4Rs in mediating DA neurotransmission by exploring their impact on DA-dependent behavior, specifically rotation frequency in the unilateral 6-hydroxydopamine-lesioned mouse model of DA-depletion. While we did not observe any differences in the degree of lesioning based on immunostaining for tyrosine hydroxylase between sexes, male mice displayed a greater number of rotations with L-DOPA compared to female mice. In contrast, we observed that IVM plus L-DOPA increased the number of rotations (per 10 min) in female, but not male mice. These findings highlight the potential role of pharmacologically targeting the purinergic receptor system in modulating DA neurotransmission as well as the importance of sex differences impacting outcome measures.

Lithocholic acid inhibits P2X2 and potentiates P2X4 receptor channel gating

The family of ATP-gated purinergic P2X receptors comprises seven bunits (P2X1-7) that are unevenly distributed in the central and peripheral nervous systems as well as other organs. Endogenous modulators of P2X receptors are phospholipids, steroids and neurosteroids. Here, we analyzed whether bile acids, which are natural products derived from cholesterol, affect P2X receptor activity. We examined the effects of primary and secondary bile acids and newly synthesized derivatives of lithocholic acid on agonist-induced responses in HEK293T cells expressing rat P2X2, P2X4 and P2X7 receptors. Electrophysiology revealed that low micromolar concentrations of lithocholic acid and its structural analog 4-dafachronic acid strongly inhibit ATP-stimulated P2X2 but potentiate P2X4 responses, whereas primary bile acids and other secondary bile acids exhibit no or reduced effects only at higher concentrations. Agonist-stimulated P2X7 responses are significantly potentiated by lithocholic acid at moderate concentrations. Structural modifications of lithocholic acid at positions C-3, C-5 or C-17 abolish both inhibitory and potentiation effects to varying degrees, and the 3α-hydroxy group contributes to the ability of the molecule to switch between potentiation and inhibition. Lithocholic acid allosterically modulates P2X2 and P2X4 receptor sensitivity to ATP, reduces the rate of P2X4 receptor desensitization and antagonizes the effect of ivermectin on P2X4 receptor deactivation. Alanine-scanning mutagenesis of the upper halve of P2X4 transmembrane domain-1 revealed that residues Phe48, Val43 and Tyr42 are important for potentiating effect of lithocholic acid, indicating that modulatory sites for lithocholic acid and ivermectin partly overlap. Lithocholic acid also inhibits ATP-evoked currents in pituitary gonadotrophs expressing native P2X2, and potentiates ATP currents in nonidentified pituitary cells expressing P2X4 receptors. These results indicate that lithocholic acid is a bioactive steroid that may help to further unveil the importance of the P2X2, and P2X4 receptors in many physiological processes.

Avermectin Derivatives, Pharmacokinetics, Therapeutic and Toxic Dosages, Mechanism of Action, and Their Biological Effects

Avermectins are a group of drugs that occurs naturally as a product of fermenting Streptomyces avermitilis, an actinomycetes, isolated from the soil. Eight different structures, including ivermectin, abamectin, doramectin, eprinomectin, moxidectin, and selamectin, were isolated and divided into four major components (A1a, A2a, B1a and B2a) and four minor components (A1b, A2b, B1b, and B2b). Avermectins are generally used as a pesticide for the treatment of pests and parasitic worms as a result of their anthelmintic and insecticidal properties. Additionally, they possess anticancer, anti-diabetic, antiviral, antifungal, and are used for treatment of several metabolic disorders. Avermectin generally works by preventing the transmission of electrical impulse in the muscle and nerves of invertebrates, by amplifying the glutamate effects on the invertebrates-specific gated chloride channel. Avermectin has unwanted effects or reactions, especially when administered indiscriminately, which include respiratory failure, hypotension, and coma. The current review examines the mechanism of actions, biosynthesis, safety, pharmacokinetics, biological toxicity and activities of avermectins.

To Inhibit or Enhance? Is There a Benefit to Positive Allosteric Modulation of P2X Receptors?

The family of ligand-gated ion channels known as P2X receptors were discovered several decades ago. Since the cloning of the seven P2X receptors (P2X1-P2X7), a huge research effort has elucidated their roles in regulating a range of physiological and pathophysiological processes. Transgenic animals have been influential in understanding which P2X receptors could be new therapeutic targets for disease. Furthermore, understanding how inherited mutations can increase susceptibility to disorders and diseases has advanced this knowledge base. There has been an emphasis on the discovery and development of pharmacological tools to help dissect the individual roles of P2X receptors and the pharmaceutical industry has been involved in pushing forward clinical development of several lead compounds. During the discovery phase, a number of positive allosteric modulators have been described for P2X receptors and these have been useful in assigning physiological roles to receptors. This review will consider the major physiological roles of P2X1-P2X7 and discuss whether enhancement of P2X receptor activity would offer any therapeutic benefit. We will review what is known about identified compounds acting as positive allosteric modulators and the recent identification of drug binding pockets for such modulators.

