IRK(1-3) and GIRK(1-4) inwardly rectifying K+ channel mRNAs are differentially expressed in the adult rat brain

Secretagogue-induced exocytosis recruits G protein-gated K+ channels to plasma membrane in endocrine cells

Stimulation-regulated fusion of vesicles to the plasma membrane is an essential step for hormone secretion but may also serve for the recruitment of functional proteins to the plasma membrane. While studying the distribution of G protein-gated K+ (KG) channels in the anterior pituitary lobe, we found KG channel subunits Kir3.1 and Kir3.4 localized on the membranes of intracellular dense core vesicles that contained thyrotropin. Stimulation of these thyrotroph cells with thyrotropin-releasing hormone provoked fusion of vesicles to the plasma membrane, increased expression of Kir3.1 and Kir3.4 subunits in the plasma membrane, and markedly enhanced KG currents stimulated by dopamine and somatostatin. These data indicate a novel mechanism for the rapid insertion of functional ion channels into the plasma membrane, which could form a new type of negative feedback control loop for hormone secretion in the endocrine system.

Effect of catecholamines on the hyperpolarization-activated cationic Ih and the inwardly rectifying potassium I(Kir) currents in the rat substantia nigra pars compacta

Whole-cell ruptured-patch and perforated-patch recordings were used in principal neurons of the rat substantia nigra pars compacta (SNc) to study the effect of catecholamines both on the hyperpolarization-activated cationic (Ih) and the inwardly rectifying potassium (I(Kir)) currents. In internal potassium, a 2 min bath application of noradrenaline (NA; 50 microM) or dopamine (DA; 50 microM) both inhibited Ih and induced an outward current associated with an increase in I(Kir) conductance. These two effects recovered poorly after wash-out. Protein kinase A (PKA), protein kinase C (PKC) and phosphatases 1 and 2A inhibitors did not modify the NA and DA effects on the amplitude of Ih and I(Kir) currents. They also had no effect on the recovery of the catecholamine responses. In perforated-patch experiments, NA and DA also induced an inhibition of Ih and revealed an outward current associated with an increase in conductance. However, both effects recovered in less than 5 min following the wash-out. These results indicate that neither PKA, PKC, nor phosphatases 1 or 2A were required in the NA and DA modulation of these two currents and that an intracellular factor, that could be either washed-out or inversely up-regulated in the ruptured-patch configuration, was implicated in the recovery of both effects. In the presence of external barium (300 microM) or internal caesium which both blocked the outward current and the increase in conductance, neither NA nor DA affected Ih, suggesting that the effect on Ih observed is secondary to the activation of the I(Kir) channels. Increasing chloride conductance of the cell by activation of GABA(A) receptors also induced an inhibition of Ih. All together these results suggest that the NA or DA induced inhibition of Ih could result from an occlusion of Ih by a space-clamp effect.

Characterization of G-protein-gated K+ channels composed of Kir3.2 subunits in dopaminergic neurons of the substantia nigra

G-protein-gated K+ (KG) channels generate slow inhibitory postsynaptic potentials in the brain. Current opinion suggests that neuronal KG channels are heterotetramers of Kir3.1 and Kir3.2. In substantia nigra (SN), however, mRNA of Kir3.1 does not express, whereas that of Kir3.2 clearly does. Therefore, we have characterized the KG channels containing Kir3.2 subunits in SN using biochemical and immunological techniques. We found that they were composed of only Kir3.2 subunits and did not contain significant amounts of either Kir3.1 or Kir3.3. Furthermore, at least some of the KG channels in SN were assemblies of the splicing variants Kir3. 2a and Kir3.2c. The channels were localized specifically at the postsynaptic membrane on the dendrites of dopaminergic neurons. Kir3. 2c, but not Kir3.2a, could bind a PDZ domain-containing protein, PSD-95. The heterologously expressed KG channels composed of Kir3.2a plus Kir3.2c or Kir3.2a alone were activated by G-protein stimulation, but expression of Kir3.2c alone was not. This study reveals that the Kir3.2 splicing variants play distinct roles in the control of function and localization of some of the KG channels in dopaminergic neurons of SN.

GIRK4 confers appropriate processing and cell surface localization to G-protein-gated potassium channels

GIRK1 and GIRK4 subunits combine to form the heterotetrameric acetylcholine-activated potassium current (IKACh) channel in pacemaker cells of the heart. The channel is activated by direct binding of G-protein Gbetagamma subunits. The GIRK1 subunit is atypical in the GIRK family in having a unique ( approximately 125-amino acid) domain in its distal C terminus. GIRK1 cannot form functional channels by itself but must combine with another GIRK family member (GIRK2, GIRK3, or GIRK4), which are themselves capable of forming functional homotetramers. Here we show, using an extracellularly Flag-tagged GIRK1 subunit, that GIRK1 requires association with GIRK4 for cell surface localization. Furthermore, GIRK1 homomultimers reside in core-glycosylated and nonglycosylated states. Coexpression of GIRK4 caused the appearance of the mature glycosylated form of GIRK1. [35S]Methionine pulse-labeling experiments demonstrated that GIRK4 associates with GIRK1 either during or shortly after subunit synthesis. Mutant and chimeric channel subunits were utilized to identify domains responsible for GIRK1 localization. Truncation of the unique C-terminal domain of Delta374-501 resulted in an intracellular GIRK1 subunit that produced normal IKACh-like channels when coexpressed with GIRK4. Chimeras containing the C-terminal domain of GIRK1 from amino acid 194 to 501 were intracellularly localized, whereas chimeras containing the C terminus of GIRK4 localized to the cell surface. Deletion analysis of the GIRK4 C terminus identified a 25-amino acid region required for cell surface targeting of GIRK1/GIRK4 heterotetramers and a 25-amino acid region required for cell surface localization of GIRK4 homotetramers. GIRK1 appeared intracellular in atrial myocytes isolated from GIRK4 knockout mice and was not maturely glycosylated, supporting an essential role for GIRK4 in the processing and cell surface localization of IKACh in vivo.

