Single-channel properties of a G-protein-coupled inward rectifier potassium channel in brain 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.

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.

Chemogenetic and Optogenetic Activation of Gαs Signaling in the Basolateral Amygdala Induces Acute and Social Anxiety-Like States

Anxiety disorders are debilitating psychiatric illnesses with detrimental effects on human health. These heightened states of arousal are often in the absence of obvious threatening cues and are difficult to treat owing to a lack of understanding of the neural circuitry and cellular machinery mediating these conditions. Activation of noradrenergic circuitry in the basolateral amygdala is thought to have a role in stress, fear, and anxiety, and the specific cell and receptor types responsible is an active area of investigation. Here we take advantage of two novel cellular approaches to dissect the contributions of G-protein signaling in acute and social anxiety-like states. We used a chemogenetic approach utilizing the Gαs DREADD (rM3Ds) receptor and show that selective activation of generic Gαs signaling is sufficient to induce acute and social anxiety-like behavioral states in mice. Second, we use a recently characterized chimeric receptor composed of rhodopsin and the β2-adrenergic receptor (Opto-β2AR) with in vivo optogenetic techniques to selectively activate Gαs β-adrenergic signaling exclusively within excitatory neurons of the basolateral amygdala. We found that optogenetic induction of β-adrenergic signaling in the basolateral amygdala is sufficient to induce acute and social anxiety-like behavior. These findings support the conclusion that activation of Gαs signaling in the basolateral amygdala has a role in anxiety. These data also suggest that acute and social anxiety-like states may be mediated through signaling pathways identical to β-adrenergic receptors, thus providing support that inhibition of this system may be an effective anxiolytic therapy.

A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ

G protein-gated K⁺ channels (GIRK; Kir3), activated by Gβγ subunits derived from G_i/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (I_evoked) and neurotransmitter-independent basal (I_basal) GIRK activities are physiologically important, but mechanisms of I_basal and its relation to I_evoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that I_basal and I_evoked are interrelated: the extent of activation by neurotransmitter (activation index, R_a) is inversely related to I_basal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates I_basal and I_evoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse I_basal-R_a correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gα_i/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gα_i/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα.

Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K⁺ channel, GIRK (G protein-gated Inwardly Rectifying K⁺ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Measurement of orexin (hypocretin) and substance P effects on constitutively active inward rectifier K(+) channels in brain neurons

Electrophysiological experiments in our laboratory have led to the discovery that the cholinergic neurons in the nucleus basalis in the rat forebrain possess constitutively active inward rectifier K(+) channels. Unlike cloned inward rectifier K(+) channels, these constitutively active inward rectifier K(+) channels were found to have unique properties, and thus were named "KirNB" (inward rectifier K(+) channels in the nucleus basalis). We found that slow excitatory transmitters, such as orexin (hypocretin) and substance P, suppress the KirNB channel, resulting in neuronal excitation. Furthermore, it was discovered that suppression of KirNB channels by these transmitters is through protein kinase C (PKC). This chapter describes detailed electrophysiological techniques for investigating the effects of orexin and substance P on constitutively active KirNB channels. For this purpose, we also present a method for culturing nucleus basalis cholinergic neurons in which KirNB channels exist. Then, we describe the procedures through which PKC has been determined to mediate inhibition of KirNB channels by orexin and substance P. There are probably many other transmitters which may produce effects on KirNB channels. This chapter will enable researchers to investigate the effects of such transmitters on KirNB channels and their roles in neuronal functions.

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.

Amplitude histogram-based method of analysis of patch clamp recordings that involve extreme changes in channel activity levels

Many ion channels show low basal activity, which is increased hundreds-fold by the relevant gating factor. A classical example is the activation G-protein-activated K(+) channels (GIRK) by Gbetagamma subunit dimer. The extent of activation (relative to basal current), R(a), is an important physiological parameter, usually readily estimated from whole cell recordings. However, calculation of R(a) often becomes non-trivial in multi-channel patches because of extreme changes in activity upon activation, from a seemingly single-channel pattern to a macroscopic one. In such cases, calculation of the net current flowing through the channels in the patch, I, before and after activation may require different methods of analysis. To address this problem, we utilized neuronal GIRK channels activated by purified Gbetagamma in excised patches of Xenopus oocytes. Channels were expressed at varying densities, from a few to several hundreds per patch. We present a simple and fast method of calculating I using amplitude histogram analysis and establish its accuracy by comparing with I calculated from event lists. This method allows the analysis of extreme changes in I in multichannel patches, which would be impossible using the standard methods of idealization and event list generation.

Noradrenergic modulation of basolateral amygdala neuronal activity: opposing influences of alpha-2 and beta receptor activation

Substantial data exists demonstrating the importance of the amygdala and the locus ceruleus (LC) in responding to stress, aversive memory formation, and the development of stress-related disorders; however, little is known about the effects of norepinephrine (NE) on amygdala neuronal activity in vivo. The basolateral nucleus of the amygdala (BLA) receives dense NE projections from the LC, NE increases in the BLA in response to stress, and the BLA can also modulate the LC via reciprocal projections. These experiments examined the effects of noradrenergic agents on spontaneous and evoked responses of BLA neurons. NE iontophoresis inhibited spontaneous firing and decreased the responsiveness of BLA neurons to electrical stimulation of entorhinal cortex and sensory association cortex (Te3). Confirmed BLA projection neurons exhibited exclusively inhibitory responses to NE. Systemic administration of propranolol, a beta-receptor antagonist, decreased the spontaneous firing rate and potentiated the NE-evoked inhibition of BLA neurons. In addition, iontophoresis of the alpha-2 agonist clonidine, footshock administration, and LC stimulation mimicked the effects of NE iontophoresis on spontaneous activity. Furthermore, the effects of LC stimulation were partially blocked by systemic administration of alpha 2 and beta receptor antagonists. This is the first study to demonstrate the actions of directly applied and stimulus-evoked NE in the BLA in vivo, and provides a mechanism by which beta receptors can mediate the important behavioral consequences of NE within the BLA. The interaction between these two structures is particularly relevant with regard to their known involvement in stress responses and stress-related disorders.

