Pore mutation in a G-protein-gated inwardly rectifying K+ channel subunit causes loss of K+-dependent inhibition in weaver hippocampus

A selectivity filter mutation provides insights into gating regulation of a K+ channel

G-protein coupled inwardly rectifying potassium (GIRK) channels are key players in inhibitory neurotransmission in heart and brain. We conducted molecular dynamics simulations to investigate the effect of a selectivity filter (SF) mutation, G154S, on GIRK2 structure and function. We observe mutation-induced loss of selectivity, changes in ion occupancy and altered filter geometry. Unexpectedly, we reveal aberrant SF dynamics in the mutant to be correlated with motions in the binding site of the channel activator Gβγ. This coupling is corroborated by electrophysiological experiments, revealing that GIRK2_wt activation by Gβγ reduces the affinity of Ba²⁺ block. We further present a functional characterization of the human GIRK2_G154S mutant validating our computational findings. This study identifies an allosteric connection between the SF and a crucial activator binding site. This allosteric gating mechanism may also apply to other potassium channels that are modulated by accessory proteins.

Gly selectivity filter (TIGYGYR) mutant of the GIRK2 channel causes rare but severe neurological disorder called the Keppen-Lubinsky syndrome. Here, the authors explore the molecular mechanism of action of this glycine to serine mutant causing disease and identify an allosteric connection between the selectivity filter and a crucial activator binding site.

Role of Baclofen in Modulating Spasticity and Neuroprotection in Spinal Cord Injury

Spinal cord injury (SCI) affects an estimated three million persons worldwide, with ∼180,000 new cases reported each year leading to severe motor and sensory functional impairments that affect personal and social behaviors. To date, no effective treatment has been made available to promote neurological recovery after SCI. Deficits in motor function is the most visible consequence of SCI; however, other secondary complications produce a significant impact on the welfare of patients with SCI. Spasticity is a neurological impairment that affects the control of muscle tone as a consequence of an insult, trauma, or injury to the central nervous system, such as SCI. The management of spasticity can be achieved through the combination of both nonpharmacological and pharmacological approaches. Baclofen is the most effective drug for spasticity treatment, and it can be administered both orally and intrathecally, depending on spasticity location and severity. Interestingly, recent data are revealing that baclofen can also play a role in neuroprotection after SCI. This new function of baclofen in the SCI scope is promising for the prospect of developing new pharmacological strategies to promote functional recovery in patients with SCI.

Excision of Trpv6 gene leads to severe defects in epididymal Ca2+ absorption and male fertility much like single D541A pore mutation

Replacement of aspartate residue 541 by alanine (D541A) in the pore of TRPV6 channels in mice disrupts Ca(2+) absorption by the epididymal epithelium, resulting in abnormally high Ca(2+) concentrations in epididymal luminal fluid and in a dramatic but incomplete loss of sperm motility and fertilization capacity, raising the possibility of residual activity of channels formed by TRPV6(D541A) proteins (Weissgerber, P., Kriebs, U., Tsvilovskyy, V., Olausson, J., Kretz, O., Stoerger, C., Vennekens, R., Wissenbach, U., Middendorff, R., Flockerzi, V., and Freichel, M. (2011) Sci. Signal. 4, ra27). It is known from other cation channels that introducing pore mutations even if they largely affect their conductivity and permeability can evoke considerably different phenotypes compared with the deletion of the corresponding protein. Therefore, we generated TRPV6-deficient mice (Trpv6(-/-)) by deleting exons encoding transmembrane domains with the pore-forming region and the complete cytosolic C terminus harboring binding sites for TRPV6-associated proteins that regulate its activity and plasma membrane anchoring. Using this strategy, we aimed to determine whether the TRPV6(D541A) pore mutant still contributes to residual channel activity and/or channel-independent functions in vivo. Trpv6(-/-) males reveal severe defects in fertility and motility and viability of sperm and a significant increase in epididymal luminal Ca(2+) concentration that is mirrored by a lack of Ca(2+) uptake by the epididymal epithelium. Therewith, Trpv6 excision affects epididymal Ca(2+) handling and male fertility to the same extent as the introduction of the D541A pore mutation, arguing against residual functions of the TRPV6(D541A) pore mutant in epididymal epithelial cells.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Somatostatin inhibits the excitability of rat small-diameter trigeminal ganglion neurons that innervate nasal mucosa and project to the upper cervical dorsal horn via activation of somatostatin 2a receptor

