Contribution of Kir3.1, Kir3.2A and Kir3.2C subunits to native G protein-gated inwardly rectifying potassium currents in cultured hippocampal neurons

From Crosstalk to Synergism: The Combined Effect of Cholesterol and PI(4,5)P2 on Inwardly Rectifying Potassium Channels

Inwardly rectifying potassium (Kir) channels are integral membrane proteins that control the flux of potassium ions across cell membranes and regulate membrane permeability. All eukaryotic Kir channels require the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for activation. In recent years, it has become evident that the function of many members of this family of channels is also mediated by another essential lipid-cholesterol. Here, we focus on members of the Kir2 and Kir3 subfamilies and their modulation by these two key lipids. We discuss how PI(4,5)P₂ and cholesterol bind to Kir2 and Kir3 channels and how they affect channel activity. We also discuss the accumulating evidence indicating that there is interplay between PI(4,5)P₂ and cholesterol in the modulation of Kir2 and Kir3 channels. In particular, we review the crosstalk between PI(4,5)P₂ and cholesterol in the modulation of the ubiquitously expressed Kir2.1 channel and the synergy between these two lipids in the modulation of the Kir3.4 channel, which is primarily expressed in the heart. Additionally, we demonstrate that there is also synergy in the modulation of Kir3.2 channels, which are expressed in the brain. These observations suggest that alterations in the relative levels PI(4,5)P₂ and cholesterol may fine-tune Kir channel activity.

Identification of a G-Protein-Independent Activator of GIRK Channels

G-protein-gated inwardly rectifying K⁺ (GIRK) channels are essential effectors of inhibitory neurotransmission in the brain. GIRK channels have been implicated in diseases with abnormal neuronal excitability, including epilepsy and addiction. GIRK channels are tetramers composed of either the same subunit (e.g., homotetramers) or different subunits (e.g., heterotetramers). Compounds that specifically target subsets of GIRK channels in vivo are lacking. Previous studies have shown that alcohol directly activates GIRK channels through a hydrophobic pocket located in the cytoplasmic domain of the channel. Here, we report the identification and functional characterization of a GIRK1-selective activator, termed GiGA1, that targets the alcohol pocket. GiGA1 activates GIRK1/GIRK2 both in vitro and in vivo and, in turn, mitigates the effects of a convulsant in an acute epilepsy mouse model. These results shed light on the structure-based development of subunit-specific GIRK modulators that could provide potential treatments for brain disorders.

GPCR-dependent biasing of GIRK channel signaling dynamics by RGS6 in mouse sinoatrial nodal cells

How G protein-coupled receptors (GPCRs) evoke specific biological outcomes while utilizing a limited array of G proteins and effectors is poorly understood, particularly in native cell systems. Here, we examined signaling evoked by muscarinic (M2R) and adenosine (A1R) receptor activation in the mouse sinoatrial node (SAN), the cardiac pacemaker. M2R and A1R activate a shared pool of cardiac G protein-gated inwardly rectifying K+ (GIRK) channels in SAN cells from adult mice, but A1R-GIRK responses are smaller and slower than M2R-GIRK responses. Recordings from mice lacking Regulator of G protein Signaling 6 (RGS6) revealed that RGS6 exerts a GPCR-dependent influence on GIRK-dependent signaling in SAN cells, suppressing M2R-GIRK coupling efficiency and kinetics and A1R-GIRK signaling amplitude. Fast kinetic bioluminescence resonance energy transfer assays in transfected HEK cells showed that RGS6 prefers Gαo over Gαi as a substrate for its catalytic activity and that M2R signals preferentially via Gαo, while A1R does not discriminate between inhibitory G protein isoforms. The impact of atrial/SAN-selective ablation of Gαo or Gαi2 was consistent with these findings. Gαi2 ablation had minimal impact on M2R-GIRK and A1R-GIRK signaling in SAN cells. In contrast, Gαo ablation decreased the amplitude and slowed the kinetics of M2R-GIRK responses, while enhancing the sensitivity and prolonging the deactivation rate of A1R-GIRK signaling. Collectively, our data show that differences in GPCR-G protein coupling preferences, and the Gαo substrate preference of RGS6, shape A1R- and M2R-GIRK signaling dynamics in mouse SAN cells.

Molecular and Cellular Mechanisms Underlying Somatostatin-Based Signaling in Two Model Neural Networks, the Retina and the Hippocampus

Neural inhibition plays a key role in determining the specific computational tasks of different brain circuitries. This functional "braking" activity is provided by inhibitory interneurons that use different neurochemicals for signaling. One of these substances, somatostatin, is found in several neural networks, raising questions about the significance of its widespread occurrence and usage. Here, we address this issue by analyzing the somatostatinergic system in two regions of the central nervous system: the retina and the hippocampus. By comparing the available information on these structures, we identify common motifs in the action of somatostatin that may explain its involvement in such diverse circuitries. The emerging concept is that somatostatin-based signaling, through conserved molecular and cellular mechanisms, allows neural networks to operate correctly.

Cholesterol intake and statin use regulate neuronal G protein-gated inwardly rectifying potassium channels

Cholesterol, a critical component of the cellular plasma membrane, is essential for normal neuronal function. Cholesterol content is highest in the brain, where most cholesterol is synthesized de novo; HMG-CoA reductase controls the synthesis rate. Despite strict control, elevated blood cholesterol levels are common and are associated with various neurological disorders. G protein-gated inwardly rectifying potassium (GIRK) channels mediate the actions of inhibitory brain neurotransmitters. Loss of GIRK function enhances neuron excitability; gain of function reduces neuronal activity. However, the effect of dietary cholesterol or HMG-CoA reductase inhibition (i.e., statin therapy) on GIRK function remains unknown. Using a rat model, we compared the effects of a high-cholesterol versus normal diet both with and without atorvastatin, a widely prescribed HMG-CoA reductase inhibitor, on neuronal GIRK currents. The high-cholesterol diet increased hippocampal CA1 region cholesterol levels and correspondingly increased neuronal GIRK currents. Both phenomena were reversed by cholesterol depletion in vitro. Atorvastatin countered the high-cholesterol diet effects on neuronal cholesterol content and GIRK currents; these effects were reversed by cholesterol enrichment in vitro. Our findings suggest that high-cholesterol diet and atorvastatin therapy affect ion channel function in the brain by modulating neuronal cholesterol levels.

