Molecular determinants for activation of G-protein-coupled inward rectifier K+ (GIRK) channels by extracellular acidosis

GABAB receptors in neocortical and hippocampal pyramidal neurons are coupled to different potassium channels

Classically, GABA_B receptors are thought to regulate neuronal excitability via G-protein-coupled inwardly rectifying potassium (GIRK) channels. Recent data, however, indicate that GABA_B receptors can also activate two-pore domain potassium channels. Here, we investigate which potassium channels are coupled to GABA_B receptors in rat neocortical layer 5 and hippocampal CA1 pyramidal neurons. Bath application of the non-specific GIRK channel blocker barium (200 μm) abolished outward currents evoked by GABA_B receptors in CA1 pyramidal, but only partially blocked GABA_B responses in layer 5 neurons. Layer 5 and CA1 pyramidal neurons also showed differential sensitivity to tertiapin-Q, a specific GIRK channel blocker. Tertiapin-Q partially blocked GABA_B responses in CA1 pyramidal neurons, but was ineffective in blocking GABA_B responses in neocortical layer 5 neurons. Consistent with the idea that GABA_B receptors are coupled to two-pore domain potassium channels, the non-specific blockers quinidine and bupivacaine partially blocked GABA_B responses in both layer 5 and CA1 neurons. Finally, we show that lowering external pH, as occurs in hypoxia, blocks the component of GABA_B responses mediated by two-pore domain potassium channels in neocortical layer 5 pyramidal neurons, while at the same time revealing a GIRK channel component. These data indicate that GABA_B receptors in neocortical layer 5 and hippocampal CA1 pyramidal neurons are coupled to different channels, with this coupling pH dependent on neocortical layer 5 pyramidal neurons. This pH dependency may act to maintain constant levels of GABA_B inhibition during hypoxia by enhancing GIRK channel function following a reduction in two-pore domain potassium channel activity.

Characterization of Rebound Depolarization in Neurons of the Rat Medial Geniculate Body In Vitro

Rebound depolarization (RD) is a response to the offset from hyperpolarization of the neuronal membrane potential and is an important mechanism for the synaptic processing of inhibitory signals. In the present study, we characterized RD in neurons of the rat medial geniculate body (MGB), a nucleus of the auditory thalamus, using whole-cell patch-clamp and brain slices. RD was proportional in strength to the duration and magnitude of the hyperpolarization; was effectively blocked by Ni(2+) or Mibefradil; and was depressed when the resting membrane potential was hyperpolarized by blocking hyperpolarization-activated cyclic nucleotide-gated (HCN) channels with ZD7288 or by activating G-protein-gated inwardly-rectifying K(+) (GIRK) channels with baclofen. Our results demonstrated that RD in MGB neurons, which is carried by T-type Ca(2+) channels, is critically regulated by HCN channels and likely by GIRK channels.

The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K⁺ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.

GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

Protons and Ca2+: ionic allies in tumor progression?

Ion channels and G-protein-coupled receptors (GPCRs) play a fundamental role in cancer progression by influencing Ca(2+) influx and signaling pathways in transformed cells. Transformed cells thrive in a hostile environment that is characterized by extracellular acidosis that promotes the pathological phenotype. The pathway(s) by which extracellular protons achieve this remain unclear. Here, a role for proton-sensing ion channels and GPCRs as mediators of the effects of extracellular protons in cancer cells is discussed.

Acidosis, acid-sensing ion channels, and neuronal cell death

Acidosis is a common feature of many neuronal diseases and often accompanied with adverse consequences such as pain and neuronal injury. Before the discovery of acid-sensing ion channels (ASICs), protons were usually considered as a modulator of other ion channels, such as voltage-gated calcium channels, N-methyl-D-aspartate, and γ-amino butyric acid(A) receptor channels. Accordingly, the functional effects of acidosis were considered as consequences of modulations of these channels. Since the first cloning of ASICs in 1997, the conventional view on acidosis-mediated pain and cell injury has been dramatically changed. To date, ASICs, which are directly activated by extracellular protons, are shown to mediate most of the acidosis-associated physiological and pathological functions. For example, ASIC1a channels are reported to mediate acidosis-induced ischemic neuronal death. In this article, we will review the possible mechanisms that underlie ASIC1a channel-mediated neuronal death and discuss ASIC1a channel modulators involved in this process.

Acid sensing by visceral afferent neurones

Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics.