This study investigated whether somatostatin (SST) modulates the excitability of nociceptive trigeminal ganglion (TRG) neurons that innervate the nasal mucosa and project to the upper cervical (C(1)) dorsal horn by using perforated-patch clamping, retrograde-labeling, and immunohistochemistry. Fluorogold (FG) retrograde labeling was used to identify the rat TRG neurons innervating the nasal mucosa, while microbeads (MB) were used to label neurons projected onto the superficial layer of the C(1) dorsal horn. FG-labeled small-diameter TRG neurons exhibited SST(2A) receptor immunoreactivity (19%) and half of these neurons were also labeled with MB. In whole-cell current-clamp mode, most (72%) of the dissociated FG-/MB-labeled TRG neurons were hyperpolarized by application of SST. The hyperpolarization was evoked by SST in a concentration-dependent manner (0.1-10 microM) and the responses were associated with a decrease in the cell input resistance. The minimum concentration to elicit a significant hyperpolarization was 1 microM. The repetitive firings during a depolarizing pulse were significantly reduced by SST (1 microM) application. The hyperpolarization and decreased firing evoked by SST were both blocked by the SST(2) receptor antagonist, CYN154806 (1 microM). Under voltage-clamp conditions, SST (1 microM) significantly increased the voltage-gated K(+) transient (I(A)) and sustained (I(K)) currents and these increases were abolished by coapplication of CYN154806 (1 microM). In the presence of both 4-aminopyridine (6 mM) and tetraethylammonium (10 mM), no significant changes in the membrane potential in response to SST application were found. These results suggest that modulation of trigeminal nociceptive transmission in the C(1) dorsal horn by activation of SST(2A) receptors occurs at the level of small-diameter TRG cell bodies and/or their afferent terminals, and that this may be related to regulation of protective upper-airway reflexes.

Inhibition of mechanosensitivity in visceral primary afferents by GABAB receptors involves calcium and potassium channels

GABA(B) receptors inhibit mechanosensitivity of visceral afferents. This is associated with reduced triggering of events that lead to gastro-esophageal reflux, with important therapeutic consequences. In other neuronal systems, GABA(B) receptor activation may be linked via G-proteins to reduced N-type Ca(2+) channel opening, increased inward rectifier K(+) channel opening, plus effects on a number of intracellular messengers. Here we aimed to determine the role of Ca(2+) and K(+) channels in the inhibition of vagal afferent mechanoreceptor function by the GABA(B) receptor agonist baclofen. The responses of three types of ferret gastro-esophageal vagal afferents (mucosal, tension and tension mucosal receptors) to graded mechanical stimuli were investigated in vitro. The effects of baclofen (200 microM) alone on these responses were quantified, and the effects of baclofen in the presence of the G-protein-coupled inward rectifier potassium channel blocker Rb(+) (4.7 mM) and/or the N-type calcium channel blocker omega-conotoxin GVIA (0.1 microM). Baclofen inhibition of mucosal receptor mechanosensitivity was abolished by both blockers. Its inhibitory effect on tension mucosal receptors was partly reduced by both. The inhibitory effect of baclofen on tension receptors was unaffected. The data indicate that the inhibitory action of GABA(B) receptors is mediated via different pathways in mucosal, tension and tension mucosal receptors via mechanisms involving both N-type Ca(2+) channels and inwardly rectifying K(+) channels and others.

