Dopamine D3 receptors regulate GABAA receptor function through a phospho-dependent endocytosis mechanism in nucleus accumbens

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly at a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory postsynaptic currents (eIPSCs) onto hippocampal principal cells. Importantly, O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude in about half of the recorded cells, mimicking forskolin. Our findings show that under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is possibly accessible to agonists, permitting strengthening of synaptic inhibition.

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly at a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory post synaptic currents (eIPSCs) onto hippocampal principal cells. Importantly, O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude, mimicking forskolin. Our findings show under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is accessible to agonists, permitting strengthening of synaptic inhibition.

Arrestin-3 Agonism at Dopamine D3 Receptors Defines a Subclass of Second-Generation Antipsychotics That Promotes Drug Tolerance

BACKGROUND

Second-generation antipsychotics (SGAs) are frontline treatments for serious mental illness. Often, individual patients benefit only from some SGAs and not others. The mechanisms underlying this unpredictability in treatment efficacy remain unclear. All SGAs bind the dopamine D3 receptor (D3R) and are traditionally considered antagonists for dopamine receptor signaling.

METHODS

Here, we used a combination of two-photon calcium imaging, in vitro signaling assays, and mouse behavior to assess signaling by SGAs at D3R.

RESULTS

We report that some clinically important SGAs function as arrestin-3 agonists at D3R, resulting in modulation of calcium channels localized to the site of action potential initiation in prefrontal cortex pyramidal neurons. We further show that chronic treatment with an arrestin-3 agonist SGA, but not an antagonist SGA, abolishes D3R function through postendocytic receptor degradation by GASP1 (G protein-coupled receptor-associated sorting protein-1).

CONCLUSIONS

These results implicate D3R-arrestin-3 signaling as a source of SGA variability, highlighting the importance of including arrestin-3 signaling in characterizations of drug action. Furthermore, they suggest that postendocytic receptor trafficking that occurs during chronic SGA treatment may contribute to treatment efficacy.

Salicylate Induced GABAAR Internalization by Dopamine D1-Like Receptors Involving Protein Kinase C (PKC) in Spiral Ganglion Neurons

BACKGROUND Sodium salicylate (SS) induces excitotoxicity of spiral ganglion neurons (SGNs) by inhibiting the response of γ-aminobutyric acid type A receptors (GABAARs). Our previous studies have shown that SS can increase the internalization of GABAARs on SGNs, which involves dopamine D1-like receptors (D1Rs) and related signaling pathways. In this study, we aimed to explore the role of D1Rs and their downstream molecule protein kinase C (PKC) in the process of SS inhibiting GABAARs. MATERIAL AND METHODS The expression of D1Rs and GABARγ2 on rat cochlear SGNs cultured in vitro was tested by immunofluorescence. Then, the SGNs were exposed to SS, D1R agonist (SKF38393), D1R antagonist (SCH23390), clathrin/dynamin-mediated endocytosis inhibitor (dynasore), and PKC inhibitor (Bisindolylmaleimide I). Western blotting and whole-cell patch clamp technique were used to assess the changes of surface and total protein of GABARγ2 and GABA-activated currents. RESULTS Immunofluorescence showed that D1 receptors (DRD1) were expressed on SGNs. Data from western blotting showed that SS promoted the internalization of cell surface GABAARs, and activating D1Rs had the same result. Inhibiting D1Rs and PKC decreased the internalization of GABAARs. Meanwhile, the phosphorylation level of GABAARγ2 S327 affected by PKC was positively correlated with the degree of internalization of GABAARs. Moreover, whole-cell patch clamp recording showed that inhibition of D1Rs or co-inhibition of D1Rs and PKC attenuated the inhibitory effect of SS on GABA-activated currents. CONCLUSIONS D1Rs mediate the GABAAR internalization induced by SS via a PKC-dependent manner and participate in the excitotoxic process of SGNs.

Cariprazine alleviates core behavioral deficits in the prenatal valproic acid exposure model of autism spectrum disorder

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction and restricted, repetitive behaviors. The unmet medical need in ASD is considerable since there is no approved pharmacotherapy for the treatment of these deficits in social communication, interaction, and behavior. Cariprazine, a dopamine D₃-preferring D₃/D₂ receptor partial agonist, is already approved for the treatment of schizophrenia and bipolar I disorder in adults; investigation in patients with ASD is warranted.

OBJECTIVES

The aim of this study was to investigate the effects of cariprazine, compared with risperidone and aripiprazole, in the rat prenatal valporic acid (VPA) exposure model on behavioral endpoints representing the core and associated symptoms of ASD.

METHODS

To induce the ASD model, time-mated Wistar rat dams were treated with VPA during pregnancy. Male offspring were assigned to groups and studied in a behavioral test battery at different ages, employing social play, open field, social approach-avoidance, and social recognition memory tests. Animals were dosed orally, once a day for 8 days, with test compounds (cariprazine, risperidone, aripiprazole) or vehicle before behavioral assessment.

RESULTS

Cariprazine showed dose-dependent efficacy on all behavioral endpoints. In the social play paradigm, only cariprazine was effective. On the remaining behavioral endpoints, including the reversal of hyperactivity, risperidone and aripiprazole displayed similar efficacy to cariprazine.

CONCLUSIONS

In the present study, cariprazine effectively reversed core behavioral deficits and hyperactivity present in juvenile and young adult autistic-like rats. These findings indicate that cariprazine may be useful in the treatment of ASD symptoms.

Ventral pallidum DRD3 potentiates a pallido-habenular circuit driving accumbal dopamine release and cocaine seeking

Drugs of abuse induce persistent remodeling of reward circuit function, a process thought to underlie the emergence of drug craving and relapse to drug use. However, how circuit-specific, drug-induced molecular and cellular plasticity can have distributed effects on the mesolimbic dopamine reward system to facilitate relapse to drug use is not fully elucidated. Here, we demonstrate that dopamine receptor D3 (DRD3)-dependent plasticity in the ventral pallidum (VP) drives potentiation of dopamine release in the nucleus accumbens during relapse to cocaine seeking after abstinence. We show that two distinct VP DRD3+ neuronal populations projecting to either the lateral habenula (LHb) or the ventral tegmental area (VTA) display different patterns of activity during drug seeking following abstinence from cocaine self-administration and that selective suppression of elevated activity or DRD3 signaling in the LHb-projecting population reduces drug seeking. Together, our results uncover how circuit-specific DRD3-mediated plasticity contributes to the process of drug relapse.

Characterization of the Functional Cross-Talk between Surface GABAA and Dopamine D5 Receptors

The γ-aminobutyric acid type A receptor (GABAAR) plays a major role in fast inhibitory synaptic transmission and is highly regulated by the neuromodulator dopamine. In this aspect, most of the attention has been focused on the classical intracellular signaling cascades following dopamine G-protein-coupled receptor activation. Interestingly, the GABAAR and dopamine D5 receptor (D5R) have been shown to physically interact in the hippocampus, but whether a functional cross-talk occurs is still debated. In the present study, we use a combination of imaging and single nanoparticle tracking in live hippocampal neurons to provide evidence that GABAARs and D5Rs form dynamic surface clusters. Disrupting the GABAAR-D5R interaction with a competing peptide leads to an increase in the diffusion coefficient and the explored area of both receptors, and a drop in immobile synaptic GABAARs. By means of patch-clamp recordings, we show that this fast lateral redistribution of surface GABAARs correlates with a robust depression in the evoked GABAergic currents. Strikingly, it also shifts in time the expression of long-term potentiation at glutamatergic synapses. Together, our data both set the plasma membrane as the primary stage of a functional interplay between GABAAR and D5R, and uncover a non-canonical role in regulating synaptic transmission.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Cariprazine alleviates core behavioral deficits in the prenatal valproic acid exposure model of autism spectrum disorder

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by deficits in social communication and interaction and restricted, repetitive behaviors. The unmet medical need in ASD is considerable since there is no approved pharmacotherapy for the treatment of these deficits in social communication, interaction, and behavior. Cariprazine, a dopamine D₃-preferring D₃/D₂ receptor partial agonist, is already approved for the treatment of schizophrenia and bipolar I disorder in adults; investigation in patients with ASD is warranted.