Acute suppression of inwardly rectifying Kir2.1 channels by direct tyrosine kinase phosphorylation

Signaling via cytosolic and receptor tyrosine kinases is associated with cell growth and differentiation but also targets onto transmitter receptors and ion channels. Here, regulation by tyrosine kinase (TK) activity was investigated for inwardly rectifying K+ (Kir2.1) channels that control membrane excitability in many central neurons. In mammalian tsA-201 cells, the membrane-permeable protein tyrosine phosphatase inhibitor, perorthovanadate (100 microM), suppressed currents through recombinant Kir2.1 channels by 60 +/- 20%. Coapplication of the TK inhibitor genistein (100 microM) completely abolished this effect. Native Kir2.1 channels in rat basophilic leukocytes were affected by manipulation of the TK and protein tyrosine phosphatase activity in a qualitatively similar manner. Site mutation of a tyrosine consensus residue for TK phosphorylation in the C-terminal domain of Kir2.1 generated channel properties indistinguishable from wild-type Kir2.1 channels. However, Kir2.1Y242F channels were no longer suppressed following exposure to perorthovanadate, indicating that the channel is a direct substrate for TKs. After coexpression of nerve growth factor receptor with Kir2.1 channels in tsA-201 cells and Xenopus oocytes, the activity of Kir2.1 was rapidly suppressed by applied nerve growth factor (0.5 microgram/ml) by 31 +/- 10 and 21 +/- 15%, respectively. Acute inhibition was also evoked by epidermal growth factor and insulin via endogenous insulin receptors, indicating that Kir2.1 channels may serve as a general target for neurotrophic growth factors in the brain.

Regulation of membrane excitability in the central nervous system by serotonin receptor subtypes

Serotonin exerts multiple electrophysiological effects on neurons of the central nervous system. It is now known that this diversity reflects at least in part the existence of multiple serotonin receptor subtypes. An example of this occurs in the CA1 region of the hippocampus where as many as ten different serotonin receptor subtypes appear to be expressed. Recent electrophysiological studies have been able to assign specific functional roles to at least 5 of these receptors. These receptors are differentially expressed in the two different cell types present in this region, pyramidal cells and GABAergic interneurons, and mediate different effects on membrane excitability. This distribution is consistent with the different functional roles played by these cells in hippocampus. Thus the differential expression of serotonin receptor subtypes in the CA1 region allows serotonin to modify the function of hippocampal neuronal networks in a manner that is both selective and precise.

Heterologous expression and coupling of G protein-gated inwardly rectifying K+ channels in adult rat sympathetic neurons

1. G protein-gated inwardly rectifying K+ (GIRK) channels were heterologously expressed in rat superior cervical ganglion (SCG) neurons by intranuclear microinjection. The properties of GIRK channels and their coupling to native receptors were characterized using the whole-cell patch-clamp technique. 2. Following coinjection of either GIRK1-2 or GIRK1-4 cDNA, application of noradrenaline (NA) produced large inwardly rectifying K+ currents. Injection of cDNA encoding individual GIRK subunits produced only small and inconsistent NA-activated inward currents. Current arising from the native expression of GIRK channels in SCG neurons was not observed. 3. NA-mediated activation of GIRK channels was abolished by pertussis toxin (PTX) pretreatment, indicating coupling via G proteins of the Gi/Go subfamily. Conversely, vasoactive intestinal peptide (VIP) activated GIRK channel currents via a cholera toxin-sensitive pathway suggesting coupling through Galphas. Pretreatment of neurons with PTX caused a significant increase in amplitude of the VIP-mediated GIRK channel currents when compared with untreated cells. 4. Application of adenosine, prostaglandin E2 and somatostatin resulted in activation of GIRK channel currents. Activation of m1 muscarinic acetylcholine receptors (i.e. application of oxotremorine M to PTX-treated neurons) failed to elicit overt GIRK channel currents. 5. GIRK channel overexpression decreased basal Ca2+ channel facilitation significantly when compared with uninjected neurons. Furthermore, the NA-mediated inhibition of Ca2+ channels was significantly attenuated. 6. In summary, the ability to heterologously express GIRK channels in adult sympathetic neurons allows the experimental alteration of receptor-G protein-effector stoichiometry. Such studies may increase our understanding of the mechanisms underlying ion channel modulation by G proteins in a neuronal environment.

The epithelial inward rectifier channel Kir7.1 displays unusual K+ permeation properties

Rat and human cDNAs were isolated that both encoded a 360 amino acid polypeptide with a tertiary structure typical of inwardly rectifying K+ channel (Kir) subunits. The new proteins, termed Kir7.1, were <37% identical to other Kir subunits and showed various unique residues at conserved sites, particularly near the pore region. High levels of Kir7.1 transcripts were detected in rat brain, lung, kidney, and testis. In situ hybridization of rat brain sections demonstrated that Kir7.1 mRNA was absent from neurons and glia but strongly expressed in the secretory epithelial cells of the choroid plexus (as confirmed by in situ patch-clamp measurements). In cRNA-injected Xenopus oocytes Kir7.1 generated macroscopic Kir currents that showed a very shallow dependence on external K+ ([K+]e), which is in marked contrast to all other Kir channels. At a holding potential of -100 mV, the inward current through Kir7.1 averaged -3.8 +/- 1.04 microA with 2 mM [K+]e and -4.82 +/- 1.87 microA with 96 mM [K+]e. Kir7.1 has a methionine at position 125 in the pore region where other Kir channels have an arginine. When this residue was replaced by the conserved arginine in mutant Kir7.1 channels, the pronounced dependence of K+ permeability on [K+]e, characteristic for other Kir channels, was restored and the Ba2+ sensitivity was increased by a factor of approximately 25 (Ki = 27 microM). These findings support the important role of this site in the regulation of K+ permeability in Kir channels by extracellular cations.

Identification of native atrial G-protein-regulated inwardly rectifying K+ (GIRK4) channel homomultimers

G-protein-regulated inwardly rectifying K+ (GIRK) channels play critical inhibitory roles throughout the nervous system, heart, and pancreas. They are believed to be heterotetramers consisting of GIRK1 (Kir3.1) and either GIRK2 (Kir3.2), GIRK3 (Kir3.3), or GIRK4 (Kir3.4) subunits. The GIRK1 subunit is hypothesized to be critical to form GIRK channels with normal channel kinetics based on heterologous expression studies. However, GIRK2 and GIRK3 proteins are present in areas of the brain where no GIRK1 has been detected. Here we demonstrate that GIRK tetramers lacking GIRK1 can be purified from bovine heart atria. We have found that only half of GIRK4 is purified as the GIRK1-GIRK4 heterotetramer, whereas the remaining GIRK4 forms a high molecular weight, SDS-resistant complex that does not contain GIRK1. These GIRK4 complexes, most likely GIRK4 homotetramers, were previously not seen because of their aberrant migration on SDS-polyacrylamide gels. We propose that all of GIRK1 and half of GIRK4 proteins in atria combine to form the heterotetramer IKACh, whereas the remaining GIRK4 forms a novel tetrameric complex. GIRK4 homotetramers form channels with unusual single channel behavior, and their contribution to native currents requires further investigation.