My close encounter with GABA(B) receptors

In this review, I summarize the sequence of events involved in characterizing the functional role of GABA(B) receptors in the CNS and their involvement in synaptic transmission. The story was launched with the realization that baclofen was a selective agonist of GABA(B) receptors. This lead to the discovery in the CNS that GABA(B) receptor activation could result in a presynaptic inhibition of transmitter release as well as a postsynaptic increase in potassium conductance. Based on this information, it was found that GABA also activated a potassium conductance. A role for GABA(B) receptors in synaptic transmission was suggested by the fact that activation of GABAergic interneurons could generate a slow IPSP mediated by an increase in potassium conductance. To link this slow IPSP to GABA(B) receptors required a selective GABA(B) antagonist. Phaclofen was the first antagonist developed and was found to antagonize the action of baclofen and the GABA(A) independent action of GABA. Most importantly, it blocked the slow IPSP. The properties of GABA(A) and GABA(B) IPSPs are remarkably different. GABA(A) IPSPs powerfully inhibit neurons and rapidly curtail excitatory inputs. This greatly enhances the precision of excitatory synaptic transmission. GABA(B) IPSPs are recruited with repetitive and synchronous activity and are postulated to modulate the rhythmic network activity of cortical tissue.

Voltage-Gated Calcium Channels Mediate Intracellular Calcium Increase inWeaverDopaminergic Neurons During Stimulation of D2and GABABReceptors

The weaver ( wv) mutation affects the pore-forming region of the inwardly rectifying potassium channel (GIRK) leading to degeneration of cerebellar granule and midbrain dopaminergic neurons. The mutated channel ( wvGIRK) loses its potassium selectivity, allowing sodium (Na+) and possibly calcium ions (Ca2+) to enter the cell. Here we performed whole cell patch-clamp recordings combined with microfluorometry to investigate possible differences in calcium ([Ca2+]i) dynamics in native dopaminergic neurons (expressing the wvGIRK2 subunits) in the midbrain slice preparation from homozygous weaver ( wv/wv) and control (+/+) mice. Under resting conditions, [Ca2+]iwas similar in wv/wv compared with +/+ neurons. Activation of wvGIRK2 channels by D2and GABABreceptors increased [Ca2+]iin wv/wv neurons, whereas activation of wild-type channels decreased [Ca2+]iin +/+ neurons. The calcium rise in wv/wv neurons was abolished by antagonists of the voltage-gated calcium channels (VGCC); voltage clamp of the neuron at −60 mV; and hyperpolarization of the neuron to −80 mV or more, in current clamp, and was unaffected by TTX. Therefore we propose that wvGIRK2 channels in native dopamine neurons are not permeable to Ca2+, and when activated by D2and GABABreceptors they mediate membrane depolarization and an indirect Ca2+influx through VGCC rather than via wvGIRK2 channels. Such calcium influx may be the trigger for calcium-mediated excitotoxicity, responsible for selective neuronal death in weaver mice.

Opioidergic modulation of excitability of rat trigeminal root ganglion neuron projections to the superficial layer of cervical dorsal horn

The aim of the present study was to investigate the effect of a micro-opioid receptor agonist DAMGO (Tyr-d-Ala-Gly-NMe-Phe-Gly-ol) on the excitability of trigeminal root ganglion (TRG) neurons, projecting onto the superficial layer of the cervical dorsal horn, by using the perforated-patch technique and to determine whether TRG neurons show the expression of mRNA or functional protein for micro-opioid receptors by using reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. TRG neurons projecting onto the superficial layer of the cervical dorsal horn were retrogradely labeled with Fluorogold (FG). The cell diameter of FG-labeled TRG neurons was small (<30 microm). Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the TRG neurons and isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement. Separation of the K(+) current components was achieved by the response to variation in the conditioning voltage. Two distinct K(+) current components, a transient (I(A)) and sustained (I(K)), were identified. DAMGO significantly increased I(A) by 57% (20 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by DAMGO administration (42%, 20 microM). The augmentation of both I(A) and I(K) was antagonized by a micro-opioid receptor antagonist, CTOP (d-Phe-Cys-Thr-d-Trp-Orn-Thr-Pen-Thr-NH(2)). Hyperpolarization of the membrane potential was elicited by DAMGO (20 microM) and the response was associated with a decrease in the input resistance. DAMGO induced hyperpolarization was blocked by CTOP. DAMGO-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by DAMGO application were observed. In the presence of BaCl(2), DAMGO evoked hyperpolarization with decreased resistance was observed. The firing rate of action potentials and the first spike duration induced by depolarizing step pulses were decreased in the presence of DAMGO. RT-PCR analysis demonstrated the expression of mRNA for micro-opioid receptors in the trigeminal ganglia. The micro-opioid receptor immunoreactivity was expressed in the small diameter FG-labeled TRG neurons. These results suggest that the activation of micro-opioid receptors inhibits the excitability of rat small diameter TRG neurons projecting on the superficial layer of the cervical dorsal horn and this inhibition is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modulation of nociceptive transmission in the trigeminal system, resulting in the functional activation of micro-opioid receptors, occurs at the level of small TRG cell bodies and/or their primary afferent terminals, which contribute to opioid analgesia in the trigeminal pain.