GIRK2 splice variants and neuronal G protein-gated K+ channels: implications for channel function and behavior

Many neurotransmitters directly inhibit neurons by activating G protein-gated inwardly rectifying K⁺ (GIRK) channels, thereby moderating the influence of excitatory input on neuronal excitability. While most neuronal GIRK channels are formed by GIRK1 and GIRK2 subunits, distinct GIRK2 isoforms generated by alternative splicing have been identified. Here, we compared the trafficking and function of two isoforms (GIRK2a and GIRK2c) expressed individually in hippocampal pyramidal neurons lacking GIRK2. GIRK2a and GIRK2c supported comparable somato-dendritic GIRK currents in Girk2^−/− pyramidal neurons, although GIRK2c achieved a more uniform subcellular distribution in pyramidal neurons and supported inhibitory postsynaptic currents in distal dendrites better than GIRK2a. While over-expression of either isoform in dorsal CA1 pyramidal neurons restored contextual fear learning in a conditional Girk2^−/− mouse line, GIRK2a also enhanced cue fear learning. Collectively, these data indicate that GIRK2 isoform balance within a neuron can impact the processing of afferent inhibitory input and associated behavior.

Biased signaling at neural melanocortin receptors in regulation of energy homeostasis

The global prevalence of obesity highlights the importance of understanding on regulation of energy homeostasis. The central melanocortin system is an important intersection connecting the neural pathways controlling satiety and energy expenditure to regulate energy homeostasis by sensing and integrating the signals of external stimuli. In this system, neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play crucial roles in the regulation of energy homeostasis. Recently, multiple intracellular signaling pathways and biased signaling at neural MCRs have been discovered, providing new insights into neural MCR signaling. This review attempts to summarize biased signaling including biased receptor mutants (both naturally occurring and lab-generated) and biased ligands at neural MCRs, and to provide a better understanding of obesity pathogenesis and new therapeutic implications for obesity treatment.

Cholesterol up-regulates neuronal G protein-gated inwardly rectifying potassium (GIRK) channel activity in the hippocampus

Hypercholesterolemia is a well known risk factor for the development of neurodegenerative disease. However, the underlying mechanisms are mostly unknown. In recent years, it has become increasingly evident that cholesterol-driven effects on physiology and pathophysiology derive from its ability to alter the function of a variety of membrane proteins including ion channels. Yet, the effect of cholesterol on G protein-gated inwardly rectifying potassium (GIRK) channels expressed in the brain is unknown. GIRK channels mediate the actions of inhibitory brain neurotransmitters. As a result, loss of GIRK function can enhance neuron excitability, whereas gain of GIRK function can reduce neuronal activity. Here we show that in rats on a high-cholesterol diet, cholesterol levels in hippocampal neurons are increased. We also demonstrate that cholesterol plays a critical role in modulating neuronal GIRK currents. Specifically, cholesterol enrichment of rat hippocampal neurons resulted in enhanced channel activity. In accordance, elevated currents upon cholesterol enrichment were also observed in Xenopus oocytes expressing GIRK2 channels, the primary GIRK subunit expressed in the brain. Furthermore, using planar lipid bilayers, we show that although cholesterol did not affect the unitary conductance of GIRK2, it significantly enhanced the frequency of channel openings. Last, combining computational and functional approaches, we identified two putative cholesterol-binding sites in the transmembrane domain of GIRK2. These findings establish that cholesterol plays a critical role in modulating GIRK activity in the brain. Because up-regulation of GIRK function can reduce neuronal activity, our findings may lead to novel approaches for prevention and therapy of cholesterol-driven neurodegenerative disease.

G Protein-Gated K+ Channel Ablation in Forebrain Pyramidal Neurons Selectively Impairs Fear Learning

BACKGROUND

Cognitive dysfunction occurs in many debilitating conditions including Alzheimer's disease, Down syndrome, schizophrenia, and mood disorders. The dorsal hippocampus is a critical locus of cognitive processes linked to spatial and contextual learning. G protein-gated inwardly rectifying potassium ion (GIRK/Kir3) channels, which mediate the postsynaptic inhibitory effect of many neurotransmitters, have been implicated in hippocampal-dependent cognition. Available evidence, however, derives primarily from constitutive gain-of-function models that lack cellular specificity.

METHODS

We used constitutive and neuron-specific gene ablation models targeting an integral subunit of neuronal GIRK channels (GIRK2) to probe the impact of GIRK channels on associative learning and memory.

RESULTS

Constitutive Girk2^-/- mice exhibited a striking deficit in hippocampal-dependent (contextual) and hippocampal-independent (cue) fear conditioning. Mice lacking GIRK2 in gamma-aminobutyric acid neurons (GAD-Cre:Girk2^flox/flox mice) exhibited a clear deficit in GIRK-dependent signaling in dorsal hippocampal gamma-aminobutyric acid neurons but no evident behavioral phenotype. Mice lacking GIRK2 in forebrain pyramidal neurons (CaMKII-Cre(+):Girk2^flox/flox mice) exhibited diminished GIRK-dependent signaling in dorsal, but not ventral, hippocampal pyramidal neurons. CaMKII-Cre(+):Girk2^flox/flox mice also displayed a selective impairment in contextual fear conditioning, as both cue fear and spatial learning were intact in these mice. Finally, loss of GIRK2 in forebrain pyramidal neurons correlated with enhanced long-term depression and blunted depotentiation of long-term potentiation at the Schaffer collateral/cornu ammonis 1 synapse in the dorsal hippocampus.

CONCLUSIONS

Our data suggest that GIRK channels in dorsal hippocampal pyramidal neurons are necessary for normal learning involving aversive stimuli and support the contention that dysregulation of GIRK-dependent signaling may underlie cognitive dysfunction in some disorders.