OBJECTIVES

The aim of this study was to investigate the effects of cariprazine, compared with risperidone and aripiprazole, in the rat prenatal valporic acid (VPA) exposure model on behavioral endpoints representing the core and associated symptoms of ASD.

METHODS

To induce the ASD model, time-mated Wistar rat dams were treated with VPA during pregnancy. Male offspring were assigned to groups and studied in a behavioral test battery at different ages, employing social play, open field, social approach-avoidance, and social recognition memory tests. Animals were dosed orally, once a day for 8 days, with test compounds (cariprazine, risperidone, aripiprazole) or vehicle before behavioral assessment.

RESULTS

Cariprazine showed dose-dependent efficacy on all behavioral endpoints. In the social play paradigm, only cariprazine was effective. On the remaining behavioral endpoints, including the reversal of hyperactivity, risperidone and aripiprazole displayed similar efficacy to cariprazine.

CONCLUSIONS

In the present study, cariprazine effectively reversed core behavioral deficits and hyperactivity present in juvenile and young adult autistic-like rats. These findings indicate that cariprazine may be useful in the treatment of ASD symptoms.

Evidence and explanation for the involvement of the nucleus accumbens in pain processing

The nucleus accumbens (NAc) is a subcortical brain structure known primarily for its roles in pleasure, reward, and addiction. Despite less focus on the NAc in pain research, it also plays a large role in the mediation of pain and is effective as a source of analgesia. Evidence for this involvement lies in the NAc’s cortical connections, functions, pharmacology, and therapeutic targeting. The NAc projects to and receives information from notable pain structures, such as the prefrontal cortex, anterior cingulate cortex, periaqueductal gray, habenula, thalamus, etc. Additionally, the NAc and other pain-modulating structures share functions involving opioid regulation and motivational and emotional processing, which each work beyond simply the rewarding experience of pain offset. Pharmacologically speaking, the NAc responds heavily to painful stimuli, due to its high density of μ opioid receptors and the activation of several different neurotransmitter systems in the NAc, such as opioids, dopamine, calcitonin gene-related peptide, γ-aminobutyric acid, glutamate, and substance P, each of which have been shown to elicit analgesic effects. In both preclinical and clinical models, deep brain stimulation of the NAc has elicited successful analgesia. The multi-functional NAc is important in motivational behavior, and the motivation for avoiding pain is just as important to survival as the motivation for seeking pleasure. It is possible, then, that the NAc must be involved in both pleasure and pain in order to help determine the motivational salience of positive and negative events.

Protrudin modulates seizure activity through GABAA receptor regulation

Epilepsy is a serious neurological disease characterized by recurrent unprovoked seizures. The exact etiology of epilepsy is not fully understood. Protrudin is a neural membrane protein and is found to be mutated in hereditary spastic paraplegia that characterized by symptoms like seizures. Here, we reported that the expression of protrudin was downregulated in the temporal neocortex of epileptic patients and in the hippocampus and cortex of pentylenetetrazol and kainic acid-kindled epileptic mouse models. Behavioral and electroencephalogram analyses indicated that overexpression of protrudin in the mouse hippocampus increased the latency of the seizure and decreased the frequency and duration of seizure activity. Using whole-cell patch clamp, overexpression of protrudin in the mouse hippocampus resulted in a reduction in action potential frequency and an increase in gamma-aminobutyric acid (GABA)ergic inhibitory current amplitude. Moreover, western blot analysis showed that the membrane expression of the GABA A receptor β2/3 subunit was also upregulated after protrudin overexpression, and coimmunoprecipitation resulted in a protein–protein interaction between protrudin, GABA_ARβ2/3 and GABA receptor-associated protein in the hippocampus of epileptic mice. These findings suggest that protrudin probably inhibits the occurrence and development of epilepsy through the regulation of GABA_A receptor-mediated synaptic transmission, and protrudin might be a promising target for the treatment of epilepsy.

Atlastin-1 modulates seizure activity and neuronal excitability

Epilepsy is a neurological disease, and the main clinical manifestation is recurrent seizures. The exact etiology of epilepsy and the pathogenesis of the disorder are not yet fully understood. Atlastin‐1, a dynamin‐like GTPase, interacts with microtubules and is responsible for vesicle formation, both of which are highly associated with the development of epilepsy. Here, we reported that the expression level of atlastin‐1 protein was reduced in the temporal neocortex of patients with temporal lobe epilepsy and in the hippocampus and adjacent cortex of a pentylenetetrazol‐kindled epileptic mouse model. Cells expressing atlastin‐1 coexpressed the inhibitory synaptic marker GAD67 in the temporal cortex and hippocampus of patients with epilepsy and an epileptic mouse model. The lentivirus‐mediated overexpression of atlastin‐1 protein in the hippocampus of mice suppressed seizure activity in behavioral experiments. Patch‐clamp recordings in the Mg²⁺‐free epilepsy cell model showed that atlastin‐1 overexpression inhibited neuronal excitability by suppressing the discharge frequency of spontaneous action potentials rather than by changing the passive and active properties of action potentials. Inhibitory synaptic transmission, but not excitatory synaptic currents, increased after atlastin‐1 overexpression. These findings suggest that atlastin‐1 likely contributes to the occurrence and development of epilepsy through inhibitory synaptic transmission.

The Role of Dopamine D3 Receptor Partial Agonism in Cariprazine-Induced Neurotransmitter Efflux in Rat Hippocampus and Nucleus Accumbens

Cariprazine is an approved antipsychotic and antidepressant which is a dopamine (DA) D₃-preferring D₃/D₂ receptor partial agonist, serotonin (5-HT) 5-HT_1A receptor partial agonist, and 5-HT_2B and 5-HT_2A receptor antagonist, a profile unique for atypical antipsychotic drugs. The purpose of this study was to clarify the effects of cariprazine and selective D₃ receptor ligands on neurotransmitter efflux in the rat nucleus accumbens (NAC) and ventral hippocampus (HIP), brain regions important for reality testing, rewarded behavior, and cognition. In vivo microdialysis was performed in awake, freely moving rats after administration of cariprazine; (+)-PD-128907 [(4aR,10bR)-3,4a,4,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride], a D₃ receptor-preferring agonist; and SB-277011A [trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolininecarboxamide hydrochloride], a selective D₃ receptor antagonist, alone or combined, and extracellular levels of multiple neurotransmitters and metabolites were measured in the NAC and HIP by ultraperformance liquid chromatography with tandem mass spectrometry. Cariprazine increased DA, norepinephrine (NE), and 5-HT efflux in both regions, whereas it increased glycine (Gly) and glutamate efflux only in the NAC and efflux of DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) only in the HIP. Similarly, SB-277011A increased DA, NE, DOPAC, and HVA, but not 5-HT, efflux in the NAC and HIP, and acetylcholine efflux in the HIP. Most of these effects of cariprazine and SB-277011A were fully or partially attenuated by the D₃ receptor agonist (+)-PD-128907, suggesting these effects of cariprazine are related to its D₃ receptor partial agonism, and that this mechanism, leading to diminished stimulation of D₃ receptors, may contribute to its efficacy in both schizophrenia and bipolar disorder. The possible role of Gly in the action of cariprazine is discussed. SIGNIFICANCE STATEMENT: The novel atypical antipsychotic drug cariprazine increased nucleus accumbens and hippocampal neurotransmitter efflux, similar to the actions of the D₃ receptor antagonist SB-277011A [trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolininecarboxamide hydrochloride]. The D₃ receptor-preferring agonist (+)-PD-128907 [(4aR, 10bR)-3,4a,4,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride], diminished the effects of both compounds on neurotransmitter efflux in both regions. These results suggested D3 receptor partial agonist activity of cariprazine, producing functional antagonism, may contribute to its efficacy in schizophrenia and bipolar disorder.

