Ion channels in presynaptic nerve terminals and control of transmitter release

The inhibitory effects of nano-Ag on voltage-gated potassium currents of hippocampal CA1 neurons

The application of the nano-sized materials continues to grow at a rapid rate in the fields of medicine, biotechnology, and environmental technology. Voltage-gated potassium currents play a key role in excitable cellular viability and function, especially in the central nervous system. The aim of this study was to investigate the actions of silver nano-particles (nano-Ag) on voltage-activated potassium currents in hippocampal CA1 neurons using whole cell patch-clamp technique. The hydrodynamic mean diameter of nano-Ag (10(-5) g mL(-1) ) was 223.9 nm in artificial cerebrospinal fluid (ACSF). Both types, transient potassium (I(A) ) and delayed rectifier potassium (I(K) ) current amplitudes were inhibited by the nano-Ag (10(-5) g mL(-1) ). The nano-Ag particles produced a hyperpolarizing shift in the activation-voltage curve of I(K) and inactivation-voltage curve of I(A) and also delayed the recovery of I(A) from inactivation. The results suggest that nano-Ag may have potential to alter the excitability of neurons by depressing the potassium channels.

GABA(B)-mediated inhibition of multiple modes of glutamate release in the nucleus of the solitary tract

In the caudal portions of the solitary tract (ST) nucleus, primary sensory afferents fall into two broad classes based on the expression of transient receptor potential vanilloid type 1 (TRPV1) receptors. Both afferent classes (TRPV1+/-) have indistinguishable glutamate release mechanisms for ST-evoked excitatory postsynaptic currents (EPSCs). However, TRPV1+ terminals release additional glutamate from a unique, TRPV1-operated vesicle pool that is temperature sensitive and facilitated by ST activity to generate asynchronous EPSCs. This study tested whether presynaptic γ-aminobutyric acid (GABA)(B) receptors inhibit both the evoked and TRPV1-operated release mechanisms on second-order ST nucleus neurons. In horizontal slices, shocks activated single ST axons and evoked the time-invariant (latency jitter <200 μs), glutamatergic EPSCs, which identified second-order neurons. Gabazine eliminated GABA(A) responses in all recordings. The GABA(B) agonist baclofen inhibited the amplitude of ST-EPSCs from both TRPV1+ and TRPV1- afferents with a similar EC(50) (∼1.2 μM). In TTX, GABA(B) activation decreased miniature EPSC (mEPSC) rates but not amplitudes, suggesting presynaptic actions downstream from terminal excitability. With calcium entry through voltage-activated calcium channels blocked by cadmium, baclofen reduced mEPSC frequency, indicating that GABA(B) reduced vesicle release by TRPV1-dependent calcium entry. GABA(B) activation also reduced temperature-evoked increases in mEPSC frequency, which relies on TRPV1. Our studies indicate that GABA(B) G protein-coupled receptors are uniformly distributed across all ST primary afferent terminals and act at multiple stages of the excitation-release cascades to suppress both action potential-triggered and TRPV1-coupled glutamate transmission pathways. Moreover, the segregated release cascades within TRPV1+ ST primary afferents represent independent, potential targets for differential modulation.

Voltage-dependent P/Q-type calcium channels at the frog neuromuscular junction

It is well known that antagonists of N-type voltage-gated calcium channels inhibit the evoked quantal release of acetylcholine in amphibian neuromuscular synapses. This, however, does not exclude the functional expression of other types of voltage-gated calcium channels in these nerve terminals. Using immunocytochemistry, we detected the expression of the alpha1A subunit of P/Q-type calcium channels (that is otherwise typical of mammalian motor nerve endings) in the frog neuromuscular junction. In addition, we demonstrated that the P/Q-type channel blocker omega-agatoxin IVA (20 nM) reduced the action potential-induced calcium transient and significantly decreased both spontaneous and evoked mediator release. Our data indicates the functional expression of P/Q-type calcium channels in the frog motor nerve ending which participate in acetylcholine release.

Regulation of neuronal ion channels via P2Y receptors

Within the last 15 years, at least 8 different G protein-coupled P2Y receptors have been characterized. These mediate slow metabotropic effects of nucleotides in neurons as well as non-neural cells, as opposed to the fast ionotropic effects which are mediated by P2X receptors. One class of effector systems regulated by various G protein-coupled receptors are voltage-gated and ligand-gated ion channels. This review summarizes the current knowledge about the modulation of such neuronal ion channels via P2Y receptors. The regulated proteins include voltage-gated Ca²⁺ and K⁺ channels, as well as N-methyl-d-aspartate, vanilloid, and P2X receptors, and the regulating entities include most of the known P2Y receptor subtypes. The functional consequences of the modulation of ion channels by nucleotides acting at pre- or postsynaptic P2Y receptors are changes in the strength of synaptic transmission. Accordingly, ATP and related nucleotides may act not only as fast transmitters (via P2X receptors) in the nervous system, but also as neuromodulators (via P2Y receptors). Hence, nucleotides are as universal transmitters as, for instance, acetylcholine, glutamate, or γ-aminobutyric acid.