Pharmacological Characterization of 5-HT1A Autoreceptor-Coupled GIRK Channels in Rat Dorsal Raphe 5-HT Neurons

G protein-activated inwardly rectifying potassium (GIRK) channels in 5-HT neurons are assumed to be principal effectors of 5-hydroxytryptamine 1A (5-HT_1A) autoreceptors, but their pharmacology, subunit composition and the role in regulation of 5-HT neuron activity have not been fully elucidated. We sought for a pharmacological tool for assessing the functional role of GIRK channels in 5-HT neurons by characterizing the effects of drugs known to block GIRK channels in the submicromolar range of concentrations. Whole-cell voltage-clamp recording in brainstem slices were used to determine concentration-response relationships for the selected GIRK channel blockers on 5-HT_1A autoreceptor-activated inwardly rectifying K⁺ conductance in rat dorsal raphe 5-HT neurons. 5-HT_1A autoreceptor-activated GIRK conductance was completely blocked by the nonselective inwardly rectifying potassium channels blocker Ba²⁺ (EC₅₀ = 9.4 μM, full block with 100 μM) and by SCH23390 (EC₅₀ = 1.95 μM, full block with 30 μM). GIRK-specific blocker tertiapin-Q blocked 5-HT_1A autoreceptor-activated GIRK conductance with high potency (EC₅₀ = 33.6 nM), but incompletely, i.e. ~16% of total conductance resulted to be tertiapin-Q-resistant. U73343 and SCH28080, reported to block GIRK channels with submicromolar EC₅₀s, were essentially ineffective in 5-HT neurons. Our data show that inwardly rectifying K⁺ channels coupled to 5-HT_1A autoreceptors display pharmacological properties generally expected for neuronal GIRK channels, but different from GIRK1-GIRK2 heteromers, the predominant form of brain GIRK channels. Distinct pharmacological properties of GIRK channels in 5-HT neurons should be explored for the development of new therapeutic agents for mood disorders.

Mechanisms underlying the activation of G-protein-gated inwardly rectifying K+ (GIRK) channels by the novel anxiolytic drug, ML297

ML297 was recently identified as a potent and selective small molecule agonist of G-protein-gated inwardly rectifying K(+) (GIRK/Kir3) channels. Here, we show ML297 selectively activates recombinant neuronal GIRK channels containing the GIRK1 subunit in a manner that requires phosphatidylinositol-4,5-bisphosphate (PIP2), but is otherwise distinct from receptor-induced, G-protein-dependent channel activation. Two amino acids unique to the pore helix (F137) and second membrane-spanning (D173) domain of GIRK1 were identified as necessary and sufficient for the selective activation of GIRK channels by ML297. Further investigation into the behavioral effects of ML297 revealed that in addition to its known antiseizure efficacy, ML297 decreases anxiety-related behavior without sedative or addictive liabilities. Importantly, the anxiolytic effect of ML297 was lost in mice lacking GIRK1. Thus, activation of GIRK1-containing channels by ML297 or derivatives may represent a new approach to the treatment of seizure and/or anxiety disorders.

Differential GABAB-receptor-mediated effects in perisomatic- and dendrite-targeting parvalbumin interneurons

Inhibitory parvalbumin-containing interneurons (PVIs) control neuronal discharge and support the generation of theta- and gamma-frequency oscillations in cortical networks. Fast GABAergic input onto PVIs is crucial for their synchronization and oscillatory entrainment, but the role of metabotropic GABA(B) receptors (GABA(B)Rs) in mediating slow presynaptic and postsynaptic inhibition remains unknown. In this study, we have combined high-resolution immunoelectron microscopy, whole-cell patch-clamp recording, and computational modeling to investigate the subcellular distribution and effects of GABA(B)Rs and their postsynaptic effector Kir3 channels in rat hippocampal PVIs. Pre-embedding immunogold labeling revealed that the receptors and channels localize at high levels to the extrasynaptic membrane of parvalbumin-immunoreactive dendrites. Immunoreactivity for GABA(B)Rs was also present at lower levels on PVI axon terminals. Whole-cell recordings further showed that synaptically released GABA in response to extracellular stimulation evokes large GABA(B)R-mediated slow IPSCs in perisomatic-targeting (PT) PVIs, but only small or no currents in dendrite-targeting (DT) PVIs. In contrast, paired recordings demonstrated that GABA(B)R activation results in presynaptic inhibition at the output synapses of both PT and DT PVIs, but more strongly in the latter. Finally, computational analysis indicated that GABA(B) IPSCs can phasically modulate the discharge of PT interneurons at theta frequencies. In summary, our results show that GABA(B)Rs differentially mediate slow presynaptic and postsynaptic inhibition in PVIs and can contribute to the dynamic modulation of their activity during oscillations. Furthermore, these data provide evidence for a compartment-specific molecular divergence of hippocampal PVI subtypes, suggesting that activation of GABA(B)Rs may shift the balance between perisomatic and dendritic inhibition.

Ras-association domain of sorting Nexin 27 is critical for regulating expression of GIRK potassium channels

G protein-gated inwardly rectifying potassium (GIRK) channels play an important role in regulating neuronal excitability. Sorting nexin 27b (SNX27b), which reduces surface expression of GIRK channels through a PDZ domain interaction, contains a putative Ras-association (RA) domain with unknown function. Deleting the RA domain in SNX27b (SNX27b-ΔRA) prevents the down-regulation of GIRK2c/GIRK3 channels. Similarly, a point mutation (K305A) in the RA domain disrupts regulation of GIRK2c/GIRK3 channels and reduces H-Ras binding in vitro. Finally, the dominant-negative H-Ras (S17N) occludes the SNX27b-dependent decrease in surface expression of GIRK2c/GIRK3 channels. Thus, the presence of a functional RA domain and the interaction with Ras-like G proteins comprise a novel mechanism for modulating SNX27b control of GIRK channel surface expression and cellular excitability.

Optical control of metabotropic glutamate receptors

G-protein coupled receptors (GPCRs), the largest family of membrane signaling proteins, respond to neurotransmitters, hormones and small environmental molecules. The neuronal function of many GPCRs has been difficult to resolve because of an inability to gate them with subtype-specificity, spatial precision, speed and reversibility. To address this, we developed an approach for opto-chemical engineering native GPCRs. We applied this to the metabotropic glutamate receptors (mGluRs) to generate light-agonized and light-antagonized “LimGluRs”. The light-agonized “LimGluR2”, on which we focused, is fast, bistable, and supports multiple rounds of on/off switching. Light gates two of the primary neuronal functions of mGluR2: suppression of excitability and inhibition of neurotransmitter release. The light-antagonized “LimGluR2block” can be used to manipulate negative feedback of synaptically released glutamate on transmitter release. We generalize the optical control to two additional family members: mGluR3 and 6. The system works in rodent brain slice and in zebrafish in vivo, where we find that mGluR2 modulates the threshold for escape behavior. These light-gated mGluRs pave the way for determining the roles of mGluRs in synaptic plasticity, memory and disease.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.

