Short-Term Desensitization of G-Protein-Activated, Inwardly Rectifying K+ (GIRK) Currents in Pyramidal Neurons of Rat Neocortex

Neuronal Adenosine A1 Receptor is Critical for Olfactory Function but Unable to Attenuate Olfactory Dysfunction in Neuroinflammation

Adenine nucleotides, such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), as well as the nucleoside adenosine are important modulators of neuronal function by engaging P1 and P2 purinergic receptors. In mitral cells, signaling of the G protein-coupled P1 receptor adenosine 1 receptor (A1R) affects the olfactory sensory pathway by regulating high voltage-activated calcium channels and two-pore domain potassium (K2P) channels. The inflammation of the central nervous system (CNS) impairs the olfactory function and gives rise to large amounts of extracellular ATP and adenosine, which act as pro-inflammatory and anti-inflammatory mediators, respectively. However, it is unclear whether neuronal A1R in the olfactory bulb modulates the sensory function and how this is impacted by inflammation. Here, we show that signaling via neuronal A1R is important for the physiological olfactory function, while it cannot counteract inflammation-induced hyperexcitability and olfactory deficit. Using neuron-specific A1R-deficient mice in patch-clamp recordings, we found that adenosine modulates spontaneous dendro-dendritic signaling in mitral and granule cells via A1R. Furthermore, neuronal A1R deficiency resulted in olfactory dysfunction in two separate olfactory tests. In mice with experimental autoimmune encephalomyelitis (EAE), we detected immune cell infiltration and microglia activation in the olfactory bulb as well as hyperexcitability of mitral cells and olfactory dysfunction. However, neuron-specific A1R activity was unable to attenuate glutamate excitotoxicity in the primary olfactory bulb neurons in vitro or EAE-induced olfactory dysfunction and disease severity in vivo. Together, we demonstrate that A1R modulates the dendro-dendritic inhibition (DDI) at the site of mitral and granule cells and impacts the processing of the olfactory sensory information, while A1R activity was unable to counteract inflammation-induced hyperexcitability.

Role of A1 receptor-activated GIRK channels in the suppression of hippocampal seizure activity

The neuromodulator adenosine is released during seizure activity to provide negative feedback suppression of ongoing activity and to delay the occurrence of the next burst of activity. Adenosine acts via multiple G-protein-coupled receptors including the A₁ receptor (A₁R) which inhibits neurotransmitter release and hyperpolarises neuronal membrane potential. The hyperpolarisation is produced, at least in part, by the activation of G-protein-activated inwardly rectifying K⁺ (GIRK) channels. We have used tertiapin-Q (TQ), a potent and selective inhibitor of GIRK channels, to assess the role of GIRK channels in controlling seizure activity in areas CA1 and CA2 of mouse hippocampal slices. TQ (100-300 nM) blocked ~50% of the adenosine-mediated membrane potential hyperpolarisation of hippocampal CA1 and CA2 neurons. TQ (100 nM) had no significant effect on synaptic transmission in area CA1 of the hippocampus but enhanced transmission in CA2, an effect prevented by blocking A₁Rs. TQ (100 nM) increased the frequency of spontaneous activity (induced by removing Mg²⁺ and increasing K⁺) and blunted the effects of exogenous adenosine on the suppression of activity. TQ had a significantly greater effect on electrically-stimulated seizure activity induced in CA2 versus that in CA1, producing a greater increase in both the duration and amplitude of the stimulated bursts. This is consistent with the greater A₁R density and A₁R activation tone in CA2. Thus GIRK channels play a role in the supressing effects of adenosine on seizure activity.

Purinergic Signaling in the Vertebrate Olfactory System

Adenosine 5'-triphosphate (ATP) is an ubiquitous co-transmitter in the vertebrate brain. ATP itself, as well as its breakdown products ADP and adenosine are involved in synaptic transmission and plasticity, neuron-glia communication and neural development. Although purinoceptors have been demonstrated in the vertebrate olfactory system by means of histological techniques for many years, detailed insights into physiological properties and functional significance of purinergic signaling in olfaction have been published only recently. We review the current literature on purinergic neuromodulation, neuron-glia interactions and neurogenesis in the vertebrate olfactory system.