State-dependent inhibition of TRPM2 channel by acidic pH

Transient receptor potential melastatin 2 (TRPM2) channel fulfills an important role in oxidative stress signaling in immune and other cells, to which local extracellular acidosis is known to occur under physiological or pathological conditions and impose significant effects on their functions. Here, we investigated whether the ADP-ribose-activated TRPM2 channel is a target for modulation by extracellular acidic pH by patch clamp recording of HEK293 cells expressing hTRPM2 channel. Induced whole cell or single channel currents were rapidly inhibited upon subsequent exposure to acidic pH. The inhibition in the steady state was complete, voltage-independent, and pH-independent in the range of pH 4.0–6.0. The inhibition was irreversible upon returning to pH 7.3, suggesting channel inactivation. In contrast, exposure of closed channels to acidic pH reduced the subsequent channel activation in a pH-dependent manner with an IC₅₀ for H⁺ of 20 μm (pH 4.7) and rendered subsequent current inhibition largely reversible, indicating differential or state-dependent inhibition and inactivation. Alanine substitution of residues in the outer vestibule of the pore including Lys⁹⁵² and Asp¹⁰⁰² significantly slowed down or reduced acidic pH-induced inhibition and prevented inactivation. The results suggest that acidic pH acts as a negative feedback mechanism where protons bind to the outer vestibule of the TRPM2 channel pore and inhibit the TRPM2 channels in a state-dependent manner.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

The slow afterhyperpolarization modulates high pH-induced changes in the excitability of rat CA1 pyramidal neurons

Extra- and intracellular recordings from the CA1 region of rat hippocampal slices were employed to examine the role of the slow afterhyperpolarization (sAHP) in modulating the increases in neuronal excitability observed on increasing extracellular pH (pHo) from 7.4 to 7.7. In the majority of experiments, an antidromic conditioning stimulus applied in the presence of D(-)-2-amino-5-phosphonopentanoic acid (D-APV), 6-cyano-7-nitroquinoxaline-2,3-dione disodium salt (CNQX) and bicuculline was employed to elicit a sAHP, and an antidromic test stimulus was applied during the sAHP. At pHo 7.4, a single conditioning stimulus elicited an action potential followed by a sAHP, which in turn inhibited the response to the test stimulus compared with the conditioning stimulus. Increasing the number of action potentials in the conditioning stimulus augmented the sAHP and further inhibited the test response, whereas isoproterenol inhibited the sAHP and prevented the relative inhibition of the test response. At pHo 7.7, a single conditioning stimulus elicited a burst of action potentials followed by a large sAHP, which in turn prevented the test stimulus from eliciting a burst of action potentials and, in extracellular recordings, further increased the inhibition of the test response. The latter effect did not solely reflect a high pHo-induced increase in the conditioning response (and, thus, the subsequent sAHP), but rather involved a more direct effect of high pHo to augment the sAHP. The results indicate that increasing pHo increases the excitability of CA1 neurons to an initial stimulus; however, a high pHo-dependent increase in the sAHP evoked by the initial stimulus limits the response to subsequent stimuli.

Hypercapnia modulates synaptic interaction of cultured brainstem neurons

CO(2) is an important metabolic product whose concentrations are constantly monitored by CO(2) chemoreceptors. However, the high systemic CO(2) sensitivity may not be achieved by the CO(2) chemoreceptors without neuronal network processes. To show modulation of network properties during hypercapnia, we studied brainstem neurons dissociated from embryonic rats (P17-19) in multielectrode arrays (MEA) after initial period (3 weeks) of culture. Spike trains of 33,622 pairs of units were analyzed using peri-event histograms (PEH). The amplitude of peri-central peaks between two CO(2)-stimulated units increased and the peak latency decreased during hypercapnia. Similar enhancement of synaptic strength was observed in those sharing a common input. These phenomena were not seen in CO(2)-unresponsive neurons. The amplitude of peri-central peaks between two CO(2) inhibited units also increased without changing latency. Over 60% CO(2)-stimulated neurons studied received mono-/oligosynaptic inputs from other CO(2)-stimulated cells, whereas only approximately 10% CO(2)-unresponsive neurons had such synaptic inputs. A small number of brainstem neurons showed electrical couplings. The coupling efficiency of CO(2)-stimulated but not CO(2)-unresponsive units was suppressed by approximately 50% with high PCO(2). Inhibitory synaptic projections were also found, which was barely affected by hypercapnia. Consistent with the strengthening of excitatory synaptic connections, CO(2) sensitivity of post-synaptic neurons was significantly higher than presynaptic neurons. The difference was eliminated with blockade of presynaptic input. Based on these indirect assessments of synaptic interaction, our PEH analysis suggests that hypercapnia appears to modulate excitatory synaptic transmissions, especially those between CO(2)-stimulated neurons.

Acid-sensing ion channel 1a is a postsynaptic proton receptor that affects the density of dendritic spines

Extracellular proton concentrations in the brain may be an important signal for neuron function. Proton concentrations change both acutely when synaptic vesicles release their acidic contents into the synaptic cleft and chronically during ischemia and seizures. However, the brain receptors that detect protons and their physiologic importance remain uncertain. Using organotypic hippocampal slices and biolistic transfection, we found the acid-sensing ion channel 1a (ASIC1a), localized in dendritic spines where it functioned as a proton receptor. ASIC1a also affected the density of spines, the postsynaptic site of most excitatory synapses. Decreasing ASIC1a reduced the number of spines, whereas overexpressing ASIC1a had the opposite effect. Ca(2+)-mediated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) signaling was probably responsible, because acid evoked an ASIC1a-dependent elevation of spine intracellular Ca(2+) concentration, and reducing or increasing ASIC1a levels caused parallel changes in CaMKII phosphorylation in vivo. Moreover, inhibiting CaMKII prevented ASIC1a from increasing spine density. These data indicate that ASIC1a functions as a postsynaptic proton receptor that influences intracellular Ca(2+) concentration and CaMKII phosphorylation and thereby the density of dendritic spines. The results provide insight into how protons influence brain function and how they may contribute to pathophysiology.