Suppression of epileptiform activity by GABA(B) receptors in wild type and weaver hippocampus 'in vitro'

Inhibition by GABA(B) receptors comprises activation of K(+) conductance and inhibition of Ca(2+) conductance, thereby reducing action potential dependent transmitter release and silencing neuronal activity. We compared epileptiform activity and its inhibition by the activation of GABA(B) receptors in homozygous weaver (wv/wv) and wild type (+/+) CA3 neurons disinhibited by GABA(A) receptor blockade. In wv/wv mice GABA(B) receptors have lost their ability to activate K(+) conductance (J. Neurosci. 18 (1998) 4001). Spontaneous synchronous burst discharges in elevated [K(+)](o) displayed only subtle differences in +/+ and wv/wv slices, except that the GABA(B) receptor agonist R-baclofen in low concentration (0.1 microM) strongly reduced the frequency of synchronous bursts in +/+ CA3 neurons, but not in wv/wv CA3 neurons. A high affinity GABA(B) antagonist, CGP55845A (0.5 microM) promoted the incidence of bursts in low [K(+)](o). Concentration dependence of the reduction of evoked EPSCs was identical in wv/wv and +/+ neurons (IC(50)=0.3 microM). Amplitudes of evoked IPSCs were reduced by 0.01 microM R-baclofen in +/+, but not in wv/wv CA3 neurons. The effect of the low concentration was abolished by Ba(2+), which is known to block Kir conductance. The data suggest that activation of Kir conductance is important for the control of GABA release by GABA(B) autoreceptors in the CA3 network. We conclude that the loss of a contribution of Kir conductance to GABA(B) receptor-mediated autoinhibition reduces the inclination towards spontaneous bursts of wv/wv CA3 pyramidal neurons.

Modulation of Chelidonii herba on GABA activated chloride current in rat PAG neurons

Modulation of Chelidonii herba on gamma-aminobutyric acid (GABA) activated chloride current in the acutely dissociated periaqueductal gray (PAG) neuron was studied by nystatin-perforated patch-clamp technique. High concentrations of Chelidonii herba elicited ion current, that was blocked by bicuculline. Low concentrations reduced the GABA activated current in PAG. Two types of inhibitory action of Chelidonii herba on GABA activated current have been implicated in PAG. One is the inhibitory action of Chelidonii herba on GABA was abolished by naltrexone and the other is that of Chelidonii herba was potentiated by naltrexone. In addition, all of two types of action of Chelidonii herba are linked to pertussis toxin-sensitive GTP-binding proteins. These results suggest that the inhibitory modulation of Chelidonii herba on GABA activated current via G-proteins in PAG neuron is an important analgesic mechanism.