Regional differences in the effects of isoflurane on neurotransmitter release

Stimulus evoked neurotransmitter release requires that Na(+) channel-dependent nerve terminal depolarization be transduced into synaptic vesicle exocytosis. Inhaled anesthetics block presynaptic Na(+) channels and selectively inhibit glutamate over GABA release from isolated nerve terminals, indicating mechanistic differences between excitatory and inhibitory transmitter release. We compared the effects of isoflurane on depolarization-evoked [(3)H]glutamate and [(14)C]GABA release from isolated nerve terminals prepared from four regions of rat CNS evoked by 4-aminopyridine (4AP), veratridine (VTD), or elevated K(+). These mechanistically distinct secretegogues distinguished between Na(+) channel- and/or Ca(2+) channel-mediated presynaptic effects. Isoflurane completely inhibited total 4AP-evoked glutamate release (IC(50) = 0.42 ± 0.03 mM) more potently than GABA release (IC(50) = 0.56 ± 0.02 mM) from cerebral cortex (1.3-fold greater potency), hippocampus and striatum, but inhibited glutamate and GABA release from spinal cord terminals equipotently. Na(+) channel-specific VTD-evoked glutamate release from cortex was also significantly more sensitive to inhibition by isoflurane than was GABA release. Na(+) channel-independent K(+)-evoked release was insensitive to isoflurane at clinical concentrations in all four regions, consistent with a target upstream of Ca(2+) entry. Isoflurane inhibited Na(+) channel-mediated (tetrodotoxin-sensitive) 4AP-evoked glutamate release (IC(50) = 0.30 ± 0.03 mM) more potently than GABA release (IC(50) = 0.67 ± 0.04 mM) from cortex (2.2-fold greater potency). The magnitude of inhibition of Na(+) channel-mediated 4AP-evoked release by a single clinical concentration of isoflurane (0.35 mM) varied by region and transmitter: Inhibition of glutamate release from spinal cord was greater than from the three brain regions and greater than GABA release for each CNS region. These findings indicate that isoflurane selectively inhibits glutamate release compared to GABA release via Na(+) channel-mediated transduction in the four CNS regions tested, and that differences in presynaptic Na(+) channel involvement determine differences in anesthetic pharmacology.

Potential role of KCNQ/M-channels in regulating neuronal differentiation in mouse hippocampal and embryonic stem cell-derived neuronal cultures

Voltage-gated K(+) channels are key regulators of neuronal excitability, playing major roles in setting resting membrane potential, repolarizing the cell membrane after action potentials and affecting transmitter release. The M-type channel or M-channel is a unique voltage- and ligand-regulated K(+) channel. It is composed of the molecular counterparts KCNQ2 and KCNQ3 (also named Kv7.2 and Kv7.3) channels and expressed in the soma and dendrites of neurons. The present investigation examined the hypothesis that KCNQ2/3 channels played a regulatory role in neuronal differentiation and maturation. In cultured mouse embryonic stem (ES) cells undergoing neuronal differentiation and primary embryonic (E15-17) hippocampal cultures, KCNQ2 and KCNQ3 channels and underlying M-currents were identified. Blocking of KCNQ channels in these cells for 5 days using the specific channel blocker XE991 (10 μM) or linopirdine (30 μM) significantly decreased synaptophysin and syntaxin expression without affecting cell viability. Chronic KCNQ2/3 channel block reduced the expression of vesicular GABA transporter (v-GAT), but not vesicular glutamate transporter (v-GluT). Enhanced ERK1/2 phosphorylation was observed in XE991- and linopirdine-treated neural progenitor cells. In electrophysiological recordings, cells undergoing chronic block of KCNQ2/3 channels showed normal amplitude of mPSCs while the frequency of mPSCs was reduced. On the other hand, KCNQ channel opener N-Ethylmaleimide (NEM, 2 μM) increased mPSC frequency. Fluorescent imaging using fluorescent styryl-dye FM4-64 revealed that chronic blockade of KCNQ2/3 channels decreased endocytosis but facilitated exocytosis. These data indicate that KCNQ2/3 channels participate in the regulation of neuronal differentiation and show a tonic regulation on pre-synaptic transmitter release and recycling in developing neuronal cells.

Multiple targets of μ-opioid receptor-mediated presynaptic inhibition at primary afferent Aδ- and C-fibers

Agonists at μ-opioid receptors (MORs) represent the gold standard for the treatment of severe pain. A key element of opioid analgesia is the depression of nociceptive information at the first synaptic relay in spinal pain pathways. The underlying mechanisms are, however, largely unknown. In spinal cord slices with dorsal roots attached prepared from young rats, we determined the inhibitory effect of the selective MOR agonist [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on monosynaptic Aδ- and C-fiber-evoked EPSCs in lamina I neurons. DAMGO depressed presynaptically Aδ- and C-fiber-mediated responses, indicating that MORs are expressed on central terminals of both fiber types. We next addressed the mechanisms of presynaptic inhibition. The effect of DAMGO at both Aδ- and C-fiber terminals was mainly mediated by an inhibition of N-type voltage-dependent Ca(2+) channels (VDCCs), and to a lesser extent of P/Q-type VDCCs. Inhibition by DAMGO was not reduced by K(+) channel blockers. The rate of miniature EPSCs was reduced by DAMGO in a dose-dependent manner. The opioid also reduced Ca(2+)-dependent, ionomycin-induced EPSCs downstream of VDCCs. DAMGO had no effect on the kinetics of vesicle exocytosis in C-fiber terminals, but decreased the rate of unloading of Aδ-fiber boutons moderately, as revealed by two-photon imaging of styryl dye destaining. Together, these results suggest that binding of opioids to MORs reduces nociceptive signal transmission at central Aδ- and C-fiber synapses mainly by inhibition of presynaptic N-type VDCCs. P/Q-type VDCCs and the transmitter release machinery are targets of opioid action as well.

Diabetes induces early transient changes in the content of vesicular transporters and no major effects in neurotransmitter release in hippocampus and retina

Diabetes induces changes in neurotransmitter release in central nervous system, which depend on the type of neurotransmitter and region studied. In this study, we evaluated the effect of diabetes (two and eight weeks duration) on basal and evoked release of [(14)C]glutamate and [(3)H]GABA in hippocampal and retinal synaptosomes. We also analyzed the effect of diabetes on the protein content of vesicular glutamate and GABA transporters, VGluT-1, VGluT-2 and VGAT, and on the α(1A) subunit of P/Q type calcium channels, which are abundant in nerve terminals. The protein content of vesicular glutamate and GABA transporters, and of the α(1A) subunit, was differently affected by diabetes in hippocampal and retinal synaptosomes. The changes were more pronounced in the retina than in hippocampus. VGluT-1 and VGluT-2 content was not affected in hippocampus. Moreover, changes occurred early, at two weeks of diabetes, but after eight weeks almost no changes were detected, with the exception of VGAT in the retina. Regarding neurotransmitter release, no major changes were detected. After two weeks of diabetes, neurotransmitter release was similar to controls. After eight weeks of diabetes, the basal release of glutamate slightly increased in hippocampus and the evoked GABA release decreased in retina. In conclusion, diabetes induces early transient changes in the content of glutamate and/or GABA vesicular transporters, and on calcium channels subunit, in retinal or hippocampal synaptosomes, but only minor changes in the release of glutamate or GABA. These results point to the importance of diabetes-induced changes in neural tissues at the presynaptic level, which may underlie alterations in synaptic transmission, particularly if they become permanent during the later stages of the disease.

