Intracellular Na+ inhibits voltage-dependent N-type Ca2+ channels by a G protein betagamma subunit-dependent mechanism

The modulation of emotional and social behaviors by oxytocin signaling in limbic network

Neuropeptides can exert volume modulation in neuronal networks, which account for a well-calibrated and fine-tuned regulation that depends on the sensory and behavioral contexts. For example, oxytocin (OT) and oxytocin receptor (OTR) trigger a signaling pattern encompassing intracellular cascades, synaptic plasticity, gene expression, and network regulation, that together function to increase the signal-to-noise ratio for sensory-dependent stress/threat and social responses. Activation of OTRs in emotional circuits within the limbic forebrain is necessary to acquire stress/threat responses. When emotional memories are retrieved, OTR-expressing cells act as gatekeepers of the threat response choice/discrimination. OT signaling has also been implicated in modulating social-exposure elicited responses in the neural circuits within the limbic forebrain. In this review, we describe the cellular and molecular mechanisms that underlie the neuromodulation by OT, and how OT signaling in specific neural circuits and cell populations mediate stress/threat and social behaviors. OT and downstream signaling cascades are heavily implicated in neuropsychiatric disorders characterized by emotional and social dysregulation. Thus, a mechanistic understanding of downstream cellular effects of OT in relevant cell types and neural circuits can help design effective intervention techniques for a variety of neuropsychiatric disorders.

The neuro-immune microenvironment of acupoints-initiation of acupuncture effectiveness

Acupuncture is a centuried and unfading treatment of traditional Chinese medicine, which has been proved to exert curative effects on various disorders. Numerous works have been put in to uncover the effective mechanisms of acupuncture. And the interdependent interaction between acupuncture and acupoint microenvironment is a crucial topic. As a benign minimally invasive stimulation, the insertion and manipulation of needle at acupoint could cause deformation of local connective tissue and secretion of various molecules, such as high mobility group box 1 and ATP. The molecules are secreted into extracellular space and bind to the corresponding receptors thus active NF-κB, MAPK, ERK pathways on mast cells, fibroblasts, keratinocytes, and monocytes/macrophages, among others. This is supposed to trigger following transcription and translation of immune factors and neural active substance, as well as promote the free ion movement (such as Ca²⁺ influx) and the expansion of blood vessels to recruit more immune cells to acupoint. Finally, acupuncture could enhance network connectivity of local microenvironment at acupoints. The earlier mentioned substances further act on a variety of receptors in local nerve endings, transmitting electrical and biochemical signals to the CNS, and giving full play to the acupuncture action. In conclusion, we portrayed a neuro-immune microenvironment network of acupoints that medicates the acupuncture action, and would lay a foundation for the systematic study of the complex network relationship of acupoints in the future.

Sodium Fluctuations in Astroglia and Their Potential Impact on Astrocyte Function

Astrocytes are the main cell type responsible for the regulation of brain homeostasis, including the maintenance of ion gradients and neurotransmitter clearance. These processes are tightly coupled to changes in the intracellular sodium (Na⁺) concentration. While activation of the sodium-potassium-ATPase (NKA) in response to an elevation of extracellular K⁺ may decrease intracellular Na⁺, the cotransport of transmitters, such as glutamate, together with Na⁺ results in an increase in astrocytic Na⁺. This increase in intracellular Na⁺ can modulate, for instance, metabolic downstream pathways. Thereby, astrocytes are capable to react on a fast time scale to surrounding neuronal activity via intracellular Na⁺ fluctuations and adjust energy production to the demand of their environment. Beside the well-documented conventional roles of Na⁺ signaling mainly mediated through changes in its electrochemical gradient, several recent studies have identified more atypical roles for Na⁺, including protein interactions leading to changes in their biochemical activity or Na⁺-dependent regulation of gene expression. In this review, we will address both the conventional as well as the atypical functions of astrocytic Na⁺ signaling, presenting the role of transporters and channels involved and their implications for physiological processes in the central nervous system (CNS). We will also discuss how these important functions are affected under pathological conditions, including stroke and migraine. We postulate that Na⁺ is an essential player not only in the maintenance of homeostatic processes but also as a messenger for the fast communication between neurons and astrocytes, adjusting the functional properties of various cellular interaction partners to the needs of the surrounding network.