Mutant mice as a model for cerebellar ataxia

Not later than two synapses after their arrival in the cerebellar cortex all excitatory afferent signals are subsequently transformed into inhibitory ones. Guaranteed by the exceedingly ordered and stereotyped synaptic arrangement of its cellular elements, the cerebellar cortex transmits this inhibitory result of cerebellar integration exclusively via Purkinje cells (PCs) in a precise temporal succession directly onto the target neurons of the deep cerebellar and vestibular nuclei. Thus the cerebellar cortex seems to produce a temporal pattern of inhibitory influence on these target neurons that modifies their excitatory action in such a way that an activation of muscle fibers occurs which progressively integrates the intended motion into the actual condition of the motoric inventory. In consequence, disturbances that affect this cerebellar inhibition will cause uncoordinated, decomposed and ataxic movements, commonly referred to as cerebellar ataxia. Electrophysiological investigations using different cerebellar mouse mutants have shown that alterations in the cerebellar inhibitory input in the target nuclei lead to diverse neuronal responses and to different consequences for the behavioural phenotype. A dependence between the reconstitution of inhibition and the behavioural outcome seems to exist. Obviously two different basic mechanisms are responsible for these observations: (1) ineffective inhibition on target neurons by surviving PCs; and (2) enhancement of intranuclear inhibition in the deep cerebellar and vestibular nuclei. Which of the two strategies evolves is dependent upon the composition of the residual cell types in the cerebellum and on the degree of PC input loss in a given area of the target nuclei. Motor behaviour seems to deteriorate under the first of these mechanisms whereas it may benefit from the second. This is substantiated by stereotaxic removal of the remaining PC input, which eliminates the influence of the first mechanism and is able to induce the second strategy. As a consequence, motor performance improves considerably. In this review, results leading to the above conclusions are presented and links forged to human cerebellar diseases.

Gamma-aminobutyric acid(B) autoreceptors in substantia nigra and neostriatum of the weaver mutant mouse

The weaver mutation causes cell loss in the center of the substantia nigra, pars compacta. We compared the depression of gamma-aminobutyric acid (GABA)(A) synaptic currents by the GABA(B) agonist R-baclofen in pars compacta neurons of weaver mice which were largely spared from cell degeneration and of wild-type mice. In weaver neurons the suppression of GABA(A) synaptic currents by R-baclofen was reduced compared to wild-type neurons. The EC(50) of R-baclofen was 6.3 times higher in weaver than in wild-type mice. In the neostriatum, which is not a target of the mutation, such a difference did not exist. We conclude that in the pars compacta the weaver mutation leads to a reduced presynaptic autoinhibition through GABA(B) receptors which may promote survival of a subset of weaver neurons in the pars compacta.

Is there a role for potassium channel openers in neuronal ion channel disorders?

Malfunction in ion channels, due to mutations in genes encoding channel proteins or the presence of autoantibodies, are increasing being implicated in causing disease conditions, termed channelopathies. Dysfunction of potassium (K(+)) channels has been associated with the pathophysiology of a number of neurological, as well as peripheral, disorders (e.g., episodic ataxia, epilepsy, neuromyotonia, Parkinson's disease, congenital deafness, long QT syndrome). K(+) channels, which demonstrate a high degree of diversity and ubiquity, are fundamental in the control of membrane depolarisation and cell excitability. A common feature of K(+) channelopathies is a reduction or loss of membrane potential repolarisation. The identification of K(+) channel subtype specific openers will allow the recovery of the mechanism(s) responsible for counteraction of uncontrolled cellular depolarisation. Synthetic agents that demonstrate K(+) channel opening properties are available for a variety of K(+) channel subtypes (e.g., K(ATP), BK(Ca), GIRK and M-channel). This study reviews the realistic therapeutic potential that may be gained in a broad spectrum of clinical conditions by K(+) channel openers. K(+) channel openers would therefore identify dysfunctional K(+) channel as therapeutic targets for clinical benefit, in addition being able to modulate normally functioning K(+) channels to gain clinical management of pathophysiological events irrespective of the cause.

The weaver mutation reverses the function of dopamine and GABA in mouse dopaminergic neurons

In the present study, we characterized the intrinsic electrophysiological properties and the membrane currents activated by dopamine (DA) D(2) and GABA(B) receptors in midbrain dopaminergic neurons, maintained in vitro in a slice preparation, from wild-type and homozygous weaver (wv/wv) mice. By using patch-clamp techniques, we found that membrane potential, apparent input resistance, and spontaneous firing of wv/wv dopaminergic neurons were similar to those of dopamine-containing cells recorded from nonaffected (+/+) animals. More interestingly, the wv/wv neurons were excited rather than inhibited by dopamine and the GABA(B) agonist baclofen. This neurotransmitter-mediated excitation was attributable to the activation of a G-protein-gated inward current that reversed polarity at a membrane potential of approximately -30 mV. We suggest that the altered behavior of the receptor-operated wv G-protein-gated inwardly rectifying K(+) channel 2 (GIRK2) might be related to the selective degeneration of the dopaminergic neurons. In addition, the wv GIRK2 would not only suppress the autoreceptor-mediated feedback inhibition of DA release but could also establish a feedforward mechanism of DA release in the terminal fields.