Small GTPase Rab11b regulates degradation of surface membrane L-type Cav1.2 channels

L-type Ca(2+) channels (LTCCs) play a critical role in Ca(2+)-dependent signaling processes in a variety of cell types. The number of functional LTCCs at the plasma membrane strongly influences the strength and duration of Ca(2+) signals. Recent studies demonstrated that endosomal trafficking provides a mechanism for dynamic changes in LTCC surface membrane density. The purpose of the current study was to determine whether the small GTPase Rab11b, a known regulator of endosomal recycling, impacts plasmalemmal expression of Ca(v)1.2 LTCCs. Disruption of endogenous Rab11b function with a dominant negative Rab11b S25N mutant led to a significant 64% increase in peak L-type Ba(2+) current (I(Ba,L)) in human embryonic kidney (HEK)293 cells. Short-hairpin RNA (shRNA)-mediated knockdown of Rab11b also significantly increased peak I(Ba,L) by 66% compared when with cells transfected with control shRNA, whereas knockdown of Rab11a did not impact I(Ba,L). Rab11b S25N led to a 1.7-fold increase in plasma membrane density of hemagglutinin epitope-tagged Ca(v)1.2 expressed in HEK293 cells. Cell surface biotinylation experiments demonstrated that Rab11b S25N does not significantly impact anterograde trafficking of LTCCs to the surface membrane but rather slows degradation of plasmalemmal Ca(v)1.2 channels. We further demonstrated Rab11b expression in ventricular myocardium and showed that Rab11b S25N significantly increases peak I(Ba,L) by 98% in neonatal mouse cardiac myocytes. These findings reveal a novel role for Rab11b in limiting, rather than promoting, the plasma membrane expression of Ca(v)1.2 LTCCs in contrast to its effects on other ion channels including human ether-a-go-go-related gene (hERG) K(+) channels and cystic fibrosis transmembrane conductance regulator. This suggests Rab11b differentially regulates the trafficking of distinct cargo and extends our understanding of how endosomal transport impacts the functional expression of LTCCs.

Localization of Kv1.3 channels in presynaptic terminals of brainstem auditory neurons

Elimination of the Kv1.3 voltage-dependent potassium channel gene produces striking changes in the function of the olfactory bulb, raising the possibility that this channel also influences other sensory systems. We have examined the cellular and subcellular localization of Kv1.3 in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, a nucleus in which neurons fire at high rates with high temporal precision. A clear gradient of Kv1.3 immunostaining along the lateral to medial tonotopic axis of the MNTB was detected. Highest levels were found in the lateral region of the MNTB, which corresponds to neurons that respond selectively to low-frequency auditory stimuli. Previous studies have demonstrated that MNTB neurons and their afferent inputs from the cochlear nucleus express three other members of the Kv1 family, Kv1.1, Kv1.2, and Kv1.6. Nevertheless, confocal microscopy of MNTB sections coimmunostained for Kv1.3 with these subunits revealed that the distribution of Kv1.3 differed significantly from other Kv1 family subunits. In particular, no axonal staining of Kv1.3 was detected, and most prominent labeling was in structures surrounding the somata of the principal neurons, suggesting specific localization to the large calyx of Held presynaptic endings that envelop the principal cells. The presence of Kv1.3 in presynaptic terminals was confirmed by coimmunolocalization with the synaptic markers synaptophysin, syntaxin, and synaptotagmin and by immunogold electron microscopy. Kv1.3 immunogold particles in the terminals were arrayed along the plasma membrane and on internal vesicular structures. To confirm these patterns of staining, we carried out immunolabeling on sections from Kv1.3(-/-) mice. No immunoreactivity could be detected in Kv1.3(-/-) mice either at the light level or in immunogold experiments. The finding of a tonotopic gradient in presynaptic terminals suggests that Kv1.3 may regulate neurotransmitter release differentially in neurons that respond to different frequencies of sound.

Control of neurotransmitter release: From Ca2+ to voltage dependent G-protein coupled receptors

This review discusses two theories that try to explain mechanisms of control of neurotransmitter release in fast synapses: the Ca(2+) hypothesis and the Ca(2+) voltage hypothesis. The review summarizes experimental results that are incompatible with predictions from the Ca(2+) hypothesis and concludes that Ca(2+) is involved in the control of the amount of release but not in the control of the time course of evoked release, i.e., initiation and termination of evoked release. Results summarizing direct effects of changes in membrane potential on the release machinery are then presented. These changes in membrane potential affect the affinity (for the transmitter) of presynaptic autoinhibitory G-protein coupled receptors (GPCRs). The voltage dependence of these GPCRs and their pivotal role in determining the time course of evoked release is discussed.

Optical mapping of release properties in synapses

Synapses are important functional units that determine how information flows through the brain. Understanding their biophysical properties and the molecules that underpin them is an important goal of cellular neuroscience. Thus, it is of interest to develop protocols that allow easy measurement of synaptic parameters in model systems that permit molecular manipulations. Here, we used a sensitive and high-time resolution optical approach that allowed us to characterize two functional parameters critical to presynaptic efficacy: vesicle fusion probability (Pv) and readily-releasable pool size (RRP). We implemented two different approaches to determine the RRP size that were in broad agreement: depletion of the RRP by high-frequency stimulation and saturation of the calcium sensor during single action potential stimuli. Our methods are based on reporters that provide a robust, quantitative, purely presynaptic readout and present a new avenue to study molecules that affect synaptic vesicle exocytosis.