Reduced P2X receptor levels are associated with antidepressant effect in the learned helplessness model

Purinergic receptors, especially P2RX, are associated to the severity of symptoms in patients suffering from depressive and bipolar disorders, and genetic deletion or pharmacological blockade of P2RX7 induces antidepressant-like effect in preclinical models. However, there is scarce evidence about the alterations in P2RX7 or P2RX4 levels and in behavioral consequences induced by previous exposure to stress, a major risk factor for depression in humans. In the present study, we evaluated the effect of imipramine (IMI) on P2RX7 and P2RX4 levels in dorsal and ventral hippocampus as well as in the frontal cortex of rats submitted to the pretest session of learned helplessness (LH) paradigm. Repeated, but not acute administration of IMI (15 mg/kg ip) reduced the levels of both P2RX7 and P2RX4 in the ventral, but not in dorsal hippocampus or frontal cortex. In addition, we tested the effect of P2RX7/P2RX4 antagonist brilliant blue G (BBG: 25 or 50 mg/kg ip) on the LH paradigm. We observed that repeated (7 days) but not acute (1 day) treatment with BBG (50 mg) reduced the number of failures to escape the shocks in the test session, a parameter mimicked by the same regimen of IMI treatment. Taken together, our data indicates that pharmacological blockade or decrease in the expression of P2RX7 is associated to the antidepressant-like behavior observed in the LH paradigm after repeated drug administration.

Its complicated: The relationship between alcohol and microglia in the search for novel pharmacotherapeutic targets for alcohol use disorders

Alcohol use disorder (AUD) is a chronic relapsing disorder with wide-ranging health consequences. Alcohol targets the central nervous system producing neurodegeneration and subsequent cognitive and behavioral deficits, but the mechanisms behind these effects remain unclear. Recently, evidence has been mounting for the role of neuroimmune activation in the pathogenesis of AUDs, but our nascent state of knowledge about the interaction of alcohol with the neuroimmune system supports that the relationship is complicated. As the resident macrophage of the central nervous system, microglia are a central focus. Human and animal research on the interplay between microglia and alcohol in AUDs has proven to be complex, and though early research focused on a pro-inflammatory phenotype of microglia, the anti-inflammatory and homeostatic roles of microglia must be considered. How these new roles for microglia should be incorporated into our thinking about the neuroimmune system in AUDs is discussed in the context of developing novel pharmacotherapies for AUDs.

Dopamine Receptor Blockade Attenuates Purinergic P2X4 Receptor-Mediated Prepulse Inhibition Deficits and Underlying Molecular Mechanisms

Sensorimotor gating refers to the ability to filter incoming sensory information in a stimulus-laden environment and disruption of this physiological process has been documented in psychiatric disorders characterized by cognitive aberrations. The effectiveness of current pharmacotherapies for treatment of sensorimotor gating deficits in the patient population still remains controversial. These challenges emphasize the need to better understand the biological underpinnings of sensorimotor gating which could lead to discovery of novel drug targets for therapeutic intervention. Notably, we recently reported a role for purinergic P2X4 receptors (P2X4Rs) in regulation of sensorimotor gating using prepulse inhibition (PPI) of acoustic startle reflex. P2X4Rs are ion channels gated by adenosine-5′-triphosphate (ATP). Ivermectin (IVM) induced PPI deficits in C57BL/6J mice in a P2X4R-specific manner. Furthermore, mice deficient in P2X4Rs [P2X4R knockout (KO)] exhibited PPI deficits that were alleviated by dopamine (DA) receptor antagonists demonstrating an interaction between P2X4Rs and DA receptors in PPI regulation. On the basis of these findings, we hypothesized that increased DA neurotransmission underlies IVM-mediated PPI deficits. To test this hypothesis, we measured the effects of D1 and D2 receptor antagonists, SCH 23390 and raclopride respectively and D1 agonist, SKF 82958 on IVM-mediated PPI deficits. To gain mechanistic insights, we investigated the interaction between IVM and dopaminergic drugs on signaling molecules linked to PPI regulation in the ventral striatum. SCH 23390 significantly attenuated the PPI disruptive effects of IVM to a much greater degree than that of raclopride. SKF 82958 failed to potentiate IVM-mediated PPI disruption. At the molecular level, modulation of D1 receptors altered IVM’s effects on dopamine and cyclic-AMP regulated phosphoprotein of 32 kDa (DARPP-32) phosphorylation. Additionally, IVM interacted with the DA receptors antagonists and SKF 82958 in phosphorylation of Ca²⁺/calmodulin kinase IIα (CaMKIIα) and its downstream target, neuronal nitric oxide synthase (nNOS). Current findings suggest an involvement for D1 and D2 receptors in IVM-mediated PPI disruption via modulation of DARPP-32, CaMKIIα and nNOS. Taken together, the findings suggest that stimulation of P2X4Rs can lead to DA hyperactivity and disruption of information processing, implicating P2X4Rs as a novel drug target for treatment of psychiatric disorders characterized by sensorimotor gating deficits.