gamma-aminobutyric acid type B receptors are expressed and functional in mammalian cardiomyocytes

gamma-Hydroxybutyrate (GHB), an anesthetic adjuvant analog of gamma-aminobutyrate (GABA), depresses cell excitability in hippocampal neurons by inducing hyperpolarization through the activation of a prominent inwardly rectifying K(+) (Kir3) conductance. These GABA type B (GABA(B))-like effects are clearly shown at high concentrations of GHB corresponding to blood levels usually reached during anesthesia and are mimicked by the GABA(B) agonist baclofen. Recent studies of native GABA(B) receptors (GABA(B)Rs) have favored the concept that GHB is also a selective agonist. Furthermore, cloning has demonstrated that GABA(B)Rs assemble heteromeric complexes from the GABA(B)R1 and GABA(B)R2 subtypes and that these assemblies are activated by GHB. The surprisingly high tissue content, together with anti-ischemic and protective effects of GHB in the heart, raises the question of a possible influence of GABA(B) agonists on excitable cardiac cells. In the present study, we provide electrophysiological evidence that GHB activates an inwardly rectifying K(+) current in rat ventricular myocytes. This effect is mimicked by baclofen, reversibly inhibited by GABA(B) antagonists, and prevented by pertussis toxin pretreatment. Both GABA(B)R1 and GABA(B)R2 are detected in cardiomyocytes by Western blotting and are shown to coimmunoprecipitate. Laser scanning confocal microscopy discloses an even distribution of the two receptors in the sarcolemma and along the transverse tubular system. Hence, we conclude that GABA(B)Rs are distributed not only in neuronal tissues but also in the heart, where they can be activated and induce electrophysiological alterations through G-protein-coupled inward rectifier potassium channels.

Impaired inhibition of epileptiform activity by baclofen, but not by adenosine in the weaver hippocampus

The weaver defect results in a loss of baclofen- and adenosine-gated K+ conductance in the hippocampus of adult homozygous (wv/wv) mice. In addition, suppression of hippocampal epileptiform activity by baclofen is impaired (Jarolimek, W., Bäurle, J., Misgeld, U., 1998. Pore mutation in a G protein-gated inwardly rectifying K+ channel subunit causes loss of K+ dependent inhibition in weaver hippocampus. Journal of Neuroscience 18, 4001-4007). We used wv/wv and wild-type (+/+) mice to determine whether K+ conductance increases are essential for the suppression of epileptiform activity by R-baclofen and adenosine in disinhibited hippocampal slices. In wv/wv mice R-baclofen was less potent by two orders of magnitude in reducing the frequency of spontaneous synchronous burst discharges than in +/+ mice. Endogenous adenosine and adenosine A1 receptor agonists differed only slightly in their efficacy to inhibit spontaneous synchronous burst discharges in wv/wv and +/+ mice. The findings on adenosine A1 receptors suggest that the varied efficacy of R-baclofen in wv/wv and +/+ mice may not be explained solely on the basis of a loss of ligand-gated K+ conductance. Therefore, we investigated the affinity of GABA(B) receptors for the antagonist CGP55845A in wv/wv and +/+ hippocampi. Schild plot analysis revealed a K(D) for the GABA(B) antagonist CGP55845A 10 fold higher in wv/wv than in +/+ mice. The data suggest that an alteration of GABA(B) receptors could contribute to the reduced efficacy of R-baclofen to suppress hippocampal epileptiform activity in weaver mice, while the suppression by adenosine remains largely unaffected.

