Presence of ethanol-sensitive glycine receptors in medium spiny neurons in the mouse nucleus accumbens

Changes in ethanol effects in knock-in mice expressing ethanol insensitive alpha1 and alpha2 glycine receptor subunits

AIMS

Glycine receptors (GlyRs) are potentiated by physiologically relevant concentrations of ethanol, and mutations in the intracellular loop of α1 and α2 subunits reduced the effect of the drug. Knock-in (KI) mice having these individual mutations revealed that α1 and α2 subunits played a role in ethanol-induced sedation and ethanol intake. In this study, we wanted to examine if the effects of stacking both mutations in a 2xKI mouse model (α1/α2) generated by a selective breeding strategy further impacted cellular and behavioral responses to ethanol.

MAIN METHODS

We used electrophysiological recordings to examine ethanol's effect on GlyRs and evaluated ethanol-induced neuronal activation using c-Fos immunoreactivity and the genetically encoded calcium indicator GCaMP6s in the nucleus accumbens (nAc). We also examined ethanol-induced behavior using open field, loss of the righting response, and drinking in the dark (DID) paradigm.

KEY FINDINGS

Ethanol did not potentiate GlyRs nor affect neuronal excitability in the nAc from 2xKI. Moreover, ethanol decreased the Ca2+ signal in WT mice, whereas there were no changes in the signal in 2xKI mice. Interestingly, there was an increase in c-Fos baseline in the 2xKI mice in the absence of ethanol. Behavioral assays showed that 2xKI mice recovered faster from a sedative dose of ethanol and had higher ethanol intake on the first test day of the DID test than WT mice. Interestingly, an open-field assay showed that 2xKI mice displayed less anxiety-like behavior than WT mice.

SIGNIFICANCE

The results indicate that α1 and α2 subunits are biologically relevant targets for regulating sedative effects and ethanol consumption.

Loss of glycine receptors in the nucleus accumbens and ethanol reward in an Alzheimer´s Disease mouse model

Alterations in cognitive and non-cognitive cerebral functions characterize Alzheimer's disease (AD). Cortical and hippocampal impairments related to extracellular accumulation of Aβ in AD animal models have been extensively investigated. However, recent reports have also implicated intracellular Aβ in limbic regions, such as the nucleus accumbens (nAc). Accumbal neurons express high levels of inhibitory glycine receptors (GlyRs) that are allosterically modulated by ethanol and have a role in controlling its intake. In the present study, we investigated how GlyRs in the 2xTg mice (AD model) affect nAc functions and ethanol intake behavior. Using transgenic and control aged-matched litter mates, we found that the GlyRα2 subunit was significantly decreased in AD mice (6-month-old). We also examined intracellular calcium dynamics using the fluorescent calcium protein reporter GCaMP in slice photometry. We also found that the calcium signal mediated by GlyRs, but not GABAAR, was also reduced in AD neurons. Additionally, ethanol potentiation was significantly decreased in accumbal neurons in the AD mice. Finally, we performed drinking in the dark (DID) experiments and found that 2xTg mice consumed less ethanol on the last day of DID, in agreement with a lower blood ethanol concentration. 2xTg mice also showed lower sucrose consumption, indicating that overall food reward was altered. In conclusion, the data support the role of GlyRs in nAc neuron excitability and a decreased glycinergic activity in the 2xTg mice that might lead to impairment in reward processing at an early stage of the disease.

Reducing the harms of alcohol: nutritional interventions and functional alcohol alternatives

The health risks and harm associated with regular alcohol consumption are well documented. In a recent WHO statement published in The Lancet Public Health alcohol consumption has been estimated to contribute worldwide to 3 million deaths in 2016 while also being responsible for 5·1% of the global burden of disease and injury. The total elimination of alcohol consumption, which has been long imbedded in human culture and society, is not practical and prohibition policies have proved historically ineffective. However, valuable strategies to reduce alcohol harms are already available and improved alternative approaches are currently being developed. Here, we will review and discuss recent advances on two main types of approaches, that is nutritional interventions and functional alcohol alternatives.