Gene-environment interactions in antisocial behavior are mediated by early-life 5-HT2A receptor activation

The ontogeny of antisocial behavior (ASB) is rooted in complex gene-environment (G×E) interactions. The best-characterized of these interplays occurs between: a) low-activity alleles of the gene encoding monoamine oxidase A (MAOA), the main serotonin-degrading enzyme; and b) child maltreatment. The purpose of this study was to develop the first animal model of this G×E interaction, to help understand the neurobiological mechanisms of ASB and identify novel targets for its therapy. Maoa hypomorphic transgenic mice were exposed to an early-life stress regimen consisting of maternal separation and daily intraperitoneal saline injections and were then compared with their wild-type and non-stressed controls for ASB-related neurobehavioral phenotypes. Maoa hypomorphic mice subjected to stress from postnatal day (PND) 1 through 7 - but not during the second postnatal week - developed overt aggression, social deficits and abnormal stress responses from the fourth week onwards. On PND 8, these mice exhibited low resting heart rate - a well-established premorbid sign of ASB - and a significant and selective up-regulation of serotonin 5-HT_2A receptors in the prefrontal cortex. Notably, both aggression and neonatal bradycardia were rescued by the 5-HT₂ receptor antagonist ketanserin (1-3 mg kg^-1, IP), as well as the selective 5-HT_2A receptor blocker MDL-100,907 (volinanserin, 0.1-0.3 mg kg^-1, IP) throughout the first postnatal week. These findings provide the first evidence of a molecular basis of G×E interactions in ASB and point to early-life 5-HT_2A receptor activation as a key mechanism for the ontogeny of this condition. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Preclinical evaluation of avermectins as novel therapeutic agents for alcohol use disorders

The deleterious effects of alcohol use disorders (AUDs) on human health have been documented worldwide. The enormous socioeconomic burden coupled with lack of efficacious pharmacotherapies underlies the need for improved treatment strategies. At present, there is a growing body of preclinical evidence that demonstrates the potential of avermectins [ivermectin (IVM), selamectin (SEL), abamectin (ABM), and moxidectin (MOX)] in treatment of AUDs. Avermectins are derived by fermentation of soil micro-organism, Streptomyces avermitilis, and have been extensively used for treatment of parasitic infections. From the mechanistic standpoint, avermectins are positive modulators of purinergic P2X4 receptors (P2X4Rs). P2X4Rs belong to P2X superfamily of cation-permeable ion channels gated by adenosine 5'-triphosphate (ATP). Building evidence has implicated a role for P2X4Rs in regulation of ethanol intake and that ethanol can inhibit ATP-gated currents in P2X4Rs. Investigations using recombinant cell models and animal models of alcohol drinking have reported that IVM, ABM, and MOX, but not SEL, were able to antagonize the inhibitory effects of ethanol on P2X4Rs in vitro and reduce ethanol intake in vivo. Furthermore, IVM was shown to reduce ethanol consumption via P2X4R potentiation in vivo, supporting the involvement of P2X4Rs in IVM's anti-alcohol effects and that P2X4Rs can be used as a platform for developing novel anti-alcohol compounds. Taken together, these findings support the utility of avermectins as a novel class of drug candidates for treatment of AUDs.

Steroid 5α-reductase 2 deficiency leads to reduced dominance-related and impulse-control behaviors

The enzyme steroid 5α-reductase 2 (5αR2) catalyzes the conversion of testosterone into the potent androgen 5α-dihydrotestosterone. Previous investigations showed that 5αR2 is expressed in key brain areas for emotional and socio-affective reactivity, yet the role of this enzyme in behavioral regulation remains mostly unknown. Here, we profiled the behavioral characteristics of 5αR2 heterozygous (HZ) and knockout (KO) mice, as compared with their wild-type (WT) littermates. While male 5αR2 KO mice displayed no overt alterations in motoric, sensory, information-processing and anxiety-related behaviors, they exhibited deficits in neurobehavioral correlates of dominance (including aggression against intruders, mating, and tube dominance) as well as novelty-seeking and risk-taking responses. Furthermore, male 5αR2 KO mice exhibited reduced D₂-like dopamine receptor binding in the shell of the nucleus accumbens - a well-recognized molecular signature of social dominance. Collectively, these results suggest that 5αR2 is involved in the establishment of social dominance and its behavioral manifestations. Further studies are warranted to understand how the metabolic actions of 5αR2 on steroid profile may be implicated in social ranking, impulse control, and the modulation of dopamine receptor expression in the nucleus accumbens.

Ivermectin and its target molecules: shared and unique modulation mechanisms of ion channels and receptors by ivermectin

Ivermectin (IVM) is an antiparasitic drug that is used worldwide and rescues hundreds of millions of people from onchocerciasis and lymphatic filariasis. It was discovered by Satoshi Ōmura and William C. Campbell, to whom the 2015 Nobel Prize in Physiology or Medicine was awarded. It kills parasites by activating glutamate-gated Cl^- channels, and it also targets several ligand-gated ion channels and receptors, including Cys-loop receptors, P2X₄ receptors and fernesoid X receptors. Recently, we found that IVM also activates a novel target, the G-protein-gated inwardly rectifying K⁺ channel, and also identified the structural determinant for the activation. In this review, we aim to provide an update and summary of recent progress in the identification of IVM targets, as well as their modulation mechanisms, through molecular structures, chimeras and site-directed mutagenesis, and molecular docking and modelling studies.

