Modulation of inwardly rectifying currents in rat sympathetic neurones by muscarinic receptors

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Inhibition of constitutive inward rectifier currents in cerebellar granule cells by pharmacological and synaptic activation of GABABreceptors

gamma-Aminobutyric acid (GABA)(B) receptors are known to enhance activation of Kir3 channels generating G-protein-dependent inward rectifier K(+)-currents (GIRK). In some neurons, GABA(B) receptors either cause a tonic GIRK activation or generate a late K(+)-dependent inhibitory postsynaptic current component. However, other neurons express Kir2 channels, which generate a constitutive inward rectifier K(+)-current (CIRK) without requiring G-protein activation. The functional coupling of CIRK with GABA(B) receptors remained unexplored so far. About 50% of rat cerebellar granule cells in the internal granular layer of P19-26 rats showed a sizeable CIRK current. Here, we have investigated CIRK current regulation by GABA(B) receptors in cerebellar granule cells, which undergo GABAergic inhibition through Golgi cells. By using patch-clamp recording techniques and single-cell reverse transcriptase-polymerase chain reaction in acute cerebellar slices, we show that granule cells co-express Kir2 channels and GABA(B) receptors. CIRK current biophysical properties were compatible with Kir2 but not Kir3 channels, and could be inhibited by the GABA(B) receptor agonist baclofen. The action of baclofen was prevented by the GABA(B) receptor blocker CGP35348, involved a pertussis toxin-insensitive G-protein-mediated pathway, and required protein phosphatases inhibited by okadaic acid. GABA(B) receptor-dependent CIRK current inhibition could also be induced by repetitive GABAergic transmission at frequencies higher than the basal autorhythmic discharge of Golgi cells. These results suggest therefore that GABA(B) receptors can exert an inhibitory control over CIRK currents mediated by Kir2 channels. CIRK inhibition was associated with an increased input resistance around rest and caused a approximately 5 mV membrane depolarization. The pro-excitatory action of these effects at an inhibitory synapse may have an homeostatic role re-establishing granule cell readiness under conditions of strong inhibition.

Cholinergic modulation of stellate cells in the mammalian ventral cochlear nucleus

The main source of excitation to the ventral cochlear nucleus (VCN) is from glutamatergic auditory nerve afferents, but the VCN is also innervated by two groups of cholinergic efferents from the ventral nucleus of the trapezoid body. One arises from collaterals of medial olivocochlear efferents, and the other arises from neurons that project solely to the VCN. This study examines the action of cholinergic inputs on stellate cells in the VCN. T stellate cells, which form one of the ascending auditory pathways to the inferior colliculus, and D stellate cells, which inhibit T stellate cells, are distinguished electrophysiologically. Whole-cell recordings from stellate cells in slices of the VCN of mice demonstrate that most T stellate cells are excited by cholinergic agonists through three types of receptors, whereas all D stellate cells tested were insensitive to cholinergic agonists. Nicotinic excitation in T stellate cells has two components. The faster component was blocked by alpha-bungarotoxin and methyllycaconitine, suggesting that receptors contained alpha7 subunits; the slower component was insensitive to both. Muscarinic receptors excite T stellate cells by blocking a voltage-insensitive, "leak" potassium conductance. Our results suggest that cholinergic efferent innervation enhances excitation by sounds of T stellate cells, opposing the inhibitory action of cholinergic innervation in the cochlea that is conveyed indirectly through the glutamatergic afferents. The inhibitory action of D stellate cells on their targets is probably not affected by cholinergic inputs. Excitation of T stellate cells by cholinergic efferents would be expected to enhance the encoding of spectral peaks in noise.