Alcohol and the dopamine system

The mesolimbic dopamine pathway plays a major role in drug reinforcement and is likely involved also in the development of drug addiction. Ethanol, like most addictive drugs, acutely activates the mesolimbic dopamine system and releases dopamine, and ethanol-associated stimuli also appear to trigger dopamine release. In addition, chronic exposure to ethanol reduces the baseline function of the mesolimbic dopamine system. The molecular mechanisms underlying ethanol´s interaction with this system remain, however, to be unveiled. Here research on the actions of ethanol in the mesolimbic dopamine system, focusing on the involvement of cystein-loop ligand-gated ion channels, opiate receptors, gastric peptides and acetaldehyde is briefly reviewed. In summary, a great complexity as regards ethanol´s mechanism(s) of action along the mesolimbic dopamine system has been revealed. Consequently, several new targets and possibilities for pharmacotherapies for alcohol use disorder have emerged.

The GlyT1-inhibitor Org 24598 facilitates the alcohol deprivation abolishing and dopamine elevating effects of bupropion + varenicline in rats

Alcohol Use Disorder (AUD) is a relapsing brain disorder that involves perturbations of brain dopamine (DA) systems, and combined treatment with varenicline + bupropion produces additive effects on accumbal DA output and abolishes the alcohol deprivation effect (ADE) in rats. Also, direct and indirect glycine receptor (GlyR) agonists raise basal DA, attenuate alcohol-induced DA release in the nucleus Accumbens (nAc) and reduce alcohol consumption in rats. This study in rats examines whether the GlyT1-inhibitor Org 24598, an indirect GlyR agonist, enhances the ADE-reducing and DA elevating action of the combined administration of varenicline + bupropion in lower doses than previously applied. Effects on voluntary alcohol consumption, the ADE and extracellular levels of glycine and DA in nAc were examined following treatment with Org 24598 6 and 9 mg/kg i.p., bupropion 3.75 mg/kg i.p. and varenicline 1.5 mg/kg s.c., in monotherapy or combined, using a two-bottle, free-choice alcohol consumption paradigm with an ADE paradigm, and in vivo microdialysis in male Wistar rats. Notably, all treatment regimens appeared to abolish the ADE but only the effect produced by the triple combination (Org24598 + varenicline + bupropion) was significant compared to vehicle. Hence, addition of Org 24598 may enhance the ADE-reducing action of varenicline + bupropion and appears to allow for a dose reduction of bupropion. Treatment with Org 24598 raised accumbal glycine levels but did not significantly alter DA output in monotherapy. Varenicline + bupropion produced a substantial elevation in accumbal DA output that was slightly enhanced following addition of Org 24598. Conceivably, the blockade of the ADE is achieved by the triple combination enhancing accumbal DA transmission in complementary ways, thereby alleviating a hypothesized hypodopaminergia and negative reinforcement to drink. Ultimately, combining an indirect or direct GlyR agonist with varenicline + bupropion may constitute a new pharmacological treatment principle for AUD, although further refinement in dosing and evaluation of other glycinergic compounds are warranted.

Regulation of ethanol-mediated dopamine elevation by glycine receptors located on cholinergic interneurons in the nucleus accumbens

Alcohol use disorder is one of the major psychiatric disorders worldwide, and there are many factors and effects contributing to the disorder, for example, the experience of ethanol reward. The rewarding and reinforcing properties of ethanol have been linked to activation of the mesolimbic dopamine system, an effect that appears to involve glycine receptors (GlyRs) in the nucleus accumbens. On which neuronal subtypes these receptors are located is, however, not known. The aim of this study was to explore the role of GlyRs on cholinergic interneurons (CIN) in sustaining extracellular dopamine levels and in ethanol-induced dopamine release. To this end, CIN were ablated by anti-choline acetyltransferase-saporin administered locally in the nucleus accumbens of male Wistar rats. Changes in dopamine levels induced by ablation, ethanol and/or a GlyR antagonist were monitored using in vivo microdialysis. The GlyRs antagonist strychnine depressed extracellular dopamine in a similar manner independent on local ablation, suggesting that GlyRs on CIN are not important for sustaining the extracellular dopamine tone. However, a low concentration of strychnine hampered ethanol-induced dopamine release in sham-treated animals, whilst no reduction was seen in ablated animals, suggesting that GlyRs located on CIN are involved in ethanol-induced dopamine release. Further, in ablated rats, ethanol-induced increases of the extracellular levels of the GlyR agonists glycine and taurine were attenuated. In conclusion, this study suggests that CIN are not important for GlyR-mediated regulation of basal dopamine output, but that CIN ablation blunts the ethanol-induced dopamine release, putatively by reducing the release of GlyR agonists.