Salicylate-Induced Ototoxicity of Spiral Ganglion Neurons: Ca2+/CaMKII-Mediated Interaction Between NMDA Receptor and GABAA Receptor

Sodium salicylate (SS) is one of the nonsteroidal anti-inflammatory drugs and widely used in clinical practice. Therefore, we aimed to investigate the potential ototoxicity mechanism of sodium salicylate: the influence of Ca²⁺/calmodulin-dependent protein kinase II (Ca²⁺/CaMKII) in interaction between NMDA receptors (NMDAR) and GABA_A receptors (GABA_AR) in rat cochlear spiral ganglion neurons (SGNs). After treatment with SS, NMDA, and an NMDAR inhibitor (APV), the changes of GABA_AR β3 (GABR β3) mRNA, surface and total protein, and GABA_AR currents in SGNs were assessed by quantitative PCR, Western blot, and whole-cell patch clamp. Mechanistically, SS and/or NMDA increased the GABR β3 mRNA expression, while decreased GABR β3 surface protein levels and GABA_AR-mediated currents. Moreover, application of SS and/or NMDA showed promotion in phosphorylation levels at S383 of GABR β3. Collectively, Ca²⁺ chelator (BAPTA) or Ca²⁺/CaMKII inhibitor (KN-93) reversed the effects of SS and/or NMDA on GABA_AR. Therefore, we hypothesize that the interaction between NMDAR and GABA_AR is involved in the SGNs damage induced by SS. In addition, the underlying molecular mechanism is related to Ca²⁺/CaMKII-mediated signaling pathway, which suggests that the interaction between calcium signal-regulated receptors mediates SS ototoxicity.

Dopaminergic-GABAergic interplay and alcohol binge drinking

The dopamine D₃ receptor (D₃R), in the nucleus accumbens (NAc), plays an important role in alcohol reward mechanisms. The major neuronal type within the NAc is the GABAergic medium spiny neuron (MSN), whose activity is regulated by dopaminergic inputs. We previously reported that genetic deletion or pharmacological blockade of D₃R increases GABA_A α6 subunit in the ventral striatum. Here we tested the hypothesis that D₃R-dependent changes in GABA_A α6 subunit in the NAc affect voluntary alcohol intake, by influencing the inhibitory transmission of MSNs. We performed in vivo and ex vivo experiments in D₃R knockout (D₃R ^-/-) mice and wild type littermates (D₃R ^+/+). Ro 15-4513, a high affinity α6-GABA_A ligand was used to study α6 activity. At baseline, NAc α6 expression was negligible in D₃R^+/+, whereas it was robust in D₃R^-/-; other relevant GABA_A subunits were not changed. In situ hybridization and qPCR confirmed α6 subunit mRNA expression especially in the NAc. In the drinking-in-the-dark paradigm, systemic administration of Ro 15-4513 inhibited alcohol intake in D₃R^+/+, but increased it in D₃R^-/-; this was confirmed by intra-NAc administration of Ro 15-4513 and furosemide, a selective α6-GABA_A antagonist. Whole-cell patch-clamp showed peak amplitudes of miniature inhibitory postsynaptic currents in NAc medium spiny neurons higher in D₃R^-/- compared to D₃R^+/+; Ro 15-4513 reduced the peak amplitude in the NAc of D₃R^-/-, but not in D₃R^+/+. We conclude that D₃R-dependent enhanced expression of α6 GABA_A subunit inhibits voluntary alcohol intake by increasing GABA inhibition in the NAc.

Drd3 Signaling in the Lateral Septum Mediates Early Life Stress-Induced Social Dysfunction

Early life stress (ELS) in the form of child abuse/neglect is associated with an increased risk of developing social dysfunction in adulthood. Little is known, however, about the neural substrates or the neuromodulatory signaling that govern ELS-induced social dysfunction. Here, we show that ELS-induced downregulation of dopamine receptor 3 (Drd3) signaling and its corresponding effects on neural activity in the lateral septum (LS) are both necessary and sufficient to cause social abnormalities in adulthood. Using in vivo Ca²⁺ imaging, we found that Drd3-expressing-LS (Drd3^LS) neurons in animals exposed to ELS show blunted activity in response to social stimuli. In addition, optogenetic activation of Drd3^LS neurons rescues ELS-induced social impairments. Furthermore, pharmacological treatment with a Drd3 agonist, which increases Drd3^LS neuronal activity, normalizes the social dysfunctions of ELS mice. Thus, we identify Drd3 in the LS as a critical mediator and potential therapeutic target for the social abnormalities caused by ELS.

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Early social deprivation (ESD) causes downregulation of Drd3 signaling in the LS

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Blunted LS Drd3 neuronal activity mediate ESD-induced social dysfunctions

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Drd3 signaling has corresponding effects on neuronal activity in the LS

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Activation of Drd3 signaling in the LS normalize social impairments of ESD mice

Abnormalities of Dopamine D3 Receptor Signaling in the Diseased Brain

Dopamine D₃ receptors (D₃R) modulate neuronal activity in several brain regions including cortex, striatum, cerebellum, and hippocampus. A growing body of evidence suggests that aberrant D₃R signaling contributes to multiple brain diseases, such as Parkinson’s disease, essential tremor, schizophrenia, and addiction. In line with these findings, D₃R has emerged as a potential target in the treatment of neurological disorders. However, the mechanisms underlying neuronal D₃R signaling are poorly understood, either in healthy or diseased brain. Here, I review the molecular mechanisms involved in D₃R signaling via monomeric D₃R and heteromeric receptor complexes (e.g., D₃R-D₁R, D₃R-D₂R, D₃R-A_2aR, and D₃R-D₃nf). I focus on D₃R signaling pathways that, according to recent reports, contribute to pathological brain states. In particular, I describe evidence on both quantitative (e.g., increased number or affinity) and qualitative (e.g., switched signaling) changes in D₃R that has been associated with brain dysfunction. I conclude with a description of basic mechanisms that modulate D₃R signaling such as desensitization, as disruption of these mechanisms may underlie pathological changes in D₃R signaling. Because several lines of evidence support the idea that imbalances in D₃R signaling alter neural function, a better understanding of downstream D₃R pathways is likely to reveal novel therapeutic strategies toward dopamine-related brain disorders.

The Small GTPase Rac1 Contributes to Extinction of Aversive Memories of Drug Withdrawal by Facilitating GABAA Receptor Endocytosis in the vmPFC

Extinction of aversive memories has been a major concern in neuropsychiatric disorders, such as anxiety disorders and drug addiction. However, the mechanisms underlying extinction of aversive memories are not fully understood. Here, we report that extinction of conditioned place aversion (CPA) to naloxone-precipitated opiate withdrawal in male rats activates Rho GTPase Rac1 in the ventromedial prefrontal cortex (vmPFC) in a BDNF-dependent manner, which determines GABA_A receptor (GABA_AR) endocytosis via triggering synaptic translocation of activity-regulated cytoskeleton-associated protein (Arc) through facilitating actin polymerization. Active Rac1 is essential and sufficient for GABA_AR endocytosis and CPA extinction. Knockdown of Rac1 expression within the vmPFC of rats using Rac1-shRNA suppressed GABA_AR endocytosis and CPA extinction, whereas expression of a constitutively active form of Rac1 accelerated GABA_AR endocytosis and CPA extinction. The crucial role of GABA_AR endocytosis in the LTP induction and CPA extinction is evinced by the findings that blockade of GABA_AR endocytosis by a dynamin function-blocking peptide (Myr-P4) abolishes LTP induction and CPA extinction. Thus, the present study provides first evidence that Rac1-dependent GABA_AR endocytosis plays a crucial role in extinction of aversive memories and reveals the sequence of molecular events that contribute to learning experience modulation of synaptic GABA_AR endocytosis.SIGNIFICANCE STATEMENT This study reveals that Rac1-dependent GABA_AR endocytosis plays a crucial role in extinction of aversive memories associated with drug withdrawal and identifies Arc as a downstream effector of Rac1 regulations of synaptic plasticity as well as learning and memory, thereby suggesting therapeutic targets to promote extinction of the unwanted memories.