Cholinergic modulation of stellate cells in the mammalian ventral cochlear nucleus

The main source of excitation to the ventral cochlear nucleus (VCN) is from glutamatergic auditory nerve afferents, but the VCN is also innervated by two groups of cholinergic efferents from the ventral nucleus of the trapezoid body. One arises from collaterals of medial olivocochlear efferents, and the other arises from neurons that project solely to the VCN. This study examines the action of cholinergic inputs on stellate cells in the VCN. T stellate cells, which form one of the ascending auditory pathways to the inferior colliculus, and D stellate cells, which inhibit T stellate cells, are distinguished electrophysiologically. Whole-cell recordings from stellate cells in slices of the VCN of mice demonstrate that most T stellate cells are excited by cholinergic agonists through three types of receptors, whereas all D stellate cells tested were insensitive to cholinergic agonists. Nicotinic excitation in T stellate cells has two components. The faster component was blocked by alpha-bungarotoxin and methyllycaconitine, suggesting that receptors contained alpha7 subunits; the slower component was insensitive to both. Muscarinic receptors excite T stellate cells by blocking a voltage-insensitive, "leak" potassium conductance. Our results suggest that cholinergic efferent innervation enhances excitation by sounds of T stellate cells, opposing the inhibitory action of cholinergic innervation in the cochlea that is conveyed indirectly through the glutamatergic afferents. The inhibitory action of D stellate cells on their targets is probably not affected by cholinergic inputs. Excitation of T stellate cells by cholinergic efferents would be expected to enhance the encoding of spectral peaks in noise.

Presence of aquaporin-4 and muscarinic receptors in astrocytes and ependymal cells in rat brain: a clue to a common function?

Using combined double immunofluorescence and laser confocal microscopy, we studied the common cellular localization of cholinergic muscarinic receptors (mAChRs) and aquaporin-4 water channels (AQP4) in the cortex, the corpus callosum and in ependymal cells of the rat brain. In the cortex, AQP4 staining was restricted to the perivascular end-feet of astrocytes. It was more widely distributed on the astrocytes of the corpus callosum. On astrocytes, mAChRs were often present in regions immunoreactive to AQP4. Ependymal cells bordering the third ventricle were also stained by both antibodies. The double staining of mAChRs with AQP4 on two different cell-types might indicate that further interactions exist which may be important in the regulation of water and electrolyte movements in the brain.

Acetylcholine modulates respiratory pattern: effects mediated by M3-like receptors in preBötzinger complex inspiratory neurons

Perturbations of cholinergic neurotransmission in the brain stem affect respiratory motor pattern both in vivo and in vitro; the underlying cellular mechanisms are unclear. Using a medullary slice preparation from neonatal rat that spontaneously generates respiratory rhythm, we patch-clamped inspiratory neurons in the preBötzinger complex (preBötC), the hypothesized site for respiratory rhythm generation, and simultaneously recorded respiratory-related motor output from the hypoglossal nerve (XIIn). Most (88%) of the inspiratory neurons tested responded to local application of acetylcholine (ACh) or carbachol (CCh) or bath application of muscarine. Bath application of 50 microM muscarine increased the frequency, amplitude, and duration of XIIn inspiratory bursts. At the cellular level, muscarine induced a tonic inward current, increased the duration, and decreased the amplitude of the phasic inspiratory inward currents in preBötC inspiratory neurons recorded under voltage clamp at -60 mV. Muscarine also induced seizure-like activity evident during expiratory periods in XIIn activity; these effects were blocked by atropine. In the presence of tetrodotoxin (TTX), local ejection of 2 mM CCh or ACh onto preBötC inspiratory neurons induced an inward current along with an increase in membrane conductance under voltage clamp and induced a depolarization under current clamp. This response was blocked by atropine in a concentration-dependent manner. Bath application of 1 microM pirenzepine, 10 microM gallamine, or 10 microM himbacine had little effect on the CCh-induced current, whereas 10 microM 4-diphenylacetoxy-N-methylpiperidine methiodide blocked the current. The current-voltage (I-V) relationship of the CCh-induced response was linear in the range of -110 to -20 mV and reversed at -11.4 mV. Similar responses were found in both pacemaker and nonpacemaker inspiratory neurons. The response to CCh was unaffected when patch electrodes contained a high concentration of EGTA (11 mM) or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (10 mM). The response to CCh was reduced greatly by substitution of 128 mM Tris-Cl for NaCl in the bath solution; the I-V curve shifted to the left and the reversal potential shifted to -47 mV. Lowering extracellular Cl(-) concentration from 140 to 70 mM had no effect on the reversal potential. These results suggest that in preBötC inspiratory neurons, ACh acts on M3-like ACh receptors on the postsynaptic neurons to open a channel permeable to Na(+) and K(+) that is not Ca(2+) dependent. This inward cation current plays a major role in depolarizing preBötC inspiratory neurons, including pacemakers, that may account for the ACh-induced increase in the frequency of respiratory motor output observed at the systems/behavioral level.