Overexpression of wild type glycine alpha 1 subunit rescues ethanol sensitivity in accumbal receptors and reduces binge drinking in mice

The nucleus accumbens (nAc) is a critical region in the brain reward system since it integrates abundant synaptic inputs contributing to the control of neuronal excitability in the circuit. The presence of inhibitory α1 glycine receptor (GlyRs) subunits, sensitive to ethanol, has been recently reported in accumbal neurons suggesting that they are protective against excessive binge consumption. In the present study, we used viral vectors (AAV) to overexpress mutant and WT α1 subunits in accumbal neurons in D1 Cre and α1 KI mice. Injection of a Cre-inducible AAV carrying an ethanol insensitive α1 subunit in D1 Cre neurons was unable to affect sensitivity to ethanol in GlyRs or affect ethanol drinking. On the other hand, using an AAV that transduced WT α1 GlyRs in GABAergic neurons in the nAc of high-ethanol consuming mice caused a reduction in ethanol intake as reflected by lowered drinking in the dark and reduced blood ethanol concentration. As expected, the AAV increased the glycine current density by 5-fold without changing the expression of GABAA receptors. Examination of the ethanol sensitivity in isolated accumbal neurons indicated that the GlyRs phenotype changed from an ethanol resistant to an ethanol sensitive type. These results support the conclusion that increased inhibition in the nAc can control excessive ethanol consumption and that selective targeting of GlyRs by pharmacotherapy might provide a mechanistic procedure to reduce ethanol binge.

The beer component hordenine inhibits alcohol addiction-associated behaviours in mice

Alcohol consumption is a widespread behaviour that may eventually result in the development of alcohol use disorder (AUD). Alcohol, however, is rarely consumed in pure form but in fruit- or corn-derived preparations, like beer. These preparations add other compounds to the consumption, which may critically modify alcohol intake and AUD risk. We investigated the effects of hordenine, a barley-derived beer compound on alcohol use-related behaviours. We found that the dopamine D2 receptor agonist hordenine (50 mg/kg) limited ongoing alcohol consumption and prophylactically diminished relapse drinking after withdrawal in mice. Although not having reinforcing effects on its own, hordenine blocked the establishment of alcohol-induced conditioned place preference (CPP). However, it independently enhanced alcohol CPP retrieval. Hordenine had a dose-dependent inhibitory effect on locomotor activity. Chronic hordenine exposure enhanced monoamine tissue levels in many brain regions. Further characterization revealed monoaminergic binding sites of hordenine and found a strong binding on the serotonin and dopamine transporters, and dopamine D3 , and adrenergic α1A and α2A receptor activation but no effects on GABAA receptor or glycinergic signalling. These findings suggest that natural ingredients of beer, like hordenine, may work as an inhibitory and use-regulating factor by their modulation of monoaminergic signalling in the brain.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Behavioral genetics of alcohol's effects in three zebrafish (Danio rerio) populations

Alcohol abuse is one of the most dangerous and serious problems for patients and society. Interpopulation studies are important in understanding how genetic background contributes to the effects of alcohol. In this study, we applied a chronic alcohol exposure protocol in three zebrafish populations (Danio rerio; both sexes; AB, TU, and outbred fish - OB). We analyzed the behavioral responses and mRNA expression involved in neurotransmitter metabolism - th1, tph1, ache, ada1, gaba1, gad1b, and bdnf. Locomotion patterns were similar between populations (increased speed after acute alcohol and unaltered locomotion after chronic and withdrawal treatments). All populations exhibited increased expression of genes associated with locomotion (th1, gad1b, and gaba1) after acute alcohol exposure. Anxiety-like responses increased in AB and TU fish during withdrawal and decreased in AB fish after acute alcohol exposure. Genes related to anxiety-like behavior (tph1 and ada1) were overexpressed in AB and TU fish after acute and withdrawal treatments, while OB fish exhibited unaltered responses. Bdnf levels decreased during withdrawal in AB and OB fish, while TU showed upregulated levels in both chronic and withdrawal treatments. Our results suggest that zebrafish populations respond differently to alcohol exposure, which may contribute to understanding the mechanisms underlying alcohol use and dependence. Moreover, we found that a more diverse genetic background (OB) was related to higher variability in behavioral and mRNA expression, demonstrating that inbred populations (AB and TU) may be useful tools in identifying alcohol use and abuse mechanisms.