Postsynaptic mechanisms underlying responsiveness of amygdaloid neurons to nociceptin/orphanin FQ

Effects of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor (ORL), were investigated in the rat lateral (AL) and central (ACe) amygdala in vitro. Approximately 98% of presumed projection neurons in the AL responded to N/OFQ with an increase in inwardly rectifying potassium conductance, resulting in an impairment in cell excitability. Half-maximal effects were obtained at 30.6 nM; the Hill coefficient was 0.63. In the ACe, 31% of the cells displayed responses similar to that in the AL, 44% were nonresponsive, and 25% responded with a small potassium current with a linear current-voltage relationship. Responses to N/OFQ were reduced by 100 microM Ba2+, were insensitive to 10 microM naloxone, and were blocked by a selective ORL antagonist, [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 (IC50 = 760 nM). Involvement of G-proteins was indicated by irreversible effects and blockade of action of N/OFQ during intracellular presence of GTP-gamma-S (100 microM) and GDP-beta-S (2 mM), respectively, and prevention of responses after incubation in pertussis toxin (500 ng/ml). These mechanisms may contribute to the role of N/OFQ in the reduction of fear responsiveness and stress that have recently been suggested on the basis of histochemical and behavioral studies.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Norepinephrine inhibits neurons of the intermediate subnucleus of the lateral septum via alpha2-adrenoceptors

The physiological and pharmacological actions of norepinephrine (NE) on neurons of the intermediate subnucleus of the lateral septum (LSI) were examined using intracellular recordings in rat brain-slices. Bath-applied NE inhibited 72.5%, excited 5.5% and had no effect on 22% of LSI neurons tested; this study focused on the inhibitory effects of NE. In current clamp recordings, 100 microM NE produced a hyperpolarization of 10.82+/-0.72 mV (n=84) with a decrease in input resistance. In voltage-clamp, NE produced a direct, post-synaptic outward current of 206.8+/-22 pA (n=37) with a 64. 3+/-4.9% increase in input conductance (IC50-17.7+/-4 microM). The NE-induced inhibition was mimicked by the alpha2-agonist, UK14,304, but not by the alpha1- or beta-adrenoceptor agonists. The alpha2-agonist, clonidine, had a weak effect in LSI neurons. Interestingly, the magnitude of the UK14,304-induced response varied between cells (ranging from 29.5 to 320% of the maximal NE inhibition), possibly suggesting the involvement of alpha2A-(high affinity for UK14,304) and non-alpha2A (low affinity for UK14,304) adrenoceptor subtypes. While the alpha2-antagonists, yohimbine, rauwolscine and idazoxan blocked NE-induced inhibition in all neurons tested, the prototypical alpha1-antagonist, prazosin produced a variable degree of block (9-58%), further indicating the possible involvement of alpha2A (prazosin-insensitive) and non-alpha2A (prazosin-sensitive) receptors. However a lack of more selective pharmacological tools precludes definitive classification of the alpha2-receptor-mediated responses into different subtypes. The alpha2-receptor-mediated current in LSI neurons displayed Ba2+-sensitive inward rectification, reversed polarity near EK and was sensitive to external K+. In conclusion, NE inhibits LSI neurons via alpha2-adrenoceptor subtypes.

Inwardly rectifying potassium (IRK) currents are correlated with IRK subunit expression in rat nucleus accumbens medium spiny neurons

Inwardly rectifying K+ (IRK) channels are critical for shaping cell excitability. Whole-cell patch-clamp and single-cell RT-PCR techniques were used to characterize the inwardly rectifying K+ currents found in projection neurons of the rat nucleus accumbens. Inwardly rectifying currents were highly selective for K+ and blocked by low millimolar concentrations of Cs+ or Ba2+. In a subset of neurons, the inwardly rectifying current appeared to inactivate at hyperpolarized membrane potentials. In an attempt to identify this subset, neurons were profiled using single-cell RT-PCR. Neurons expressing substance P mRNA exhibited noninactivating inward rectifier currents, whereas neurons expressing enkephalin mRNA exhibited inactivating inward rectifier currents. The inactivation of the inward rectifier was correlated with the expression of IRK1 mRNA. These results demonstrate a clear physiological difference in the properties of medium spiny neurons and suggest that this difference could influence active state transitions driven by cortical and hippocampal excitatory input.

The dependence of Ag+ block of a potassium channel, murine kir2.1, on a cysteine residue in the selectivity filter

Externally applied Ag+ (100-200 nM) irreversibly blocked the strong inwardly rectifying K+ channel, Kir2.1. Mutation to serine of a cysteine residue at position 149 in the pore-forming H5 region of Kir2.1 abolished Ag+ blockage. To determine how many of the binding sites must be occupied by Ag+ before the channel is blocked, we measured the rate of channel block and found that our results were best fitted assuming that only one Ag+ ion need bind to eliminate channel current. We tested our hypothesis further by constructing covalently linked dimers and tetramers of Kir2.1 in which cysteine had been replaced by serine in one (dimer) or three (tetramer) of the linked subunits. When expressed, these constructs yielded functional channels with either two (dimer) or one (tetramer) cysteines per channel at position 149. Blockage in the tetramer was complete after sufficient exposure to 200 nM Ag+, a result that is also consistent with only one Ag+ being required to bind to Cys149 to block fully. The rate of development of blockage was 16 times slower than in wild-type channels; the rate was 4 times slower in channels formed from dimers.

Characterization of murine Girk2 transcript isoforms: structure and differential expression

A mutation in the G-protein-linked inwardly rectifying K+ channel 2 gene (Girk2) is the cause of the weaver mouse phenotype. We determined that the originally published Girk2 transcript is composed of five exons. The primary coding exon (designated exon 4a in our system) encodes over two-thirds of the protein. Five different full-length Girk2 transcript isoforms (designated Girk2-1, Girk2A-1, Girk2A-2, Girk2B, and Girk2C) originating from different transcriptional start sites and/or alternative splicing were isolated by cDNA RACE. Several of the transcripts were predicted to encode truncated proteins that may lack some of the G-proteincoupling sequence. Northern blotting and in situ hybridization studies with transcript-specific probes indicated that the transcripts were differentially expressed in both normal and weaver mice. All transcripts tested were expressed in the three major targets of action of the weaver mutation: cerebellum, substantia nigra, and testis. Two of the transcripts, Girk2A-1 and Girk2A-2, encode identical proteins and have a distinct pattern of expression in testis, which suggests that they are associated with specific stages of spermatogenesis. An additional transcript, Girk2D, appears to be brain-specific, not polyadenylated, and highly expressed in cerebellar granule cells.