Synergistic roles for G-protein γ3 and γ7 subtypes in seizure susceptibility as revealed in double knock-out mice

The functions of different G-protein αβγ subunit combinations are traditionally ascribed to their various α components. However, the discovery of similarly diverse γ subtypes raises the possibility that they may also contribute to specificity. To test this possibility, we used a gene targeting approach to determine whether the closely related γ(3) and γ(7) subunits can perform functionally interchangeable roles in mice. In contrast to single knock-out mice that show normal survival, Gng3(-/-)Gng7(-/-) double knock-out mice display a progressive seizure disorder that dramatically reduces their median life span to only 75 days. Biochemical analyses reveal that the severe phenotype is not due to redundant roles for the two γ subunits in the same signaling pathway but rather is attributed to their unique actions in different signaling pathways. The results suggest that the γ(3) subunit is a component of a G(i/o) protein that is required for γ-aminobutyric acid, type B, receptor-regulated neuronal excitability, whereas the γ(7) subunit is a component of a G(olf) protein that is responsible for A(2A) adenosine or D(1) dopamine receptor-induced neuro-protective response. The development of this mouse model offers a novel experimental framework for exploring how signaling pathways integrate to produce normal brain function and how their combined dysfunction leads to spontaneous seizures and premature death. The results underscore the critical role of the γ subunit in this process.

Alternative G protein coupling and biased agonism: new insights into melanocortin-4 receptor signalling

The melanocortin-4 receptor (MC4R) is a prototypical G protein-coupled receptor (GPCR) that plays a considerable role in controlling appetite and energy homeostasis. Signalling initiated by MC4R is orchestrated by multiple agonists, inverse agonism and by interactions with accessory proteins. The exact molecular events translating MC4R signalling into its physiological role, however, are not fully understood. This review is an attempt to summarize new aspects of MC4R signalling in the context of its recently discovered alternative G protein coupling, and to give a perspective on how future research could improve our knowledge about the intertwining molecular mechanisms that are responsible for the regulation of energy homeostasis by the melanocortin system.

G protein {beta}{gamma} gating confers volatile anesthetic inhibition to Kir3 channels

G protein-activated inwardly rectifying potassium (GIRK or Kir3) channels are directly gated by the βγ subunits of G proteins and contribute to inhibitory neurotransmitter signaling pathways. Paradoxically, volatile anesthetics such as halothane inhibit these channels. We find that neuronal Kir3 currents are highly sensitive to inhibition by halothane. Given that Kir3 currents result from increased Gβγ available to the channels, we asked whether reducing available Gβγ to the channel would adversely affect halothane inhibition. Remarkably, scavenging Gβγ using the C-terminal domain of β-adrenergic receptor kinase (cβARK) resulted in channel activation by halothane. Consistent with this effect, channel mutants that impair Gβγ activation were also activated by halothane. A single residue, phenylalanine 192, occupies the putative Gβγ gate of neuronal Kir3.2 channels. Mutation of Phe-192 at the gate to other residues rendered the channel non-responsive, either activated or inhibited by halothane. These data indicated that halothane predominantly interferes with Gβγ-mediated Kir3 currents, such as those functioning during inhibitory synaptic activity. Our report identifies the molecular correlate for anesthetic inhibition of Kir3 channels and highlights the significance of these effects in modulating neurotransmitter-mediated inhibitory signaling.

GABAB receptor coupling to G-proteins and ion channels

GABA(B) receptors have been found to play a key role in regulating membrane excitability and synaptic transmission in the brain. The GABA(B) receptor is a G-protein coupled receptor (GPCR) that associates with a subset of G-proteins (pertussis toxin sensitive Gi/o family), that in turn regulate specific ion channels and trigger cAMP cascades. In this review, we describe the relationships between the GABA(B) receptor, its effectors and associated proteins that mediate GABA(B) receptor function within the brain. We discuss a unique feature of the GABA(B) receptor, the requirement for heterodimerization to produce functional receptors, as well as an increasing body of evidence that suggests GABA(B) receptors comprise a macromolecular signaling heterocomplex, critical for efficient targeting and function of the receptors. Within this complex, GABA(B) receptors associate specifically with Gi/o G-proteins that regulate voltage-gated Ca(2+) (Ca(V)) channels, G-protein activated inwardly rectifying K(+) (GIRK) channels, and adenylyl cyclase. Numerous studies have revealed that lipid rafts, scaffold proteins, targeting motifs in the receptor, and regulators of G-protein signaling (RGS) proteins also contribute to the function of GABA(B) receptors and affect cellular processes such as receptor trafficking and activity-dependent desensitization. This complex regulation of GABA(B) receptors in the brain may provide opportunities for new ways to regulate GABA-dependent inhibition in normal and diseased states of the nervous system.

Morphine- and CaMKII-dependent enhancement of GIRK channel signaling in hippocampal neurons

G-protein-gated inwardly rectifying potassium (GIRK) channels, which help control neuronal excitability, are important for the response to drugs of abuse. Here, we describe a novel pathway for morphine-dependent enhancement of GIRK channel signaling in hippocampal neurons. Morphine treatment for ∼20 h increased the colocalization of GIRK2 with PSD95, a dendritic spine marker. Western blot analysis and quantitative immunoelectron microscopy revealed an increase in GIRK2 protein and targeting to dendritic spines. In vivo administration of morphine also produced an upregulation of GIRK2 protein in the hippocampus. The mechanism engaged by morphine required elevated intracellular Ca(2+) and was insensitive to pertussis toxin, implicating opioid receptors that may couple to Gq G-proteins. Met-enkephalin, but not the μ-selective (DAMGO) and δ-selective (DPDPE) opioid receptor agonists, mimicked the effect of morphine, suggesting involvement of a heterodimeric opioid receptor complex. Peptide (KN-93) inhibition of CaMKII prevented the morphine-dependent change in GIRK localization, whereas expression of a constitutively activated form of CaMKII mimicked the effects of morphine. Coincident with an increase in GIRK2 surface expression, functional analyses revealed that morphine treatment increased the size of serotonin-activated GIRK currents and Ba(2+)-sensitive basal K(+) currents in neurons. These results demonstrate plasticity in neuronal GIRK signaling that may contribute to the abusive effects of morphine.

