RGS2 modulates coupling between GABAB receptors and GIRK channels in dopamine neurons of the ventral tegmental area

Regulators of G protein signaling in neuropsychiatric disorders

Regulators of G protein signaling (RGS) comprise a diverse group of about 40 proteins which determine signaling amplitude and duration via modulation of receptor/G protein or receptor/effector coupling. Several members of the RGS family are expressed in the brain, where they have precise roles in regulation of important physiological processes. The unique functions of each RGS can be attributed to its structure, distinct pattern of expression, and regulation, and its preferential interactions with receptors, Galpha subunits and other signaling proteins. Evidence suggests dysfunction of RGS proteins is related to several neuropathological conditions. Moreover, clinical and preclinical work reveals that the efficacy and/or side effects of treatments are highly influenced by RGS activity. This article summarizes findings on RGS proteins in vulnerability to several neuropsychiatric disorders, the mechanism via which RGS proteins control neuronal responses and their potential use as drug targets.

Mapping a barbiturate withdrawal locus to a 0.44 Mb interval and analysis of a novel null mutant identify a role for Kcnj9 (GIRK3) in withdrawal from pentobarbital, zolpidem, and ethanol

Here, we map a quantitative trait locus (QTL) with a large effect on predisposition to barbiturate (pentobarbital) withdrawal to a 0.44 Mb interval of mouse chromosome 1 syntenic with human 1q23.2. We report a detailed analysis of the genes within this interval and show that it contains 15 known and predicted genes, 12 of which demonstrate validated genotype-dependent transcript expression and/or nonsynonymous coding sequence variation that may underlie the influence of the QTL on withdrawal. These candidates are involved in diverse cellular functions including intracellular trafficking, potassium conductance and spatial buffering, and multimolecular complex dynamics, and indicate both established and novel aspects of neurobiological response to sedative-hypnotics. This work represents a substantial advancement toward identification of the gene(s) that underlie the phenotypic effects of the QTL. We identify Kcnj9 as a particularly promising candidate and report the development of a Kcnj9-null mutant model that exhibits significantly less severe withdrawal from pentobarbital as well as other sedative-hypnotics (zolpidem and ethanol) versus wild-type littermates. Reduced expression of Kcnj9, which encodes GIRK3 (Kir3.3), is associated with less severe sedative-hypnotic withdrawal. A multitude of QTLs for a variety of complex traits, including diverse responses to sedative-hypnotics, have been detected on distal chromosome 1 in mice, and as many as four QTLs on human chromosome 1q have been implicated in human studies of alcohol dependence. Thus, our results will be primary to additional efforts to identify genes involved in a wide variety of behavioral responses to sedative-hypnotics and may directly facilitate progress in human genetics.

CRF enhancement of GIRK channel-mediated transmission in dopamine neurons

Dopamine neurons in the ventral midbrain contribute to learning and memory of natural and drug-related rewards. Corticotropin-releasing factor (CRF), a stress-related peptide, is thought to be involved in aspects of relapse following drug withdrawal, but the cellular actions are poorly understood. This study investigates the action of CRF on G-protein-linked inhibitory postsynaptic currents (IPSCs) mediated by GIRK (Kir3) channels in dopamine neurons. CRF enhanced the amplitude and slowed the kinetics of IPSCs following activation of D2-dopamine and GABA(B) receptors. This action was postsynaptic and dependent on the CRF(1) receptor. The enhancement induced by CRF was attenuated by repeated in vivo exposures to psychostimulants or restraint stress. The results indicate that CRF influences dopamine- and GABA-mediated inhibition in the midbrain, suggesting implications for the chronic actions of psychostimulants and stress on dopamine-mediated behaviors.

Subcellular compartment-specific molecular diversity of pre- and post-synaptic GABA-activated GIRK channels in Purkinje cells