Morphological and physiological properties of serotonergic neurons in dissociated cultures from the postnatal rat dorsal raphe nucleus

We have developed dissociated primary cultures of the dorsal raphe nucleus from postnatal 9-12-day-old rats. The nucleus was dissected out from brain slices, dissociated, and cultured over a glial feeder layer. Serotonin immunocytochemistry revealed that 62% of cultured neurons were serotonergic. There was no significant difference in diameters between serotonergic and non-serotonergic neurons. With the whole-cell patch-clamp method, cultured neurons were tested for responses to 8-hydroxydipropylaminotetraline (8-OH-DPAT, a selective agonist for 5-HT(1A)), and then treated with serotonin immunocytochemistry. Ninety-two percent of neurons responding to 8-OH-DPAT were serotonergic. These results were used to identify serotonergic neurons. In most cases, serotonergic neurons did not show spontaneous firings of action potentials. Constant current depolarizations elicited trains of action potentials that usually did not show marked adaptation. Application of 8-OH-DPAT inhibited action potential firing. The current-voltage relation of the 8-OH-DPAT-induced current indicated an inward rectification with its reversal potential near E(K). Serotonergic neurons were depolarized by phenylephrine, bombesin, and gastrin-releasing peptide. This culture system will serve as a useful tool for elucidating the cellular, physiological, and molecular properties of brain serotonergic neurons.

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.

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.

Orexin (hypocretin) effects on constitutively active inward rectifier K+ channels in cultured nucleus basalis neurons

Orexins are excitatory transmitters implicated in sleep disorders. Because orexins were discovered only recently, their ionic and signal transduction mechanisms have not been well clarified. We recently reported that orexin A (OXA) inhibits G protein-coupled inward rectifier K+ (GIRK) channels in cultured locus coeruleus and nucleus tuberomammillaris neurons. Other work in our laboratory revealed the existence of a novel inward rectifier K+ channel (KirNB), which is located in cholinergic neurons of the nucleus basalis (NB) and possesses unique single-channel characteristics. The mean open time is considerably shorter in KirNB than in Kir2.0 channels. Constitutive activity and a smaller unitary conductance set KirNB apart from cloned Kir3.0 channels. Previously, we found that substance P excites NB neurons by inhibiting KirNB channels. Here we show that orexins suppress KirNB channel activity, likely leading to neuronal excitation. Electrophysiological studies were performed on cultured NB neurons from the basal forebrain. OXA application decreased whole cell conductance through a pertussis toxin (PTX)-insensitive G protein. The OXA-suppressed current was inwardly rectifying with a reversal potential around E(K). Single-channel recordings of NB neurons revealed that constitutively active KirNB channels were transiently inhibited by OXA. Okadaic acid pretreatment abolished the recovery. The results suggest that OXA inhibition of KirNB is mediated by a PTX-insensitive G protein (i.e., G(q/11)), which eventually results in channel phosphorylation. Recovery from this inhibition is by dephosphorylation. These results, taken together with our previous study, suggest that orexin receptors can elicit neuronal excitation through at least two families of inward rectifier K+ channels: GIRK and KirNB channels.

Single-cell RT-PCR analysis of GIRK channels expressed in rat locus coeruleus and nucleus basalis neurons

G protein-coupled inward rectifier potassium channels (GIRK, Kir3) play a crucial role in determining neuronal excitability. Currently, four mammalian GIRK members (GIRK1-4) have been genetically identified. We have been investigating physiological properties of GIRKs in cultured noradrenergic neurons from the locus coeruleus (LC) and cholinergic neurons from the nucleus basalis (NB). Yet, precise information is lacking about which types of GIRK channels are present in these neurons. We performed single-cell RT-PCR on these cultured neurons. In 13 noradrenergic LC neurons, GIRK1, GIRK2, GIRK3, and GIRK4 mRNAs existed in 12, 13, nine, and six neurons, respectively. In six cholinergic NB neurons, GIRK1, GIRK2, GIRK3, and GIRK4 mRNAs existed in six, four, one, and three neurons, respectively. Therefore, GIRK1 and GIRK2 mRNAs are most frequently encountered in both LC and NB neurons.

Chronic exposure to cold stress alters electrophysiological properties of locus coeruleus neurons recorded in vitro

Chronic stress exposure can alter central noradrenergic function. Previously, we reported that in chronically cold-exposed rats the release of norepinephrine and electrophysiological activation of locus coeruleus (LC) neurons is enhanced in response to multiple excitatory stimuli without alterations in basal activity. In the present studies, we used in vitro intracellular recording techniques to explore the effect of chronic cold exposure on the basal and evoked electrophysiological properties of LC neurons in horizontal slices of the rat brainstem. Consistent with our findings from in vivo experiments, chronic cold exposure did not affect basal firing rate. Furthermore, gross morphology of LC neurons and spike waveform characteristics were similar in slices from control and previously cold-exposed rats. However, excitability in response to intracellular current injection and input resistance were larger in slices from previously cold-exposed rats. In addition, the accommodation of spike firing in response to sustained current injection was smaller and the period of postactivation inhibition appeared to be less in LC neurons from cold-exposed rats. These data demonstrate that the stress-evoked sensitization of LC neurons observed in vivo is at least in part maintained in the slice preparation and suggest that alterations in electrophysiological properties of LC neurons contribute to the chronic stress-induced sensitization of central noradrenergic function observed in vivo. Furthermore, the present data suggest that an alteration in autoinhibitory control of LC activity is involved in the chronic stress-induced alterations. The enhanced functional capacity of LC neurons following cold exposure of rats may represent a unique model to study the mechanisms underlying the alterations in central noradrenergic function observed in humans afflicted with mood and anxiety disorders.