Roles of KChIP1 in the regulation of GABA-mediated transmission and behavioral anxiety

K⁺ channel interacting protein 1 (KChIP1) is a neuronal calcium sensor (NCS) protein that interacts with multiple intracellular molecules. Its physiological function, however, remains largely unknown. We report that KChIP1 is predominantly expressed at GABAergic synapses of a subset of parvalbumin-positive neurons in the brain. Forced expression of KChIP1 in cultured hippocampal neurons increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), reduced paired pulse facilitation of autaptic IPSCs, and decreases potassium current density. Furthermore, genetic ablation of KChIP1 potentiated potassium current density in neurons and caused a robust enhancement of anxiety-like behavior in mice. Our study suggests that KChIP1 is a synaptic protein that regulates behavioral anxiety by modulating inhibitory synaptic transmission, and drugs that act on KChIP1 may help to treat patients with mood disorders including anxiety.

Molecular diversity of spider venom

Spider venom, a factor that has played a decisive role in the evolution of one of the most successful groups of living organisms, is reviewed. Unique molecular diversity of venom components including substances of variable structure (from simple low molecular weight compounds to large multidomain proteins) with different functions is considered. Special attention is given to the structure, properties, and biosynthesis of toxins of polypeptide nature.

Regional differences in nerve terminal Na+ channel subtype expression and Na+ channel-dependent glutamate and GABA release in rat CNS

We tested the hypothesis that expression of pre-synaptic voltage-gated sodium channel (Na(v)) subtypes coupled to neurotransmitter release differs between transmitter types and CNS regions in a nerve terminal-specific manner. Na(v) coupling to transmitter release was determined by measuring the sensitivity of 4-aminopyridine (4AP)-evoked [(3)H]glutamate and [(14)C]GABA release to the specific Na(v) blocker tetrodotoxin (TTX) for nerve terminals isolated from rat cerebral cortex, hippocampus, striatum and spinal cord. Expression of various Na(v) subtypes was measured by immunoblotting using subtype-specific antibodies. Potencies of TTX for inhibition of glutamate and GABA release varied between CNS regions. However, the efficacies of TTX for inhibition of 4AP-evoked glutamate release were greater than for inhibition of GABA release in all regions except spinal cord. The relative nerve terminal expression of total Na(v) subtypes as well as of specific subtypes varied considerably between CNS regions. The region-specific potencies of TTX for inhibition of 4AP-evoked glutamate release correlated with greater relative expression of total nerve terminal Na(v) and Na(v)1.2. Nerve terminal-specific differences in the expression of specific Na(v) subtypes contribute to transmitter-specific and regional differences in pharmacological sensitivities of transmitter release.

Apamin reduces neuromuscular transmission by activating inhibitory muscarinic M(2) receptors on motor nerve terminals

This study was undertaken to investigate the mechanism by which the toxin from the bee venom, apamin, might exert beneficial effects in patients suffering from myotonic dystrophy. The effects of apamin were compared with those produced by another potassium channel blocker, 4-aminopyridine, on rat hemidiaphragm preparations stimulated at a 100 Hz frequency via the phrenic nerve. Apamin and 4-aminopyridine increased nerve-evoked tetanic fade without changing the maximal tetanic tension. The inhibitory effect of apamin was mimicked by acetylcholine. In contrast with apamin, 4-aminopyridine increased the amplitude of muscle contractions induced by nerve stimulation at 0.2 Hz frequency. All these compounds were devoid of effect when diaphragm muscle fibres were stimulated directly in the presence of the neuromuscular blocker, D-tubocurarine. The muscarinic M(2) receptor antagonist, methoctramine, prevented the inhibitory effects of both apamin and acetylcholine. Blockade of presynaptic facilitatory muscarinic M(1) and nicotinic receptors respectively with pirenzepine and hexamethonium increased apamin-induced tetanic fade. Data suggest that apamin inhibits neuromuscular transmission by a mechanism independent of the blockade of Ca(2+)-activated K(+) channels, which might involve the activation of inhibitory muscarinic M(2) receptors on motor nerve terminals. Such a mechanism may be the origin of the beneficial effect of apamin controlling muscle excitability in patients suffering from myotonic diseases.

Extrinsic afferent nerve sensitivity and enteric neurotransmission in murine jejunum in vitro

Enteric and extrinsic sensory neurons respond to similar stimuli. Thus they may be activated in series or in parallel. Because signal transmission via synapses or mediator release would depend on calcium, we investigated its role for extrinsic afferent sensitivity to chemical and mechanical stimulation. Extracellular multiunit afferent recordings were made in vitro from paravascular nerve bundles supplying the mouse jejunum. Intraluminal pressure and afferent nerve responses were recorded under control conditions and under four conditions designed to interfere with enteric neurotransmission. We found that phasic intestinal contractions ceased after switching perfusion to Ca(2+)-free buffer with or without a purinergic P2 receptor antagonist, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) or cadmium (blocking all Ca(2+)-channels) but not following omega-conotoxin GVIA (N-type Ca(2+)-channel blocker). Luminal HCl (pH 2) and 5-HT (500 microM) evoked peak firing of 17 +/- 4 impulses per second (imp/s) (n = 10) and 21 +/- 4 imp/s (n = 13) under control conditions. These responses were reduced to 4 +/- 2 imp/s and 5 +/- 2 imp/s by cadmium (n = 7, P < 0.05), to 7 +/- 2 imp/s and 6 +/- 1 imp/s by Ca(2+)-free perfusion (n = 6, P < 0.05), and to 3 +/- 1 imp/s and 4 +/- 1 imp/s by Ca(2+)-free perfusion with PPADS (n = 6, P < 0.05). Responses were unchanged by omega-conotoxin GVIA. Mechanical ramp distension of the intestinal segment to 60 cmH(2)O was not altered by any of the experimental conditions. We concluded that HCl and 5-HT activate extrinsic afferents via a calcium-dependent mechanism, which is unlikely to involve enteric neurons carrying N-type calcium channels. Extrinsic mechanosensitivity is independent of enteric neurotransmission. It appears that cross talk from the enteric to the extrinsic nervous system does not mediate extrinsic afferent sensitivity.