Behavioral effects of D3 receptor inhibition and 5-HT4 receptor activation on animals undergoing chronic cannabinoid exposure during adolescence

Chronic exposure to cannabinoids during adolescence results in long-lasting behavioral deficits that match some symptomatologic aspects of schizophrenia. The aim of this study was to investigate the reversibility of the emotional and the cognitive effects of chronic exposure to cannabinoids during adolescence, via subsequent modulation of the serotoninergic 5-HT4 and dopaminergic D3 receptors. RS67333 as a 5-HT4 agonist and U-99194A as a D3 antagonist were administered separately at 1 mg/kg and 20 mg/kg, and in combination at 0.5 mg/kg and 10 mg/kg to adult animals undergoing chronic treatment with the synthetic cannabinoid receptor agonist WIN55,212-2 (1 mg/kg) during adolescence. Animals were tested for anxiety-like behavior and episodic-like memory in the open field and novel object recognition tests respectively 30 minutes after the last drug administration. Chronic WIN55,212-2 treated animals exhibited a lasting disruption of episodic memory and increased anxiety levels. The effect on episodic-like memory were partially restored by acute administration of RS67333 and U-99194A and completely by administration of both drugs in combination at lower doses. However, only RS67333 (20 mg/kg) improved the anxiogenic-like effect of WIN55,212-2. These findings give further support that chronic exposure to cannabinoids during adolescence may be used as an animal model for schizophrenia, and highlight D3 and 5-HT4 receptors as potential targets for an enhanced treatment of the cognitive aspect of this disease.

The microRNA miR-124 suppresses seizure activity and regulates CREB1 activity

miR-124, a brain-specific microRNA, was originally considered as a key regulator in neuronal differentiation and the development of the nervous system. Here we showed that miR-124 expression was suppressed in patients with epilepsy and rats after drug induced-seizures. Intrahippocampal administration of a miR-124 duplex led to alleviated seizure severity and prolonged onset latency in two rat models (pentylenetetrazole- and pilocarpine-induced seizures), while miR-124 inhibitor led to shortened onset latency in pilocarpine-induced seizure rat models. Moreover, the result of local field potentials (LFPs) records further demonstrated miR-124 may have anti-epilepsy function. Inhibition of neuronal firing by miR-124 was associated with the suppression of mEPSC, AMPAR- and NMDAR-mediated currents, which were accompanied by decreased surface expression of NMDAR. In addition, miR-124 injection resulted in decreased activity and expression of cAMP-response element-binding protein1 (CREB1). a key regulator in epileptogenesis. A dual-luciferase reporter assay was used to confirm that miR-124 targeted directly the 3'UTR of CREB1 gene and repressed the CREB1 expression in HEK293T cells. Immunoprecipitation studies confirmed that the CREB1 antibody effectively precipitated CREB1 and NMDAR1 but not GLUR1 from rat brain hippocampus. These results revealed a previously unknown function of miR-124 in neuronal excitability and provided a new insight into molecular mechanisms underlying epilepsy.

Melanin-concentrating hormone is necessary for olanzapine-inhibited locomotor activity in male mice

Olanzapine (OLZ), an atypical antipsychotic, can be effective in treating patients with restricting type anorexia nervosa who exercise excessively. Clinical improvements include weight gain and reduced pathological hyperactivity. However the neuronal populations and mechanisms underlying OLZ actions are not known. We studied the effects of OLZ on hyperactivity using male mice lacking the hypothalamic neuropeptide melanin-concentrating hormone (MCHKO) that are lean and hyperactive. We compared the in vivo effects of systemic or intra-accumbens nucleus (Acb) OLZ administration on locomotor activity in WT and MCHKO littermates. Acute systemic OLZ treatment in WT mice significantly reduced locomotor activity, an effect that is substantially attenuated in MCHKO mice. Furthermore, OLZ infusion directly into the Acb of WT mice reduced locomotor activity, but not in MCHKO mice. To identify contributing neuronal mechanisms, we assessed the effect of OLZ treatment on Acb synaptic transmission ex vivo and in vitro. Intraperitoneal OLZ treatment reduced Acb GABAergic activity in WT but not MCHKO neurons. This effect was also seen in vitro by applying OLZ to acute brain slices. OLZ reduced the frequency and amplitude of GABAergic activity that was more robust in WT than MCHKO Acb. These findings indicate that OLZ reduced Acb GABAergic transmission and that MCH is necessary for the hypolocomotor effects of OLZ.

Modulation of AMPA receptor mediated current by nicotinic acetylcholine receptor in layer I neurons of rat prefrontal cortex

Layer I neurons in the prefrontal cortex (PFC) exhibit extensive synaptic connections with deep layer neurons, implying their important role in the neural circuit. Study demonstrates that activation of nicotinic acetylcholine receptors (nAChRs) increases excitatory neurotransmission in this layer. Here we found that nicotine selectively increased the amplitude of AMPA receptor (AMPAR)-mediated current and AMPA/NMDA ratio, while without effect on NMDA receptor-mediated current. The augmentation of AMPAR current by nicotine was inhibited by a selective α7-nAChR antagonist methyllycaconitine (MLA) and intracellular calcium chelator BAPTA. In addition, nicotinic effect on mEPSC or paired-pulse ratio was also prevented by MLA. Moreover, an enhanced inward rectification of AMPAR current by nicotine suggested a functional role of calcium permeable and GluA1 containing AMPAR. Consistently, nicotine enhancement of AMPAR current was inhibited by a selective calcium-permeable AMPAR inhibitor IEM-1460. Finally, the intracellular inclusion of synthetic peptide designed to block GluA1 subunit of AMPAR at CAMKII, PKC or PKA phosphorylation site, as well as corresponding kinase inhibitor, blocked nicotinic augmentation of AMPA/NMDA ratio. These results have revealed that nicotine increases AMPAR current by modulating the phosphorylation state of GluA1 which is dependent on α7-nAChR and intracellular calcium.

Dopamine D3 receptor-dependent changes in alpha6 GABAA subunit expression in striatum modulate anxiety-like behaviour: Responsiveness and tolerance to diazepam

Increasing evidence indicates that central dopamine (DA) neurotransmission is involved in pathophysiology of anxiety, in particular the DA receptor subtype 3 (D3R). We previously reported that D3R null mice (D3R(-/-)) exhibit low baseline anxiety levels and that acutely administrated diazepam is more effective in D3R(-/-) than in wild type (WT) when tested in the elevated plus maze test (EPM). Here we tested the hypothesis that genetic deletion or pharmacological blockade of D3R affect GABAA subunit expression, which in turn modulates anxiety-like behaviour as well as responsiveness and tolerance to diazepam. D3R(-/-) mice exhibited tolerance to diazepam (0.5mg/kg, i.p.), assessed by EPM, as fast as after 3 day-treatment, performing similarly to untreated D3R(-/-) mice; conversely, WT exhibited tolerance to diazepam after a 14-21 day-treatment. Analysis of GABAA α6 subunit mRNA expression by qPCR in striatum showed that it was about 15-fold higher in D3R(-/-) than in WT. Diazepam treatment did not modify α6 expression in D3R(-/-), but progressively increased α6 expression in WT, to the level of untreated D3R(-/-) after 14-21 day-treatment. BDNF mRNA expression in striatum was remarkably (>10-fold) increased after 3 days of diazepam-treatment in both WT and D3R(-/-); such expression level, however, slowly declined below control levels, by 14-21 days. Following a 7 day-treatment with the selective D3R antagonist SB277011A, WT exhibited a fast tolerance to diazepam accompanied by a robust increase in α6 subunit expression. In conclusion, genetic deletion or pharmacological blockade of D3R accelerate the development of tolerance to repeated administrations of diazepam and increase α6 subunit expression, a GABAA subunit that has been linked to diazepam insensitivity. Modulation of GABAA receptor by DA transmission may be involved in the mechanisms of anxiety and, if occurring in humans, may have therapeutic relevance following repeated use of drugs targeting D3R.