Modulatory Actions of the Glycine Receptor β Subunit on the Positive Allosteric Modulation of Ethanol in α2 Containing Receptors

Alpha1-containing glycine receptors (GlyRs) are major mediators of synaptic inhibition in the spinal cord and brain stem. Recent studies reported the presence of α2-containing GlyRs in other brain regions, such as nucleus accumbens and cerebral cortex. GlyR activation decreases neuronal excitability associated with sensorial information, motor control, and respiratory functions; all of which are significantly altered during ethanol intoxication. We evaluated the role of β GlyR subunits and of two basic amino acid residues, K389 and R390, located in the large intracellular loop (IL) of the α2 GlyR subunit, which are important for binding and functional modulation by Gβγ, the dimer of the trimeric G protein conformation, using HEK-293 transfected cells combined with patch clamp electrophysiology. We demonstrate a new modulatory role of the β subunit on ethanol sensitivity of α2 subunits. Specifically, we found a differential allosteric modulation in homomeric α2 GlyRs compared with the α2β heteromeric conformation. Indeed, while α2 was insensitive, α2β GlyRs were substantially potentiated by ethanol, GTP-γ-S, propofol, Zn²⁺ and trichloroethanol. Furthermore, a Gβγ scavenger (ct-GRK2) selectively attenuated the effects of ethanol on recombinant α2β GlyRs. Mutations in an α2 GlyR co-expressed with the β subunit (α2AAβ) specifically blocked ethanol sensitivity, but not propofol potentiation. These results show a selective mechanism for low ethanol concentration effects on homomeric and heteromeric conformations of α2 GlyRs and provide a new mechanism for ethanol pharmacology, which is relevant to upper brain regions where α2 GlyRs are abundantly expressed.

Contribution of GlyR α3 Subunits to the Sensitivity and Effect of Ethanol in the Nucleus Accumbens

The glycine receptor (GlyR), a ligand-gated ion channel, is critical for inhibitory neurotransmission in brainstem, spinal cord, and in supraspinal regions. Recent data from several laboratories have shown that GlyRs are expressed in the brain reward circuitry and that α1 and α2 are the principal subunits expressed in the nucleus accumbens (nAc). In the present study, we studied the sensitivity to ethanol of homomeric and heteromeric α3 GlyR subunits in HEK293 cells and dissociated neurons from the nAc. Finally, we explored ethanol-related behaviors in a Glra3 knockout mouse (Glra3^–/–). Studies in HEK293 cells showed that while homomeric α3 GlyR subunits were insensitive to ethanol, heteromeric α3β GlyR subunits showed higher sensitivity to ethanol. Additionally, using electrophysiological recordings in dissociated accumbal neurons, we found that the glycine current density increased in Glra3^–/– mice and the GlyRs were less affected by ethanol and picrotoxin. We also examined the effect of ethanol on sedation and drinking behavior in Glra3^–/– mice and found that the duration in the loss of righting reflex (LORR) was unchanged compared to wild-type (WT) mice. On the other hand, using the drinking in the dark (DID) paradigm, we found that Glra3^–/– mice have a larger ethanol consumption compared to WT mice, and that this was already high during the first days of exposure to ethanol. Our results support the conclusion that heteromeric α3β, but not homomeric α3, GlyRs are potentiated by ethanol. Also, the increase in GlyR and GABA_AR mediated current densities in accumbal neurons in the KO mice support the presence of compensatory changes to α3 knock out. The increase in ethanol drinking in the Glra3^–/– mice might be associated to the reduction in β and compensatory changes in other subunits in the receptor arrangement.

Synaptic dysregulation and hyperexcitability induced by intracellular amyloid beta oligomers

Intracellular amyloid beta oligomer (iAβo) accumulation and neuronal hyperexcitability are two crucial events at early stages of Alzheimer's disease (AD). However, to date, no mechanism linking iAβo with an increase in neuronal excitability has been reported. Here, the effects of human AD brain-derived (h-iAβo) and synthetic (iAβo) peptides on synaptic currents and action potential firing were investigated in hippocampal neurons. Starting from 500 pM, iAβo rapidly increased the frequency of synaptic currents and higher concentrations potentiated the AMPA receptor-mediated current. Both effects were PKC-dependent. Parallel recordings of synaptic currents and nitric oxide (NO)-associated fluorescence showed that the increased frequency, related to pre-synaptic release, was dependent on a NO-mediated retrograde signaling. Moreover, increased synchronization in NO production was also observed in neurons neighboring those dialyzed with iAβo, indicating that iAβo can increase network excitability at a distance. Current-clamp recordings suggested that iAβo increased neuronal excitability via AMPA-driven synaptic activity without altering membrane intrinsic properties. These results strongly indicate that iAβo causes functional spreading of hyperexcitability through a synaptic-driven mechanism and offers an important neuropathological significance to intracellular species in the initial stages of AD, which include brain hyperexcitability and seizures.