Cesium induces spontaneous epileptiform activity without changing extracellular potassium regulation in rat hippocampus

Cesium has been widely used to study the roles of the hyperpolarization-activated (I(h)) and inwardly rectifying potassium (K(IR)) channels in many neuronal and nonneuronal cell types. Recently, extracellular application of cesium has been shown to produce epileptiform activity in brain slices, but the mechanisms for this are not known. It has been proposed that cesium blocks the K(IR) in glia, resulting in an abnormal accumulation of potassium in the extracellular space and inducing epileptiform activity. This hypothesis has been tested in hippocampal slices and cultured hippocampal neurons using potassium-sensitive microelectrodes. In the present study, application of cesium produced spontaneous epileptiform discharges at physiological extracellular potassium concentration ([K(+)](o)) in the CA1 and CA3 regions of hippocampal slices. This epileptiform activity was not mimicked by increasing the [K(+)](o). The epileptiform discharges induced by cesium were not blocked by the N-methyl-D- aspartate (NMDA) receptor antagonist AP-5, but were blocked by the non-NMDA receptor antagonist CNQX. In the dentate gyrus, cesium induced the appearance of spontaneous nonsynaptic field bursts in 0 added calcium and 3 mM potassium. Moreover, cesium increased the frequency of field bursts already present. In contrast, ZD-7288, a specific I(h) blocker, did not cause spontaneous epileptiform activity in CA1 and CA3, nor did it affect the field bursts in the dentate gyrus, suggesting that cesium induced epileptiform activity is not directly related to blockade of the I(h). When potassium-sensitive microelectrodes were used to measure [K(+)](o), there was no significant increase in [K(+)](o) in CA1 and CA3 after cesium application. In the dentate gyrus, cesium did not change the baseline level of [K(+)](o) or the rate of [K(+)](o) clearance after the field bursts. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also produced cellular burst activity without significantly changing the resting membrane potential, which might indicate an increase in [K(+)](o). Our results suggest that cesium causes epileptiform activity by a mechanism unrelated to an alteration in [K(+)](o) regulation.

The weaver GIRK2 mutation leads to decreased levels of serum thyroid hormone: characterization of the effect on midbrain dopaminergic neuron survival

The selective neurodegenerative changes occurring in the weaver mutant cerebellum and midbrain are linked to a point mutation in an inward rectifying potassium channel (GIRK2). However, given that GIRK2 is widely expressed in the CNS, it is not understood why this mutation only leads to neuroanatomically selective and developmentally specific neuronal cell death. Here we show that the phenotype of the weaver mutant mouse includes hypothyroidism, which is associated with delays in somatic development and decreased expression of striatal transforming growth factor alpha (TGF-alpha). Since thyroid hormone has major effects on brain development, further studies were performed to address whether some of pathological changes detected the weaver mutant mouse are due to the reduced thyroid hormone levels. We observed that daily thyroid hormone replacement was able to stimulate somatic growth and restore TGF-alpha expression to wild-type levels, indicating that while these mice are responsive to thyroid hormone they possibly have a defect in the ability to regulate its release at the level of the hypothalamic pituitary axis. However, when we assessed whether thyroid hormone replacement could rescue midbrain dopaminergic neurons we found that this treatment accelerated rather than attenuated neurodegeneration. We did not observe that thyroid hormone was able to directly regulate expression of GIRK2 mRNA levels in the midbrain and therefore, speculate that the mechanism by which thyroid hormone accelerates midbrain dopaminergic neurodegeneration is by enhancing the maturation of the striatonigral inputs. In summary, we detected reduced levels of serum thyroid hormone in the weaver mutant mouse, which appears to be responsible for delays in somatic growth and the onset of neurodegenerative changes in the midbrain.

Role of heteromer formation in GABAB receptor function

Recently, GBR1, a seven-transmembrane domain protein with high affinity for gamma-aminobutyric acid (GABA)B receptor antagonists, was identified. Here, a GBR1-related protein, GBR2, was shown to be coexpressed with GBR1 in many brain regions and to interact with it through a short domain in the carboxyl-terminal cytoplasmic tail. Heterologously expressed GBR2 mediated inhibition of adenylyl cyclase; however, inwardly rectifying potassium channels were activated by GABAB receptor agonists only upon coexpression with GBR1 and GBR2. Thus, the interaction of these receptors appears to be crucial for important physiological effects of GABA and provides a mechanism in receptor signaling pathways that involve a heterotrimeric GTP-binding protein.

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

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