Two different inward rectifier K+ channels are effectors for transmitter-induced slow excitation in brain neurons

Substance P (SP) excites large neurons of the nucleus basalis (NB) by inhibiting an inward rectifier K(+) channel (Kir). The properties of the Kir in NB (KirNB) in comparison with the G protein-coupled Kir (GIRK) were investigated. Single-channel recordings with the cell-attached mode showed constitutively active KirNB channels, which were inhibited by SP. When the recording method was changed from the on-cell to the inside-out mode, the channel activity of KirNB remained intact with its constitutive activity unaltered. Application of Gbeta(1gamma2) to inside-out patches induced activity of a second type of Kir (GIRK). Application of Gbeta(1gamma2), however, did not change the KirNB activity. Sequestering Gbeta(1gamma2) with Galpha(i2) abolished the GIRK activity, whereas the KirNB activity was not affected. The mean open time of KirNB channels (1.1 ms) was almost the same as that of GIRKs. The unitary conductance of KirNB was 23 pS (155 mM [K(+)](o)), whereas that of the GIRK was larger (32-39 pS). The results indicate that KirNB is different from GIRKs and from any of the classical Kirs (IRKs). Whole-cell current recordings revealed that application of muscarine to NB neurons induced a GIRK current, and this GIRK current was also inhibited by SP. Thus, SP inhibits both KirNB and GIRKs. We conclude that the excitatory transmitter SP has two types of Kirs as its effectors: the constitutively active, Gbetagamma-independent KirNB channel and the Gbetagamma-dependent GIRK.

Opioid receptor modulation of a metabolically sensitive ion channel in rat amygdala neurons

We have used single-channel patch-clamp recordings to study opiate receptor effects on freshly dissociated neurons from the rat amygdalohippocampal area (also called the posterior nucleus of the amygdala), an output nucleus of the amygdala implicated in appetitive behaviors. Dissociated cells included a distinct subpopulation that was 30-40 micrometer in diameter, multipolar or pyramidal in shape, and immunoreactive for neuron-specific enolase, mu opioid receptors, and galanin. In whole-cell perforated-patch recordings, these cells responded to low concentrations of mu opioid agonists with a hyperpolarization. In cell-attached single channel recordings, these cells expressed a large variety of K(+)-permeable ion channels, including 20-100 pS inward rectifiers and 150-200 pS apparent Ca(2+)-activated K(+) channels, none of which appeared sensitive to the presence of opioid drugs. In contrast, a 130 pS inwardly rectifying channel was selectively activated by mu opioid receptors in this same subpopulation of cells and was active only in the presence of opioid agonists, and inhibited in the presence of antagonists. Channels identical to the 130 pS channel in conductance and voltage sensitivity were activated in the absence of opioids, when the cells were treated with glucose-free medium or with the metabolic inhibitor rotenone. The sulfonylurea drug tolbutamide inhibited 130 pS channel openings elicited by opioids. Thus, a subpopulation of amygdala projection neurons expresses a metabolically sensitive ion channel that is selectively modulated by opiate receptors. This mechanism may allow opioid neurotransmitters to regulate ingestive behaviors, and thus, opiate drugs to influence reward pathways.

Expression of mRNA and functional alpha(1)-adrenoceptors that suppress the GIRK conductance in adult rat locus coeruleus neurons

1. Locus coeruleus neurons in adult rats express binding sites and mRNA for alpha(1)-adrenoceptors even though the depolarizing effect of alpha(1)-adrenoceptor agonists on neonatal neurons disappears during development. 2. In this study intracellular microelectrodes were used to record from locus coeruleus neurons in brain slices of adult rats and reverse transcription-polymerase chain reaction (RT - PCR) was used to investigate the mRNA expression of alpha(1)- and alpha(2)-adrenoceptors in juvenile and adult rats. 3. The alpha(1)-adrenoceptor agonist phenylephrine had no effect on the membrane conductance of locus coeruleus neurons (V(hold) -60 mV) but decreased the G protein coupled, inward rectifier potassium (GIRK) conductance induced by alpha(2)-adrenoceptor or mu-opioid agonists. The GIRK conductance induced by noradrenaline was increased in amplitude when alpha(1)-adrenoceptors were blocked with prazosin. 4. RT - PCR of total cellular RNA isolated from microdissected locus coeruleus tissue demonstrated strong mRNA expression of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in both juvenile and adult rats. However, only mRNA transcripts for the alpha(1b)-adrenoceptors were consistently detected in cytoplasmic samples taken from single locus coeruleus neurons of juvenile rats, suggesting that this subtype may be responsible for the physiological effects seen in juvenile rats. 5. Juvenile and adult locus coeruleus tissue expressed mRNA for the alpha(2a)- and alpha(2c)-adrenoceptors while the alpha(2b)-adrenoceptor was only weakly expressed in juveniles and was not detected in adults. 6. The results of this study show that alpha(1)-adrenoceptors expressed in adult locus coeruleus neurons function to suppress the GIRK conductance that is activated by mu-opioid and alpha(2)-adrenoceptors.