P2X4: A fast and sensitive purinergic receptor

Extracellular nucleotides have been recognized as important mediators of activation, triggering multiple responses via plasma membrane receptors known as P2 receptors. P2 receptors comprise P2X ionotropic receptors and G protein-coupled P2Y receptors. P2X receptors are expressed in many tissues, where they are involved in a number of functions including synaptic transmission, muscle contraction, platelet aggregation, inflammation, macrophage activation, differentiation and proliferation, neuropathic and inflammatory pain. P2X4 is one of the most sensitive purinergic receptors (at nanomolar ATP concentrations), about one thousand times more than the archetypal P2X7. P2X4 is widely expressed in central and peripheral neurons, in microglia, and also found in various epithelial tissues and endothelial cells. It localizes on the plasma membrane, but also in intracellular compartments. P2X4 is preferentially localized in lysosomes, where it is protected from proteolysis by its glycosylation. High ATP concentration in the lysosomes does not activate P2X4 at low pH; P2X4 gets activated by intra-lysosomal ATP only in its fully dissociated tetra-anionic form, when the pH increases to 7.4. Thus, P2X4 is functioning as a Ca2+-channel after the fusion of late endosomes and lysosomes. P2X4 modulates major neurotransmitter systems and regulates alcohol-induced responses in microglia. P2X4 is one of the key receptors mediating neuropathic pain. However, injury-induced upregulation of P2X4 expression is gender dependent and plays a key role in pain difference between males and females. P2X4 is also involved in inflammation. Extracellular ATP being a pro-inflammatory molecule, P2X4 can trigger inflammation in response to high ATP release. It is therefore involved in multiple pathologies, like post-ischemic inflammation, rheumatoid arthritis, airways inflammation in asthma, neurodegenerative diseases and even metabolic syndrome. Although P2X4 remains poorly characterized, more studies are needed as it is likely to be a potential therapeutic target in these multiple pathologies.

Preclinical development of moxidectin as a novel therapeutic for alcohol use disorder

Current pharmacotherapies for alcohol used disorder (AUD) are few and relatively ineffective illustrating the need for the development of new, effective medications. Using a translational approach, our laboratory reported that ivermectin, an FDA-approved, human and animal anti-parasitic agent, can significantly reduce ethanol intake in male and female mice across different drinking paradigms. Extending this line of investigation, the current paper investigated the utility of moxidectin (MOX), an analogue of ivermectin, to reduce ethanol intake. Notably, MOX is widely held to have lower neurotoxicity potential and improved margin of safety compared to ivermectin. Using a 24-h-two-bottle choice paradigm, MOX significantly reduced ethanol intake in a dose dependent manner in both male and female C57BL/6J mice, respectively (1.25-7.5 mg/kg) and (1.25-10 mg/kg). Further, multi-day administration of MOX (2.5 mg/kg; intraperitoneal injection) for 5 consecutive days significantly reduced ethanol intake in both the 24-h-two-bottle choice and Drinking-in-the-Dark paradigms in female mice. No overt signs of behavioral toxicity were observed. Notably in both male and female mice, MOX significantly reduced ethanol intake starting approximately 4 h post-injection. Using a Xenopus oocyte expression system, we found that MOX significantly potentiated P2X4 receptor (P2X4R) function and antagonized the inhibitory effects of ethanol on ATP-gated currents in P2X4Rs. This latter finding represents the first report of MOX having activity on P2X4Rs. In addition, MOX potentiated GABA_A receptors, but to a lesser degree as compared to ivermectin supporting the hypothesis that MOX would be advantageous (compared to ivermectin) with respect to reducing contraindications. Overall, the results illustrate the potential for development of MOX as a novel pharmacotherapy for the treatment of AUD.

P2X4 Receptor Reporter Mice: Sparse Brain Expression and Feeding-Related Presynaptic Facilitation in the Arcuate Nucleus

P2X4 receptors are ATP-gated cation channels that are widely expressed in the nervous system. To identify P2X4 receptor-expressing cells, we generated BAC transgenic mice expressing tdTomato under the control of the P2X4 receptor gene (P2rx4). We found sparse populations of tdTomato-positive neurons in most brain areas with patterns that matched P2X4 mRNA distribution. tdTomato expression within microglia was low but was increased by an experimental manipulation that triggered microglial activation. We found surprisingly high tdTomato expression in the hypothalamic arcuate nucleus (Arc) (i.e., within parts of the neural circuitry controlling feeding). Immunohistochemistry and genetic crosses of P2rx4 tdTomato mice with cell-specific GFP reporter lines showed that the tdTomato-expressing cells were mainly AgRP-NPY neurons and tanycytes. There was no electrophysiological evidence for functional expression of P2X4 receptors on AgRP-NPY neuron somata, but instead, we found clear evidence for functional presynaptic P2X4 receptor-mediated responses in terminals of AgRP-NPY neurons onto two of their postsynaptic targets (Arc POMC and paraventricular nucleus neurons), where ATP dramatically facilitated GABA release. The presynaptic responses onto POMC neurons, and the expression of tdTomato in AgRP-NPY neurons and tanycytes, were significantly decreased by food deprivation in male mice in a manner that was partially reversed by the satiety-related peptide leptin. Overall, we provide well-characterized tdTomato reporter mice to study P2X4-expressing cells in the brain, new insights on feeding-related regulation of presynaptic P2X4 receptor responses, and the rationale to explore extracellular ATP signaling in the control of feeding behaviors.