Ionic signalling in astroglia beyond calcium

Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal excitability through regulation of extracellular ion concentrations, as well as assisting and modulating synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic actions are initiated and controlled by intercellular signalling of ions, including Ca²⁺ , Na⁺ , Cl^- , H⁺ and possibly K⁺ . This review summarises current knowledge on ionic signals mediated by the major monovalent ions, which occur in microdomains, as global events, or as propagating intercellular waves and thereby represent the substrate for astroglial excitability.

Effect of Electroacupuncture on 99mTc-Sodium Pertechnetate Uptake and Extracellular Fluid Free Molecules in the Stomach in Acupoint ST36 and ST39

Electroacupuncture (EA) is a therapeutic modality in which the electrical stimulation is integrated with concepts of acupuncture to treat diseases. This study was designed to evaluate the connection between the electro-acupuncture induced increase in Na^99mTcO4 uptake in the stomach wall, and the ionic molecule levels in the extracellular fluid in the acupoints. Wistar rats were treated by 2 or 100 Hz EA at Zusanli (ST 36) and Xiajuxu (ST 39) bilaterally for 60 minutes. The accumulation of Na^99mTcO4 in the gastric wall and the free ions, including Ca²⁺, K⁺, Na⁺, and Cl⁻, in the acupoints were measured every 60 minutes. The radioactivity uptake in the stomach was significantly increased during EA, reaching peak at 180 minutes after the EA. The concentration of extracellular ions was also significantly increased during EA. The Ca²⁺ level continued to rise until 60 minutes after EA, then started to decrease at 120 minutes post-EA. The results suggest this up-regulatory effect of EA on gastric activity might be triggered by the increase of the extracellular ion levels, this effect lasts longer than stimulating the release of transmembrane Ca²⁺ flow alone. This might aid in providing a better understanding of the long-lasting effect claimed in acupuncture treatment.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

The Epithelial Sodium Channel and the Processes of Wound Healing

The epithelial sodium channel (ENaC) mediates passive sodium transport across the apical membranes of sodium absorbing epithelia, like the distal nephron, the intestine, and the lung airways. Additionally, the channel has been involved in the transduction of mechanical stimuli, such as hydrostatic pressure, membrane stretch, and shear stress from fluid flow. Thus, in vascular endothelium, it participates in the control of the vascular tone via its activity both as a sodium channel and as a shear stress transducer. Rather recently, ENaC has been shown to participate in the processes of wound healing, a role that may also involve its activities as sodium transporter and as mechanotransducer. Its presence as the sole channel mediating sodium transport in many tissues and the diversity of its functions probably underlie the complexity of its regulation. This brief review describes some aspects of ENaC regulation, comments on evidence about ENaC participation in wound healing, and suggests possible regulatory mechanisms involved in this participation.