Modulation of inward rectifier potassium channel by toosendanin, a presynaptic blocker

The effect of toosendanin, a presynaptic blocker, on the inward rectifier potassium channel (K(Kir)) of hippocampal CA1 pyramidal neurons of rats was studied by the single-channel patch-clamp technique. The results showed that toosendanin had an inhibitory effect on K(Kir) in an excised inside-out patch of the neuron under a symmetrical 150 mM K(+) condition. By decreasing the slower open time constant and increasing the slower close time constant, toosendanin (1x10(-6)-1x10(-4) g/ml) significantly reduced the open probability of the channel in a concentration-dependent manner. Meanwhile, a dose-dependent reduction in unitary conductance of the channel was also detected after toosendanin application. These data offer an explanation for toosendanin-induced facilitation of neurotransmitter release and antibotulismic effect of the drug.

Somatostatin activates two types of inwardly rectifying K+ channels in MIN-6 cells

Western blotting revealed the presence of five somatostatin receptor types, sst1, sst2, sst3, sst4 and sst5, in the mouse pancreatic -cell line MIN-6. In MIN-6 cells, glucose-induced electrical activity was potently (pEC50 = 12.7) and irreversibly reduced by somatostatin (SRIF-14); this was associated with hyperpolarization of the membrane potential (pEC50 = 11.2) and a decrease in the input resistance (pEC50 = 12.7). The effects of SRIF-14 were mimicked by 100 nM L-362,855 (a partial agonist at sst5 receptors), but not BIM-23027 or NNC-26,9100 (selective agonists at sst2 and sst4 receptors, respectively). CH-275 at 100 nM (a selective agonist at sst1 receptors) partially inhibited electrical activity but without membrane potential hyperpolarization. One hundred nanomolar SRIF-28 activated an inwardly rectifying K+ current (ISRIF) ISRIF was activated neither by 1 M BIM-23056 nor CYN-154806 (antagonists at sst5 and sst2 receptors, respectively). The activation of ISRIF by 100 nM SRIF-28 was, however, inhibited 93 % by BIM-23056; CYN-154806 had no effect. Both 100 nM glibenclamide and 200 M tolbutamide, blockers of the -cell ATP-sensitive K+ channel (K-ATP), reduced ISRIF by ~44 %, whereas 1 mM Ba2+ abolished ISRIF. In cell-attached patches, 100 nM SRIF-14 activated two types of single-channel currents whose properties were consistent with those of K-ATP and GIRK channels. In conclusion, somatostatin can inhibit glucose-induced electrical activity in MIN-6 cells by the combined activation of K-ATP and GIRK channels. Studies with selective agonists and antagonists are consistent with this effect being mediated by the sst5 receptor.

Altered gating of opiate receptor-modulated K+ channels on amygdala neurons of morphine-dependent rats

The molecular mechanism of tolerance to opiate drugs is poorly understood. We have used single-channel patch-clamp recordings to study opiate receptor effects on dissociated neurons from rat amygdala, a limbic region implicated in addiction processes. A 130-pS inwardly rectifying K(+)-preferring cation channel was activated by mu opioid receptors in a membrane-delimited manner. After chronic treatment of the rats with morphine, channel gating changed markedly, with an approximately 100-fold decrease in open probability at a given morphine concentration. The change in channel gating correlated both in time course and in dose of morphine treatment with the development of functional opiate dependence and appeared to arise at a step after G-protein activation and before channel permeation by K(+). This decreased receptor-channel coupling appears to be large enough to account quantitatively for opiate tolerance and may represent one of the mechanisms through which tolerance occurs.

Morphine-6 beta-glucuronide has a higher efficacy than morphine as a mu-opioid receptor agonist in the rat locus coeruleus

1. The pharmacological properties of the active morphine metabolite, morphine-6 beta-D-glucuronide (M6G), and the parent compound were compared in rat locus coeruleus neurons by electrophysiological recording in brain slices. 2. M6G and morphine activated potassium currents in voltage clamped neurons, which were blocked by the opioid receptor antagonist naloxone. 3. Both M6G and morphine behaved as partial agonists that produced maximal responses smaller than the system maximum, which was measured using [Met(5)]-enkephalin. M6G produced a larger maximal response (78%) than morphine (62%), which we estimated was due to a 2 - 4 fold difference in the relative efficacy of the agonists. 4. 3-O-methoxynaltrexone, which has been reported to behave as a selective antagonist of a M6G preferring receptor, was equally effective at blocking currents produced by M6G and the selective mu-opioid receptor agonist DAMGO. 5. M6G currents were occluded by a prior application of morphine, and were reduced when mu-opioid receptors were desensitized by using [Met(5)]-enkephalin. 6. Morphine-3 beta-D-glucuronide did not affect action potential firing or membrane currents in locus coeruleus neurons and had no effect on currents produced by M6G. 7. These results show that the relative efficacy of M6G is higher than morphine in locus coeruleus neurons, contrary to what has been shown using mu-opioid receptors expressed in cell clones.

Slow modal gating of single G protein-activated K+ channels expressed in Xenopus oocytes

The slow kinetics of G protein-activated K+ (GIRK) channels expressed in Xenopus oocytes were studied in single-channel, inside-out membrane patches. Channels formed by GIRK1 plus GIRK4 subunits, which are known to form the cardiac acetylcholine (ACh)-activated GIRK channel (KACh), were activated by a near-saturating dose of G protein betagamma subunits (Gbetagamma; 20 nM). The kinetic parameters of the expressed GIRK1/4 channels were similar to those of cardiac KACh. GIRK1/4 channels differed significantly from channels formed by GIRK1 with the endogenous oocyte subunit GIRK5 (GIRK1/5) in some of their kinetic parameters and in a 3-fold lower open probability, Po. The unexpectedly low Po (0.025) of GIRK1/4 was due to the presence of closures of hundreds of milliseconds; the channel spent approximately 90 % of the time in the long closed states. GIRK1/4 channels displayed a clear modal behaviour: on a time scale of tens of seconds, the Gbetagamma-activated channels cycled between a low-Po mode (Po of about 0.0034) and a bursting mode characterized by an approximately 30-fold higher Po and a different set of kinetic constants (and, therefore, a different set of channel conformations). The available evidence indicates that the slow modal transitions are not driven by binding and unbinding of Gbetagamma. The GTPgammaS-activated Galphai1 subunit, previously shown to inhibit GIRK channels, substantially increased the time spent in closed states and apparently shifted the channel to a mode similar, but not identical, to the low-Po mode. This is the first demonstration of slow modal transitions in GIRK channels. The detailed description of the slow gating kinetics of GIRK1/4 may help in future analysis of mechanisms of GIRK gating.