SIGNIFICANCE STATEMENT

Cells expressing ATP-gated P2X4 receptors have proven problematic to identify and study in brain slice preparations because P2X4 expression is sparse. To address this limitation, we generated and characterized BAC transgenic P2rx4 tdTomato reporter mice. We report the distribution of tdTomato-expressing cells throughout the brain and particularly strong expression in the hypothalamic arcuate nucleus. Together, our studies provide a new, well-characterized tool with which to study P2X4 receptor-expressing cells. The electrophysiological studies enabled by this mouse suggest previously unanticipated roles for ATP and P2X4 receptors in the neural circuitry controlling feeding.

Role of purinergic P2X4 receptors in regulating striatal dopamine homeostasis and dependent behaviors

Purinergic P2X4 receptors (P2X4Rs) belong to the P2X superfamily of ion channels regulated by ATP. We recently demonstrated that P2X4R knockout (KO) mice exhibited deficits in sensorimotor gating, social interaction, and ethanol drinking behavior. Dopamine (DA) dysfunction may underlie these behavioral changes, but there is no direct evidence for P2X4Rs' role in DA neurotransmission. To test this hypothesis, we measured markers of DA function and dependent behaviors in P2X4R KO mice. P2X4R KO mice exhibited altered density of pre-synaptic markers including tyrosine hydroxylase, dopamine transporter; post-synaptic markers including dopamine receptors and phosphorylation of downstream targets including dopamine and cyclic-AMP regulated phosphoprotein of 32 kDa and cyclic-AMP-response element binding protein in different parts of the striatum. Ivermectin, an allosteric modulator of P2X4Rs, significantly affected dopamine and cyclic AMP regulated phosphoprotein of 32 kDa and extracellular regulated kinase1/2 phosphorylation in the striatum. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Using the 6-hydroxydopamine model of DA depletion, P2X4R KO mice exhibited an attenuated levodopa (L-DOPA)-induced motor behavior, whereas ivermectin enhanced this behavior. Collectively, these findings identified an important role for P2X4Rs in maintaining DA homeostasis and illustrate how this association is important for CNS functions including motor control and sensorimotor gating. We propose that P2X4 receptors (P2X4Rs) regulate dopamine (DA) homeostasis and associated behaviors. Pre-synaptic and post-synaptic DA markers were significantly altered in the dorsal and ventral striatum of P2X4R KO mice, implicating altered DA neurotransmission. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Ivermectin (IVM), a positive modulator of P2X4Rs, enhanced levodopa (L-DOPA)-induced motor behavior. These studies highlight potential interactions between P2X4Rs and DA system.

A Pilot Study of the Safety and Initial Efficacy of Ivermectin for the Treatment of Alcohol Use Disorder

BACKGROUND

Ivermectin (IVM) is an antiparasitic agent that has been shown to reduce alcohol intake in mice, suggesting IVM as a potential treatment for alcohol use disorder (AUD). However, the safety profile of IVM administered in combination with an intoxicating dose of alcohol has not been characterized in humans.

METHODS

This pilot project sought to provide the first clinical evidence that IVM could be repositioned as an AUD pharmacotherapy by examining (i) the safety of combining IVM (30 mg oral , once a day [QD]) with an intoxicating dose of intravenous alcohol (0.08 g/dl) and (ii) the effects of IVM on alcohol cue-induced craving and subjective response to alcohol. Eleven individuals with AUD participated in a randomized, placebo-controlled, crossover study in which they received the study medication, participated in a cue exposure paradigm followed by intravenous alcohol administration, and remained in an inpatient unit overnight for observation.

RESULTS

IVM treatment, versus placebo, did not increase the number or severity of adverse effects during alcohol administration or throughout the visit. However, IVM did not reduce cue-induced craving nor did it significantly affect subjective response to alcohol.

CONCLUSIONS

These results suggest that IVM (30 mg oral, QD) is safe in combination with an intoxicating dose of alcohol, but do not provide evidence that this dose of IVM is effective in reducing alcohol craving or its reinforcing effects. Given the preclinical data suggesting IVM is effective in reducing alcohol consumption in mice, additional studies testing larger samples and alternate dosing regimens are warranted to further characterize the potential efficacy of IVM as an AUD treatment.

Extracellular nucleotides as novel, underappreciated pro-metastatic factors that stimulate purinergic signaling in human lung cancer cells

Background

One of the challenging problems of current radio-chemotherapy is recurrence and metastasis of cancer cells that survive initial treatment. We propose that one of the unwanted effects of radiochemotherapy is the release from damaged (“leaky”) cells of nucleotides such as ATP and UTP that exert pro-metastatic functions and can directly stimulate chemotaxis of cancer cells.

Methods

To address this problem in a model of human lung cancer (LC), we employed several complementary in vitro and in vivo approaches to demonstrate the role of extracellular nucleotides (EXNs) in LC cell line metastasis and tumor progression. We measured concentrations of EXNs in several organs before and after radiochemotherapy. The purinergic receptor agonists and antagonists, inhibiting all or selected subtypes of receptors, were employed in in vitro and in vivo pro-metastatic assays.

Results

We found that EXNs accumulate in several organs in response to radiochemotherapy, and RT-PCR analysis revealed that most of the P1 and P2 receptor subtypes are expressed in human LC cells. EXNs were found to induce chemotaxis and adhesion of LC cells, and an autocrine loop was identified that promotes the proliferation of LC cells. Most importantly, metastasis of these cells could be inhibited in immunodeficient mice in the presence of specific small molecule inhibitors of purinergic receptors.