Neuroprotective potential of adenosine A1 receptor partial agonists in experimental models of cerebral ischemia

Cerebral ischemia is the second most common cause of death and a major cause of disability worldwide. Available therapies are based only on anticoagulants or recombinant tissue plasminogen activator. Extracellular adenosine increases during ischemia and acts as a neuroprotective endogenous agent mainly by activating adenosine A₁ receptors (A₁ Rs) which control calcium influx, glutamate release, membrane potential, and metabolism. Accordingly, in many experimental paradigms it has been already demonstrated that the stimulation of A₁ R with full agonists is able to reduce ischemia-related structural and functional brain damage; unfortunately, cardiovascular side effects and desensitization of A₁ R induced by these compounds have strongly limited their exploitation in stroke therapy so far. Among the newly emerging compounds, A₁ R partial agonists could be almost free of side effects and equally effective. Therefore, we decided to evaluate the neuroprotective potential of two A₁ R partial agonists, namely 2'-dCCPA and 3'-dCCPA, in in vitro and ex vivo experimental models of cerebral ischemia. Within the experimental paradigm of oxygen-glucose deprivation in vitro in human neuroblastoma (SH-SY5Y) cells both A₁ R partial agonists increased cell viability. Considering the high level of expression of A₁ Rs in the hippocampus and the susceptibility of CA1 region to hypoxia, we performed electrophysiological experiments in this subfield. The application of 7 min of oxygen-glucose deprivation constantly produces an irreversible synaptic failure in all the C57Bl/6 mice hippocampal slices evaluated; both tested compounds allowed a significant recovery of synaptic transmission. These findings demonstrate that A₁ R and its partial agonists are still of interest for cerebral ischemia therapy. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Adenosine A1 receptor activates background potassium channels and modulates information processing in olfactory bulb mitral cells

KEY POINTS

Adenosine is a widespread neuromodulator in the mammalian brain, but whether it affects information processing in sensory system(s) remains largely unknown. Here we show that adenosine A₁ receptors hyperpolarize mitral cells, one class of principal neurons that propagate odour information from the olfactory bulb to higher brain areas, by activation of background K⁺ channels. The adenosine-modulated background K⁺ channels belong to the family of two-pore domain K⁺ channels. Adenosine reduces spontaneous activity of mitral cells, whereas action potential firing evoked by synaptic input upon stimulation of sensory neurons is not affected, resulting in a higher ratio of evoked firing (signal) over spontaneous firing (noise) and hence an improved signal-to-noise ratio. The study shows for the first time that adenosine influences fine-tuning of the input-output relationship in sensory systems.

ABSTRACT

Neuromodulation by adenosine is of critical importance in many brain regions, but the role of adenosine in olfactory information processing has not been studied so far. We investigated the effects of adenosine on mitral cells, which are projection neurons of the olfactory bulb. Significant expression of A₁ and A_2A receptors was found in mitral cells, as demonstrated by in situ hybridization. Application of adenosine in acute olfactory bulb slices hyperpolarized mitral cells in wild-type but not in adenosine A₁ receptor knockout mice. Adenosine-induced hyperpolarization was mediated by background K⁺ currents that were reduced by halothane and bupivacaine, which are known to inhibit two-pore domain K⁺ (K2P) channels. In mitral cells, electrical stimulation of axons of olfactory sensory neurons evoked synaptic currents, which can be considered as input signals, while spontaneous firing independent of sensory input can be considered as noise. Synaptic currents were not affected by adenosine, while adenosine reduced spontaneous firing, leading to an increase in the signal-to-noise ratio of mitral cell firing. Our findings demonstrate that A₁ adenosine receptors activate two-pore domain K⁺ channels, which increases the signal-to-noise ratio of the input-output relationship in mitral cells and thereby modulates information processing in the olfactory bulb.

Mechanisms of fast desensitization of GABA(B) receptor-gated currents

GABA(B) receptors (GABA(B)Rs) regulate the excitability of most neurons in the central nervous system by modulating the activity of enzymes and ion channels. In the sustained presence of the neurotransmitter γ-aminobutyric acid, GABA(B)Rs exhibit a time-dependent decrease in the receptor response-a phenomenon referred to as homologous desensitization. Desensitization prevents excessive receptor influences on neuronal activity. Much work focused on the mechanisms of GABA(B)R desensitization that operate at the receptor and control receptor expression at the plasma membrane. Over the past few years, it became apparent that GABA(B)Rs additionally evolved mechanisms for faster desensitization. These mechanisms operate at the G protein rather than at the receptor and inhibit G protein signaling within seconds of agonist exposure. The mechanisms for fast desensitization are ideally suited to regulate receptor-activated ion channel responses, which influence neuronal activity on a faster timescale than effector enzymes. Here, we provide an update on the mechanisms for fast desensitization of GABA(B)R responses and discuss physiological and pathophysiological implications.