Voltage-Induced Ca²⁺ Release in Postganglionic Sympathetic Neurons in Adult Mice

Recent studies have provided evidence that depolarization in the absence of extracellular Ca²⁺ can trigger Ca²⁺ release from internal stores in a variety of neuron subtypes. Here we examine whether postganglionic sympathetic neurons are able to mobilize Ca²⁺ from intracellular stores in response to depolarization, independent of Ca²⁺ influx. We measured changes in cytosolic ΔF/F₀ in individual fluo-4 –loaded sympathetic ganglion neurons in response to maintained K⁺ depolarization in the presence (2 mM) and absence of extracellular Ca²⁺ ([Ca²⁺]_e). Progressive elevations in extracellular [K⁺]_e caused increasing membrane depolarizations that were of similar magnitude in 0 and 2 mM [Ca²⁺]_e. Peak amplitude of ΔF/F₀ transients in 2 mM [Ca²⁺]_e increased in a linear fashion as the membrane become more depolarized. Peak elevations of ΔF/F₀ in 0 mM [Ca²⁺]_e were ~5–10% of those evoked at the same membrane potential in 2 mM [Ca²⁺]_e and exhibited an inverse U-shaped dependence on voltage. Both the rise and decay of ΔF/F₀ transients in 0 mM [Ca²⁺]_e were slower than those of ΔF/F₀ transients evoked in 2 mM [Ca²⁺]_e. Rises in ΔF/F₀ evoked by high [K⁺]_e in the absence of extracellular Ca²⁺ were blocked by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺ ATPase, or the inositol 1,4,5-triphosphate (IP₃) receptor antagonists 2-aminoethoxydiphenyl borate and xestospongin C, but not by extracellular Cd²⁺, the dihydropyridine antagonist nifedipine, or by ryanodine at concentrations that caused depletion of ryanodine-sensitive Ca²⁺ stores. These results support the notion that postganglionic sympathetic neurons possess the ability to release Ca²⁺ from IP₃-sensitive internal stores in response to membrane depolarization, independent of Ca²⁺ influx.

Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation-chloride co-transporter NKCC1

The Na(+)-K(+)-2Cl(-) co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na(+), 1 K(+) and 2 Cl(-), it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca(2+)-dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Sodium dynamics: another key to astroglial excitability?

Astroglial excitability is largely mediated by fluctuations in intracellular ion concentrations. In addition to generally acknowledged Ca²⁺ excitability of astroglia, recent studies have demonstrated that neuronal activity triggers transient increases in the cytosolic Na⁺ concentration ([Na⁺](i)) in perisynaptic astrocytes. These [Na⁺](i) transients are controlled by multiple Na⁺-permeable channels and Na⁺-dependent transporters; spatiotemporally organized [Na⁺](i) dynamics in turn regulate diverse astroglial homeostatic responses such as metabolic/signaling utilization of lactate and glutamate, transmembrane transport of neurotransmitters and K⁺ buffering. In particular, near-membrane [Na⁺](i) transients determine the rate and the direction of the transmembrane transport of GABA and Ca²⁺. We discuss here the role of Na⁺ in the regulation of various systems that mediate fast bidirectional communication between neurones and glia at the single synapse level.

Stargazin modulates neuronal voltage-dependent Ca(2+) channel Ca(v)2.2 by a Gbetagamma-dependent mechanism

Loss of neuronal protein stargazin (gamma(2)) is associated with recurrent epileptic seizures and ataxia in mice. Initially, due to homology to the skeletal muscle calcium channel gamma(1) subunit, stargazin and other family members (gamma(3-8)) were classified as gamma subunits of neuronal voltage-gated calcium channels (such as Ca(V)2.1-Ca(V)2.3). Here, we report that stargazin interferes with G protein modulation of Ca(V)2.2 (N-type) channels expressed in Xenopus oocytes. Stargazin counteracted the Gbetagamma-induced inhibition of Ca(V)2.2 channel currents, caused either by coexpression of the Gbetagamma dimer or by activation of a G protein-coupled receptor. Expression of high doses of Gbetagamma overcame the effects of stargazin. High affinity Gbetagamma scavenger proteins m-cbetaARK and m-phosducin produced effects similar to stargazin. The effects of stargazin and m-cbetaARK were not additive, suggesting a common mechanism of action, and generally independent of the presence of the Ca(V)beta(3) subunit. However, in some cases, coexpression of Ca(V)beta(3) blunted the modulation by stargazin. Finally, the Gbetagamma-opposing action of stargazin was not unique to Ca(V)2.2, as stargazin also inhibited the Gbetagamma-mediated activation of the G protein-activated K(+) channel. Purified cytosolic C-terminal part of stargazin bound Gbetagamma in vitro. Our results suggest that the regulation by stargazin of biophysical properties of Ca(V)2.2 are not exerted by direct modulation of the channel but via a Gbetagamma-dependent mechanism.