Conclusions

Based on this result, EXNs are novel pro-metastatic factors released particularly during radiochemotherapy, and inhibition of their pro-metastatic effects via purinergic signaling could become an important part of anti-metastatic treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0469-z) contains supplementary material, which is available to authorized users.

Oral delivery of ivermectin using a fast dissolving oral film: Implications for repurposing ivermectin as a pharmacotherapy for alcohol use disorder

Individuals suffering from an alcohol-use disorder (AUD) constitute a major health concern. Preclinical studies in our laboratory show that acute and chronic intraperitoneal (i.p.) administration of ivermectin (IVM) reduces alcohol intake and preference in mice. To enable clinical investigation to use IVM for the treatment of an AUD, development of an oral formulation that can be used in animals as well as long-term preclinical toxicology studies are required. The present work explores the use of a promising alternative dosage form of IVM, fast-dissolving oral films (Cure Pharmaceutical®), to test the efficacy and safety of oral IVM in conjunction with alcohol exposure. We tested the effect of IVM (0.21 mg) using a fast-dissolving oral film delivery method on reducing 10% v/v alcohol (10E) intake in female C57BL/6 mice using a 24-h access two-bottle choice paradigm for 6 weeks (5 days per week). Differences in ethanol intake, preference for ethanol, water intake, fluid intake, food intake, changes in mouse and organ weights, as well as histological changes to kidney, liver, and brain were analyzed. The IVM group drank significantly less ethanol over the 30-day period compared to the placebo (blank strip) and the no-treatment groups. Organ weights did not differ between the groups. Histological evaluation showed no differences in the brain and kidney between groups. In the liver, there was a slight increase in the incidence of microvesicular fatty and degenerative changes of the animals receiving the thin strips. No overt hepatocellular necrosis or perivascular inflammation was noted. Overall, these data support the use of this novel method of oral drug delivery for longer-term studies and should facilitate FDA required preclinical testing that is necessary to repurpose IVM for treatment of an AUD.

Multiday administration of ivermectin is effective in reducing alcohol intake in mice at doses shown to be safe in humans

Ivermectin (IVM), an FDA approved anthelmintic agent, can significantly reduce ethanol intake in mice following acute administration. The current study evaluates the sustainability and safety of multiday IVM administration in reducing 10% v/v ethyl alcohol (10E) intake in mice at a dose shown to be safe in humans. We tested the effect of 10-day administration of IVM (3.0 mg/kg/day; intraperitoneally) on reducing 10E intake in C57BL/6J mice using a 24-h, two-bottle choice paradigm. On the 10th day of IVM administration, mice were sacrificed at 0, 0.5, 2, 8, 32, 48, and 72 h after injection. Brain tissue and plasma samples were collected and analyzed using liquid chromatography with tandem mass spectrometry (LC-MS/MS). Analysis of variance (ANOVA) was used to assess the effect of 10-day IVM administration on 10E intake, 10E preference, water intake, and total fluid intake with Dunnett's multiple comparison post-hoc test. Individual Student's t-tests were also used to further quantify changes in these dependent variables. IVM significantly decreased 10E intake over a 9-day period (P<0.01). Pre-IVM 10E intake was 9.1±3.2 g/kg/24 h. Following the 9th day of IVM injections, intake dropped by almost 30% (P<0.05). IVM had no effect on total water intake or mouse weight throughout the study; however, there was a significant decrease in both preference for 10E (P<0.01) and total fluid intake (P<0.05). Multiday administration of IVM significantly reduces 10E intake and preference in animals without causing any apparent adverse effects at a dose shown to be safe in humans.

Purinergic system dysfunction in mood disorders: a key target for developing improved therapeutics

Uric acid and purines (such as adenosine) regulate mood, sleep, activity, appetite, cognition, memory, convulsive threshold, social interaction, drive, and impulsivity. A link between purinergic dysfunction and mood disorders was first proposed a century ago. Interestingly, a recent nationwide population-based study showed elevated risk of gout in subjects with bipolar disorder (BD), and a recent meta-analysis and systematic review of placebo-controlled trials of adjuvant purinergic modulators confirmed their benefits in bipolar mania. Uric acid may modulate energy and activity levels, with higher levels associated with higher energy and BD spectrum. Several recent genetic studies suggest that the purinergic system - particularly the modulation of P1 and P2 receptor subtypes - plays a role in mood disorders, lending credence to this model. Nucleotide concentrations can be measured using brain spectroscopy, and ligands for in vivo positron emission tomography (PET) imaging of adenosine (P1) receptors have been developed, thus allowing potential target engagement studies. This review discusses the key role of the purinergic system in the pathophysiology of mood disorders. Focusing on this promising therapeutic target may lead to the development of therapies with antidepressant, mood stabilization, and cognitive effects.