Presence of ethanol sensitive and insensitive glycine receptors in the ventral tegmental area and prefrontal cortex in mice

BACKGROUND AND PURPOSE

Previous studies showed that glycine receptors (GlyRs) composed of α1 and β subunits are primarily found in spinal cord and brainstem and are potentiated by ethanol (10-100 mM). However, much less is known about the presence, composition, and ethanol sensitivity of GlyRs in higher CNS regions. In the present study, we examined two regions of the brain reward system, the ventral tegmental area (VTA) and the prefrontal cortex (PFC), to determine their GlyR subunit composition and sensitivity to ethanol.

EXPERIMENTAL APPROACH

To achieve these aims, we used Western blot, immunohistochemistry and electrophysiological techniques in three different models: Wild-type C57BL/6, GlyR α1 knock-in and GlyR α2 knockout mice.

KEY RESULTS

Similar levels of α and β GlyR subunits were detected in both brain regions, and electrophysiological recordings demonstrated the presence of glycine-activated currents in both areas. The sensitivity of GlyRs to glycine was lower in the PFC compared to VTA. Picrotoxin blocked the glycine-activated current in the PFC and VTA only partially, indicating that both regions express heteromeric αβ receptors. Interestingly, GlyRs in VTA neurons, but not in PFC neurons, were potentiated by ethanol.

CONCLUSION AND IMPLICATIONS

GlyRs in VTA neurons from WT and α2 KO mice were potentiated by ethanol, but not in neurons from the α1 KI mice, supporting the conclusion that α1 GlyRs are predominantly expressed in the VTA. By contrast, GlyRs in PFC neurons were not potentiated in any of the mouse models studied, suggesting the presence of either α2/α3/α4 rather than α1 GlyR subunits.

Reduced sedation and increased ethanol consumption in knock-in mice expressing an ethanol insensitive alpha 2 subunit of the glycine receptor

Previous studies have shown the presence of several subunits of the inhibitory glycine receptor (GlyR) in the reward system, specifically in medium spiny neurons (MSNs) of the nucleus Accumbens (nAc). It was suggested that GlyR α1 subunits regulate nAc excitability and ethanol consumption. However, little is known about the role of the α2 subunit in the adult brain since it is a subunit highly expressed during early brain development. In this study, we used genetically modified mice with a mutation (KR389-390AA) in the intracellular loop of the GlyR α2 subunit which results in a heteromeric α2β receptor that is insensitive to ethanol. Using this mouse model denoted knock-in α2 (KI α2), our electrophysiological studies showed that neurons in the adult nAc expressed functional KI GlyRs that were rather insensitive to ethanol when compared with WT GlyRs. In behavioral tests, the KI α2 mice did not show any difference in basal motor coordination, locomotor activity, or conditioned place preference compared with WT littermate controls. In terms of ethanol response, KI α2 male mice recovered faster from the administration of ataxic and sedative doses of ethanol. Furthermore, KI α2 mice consumed higher amounts of ethanol in the first days of the drinking in the dark protocol, as compared with WT mice. These results show that the α2 subunit is important for the potentiation of GlyRs in the adult brain and this might result in reduced sedation and increased ethanol consumption.

Effects of systemic glycine on accumbal glycine and dopamine levels and ethanol intake in male Wistar rats

Approved medications for alcohol use disorder (AUD) display modest effect sizes. Pharmacotherapy aimed at the mechanism(s) by which ethanol activates the dopamine reward pathway may offer improved outcomes. Basal and ethanol-induced accumbal dopamine release in the rat involve glycine receptors (GlyR) in the nucleus accumbens (nAc). Glycine transporter 1 (GlyT-1) inhibitors, which raise extracellular glycine levels, have repeatedly been shown to decrease ethanol intake in the rat. To further explore the rational for elevating glycine levels in the treatment of AUD, this study examined accumbal extracellular glycine and dopamine levels and voluntary ethanol intake and preference in the rat, after systemic treatment with glycine. The effects of three different doses of glycine i.p. on accumbal glycine and dopamine levels were examined using in vivo microdialysis in Wistar rats. In addition, the effects of the intermediate dose of glycine on voluntary ethanol intake and preference were examined in a limited access two-bottle ethanol/water model in the rat. Systemic glycine treatment increased accumbal glycine levels in a dose-related manner, whereas accumbal dopamine levels were elevated in a subpopulation of animals, defined as dopamine responders. Ethanol intake and preference decreased after systemic glycine treatment. These results give further support to the concept of elevating central glycine levels to reduce ethanol intake and indicate that targeting the glycinergic system may represent a pharmacologic treatment principle for AUD.