Short communication: genetic ablation of L-type Ca2+ channels abolishes depolarization-induced Ca2+ release in arterial smooth muscle

RATIONALE

In arterial myocytes, membrane depolarization-induced Ca(2+) release (DICR) from the sarcoplasmic reticulum (SR) occurs through a metabotropic pathway that leads to inositol trisphosphate synthesis independently of extracellular Ca(2+) influx. Despite the fundamental functional relevance of DICR, its molecular bases are not well known.

OBJECTIVE

Biophysical and pharmacological data have suggested that L-type Ca(2+) channels could be the sensors coupling membrane depolarization to SR Ca(2+) release. This hypothesis was tested using smooth muscle-selective conditional Ca(v)1.2 knockout mice.

METHODS AND RESULTS

In aortic myocytes, the decrease of Ca(2+) channel density was paralleled by the disappearance of SR Ca(2+) release induced by either depolarization or Ca(2+) channel agonists. Ca(v)1.2 channel deficiency resulted in almost abolition of arterial ring contraction evoked by DICR. Ca(2+) channel-null cells showed unaltered caffeine-induced Ca(2+) release and contraction.

CONCLUSION

These data suggest that Ca(v)1.2 channels are indeed voltage sensors coupled to the metabolic cascade, leading to SR Ca(2+) release. These findings support a novel, ion-independent, functional role of L-type Ca(2+) channels linked to intracellular signaling pathways in vascular myocytes.

Regulation of podosome formation in macrophages by a splice variant of the sodium channel SCN8A

Voltage-gated sodium channels initiate electrical signaling in excitable cells such as muscle and neurons. They also are expressed in non-excitable cells such as macrophages and neoplastic cells. Previously, in macrophages, we demonstrated expression of SCN8A, the gene that encodes the channel NaV1.6, and intracellular localization of NaV1.6 to regions near F-actin bundles, particularly at areas of cell attachment. Here we show that a splice variant of NaV1.6 regulates cellular invasion through its effects on podosome and invadopodia formation in macrophages and melanoma cells. cDNA sequence analysis of SCN8A from THP-1 cells, a human monocyte-macrophage cell line, confirmed the expression of a full-length splice variant that lacks exon 18. Immunoelectron microscopy demonstrated NaV1.6-positive staining within the electron dense podosome rosette structure. Pharmacologic antagonism with tetrodotoxin (TTX) in differentiated THP-1 cells or absence of functional NaV1.6 through a naturally occurring mutation (med) in mouse peritoneal macrophages inhibited podosome formation. Agonist-mediated activation of the channel with veratridine caused release of sodium from cationic vesicular compartments, uptake by mitochondria, and mitochondrial calcium release through the Na/Ca exchanger. Invasion by differentiated THP-1 and HTB-66 cells, an invasive melanoma cell line, through extracellular matrix was inhibited by TTX. THP-1 invasion also was inhibited by small hairpin RNA knockdown of SCN8A. These results demonstrate that a variant of NaV1.6 participates in the control of podosome and invadopodia formation and suggest that intracellular sodium release mediated by NaV1.6 may regulate cellular invasion of macrophages and melanoma cells.

Expanding the neuron's calcium signaling repertoire: intracellular calcium release via voltage-induced PLC and IP3R activation

Neuronal calcium acts as a charge carrier during information processing and as a ubiquitous intracellular messenger. Calcium signals are fundamental to numerous aspects of neuronal development and plasticity. Specific and independent regulation of these vital cellular processes is achieved by a rich bouquet of different calcium signaling mechanisms within the neuron, which either can operate independently or may act in concert. This study demonstrates the existence of a novel calcium signaling mechanism by simultaneous patch clamping and calcium imaging from acutely isolated central neurons. These neurons possess a membrane voltage sensor that, independent of calcium influx, causes G-protein activation, which subsequently leads to calcium release from intracellular stores via phospholipase C and inositol 1,4,5-trisphosphate receptor activation. This allows neurons to monitor activity by intracellular calcium release without relying on calcium as the input signal and opens up new insights into intracellular signaling, developmental regulation, and information processing in neuronal compartments lacking calcium channels.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