Electrophysiological roles of L-type channels in different classes of guinea pig sympathetic neuron

The electrophysiological consequences of blocking Ca(2+) entry through L-type Ca(2+) channels have been examined in phasic (Ph), tonic (T), and long-afterhyperpolarizing (LAH) neurons of intact guinea pig sympathetic ganglia isolated in vitro. Block of Ca(2+) entry with Co(2+) or Cd(2+) depolarized T and LAH neurons, reduced action potential (AP) amplitude in Ph and LAH neurons, and increased AP half-width in Ph neurons. The afterhyperpolarization (AHP) and underlying Ca(2+)-dependent K(+) conductances (gKCa1 and gKCa2) were reduced markedly in all classes. Addition of 10 microM nifedipine increased input resistance in LAH neurons, raised AP threshold in Ph and LAH neurons, and caused a small increase in AP half-width in Ph neurons. AHP amplitude and the amplitude and decay time constant of gKCa1 were reduced by nifedipine in all classes; the slower conductance, gKCa2, which underlies the prolonged AHP in LAH neurons, was reduced by 40%. Surprisingly, AHP half-width was lengthened by nifedipine in a proportion of neurons in all classes; despite this, neuron excitability was increased during a maintained depolarization. Nifedipine's effects on AHP half-width were not mimicked by 2 mM Cs(+) or 2 mM anthracene-9-carboxylic acid, a blocker of Cl(-) channels, and it did not modify transient outward currents of the A or D types. The effects of 100 microM Ni(2+) differed from those of nifedipine. Thus in Ph neurons, Ca(2+) entry through L-type channels during a single action potential contributes to activation of K(+) conductances involved in both the AP and AHP, whereas in T and LAH neurons, it acts only on gKCa1 and gKCa2. These results differ from the results in rat superior cervical ganglion neurons, in which L-type channels are selectively coupled to BK channels, and in hippocampal neurons, in which L-type channels are selectively coupled to SK channels. We conclude that the sources of Ca(2+) for activating the various Ca(2+)-activated K(+) conductances are distinct in different types of neuron.

Selective activation of heterologously expressed G protein-gated K+ channels by M2 muscarinic receptors in rat sympathetic neurones

1. G protein-regulated inward rectifier K+ (GIRK) channels were over-expressed in dissociated rat superior cervical sympathetic (SCG) neurones by co-transfecting green fluorescent protein (GFP)-, GIRK1- and GIRK2-expressing plasmids using the biolistic technique. Membrane currents were subsequently recorded with whole-cell patch electrodes. 2. Co-transfected cells had larger Ba2+-sensitive inwardly rectifying currents and 13 mV more negative resting potentials (in 3 mM [K+]o) than non-transfected cells, or cells transfected with GIRK1 or GIRK2 alone. 3. Carbachol (CCh, 1-30 microM) increased the inwardly rectifying current in 70 % of GIRK1+ GIRK2-transfected cells by 261 +/- 53 % (n = 6, CCh 30 microM) at -120 mV, but had no effect in non-transfected cells or in cells transfected with GIRK1 or GIRK2 alone. Pertussis toxin prevented the effect of carbachol but had no effect on basal currents. 4. The effect of CCh was antagonized by 6 nM tripitramine but not by 100 nM pirenzepine, consistent with activation of endogenous M2 muscarinic acetylcholine receptors. 5. In contrast, inhibition of the voltage-activated Ca2+ current by CCh was antagonized by 100 nM pirenzepine but not by 6 nM tripitramine, indicating that it was mediated by M4 muscarinic acetylcholine receptors. 6. We conclude that endogenous M2 and M4 muscarinic receptors selectively couple to GIRK currents and Ca2+ currents respectively, with negligible cross-talk.