Metabotropic glutamate group II receptors activate a G protein-coupled inwardly rectifying K+ current in neurones of the rat cerebellum

1. The effects of the group II metabotropic glutamate receptor (mGluR) agonists DCG-IV and LY354740 were examined in neurones freshly dissociated from the rat cerebellum and olfactory bulb, using the whole-cell configuration of the patch-clamp technique. 2. Under experimental conditions in which K+ currents would be inward, rapid application of DCG-IV and LY354740 to interneurones expressing the group II mGluRs induced an inward current in a subpopulation of interneurones of the cerebellum, the unipolar brush cells. 3. The currents induced by DCG-IV and LY354740 had the major characteristics of a G protein-coupled inwardly rectifying K+ channel (GIRK) current; namely, rapid activation and deactivation upon agonist application and removal, G protein dependence, strong inward rectification, Cs+ and Ba2+ sensitivity, and K+ selectivity. 4. In Golgi cells of the cerebellum and interneurones of the accessory olfactory bulb, which also express group II mGluRs, LY354740 did not induce GIRK activation but inhibited voltage-gated Ca2+ channel currents. 5. These results demonstrate that, in unipolar brush cells, native group II mGluRs can functionally couple to activation of GIRKs. Thus, the absence of coupling in the majority of CNS neurones examined to date may be due to restricted cellular co-localization or co-expression of the appropriate proteins.

Kir2.4: a novel K+ inward rectifier channel associated with motoneurons of cranial nerve nuclei

Members of the Kir2 subfamily of inwardly rectifying K+ channels characterized by their strong current rectification are widely expressed both in the periphery and in the CNS in mammals. We have cloned from rat brain a fourth subfamily member, designated Kir2.4 (IRK4), which shares 53-63% similarity to Kir2.1, Kir2.2, or Kir2.3 on the amino acid level. In situ hybridization analysis identifies Kir2.4 as the most restricted of all Kir subunits in the brain. Kir2. 4 transcripts are expressed predominantly in motoneurons of cranial nerve motor nuclei within the general somatic and special visceral motor cell column and thus are uniquely related to a functional system. Heterologous expression of Kir2.4 in Xenopus oocytes and mammalian cells gives rise to low-conductance channels (15 pS), with an affinity to the channel blockers Ba2+ (Ki = 390 microM) and Cs+ (Ki = 8.06 mM) 30-50-fold lower than in other Kir channels. Low Ba2+ sensitivity allows dissection of Kir2.4 currents from other Kir conductances in hypoglossal motoneurons (HMs) in rat brainstem slices. The finding that Ba2+-mediated block of Kir2.4 in HMs evokes tonic activity and increases the frequency of induced spike discharge indicates that Kir2.4 channels are of major importance in controlling excitability of motoneurons in situ.

Ontogeny of gene expression of Kir channel subunits in the rat

We report the detailed gene expression of all subunits within the Kir2 and Kir3 inwardly rectifying K+ channel subfamilies in the developing rat. Using in situ hybridization, onset of expression and cellular distribution of transcripts in embryonic and postnatal rat brains as well as in peripheral tissues is evaluated. Beginning at embryonic day 13 (E13), except "forebrain" Kir2.3 subunits which are absent from the body and brain until E21, all subunits appear with distinct and mainly nonoverlapping expression patterns. During ontogenic development, expression in the CNS becomes more widespread, leading to widely overlapping mRNA patterns as observed in the adult rat. Subunits are mainly found in regions of the developing brain that are also positive in the adult. Most subunits, in particular Kir3.2 and Kir3.4, are expressed transiently in distinct brain nuclei during ontogeny. Appearance of Kir transcripts is not generally related to the progressive and recessive phases during neurogenesis, but rather regulated differentially for each subunit and any specific group of neurons. It is demonstrated for the first time that several subunits, and most abundantly Kir2.2, are present early in the peripheral nervous system, i.e., in dorsal root-, sensory cranial-, and sympathetic ganglia. Also, of all subunits Kir3.3 is ubiquitously expressed in the entire embryonic nervous system and throughout the body. In summary, analysis of ontogenic Kir channel expression helps deciphering the importance of Kir channels (as exemplified for the defective weaver Kir3.2 gene) during proliferation, differentiation, and synaptogenesis in the CNS.

Effects of clozapine on the delta- and kappa-opioid receptors and the G-protein-activated K+ (GIRK) channel expressed in Xenopus oocytes

1. To investigate the effects of clozapine, an atypical antipsychotic, on the cloned mu-, delta- and kappa-opioid receptors and G-protein-activated inwardly rectifying K+ (GIRK) channel, we performed the Xenopus oocyte functional assay with each of the three opioid receptor mRNAs and/or the GIRK1 mRNA. 2. In the oocytes co-injected with either the delta- or kappa-opioid receptor mRNA and the GIRK1 mRNA, application of clozapine induced inward currents which were attenuated by naloxone, an opioid-receptor antagonist, and blocked by Ba2+, which blocks the GIRK channel. Since the opioid receptors functionally couple to the GIRK channel, these results indicate that clozapine activates the delta- and kappa-opioid receptors and that the inward-current responses are mediated by the GIRK channel. The action of clozapine at the delta-opioid receptor was more potent and efficacious than that at the kappa-opioid receptor. In the oocytes co-injected with the mu-opioid receptor and GIRK1 mRNAs, application of clozapine (100 microM) did not induce an inward current, suggesting that clozapine could not activate the mu-opioid receptor. 3. Application of clozapine caused a reduction of the basal inward current in the oocytes injected with the GIRK1 mRNA alone, but caused no current response in the uninjected oocytes. These results indicate that clozapine blocks the GIRK channel. 4. To test the antagonism of clozapine for the mu- and kappa-opioid receptors, we applied clozapine together with each selective opioid agonist to the oocytes co-injected with either the mu- or kappa-opioid receptor mRNA and the GIRK1 mRNA. Each of the peak currents induced by each selective opioid agonist together with clozapine was almost equal to the responses to a selective opioid agonist alone. These results indicate that clozapine has no significant antagonist effect on the mu- and kappa-opioid receptors. 5. We conclude that clozapine acts as a delta- and kappa-agonist and as a GIRK channel blocker. Our results suggest that the efficacy and side effects of clozapine under clinical conditions may be partly due to activation of the delta-opioid receptor and blockade of the GIRK channel.

Abnormal heart rate regulation in GIRK4 knockout mice

Acetylcholine (ACh) released from the stimulated vagus nerve decreases heart rate via modulation of several types of ion channels expressed in cardiac pacemaker cells. Although the muscarinic-gated potassium channel I(KACh) has been implicated in vagally mediated heart rate regulation, questions concerning the extent of its contribution have remained unanswered. To assess the role of I(KACh) in heart rate regulation in vivo, we generated a mouse line deficient in I(KACh) by targeted disruption of the gene coding for GIRK4, one of the channel subunits. We analyzed heart rate and heart rate variability at rest and after pharmacological manipulation in unrestrained conscious mice using electrocardiogram (ECG) telemetry. We found that I(KACh) mediated approximately half of the negative chronotropic effects of vagal stimulation and adenosine on heart rate. In addition, this study indicates that I(KACh) is necessary for the fast fluctuations in heart rate responsible for beat-to-beat control of heart activity, both at rest and after vagal stimulation. Interestingly, noncholinergic systems also appear to modulate heart activity through I(KACh). Thus, I(KACh) is critical for effective heart rate regulation in mice.