Bestrophin 2: an anion channel associated with neurogenesis in chemosensory systems

The chemosensory neuroepithelia of the vertebrate olfactory system share a life-long ability to regenerate. Novel neurons proliferate from basal stem cells that continuously replace old or damaged sensory neurons. The sensory neurons of the mouse and rat olfactory system specifically express bestrophin 2, a member of the bestrophin family of calcium-activated chloride channels. This channel was recently proposed to operate as a transduction channel in olfactory sensory cilia. We raised a polyclonal antibody against bestrophin 2 and characterized the expression pattern of this protein in the mouse main olfactory epithelium, septal organ of Masera, and vomeronasal organ. Comparison with the maturation markers growth-associated protein 43 and olfactory marker protein revealed that bestrophin 2 was expressed in developing sensory neurons of all chemosensory neuroepithelia, but was restricted to proximal cilia in mature sensory neurons. Our results suggest that bestrophin 2 plays a critical role during differentiation and growth of axons and cilia. In mature olfactory receptor neurons, it appears to support growth and function of sensory cilia.

SP protects cerebellar granule cells against beta-amyloid-induced apoptosis by down-regulation and reduced activity of Kv4 potassium channels

The tachykinin endecapeptide substance P (SP) has been demonstrated to exert a functional role in neurodegenerative disorders, including Alzheimer's disease (AD). Aim of the present study was to evaluate the SP neuroprotective potential against apoptosis induced by the neurotoxic beta-amyloid peptide (A beta) in cultured rat cerebellar granule cells (CGCs). We found that SP protects CGCs against both A beta(25-35)- and A beta(1-42)-induced apoptotic CGCs death as revealed by live/dead cell assay, Hoechst staining and caspase(s)-induced PARP-1 cleavage, through an Akt-dependent mechanism. Since in CGCs the fast inactivating or A-type K(+) current (I(KA)) was potentiated by A beta treatment through up-regulation of Kv4 subunits, we investigated whether I(KA) and the related potassium channel subunits could be involved in the SP anti-apoptotic activity. Patch-clamp experiments showed that the A beta-induced increase of I(KA) current amplitude was reversed by SP treatment. In addition, as revealed by Western blot analysis and immunofluorescence studies, SP prevented the up-regulation of Kv4.2 and Kv4.3 channel subunits expression. These results indicate that SP plays a role in the regulation of voltage-gated potassium channels in A beta-mediated neuronal death and may represent a new approach in the understanding and treatment of AD.

Sodium channels and the synaptic mechanisms of inhaled anaesthetics

General anaesthetics act in an agent-specific manner on synaptic transmission in the central nervous system by enhancing inhibitory transmission and reducing excitatory transmission. The synaptic mechanisms of general anaesthetics involve both presynaptic effects on transmitter release and postsynaptic effects on receptor function. The halogenated volatile anaesthetics inhibit neuronal voltage-gated Na(+) channels at clinical concentrations. Reductions in neurotransmitter release by volatile anaesthetics involve inhibition of presynaptic action potentials as a result of Na(+) channel blockade. Although voltage-gated ion channels have been assumed to be insensitive to general anaesthetics, it is now evident that clinical concentrations of volatile anaesthetics inhibit Na(+) channels in isolated rat nerve terminals and neurons, as well as heterologously expressed mammalian Na(+) channel alpha subunits. Voltage-gated Na(+) channels have emerged as promising targets for some of the effects of the inhaled anaesthetics. Knowledge of the synaptic mechanisms of general anaesthetics is essential for optimization of anaesthetic techniques for advanced surgical procedures and for the development of improved anaesthetics.

Voltage-gated potassium channel as a facilitator of exocytosis

Voltage-gated ion channels are well characterized for their function in excitability signals. Accumulating studies, however, have established an ion-independent function for the major classes of ion channels in cellular signaling. During the last few years we established a novel role for Kv2.1, a voltage-gated potassium (Kv) channel, classically known for its role of repolarizing the membrane potential, in facilitation of exocytosis. Kv2.1 induces facilitation of depolarization-induced release through its direct interaction with syntaxin, a protein component of the exocytotic machinery, independently of the potassium ion flow through the channel's pore. Here, we review our recent studies, further characterize the phenomena (using chromaffin cells and carbon fiber amperometry), and suggest plausible mechanisms that can underlie this facilitation of release.

Plasticity in glutamatergic NTS neurotransmission

Changes in the physiological state of an animal or human can result in alterations in the cardiovascular and respiratory system in order to maintain homeostasis. Accordingly, the cardiovascular and respiratory systems are not static but readily adapt under a variety of circumstances. The same can be said for the brainstem circuits that control these systems. The nucleus tractus solitarius (NTS) is the central integration site of baroreceptor and chemoreceptor sensory afferent fibers. This central nucleus, and in particular the synapse between the sensory afferent and second-order NTS cell, possesses a remarkable degree of plasticity in response to a variety of stimuli, both acute and chronic. This brief review is intended to describe the plasticity observed in the NTS as well as the locus and mechanisms as they are currently understood. The functional consequence of NTS plasticity is also discussed.

Energetics of the cleft closing transition and the role of electrostatic interactions in conformational rearrangements of the glutamate receptor ligand binding domain

The ionotropic glutamate receptors are localized in the pre- and postsynaptic membrane of neurons in the brain. Activation by the principal excitatory neurotransmitter glutamate allows the ligand binding domain to change conformation, communicating opening of the channel for ion conduction. The free energy of the GluR2 S1S2 ligand binding domain (S1S2) closure transition was computed using a combination of thermodynamic integration and umbrella sampling modeling methods. A path that involves lowering the charge on E705 was chosen to clarify the role of this binding site residue. A continuum electrostatics approach in S1S2 is used to show E705, located in the ligand binding cleft, stabilizes the closed conformation of S1S2 via direct interactions with other protein residues, not through the ligand. In the closed conformation, in the absence of a ligand, S1S2 is somewhat more closed than what has been reported in X-ray structures. A semiopen conformation has been identified which is characterized by disruption of a single cross-cleft interaction and differs only slightly in energy from the fully closed S1S2. The fully open S1S2 conformation exhibits a wide energy well and shares structural similarity with the apo S1S2 crystal structure. Hybrid continuum electrostatics/MD calculations along the chosen closure transition pathway reveal solvation energies, and electrostatic interaction energies between two lobes of the protein increase the relative energetic difference between the open and closed conformational states. By analyzing the role of several cross-cleft contacts as well as other binding site residues, we demonstrate how S1S2 interactions facilitate formation of the closed conformation of the GluR2 ligand binding domain.