Orchestration of "presto" and "largo" synchrony in up-down activity of cortical networks

It has been demonstrated using single-cell and multiunit electrophysiology in layer III entorhinal cortex and disinhibited hippocampal CA3 slices that the balancing of the up-down activity is characterized by both GABA_A and GABA_B mechanisms. Here we report novel results obtained using multi-electrode array (60 electrodes) simultaneous recordings from reverberating postnatal neocortical networks containing 19.2 ± 1.4% GABAergic neurons, typical of intact tissue. We observed that in each spontaneous active-state the total number of spikes in identified clusters of excitatory and inhibitory neurons is almost equal, thus suggesting a balanced average activity. Interestingly, in the active-state, the early phase is sustained by only 10% of the total spikes and the firing rate follows a sigmoidal regenerative mode up to peak at 35 ms with the number of excitatory spikes greater than inhibitory, therefore indicating an early unbalance. Concentration-response pharmacology of up- and down-state lifetimes in clusters of excitatory (n = 1067) and inhibitory (n = 305) cells suggests that, besides the GABA_A and GABA_B mechanisms, others such as GAT-1-mediated uptake, I_h, I_NaP and I_M ion channel activity, robustly govern both up- and down-activity. Some drugs resulted to affect up- and/or down-states with different IC₅₀s, providing evidence that various mechanisms are involved. These results should reinforce not only the role of synchrony in CNS networks, but also the recognized analogies between the Hodgkin–Huxley action potential and the population bursts as basic mechanisms for originating membrane excitability and CNS network synchronization, respectively.

Emerging roles for G protein-gated inwardly rectifying potassium (GIRK) channels in health and disease

G protein-gated inwardly rectifying potassium (GIRK) channels hyperpolarize neurons in response to activation of many different G protein-coupled receptors and thus control the excitability of neurons through GIRK-mediated self-inhibition, slow synaptic potentials and volume transmission. GIRK channel function and trafficking are highly dependent on the channel subunit composition. Pharmacological investigations of GIRK channels and studies in animal models suggest that GIRK activity has an important role in physiological responses, including pain perception and memory modulation. Moreover, abnormal GIRK function has been implicated in altering neuronal excitability and cell death, which may be important in the pathophysiology of diseases such as epilepsy, Down's syndrome, Parkinson's disease and drug addiction. GIRK channels may therefore prove to be a valuable new therapeutic target.

Pertussis toxin-sensitive signaling of melanocortin-4 receptors in hypothalamic GT1-7 cells defines agouti-related protein as a biased agonist

Melanocortin-4 receptor (MC4R)-induced anorexigenic signaling in the hypothalamus controls body weight and energy homeostasis. So far, MC4R-induced signaling has been exclusively attributed to its coupling to G(s) proteins. In line with this monogamous G protein coupling profile, most MC4R mutants isolated from obese individuals showed a reduced ability to activate G(s). However, some mutants displayed enhanced G(s) coupling, suggesting that signaling pathways independent of G(s) may be involved in MC4R-mediated anorexigenic signaling. Here we report that the G(s) signaling-deficient MC4R-D90N mutant activates G proteins in a pertussis toxin-sensitive manner, indicating that this mutant is able to selectively interact with G(i/o) proteins. Analyzing a hypothalamic cell line (GT1-7 cells), we observed activation of pertussis toxin-sensitive G proteins by the wild-type MC4R as well, reflecting multiple coupling of the MC4R to G(s) and G(i/o) proteins in an endogenous cell system. Surprisingly, the agouti-related protein, which has been classified as a MC4R antagonist, selectively activates G(i/o) signaling in GT1-7 cells. Thus, the agouti-related protein antagonizes melanocortin-dependent G(s) activation not only by competitive antagonism but additionally by initiating G(i/o) protein-induced signaling as a biased agonist.

Coregulation of natively expressed pertussis toxin-sensitive muscarinic receptors with G-protein-activated potassium channels

Many inhibitory neurotransmitters in the brain activate Kir3 channels by stimulating pertussis toxin (PTX)-sensitive G-protein-coupled receptors. Here, we investigated the regulation of native muscarinic receptors and Kir3 channels expressed in NGF-differentiated PC12 cells, which are similar to sympathetic neurons. Quantitative reverse transcription-PCR and immunocytochemistry revealed that NGF treatment significantly upregulated mRNA and protein for m2 muscarinic receptors, PTX-sensitive G alpha(o) G-proteins, and Kir3.2c channels. Surprisingly, these upregulated muscarinic receptor/Kir3 signaling complexes were functionally silent. Ectopic expression of m2 muscarinic receptors or Kir3.2c channels was unable to produce muscarinic receptor-activated Kir3 currents with oxotremorine. Remarkably, pretreatment with muscarinic (m2/m4) receptor antagonists resulted in robust oxotremorine-activated Kir3 currents. Thus, sustained cholinergic stimulation of natively expressed m2/m4 muscarinic receptors controlled cell surface expression and functional coupling of both receptors and Kir3 channels. This new pathway for controlling Kir3 signaling could help limit the potential harmful effects of excessive Kir3 activity in the brain.