Chronic pramipexole treatment increases tolerance for sucrose in normal and ventral tegmental lesioned rats

The loss of dopamine neurons observed in Parkinson's disease (PD) elicits severe motor control deficits which are reduced by the use of dopamine agonists. However, recent works have indicated that D3-preferential agonists such as pramipexole can induce impulse control disorders (ICDs) such as food craving or compulsive eating. In the present study, we performed an intermittent daily feeding experiment to assess the effect of chronic treatment by pramipexole and VTA bilateral lesion on tolerance for sucrose solution. The impact of such chronic treatment on spontaneous locomotion and spatial memory was also examined. Changes in sucrose tolerance could indicate the potential development of a change in food compulsion or addiction related to the action of pramipexole. Neither the bilateral lesion of the VTA nor chronic treatment with pramipexole altered the spontaneous locomotion or spatial memory in rats. Rats without pramipexole treatment quickly developed a stable intake of sucrose solution in the 12 h access phase. On the contrary, when under daily pramipexole treatment, rats developed a stronger and ongoing escalation of their sucrose solution intakes. In addition, we noted that the change in sucrose consumption was sustained by an increase of the expression of the Dopamine D3 receptor in the core and the shell regions of the nucleus accumbens. The present results may suggest that long-term stimulation of the Dopamine D3 receptor in animals induces a strong increase in sucrose consumption, indicating an effect of this receptor on certain pathological aspects of food eating.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Contribution of adenylyl cyclase modulation of pre- and postsynaptic GABA neurotransmission to morphine antinociception and tolerance

Opioid inhibition of presynaptic GABA release in the ventrolateral periaqueductal gray (vlPAG) activates the descending antinociception pathway. Tolerance to repeated opioid administration is associated with upregulation of adenylyl cyclase activity. The objective of these studies was to test the hypothesis that adenylyl cyclase contributes to opioid tolerance by modulating GABA neurotransmission. Repeated microinjections of morphine or the adenylyl cyclase activator NKH477 into the vlPAG decreased morphine antinociception as would be expected with the development of tolerance. Conversely, microinjection of the adenylyl cyclase inhibitor SQ22536 reversed both the development and expression of morphine tolerance. These behavioral results indicate that morphine tolerance is dependent on adenylyl cyclase activation. Electrophysiological experiments revealed that acute activation of adenylyl cyclase with forskolin increased the frequency of presynaptic GABA release. However, recordings from rats treated with repeated morphine administration did not exhibit increased basal miniature inhibitory postsynaptic current (mIPSC) frequency but showed a decrease in mean amplitude of mIPSCs indicating that repeated morphine administration modulates postsynaptic GABAA receptors without affecting the probability of presynaptic GABA release. SQ22536 reversed this change in mIPSC amplitude and inhibited mIPSC frequency selectively in morphine tolerant rats. Repeated morphine or NKH477 administration also decreased antinociception induced by microinjection of the GABAA receptor antagonist bicuculline, further demonstrating changes in GABA neurotransmission with morphine tolerance. These results show that the upregulation of adenylyl cyclase caused by repeated vlPAG morphine administration produces antinociceptive tolerance by modulating both pre- and postsynaptic GABA neurotransmission.

Selective modulation of GABAergic tonic current by dopamine in the nucleus accumbens of alcohol-dependent rats

The nucleus accumbens (NAcc) is a key structure of the mesolimbic dopaminergic reward system and plays an important role in mediating alcohol-seeking behaviors. Alterations in glutamatergic and GABAergic signaling were recently demonstrated in the NAcc of rats after chronic intermittent ethanol (CIE) treatment, a model of alcohol dependence. Here we studied dopamine (DA) modulation of GABAergic signaling and how this modulation might be altered by CIE treatment. We show that the tonic current (I(tonic)) mediated by extrasynaptic γ-aminobutyric acid type A receptors (GABA(A)Rs) of medium spiny neurons (MSNs) in the NAcc core is differentially modulated by DA at concentrations in the range of those measured in vivo (0.01-1 μM), without affecting the postsynaptic kinetics of miniature inhibitory postsynaptic currents (mIPSCs). Use of selective D1 receptor (D1R) and D2 receptor (D2R) ligands revealed that I(tonic) potentiation by DA (10 nM) is mediated by D1Rs while I(tonic) depression by DA (0.03-1 μM) is mediated by D2Rs in the same MSNs. Addition of guanosine 5'-O-(2-thiodiphosphate) (GDPβS) to the recording pipettes eliminated I(tonic) decrease by the selective D2R agonist quinpirole (5 nM), leaving intact the quinpirole effect on mIPSC frequency. Recordings from CIE and vehicle control (CIV) MSNs during application of D1R agonist (SKF 38393, 100 nM) or D2R agonist (quinpirole, 2 nM) revealed that SKF 38393 potentiated I(tonic) to the same extent, while quinpirole reduced I(tonic) to a similar extent, in both groups of rats. Our data suggest that the selective modulatory effects of DA on I(tonic) are unaltered by CIE treatment and withdrawal.

The ability of BDNF to modify neurogenesis and depressive-like behaviors is dependent upon phosphorylation of tyrosine residues 365/367 in the GABA(A)-receptor γ2 subunit

Brain-derived neurotrophic factor (BDNF) is a potent regulator of neuronal activity, neurogenesis, and depressive-like behaviors; however, downstream effectors by which BDNF exerts these varying actions remain to be determined. Here we reveal that BDNF induces long-lasting enhancements in the efficacy of synaptic inhibition by stabilizing γ2 subunit-containing GABA(A) receptors (GABA(A)Rs) at the cell surface, leading to persistent reductions in neuronal excitability. This effect is dependent upon enhanced phosphorylation of tyrosines 365 and 367 (Y365/7) in the GABA(A)R γ2 subunit as revealed using mice in which these residues have been mutated to phenyalanines (Y365/7F). Heterozygotes for this mutation exhibit an antidepressant-like phenotype, as shown using behavioral-despair models of depression. In addition, heterozygous Y365/7F mice show increased levels of hippocampal neurogenesis, which has been strongly connected with antidepressant action. Both the antidepressant phenotype and the increased neurogenesis seen in these mice are insensitive to further modulation by BDNF, which produces robust antidepressant-like activity and neurogenesis in wild-type mice. Collectively, our results suggest a critical role for GABA(A)R γ2 subunit Y365/7 phosphorylation and function in regulating the effects of BDNF.

Metabotropic regulation of extrasynaptic GABAA receptors

A large body of work now shows the importance of GABA_A receptor-mediated tonic inhibition in regulating CNS function. However, outside of pathological conditions, there is relatively little evidence that the magnitude of tonic inhibition is itself under regulation. Here we review the mechanisms by which tonic inhibition is known to be modulated, and outline the potential behavioral consequences of this modulation. Specifically, we address the ability of protein kinase A and C to phosphorylate the extrasynaptic receptors responsible for the tonic GABA_A current, and how G-protein coupled receptors can regulate tonic inhibition through these effectors. We then speculate about the possible functional consequences of regulating the magnitude of the tonic GABA_A current.

Agonist-dependent endocytosis of γ-aminobutyric acid type A (GABAA) receptors revealed by a γ2(R43Q) epilepsy mutation

GABA-gated chloride channels (GABAARs) trafficking is involved in the regulation of fast inhibitory transmission. Here, we took advantage of a γ2(R43Q) subunit mutation linked to epilepsy in humans that considerably reduces the number of GABAARs on the cell surface to better understand the trafficking of GABAARs. Using recombinant expression in cultured rat hippocampal neurons and COS-7 cells, we showed that receptors containing γ2(R43Q) were addressed to the cell membrane but underwent clathrin-mediated dynamin-dependent endocytosis. The γ2(R43Q)-dependent endocytosis was reduced by GABAAR antagonists. These data, in addition to a new homology model, suggested that a conformational change in the extracellular domain of γ2(R43Q)-containing GABAARs increased their internalization. This led us to show that endogenous and recombinant wild-type GABAAR endocytosis in both cultured neurons and COS-7 cells can be amplified by their agonists. These findings revealed not only a direct relationship between endocytosis of GABAARs and a genetic neurological disorder but also that trafficking of these receptors can be modulated by their agonist.