Dopamine activates inward rectifier K+ channel in acutely dissociated rat substantia nigra neurones

The effect of dopamine (DA) was investigated on acutely dissociated rat substantia nigra pars compacta (SNc) neurones by using patch clamp recording. The SNc neurones could be classified into two groups. About 75% of large neurones (>30 microm in diameter) were tyrosine hydroxylase (TH) positive while almost all small neurones (<20 microm) were TH negative. In the large neurones, DA hyperpolarized the membrane, resulting in a reduction of the frequency of spontaneous action potentials in current-clamp mode and induced an inward rectifier K+ current in voltage-clamp mode. Quinpirole, a D2 receptor agonist, mimicked the DA action. S(-)-sulpiride, a D2 receptor antagonist, inhibited the DA-induced current (I(DA)) more effectively than SKF83566, a D1 receptor antagonist. Intracellular application of either guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS) or pertussis toxin (IAP) suppressed I(DA). Guanosine 5'-O-(3-thiotriphosphate) (GTP-gammaS) sustained the DA response. Modulators for cAMP such as forskolin and isobutylmethylxathine, H-89, a protein kinase A inhibitor, and chelerythrine, a protein kinase C inhibitor, had no effect on I(DA). The frequency of DA-induced single channel currents in the inside-out patch configuration, for which the unitary conductance was 56.6pS, was greatly reduced by the replacement of GTP with GDP perfused at the cytosolic side. These results suggest that DA acts on a D2-like receptor and activates directly an IAP-sensitive G protein coupled with inward rectifier K+ channels, resulting in a decrease in the spontaneous firing activities of rat SNc dopaminergic neurones.

Variance analysis of current fluctuations of adenosine- and baclofen-activated GIRK channels in dissociated neocortical pyramidal cells

Whole cell recordings were obtained from pyramidal cell somata acutely isolated from rat neocortex. In voltage-clamp mode, adenosine (0.3-1000 microM), and the GABA(B) receptor agonist, baclofen (1-300 microM), induced K+ current responses mediated by G protein-activated inwardly rectifying K+ (GIRK) channels. In our preparation, adenosine activated GIRK currents with an average EC(50) of 2 microM. Baclofen had an average EC50 of 26 microM. To estimate and compare unitary conductance and density of GIRK channels activated by either adenosine or baclofen, we performed variance analysis of current fluctuations associated with the application of the two agonists at increasing concentrations. Irrespective of the agonist tested, GIRK channels displayed an average single-channel conductance of 25 pS at our recording conditions ([K+]o: 60 mM). Assuming that GIRK channel conductance increases in proportion to the square root of [K+]o, this would translate into 5-6 pS at physiological ion gradients. GIRK channels activated by adenosine or baclofen were not only identical in terms of unitary conductance, they also displayed the same average density of 0.5 channels micron(-2) for both agonists. Our data strongly suggest that the two compounds recruit the same type of channel and thus most likely share a common transduction and effector system.

Mu-opioid agonist-induced activation of G-protein-coupled inwardly rectifying potassium current in rat periaqueductal gray neurons

The characteristics of the inwardly rectifying K+ current activated by a mu-type opioid agonist, D-Ala2,N-MePhe4,Gly5-ol-enkephalin (DAMGO), were examined in the acutely dissociated rat periaqueductal gray neurons using the nystatin-perforated and the conventional whole-cell recording modes under voltage-clamp conditions. DAMGO activated inward currents in a concentration- and voltage-dependent manner. The DAMGO-induced current was an inwardly rectifying K+ current (I(DAMGO)) which was sensitive to K+ channel blockers, quinine and Ba2+ but insensitive to Cs+ and tetraethylammonium. In the conventional whole-cell clamp mode, guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDPbetas, 0.4 mM) inhibited the amplitude of I(DAMGO) to 28% of that of the initial current. After the intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate) tetralithium salt (GTPgammas, 0.4 mM) for 1 min, the first application of DAMGO irreversibly activated I(DAMGO). By the extracellular application of N-ethylmaleimide at a concentration of 50 microM for 2 min, I(DAMGO) was completely abolished. When a conventional whole-cell patch was made with a patch-pipette containing 1 microg/ml of pertussis toxin together with 1 mM of beta-nicotinamide adenine dinucleotide, I(DAMGO) gradually declined to about 41% of its initial amplitude. The extracellular application of second messenger modulators including protein kinase inhibitor (staurosporin), protein kinase A activators (forskolin, 3-isobutyl-l-methyl-xanthine and dibutyryladenosine 3'5'-cyclic monophosphate) and protein kinase C activators (phorbol-12-myristate-13-acetate and 1-oleoyl-2-acetyl-sn-glycerol) had no effect on I(DAMGO). These results suggest that (i) DAMGO-activated inwardly rectifying K+ current is mediated by pertussis toxin-sensitive guanine nucleotide binding proteins (G-proteins); (ii) the types of G protein involved in I(DAMGO) are Gi and/or Go; and (iii) the G-proteins exert their roles in I(DAMGO) without any mediation of the second messenger systems.