Receptor-specific inhibition of GABAB-activated K+ currents by muscarinic and metabotropic glutamate receptors in immature rat hippocampus

It has been shown that the activation of G(q)-coupled receptors (G(q)PCRs) in cardiac myocytes inhibits the G protein-gated inwardly rectifying K(+) current (I(GIRK)) via receptor-specific depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)). In this study, we investigated the mechanism of the receptor-mediated regulation of I(GIRK) in acutely isolated hippocampal CA1 neurons by the muscarinic receptor agonist, carbachol (CCh), and the group I metabotropic glutamate receptor (mGluR) agonist, 3,5-dihydroxyphenylglycine (DHPG). I(GIRK) was activated by the GABA(B) receptor agonist, baclofen. When baclofen was repetitively applied at intervals of 2-3 min, the amplitude of the second I(GIRK) was 92.3 +/- 1.7% of the first I(GIRK) in control. Pretreatment of neurons with CCh or DHPG prior to the second application of baclofen caused a reduction in the amplitude of the second I(GIRK) to 54.8 +/- 1.3% and 51.4 +/- 0.6%, respectively. In PLCbeta1 knockout mice, the effect of CCh on I(GIRK) was significantly reduced, whereas the effect of DHPG remained unchanged. The CCh-mediated inhibition of I(GIRK) was almost completely abolished by PKC inhibitors and pipette solutions containing BAPTA. The DHPG-mediated inhibition of I(GIRK) was attenuated by the inhibition of phospholipase A(2) (PLA(2)), or the sequestration of arachidonic acid. We confirmed that DHPG eliminated the inhibition of I(GIRK) by arachidonic acid. These results indicate that muscarinic inhibition of I(GIRK) is mediated by the PLC/PKC signalling pathway, while group I mGluR inhibition of I(GIRK) occurs via the PLA(2)-dependent production of arachidonic acid. These results present a novel receptor-specific mechanism for crosstalk between G(q)PCRs and GABA(B) receptors.

Ts65Dn, a Mouse Model of Down Syndrome, Exhibits Increased GABAB-Induced Potassium Current

Down syndrome (DS) is the most common nonheritable cause of mental retardation. DS is the result of the presence of an extra chromosome 21 and its phenotype may be a consequence of overexpressed genes from that chromosome. One such gene is Kcnj6/Girk2, which encodes the G-protein-coupled inward rectifying potassium channel subunit 2 (GIRK2). We have recently shown that the DS mouse model, Ts65Dn, overexpresses GIRK2 throughout the brain and in particular the hippocampus. Here we report that this overexpression leads to a significant increase (∼2-fold) in GABAB-mediated GIRK current in primary cultured hippocampal neurons. The dose response curves for peak and steady-state GIRK current density is significantly shifted left toward lower concentrations of baclofen in Ts65Dn neurons compared with diploid controls, consistent with increased functional expression of GIRK channels. Stationary fluctuation analysis of baclofen-induced GIRK current from Ts65Dn neurons indicated no significant change in single-channel conductance compared with diploid. However, significant increases in GIRK channel density was found in Ts65Dn neurons. In normalized baclofen-induced GIRK current and GIRK current kinetics no difference was found between diploid and Ts65Dn neurons, which suggests unimpaired mechanisms of interaction between GIRK channel and GABAB receptor. These results indicate that increased expression of GIRK2 containing channels have functional consequences that likely affect the balance between excitatory and inhibitory neuronal transmission.

Complex Genetics of Interactions of Alcohol and CNS Function and Behavior

This work summarizes the proceedings of a symposium at the 2004 RSA Meeting in Vancouver, Canada. The organizers were R. W. Williams and D. B. Matthews; the Chair was M. F. Miles. The presentations were (1) WebQTL: A resource for analysis of gene expression variation and the genetic dissection of alcohol related phenotypes, by E. J. Chesler, (2) The marriage of microarray and qtl analyses: what's to gain, by J. K. Belknap, (3) Use of WebQTL to identify QTLs associated with footshock stress and ethanol related behaviors, by D. B. Matthews, (4) A high throughput strategy for the detection of quantitative trait genes, by R. J. Hitzemann, and (5) The use of gene arrays in conjunction with transgenic and selected animals to understand anxiety in alcoholism, by. B. Tabakoff.

Compartment-dependent colocalization of Kir3.2-containing K+ channels and GABAB receptors in hippocampal pyramidal cells

G-protein-coupled inwardly rectifying K+ channels (Kir3 channels) coupled to metabotropic GABAB receptors are essential for the control of neuronal excitation. To determine the distribution of Kir3 channels and their spatial relationship to GABAB receptors on hippocampal pyramidal cells, we used a high-resolution immunocytochemical approach. Immunoreactivity for the Kir3.2 subunit was most abundant postsynaptically and localized to the extrasynaptic plasma membrane of dendritic shafts and spines of principal cells. Quantitative analysis of immunogold particles for Kir3.2 revealed an enrichment of the protein around putative glutamatergic synapses on dendritic spines, similar to that of GABA(B1). Consistent with this observation, a high degree of coclustering of Kir3.2 and GABA(B1) was revealed around excitatory synapses by the highly sensitive SDS-digested freeze-fracture replica immunolabeling. In contrast, in dendritic shafts receptors and channels were found to be mainly segregated. These results suggest that Kir3.2-containing K+ channels on dendritic spines preferentially mediate the effect of GABA, whereas channels on dendritic shafts are likely to be activated by other neurotransmitters as well. Thus, Kir3 channels, localized to different subcellular compartments of hippocampal principal cells, appear to be differentially involved in synaptic integration in pyramidal cell dendrites.

Abnormal expression of the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) in hippocampus, frontal cortex, and substantia nigra of Ts65Dn mouse: a model of Down syndrome

Ts65Dn, a mouse model of Down syndrome (DS), demonstrates abnormal hippocampal synaptic plasticity and behavioral abnormalities related to spatial learning and memory. The molecular mechanisms leading to these impairments have not been identified. In this study, we focused on the G-protein-activated inwardly rectifying potassium channel 2 (GIRK2) gene that is highly expressed in the hippocampus region. We studied the expression pattern of GIRK subunits in Ts65Dn and found that GIRK2 was overexpressed in all analyzed Ts65Dn brain regions. Interestingly, elevated levels of GIRK2 protein in the Ts65Dn hippocampus and frontal cortex correlated with elevated levels of GIRK1 protein. This suggests that heteromeric GIRK1-GIRK2 channels are overexpressed in Ts65Dn hippocampus and frontal cortex, which could impair excitatory input and modulate spike frequency and synaptic kinetics in the affected regions. All GIRK2 splicing isoforms examined were expressed at higher levels in the Ts65Dn in comparison to the diploid hippocampus. The pattern of GIRK2 expression in the Ts65Dn mouse brain revealed by in situ hybridization and immunohistochemistry was similar to that previously reported in the rodent brain. However, in the Ts65Dn mouse a strong immunofluorescent staining of GIRK2 was detected in the lacunosum molecular layer of the CA3 area of the hippocampus. In addition, tyrosine hydroxylase containing dopaminergic neurons that coexpress GIRK2 were more numerous in the substantia nigra compacta and ventral tegmental area in the Ts65Dn compared to diploid controls. In summary, the regional localization and the increased brain levels coupled with known function of the GIRK channel may suggest an important contribution of GIRK2 containing channels to Ts65Dn and thus to DS neurophysiological phenotypes.