Altered cortical GABAA receptor composition, physiology, and endocytosis in a mouse model of a human genetic absence epilepsy syndrome

Patients with generalized epilepsy exhibit cerebral cortical disinhibition. Likewise, mutations in the inhibitory ligand-gated ion channels, GABAA receptors (GABAARs), cause generalized epilepsy syndromes in humans. Recently, we demonstrated that heterozygous knock-out (Hetα1KO) of the human epilepsy gene, the GABAAR α1 subunit, produced absence epilepsy in mice. Here, we determined the effects of Hetα1KO on the expression and physiology of GABAARs in the mouse cortex. We found that Hetα1KO caused modest reductions in the total and surface expression of the β2 subunit but did not alter β1 or β3 subunit expression, results consistent with a small reduction of GABAARs. Cortices partially compensated for Hetα1KO by increasing the fraction of residual α1 subunit on the cell surface and by increasing total and surface expression of α3, but not α2, subunits. Co-immunoprecipitation experiments revealed that Hetα1KO increased the fraction of α1 subunits, and decreased the fraction of α3 subunits, that associated in hybrid α1α3βγ receptors. Patch clamp electrophysiology studies showed that Hetα1KO layer VI cortical neurons exhibited reduced inhibitory postsynaptic current peak amplitudes, prolonged current rise and decay times, and altered responses to benzodiazepine agonists. Finally, application of inhibitors of dynamin-mediated endocytosis revealed that Hetα1KO reduced base-line GABAAR endocytosis, an effect that probably contributes to the observed changes in GABAAR expression. These findings demonstrate that Hetα1KO exerts two principle disinhibitory effects on cortical GABAAR-mediated inhibitory neurotransmission: 1) a modest reduction of GABAAR number and 2) a partial compensation with GABAAR isoforms that possess physiological properties different from those of the otherwise predominant α1βγ GABAARs.

Receptor-specific modulation of risk-based decision making by nucleus accumbens dopamine

The nucleus accumbens (NAc) serves as an integral node within cortico-limbic circuitry that regulates various forms of cost-benefit decision making. The dopamine (DA) system has also been implicated in enabling organisms to overcome a variety of costs to obtain more valuable rewards. However, it remains unclear how DA activity within the NAc may regulate decision making involving reward uncertainty. This study investigated the contribution of different DA receptor subtypes in the NAc to risk-based decision making, assessed with a probabilistic discounting task. In well-trained rats, D1 receptor blockade with SCH 23,390 decreased preference for larger, uncertain rewards, which was associated with enhanced negative-feedback sensitivity (ie, an increased tendency to select a smaller/certain option after an unrewarded risky choice). Treatment with a D1 agonist (SKF 81,297) optimized decision making, increasing choice of the risky option when reward probability was high, and decreasing preference under low probability conditions. In stark contrast, neither blockade of NAc D2 receptors with eticlopride, nor stimulation of these receptors with quinpirole or bromocriptine influenced risky choice. In comparison, infusion of the D3-preferring agonist PD 128,907 decreased reward sensitivity and risky choice. Collectively, these results show that mesoaccumbens DA refines risk-reward decision biases via dissociable mechanisms recruiting D1 and D3, but not D2 receptors. D1 receptor activity mitigates the effect of reward omissions on subsequent choices to promote selection of reward options that may have greater long-term utility, whereas excessive D3 receptor activity blunts the impact that larger/uncertain rewards have in promoting riskier choices.

Cholinergic interneurons suppress action potential initiation of medium spiny neurons in rat nucleus accumbens shell

Acetylcholine plays a crucial role in the regulation of neural functions, including dopamine release, synaptic activity, and intrinsic electrophysiological properties of the nucleus accumbens (NAc) shell. Although the effects of acetylcholine on the action potential properties of NAc medium spiny (MS) neurons have been reported, how intrinsic acetylcholine released from NAc cholinergic interneurons regulates the neural activity of MS neurons is still an open issue. To explore the cholinergic effects on the subthreshold responses and action potential properties of MS neurons in the NAc shell, we first tested the effects of carbachol, a non-selective cholinergic agonist, on MS neuronal activity. Then, we tested the effects of the activation of cholinergic interneurons on the electrophysiological properties of MS neurons via multiple whole-cell patch-clamp recordings. Bath application of carbachol induced resting membrane potential depolarization accompanied by an increase in the voltage response to negative current injection. These increases were blocked by the pre-application of pirenzepine, an M1 muscarinic receptor antagonist. In spite of the facilitative effect on voltage responses of negative current injection, carbachol diminished the characteristic slowly-depolarizing ramp potentials, which respond to positive current pulse injection. Thus, carbachol increased the rheobase and shifted the frequency-current curve toward the right. Repetitive spike firing of a cholinergic interneuron following positive current injection induced a similar increase in the rheobase, which delayed the action potential initiation in 38.9% MS neurons. In contrast to the bath application of carbachol, cholinergic interneuronal stimulation had little effect on the resting membrane potential in MS neurons. These results suggest that the acetylcholine released from a cholinergic interneuron is sufficient to suppress the repetitive spike firing of the adjacent MS neurons, although the depolarization of the resting membrane potential may require simultaneous activation of multiple cholinergic interneurons.

Aβ impairs nicotinic regulation of inhibitory synaptic transmission and interneuron excitability in prefrontal cortex

Background

Accumulation of β-amyloid (Aβ) and cholinergic deficiency are two prominent features of Alzheimer’s disease (AD). To understand how Aβ-induced dysfunction of the nicotinic system may contribute to cognitive impairment in AD, we examined the effect of Aβ on nicotinic regulation of synaptic transmission and neuronal excitability in prefrontal cortex (PFC), a brain region critical for cognitive processes.

Results

We found that activation of nicotinic acetylcholine receptors (nAChRs) with nicotine increased the inhibitory postsynaptic currents recorded in PFC pyramidal neurons, which was associated with the nicotine-induced increase in the excitability of PFC layer I GABAergic interneurons. Both effects of nicotine were disrupted by Aβ. However, Aβ did not impair nicotinic regulation of excitatory neurotransmission in PFC interneurons. The nicotinic effect on synaptic inhibition was also lost in transgenic mice with five familial Alzheimer’s disease mutations. Inhibiting PKC attenuated nicotinic regulation of inhibitory, but not excitatory, neurotransmission.

Conclusions

Our study suggests that Aβ selectively impairs nicotinic regulation of inhibitory inputs to PFC pyramidal neurons, which might be due to its interference with PKC activation. Thus, in the PFC circuits of AD, the balance between inhibition and excitation under the control of nAChRs may be disturbed by Aβ.

Stabilization of GABA(A) receptors at endocytic zones is mediated by an AP2 binding motif within the GABA(A) receptor β3 subunit

The strength of synaptic inhibition can be controlled by the stability and endocytosis of surface and synaptic GABA(A) receptors (GABA(A)Rs), but the surface receptor dynamics that underpin GABA(A)R recruitment to dendritic endocytic zones (EZs) have not been investigated. Stabilization of GABA(A)Rs at EZs is likely to be regulated by receptor interactions with the clathrin-adaptor AP2, but the molecular determinants of these associations remain poorly understood. Moreover, although surface GABA(A)R downmodulation plays a key role in pathological disinhibition in conditions such as ischemia and epilepsy, whether this occurs in an AP2-dependent manner also remains unclear. Here we report the characterization of a novel motif containing three arginine residues (405RRR407) within the GABA(A)R β3-subunit intracellular domain (ICD), responsible for the interaction with AP2 and GABA(A)R internalization. When this motif is disrupted, binding to AP2 is abolished in vitro and in rat brain. Using single-particle tracking, we reveal that surface β3-subunit-containing GABA(A)Rs exhibit highly confined behavior at EZs, which is dependent on AP2 interactions via this motif. Reduced stabilization of mutant GABA(A)Rs at EZs correlates with their reduced endocytosis and increased steady-state levels at synapses. By imaging wild-type or mutant super-ecliptic pHluorin-tagged GABA(A)Rs in neurons, we also show that, under conditions of oxygen-glucose deprivation to mimic cerebral ischemia, GABA(A)Rs are depleted from synapses in dendrites, depending on the 405RRR407 motif. Thus, AP2 binding to an RRR motif in the GABA(A)R β3-subunit ICD regulates GABA(A)R residency time at EZs, steady-state synaptic receptor levels, and pathological loss of GABA(A)Rs from synapses during simulated ischemia.