Activation of G protein-gated inwardly-rectifying K(+) (GIRK or Kir3) channels by metabotropic gamma-aminobutyric acid (B) (GABA(B)) receptors is an essential signalling pathway controlling neuronal excitability and synaptic transmission in the brain. To investigate the relationship between GIRK channel subunits and GABA(B) receptors in cerebellar Purkinje cells at post- and pre-synaptic sites, we used biochemical, functional and immunohistochemical techniques. Co-immunoprecipitation analysis demonstrated that GIRK subunits are co-assembled with GABA(B) receptors in the cerebellum. Immunoelectron microscopy showed that the subunit composition of GIRK channels in Purkinje cell spines is compartment-dependent. Thus, at extrasynaptic sites GIRK channels are formed by GIRK1/GIRK2/GIRK3, post-synaptic densities contain GIRK2/GIRK3 and dendritic shafts contain GIRK1/GIRK3. The post-synaptic association of GIRK subunits with GABA(B) receptors in Purkinje cells is supported by the subcellular regulation of the ion channel and the receptor in mutant mice. At pre-synaptic sites, GIRK channels localized to parallel fibre terminals are formed by GIRK1/GIRK2/GIRK3 and co-localize with GABA(B) receptors. Consistent with this morphological evidence we demonstrate their functional interaction at axon terminals in the cerebellum by showing that GIRK channels play a role in the inhibition of glutamate release by GABA(B) receptors. The association of GIRK channels and GABA(B) receptors with excitatory synapses at both post- and pre-synaptic sites indicates their intimate involvement in the modulation of glutamatergic neurotransmission in the cerebellum.

RGS9-2: probing an intracellular modulator of behavior as a drug target

Regulators of G-protein signaling (RGS proteins) comprise a large family of signal transduction molecules that modulate G-protein-coupled-receptor (GPCR) function. Among the RGS proteins expressed in the brain, RGS9-2 is very abundant in the striatum, a brain region involved in movement, motivation, mood and addiction. This protein negatively modulates signal transduction thus playing a key part in striatal function and resultant behavioral responses. In particular, there is evidence of important interactions with mu-opioid- and dopamine D(2)-receptor signaling pathways. Several studies indicate that manipulations of RGS9-2 levels in the striatum might greatly affect pharmacological responses. These findings indicate that treatment strategies targeting RGS9-2 levels or activity might be used to enhance responses to drugs acting at GPCRs and/or prevent undesired drug actions.

Gamma-butyrolactone (GBL) disruption of passive avoidance learning in the day-old chick appears to be due to its effect on GABAB not gamma-hydroxybutyric [corrected] acid (GHB) receptors

Gamma-butyrolactone (GBL) is a prodrug to gamma-hydroxybutyric acid (GHB) and metabolises to GHB when ingested. Discrimination stimulus studies report generalisation of effects of GHB to GBL. While amnesia is one of the most commonly reported symptoms of GHB's ingestion in human users, as yet few studies have examined this effect. Although an endogenous GHB specific receptor is present in the brain, several studies have indicated that the clinical effects of exogenous doses of GBL/GHB are due to its action on GABA(B) receptors rather than on the GHB receptor. In this series of studies, New Hampshire x White leghorn cockerels were trained using a modified version of the passive avoidance learning task. Subcutaneous injections of GBL induced a memory deficit by 10 min post-training, which persisted for at least 24 h. No effect on memory was seen with administration of the specific GHB agonist NCS-356 (gamma-p-chlorophenyl-trans-4-hydroxycrotonate). The GBL-induced memory deficit appeared similar to the deficit produced by baclofen, where the antagonist facilitated learning. Additionally, GBL-induced memory deficit was ameliorated by application of a GABA(B) antagonist. The results support the hypothesis that GBL exerts its influence on memory via the GABA(B) receptor rather than by the specific GHB receptor.

cGMP signalling in the mammalian brain: role in synaptic plasticity and behaviour

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays a crucial role in the control of cardiovascular and gastrointestinal homeostastis, but its effects on neuronal functions are less established. This review summarizes recent biochemical and functional data on the role of the cGMP signalling pathway in the mammalian brain, with a focus on the regulation of synaptic plasticity, learning, and other complex behaviours. Expression profiling, along with pharmacological and genetic manipulations, indicates important functions of nitric oxide (NO)-sensitive soluble guanylyl cyclases (sGCs), cGMP-dependent protein kinases (cGKs), and cGMP-regulated phosphodiesterases (PDEs) as generators, effectors, and modulators of cGMP signals in the brain, respectively. In addition, neuronal cGMP signalling can be transmitted through cyclic nucleotide-gated (CNG) or hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. The canonical NO/sGC/cGMP/cGK pathway modulates long-term changes of synaptic activity in the hippocampus, amygdala, cerebellum, and other brain regions, and contributes to distinct forms of learning and memory, such as fear conditioning, motor adaptation, and object recognition. Behavioural studies indicate that cGMP signalling is also involved in anxiety, addiction, and the pathogenesis of depression and schizophrenia. At the molecular level, different cGK isoforms appear to mediate effects of cGMP on presynaptic transmitter release and postsynaptic functions. The cGKs have been suggested to modulate cytoskeletal organization, vesicle and AMPA receptor trafficking, and gene expression via phosphorylation of various substrates including VASP, RhoA, RGS2, hSERT, GluR1, G-substrate, and DARPP-32. These and other components of the cGMP signalling cascade may be attractive new targets for the treatment of cognitive impairment, drug abuse, and psychiatric disorders.