Developmental expression of the GIRK family of inward rectifying potassium channels: implications for abnormalities in the weaver mutant mouse

G-protein-gated inward rectifying potassium channels (GIRKs) are a newly identified gene family. These gene products are thought to form functional channels through the assembly of heteromeric subunits. Recently, it has been demonstrated that a point mutation in the GIRK2 gene, one of the GIRK family members, is the cause of the neurological and reproductive defects observed in the weaver (wv) mutant mouse. The mechanism(s) by which a single amino acid substitution in GIRK2 protein leads to the severe phenotypes in the wv / wv mouse is not fully understood. However, it implicates the importance of GIRK channels in neuronal development. To characterize the mRNA expression patterns of GIRK1-3 during mouse brain development we have used in situ hybridization analyses. We found that the expression of all three genes showed developmental regulation. In most areas that showed expression, the levels of GIRK1-3 transcripts reached their peak at around postnatal day 10 (P10). In general, GIRK1 showed the least fluctuation in its levels of expression during development, while dynamic changes were found with the levels of GIRK2 and GIRK3 transcripts. GIRK3 becomes the predominant inward rectifying K+-channel in the brain at later postnatal ages. In the CNS regions affected in the wv / wv mouse, GIRK2 is the predominant inward rectifying channel that is expressed. This suggests that the presence of the other subtypes are able to compensate for the mutated GIRK2 channel in weaver neurons that survive.

On the nature of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus in vitro

1. Intracellular sharp electrode current clamp and discontinuous single electrode voltage clamp recordings were made from thalamocortical neurones (n = 57) of the cat ventrobasal thalamus in order to investigate the mechanism underlying anomalous rectification. 2. Under current clamp conditions, voltage-current (V-I) relationships in a potential range of -55 to -110 mV demonstrated anomalous rectification with two components: fast rectification, which controlled the peak of negative voltage deviations, and time-dependent rectification. Time-dependent rectification was apparent as a depolarizing sag generated during the course of negative voltage deviations, was first formed at potentials in the range -60 to -70 mV, and was sensitive to 3 mM Cs+ (n = 6). Similarly, under voltage clamp conditions, instantaneous and steady-state I-V relationships demonstrated anomalous rectification. A slowly activating inward current with an activation threshold in the range of -65 to -70 mV formed time-dependent rectification. This current was sensitive to Cs+ (3 mM) (n = 3) and had properties similar to the slow inward mixed cationic current (Ih). 3. 4-(N-Ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288) (100-300 microM) irreversibly blocked time-dependent rectification mediated by Ih (n = 23 of 25 neurones), and led to a hyperpolarization of the resting membrane potential (6.8 +/- 0.5 mV). In the presence of ZD 7288, V-I and I-V relationships, exhibited fast anomalous rectification, first activated from potential more negative than -80 mV. 4. Ba2+ (100 microM) (n = 8), in the continuous presence of ZD 7288, reversibly linearized peak V-I and instantaneous I-V relationships over a potential range of -70 to -120 mV, and led to a membrane depolarization (13.3 +/- 4.2 mV) or tonic inward current (192 +/- 36 pA). 5. The co-application of ZD 7288 and Ba2+ revealed a depolarizing sag in negative voltage deviations under current clamp conditions, or a large inward current with kinetics two to three times slower than those of Ih under voltage clamp conditions. This novel form of time-dependent rectification was first apparent at potentials more negative than about -85 mV, was sensitive to 5 mM Cs+ (n = 4), and is termed Ih,slow. Ih,slow tail currents reversed between -65.3 and -56.6 mV (with potassium acetate electrodes, n = 3) or -57.6 and -50.3 mV (with KCl electrodes, n = 3). 6. Computer simulations confirmed that the pattern of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus is mediated by the concerted action of Ih and a Ba(2+)-sensitive current with properties similar to an inwardly rectifying K+ current (IKIR).

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.

Diverse cell death pathways result from a single missense mutation in weaver mouse

Neuronal death affects selectively granule cell precursors of the cerebellum and the dopaminergic neurons of midbrain in the weaver mutant mouse. The weaver phenotype is associated with a missense mutation in the gene coding for the GIRK2 potassium channel, which results in chronic depolarization. Using DNA gel electrophoresis, electron microscopy (EM), the in situ end-labeling (ISEL) technique at the light and EM level, and immunohistochemistry for apoptosis-related proteins c-Jun and proliferating cell nuclear antigen (PCNA), we have investigated the mechanisms of cell death in cerebellum and substantia nigra. Between postnatal day P1 and P21, in the external germinal layer of the cerebellum, most degenerating granule cell precursors were found to aggregate to form clusters. Degenerating cells exhibited strong nuclear staining for ISEL, c-Jun, and PCNA and had a typical apoptotic morphology by EM. Increased c-Jun and ISEL staining were also occasionally seen in Purkinje cells. Between P14 and P21, when dopaminergic neurons start to degenerate, staining for ISEL, c-Jun, and PCNA in weaver substantia nigra was the same as in controls. By EM, however, we found only in weaver mice numerous dopaminergic cells that showed extensive vacuolar and autophagic changes of cytoplasm, preservation of membrane and organelle integrity, and absence of chromatin condensation or DNA fragmentation by EM-ISEL. The combination of vacuolar and autophagic changes identifies a novel type of non-necrotic, nonapoptotic cell death. After biochemical analysis of DNA, a clear-cut laddering, suggestive of oligonucleosomal fragmentation, was present in samples from weaver cerebellum. Cell death diversity appears to be influenced by specific features of target cells. These findings may be relevant for understanding the mechanisms of cell death in neurodegenerative diseases.

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.

Defective gamma-aminobutyric acid type B receptor-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from weaver and Girk2 null mutant mice

Stimulation of inhibitory neurotransmitter receptors, such as gamma-aminobutyric acid type B (GABAB) receptors, activates G protein-gated inwardly rectifying K+ channels (GIRK) which, in turn, influence membrane excitability. Seizure activity has been reported in a Girk2 null mutant mouse lacking GIRK2 channels but showing normal cerebellar development as well as in the weaver mouse, which has mutated GIRK2 channels and shows abnormal development. To understand how the function of GIRK2 channels differs in these two mutant mice, we compared the G protein-activated inwardly rectifying K+ currents in cerebellar granule cells isolated from Girk2 null mutant and weaver mutant mice with those from wild-type mice. Activation of GABAB receptors in wild-type granule cells induced an inwardly rectifying K+ current, which was sensitive to pertussis toxin and inhibited by external Ba2+ ions. The amplitude of the GABAB receptor-activated current was severely attenuated in granule cells isolated from both weaver and Girk2 null mutant mice. By contrast, the G protein-gated inwardly rectifying current and possibly the agonist-independent basal current appeared to be less selective for K+ ions in weaver but not Girk2 null mutant granule cells. Our results support the hypothesis that a nonselective current leads to the weaver phenotype. The loss of GABAB receptor-activated GIRK current appears coincident with the absence of GIRK2 channel protein and the reduction of GIRK1 channel protein in the Girk2 null mutant mouse, suggesting that GABAB receptors couple to heteromultimers composed of GIRK1 and GIRK2 channel subunits.