Molecular and cellular diversity of neuronal G-protein-gated potassium channels

Neuronal G-protein-gated potassium (GIRK) channels mediate the inhibitory effects of many neurotransmitters. Although the overlapping distribution of GIRK subunits suggests that channel composition varies in the CNS, little direct evidence supports the existence of structural or functional diversity in the neuronal GIRK channel repertoire. Here we show that the GIRK channels linked to GABAB receptors differed in two neuron populations. In the substantia nigra, GIRK2 was the principal subunit, and it was found primarily in dendrites of neurons in the substantia nigra pars compacta (SNc). Baclofen evoked prominent barium-sensitive outward current in dopamine neurons of the SNc from wild-type mice, but this current was completely absent in neurons from GIRK2 knock-out mice. In the hippocampus, all three neuronal GIRK subunits were detected. The loss of GIRK1 or GIRK2 was correlated with equivalent, dramatic reductions in baclofen-evoked current in CA1 neurons. Virtually all of the barium-sensitive component of the baclofen-evoked current was eliminated with the ablation of both GIRK2 and GIRK3, indicating that channels containing GIRK3 contribute to the postsynaptic inhibitory effect of GABAB receptor activation. The impact of GIRK subunit ablation on baclofen-evoked current was consistent with observations that GIRK1, GIRK2, and GABAB receptors were enriched in lipid rafts isolated from mouse brain, whereas GIRK3 was found primarily in higher-density membrane fractions. Altogether, our data show that different GIRK channel subtypes can couple to GABAB receptors in vivo. Furthermore, subunit composition appears to specify interactions between GIRK channels and organizational elements involved in channel distribution and efficient receptor coupling.

Mutation of critical GIRK subunit residues disrupts N- and C-termini association and channel function

The subfamily of G-protein-linked inwardly rectifying potassium channels (GIRKs) is coupled to G-protein receptors throughout the CNS and in the heart. We used mutational analysis to address the role of a specific hydrophobic region of the GIRK1 subunit. Deletion of the GIRK1 C-terminal residues 330-384, as well as the point mutation I331R, resulted in a decrease in channel function when coexpressed with GIRK4 in oocytes and in COS-7 cells. Surface protein expression of GIRK1 I331R coexpressed with GIRK4 was comparable with wild type, indicating that subunits assemble and are correctly localized to the membrane. Subsequent mutation of homologous residues in both the GIRK4 subunit and Kir2.1 (Gbetagamma-independent inward rectifier) also resulted in a decrease in channel function. Intracellular domain associations resulted in the coimmunoprecipitation of the GIRK1 N and C termini and GIRK4 N and C termini. The point mutation I331R in the GIRK1 C terminus or L337R in the GIRK4 C terminus decreased the association between the N and C termini. Mutation of a GIRK1 N-terminal hydrophobic residue, predicted structurally to interact with the C-terminal domain, also resulted in a decrease in channel function and termini association. We hypothesize that the hydrophobic nature of this GIRK1 subunit region is critical for interaction between adjacent termini and is permissive for channel gating. In addition, the homologous mutation in cytoplasmic domains of Kir2.1 (L330R) did not disrupt association, suggesting that the overall structural integrity of this region is critical for inward rectifier function.

PKC-delta sensitizes Kir3.1/3.2 channels to changes in membrane phospholipid levels after M3 receptor activation in HEK-293 cells

G protein-gated inward rectifier (Kir3) channels are inhibited by activation of G(q/11)-coupled receptors and this has been postulated to involve the signaling molecules protein kinase C (PKC) and/or phosphatidylinositol 4,5-bisphosphate (PIP(2)). Their precise roles in mediating the inhibition of this family of channels remain controversial. We examine here their relative roles in causing inhibition of Kir3.1/3.2 channels stably expressed in human embryonic kidney (HEK)-293 cells after muscarinic M(3) receptor activation. In perforated patch mode, staurosporine prevented the G(q/11)-mediated, M(3) receptor, inhibition of channel activity. Recovery from M(3)-mediated inhibition was wortmannin sensitive. Whole cell currents, where the patch pipette was supplemented with PIP(2), were still irreversibly inhibited by M(3) receptor stimulation. When adenosine A(1) receptors were co-expressed, inclusion of PIP(2) rescued the A(1)-mediated response. Recordings from inside-out patches showed that catalytically active PKC applied directly to the intracellular membrane face inhibited the channels: a reversible effect modulated by okadaic acid. Generation of mutant heteromeric channel Kir3.1S185A/Kir3.2C-S178A, still left the channel susceptible to receptor, pharmacological, and direct kinase-mediated inhibition. Biochemically, labeled phosphate is incorporated into the channel. We suggest that PKC-delta mediates channel inhibition because recombinant PKC-delta inhibited channel activity, M(3)-mediated inhibition of the channel, was counteracted by overexpression of two types of dominant negative PKC-delta constructs, and, by using confocal microscopy, we have demonstrated translocation of green fluorescent protein-tagged PKC-delta to the plasma membrane on M(3) receptor stimulation. Thus Kir3.1/3.2 channels are sensitive to changes in membrane phospholipid levels but this is contingent on the activity of PKC-delta after M(3) receptor activation in HEK-293 cells.