Behavioral characterization of mice lacking GIRK/Kir3 channel subunits

G protein-gated inwardly rectifying K(+) (GIRK/Kir3) channels mediate the postsynaptic inhibitory effects of many neurotransmitters and drugs of abuse. The lack of drugs selective for GIRK channels has hindered our ability to study their contributions to behavior. Here, we assessed the impact of GIRK subunit ablation on several behavioral endpoints. Mice were evaluated with respect to open-field motor activity and habituation, anxiety-related behavior, motor co-ordination and ataxia and operant performance. GIRK3 knockout ((-/-)) mice behaved indistinguishably from wild-type mice in this panel of tests. GIRK1(-/-) mice and GIRK2(-/-) mice, however, showed elevated motor activity and delayed habituation to an open field. GIRK2(-/-) mice, and to a lesser extent GIRK1(-/-) mice, also displayed reduced anxiety-related behavior in the elevated plus maze. Both GIRK1(-/-) mice and GIRK2(-/-) mice displayed marked resistance to the ataxic effects of the GABA(B) receptor agonist baclofen in the rotarod test. All GIRK(-/-) mice were able to learn an operant task using food as the reinforcing agent. Within-session progressive ratio scheduling, however, showed elevated lever press behavior in GIRK2(-/-) mice and, to a lesser extent, in GIRK1(-/-) mice. Phenotypic differences between mice lacking GIRK1, GIRK2 and GIRK3 correlate well with the known impact of GIRK subunit ablation on neurotransmitter-gated GIRK currents, arguing that most neuronal GIRK channels contain GIRK1 and/or GIRK2. Altogether, our data suggest that GIRK channels make important contribution to a range of behaviors and may represent points of therapeutic intervention in disorders of anxiety, spasticity and reward.

Pre- and postsynaptic regulation of locus coeruleus neurons after chronic morphine treatment: a study of GIRK-knockout mice

While the acute inhibitory effect of opioids on locus coeruleus (LC) neurons is mediated primarily by the activation of G protein-gated inwardly-rectifying K(+) (GIRK) channels, the 3'-5'-cyclic adenosine monophosphate (cAMP) system has been implicated in the effects of chronic morphine exposure. Presently, the impact of chronic morphine treatment on GIRK-dependent and GIRK-independent mechanisms underlying the opioid-induced inhibition of LC neurons is unclear. Here, opioid-induced postsynaptic inhibition was studied in LC neurons from wild-type and GIRK2/GIRK3(-/-) mice at baseline and following chronic morphine treatment. The postsynaptic inhibition of LC neurons caused by the opioid agonist [Met](5) enkephalin (ME) was unaffected by chronic morphine treatment in mice of either genotype. Furthermore, chronic morphine treatment had no effect on the forskolin augmentation of the ME-induced current in wild-type LC neurons and only a minor effect on the ME-induced current in LC neurons from GIRK2/GIRK3(-/-) mice. Chronic morphine treatment did, however, lead to an increased frequency of spontaneous excitatory postsynaptic currents (EPSCs) in the LC. Interestingly, while forskolin augmented the EPSC frequency similarly in untreated and morphine-treated wild-type mice, as well as untreated GIRK2/GIRK3(-/-) mice, it failed to increase the frequency of EPSCs in morphine-treated GIRK2/GIRK3(-/-) mice. Altogether, the findings suggest that chronic morphine treatment exerts little impact on ion channels and signaling pathways that mediate the postsynaptic inhibitory effects of opioids but does enhance excitatory neurotransmission in the mouse LC.