Changes in neuronal excitability and synaptic transmission in nucleus accumbens in a transgenic Alzheimer's disease mouse model

Several previous studies showed that hippocampus and cortex are affected in Alzheimer's disease (AD). However, other brain regions have also been found to be affected and could contribute with new critical information to the pathophysiological basis of AD. For example, volumetric studies in humans have shown a significant atrophy of the striatum, particularly in the nucleus Accumbens (nAc). The nAc is a key component of the limbic reward system and it is involved in cognition and emotional behaviors such as pleasure, fear, aggression and motivations, all of which are affected in neurodegenerative diseases such as AD. However, its role in AD has not been extensively studied. Therefore, using an AD mouse model, we investigated if the nAc was affected in 6 months old transgenic 2xTg (APP/PS1) mice. Immunohistochemistry (IHC) analysis in 2xTg mice showed increased intraneuronal Aβ accumulation, as well as occasional extracellular amyloid deposits detected through Thioflavin-S staining. Interestingly, the intracellular Aβ pathology was associated to an increase in membrane excitability in dissociated medium spiny neurons (MSNs) of the nAc. IHC and western blot analyses showed a decrease in glycine receptors (GlyR) together with a reduction in the pre- and post-synaptic markers SV2 and gephyrin, respectively, which correlated with a decrease in glycinergic miniature synaptic currents in nAc brain slices. Additionally, voltage-clamp recordings in dissociated MSNs showed a decrease in AMPA- and Gly-evoked currents. Overall, these results showed intracellular Aβ accumulation together with an increase in excitability and synaptic alterations in this mouse model. These findings provide new information that might help to explain changes in motivation, anhedonia, and learning in the onset of AD pathogenesis.

Ethanol consumption and sedation are altered in mice lacking the glycine receptor α2 subunit

BACKGROUND AND PURPOSE

The precise mechanism/s of action of ethanol, although studied for many years, are not well understood. Like other drugs of abuse, ethanol affects dopamine levels in the nucleus accumbens (nAc), an important region of the mesolimbic system, causing a reinforcing effect. It has been shown that glycine receptors (GlyRs) present in the nAc are potentiated by clinically relevant concentrations of ethanol, where α1 and α2 are the predominant subunits expressed.

EXPERIMENTAL APPROACH

Using a combination of electrophysiology and behavioural assays, we studied the involvement of GlyR α2 subunits on the effects of low and high doses of ethanol, as well as on consumption using mice lacking the GlyR α2 subunit (male Glra2-/Y and female Glra2-/- ).

KEY RESULTS

GlyR α2 subunits exist in accumbal neurons, since the glycine-evoked currents and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) in Glra2-/Y mice were drastically decreased. In behavioural studies, differences in ethanol consumption and sedation were observed between wild-type (WT) and Glra2 knockout (KO) mice. Using the drinking in the dark (DID) paradigm, we found that Glra2-/Y mice presented a binge-like drinking behaviour immediately when exposed to ethanol rather than the gradual consumption seen in WT animals. Interestingly, the effect of knocking out Glra2 in female (Glra2-/- ) mice was less evident, since WT female mice already showed higher DID.

CONCLUSION AND IMPLICATIONS

The differences in ethanol consumption between WT and KO mice provide additional evidence supporting the conclusion that GlyRs are biologically relevant targets for the sedative and rewarding properties of ethanol.