Excitatory effects of muscarine on septohippocampal neurons: involvement of M3 receptors

Cholinergic mechanisms in the septohippocampal pathway contribute to several cognitive functions and impaired cholinergic transmission in this pathway may be related to the memory loss and dementia that accompanies normal aging and Alzheimer's disease and behavioral studies suggest that muscarinic mechanisms in the medial septum/diagonal band of Broca (MSDB) may contribute to these functions. The goal of the present study was to begin a characterization of the physiological and pharmacological effects of muscarine on antidromically identified septohippocampal neurons (SHNs). Muscarinic agonists produced a concentration-dependent excitation in >90% of SHNs tested using extracellular recordings in an in vitro rat brain slice preparation. The SHNs excited by muscarine had a broad range of conduction velocities (0.2 to 3.7 m/s; mean: 1.6+/-0.06 m/s; n=110), suggesting involvement of neurons with both slow (possibly cholinergic) and fast (possibly GABAergic) conducting fibers. The muscarine-induced excitations in SHNs were found not to be mediated via M1, M2 or M4 receptors, as they were not blocked by the M1-selective antagonists, pirenzepine or telenzepine or by the M2/M4-selective antagonist, methoctramine. In contrast, the M3-selective antagonist, 4-DAMP-mustard, blocked muscarinic excitations in a majority of SHNs, indicating the presence of M3 as well as non-M3-type responses. McN-A-343, an M1 and M5-selective agonist, excited 33% of neurons tested, confirming involvement of non-M3 receptors (possibly M5) and M3 receptors. Since the cholinergic and GABAergic MSDB neurons together innervate almost every type of hippocampal neuron, the effects of muscarine on SHNs would also have a profound effect on hippocampal circuitry.

ANG II AT1 receptors induce depolarization and inward current in rat median preoptic neurons in vitro

To examine ANG II receptors in rat median preoptic (MnPO) neurons, we used patch-clamp whole cell recordings in a parasagittal brain slice preparation. Lucifer yellow-filled neurons displayed a simple morphology with two to three aspiny dendrites. Bath-applied ANG II (1-2,000 nM for 30 s) induced a response in 37 of 70 cells. In current-clamp recordings, cells displayed a prolonged (10- to 30-min) depolarizing plateau with action potential discharges and an associated reduction in postburst afterhyperpolarization and spike frequency adaptation. In voltage-clamp recordings (holding potential -65 mV), cells displayed tetrodotoxin-resistant inward currents of 7. 6 +/- 1.9 (n = 5), 9.9 +/- 1.9 (n = 9), and 9.2 +/- 2.2 pA (n = 6) at 10, 200, and 2,000 nM, respectively. Responses were blockable by pretreatment with losartan (2 microM; n = 6) but not by PD-123177 (20 microM; n = 3). Net ANG II-induced current revealed a 7.8 +/- 0. 9% reduction in membrane conductance, decreasing but not reversing at hyperpolarized levels. Neurons expressing a strong hyperpolarization-activated, time-independent inward rectification were more likely to respond to ANG II. There was no correlation between the response of a neuron to ANG II and its response to norepinephrine.

Encoding properties induced by a persistent voltage-gated muscarinic sodium current in rabbit sympathetic neurones

1. A time- and voltage-dependent Na(+)-selective current termed INa,M is activated by muscarinic agonists or splanchnic nerve stimulation in sympathetic neurones of rabbit coeliac and superior mesenteric ganglia. The firing patterns induced by INa,M were investigated in patch-clamped neurones within intact ganglia, and compared with those generated by a neuronal model including INa,M. 2. INa,M was characterized by voltage-dependent low threshold activation and high-threshold inactivation functions. The overlapping functions produced a persistent U-shaped current between -100 and -20 mV, which peaked at the cell resting potential. The activation and inactivation kinetics were fitted to single exponentials with time constants of approximately 100 and 400 ms, respectively. 3. Activating INa,M with muscarinic agonists or nerve stimulation depolarized and fired the neurones. The depolarization was paralleled by an apparent increase in input membrane resistance. The model showed that this paradox resulted from the turning off of INa,M during resistance tests, which also accounted for the all-or-none slow hyperpolarizing responses to current pulses. 4. INa,M gave the neurones an N-shaped I-V relationship capable of producing complex firing patterns. Under given conditions, carbachol-treated neurones could either fire regularly or remain silent at approximately -80 mV, i.e. they displayed bistability. Transitions from one state to the other were triggered with short current pulses. The transitions resulted from the turning on and off of INa,M. 5. Firing reduced INa,M, an effect abolished by blocking Ca2+ channels or adding BAPTA (40 mM) to the pipette. The Ca(2+)-related negative regulation of INa,M may have mediated endogenous bursting activity. Burst firing was generated by the model upon introducing Ca2+ regulation of INa,M. 6. The results demonstrate that INa,M gives prevertebral sympathetic neurones a wide repertoire of firing patterns: pacemaker-like properties, bistability and burst firing capability. They suggest that the INa,M-related encoding properties may provide sympathetic neurotransmission with new potentialities.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Identification of two nervous system-specific members of the erg potassium channel gene family