Human gamma-aminobutyric acid type B receptors are differentially expressed and regulate inwardly rectifying K+ channels

gamma-Aminobutyric acid type B receptors (GABABRs) are involved in the fine tuning of inhibitory synaptic transmission. Presynaptic GABABRs inhibit neurotransmitter release by down-regulating high-voltage activated Ca2+ channels, whereas postsynaptic GABABRs decrease neuronal excitability by activating a prominent inwardly rectifying K+ (Kir) conductance that underlies the late inhibitory postsynaptic potentials. Here we report the cloning and functional characterization of two human GABABRs, hGABABR1a (hR1a) and hGABABR1b (hR1b). These receptors closely match the pharmacological properties and molecular weights of the most abundant native GABABRs. We show that in transfected mammalian cells hR1a and hR1b can modulate heteromeric Kir3.1/3.2 and Kir3.1/3.4 channels. Heterologous expression therefore supports the notion that Kir3 channels are the postsynaptic effectors of GABABRs. Our data further demonstrate that in principle either of the cloned receptors could mediate inhibitory postsynaptic potentials. We find that in the cerebellum hR1a and hR1b transcripts are largely confined to granule and Purkinje cells, respectively. This finding supports a selective association of hR1b, and not hR1a, with postsynaptic Kir3 channels. The mapping of the GABABR1 gene to human chromosome 6p21.3, in the vicinity of a susceptibility locus (EJM1) for idiopathic generalized epilepsies, identifies a candidate gene for inherited forms of epilepsy.

Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors

We characterized potassium current activated by G-protein-coupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba2+. The conductance had identical properties to that activated by GABAB receptors in the same cells. In one-half of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba2+-sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABAB-activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The half-maximally effective agonist concentrations were approximately 75 nM for somatostatin, approximately 100 nM for serotonin, and approximately 400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-to-channel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

Pre- and postsynaptic inhibitory actions of methionine-enkephalin on identified bulbospinal neurons of the rat RVL

The effects of methionine-enkephalin (ME) on visualized bulbospinal neurons of the rostral ventrolateral medulla (RVL) were characterized in thin slices at 32 degrees C using the whole cell patch-clamp technique. Thirty-five percent of the recorded neurons were found to be tyrosine hydroxylase immunoreactive (C1 neurons). In voltage-clamp recordings, ME (3 microM) induced an outward current in 66% of RVL bulbospinal neurons. A similar percentage of C1 and non-C1 neurons were opioid sensitive. The current induced by ME was inwardly rectifying, reversed close to the potassium equilibrium potential, and was blocked by barium. Most spontaneous postsynaptic currents recorded in these neurons were tetrodotoxin (TTX)-resistant miniature postsynaptic currents (mPSCs). Approximately, 75% of mPSCs had rapid kinetics (decay time = 4.7 ms) and were glutamatergic [miniature excitatory postsynaptic currents (mEPSCs)] because they were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). The remaining mPSCs had much slower kinetics (decay time = 19.6 ms) and were GABAergic [miniature inhibitory postsynaptic currents (mIPSCs)] as they were blocked by gabazine (3 microM) but not by strychnine (3-10 microM). ME decreased the frequency of mEPSCs and mIPSCs by 69 and 43%, respectively. The inhibitory effects of ME were mimicked by the selective mu-opioid receptor agonist endomorphin-1 (EM, 3 microM) and were blocked by naloxone (1 microM). In the absence of TTX, excitatory PSCs evoked by focal electrical stimulation were isolated by application of gabazine and strychnine. EM reduced the amplitude of the evoked EPSCs by 41% without changing their decay time. We conclude that opioids inhibit the majority of RVL C1 and non-C1 bulbospinal neurons by activating a potassium conductance postsynaptically and by decreasing the presynaptic release of glutamate. These cellular mechanisms could explain the depressive cardiovascular effects and the sympathoinhibition produced by opioid transmitters in the RVL, in particular during hypotensive hemorrhage.

Gi-3 protein mediates the increase in voltage-gated K+ currents by somatostatin on cultured ovine somatotrophs

Voltage-gated K+ currents in rat somatotrophs are increased by somatostatin (SRIF) through unidentified G protein. In this experiment, somatotroph-enriched cells (up to 85%) were obtained from ovine pituitary glands and further identified by the increase in K+ currents by SRIF. The whole cell recording was employed to study the voltage-gated K+ currents. A reversible increase in K+ currents (up to 150% of control) was obtained in response to local application of SRIF (10 nM) but not vehicle. When the guanosine 5'-O-(3-thiotriphosphate) was included in the pipette solution (200 microM), the recovery phase of K+ current response to SRIF was abolished. Inclusion of guanosine 5'-O-(2-thiodiphosphate) (200 microM) in pipette solution blocked the K+ current response to SRIF. Intracellular dialysis of antibodies against alphao-, alphai-, alphai-1-2-, or alphai-3-subunits of G proteins via patch pipettes was confirmed by immunofluorescent staining of the antibodies. Antibody dialysis alone did not modify voltage-gated K+ currents. Dialysis of anti-alphai or anti-alphai-3 antibodies significantly attenuated the increase in K+ currents that was obtained after application of 10 or 100 nM SRIF. Dialysis with anti-alphao, anti-alphai-1-2, or heat-inactivated (60 degreesC for 10 min) anti-alphai antibodies did not diminish the effect of SRIF on K+ currents. We conclude that the Gi-3 protein mediates the effect of SRIF on voltage-gated K+ currents in ovine somatotrophs.

alpha2-Adrenoceptor-mediated potassium currents in acutely dissociated rat locus coeruleus neurones