Expression and clustered distribution of an inwardly rectifying potassium channel, KAB-2/Kir4.1, on mammalian retinal Müller cell membrane: their regulation by insulin and laminin signals

Inwardly rectifying potassium (K+) channels (Kir) in Müller cells, the dominant glial cells in the retina, are supposed to be responsible for the spatial buffering action of K+ ions. The molecular properties and subcellular localization of Müller cell Kir channels in rat and rabbit retinas were examined by using electrophysiological, molecular biological, and immunostaining techniques. Only a single population of Kir channel activity, the properties of which were identical to those of KAB-2/Kir4.1 expressed in HEK293T cells, could be recorded from endfoot to the distal portion of Müller cells. Consistently, Northern blot, in situ hybridization, and RT-PCR analyses indicated expression of Kir4. 1 in Müller cells per se. The Kir4.1 immunoreactivity was distributed in clusters throughout Müller cell membrane. The Kir4.1 expression in Müller cells disappeared promptly after culturing. When the dissociated Müller cells were cultured on laminin-coated dishes in the presence of insulin, Kir4.1 immunoreactivity was detected in a clustered manner on the cell membrane. Because insulin and laminin exist in the surrounding of Müller cells in the retina, these substances possibly may be physiological regulators of expression and distribution of Kir4.1 in Müller cells in vivo.

An immunocytochemical study on the distribution of two G-protein-gated inward rectifier potassium channels (GIRK2 and GIRK4) in the adult rat brain

G-protein-gated inward rectifier potassium channels mediate the synaptic actions of numerous neurotransmitters in the mammalian brain, and were recently shown to be candidates for genetic mutations leading to neuronal cell death. This report describes the localization of G-protein-gated inward rectifier potassium channel-2 and G-protein-gated inward rectifier potassium channel-4 proteins in the rat brain, as assessed by immunocytochemistry. G-protein-gated inward rectifier potassium channel-2 immunoreactivity was widely distributed throughout the brain, with the strongest staining seen in the hippocampus, septum, granule cell layer of the cerebellum, amygdala and substantia nigra pars compacta. In contrast, G-protein-gated inward rectifier potassium channel-4 immunoreactivity was restricted to some neuronal populations, such as Purkinje cells and neurons of the globus pallidus and the ventral pallidum. The presence of G-protein-gated inward rectifier potassium channel-2 immunoreactivity in substantia nigra pars compacta dopaminergic neurons was confirmed by showing its co-localization with tyrosine hydroxylase by double immunocytochemistry, and also by selectively lesioning dopaminergic neurons with the neurotoxin 6-hydroxydopamine. At the cellular level both proteins were localized in neuronal cell bodies and dendrites, but clear differences were seen in the degree of dendritic staining among neuronal groups. For some neuronal groups the staining of distal dendrites (notably dendritic spines) was strong, while for others the cell body and proximal dendrites were preferentially labelled. In addition, some of the results suggest that G-protein-gated inward rectifier potassium channel-2 protein could be localized in distal axonal terminal fields. A knowledge of the distribution of G-protein-gated inward rectifier potassium channel proteins in the brain could help to elucidate their physiological roles and to evaluate their potential involvement in neurodegenerative processes in animal models and human diseases.

G protein-coupled inwardly rectifying K+ channels (GIRKs) mediate postsynaptic but not presynaptic transmitter actions in hippocampal neurons

To study the role of G protein-coupled, inwardly rectifying K+ (GIRK) channels in mediating neurotransmitter actions in hippocampal neurons, we have examined slices from transgenic mice lacking the GIRK2 gene. The outward currents evoked by agonists for GABA(B) receptors, 5HT1A receptors, and adenosine A1 receptors were essentially absent in mutant mice, while the inward current evoked by muscarinic receptor activation was unaltered. In contrast, the presynaptic inhibitory action of a number of presynaptic receptors on excitatory and inhibitory terminals was unaltered in mutant mice. These included GABA(B), adenosine, muscarinic, metabotropic glutamate, and NPY receptors on excitatory synapses and GABA(B) and opioid receptors on inhibitory synapses. These findings suggest that a number of G protein-coupled receptors activate the same class of postsynaptic K+ channel, which contains GIRK2. In addition, the GIRK2 channels play no role in the inhibition mediated by presynaptic G protein-coupled receptors, suggesting that the same receptor can couple to different effector systems according to its subcellular location in the neuron.

Activation of heteromeric G protein-gated inward rectifier K+ channels overexpressed by adenovirus gene transfer inhibits the excitability of hippocampal neurons

G protein-gated inward rectifier K+ channel subunits 1-4 (GIRK1-4) have been cloned from neuronal and atrial tissue and function as heterotetramers. To examine the inhibition of neuronal excitation by GIRKs, we overexpressed GIRKs in cultured hippocampal neurons from 18 day rat embryos, which normally lack or show low amounts of GIRK protein and currents. Adenoviral recombinants containing the cDNAs for GIRK1, GIRK2, GIRK4, and the serotonin 1A receptor were constructed. Typical GIRK currents could be activated by endogenous GABAB, serotonin 5-HT1A, and adenosine A1 receptors in neurons coinfected with GIRK1+2 or GIRK1+4. Under current clamp, GIRK activation increased the cell membrane conductance by 1- to 2-fold, hyperpolarized the cell by 11-14 mV, and inhibited action potential firing by increasing the threshold current for firing by 2- to 3-fold. These effects were not found in non- and mock-infected neurons, and were similar to the effects of muscarinic stimulation of native GIRK currents in atrial myocytes. Two inhibitory effects of GIRK activation, hyperpolarization and diminution of depolarizing pulses, were simulated from the experimental data. These inhibitory effects are physiologically important in the voltage range between the resting membrane potential and the potential where voltage-gated Na+ and K+ currents are activated; that is where GIRK currents are outward.