Two new potassium channel genes, erg2 and erg3, that are expressed in the nervous system of the rat were identified. These two genes form a small gene family with the previously described erg1 (HERG) gene. The erg2 and erg3 genes are expressed exclusively in the nervous system, in marked contrast to erg1, which is expressed in both neural and non-neural tissues. All three genes are expressed in peripheral sympathetic ganglia. The erg3 channel produces a current that has a large transient component at positive potentials, whereas the other two channels are slowly activating delayed rectifiers. Expression of the erg1 gene in the sympathetic nervous system has potential implications for the etiology of the LQT2 form of the human genetic disease long QT syndrome.

Identification of two nervous system-specific members of the erg potassium channel gene family

Two new potassium channel genes, erg2 and erg3, that are expressed in the nervous system of the rat were identified. These two genes form a small gene family with the previously described erg1 (HERG) gene. The erg2 and erg3 genes are expressed exclusively in the nervous system, in marked contrast to erg1, which is expressed in both neural and non-neural tissues. All three genes are expressed in peripheral sympathetic ganglia. The erg3 channel produces a current that has a large transient component at positive potentials, whereas the other two channels are slowly activating delayed rectifiers. Expression of the erg1 gene in the sympathetic nervous system has potential implications for the etiology of the LQT2 form of the human genetic disease long QT syndrome.

A novel slow hyperpolarization-activated potassium current (IK(SHA)) from a mouse hippocampal cell line

1. A slow hyperpolarization-activated inwardly rectifying K+ current (IK(SHA)) with novel characteristics was identified from the mouse embryonic hippocampus x neuroblastoma cell line HN9.10e. 2. The non-inactivating current activated negative to a membrane potential of -80 mV with slow and complex activation kinetics (tau act approximately 1-7 s) and a characteristic delay of 1-10 s (-80 to -140 mV) that was linearly dependent on the membrane potential. 3. Tail currents and instantaneous open channel currents determined through fast voltage ramps reversed at the K+ equilibrium potential (EK) indicating that primarily K+, but not Na+, permeated the channels. 4. IK(SHA) was unaffected by altering the intracellular Ca2+ concentration between approximately 0 and 10 microM, but was susceptible to block by 5 mM extracellular Ca2+, Ba2+ (Ki = 0.42 mM), and Cs+ (Ki = 2.77 mM) 5. In cells stably transformed with M2 muscarinic receptors, IK(SHA) was rapidly, but reversibly, suppressed by application of micromolar concentrations of muscarine. 6. At the single channel level K(SHA) channel openings were observed with the characteristic delay upon membrane hyperpolarization. Analysis of unitary currents revealed an inwardly rectifying I-V profile and a channel slope conductance of 7 pS. Channel activity persisted in the inside-out configuration for many minutes. 7. It is concluded that IK(SHA) in HN9.10e cells represents a novel K+ current, which is activated upon membrane hyperpolarization. It is functionally different from both classic inwardly rectifying IKir currents and other cationic hyperpolarization-activated IH currents that have been previously described in neuronal or glial cells.