GABAB receptor phosphorylation regulates KCTD12-induced K⁺ current desensitization

GABAB receptors assemble from GABAB1 and GABAB2 subunits. GABAB2 additionally associates with auxiliary KCTD subunits (named after their K(+) channel tetramerization-domain). GABAB receptors couple to heterotrimeric G-proteins and activate inwardly-rectifying K(+) channels through the βγ subunits released from the G-protein. Receptor-activated K(+) currents desensitize in the sustained presence of agonist to avoid excessive effects on neuronal activity. Desensitization of K(+) currents integrates distinct mechanistic underpinnings. GABAB receptor activity reduces protein kinase-A activity, which reduces phosphorylation of serine-892 in GABAB2 and promotes receptor degradation. This form of desensitization operates on the time scale of several minutes to hours. A faster form of desensitization is induced by the auxiliary subunit KCTD12, which interferes with channel activation by binding to the G-protein βγ subunits. Here we show that the two mechanisms of desensitization influence each other. Serine-892 phosphorylation in heterologous cells rearranges KCTD12 at the receptor and slows KCTD12-induced desensitization. Likewise, protein kinase-A activation in hippocampal neurons slows fast desensitization of GABAB receptor-activated K(+) currents while protein kinase-A inhibition accelerates fast desensitization. Protein kinase-A fails to regulate fast desensitization in KCTD12 knock-out mice or knock-in mice with a serine-892 to alanine mutation, thus demonstrating that serine-892 phosphorylation regulates KCTD12-induced desensitization in vivo. Fast current desensitization is accelerated in hippocampal neurons carrying the serine-892 to alanine mutation, showing that tonic serine-892 phosphorylation normally limits KCTD12-induced desensitization. Tonic serine-892 phosphorylation is in turn promoted by assembly of receptors with KCTD12. This cross-regulation of serine-892 phosphorylation and KCTD12 activity sharpens the response during repeated receptor activation.

Short-term desensitization of muscarinic K+ current in the heart

Acetylcholine (ACh) rapidly increases cardiac K(+) currents (IKACh) by activating muscarinic K(+) (KACh) channels followed by a gradual amplitude decrease within seconds. This phenomenon is called short-term desensitization and its precise mechanism and physiological role are still unclear. We constructed a mathematical model for IKACh to examine the conditions required to reconstitute short-term desensitization. Two conditions were crucial: two distinct muscarinic receptors (m2Rs) with different affinities for ACh, which conferred an IKACh response over a wide range of ACh concentrations, and two distinct KACh channels with different affinities for the G-protein βγ subunits, which contributed to reconstitution of the temporal behavior of IKACh. Under these conditions, the model quantitatively reproduced several unique properties of short-term desensitization observed in myocytes: 1), the peak and quasi-steady states with 0.01-100 μM [ACh]; 2), effects of ACh preperfusion; and 3), recovery from short-term desensitization. In the presence of 10 μM ACh, the IKACh model conferred recurring spontaneous firing after asystole of 8.9 s and 10.7 s for the Demir and Kurata sinoatrial node models, respectively. Therefore, two different populations of KACh channels and m2Rs may participate in short-term desensitization of IKACh in native myocytes, and may be responsible for vagal escape at nodal cells.

Opposite effects of KCTD subunit domains on GABA(B) receptor-mediated desensitization

GABA(B) receptors assemble from principle and auxiliary subunits. The principle subunits GABA(B1) and GABA(B2) form functional heteromeric GABA(B(1,2)) receptors that associate with homotetramers of auxiliary KCTD8, -12, -12b, or -16 (named after their K(+) channel tetramerization domain) subunits. These auxiliary subunits constitute receptor subtypes with distinct functional properties. KCTD12 and -12b generate desensitizing receptor responses while KCTD8 and -16 generate largely non-desensitizing receptor responses. The structural elements of the KCTDs underlying these differences in desensitization are unknown. KCTDs are modular proteins comprising a T1 tetramerization domain, which binds to GABA(B2), and a H1 homology domain. KCTD8 and -16 contain an additional C-terminal H2 homology domain that is not sequence-related to the H1 domains. No functions are known for the H1 and H2 domains. Here we addressed which domains and sequence motifs in KCTD proteins regulate desensitization of the receptor response. We found that the H1 domains in KCTD12 and -12b mediate desensitization through a particular sequence motif, T/NFLEQ, which is not present in the H1 domains of KCTD8 and -16. In addition, the H2 domains in KCTD8 and -16 inhibit desensitization when expressed C-terminal to the H1 domains but not when expressed as a separate protein in trans. Intriguingly, the inhibitory effect of the H2 domain is sequence-independent, suggesting that the H2 domain sterically hinders desensitization by the H1 domain. Evolutionary analysis supports that KCTD12 and -12b evolved desensitizing properties by liberating their H1 domains from antagonistic H2 domains and acquisition of the T/NFLEQ motif.