P2X4 receptors (P2X4Rs) represent a novel target for the development of drugs to prevent and/or treat alcohol use disorders

Alcohol use disorders (AUDs) have a staggering socioeconomic impact. Few therapeutic options are available, and they are largely inadequate. These shortcomings highlight the urgent need to develop effective medications to prevent and/or treat AUDs. A critical barrier is the lack of information regarding the molecular target(s) by which ethanol (EtOH) exerts its pharmacological activity. This review highlights findings implicating P2X4 receptors (P2X4Rs) as a target for the development of therapeutics to treat AUDs and discusses the use of ivermectin (IVM) as a potential clinical tool for treatment of AUDs. P2XRs are a family of ligand-gated ion channels (LGICs) activated by extracellular ATP. Of the P2XR subtypes, P2X4Rs are expressed the most abundantly in the CNS. Converging evidence suggests that P2X4Rs are involved in the development and progression of AUDs. First, in vitro studies report that pharmacologically relevant EtOH concentrations can negatively modulate ATP-activated currents. Second, P2X4Rs in the mesocorticolimbic dopamine system are thought to play a role in synaptic plasticity and are located ideally to modulate brain reward systems. Third, alcohol-preferring (P) rats have lower functional expression of the p2rx4 gene than alcohol-non-preferring (NP) rats suggesting an inverse relationship between alcohol intake and P2X4R expression. Similarly, whole brain p2rx4 expression has been shown to relate inversely to innate 24 h alcohol preference across 28 strains of rats. Fourth, mice lacking the p2rx4 gene drink more EtOH than wildtype controls. Fifth, IVM, a positive modulator of P2X4Rs, antagonizes EtOH-mediated inhibition of P2X4Rs in vitro and reduces EtOH intake and preference in vivo. These findings suggest that P2X4Rs contribute to EtOH intake. The present review summarizes recent findings focusing on the P2X4R as a molecular target of EtOH action, its role in EtOH drinking behavior and modulation of its activity by IVM as a potential therapy for AUDs.

Imaging P2X4 receptor subcellular distribution, trafficking, and regulation using P2X4-pHluorin

A P2X4 receptor labeled with the pH-sensitive GFP superecliptic pHluorin represents a useful probe to investigate P2X4 receptor distribution, trafficking, and up-regulation.

P2X4 receptors are adenosine triphosphate (ATP)-gated cation channels present on the plasma membrane (PM) and also within intracellular compartments such as vesicles, vacuoles, lamellar bodies (LBs), and lysosomes. P2X4 receptors in microglia are up-regulated in epilepsy and in neuropathic pain; that is to say, their total and/or PM expression levels increase. However, the mechanisms underlying up-regulation of microglial P2X4 receptors remain unclear, in part because it has not been possible to image P2X4 receptor distribution within, or trafficking between, cellular compartments. Here, we report the generation of pH-sensitive fluorescently tagged P2X4 receptors that permit evaluations of cell surface and total receptor pools. Capitalizing on information gained from zebrafish P2X4.1 crystal structures, we designed a series of mouse P2X4 constructs in which a pH-sensitive green fluorescent protein, superecliptic pHluorin (pHluorin), was inserted into nonconserved regions located within flexible loops of the P2X4 receptor extracellular domain. One of these constructs, in which pHluorin was inserted after lysine 122 (P2X4-pHluorin123), functioned like wild-type P2X4 in terms of its peak ATP-evoked responses, macroscopic kinetics, calcium flux, current–voltage relationship, and sensitivity to ATP. P2X4-pHluorin123 also showed pH-dependent fluorescence changes, and was robustly expressed on the membrane and within intracellular compartments. P2X4-pHluorin123 identified cell surface and intracellular fractions of receptors in HEK-293 cells, hippocampal neurons, C8-B4 microglia, and alveolar type II (ATII) cells. Furthermore, it showed that the subcellular fractions of P2X4-pHluorin123 receptors were cell and compartment specific, for example, being larger in hippocampal neuron somata than in C8-B4 cell somata, and larger in C8-B4 microglial processes than in their somata. In ATII cells, P2X4-pHluorin123 showed that P2X4 receptors were secreted onto the PM when LBs undergo exocytosis. Finally, the use of P2X4-pHluorin123 showed that the modulator ivermectin did not increase the PM fraction of P2X4 receptors and acted allosterically to potentiate P2X4 receptor responses. Collectively, our data suggest that P2X4-pHluorin123 represents a useful optical probe to quantitatively explore P2X4 receptor distribution, trafficking, and up-regulation.

Avermectins differentially affect ethanol intake and receptor function: implications for developing new therapeutics for alcohol use disorders

Our laboratory is investigating ivermectin (IVM) and other members of the avermectin family as new pharmaco-therapeutics to prevent and/or treat alcohol use disorders (AUDs). Earlier work found that IVM significantly reduced ethanol intake in mice and that this effect likely reflects IVM's ability to modulate ligand-gated ion channels. We hypothesized that structural modifications that enhance IVM's effects on key receptors and/or increase its brain concentration should improve its anti-alcohol efficacy. We tested this hypothesis by comparing the abilities of IVM and two other avermectins, abamectin (ABM) and selamectin (SEL), to reduce ethanol intake in mice, to alter modulation of GABAARs and P2X4Rs expressed in Xenopus oocytes and to increase their ability to penetrate the brain. IVM and ABM significantly reduced ethanol intake and antagonized the inhibitory effects of ethanol on P2X4R function. In contrast, SEL did not affect either measure, despite achieving higher brain concentrations than IVM and ABM. All three potentiated GABAAR function. These findings suggest that chemical structure and effects on receptor function play key roles in the ability of avermectins to reduce ethanol intake and that these factors are more important than brain penetration alone. The direct relationship between the effect of these avermectins on P2X4R function and ethanol intake suggest that the ability to antagonize ethanol-mediated inhibition of P2X4R function may be a good predictor of the potential of an avermectin to reduce ethanol intake and support the use of avermectins as a platform for developing novel drugs to prevent and/or treat AUDs.