Dominant role of betagamma subunits of G-proteins in oxytocin-evoked burst firing

Pulsatile neuropeptide secretion is associated with burst firing patterns; however, intracellular signaling cascades leading to bursts remain unclear. We explored mechanisms underlying burst firing in oxytocin (OT) neurons in the supraoptic nucleus in brain slices from lactating rats. Application of 10 pm OT for 30 min or progressively rising OT concentrations from 1 to 100 pm induced burst firing in OT neurons in patch-clamp recordings. Burst generation was blocked by OT antagonist and ionotropic glutamate receptor blockers or tetanus toxin. Blocking G-protein activation with suramin or intracellular GDP-beta-S, but not intracellularly administered antibody against the OT-receptor (OTR) C terminus, blocked bursts. Moreover, pretreatment of slices with pertussis toxin, an inhibitor of G(i/o)-proteins, did not block OT-evoked bursts, suggesting that G(i)/G(o) activation is unnecessary for burst generation. Thus, we further examined G alpha(q/11)-associated signaling pathways in OT-evoked bursts. Inhibition of phospholipase C or RhoA/Rho kinase did not block bursts. Activation of G betagamma subunits using myristoylated G betagamma-binding peptide (mSIRK) caused bursts, whereas intracellularly loaded antibody against G beta subunit blocked OT-evoked bursts. Blocking Src family kinase, but not phosphatidylinositol 3-kinase, occluded OT-evoked bursts. Similar to the effects of OT on EPSCs, mSIRK inhibited tonic EPSCs and elicited EPSC clustering. Finally, suckling caused dissociation of OTRs and G beta subunits from G alpha(q/11) subunits shown by coimmunoprecipitation and immunocytochemistry, supporting crucial roles for OTRs and G betagamma subunits in the milk-ejection reflex. We conclude that G betagamma subunits play a dominant role in burst firing evoked by applied OT or by suckling.

Intracellular monovalent ions as second messengers

It is generally accepted that electrochemical gradients of monovalent ions across the plasma membrane, created by the coupled function of pumps, carriers and channels, are involved in the maintenance of resting and action membrane potential, cell volume adjustment, intracellular Ca(2+ )handling and accumulation of glucose, amino acids, nucleotides and other precursors of macromolecular synthesis. In the present review, we summarize data showing that side-by-side with these classic functions, modulation of the intracellular concentration of monovalent ions in a physiologically reasonable range is sufficient to trigger numerous cellular responses, including changes in enzyme activity, gene expression, protein synthesis, cell proliferation and death. Importantly, the engagement of monovalent ions in regulation of the above-listed cellular responses occurs at steps upstream of Ca(2+) (i) and other key intermediates of intracellular signaling, which allows them to be considered as second messengers. With the exception of HCO (3) (-) -sensitive soluble adenylyl cyclase, the molecular origin of sensors involved in the function of monovalent ions as second messengers remains unknown.

Kinetic modeling of Na(+)-induced, Gbetagamma-dependent activation of G protein-gated K(+) channels