Positive and negative coupling of the metabotropic glutamate receptors to a G protein-activated K+ channel, GIRK, in Xenopus oocytes

Metabotropic glutamate receptors (mGluRs) control intracellular signaling cascades through activation of G proteins. The inwardly rectifying K+ channel, GIRK, is activated by the beta gamma subunits of G proteins and is widely expressed in the brain. We investigated whether an interaction between mGluRs and GIRK is possible, using Xenopus oocytes expressing mGluRs and a cardiac/brain subunit of GIRK, GIRK1, with or without another brain subunit, GIRK2. mGluRs known to inhibit adenylyl cyclase (types 2, 3, 4, 6, and 7) activated the GIRK channel. The strongest response was observed with mGluR2; it was inhibited by pertussis toxin (PTX). This is consistent with the activation of GIRK by Gi/Go-coupled receptors. In contrast, mGluR1a and mGluR5 receptors known to activate phospholipase C, presumably via G proteins of the Gq class, inhibited the channel's activity. The inhibition was preceded by an initial weak activation, which was more prominent at higher levels of mGluR1a expression. The inhibition of GIRK activity by mGluR1a was suppressed by a broad-specificity protein kinase inhibitor, staurosporine, and by a specific protein kinase C (PKC) inhibitor, bis-indolylmaleimide, but not by PTX, Ca(2-)chelation, or calphostin C. Thus, mGluR1a inhibits the GIRK channel primarily via a pathway involving activation of a PTX-insensitive G protein and, eventually, of a subtype of PKC, possibly PKC-mu. In contrast, the initial activation of GIRK1 caused by mGluR1a was suppressed by PTX but not by the protein kinase inhibitors. Thus, this activation probably results from a promiscuous coupling of mGluR1a to a Gi/Go protein. The observed modulations may be involved in the mGluRs effects on neuronal excitability in the brain. Inhibition of GIRK by phospholipase C-activating mGluRs bears upon the problem of specificity of G protein (GIRK interaction) helping to explain why receptors coupled to Gq are inefficient in activating GIRK.

Receptor-regulated ion channels

Recent advances in the study of receptor-regulated ion channels include the cloning of the genes encoding three types of potassium channel that are favorite targets of receptors for transmitters and hormones. Studies of these channels have also provided a strong indication that G-protein betagamma subunits may gate ion channels via direct protein-protein interactions. Similarities between channel regulation by natriuretic peptides and channel regulation by secreted peptide products of the Alzheimer's beta-amyloid precursor protein offer hints for the existence of a receptor for the latter. There are also other novel examples of channel regulation in excitable and nonexcitable cells, including liver cells and blood cells.

Partial structure, chromosome localization, and expression of the mouse Girk4 gene

The G protein-gated potassium channel IKACh constitutes part of a signaling pathway that mediates the negative chronotropic and inotropic effects of acetylcholine on cardiac physiology. Similar or identical ion channels regulate the excitability of many neurons in response to neurotransmitters. IKACh is composed of two homologous subunits, GIRK1 and GIRK4. Here we describe a partial genomic structure of the mouse Girk4 gene. Two exons containing the complete protein-coding sequence were identified. Girk4 was mapped to mouse chromosome 9 (13 cM), consistent with the mapping of human GIRK4 to chromosome 11q23-ter. GIRK4 mRNA was found mainly in mouse heart, with trace levels detected in brain, kidney, lung, and spleen. No detectable levels were observed in skeletal muscle, liver, and testis. The onset of GIRK4 mRNA expression in the developing mouse occurs between Embryonic Days 7 and 11, consistent with the appearance and function of the mouse heart.

Localization and developmental changes of the expression of two inward rectifying K(+)-channel proteins in the rat brain

We have raised affinity-purified polyclonal antibodies specific for the inward rectifying K+ channel (IRK1/Kir2.1) and the G protein-activated inward rectifying K+ channel (GIRK1/Kir3.1) examined their distributions in the rat brain immunohistochemically. The regional expression pattern of the IRK1 and GIRK1 proteins were similar to those of mRNA of the previous in situ hybridization study. The subcellular distribution was studied in the cerebellum; cerebral cortex and hippocampus. In the cerebellum, the IRK1 protein was clearly detected in the somata and proximal dendrites of Purkinje cells, while the GIRK1 protein was present in the somata and clustered dendrites of granule cells. In the cerebral cortex and hippocampus, both IRK1- and GIRK1-immunoreactivities were detected in the somata and apical dendrites of the pyramidal cells. The presence of IRK1 or GIRK1 proteins in the axons could not proved by the present study. The developmental changes of the expression pattern of the GIRK1 protein were also investigated in the hippocampus and in the cerebellum of postnatal day (P) 7 to P17 rats. The GIRK1 protein was detected neither in the subgranular zone of the dentate gyrus nor in the proliferative zone of the external granule cell layer of the cerebellum, in which granule cell precursors are reported to proliferate, while it was clearly detected in the adjacent layer in which postmitotic but immature cells exist. These results imply that the expression of the GIRK1 protein starts just after the neuronal precursors finished the last mitotic cell division.

Effects of serotonin on caudal raphe neurons: activation of an inwardly rectifying potassium conductance

We used whole cell current- and voltage-clamp recording in neonatal rat brain stem slices to characterize firing properties and effects of serotonin (5-HT) on neurons (n = 225) in raphe pallidus (RPa) and raphe obscurus (ROb). Of a sample of 51 Lucifer yellow-filled neurons recovered after immunohistochemical processing to detect tryptophan hydroxylase (TPH), 34 were found to be TPH immunoreactive (i.e., serotonergic). Serotonergic neurons had long-duration action potentials and fired spontaneously at low frequency (approximately 1 Hz) in a pattern that was often irregular; at higher firing frequencies the discharge became more regular. These neurons displayed spike frequency adaptation, with maximal steady-state firing rates of < 4 Hz. The overwhelming majority of identified serotonergic neurons was hyperpolarized by bath-applied 5-HT (94%; n = 32 of 34); conversely, most cells in this sample that were hyperpolarized by 5-HT were serotonergic (78%; n= 32 of 41). TPH-immunonegative neurons were separated into two populations. One group had properties that were indistinguishable from those of serotonergic caudal raphe neurons. The other group was truly distinct; those neurons had more hyperpolarized resting membrane potentials, were not spontaneously active, had shorter-duration action potentials, and were depolarized by 5-HT. Caudal raphe neurons responded to 5-HT (1-5 microM) with membrane hyperpolarization in current clamp (-13.4 +/- 1.1 mV, mean +/- SE) or with outward current in voltage clamp (16.0 +/- 1.4 pA). The current induced by 5-HT was inwardly rectifying and associated with an increase in peak conductance and was highly selective for K+. It was completely blocked by 0.2 mM Ba2+ but not by glibenclamide, an inhibitor of ATP-sensitive K+ channels. Effects of 5-HT were dose dependent, with an EC50 of 0.1-0.3 microM. The 5-HT1A agonist 8-OH-DPAT mimicked, and the 5-HT1A antagonists (+)WAY 10,0135 and NAN 190 blocked, effects of 5-HT. The 5-HT2A/C antagonist ketanserin did not inhibit the effects of 5-HT. Fewer 5-HT-responsive neurons were encountered in slices exposed acutely to pertussis toxin (approximately 13%) than in adjacent control slices not exposed to pertussis toxin (approximately 85%). In addition, in neurons recorded with pipettes containing GTP gamma S (0.1 mM), 5-HT induced an inwardly rectifying current that did not reverse on washing. In many cells recorded with GTP gamma S, a current developed in the absence of agonist that had properties identical to those of the 5-HT-sensitive current; when followed for extended periods, the agonist-independent GTP gamma S-induced conductance desensitized, returning toward control levels with a time constant of approximately 18 min. Together these results indicate that serotonergic neurons of ROb and RPa are spontaneously active in a neonatal rat brain stem slice preparation and that hyperpolarization of those neurons by 5-HT1A receptor stimulation is due to pertussis toxin-sensitive G protein-mediated activation of an inwardly rectifying K+ conductance. In addition, we identified a group of nonserotonergic medullary raphe neurons that had distinct electrophysiological properties and that was depolarized by 5-HT.