Modulation of glycine receptor single-channel conductance by intracellular phosphorylation

Glycine receptors (GlyRs) are anion-permeable pentameric ligand-gated ion channels (pLGICs). The GlyR activation is critical for the control of key neurophysiological functions, such as motor coordination, respiratory control, muscle tone and pain processing. The relevance of the GlyR function is further highlighted by the presence of abnormal glycinergic inhibition in many pathophysiological states, such as hyperekplexia, epilepsy, autism and chronic pain. In this context, previous studies have shown that the functional inhibition of  GlyRs containing the α3 subunit is a pivotal mechanism of pain hypersensitivity. This pathway involves the activation of EP2 receptors and the subsequent PKA-dependent phosphorylation of α3GlyRs within the intracellular domain (ICD), which decrease the GlyR-associated currents and enhance neuronal excitability. Despite the importance of this mechanism of glycinergic dis-inhibition associated with dysfunctional α3GlyRs, our current understanding of the molecular events involved is limited. Here, we report that the activation of PKA signaling pathway decreases the unitary conductance of α3GlyRs. We show in addition that the substitution of the PKA-targeted serine with a negatively charged residue within the ICD of α3GlyRs and of chimeric receptors combining bacterial GLIC and α3GlyR was sufficient to generate receptors with reduced conductance. Thus, our findings reveal a potential biophysical mechanism of glycinergic dis-inhibition and suggest that post-translational modifications of the ICD, such as phosphorylation, may shape the conductance of other pLGICs.

Influence of nonsynaptic α1 glycine receptors on ethanol consumption and place preference

Here, we used knock-in (KI) mice that have ethanol-insensitive alpha 1 glycine receptors (GlyRs) (KK385/386AA) to examine how alpha 1 GlyRs might affect binge drinking and conditioned place preference. Data show that tonic alpha 1 GlyR-mediated currents were exclusively sensitive to ethanol only in wild-type mice. Behavioral studies showed that the KI mice have a higher intake of ethanol upon first exposure to drinking and greater conditioned place preference to ethanol. This study suggests that nonsynaptic alpha 1-containing GlyRs have a role in motivational and early reinforcing effects of ethanol.

High ethanol sensitive glycine receptors regulate firing in D1 medium spiny neurons in the nucleus accumbens

Inhibitory glycine receptors (GlyRs) are widely expressed in spinal cord and brain stem. They are also expressed in the nucleus Accumbens (nAc) where they have been implicated in the release of dopamine from the ventral tegmental area to the nAc in the presence of ethanol. One of the major types of neurons in the nAc are the Dopamine 1 receptor-expressing (D1+) medium spiny neurons (MSNs) that are activated when addictive drugs, like ethanol, are administrated. Thus, D1(+) MSNs are a relevant target for the study of ethanol effects. Here, using electrophysiological recordings, we report that GlyRs in D1(+) MSNs are highly sensitive to ethanol, with potentiation starting at 5 mM (26 ± 5%). Single channel recordings in D1(+) MSNs showed that 10 mM ethanol increased the open probability of the channel (0.22 ± 0.05 versus 0.66 ± 0.16), but did not affect channel conductance (~40 pS). A glycinergic mediated tonic current in D1(+) MSNs was potentiated by 10 and 50 mM ethanol causing a reduction in the excitability of these cells. A 34 ± 7% reduction in action potential firing was observed in these neurons in the presence of 50 mM ethanol. Interestingly, no effects of ethanol were detected in the presence of strychnine or in D1(-) MSNs in the nAc. These results indicate that GlyRs present in D1(+) MSNs are sensitive to low concentrations of ethanol, and that potentiation of this inhibitory current regulates the activation of nAc, acting as a homeostatic signal that would prevent over-activation of the reward system when drugs like ethanol are consumed.

Activation of glycine receptors in the lateral habenula rescues anxiety- and depression-like behaviors associated with alcohol withdrawal and reduces alcohol intake in rats

The lateral habenula (LHb) is activated by a range of aversive states including those related to alcohol withdrawal and has glycine receptors (GlyRs), a sensitive target of alcohol. However, whether GlyRs in the LHb contribute to alcohol-related behaviors is unknown. Here, we report that rats experiencing withdrawal from chronic alcohol consumption showed higher anxiety and sensitivity to stress compared to their alcohol-naïve counterparts. Intra-LHb injection of glycine attenuated these aberrant behaviors and reduced alcohol intake upon alcohol re-access. Glycine's effect was blocked by strychnine, a GlyR antagonist, indicating that it was mediated by strychnine-sensitive GlyRs. Conversely, intra-LHb strychnine elicited anxiety- and depression-like behaviors in Naïve rats but not in withdrawal rats. Additionally, both the frequency and the amplitude of the spontaneous IPSCs were lower in LHb neurons in slices of withdrawal rats compared to naïve rats. Also, there were sporadic strychnine-sensitive synaptic events in some LHb neurons. Bath perfusion of strychnine induced a depolarizing inward current and increased action potential firings in LHb neurons. By contrast, bath perfusion of glycine or sarcosine, a glycine transporter subtype 1 inhibitor, inhibited LHb activity. Collectively, these data reveal that LHb neurons are under the tonic glycine inhibition both in physiological and pathological conditions. Activation of GlyRs reverses LHb hyperactivity, alleviates aberrant behaviors, and reduces alcohol intake, thus highlighting the GlyRs in the LHb as a potential therapeutic target for alcohol-use disorders.