Neuromuscular synaptic function in mice lacking major subsets of gangliosides

Gangliosides are a family of sialylated glycosphingolipids enriched in the outer leaflet of neuronal membranes, in particular at synapses. Therefore, they have been hypothesized to play a functional role in synaptic transmission. We have measured in detail the electrophysiological parameters of synaptic transmission at the neuromuscular junction (NMJ) ex vivo of a GD3-synthase knockout mouse, expressing only the O- and a-series gangliosides, as well as of a GM2/GD2-synthase*GD3-synthase double-knockout (dKO) mouse, lacking all gangliosides except GM3. No major synaptic deficits were found in either null-mutant. However, some extra degree of rundown of acetylcholine release at high intensity use was present at the dKO NMJ and a temperature-specific increase in acetylcholine release at 35 degrees C was observed in GD3-synthase knockout NMJs, compared with wild-type. These results indicate that synaptic transmission at the NMJ is not crucially dependent on the particular presence of most ganglioside family members and remains largely intact in the sole presence of GM3 ganglioside. Rather, presynaptic gangliosides appear to play a modulating role in temperature- and use-dependent fine-tuning of transmitter output.

Adenosine A1 receptors inhibit GABAergic transmission in rat tuberomammillary nucleus neurons

The adenosinergic modulation of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) was investigated in mechanically dissociated rat tuberomammillary nucleus (TMN) neurons using a conventional whole-cell patch clamp technique. Adenosine (100 microM) reversibly decreased mIPSC frequency without affecting the current amplitude, indicating that adenosine acts presynaptically to decrease the probability of spontaneous GABA release. The adenosine action on GABAergic mIPSC frequency was completely blocked by 1 microM DPCPX, a selective A(1) receptor antagonist, and mimicked by 1 microM CPA, a selective A(1) receptor agonist. This suggests that presynaptic A(1) receptors were responsible for the adenosine-mediated inhibition of GABAergic mIPSC frequency. CPA still decreased GABAergic mIPSC frequency even either in the presence of 200 microM Cd(2+), a general voltage-dependent Ca(2+) channel blocker, or in the Ca(2+)-free external solution. However, the inhibitory effect of CPA on GABAergic mIPSC frequency was completely occluded by 1 mM Ba(2+), a G-protein coupled inwardly rectifying K(+) (GIRK) channel blocker. In addition, the CPA-induced decrease in mIPSC frequency was completely occluded by either 100 microM SQ22536, an adenylyl cyclase (AC) inhibitor, or 1 muM KT5720, a specific protein kinase A (PKA) inhibitor. The results suggest that the activation of presynaptic A(1) receptors decreases spontaneous GABAergic transmission onto TMN neurons via the modulation of GIRK channels as well as the AC/cAMP/PKA signal transduction pathway. This adenosine A(1) receptor-mediated modulation of GABAergic transmission onto TMN neurons may play an important role in the fine modulation of the excitability of TMN histaminergic neurons as well as the regulation of sleep-wakefulness.

Presynaptic melanocortin-4 receptors on vagal afferent fibers modulate the excitability of rat nucleus tractus solitarius neurons

The nucleus tractus solitarius (NTS) integrates visceral sensory signals with information from the forebrain to control homeostatic functions, including food intake. Melanocortin 3/4 receptor (MC3/4R) ligands administered directly to the caudal brainstem powerfully modulate meal size but not frequency, suggesting the enhancement of visceral satiety signals. Using whole-cell recordings from rat brainstem slices, we examined the effects of melanocortin ligands, alpha-melanocyte-stimulating hormone (alphaMSH) and melanotan II (MTII), on EPSC in NTS neurons. Thirty-two percent of NTS neurons responded to perfusion with MTII or alphaMSH with either an increase (24%) or a decrease (8%) in the frequency, but not amplitude, of spontaneous EPSCs; the effects of MTII were abolished by pretreatment with SHU9119. After surgical vagal deafferentation, only four of 34 (9%) NTS neurons responded to MTII with an increase in EPSC frequency. When EPSCs were evoked by electrical stimulation of the tractus solitarius in Krebs' solution with 2.4 mm Ca(2+)(e), alphaMSH and MTII increased the amplitude in six of the 28 neurons tested, decreased amplitude in 14 with no effect in the remaining eight neurons. In four of six neurons unresponsive to MTII, decreasing Ca(2+)(e) levels to 1.5 mM uncovered an excitatory effect of MTII on EPSC amplitude. Reverse transcription-PCR analysis revealed the presence of MC4R, but not MC3R, in nodose ganglia. These results show that MC4R signaling leads mainly to presynaptic modulation of glutamatergic synaptic transmission and suggest that melanocortinergic-induced decrease of food intake may occur via enhancement of vagal afferent satiation signals from the gastrointestinal tract.