1. The noradrenaline (NA)-activated response was investigated in neurones acutely dissociated from the rat locus coeruleus (LC) using nystatin-perforated, conventional whole-cell and inside-out patch recording modes under current- and voltage-clamp conditions. 2. Under current-clamp conditions, NA hyperpolarized the LC neurones, abolishing the spontaneous action potentials. In voltage-clamp studies, NA induced an inwardly rectifying K+ current (INA) in a concentration-dependent manner with a half-maximum effective concentration of 2.2 x 10(-7) M. 3. INA was mimicked by the alpha2-agonist UK14304 but was inhibited by either the alpha2B/alpha2C antagonist ARC239 or the alpha1- and alpha2B/alpha2C antagonist prazosin, suggesting the contribution of alpha2B/alpha2C adrenoceptors. 4. INA was inhibited by the intracellular application of GDPbetaS but fully activated by intracellular perfusion of GTPgammaS. 5. In the inside-out recording mode, the application of GTP to the cytoplasmic side of the patch membrane markedly enhanced the open probability of the NA-activated single channels which represented the inwardly rectifying properties. 6. These results indicate that the activation of alpha2B/alpha2C adrenoceptors coupled with GTP-binding protein directly activates the inwardly rectifying K+ currents in rat LC neurones, thus resulting in a decrease in the spontaneous firing activities.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

Voltage-gated and inwardly rectifying potassium channels

This lecture is dedicated to Max Delbrück and Seymour Benzer. Max Delbrück was our graduate advisor. He introduced us to a variety of biophysical problems, and taught us ways of thinking about these problems by example. Potassium channels was one of the topics included in his journal club in the early seventies; Max also carefully considered the feasibility of purifying potassium channels then. It was in Seymour Benzer's laboratory that we began to look for Drosophila mutants that affect synaptic transmission at the larval neuromuscular junction. Shaker was the first behavioural mutant we tested that gave a robust phenotype, a phenotype that could be mimicked by treating wild-type preparations with a potassium channel blocker. This mutant fly has led us to our subsequent molecular studies of potassium channels. Since we settled in the University of California, San Francisco, and began to study neural development as well as potassium channels, we have settled into the pattern of each attending meetings and presenting our studies on one of these two areas so as to avoid both being away from home and our children at the same time. In following this pattern, I will be presenting the studies of potassium channels as part of our long-term collaboration. In this talk I will first briefly take you through the path that led us to the molecular studies of potassium channels and then discuss the diversity and modulation of these potassium channels at the molecular and physiological level.

Different G proteins mediate somatostatin-induced inward rectifier K+ currents in murine brain and endocrine cells

1. Types of G proteins (G protein alpha-subunit subtypes) which mediate the activation of inward rectifier K+ currents by somatostatin (somatotrophin release-inhibiting factor, SRIF) were determined in cultured locus coeruleus neurones from newborn rats and in AtT-20 cells (a mouse pituitary cell line). 2. The whole-cell patch clamp technique was used together with injection of antibodies against pertussis toxin (PTX)-sensitive G protein alpha-subunits or with injection of antisense (or sense) oligonucleotides against these G proteins. 3. In locus coeruleus neurones, the SRIF-induced activation of inward rectifier K+ currents was inhibited by anti-G alpha i1/G alpha i2 antibody injection, but not by anti-G alpha i3 or by anti-G alpha o/G alpha i3 antibody injection, suggesting that the SRIF response is mediated through G alpha i1 and/or G alpha i2. 4. The SRIF-induced activation of the inward rectifier was suppressed in locus coeruleus neurones after injection of antisense oligonucleotides against G alpha i2, but not by injection of sense oligonucleotides against G alpha i2. Injection of antisense (or sense) oligonucleotides against G alpha i1, G alpha i3 and G alpha O (common) had no effect. These results suggest that G alpha i2 is involved in this SRIF response. 5. In AtT-20 cells, the SRIF-induced activation of inward rectifier K+ currents was suppressed by injection of anti-G alpha i3 antibody, but not by injection of anti-G alpha i1/G alpha i2 antibody. 6. The above results indicate that Gi mediates the SRIF effects on inward rectifier K+ currents. However, different subtypes of Gi are involved in the brain neurones and in the endocrine cells: Gi2 in locus coeruleus neurones and Gi3 in AtT-20 cells.

Endogenous GABA activates small-conductance K+ channels underlying slow IPSCs in rat hippocampal neurons

The objective of this study was to determine the properties of K+ channels activated by endogenously released trasmitter under synaptic conditions. First, the levels of gamma-aminobutyric acid (GABA) were depleted in hippocampal nerve endings to establish the relative contribution of endogenously released GABA to the activation of GABA(B) receptors mediating slow inhibitory postsynaptic currents (IPSCs). Inhibition of glutamic acid decarboxylase and GABA reuptake effectively depleted >85% of the releasable GABA pool, producing parallel reductions of GABA(A) and GABA(B) receptor-mediated IPSCs, indicating that both classes of receptors are activated synaptically by endogenously released GABA. Whole cell patch-clamp recordings of stimulus-evoked slow IPSCs at potentials hyperpolarized from the potassium reversal potential were consistent with the activation of a nonrectifying (n = 3) or slightly outwardly rectifying (n = 4) K+ conductance by the endogenously released GABA. Spectral analysis of the decay phase of GABA(B) IPSCs revealed several time constants indicating complex underlying channel kinetics. Nonstationary variance analysis yielded a small unitary conductance in the range of 5-13 pS, consistent with a large number of channels activated during evoked currents. These results indicate that in granule cells of the dentate gyrus, GABA released synaptically from interneuron terminals activates an unusually small K+ conductance, with no or slight outward rectification. This conductance is therefore unlike those typically reported for neuronal G protein-coupled K+ channels or those activated by exogenously applied baclofen with larger, inwardly rectifying conductances.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

Single-channel properties of the nonselective cation conductance induced by neurotensin in dopaminergic neurons