Acid-induced pain and its modulation in humans

Despite the discovery of ion channels that are activated by protons, we still know relatively little about the signaling of acid pain. We used a novel technique, iontophoresis of protons, to investigate acid-induced pain in human volunteers. We found that transdermal iontophoresis of protons consistently caused moderate pain that was dose-dependent. A marked desensitization occurred with persistent stimulation, with a time constant of approximately 3 min. Recovery from desensitization occurred slowly, over many hours. Acid-induced pain was significantly augmented in skin sensitized by acute topical application of capsaicin. However, skin desensitized by repeated capsaicin application showed no significant reduction in acid-induced pain, suggesting that both capsaicin-sensitive and insensitive sensory neurons contribute to acid pain. Furthermore, topical application of non-steroidal anti-inflammatory drugs (NSAIDs) significantly attenuated acid-evoked pain but did not affect the heat pain threshold, suggesting a specific interaction between NSAIDs and peripheral acid sensors. Subcutaneous injection of amiloride (1 mm) also significantly inhibited the pain induced by iontophoresis of acid, suggesting an involvement of acid-sensing ion channel (ASIC) receptors. Conversely, iontophoresis of acid over a wide range of skin temperatures from 4 to 40 degrees C produced only minor changes in the induced pain. Together these data suggest a prominent role for ASIC channels and only a minor role for transient receptor potential vanilloid receptor-1 as mediators of cutaneous acid-induced pain.

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.

Molecular basis for the inhibition of G protein-coupled inward rectifier K(+) channels by protein kinase C

G protein-coupled inward rectifier K(+) (GIRK) channels regulate cellular excitability and neurotransmission. The GIRK channels are activated by a number of inhibitory neurotransmitters through the G protein betagamma subunit (G(betagamma)) after activation of G protein-coupled receptors and inhibited by several excitatory neurotransmitters through activation of phospholipase C. If the inhibition is produced by PKC, there should be PKC phosphorylation sites in GIRK channel proteins. To identify the PKC phosphorylation sites, we performed systematic mutagenesis analysis on GIRK4 and GIRK1 subunits expressed in Xenopus oocytes. Our data showed that the heteromeric GIRK1/GIRK4 channels were inhibited by a PKC activator phorbol 12-myristate 13-acetate (PMA) through reduction of single channel open-state probability. Direct application of the catalytic subunit of PKC to excised patches had a similar inhibitory effect. This inhibition was greatly eliminated by mutation of Ser-185 in GIRK1 and Ser-191 in GIRK4 that remained G protein sensitive. The PKC-dependent phosphorylation seems to mediate the channel inhibition by the excitatory neurotransmitter substance P (SP) as specific PKC inhibitors and mutation of these PKC phosphorylation sites abolished the SP-induced inhibition of GIRK1/GIRK4 channels. Thus, these results indicate that the PKC-dependent phosphorylation underscores the inhibition of GIRK channels by SP, and Ser-185 in GIRK1 and Ser-191 in GIRK4 are the PKC phosphorylation sites.

Inhibition of G-protein-coupled inward rectifying K+ channels by intracellular acidosis

G-protein-coupled inward rectification K(+) (GIRK) channels play an important role in modulation of synaptic transmission and cellular excitability. The GIRK channels are regulated by diverse intra- and extracellular signaling molecules. Previously, we have shown that GIRK1/GIRK4 channels are activated by extracellular protons. The channel activation depends on a histidine residue in the M1-H5 linker and may play a role in neurotransmission. Here, we show evidence that the heteromeric GIRK1/GIRK4 channels are inhibited by intracellular acidification. This inhibition was produced by selective decrease in the channel open probability with a modest drop in the single-channel conductance. The inhibition does not seem to require G-proteins as it was seen in two G-protein coupling-defective GIRK mutants and in excised patches in the absence of exogenous G-proteins. Three histidine residues in intracellular domains were critical for the inhibition. Individual mutation of His-64, His-228, or His-352 in GIRK4 abolished or greatly diminished the inhibition in homomeric GIRK4. Mutations of any of these histidine residues in GIRK4 or their counterparts in GIRK1 were sufficient to eliminate the pH(i) sensitivity of the heteromeric GIRK1/GIRK4 channels. Thus, the molecular and biophysical bases for the inhibition of GIRK channels by intracellular protons are illustrated. Because of the inequality of the pH(i) and pH(o) in most cells and their relatively independent controls by cellular versus systemic mechanisms, such pH(i) sensitivity may allow these channels to regulate cellular excitability in certain physiological and pathophysiological conditions when intracellular acidosis occurs.