Endogenous RGS proteins enhance acute desensitization of GABA(B) receptor-activated GIRK currents in HEK-293T cells

The coupling of GABA(B) receptors to G-protein-gated inwardly rectifying potassium (GIRK) channels constitutes an important inhibitory pathway in the brain. Here, we examined the mechanism underlying desensitization of agonist-evoked currents carried by homomeric GIRK2 channels expressed in HEK-293T cells. The canonical GABA(B) receptor agonist baclofen produced GIRK2 currents that decayed by 57.3+/-1.4% after 60 s of stimulation, and then deactivated rapidly (time constant of 3.90+/-0.21 s) upon removal of agonist. Surface labeling studies revealed that GABA(B) receptors, in contrast to micro opioid receptors (MOR), did not internalize with a sustained stimulation for 10 min, excluding receptor redistribution as the primary mechanism for desensitization. Furthermore, heterologous desensitization was observed between GABA(B) receptors and MOR, implicating downstream proteins, such G-proteins or the GIRK channel. To investigate the G-protein turnover cycle, the non-hydrolyzable GTP analogue (GTPgammaS) was included in the intracellular solution and found to attenuate desensitization to 38.3+/-2.0%. The extent of desensitization was also reduced (45.3+/-1.3%) by coexpressing a mutant form of the Galphaq G-protein subunit that has been designed to sequester endogenous RGS proteins. Finally, reconstitution of GABA(B) receptors with Galphao G-proteins rendered insensitive to RGS resulted in significantly less desensitization (28.5+/-3.2%). Taken together, our results demonstrate that endogenous levels of RGS proteins effectively enhance GABA(B) receptor-dependent desensitization of GIRK currents.

Development of GABAB subunits and functional GABAB receptors in rat cultured hippocampal neurons

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) play a critical role in inhibitory synaptic transmission in the hippocampus but the ontogeny of their subunit synthesis and synaptic localisation has not been determined. Here we report the distributions and developmental profiles of GABA(B1) and GABA(B2) subunits in cultured rat embryonic hippocampal neurons. Limited levels of GABA(B1) and GABA(B2) immunoreactivity were present at 3 days in vitro (DIV). At 7 DIV, when baclofen-evoked inwardly rectifying K(+) channel-mediated responses first appear in the cells, there was a more widespread expression within the soma and proximal dendrites. Levels of the K(+) channel GIRK 1 were relatively constant at all time points suggesting channel availability does not limit the appearance of functional GABA(B)Rs. At 14 DIV the staining displayed a punctate dendritic distribution and near maximal GABA(B)R-mediated electrophysiological responses were obtained. About half of the puncta for each GABA(B)R subunit in dendrites co-localised with the synaptic marker SV2a suggesting that these subunits are at or very near to synapses. Interestingly, at all ages strong GABA(B)R immunoreactivity was also present in the nuclei of neurons. These results provide an important developmental baseline for future studies aimed at investigating, for example, the trafficking and functional regulation of these receptors.

Role of G protein-gated inwardly rectifying potassium channels in P2Y12 receptor-mediated platelet functional responses

The role of the G(i)-coupled platelet P2Y(12) receptor in platelet function has been well established. However, the functional effector or effectors contributing directly to alphaIIbbeta3 activation in human platelets has not been delineated. As the P2Y(12) receptor has been shown to activate G protein-gated, inwardly rectifying potassium (GIRK) channels, we investigated whether GIRK channels mediate any of the functional responses of the platelet P2Y(12) receptor. Western blot analysis revealed that platelets express GIRK1, GIRK2, and GIRK4. In aspirin-treated and washed human platelets, 2 structurally distinct GIRK inhibitors, SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride) and U50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(pyrrolidinyl)cyclohexyl] benzeneacetamide methanesulfonate), inhibited adenosine diphosphate (ADP)-, 2-methylthioADP (2-MeSADP)-, U46619-, and low-dose thrombin-mediated platelet aggregation. However, the GIRK channel inhibitors did not affect platelet aggregation induced by high concentrations of thrombin, AYPGKF, or convulxin. Furthermore, the GIRK channel inhibitors reversed SFLLRN-induced platelet aggregation, inhibited the P2Y(12)-mediated potentiation of dense granule secretion and Akt phosphorylation, and did not affect the agonist-induced G(q)-mediated platelet shape change and intracellular calcium mobilization. Unlike AR-C 69931MX, a P2Y(12) receptor-selective antagonist, the GIRK channel blockers did not affect the ADP-induced adenlylyl cyclase inhibition, indicating that they do not directly antagonize the P2Y(12) receptor. We conclude that GIRK channels are important functional effectors of the P2Y(12) receptor in human platelets.

Rapid desensitization of G protein-gated inwardly rectifying K(+) currents is determined by G protein cycle

Activation of G protein-gated inwardly rectifying K(+) (GIRK) channels, found in the brain, heart, and endocrine tissue, leads to membrane hyperpolarization that generates neuronal inhibitory postsynaptic potentials, slows the heart rate, and inhibits hormone release. During stimulation of G(i/o)-coupled receptors and subsequent channel activation, it has been observed that the current desensitizes. In this study we examined mechanisms underlying fast desensitization of cloned heteromeric neuronal Kir3.1+3.2A and atrial Kir3.1+3.4 channels and also homomeric Kir3.0 currents in response to stimulation of several G(i/o) G protein-coupled receptors (GPCRs) expressed in HEK-293 cells (adenosine A(1), adrenergic alpha(2A), dopamine D(2S), M(4) muscarinic, and GABA(B1b/2) receptors). We found that all agonist-induced currents displayed a similar degree of desensitization except the adenosine A(1) receptor, which exhibits an additional desensitizing component. Using the nonhydrolyzable GTP analog guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), we found that this is due to a receptor-dependent, G protein-independent process. Using Ca(2+) imaging we showed that desensitization is unlikely to be accounted for solely by phospholipase C activation and phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis. We examined the contribution of the G protein cycle and found the following. First, agonist concentration is strongly correlated with degree of desensitization. Second, competitive inhibition of GDP/GTP exchange by using nonhydrolyzable guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) has two effects, a slowing of channel activation and an attenuation of the fast desensitization phenomenon. Finally, using specific Galpha subunits we showed that ternary complexes with fast activation rates display more prominent desensitization than those with slower activation kinetics. Together our data suggest that fast desensitization of GIRK currents is accounted for by the fundamental properties of the G protein cycle.