Regulation of neuronal GABA(B) receptor functions by subunit composition

GABA(B) receptors (GABA(B)Rs) are G protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the CNS. In the past 5 years, notable advances have been made in our understanding of the molecular composition of these receptors. GABA(B)Rs are now known to comprise principal and auxiliary subunits that influence receptor properties in distinct ways. The principal subunits regulate the surface expression and the axonal versus dendritic distribution of these receptors, whereas the auxiliary subunits determine agonist potency and the kinetics of the receptor response. This Review summarizes current knowledge on how the subunit composition of GABA(B)Rs affects the distribution of these receptors, neuronal processes and higher brain functions.

Dopaminergic-induced changes in Mauthner cell excitability disrupt prepulse inhibition in the startle circuit of goldfish

Prepulse inhibition (PPI) is a widespread sensorimotor gating phenomenon characterized by a decrease in startle magnitude if a nonstartling stimulus is presented 20-1,000 ms before a startling stimulus. Dopaminergic agonists disrupt behavioral PPI in various animal models. This provides an important neuropharmacological link to schizophrenia patients that typically show PPI deficits at distinct (60 ms) prepulse-pulse intervals. Here, we study time-dependent effects of dopaminergic modulation in the goldfish Mauthner cell (M-cell) startle network, which shows PPI-like behavioral and physiological startle attenuations. The unique experimental accessibility of the M-cell system allows investigating the underlying cellular mechanism with physiological stimuli in vivo. Our results show that the dopaminergic agonist apomorphine (2 mg/kg body wt) reduced synaptic M-cell PPI by 23.6% (n = 18; P = 0.009) for prepulse-pulse intervals of 50 ms, whereas other intervals showed no reduction. Consistently, application of the dopamine antagonist haloperidol (0.4 mg/kg body wt) restored PPI to control level. Current ramp injections while recording M-cell membrane potential revealed that apomorphine acts through a postsynaptic, time-dependent mechanism by deinactivating a M-cell membrane nonlinearity, effectively increasing input resistance close to threshold. This increase is most pronounced for prepulse-pulse intervals of 50 ms (47.9%, n = 8; P < 0.05) providing a time-dependent, cellular mechanism for dopaminergic disruption of PPI. These results provide, for the first time, direct evidence of dopaminergic modulation of PPI in the elementary startle circuit of vertebrates and reemphasize the potential of characterizing temporal aspects of PPI at the physiological level to understand its underlying mechanisms.

Nonenzymatic rapid control of GIRK channel function by a G protein-coupled receptor kinase

G protein-coupled receptors (GPCRs) respond to agonists to activate downstream enzymatic pathways or to gate ion channel function. Turning off GPCR signaling is known to involve phosphorylation of the GPCR by GPCR kinases (GRKs) to initiate their internalization. The process, however, is relatively slow and cannot account for the faster desensitization responses required to regulate channel gating. Here, we show that GRKs enable rapid desensitization of the G protein-coupled potassium channel (GIRK/Kir3.x) through a mechanism independent of their kinase activity. On GPCR activation, GRKs translocate to the membrane and quench channel activation by competitively binding and titrating G protein βγ subunits away from the channel. Of interest, the ability of GRKs to effect this rapid desensitization depends on the receptor type. The findings thus reveal a stimulus-specific, phosphorylation-independent mechanism for rapidly downregulating GPCR activity at the effector level.

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

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

1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122) selectively inhibits Kir3 and BK channels in a phospholipase C-independent fashion