G protein-activated K(+)(GIRK) channels are activated by numerous neurotransmitters that act on Gi/o proteins, via a direct interaction with the Gbetagamma subunit of G proteins. In addition, GIRK channels are positively regulated by intracellular Na(+) via a direct interaction (fast pathway) and via a GGbetagamma-dependent mechanism (slow pathway). The slow modulation has been proposed to arise from the recently described phenomenon of Na(+)-induced reduction of affinity of interaction between GalphaGDP and Gbetagamma subunits of G proteins. In this scenario, elevated Na(+) enhances basal dissociation of G protein heterotrimers, elevating free cellular Gbetagamma and activating GIRK. However, it is not clear whether this hypothesis can account for the quantitative and kinetic aspects of the observed regulation. Here, we report the development of a quantitative model of slow, Na(+)-dependent, G protein-mediated activation of GIRK. Activity of GIRK1F137S channels, which are devoid of direct interaction with Na(+), was measured in excised membrane patches and used as an indicator of free GGbetagamma levels. The change in channel activity was used to calculate the Na(+)-dependent change in the affinity of G protein subunit interaction. Under a wide range of initial conditions, the model predicted that a relatively small decrease in the affinity of interaction of GalphaGDP and GGbetagamma (about twofold under most conditions) accounts for the twofold activation of GIRK induced by Na(+), in agreement with biochemical data published previously. The model also correctly described the slow time course of Na(+) effect and explained the previously observed enhancement of Na(+)-induced activation of GIRK by coexpressed Galphai3. This is the first quantitative model that describes the basal equilibrium between free and bound G protein subunits and its consequences on regulation of a GGbetagamma effector.

Adenylyl cyclase type-VIII activity is regulated by G(betagamma) subunits

The Ca2+-activated adenylyl cyclase type VIII (AC-VIII) has been implicated in several forms of neural plasticity, including drug addiction and learning and memory. It has not been clear whether Gi/o proteins and G-protein coupled receptors regulate the activity of AC-VIII. Here we show in intact mammalian cell system that AC-VIII is inhibited by mu-opioid receptor activation and that this inhibition is pertussis toxin sensitive. Moreover, we show that G(betagamma) subunits inhibit AC-VIII activity, while constitutively active alphai/o subunits do not. Different Gbeta isoforms varied in their efficacies, with Gbeta1gamma2 or Gbeta2gamma2 being more efficient than Gbeta3gamma2 and Gbeta4gamma2, while Gbeta5 (transfected with gamma2) had no effect. As for the Ggamma subunits, Gbeta1 inhibited AC-VIII activity in the presence of all gamma subunits tested except for gamma5 that had only a marginal activity. Moreover, cotransfection with proteins known to serve as scavengers of Gbetagamma dimers, or to reduce Gbetagamma plasma membrane anchorage, markedly attenuated the mu-opioid receptor-induced inhibition of AC-VIII. These results demonstrate that Gbetagamma (originating from agonist activation of these receptors) and probably not Galphai/o subunits are involved in the agonist inhibition of AC-VIII.

Presynaptic NMDA autoreceptors facilitate axon excitability: a new molecular target for the anticonvulsant gabapentin

Gabapentin is a drug with anticonvulsant and analgesic properties causing the reduction of neurotransmitter release. We show that one of the mechanisms implicated in this effect of gabapentin is the reduction of the axon excitability measured as an amplitude change of the presynaptic fibre volley (FV) in the CA1 area of rat hippocampal slices. Interestingly, we found that gabapentin-induced depression of FV is mimicked and occluded by NMDA receptor (NMDA-R) antagonists, indicating that these receptors are located presynaptically and are activated by ambient levels of glutamate. Conversely, NMDA application (20 microM, 10 min) elicits a reversible FV potentiation which is reduced by gabapentin. Both NMDA- and gabapentin-induced FV changes are partially explained by modifications in the firing threshold of individual fibres. Increasing [K(+)](o) does not mimic or occlude (at a concentration of 6.5 mM) the effect of NMDA on FV amplitude, which makes it unlikely that a rise in [K(+)](o) induced by NMDA receptor activation could indirectly participate in the potentiation of the FV. The NMDA-induced FV potentiation is independent of extracellular calcium presence but is completely inhibited in a low-Na(+) solution (50% reduction) or under NMDA channel block (high Mg(2+) or MK 801). These findings suggest that sodium entry through presynaptic NMDA-R channels facilitates axon excitability. The interaction of gabapentin with this newly described mechanism might contribute to its therapeutic benefits.