Chronic Oxycodone Self-administration Altered Reward-related Genes in the Ventral and Dorsal Striatum of C57BL/6J Mice: An RNA-seq Analysis

Prescription opioid abuse, for example of oxycodone, is a pressing public health issue. This study focuses on how chronic oxycodone self-administration (SA) affects the reward pathways in the mouse brain. In this study, we tested the hypothesis that the expression of reward-related genes in the ventral and dorsal striatum, areas involved in different aspects of opioid addiction models, was altered within 1 h after chronic oxycodone SA, using transcriptome-wide sequencing (RNA-seq). Based on results from earlier human genetic and rodent preclinical studies, we focused on a set of genes that may be associated with the development of addictive diseases and the rewarding effect of drugs of abuse, primarily in the opioid, stress response and classical neurotransmitter systems. We found that 32 transcripts in the ventral striatum, and 7 in the dorsal striatum, were altered significantly in adult mice that had self-administered oxycodone (n = 5) for 14 consecutive days (4 h/day) compared with yoked saline controls (n = 5). The following 5 genes in the ventral striatum showed experiment-wise significant changes: proopiomelanocortin (Pomc) and serotonin 5-HT-2A receptor (Htr2a) were upregulated; serotonin receptor 7 (Htr7), galanin receptor1 (Galr1) and glycine receptor 1 (Glra1) were downregulated. Some genes detected by RNA-seq were confirmed by quantitative polymerase chain reaction (qPCR). Conclusion: A RNA-seq study shows that chronic oxycodone SA alters the expression of several reward-related genes in the dorsal and ventral striatum. These results suggest potential mechanisms underlying neuronal adaptation to chronic oxycodone self-exposure, of relevance to our mechanistic understanding of prescription opioid abuse.

Presence of Inhibitory Glycinergic Transmission in Medium Spiny Neurons in the Nucleus Accumbens

It is believed that the rewarding actions of drugs are mediated by dysregulation of the mesolimbic dopaminergic system leading to increased levels of dopamine in the nucleus accumbens (nAc). It is widely recognized that GABAergic transmission is critical for neuronal inhibition within nAc. However, it is currently unknown if medium spiny neurons (MSNs) also receive inhibition by means of glycinergic synaptic inputs. We used a combination of proteomic and electrophysiology studies to characterize the presence of glycinergic input into MSNs from nAc demonstrating the presence of glycine transmission into nAc. In D1 MSNs, we found low frequency glycinergic miniature inhibitory postsynaptic currents (mIPSCs) which were blocked by 1 μM strychnine (STN), insensitive to low (10, 50 mM) and high (100 mM) ethanol (EtOH) concentrations, but sensitive to 30 μM propofol. Optogenetic experiments confirmed the existence of STN-sensitive glycinergic IPSCs and suggest a contribution of GABA and glycine neurotransmitters to the IPSCs in nAc. The study reveals the presence of glycinergic transmission in a non-spinal region and opens the possibility of a novel mechanism for the regulation of the reward pathway.

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.

Alcohol and the Brain: Neuronal Molecular Targets, Synapses, and Circuits

Ethanol is one of the most commonly abused drugs. Although environmental and genetic factors contribute to the etiology of alcohol use disorders, it is ethanol's actions in the brain that explain (1) acute ethanol-related behavioral changes, such as stimulant followed by depressant effects, and (2) chronic changes in behavior, including escalated use, tolerance, compulsive seeking, and dependence. Our knowledge of ethanol use and abuse thus relies on understanding its effects on the brain. Scientists have employed both bottom-up and top-down approaches, building from molecular targets to behavioral analyses and vice versa, respectively. This review highlights current progress in the field, focusing on recent and emerging molecular, cellular, and circuit effects of the drug that impact ethanol-related behaviors. The focus of the field is now on pinpointing which molecular effects in specific neurons within a brain region contribute to behavioral changes across the course of acute and chronic ethanol exposure.