Contribution of P2X4 receptors to ethanol intake in male C57BL/6 mice

P2X receptors (P2XRs) are a family of cation-permeable ligand-gated ion channels activated by synaptically released extracellular adenosine 5'-triphosphate. The P2X4 subtype is abundantly expressed in the central nervous system and is sensitive to low intoxicating ethanol concentrations. Genetic meta-analyses identified the p2rx4 gene as a candidate gene for innate alcohol intake and/or preference. The current study used mice lacking the p2rx4 gene (knockout, KO) and wildtype (WT) C57BL/6 controls to test the hypothesis that P2X4Rs contribute to ethanol intake. The early acquisition and early maintenance phases of ethanol intake were measured with three different drinking procedures. Further, we tested the effects of ivermectin (IVM), a drug previously shown to reduce ethanol's effects on P2X4Rs and to reduce ethanol intake and preference, for its ability to differentially alter stable ethanol intake in KO and WT mice. Depending on the procedure and the concentration of the ethanol solution, ethanol intake was transiently increased in P2X4R KO versus WT mice during the acquisition of 24-h and limited access ethanol intake. IVM significantly reduced ethanol intake in P2X4R KO and WT mice, but the degree of reduction was 50 % less in the P2X4R KO mice. Western blot analysis identified significant changes in γ-aminobutyric acidA receptor α1 subunit expression in brain regions associated with the regulation of ethanol behaviors in P2X4R KO mice. These findings add to evidence that P2X4Rs contribute to ethanol intake and indicate that there is a complex interaction between P2X4Rs, ethanol, and other neurotransmitter receptor systems.

Sociocommunicative and sensorimotor impairments in male P2X4-deficient mice

Purinergic P2X receptors are a family of ligand-gated ion channels gated by extracellular adenosine 5'-triphosphate (ATP). Of the seven P2X subtypes, P2X4 receptors (P2X4Rs) are richly expressed in the brain, yet their role in behavioral organization remains poorly understood. In this study, we examined the behavioral responses of P2X4R heterozygous (HZ) and knockout (KO) mice in a variety of testing paradigms designed to assess complementary aspects of sensory functions, emotional reactivity, and cognitive organization. P2X4R deficiency did not induce significant alterations of locomotor activity and anxiety-related indices in the novel open field and elevated plus-maze tests. Conversely, P2X4R KO mice displayed marked deficits in acoustic startle reflex amplitude, as well as significant sensorimotor gating impairments, as assessed by the prepulse inhibition of the startle. In addition, P2X4R KO mice displayed enhanced tactile sensitivity, as signified by a lower latency in the sticky-tape removal test. Moreover, both P2X4R HZ and KO mice showed significant reductions in social interaction and maternal separation-induced ultrasonic vocalizations in pups. Notably, brain regions of P2X4R KO mice exhibited significant brain-regional alterations in the subunit composition of glutamate ionotropic receptors. These results collectively document that P2X4-deficient mice exhibit a spectrum of phenotypic abnormalities partially akin to those observed in other murine models of autism-spectrum disorder. In conclusion, our findings highlight a putative role of P2X4Rs in the regulation of perceptual and sociocommunicative functions and point to these receptors as putative targets for disturbances associated with neurodevelopmental disorders.

Tryptophan 46 is a site for ethanol and ivermectin action in P2X4 receptors

ATP-gated purinergic P2X4 receptors (P2X4Rs) are the most alcohol-sensitive P2XR subtype. We recently reported that ivermectin (IVM), an antiparasitic used in animals and humans, antagonized ethanol inhibition of P2X4Rs. Furthermore, IVM reduced ethanol intake in mice. The first molecular model of the rat P2X4R, built onto the X-ray crystal structure of zebrafish P2X4R, revealed an action pocket for both ethanol and IVM formed by Asp331, Met336 in TM2 and Trp46, and Trp50 in TM1 segments. The role of Asp331 and Met336 was experimentally confirmed. The present study tested the hypothesis that Trp46 plays a role in ethanol and IVM modulation of P2X4Rs. Trp46 was mutated to residues with different physicochemical properties and the resultant mutants tested for ethanol and IVM responses using Xenopus oocyte expression system and two-electrode voltage clamp. Nonaromatic substitutions at position 46 reduced ethanol inhibition at higher concentrations and switched IVM potentiation to inhibition. Simultaneous substitution of alanine at positions Trp46 and Met336 also resulted in similar changes in ethanol and IVM responses. Furthermore, a new molecular model based on the open pore conformation of zebrafish P2X4R suggested a role for Tyr42 that was further supported experimentally. Our previous and current findings, combined with our preliminary evidence of increased ethanol consumption in P2X4R knockout mice, suggest that the ethanol and IVM action pocket in P2X4Rs formed by positions 42, 46, 331, and 336 presents a potential target for medication development for alcohol use disorders.