Direct interaction of endogenous Kv channels with syntaxin enhances exocytosis by neuroendocrine cells

K⁺ efflux through voltage-gated K⁺ (Kv) channels can attenuate the release of neurotransmitters, neuropeptides and hormones by hyperpolarizing the membrane potential and attenuating Ca²⁺ influx. Notably, direct interaction between Kv2.1 channels overexpressed in PC12 cells and syntaxin has recently been shown to facilitate dense core vesicle (DCV)-mediated release. Here, we focus on endogenous Kv2.1 channels and show that disruption of their interaction with native syntaxin after ATP-dependent priming of the vesicles by Kv2.1 syntaxin–binding peptides inhibits Ca²⁺ -triggered exocytosis of DCVs from cracked PC12 cells in a specific and dose-dependent manner. The inhibition cannot simply be explained by the impairment of the interaction of syntaxin with its SNARE cognates. Thus, direct association between endogenous Kv2.1 and syntaxin enhances exocytosis and in combination with the Kv2.1 inhibitory effect to hyperpolarize the membrane potential, could contribute to the known activity dependence of DCV release in neuroendocrine cells and in dendrites where Kv2.1 commonly expresses and influences release.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

G-protein-coupled GABAB receptors inhibit Ca2+ channels and modulate transmitter release in descending turtle spinal cord terminal synapsing motoneurons

Presynaptic gamma-aminobutyric acid type B receptors (GABA(B)Rs) regulate transmitter release at many central synapses by inhibiting Ca(2+) channels. However, the mechanisms by which GABA(B)Rs modulate neurotransmission at descending terminals synapsing on motoneurons in the spinal cord remain unexplored. To address this issue, we characterized the effects of baclofen, an agonist of GABA(B)Rs, on the monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in motoneurons by stimulation of the dorsolateral funiculus (DLF) terminals in a slice preparation from the turtle spinal cord. We found that baclofen depressed neurotransmission in a dose-dependent manner (IC(50) of approximately 2 microM). The membrane time constant of the motoneurons did not change, whereas the amplitude ratio of the evoked EPSPs in response to a paired pulse was altered in the presence of the drug, suggesting a presynaptic mechanism. Likewise, the use of N- and P/Q-type Ca(2+) channel antagonists (omega-conotoxin GVIA and omega-agatoxin IVA, respectively) also depressed EPSPs significantly. Therefore, these channels are likely involved in the Ca(2+) influx that triggers transmitter release from DLF terminals. To determine whether the N and P/Q channels were regulated by GABA(B)R activation, we analyzed the action of the toxins in the presence of baclofen. Interestingly, baclofen occluded omega-conotoxin GVIA action by approximately 50% without affecting omega-agatoxin IVA inhibition, indicating that the N-type channels are the target of GABA(B)Rs. Lastly, the mechanism underlying this effect was further assessed by inhibiting G-proteins with N-ethylmaleimide (NEM). Our data show that EPSP depression caused by baclofen was prevented by NEM, suggesting that GABA(B)Rs inhibit N-type channels via G-protein activation.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Differential role of N-type calcium channel splice isoforms in pain

N-type calcium channels are essential mediators of spinal nociceptive transmission. The core subunit of the N-type channel is encoded by a single gene, and multiple N-type channel isoforms can be generated by alternate splicing. In particular, cell-specific inclusion of an alternatively spliced exon 37a generates a novel form of the N-type channel that is highly enriched in nociceptive neurons and, as we show here, downregulated in a neuropathic pain model. Splice isoform-specific small interfering RNA silencing in vivo reveals that channels containing exon 37a are specifically required for mediating basal thermal nociception and for developing thermal and mechanical hyperalgesia during inflammatory and neuropathic pain. In contrast, both N-type channel isoforms (e37a- and e37b-containing) contribute to tactile neuropathic allodynia. Hence, exon 37a acts as a molecular switch that tailors the channels toward specific roles in pain.

Effects of various K+ channel blockers on spontaneous glycine release at rat spinal neurons

Molecular biology approaches have identified more than 70 different K+ channel genes that assemble to form diverse functional classes of K+ channels. Although functional K+ channels are present within presynaptic nerve endings, direct studies of their precise identity and function have been generally limited to large, specialized presynaptic terminals such as basket cell terminals and Calyx of Held. In the present study, therefore, we investigated the functional K+ channel subtypes on the small glycinergic nerve endings (< 1 microm diameter) projecting to spinal sacral dorsal commissural nucleus (SDCN) neurons. In the presence of TTX, whole-cell patch recording of mIPSCs was made from mechanically dispersed SDCN neurons in which functional nerve endings remain attached. Glycinergic responses were isolated by blocking glutamatergic and GABAergic inputs with CNQX, AP5 and bicuculline. The K+ channel blockers, 4-AP, TEA, delta-dendrotoxin, margatoxin, iberiotoxin, charybdotoxin and apamin, significantly increased 'spontaneous' mIPSC frequency without affecting mIPSC amplitude. The results suggest the existence of the following K+ channel subtypes on glycinergic nerve endings that are involved in regulating 'spontaneous' glycine release (mIPSCs): the Shaker-related K+ channels Kv1.1, Kv1.2, Kv1.3, Kv1.6 and Kv1.7 and the intracellular Ca2+ -sensitive K+ channels BKCa, IKCa and SKCa. Ca2+ channel blockers by themselves, including L-type (nifedipine), P/Q-type (omega-agatoxin IVA, AgTX) and N-type (omega-conotoxin GVIA, CgTX), did not alter the 'spontaneous' mIPSC frequency or amplitude, but inhibited the increase of the mIPSC frequency evoked by 4-AP, indicating the participation of L-, P/Q- and N-type Ca2+ channels regulating 'spontaneous' glycine release from the nerve terminals.

Isoflurane inhibits NaChBac, a prokaryotic voltage-gated sodium channel

Volatile anesthetics inhibit mammalian voltage-gated Na(+) channels, an action that contributes to their presynaptic inhibition of neurotransmitter release. We measured the effects of isoflurane, a prototypical halogenated ether volatile anesthetic, on the prokaryotic voltage-gated Na(+) channel from Bacillus halodurans (NaChBac). Using whole-cell patch-clamp recording, human embryonic kidney 293 cells transfected with NaChBac displayed large inward currents (I(Na)) that activated at potentials of -60 mV or higher with a peak voltage of activation of 0 mV (from a holding potential of -80 mV) or -10 mV (from a holding potential of -100 mV). Isoflurane inhibited I(Na) in a concentration-dependent manner over a clinically relevant concentration range; inhibition was significantly more potent from a holding potential of -80 mV (IC(50) = 0.35 mM) than from -100 mV (IC(50) = 0.48 mM). Isoflurane positively shifted the voltage dependence of peak activation, and it negatively shifted the voltage dependence of end steady-state activation. The voltage dependence of inactivation was negatively shifted with no change in slope factor. Enhanced inactivation of I(Na) was 8-fold more sensitive to isoflurane than reduction of channel opening. In addition to tonic block of closed and/or open channels, isoflurane enhanced use-dependent block by delaying recovery from inactivation. These results indicate that a prokaryotic voltage-gated Na(+) channel, like mammalian voltage-gated Na(+) channels, is inhibited by clinical concentrations of isoflurane involving multiple state-dependent mechanisms. NaChBac should provide a useful model for structure-function studies of volatile anesthetic actions on voltage-gated ion channels.