Slow nonselective cation conductances play a central role in determining the excitability of many neurons, but heretofore this channel type has not been analyzed at the single-channel level. Neurotensin (NT) excites cultured dopaminergic neurons from the ventral tegmental area primarily by increasing such a cation conductance. Using the outside-out configuration of the patch clamp, we elicited single-channel activity of this NT-induced cation channel. Channel activity was blocked by the nonpeptide NT antagonist SR48692, indicating that the response was mediated by NT receptors. The channel opened in both solitary form and in bursts. The reversal potential was -4.2 +/- 1.7 mV, and the elementary conductance was 31 pS at -67 mV with [Na+]o = 140 mM, [Cs+]o = 5 mM, [Na+]i = 88 mM, and [Cs+]i = 74 mM. Thus, the channel was permeable to both Na+ and Cs+. From these characteristics, it is likely that this channel is responsible for the whole-cell current we studied previously. In guanosine 5'-[gamma-thio]triphosphate-loaded cells, NT irreversibly activated about half of the channel activity, suggesting that at least part of the response was mediated by a G protein. Similar channel activity could be induced occasionally in the cell-attached configuration by applying NT outside the patch region.

GABAB receptor-activated inwardly rectifying potassium current in dissociated hippocampal CA3 neurons

GABA and the GABAB receptor agonist baclofen activated a potassium conductance in acutely dissociated hippocampal CA3 neurons. Baclofen-activated current required internal GTP, was purely potassium selective, and showed strong inward rectification. As with acetylcholine-activated current in atrial myocytes, external Cs+ blocked inward but not outward current in a highly voltage-dependent manner, whereas Ba2+ blocked with no voltage dependence. Unlike the cardiac current, however, the baclofen-activated current showed no intrinsic voltage-dependent relaxation. With fast solution exchange, current was activated by baclofen or GABA with a lag of approximately 50 msec followed by an exponential phase (time constant approximately 225 msec at saturating agonist concentrations); deactivation was preceded by a lag of approximately 150 msec and occurred with a time constant of approximately 1 sec. GABA activated the potassium conductance with a half maximally effective concentration (EC50) of 1.6 microM, much lower than that for activation of GABAA receptor-activated chloride current in the same cells (EC50 approximately 25 microM). At low GABA concentrations, activation of the GABAB current had a Hill coefficient of 1.4-2.1, suggesting cooperativity in the receptor-to-channel pathway. Although the maximal conductance activated by GABAB receptors is much smaller than that activated by GABAA receptors, its higher sensitivity to GABA and slower time course make it well suited to respond to low concentrations of extra-synaptic GABA.

A technique for culturing brain nuclei from postnatal rats

The technique for culturing brain nuclei from postnatal rats is described in detail. Key features of this simple method of culturing brain nuclei are: (a) to make brain slices of a particular brain region and to isolate the brain nucleus under direct visualization using a dissecting microscope; (b) to use papain for dissociation; (c) to grow neuron cultures over a glial feeder layer; and (d) to use rat serum (prepared in the laboratory) in the culture medium. We have developed neuronal cultures from several types of brain nuclei (such as the locus coeruleus) and identified the type of neurons immunocytochemically and histochemically. With this culture method, we can obtain high purity cultures of specific types of brain neurons. Our brain nucleus cultures are excellent materials for cellular and molecular physiological studies. Long-term changes of a single neuron belonging to a particular neuron type can be observed. Experiments using the patch clamp technique and intracellular injection of antibodies and antisense oligonucleotides are feasible.

Activation of G protein-coupled inward rectifier K+ channels in brain neurons requires association of G protein beta gamma subunits with cell membrane

In cultured noradrenergic neurons from the rat locus coeruleus, application of recombinant G protein beta 1 gamma 2 subunits (30 nM) to the cytoplasmic side induced single channel activity similar to the somatostatin-induced single channel activity of G protein-coupled inward rectifier potassium channels (Kir (G)). In contrast, recombinant GTP gamma S-activated, myristoylated alpha i2 (100 nM) did not activate this brain Kir (G). Application of beta 1 gamma 2 C68S (30 nM or 150 nM), in which the cysteine residue fourth from the carboxyl terminus of gamma 2 was replaced by serine, failed to activate the brain Kir(G). This mutant lacks prenylation which is required for the association of beta gamma subunit with the cell membrane. Thus, our results suggest that the association of beta gamma subunit with the cell membrane is a prerequisite for activating Kir(G) channels.

Localization and interaction of epitope-tagged GIRK1 and CIR inward rectifier K+ channel subunits

GIRK1 and CIR are G-protein activated inward rectifier K+ channel subunits that combine to form the heteromultimer IKACh, the G beta gamma-activated atrial K channel responsible for the vagal slowing of heart rate. Epitope-tagged channel subunits were constructed by the introduction of distinct six amino acid epitopes into the C-termini or putative extracellular domains of GIRK1 and CIR. Carboxyl-terminal tagged subunits were activated by purified G beta gamma subunits in inside-out patches when expressed in Cos cells. Interestingly, insertion of three amino acids into the putative extracellular domain of GIRK1 resulted in an inactive subunit that acted as a dominant negative subunit when coexpressed with wild type GIRK1 and CIR in Xenopus oocytes. The epitope-tagged CIR-AU1 subunit coimmunoprecipitated GIRK1-AU5 from metabolically labeled Cos cells. Immunofluorescence labeling of Cos cells localized GIRK1-AU5 to internal cytoskeletal structures that co-stained with antibodies against the intermediate filament protein, vimentin. CIR-AU1 localized primarily to the plasma membrane. Double immunofluorescence labeling showed that GIRK1-AU5 plasma membrane staining was detectable only when coexpressed with CIR-AU1.