1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122) is widely used to inhibit phospholipase C (PLC)-mediated signaling, but we and others have also reported a PLC-independent block of Kir3 channels in native cells. To elaborate on this major side effect, we examined the action of U73122 and 1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-2,5-pyrollidinedione (U73343), a structurally related but not PLC-inhibiting analog, on Kir1.1, Kir2.1, or Kir3.1/3.2 channels expressed in HEK293 cells. Both compounds (10 microM) displayed an unusual degree of selectivity for Kir3, superior even to that of tertiapin, which discriminates between Kir3 and Kir2 but also inhibits Kir1.1. Recordings from mutant Kir2 and Kir3 channels showed that U73122 is unlikely to block Kir3 by interfering with binding of phosphatidylinositol 4,5-bisphosphate, and U73122 did not seem to act like a pore blocker. U73122 and U73343 also unexpectedly suppressed Ca(2+)-activated K(+) channels of the large-conductance type (MaxiK, BK) in a PLC-independent fashion. In single-channel recordings, both compounds significantly decreased open probability of BK channels and slowed their ultrafast gating ("flickering") at very depolarized potentials. Alignment of the amino acid sequences of Kir3 and BK channels suggested that the highly selective effect of U73122/U73343 is mediated by a homologous domain within the long C-terminal ends. In fact, mutations in the C-terminal region of Kir2 and Kir3 channels significantly altered their sensitivity to the two compounds. Our data strongly caution against the use of U73122 when exploring signaling pathways involving Kir3 and BK channels. However, the apparent binding of U73122/U73343 to a common structural motif might be exploited to develop drugs selectively targeting Kir3 and BK channels.

Unexpected suppression of neuronal G protein-activated, inwardly rectifying K+ current by common phospholipase C inhibitor

Stabilization of the binding of phosphatidylinositol bisphosphate (PIP(2)) to G protein-coupled inward rectifier K+ (GIRK) channels is essential for their activation, whereas hydrolysis of PIP(2) by phospholipase C (PLC) inhibits channel activity. Apparently inconsistent with this mechanism, we found that the commonly used PLC inhibitor, U73122 (1 microM), produced a significant reduction in the amplitude of baclofen (20 microM)-evoked GIRK currents in whole-cell recordings from acutely isolated rat neocortical pyramidal cells. Also, U73122 reduced the percentage of baclofen-responsive neurons from 100% (n=40) to 56% (n=25). Since NCDC (100 microM), a PLC inhibitor of another molecular class, displayed no effect on GIRK current amplitude or responsiveness (100%, n=6), inhibition of PLC is unlikely to account for the effects of U73122 in our preparation. Lending further support to this notion, the structurally closely related compound, U73343, which does not inhibit PLC, proved to be even more efficient in suppressing GIRK current as compared to U73122. In neurons, in which GIRK channels were irreversibly activated by GTPgammaS (n=10), the depressant action of U71322 was fully preserved. These findings hint at a direct interaction of U73122 with the GIRK channel or a closely associated protein. Caution is therefore warranted when employing this compound to examine the role of PLC and PIP(2) in the regulation of GIRK channel activity.

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

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

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

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

Acute desensitization of GIRK current in rat atrial myocytes is related to K+ current flow

We have investigated the acute desensitization of acetylcholine-activated GIRK current (I(K(ACh))) in cultured adult rat atrial myocytes. Acute desensitization of I(K(ACh)) is observed as a partial relaxation of current with a half-time of < 5 s when muscarinic M2 receptors are stimulated by a high concentration (> 2 micromol l(-1)) of ACh. Under this condition experimental manoeuvres that cause a decrease in the amplitude of I(K(ACh)), such as partial block of M2 receptors by atropine, intracellular loading with GDP-beta-S, or exposure to Ba2+, caused a reduction in desensitization. Acute desensitization was also identified as a decrease in current amplitude and a blunting of the response to saturating [ACh] (20 micromol l(-1)) when the current had been partially activated by a low concentration of ACh or by stimulation of adenosine A1 receptors. A reduction in current analogous to acute desensitization was observed when ATP-dependent K+ current (I(K(ATP))) was activated either by mitochondrial uncoupling using 2,4-dinitrophenole (DNP) or by the channel opener rilmakalim. Adenovirus-driven overexpression of Kir2.1, a subunit of constitutively active inwardly rectifying K+ channels, resulted in a large Ba2+-sensitive background K+ current and a dramatic reduction of ACh-activated current. Adenovirus-driven overexpression of GIRK4 (Kir3.4) subunits resulted in an increased agonist-independent GIRK current paralleled by a reduction in I(K(ACh)) and removal of the desensitizing component. These data indicate that acute desensitization depends on K+ current flow, independent of the K+ channel species, suggesting that it reflects a reduction in electrochemical driving force rather than a bona fide signalling mechanism. This is supported by the observation that desensitization is paralleled by a significant negative shift in reversal potential of I(K(ACh)). Since the ACh-induced hyperpolarization shows comparable desensitization properties as I(K(ACh)), this novel current-dependent desensitization is a physiologically relevant process, shaping the time course of parasympathetic bradycardia.