Disrupted GABAAR trafficking and synaptic inhibition in a mouse model of Huntington's disease

Growing evidence suggests that Huntington's disease (HD), a neurodegenerative movement disorder caused by the mutant huntingtin (htt) with an expanded polyglutamine (polyQ) repeat, is associated with the altered intracellular trafficking and synaptic function. GABA(A) receptors, the key determinant of the strength of synaptic inhibition, have been found to bind to the huntingtin associated protein 1 (HAP1). HAP1 serves as an adaptor linking GABA(A) receptors to the kinesin family motor protein 5 (KIF5), controlling the transport of GABA(A) receptors along microtubules in dendrites. In this study, we found that GABA(A)R-mediated synaptic transmission is significantly impaired in a transgenic mouse model of HD expressing polyQ-htt, which is accompanied by the diminished surface expression of GABA(A) receptors. Moreover, the GABA(A)R/HAP1/KIF5 complex is disrupted and dissociated from microtubules in the HD mouse model. These results suggest that GABA(A)R trafficking and function is impaired in HD, presumably due to the interference of KIF5-mediated microtubule-based transport of GABA(A) receptors. The diminished inhibitory synaptic efficacy could contribute to the loss of the excitatory/inhibitory balance, leading to increased neuronal excitotoxicity in HD.

Molecular characterization of individual D3 dopamine receptor-expressing cells isolated from multiple brain regions of a novel mouse model

Among dopamine receptors, the expression and function of the D3 receptor subtype is not well understood. The receptor has the highest affinity for dopamine and many drugs that target dopamine receptors.In this paper, we examined, at the single cell level, the characteristics of D3 receptor-expressing cells isolated from different brain regions of male and female mice that were either 35 or 70 days old. The brain regions included nucleus accumbens, Islands of Calleja, olfactory tubercle,retrosplenial cortex, dorsal subiculum, mammillary body,amygdala and septum. The expression analysis was done in the drd3-enhanced green fluorescent protein transgenic mice that report the endogenous expression of D3 receptor mRNA. Using single cell reverse transcriptase PCR, we determined if the D3 receptor-expressing fluorescent cells in these mice were neurons or glia and if they were glutamatergic, GABAergic or catecholaminergic. Next, we determined if the fluorescent cells co-expressed the four other dopamine receptor subtypes, adenylate cyclase V(ACV) isoform, and three different isoforms of G protein coupled inward rectifier potassium (GIRK) channels. The results suggest that D3 receptor is expressed in neurons,with region-specific expression in glutamatergic and GABAergic neurons. The D3 receptor primarily coexpressed with D1 and D2 dopamine receptors with regional, sex and age-dependent differences in the coexpression pattern. The percentage of cells co-expressing D3 receptor and ACV or GIRK channels varied significantly by brain region, sex and age. The molecular characterization of D3 receptor-expressing cells in mouse brain reported here will facilitate the characterization of D(3) receptor function in physiology and pathophysiology.

D(2)-like dopamine receptors differentially regulate unitary IPSCs depending on presynaptic GABAergic neuron subtypes in rat nucleus accumbens shell

In the nucleus accumbens (NAc), a medium spiny (MS) neuron receives GABAergic inputs from two major sources: fast-spiking (FS) neurons and other, adjacent MS neurons. These two types of inhibitory synapses are considered to play different roles in output activities, i.e., FS→MS connections suppress output from the NAc whereas MS→MS connections contribute to lateral inhibition. In the present study, we focused on the electrophysiological properties of unitary inhibitory postsynaptic currents (uIPSCs) obtained from MS→MS connections and FS→MS connections and examined the effects of quinpirole, a dopamine D(2)-like receptor agonist, on uIPSCs with multiple whole cell patch-clamp recording. Application of quinpirole (1 μM) reliably suppressed the amplitude of uIPSCs by 29.6% in MS→MS connections, with increases in paired-pulse ratio and failure rate. The suppressive effects of quinpirole on uIPSCs were mimicked by 1 μM PD128907, a D(2/3) receptor agonist, whereas quinpirole-induced suppression of uISPCs was blocked by preapplication of 1 μM sulpiride or 10 μM nafadotride, both D(2/3) receptor antagonists. On the other hand, quinpirole (1 μM) had divergent effects on FS→MS connections, i.e., quinpirole increased uIPSC amplitude in 38.1% of FS→MS connections and 23.8% of FS→MS connections were suppressed by quinpirole. Analysis of coefficient of variation in uIPSC amplitude implied the involvement of presynaptic mechanisms in quinpirole-induced effects on uIPSCs. These results suggest that activation of D(2)-like receptors facilitates outputs from MS neurons in the NAc by reducing lateral inhibition during a dormant period of FS neuron activities.

Glycine receptor internalization by protein kinases activation

Although glycine-induced currents in the central nervous system have been proven to be modulated by protein kinases A (PKA) and C (PKC), the mechanism is not well understood. In order to better comprehend the mechanism involved in this phenomenon, we tested the PKA and PKC activation effect on the specific [(3) H]glycine and [(3) H]strychnine binding to postsynaptic glycine receptor (GlyR) in intact rat retina. The specific binding constituted about 20% of the total radioligand binding. Kinetic analysis of the specific binding exhibited a sigmoidal behavior with three glycine and two strychnine binding sites and affinities of 212 nM for [(3) H]glycine and 50 nM for [(3) H]strychnine. Specific radioligand binding was decreased (60-85%) by PKA and PKC activation, an effect that was blocked by specific kinases inhibitors, as well as by cytochalasin D. GlyR expressed in the plasma membrane decreased about 50% in response to kinases activation, which was consistent with an increase of the receptor in the microsomal fraction when PKA was activated. Moreover, immunoprecipitation studies indicated that these kinases lead to a time-dependent receptor phosphorylation. Our results suggest that in retina, GlyR is cross-regulated by G protein-coupled receptors, activating PKA and PKC.

GABA(A) Receptors: Post-Synaptic Co-Localization and Cross-Talk with Other Receptors

γ-Aminobutyric acid type A receptors (GABA_ARs) are the major inhibitory neurotransmitter receptors in the central nervous system, and importantly contribute to the functional regulation of the nervous system. Several studies in the last few decades have convincingly shown that GABA can be co-localized with other neurotransmitters in the same synapse, and can be co-released with these neurotransmitters either from the same vesicles or from different vesicle pools. The co-released transmitters may act on post-synaptically co-localized receptors resulting in a simultaneous activation of both receptors. Most of the studies investigating such co-activation observed a reduced efficacy of GABA for activating GABA_ARs and thus, a reduced inhibition of the post-synaptic neuron. Similarly, in several cases activation of GABA_ARs has been reported to suppress the response of the associated receptors. Such a receptor cross-talk is either mediated via a direct coupling between the two receptors or via the activation of intracellular signaling pathways and is used for fine tuning of inhibition in the nervous system. Recently, it was demonstrated that a direct interaction of different receptors might already occur in intracellular compartments and might also be used to specifically target the receptors to the cell membrane. In this article, we provide an overview on such cross-talks between GABA_ARs and several other neurotransmitter receptors and briefly discuss their possible physiological and clinical importance.