GIRK1 immunoreactivity is present predominantly in dendrites, dendritic spines, and somata in the CA1 region of the hippocampus

Electron microscopic analysis of the CA1 region of the rat hippocampus revealed that specific immunoreactivity (IR) for a G protein-gated, inwardly rectifying potassium channel (GIRK1) was present exclusively in neurons and predominantly located in spiny dendrites of pyramidal cells. Within stratum lacunosum-moleculare and the superficial stratum radiatum, GIRK1-IR was often present immediately adjacent to asymmetric (excitatory-type) postsynaptic densities in dendritic spines. The subcellular localization of GIRK1-IR in the Golgi apparatus of pyramidal cell somata and in the plasma membrane of dendrites and dendritic spines confirms the hypothesis that GIRK1 is synthesized by pyramidal cells and transported to the more distal dendritic processes. G protein-coupled receptor activation of a dendritic potassium conductance would attenuate the propagation of excitatory synaptic inputs and thereby produce postsynaptic inhibition. Thus, these results show that the GIRK family of channels joins the list of voltage-sensitive channels now known to be expressed in dendritic spines.

Subunit interactions in the assembly of neuronal Kir3.0 inwardly rectifying K+ channels

Cardiac G protein-activated Kir (GIRK) channels may assemble as heterotetrameric polypeptides from two subunits, Kir3.1 and Kir3.4. For a functional comparison with native channels in the CNS we investigated all possible combinations of heteromeric channel formation from brain Kir3.1, Kir3.2, Kir3.3, and Kir3.4 subunits in mRNA-injected Xenopus oocytes. Analysis of macroscopic current amplitudes and channel gating kinetics indicated that individual subunits or combinations of Kir3.2, Kir3.3, and Kir3.4 formed functional channels ineffectively. Each of these subunits gave rise to prominent currents with distinct characteristics only in the presence of Kir3.1 subunits. Functional expression of concatemeric constructs between Kir3.1 and Kir3.2/3.4 subunits as well as coimmunoprecipitations with subunit-specific antibodies confirmed heteromeric channel formation. Mutational swapping between subunits of a single pore loop residue (Kir3.1F137S; Kir3.3S114F; a phenylalanine confers slow channel gating in Kir3.1 subunits) revealed that Kir3.1 subunits are an important constituent for native heteromeric channels and dominate their functional properties. However, homomeric channels from Kir3.1 subunits in vivo may not exist due to the spatial conflict of bulky phenylalanines in the pore structure.

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.

Heteromultimerization of G-protein-gated inwardly rectifying K+ channel proteins GIRK1 and GIRK2 and their altered expression in weaver brain

The weaver (wv) gene (GIRK2) is a member of the G-protein-gated inwardly rectifying potassium (GIRK) channel family, known effectors in the signal transduction pathway of neurotransmitters such as acetylcholine, dopamine, opioid peptides, and substance P in modulation of neurotransmitter release and neuronal excitability. GIRK2 immunoreactivity is found in but not limited to brain regions known to be affected in wv mice, such as the cerebellar granule cells and dopaminergic neurons in the substantia nigra pars compacta. It is also observed in the ventral tegmental area, hippocampus, cerebral cortex, and thalamus. GIRK2 and GIRK1, a related family member, have overlapping yet distinct distributions in rat and mouse brains. In regions where both channel proteins are expressed, such as the cerebral cortex, hippocampus, and cerebellum, they can be co-immunoprecipitated, indicating that they interact to form heteromeric channels in vivo. In the brain of the wv mouse, GIRK2 expression is decreased dramatically. In regions where GIRK1 and GIRK2 distributions overlap, both GIRK1 and GIRK2 expressions are severely disrupted, probably because of their co-assembly. The expression patterns of these GIRK channel subunits provide a basis for consideration of the machinery for neuronal signaling as well as the differential effects of the wv mutation in various neurons.

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.

Time resolved kinetics of direct G beta 1 gamma 2 interactions with the carboxyl terminus of Kir3.4 inward rectifier K+ channel subunits

The direct interaction of recombinant G beta 1 gamma 2 proteins with the carboxyl terminal domain of a G protein-gated inward rectifier K channel subunit, Kir3.4 (GIRK4), was measured in real time using biosensor chip technology. The carboxyl terminus of Kir3.4 (a.a. 186-419) was expressed in bacteria as a glutathione-S-transferase (GST) fusion protein, GST-Kir3. 4ct. GST-Kir3.4ct was immobilized to the surface of a biosensor chip by high affinity binding of the GST domain to a covalently attached anti-GST antibody. The association and dissociation rates of G beta 1 gamma 2 dimers with the immobilized Kir3.4ct domain were temporally resolved as a change in refractive index detected by surface plasmon resonance. Specific binding of G beta 1 gamma 2 dimers to Kir3.4ct was characterized by a dissociation rate (kd) of approximately 0.003 s-1. Association kinetics were dominated by a concentration-independent component (time constant approximately 50 s) which complicates models of binding and may indicate conformational changes during binding of G beta 1 gamma 2 to Kir3.4ct. The estimated equilibrium dissociation binding constant (Kd) was approximately 800 nM. These studies demonstrate that G beta gamma dimers interact directly with the Kir3.4 channel subunit, and suggest interesting details in the interaction with the major cytosolic carboxyl terminal domain. The slow G beta 1 gamma 2 dissociation rate measured on the sensor chip is similar in magnitude to a slow component of channel deactivation measured electrophysiologically in Xenopus oocytes expressing Kir3.1/3.4 multimeric channels and a G protein-coupled receptor. Biosensor-based experiments such as those described here will complement electrophysiological studies on the molecular basis of G protein interactions with Kir channels and other ion channel proteins.