Inactivation of the maternal fragile X gene results in sensitization of GABAB receptor function in the offspring

Fragile X syndrome is an X-linked disorder caused by the inactivation of the FMR1 gene, with symptoms ranging from impaired cognitive functions to seizures, anxiety, sensory abnormalities, and hyperactivity. Although fragile X syndrome is considered a typical Mendelian disorder, we have recently reported that the environment, specifically the fmr1(+/-) or fmr1(-/-) [H or knockout (KO)] maternal environment, elicits on its own a partial fragile X-like phenotype and can contribute to the overall phenotype of fmr1(-/0) (KO) male offspring. Genetically fmr1(+/0) (WT) males born to H females (H(maternal) > WT(offspring)), similar to KO male offspring born to H and KO mothers (H > KO and KO > KO), exhibit locomotor hyperactivity. These mice also showed reduced D(2) autoreceptor function, indicating a possible diminished feedback inhibition of dopamine (DA) release in the nigrostriatal and mesolimbic systems. The GABAergic system also regulates DA release, in part via presynaptic GABA(B) receptors (Rs) located on midbrain dopaminergic neurons. Here, we show that the locomotor inhibitory effect of the GABA(B)R agonist baclofen [4-amino-3-(4-chlorophenyl)-butanoic acid] is enhanced in all progeny of mutant mothers (H > WT, H > KO, and KO > KO) compared with WT > WT mice, irrespective of their own genotype. However, increased sensitivity to baclofen was selective and limited to the locomotor response because the muscle-relaxant and sedative effects of the drug were not altered by the maternal environment. These data show that GABA(B)R sensitization, traditionally induced pharmacologically, can also be elicited by the fmr1-deficient maternal environment.

Addictive drugs modulate GIRK-channel signaling by regulating RGS proteins

Regulator of G-protein signaling (RGS) proteins are strong modulators of G-protein-mediated pathways in the nervous system. One function of RGS proteins is to accelerate the activation-deactivation kinetics of G-protein-coupled inwardly rectifying potassium (GIRK) channels. The opening of GIRK channels reduces the firing rates of neurons. Recent studies indicate that RGS proteins also modulate the coupling efficiency between gamma-aminobutyric acid type B (GABA(B)) receptors and GIRK channels in dopamine neurons of the ventral tegmental area (VTA), the initial target for addictive drugs in the brain reward pathway. Chronic drug exposure can dynamically regulate the expression levels of RGS. Functional and behavioral studies now reveal that levels of RGS2 protein, through selective association with GIRK3, critically determine whether GABA(B) agonists are excitatory or inhibitory in the VTA. The regulation of RGS protein in the reward pathway might underlie adaptation to different types of addictive drugs.

Searching the principal genes for neural differentiation of mouse ES cells by factorizing eigengenes of clusters

A time-series microarray experiment is useful to study the changes in the expression of a large number of genes over time. Many methods for clustering genes using gene expression profiles have been suggested, but it is not easy to interpret the biological significance of the results or utilize these methods for understanding the dynamics of gene regulatory systems. In this study, we introduce an algorithm for readjusting the boundaries of clusters by adopting the advantages of both k-means and singular value decomposition (SVD). In addition, we suggest a methodology for searching the principal genes that can be the most crucial genes in regulation of clusters. We found 34 principal genes from 171 clusters having strong concentratedness in their expression patterns and distinct ranges of oscillatory phases, by using a time-series microarray dataset of mouse embryonic stem (ES) cells after induction of dopaminergic neural differentiation. The biological significance of the principal genes examined in the literature supports the feasibility of our algorithms in that the hierarchy of clusters may lead the manifestation of the phenotypes, e.g., the development of the nervous system.

Absence and rescue of morphine withdrawal in GIRK/Kir3 knock-out mice

Although morphine induces both analgesia and dependence through mu-opioid receptors (MORs), the respective contributions of the intracellular effectors engaged by MORs remain unknown. To examine the contribution of G-protein-gated inwardly rectifying K(+) (GIRK, Kir3) channels to morphine dependence and analgesia, we quantified naloxone-precipitated withdrawal behavior and morphine analgesia using GIRK knock-out ((-/-)) mice. The morphine withdrawal syndrome was strongly attenuated, whereas morphine analgesia was mostly preserved in mice lacking both GIRK2 and GIRK3 (GIRK2/3(-/-) mice). In acute slices containing the locus ceruleus (LC) from GIRK2/3(-/-) mice, the increase in spontaneous firing typically associated with morphine withdrawal was absent. Moreover, although morphine elicited normal presynaptic inhibition in the LC, postsynaptic GIRK currents were completely abolished in GIRK2/3(-/-) mice. Altogether, these data suggested that morphine-evoked postsynaptic inhibition of the LC was required for the induction of dependence. Consistent with this hypothesis, morphine withdrawal behavior was rescued in GIRK2/3(-/-) mice by ablation of adrenergic fibers using the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine. Our data suggest that inhibition of adrenergic tone is required for the induction of dependence, and that channels containing GIRK2 and GIRK3 serve as an inhibitory gate.