Dopamine D3-like receptors modulate anxiety-like behavior and regulate GABAergic transmission in the rat lateral/basolateral amygdala

Central among the brain regions that regulate fear/anxiety behaviors is the lateral/basolateral amygdala (BLA). BLA output is tightly controlled by the relative activity of two populations of inhibitory GABAergic interneurons, local feedback cells distributed throughout the nucleus, and feedforward cells found along the lateral paracapsular border of this subdivision. Recent studies suggest that dopamine (DA) can modulate the BLA GABAergic system, thus linking fear/anxiety states with mesolimbic reward/attentional processes. However, the precise dopaminergic mechanisms regulating the activity of the two BLA GABAergic neuron populations have not been fully explored. We therefore examined the effects of DA D3-like receptors on BLA-dependent anxiety-like behavior and neurophysiology. After confirming the presence of D3-like receptors within the BLA, we found that microinjection of a D3-selective antagonist into the BLA decreased anxiety-like behavior expressed in both the light/dark transition test and the elevated plus maze. Consistent with this, we found that in vitro D3-like receptor activation selectively inhibits synaptic transmission at both BLA feedback and feedforward GABAergic interneuron populations, with no effect on glutamatergic transmission. This inhibition of GABAergic transmission is a result of a D3-like receptor-mediated, dynamin-dependent process that presumably reflects endocytosis of postsynaptic GABA(A) receptors found on principal BLA neurons. Because environmental cues alter both DA release and relative activity states of the BLA, our data strongly suggest that DA, potentially acting through D3-like receptors, may suppress the relative contribution by inhibitory processes in the BLA and modify the expression of BLA-related behaviors.

The cell biology of synaptic inhibition in health and disease

Fast synaptic inhibition is largely mediated by GABA(A) receptors (GABA(A)Rs), ligand-gated chloride channels that play an essential role in the control of cell and network activity in the brain. Recent work has demonstrated that the delivery, number and stability of GABA(A)Rs at inhibitory synapses play a key role in the dynamic regulation of inhibitory synaptic efficacy and plasticity. The regulatory pathways essential for the fine-tuning of synaptic inhibition have also emerged as key sites of vulnerability during pathological changes in cell excitability in disease states.

Dopamine-dependent tuning of striatal inhibitory synaptogenesis

Dopaminergic projections to the striatum, crucial for the correct functioning of this brain region in adulthood, are known to be established early in development, but their role is currently uncharacterized. We demonstrate here that dopamine, by activating D(1)- and/or D(2)-dopamine receptors, decreases the number of functional GABAergic synapses formed between the embryonic precursors of the medium spiny neurons, the principal output neurons of the striatum, with associated changes in spontaneous synaptic activity. Activation of these receptors reduces the size of postsynaptic GABA(A) receptor clusters and their overall cell-surface expression, without affecting the total number of clusters or the size or number of GABAergic nerve terminals. These changes result from an increased internalization of GABA(A) receptors, and are mediated by distinct signaling pathways converging at the level of GABA(A) receptors to cause a transient PP2A/PP1-dependent dephosphorylation. Thus, tonic D(1)- and D(2)-receptor activity limits the extent of collateral inhibitory synaptogenesis between medium spiny neurons, revealing a novel role of dopamine in controlling the development of intrinsic striatal microcircuits.

(+)-UH 232, a partial agonist of the D3 dopamine receptors, attenuates cognitive effects of angiotensin IV and des-Phe(6)-angiotensin IV in rats

We have recently found that postsynaptic D3 dopamine (DA) receptors appear not to participate in the memory enhancing effects of the angiotensin AT4 receptor agonists angiotensin IV (Ang IV) and des-Phe(6)-Ang IV. In this study we evaluated role of the presynaptic DA D3 receptors in these effects. For that purpose effect of (+)-UH 232, a selective D3 DA receptors partial agonist preferring presynaptic sites, on the pro-cognitive action of intracerebroventricularly (icv) injected Ang IV and des-Phe(6)-Ang IV was examined. Male Wistar rats weighing 180-200 g were used. Both peptides given at the dose of 1 nmol facilitated recall of a passive avoidance (PA) behaviour, improved object recognition (OR), and increased apomorphine-induced stereotype behaviour. In the auxiliary tests performed to control for the unspecific influence of motor (open field, OF) and emotional ('plus' maze, PM) effects of our treatments on the results of the memory tests they had either no (OF) or negligible (PM) effects. Intraperitoneal pre-treatment of the animals with an ineffective on its own dose (1 mg/kg) of (+)-UH 232 abolished or markedly diminished effects of both peptides on PA and OR but did not influence enhancement of stereotypy caused by the peptides.

Delivery of GABAARs to synapses is mediated by HAP1-KIF5 and disrupted by mutant huntingtin

The density of GABA(A) receptors (GABA(A)Rs) at synapses regulates brain excitability, and altered inhibition may contribute to Huntington's disease, which is caused by a polyglutamine repeat in the protein huntingtin. However, the machinery that delivers GABA(A)Rs to synapses is unknown. We demonstrate that GABA(A)Rs are trafficked to synapses by the kinesin family motor protein 5 (KIF5). We identify the adaptor linking the receptors to KIF5 as the huntingtin-associated protein 1 (HAP1). Disrupting the HAP1-KIF5 complex decreases synaptic GABA(A)R number and reduces the amplitude of inhibitory postsynaptic currents. When huntingtin is mutated, as in Huntington's disease, GABA(A)R transport and inhibitory synaptic currents are reduced. Thus, HAP1-KIF5-dependent GABA(A)R trafficking is a fundamental mechanism controlling the strength of synaptic inhibition in the brain. Its disruption by mutant huntingtin may explain some of the defects in brain information processing occurring in Huntington's disease and provides a molecular target for therapeutic approaches.

The role of GABAAR phosphorylation in the construction of inhibitory synapses and the efficacy of neuronal inhibition

GABA(A)Rs [GABA (gamma-aminobutyric acid) type-A receptors] are heteropentameric chloride-selective ligand-gated ion channels that mediate fast inhibition in the brain and are key therapeutic targets for benzodiazepines, barbiturates, neurosteroids and general anaesthetics. In the brain, most of the benzodiazepine-sensitive synaptic receptor subtypes are assembled from alpha(1-3), beta(1-3) and gamma(2) subunits. Although it is evident that the pharmacological manipulation of GABA(A)R function can have profound effects on behaviour, the endogenous mechanisms that neurons use to promote sustained changes in the efficacy of neuronal inhibition remain to be documented. It is increasingly clear that GABA(A)Rs undergo significant rates of constitutive endocytosis and regulate recycling processes that can determine the efficacy of synaptic inhibition. Their endocytosis is regulated via the direct binding of specific endocytosis motifs within the intracellular domains of receptor beta(1-3) and gamma(2) subunits to the clathrin adaptor protein AP2 (adaptor protein 2). These binding motifs contain major sites of both serine and tyrosine phosphorylation within GABA(A)Rs. Their phosphorylation can have dramatic effects on binding to AP2. In the present review, we evaluate the role that these phospho-dependent interactions play in regulating the construction of inhibitory synapses, efficacy of neuronal inhibition and neuronal structure.

Antipsychotic drug development

Schizophrenia typically manifests itself with a wide array of symptoms--positive, negative, cognitive, and affective--and may also involve neurodevelopmental and neurodegenerative aspects. Each of these symptom dimensions may be derived from pathology at one or more receptor types, localized in different regions of the brain. The absence of a single therapeutic target for schizophrenia has therefore prompted the de-emphasis of selective "magic bullets" and a critical re-examination of the intramolecular polypharmacy afforded by antipsychotics. In this chapter, we present a review of some of the receptor targets that are currently thought to mediate symptoms of schizophrenia, and discuss their possible implications for future antipsychotic drug development. Therapeutic strategies for schizophrenia that successfully exploit the multifunctionality of antipsychotics will take into account the entire receptor activity "portfolio" of the agent and provide a total therapeutic response that, like the elephant of the Buddhist parable, is greater than the sum of its parts.