Substance P provides neuroprotection in cerebellar granule cells through Akt and MAPK/Erk activation: Evidence for the involvement of the delayed rectifier potassium current

In the current study, we have evaluated the ability of substance P (SP) and other neurokinin 1 receptor (NK1) agonists to protect, in a dose- and time-dependent manner, primary cultures of rat cerebellar granule cells (CGCs) from serum and potassium deprivation-induced cell death (S-K5). We also established the presence of SP high affinity NK1 transcripts and the NK1 protein localization in the membrane of a sub-population of CGCs. Moreover, SP significantly and dose-dependently reduced the Akt 1/2 and Erk1/2 dephosphorylation induced by S-K5 conditions, as demonstrated by Western blot analysis. Surprisingly, in SP-treated CGCs caspase-3 activity was not inhibited, while the calpain-1 activity was moderately reduced. Corroborating this result, SP blocked calpain-mediated cleavage of tau protein, as demonstrated by the reduced appearance of a diagnostic fragment of 17 kDa by Western blot analysis. In addition, SP induced a significant reduction of the delayed rectifier K+ currents (Ik) in about 42% of the patched neurons, when these were evoked with depolarizing potential steps. Taken together, the present results demonstrate that the activation of NK1 receptors expressed in CGCs promote the neuronal survival via pathways involving Akt and Erk activation and by inhibition of Ik which can contribute to the neuroprotective effect of the peptide.

Characterization of Na+-Activated K+ Currents in Larval Lamprey Spinal Cord Neurons

Potassium channels play an important role in controlling neuronal firing and synaptic interactions. Na+-activated K+ ( KNa) channels have been shown to exist in neurons in different regions of the CNS, but their physiological function has been difficult to assess. In this study, we have examined if neurons in the spinal cord possess KNa currents. We used whole cell recordings from isolated spinal cord neurons in lamprey. These neurons display two different KNa currents. The first was transient and activated by the Na+ influx during the action potentials, and it was abolished when Na+ channels were blocked by tetrodotoxin. The second KNa current was sustained and persisted in tetrodotoxin. Both KNa currents were abolished when Na+ was substituted with choline or N-methyl-d-glucamine, indicating that they are indeed dependent on Na+ influx into neurons. When Na+ was substituted with Li+, the amplitude of the inward current was unchanged, whereas the transient KNa current was reduced but not abolished. This suggests that the transient KNa current is partially activated by Li+. These two KNa currents have different roles in controlling the action potential waveform. The transient KNa appears to act as a negative feedback mechanism sensing the Na+ influx underlying the action potential and may thus be critical for setting the amplitude and duration of the action potential. The sustained KNa current has a slow kinetic of activation and may underlie the slow Ca2+-independent afterhyperpolarization mediated by repetitive firing in lamprey spinal cord neurons.

Involvement of KCNQ2 subunits in [3H]dopamine release triggered by depolarization and pre-synaptic muscarinic receptor activation from rat striatal synaptosomes

KCNQ2 and KCNQ3 subunits encode for the muscarinic-regulated current (I(KM)), a sub-threshold voltage-dependent K+ current regulating neuronal excitability. In this study, we have investigated the involvement of I(KM) in dopamine (DA) release from rat striatal synaptosomes evoked by elevated extracellular K+ concentrations ([K+]e) and by muscarinic receptor activation. [3H]dopamine ([3H]DA) release triggered by 9 mmol/L [K+]e was inhibited by the I(KM) activator retigabine (0.01-30 micromol/L; Emax = 54.80 +/- 3.85%; IC50 = 0.50 +/- 0.36 micromol/L). The I(KM) blockers tetraethylammonium (0.1-3 mmol/L) and XE-991 (0.1-30 micromol/L) enhanced K+-evoked [3H]DA release and prevented retigabine-induced inhibition of depolarization-evoked [3H]DA release. Retigabine-induced inhibition of K+-evoked [3H]DA release was also abolished by synaptosomal entrapment of blocking anti-KCNQ2 polyclonal antibodies, an effect prevented by antibody pre-absorption with the KCNQ2 immunizing peptide. Furthermore, the cholinergic agonist oxotremorine (OXO) (1-300 micromol/L) potentiated 9 mmol/L [K+]e-evoked [3H]DA release (Emax = 155 +/- 9.50%; EC50 = 25 +/- 1.80 micromol/L). OXO (100 micromol/L)-induced [3H]DA release enhancement was competitively inhibited by pirenzepine (1-10 nmol/L) and abolished by the M3-preferring antagonist 4-diphenylacetoxy N-methylpiperidine methiodide (1 micromol/L), but was unaffected by the M1-selective antagonist MT-7 (10-100 nmol/L) or by Pertussis toxin (1.5-3 microg/mL), which uncouples M2- and M4-mediated responses. Finally, OXO-induced potentiation of depolarization-induced [3H]DA release was not additive to that produced by XE-991 (10 micromol/L), was unaffected by retigabine (10 micromol/L), and was abolished by synaptosomal entrapment of anti-KCNQ2 antibodies. Collectively, these findings indicate that, in rat striatal nerve endings, I(KM) channels containing KCNQ2 subunits regulate depolarization-induced DA release and that I(KM) suppression is involved in the reinforcement of depolarization-induced DA release triggered by the activation of pre-synaptic muscarinic heteroreceptors.