Corticotropin releasing hormone inhibits an inwardly rectifying potassium current in rat corticotropes

1. The perforated-patch-clamp technique was used to identify an inwardly rectifying K+ current (IK(IR)) in cultured rat anterior pituitary cells highly enriched in corticotropes. IK(IR) was rapidly activating and highly selective for K+. The K+ conductance was approximately proportional to the square root of the extracellular K+ concentration. 2. IK(IR) was blocked in a voltage-dependent manner by external Ba2+ and Cs+, slightly attenuated by 5 mM 4-aminopyridine (15% inhibition) and insensitive to 10 mM tetraethylammonium, 2 mM Ca2+, 1 mM Cd2+ and 50 microM La3+. 3. In physiological saline, 100 microM Ba2+, which inhibits 86% of IK(IR) at the cell resting potential, depolarized cells by 6.1 +/- 0.7 mV from a mean resting potential of -59.6 +/- 0.8 mV. 4. Corticotropin releasing hormone (CRH), which activates adenylyl cyclase and stimulates adrenocorticotropic hormone (ACTH) secretion from corticotropes, inhibited IK(IR) by 25% and depolarized the cells by 10.2 +/- 1.0 mV. Dibutyryl cAMP ((Bu)2cAMP) mimicked these effects. 5. The membrane depolarization evoked by Ba2+ or CRH increased the cell firing frequency. Comparison of cells exhibiting a membrane potential of approximately -50 mV revealed that spike frequency in the presence of CRH (109 +/- 7 spikes (5 min)-1) was greater than in control (60 +/- 5 spikes (5 min)-1) or Ba(2+)-treated (77 +/- 15 spikes (5 min)-1) corticotropes. 6. The data suggest that IK(IR) contributes to maintenance of the resting membrane potential of rat corticotropes. Inhibition of IK(IR) plays a role in, but does not account for all of, the membrane depolarization and enhancement of firing frequency evoked by CRH.

Regulation of IRK3 inward rectifier K+ channel by m1 acetylcholine receptor and intracellular magnesium

Inward rectifier K+ channels control the cell's membrane potential and neuronal excitability. We report that the IRK3 but not the IRK1 inward rectifier K+ channel activity is inhibited by m1 muscarinic acetylcholine receptor. This m1 modulation cannot be accounted for by protein kinase C, Ca2+, or channel phosphorylation, but can be mimicked by Mg2+. Based on quantitative analyses of IRK3 and two different IRK1 mutant channels bestowed with sensitivity to m1 modulation, we suggest that the resting Mg2+ level causes chronic inhibition of IRK3 channels, and m1 receptor stimulation may lead to an increase of cytoplasmic Mg2+ concentration and further channel inhibition, due to the ability of Mg2+ to lead these channels into a prolonged inactivated state.

On-going and reflex synaptic events in rat superior cervical ganglion cells

1. Synaptic events evoked by brief noxious cutaneous stimuli were recorded in sympathetic neurones in the superior cervical ganglion of anaesthetized rats. 2. On-going excitatory synaptic potentials (ESPs) and/or action potentials (APs) were recorded in 69% of neurones at mean frequencies that varied from 0.01 to 6.3 Hz in different cells. From histograms of ESP amplitude during membrane hyperpolarization, it appears that most cells received one (52%), or two or more (36%), suprathreshold inputs and several subthreshold inputs with overlapping amplitudes. 3. Pinching the skin for 1-3 s evoked either a brief burst of synaptic events (lasting about 300 ms) preceding a few seconds of inhibition (burst-inhibitory (BI) neurones), or simply an excitation (excitatory (E) neurones), or no response (O neurones). In 60% of BI neurones, a second burst occurred after the end of the pinch. 4. BI neurones had a higher frequency of on-going synaptic activity (2.9 +/- 0.5 Hz, n = 15) than E neurones (0.2 +/- 0.1 Hz, n = 5) or O (0.2 +/- 0.1 Hz, n = 5) neurones. Most neurones with two or more suprathreshold inputs were BI neurones. In 20% of neurones (all BI with high rates of synaptic activity), several other inputs had ESPs with amplitudes close to threshold. 5. Subthreshold and suprathreshold inputs responded in the same way in only 45% of neurones, but suprathreshold inputs were excited in 73% of BI and all E neurones. The order of recruitment of different inputs varied from trial to trial. If classification was based only on suprathreshold responses, there were 36% BI, 32% E and 32% O neurones. 6. In the majority of neurones, postganglionic discharge was initiated exclusively by suprathreshold inputs, even during reflex excitation. 7. Qualitatively similar, but smaller, responses were evoked by a puff of air on the abdomen in 71% of cells tested. 8. The data suggest that the natural discharge of SCG neurones is largely determined by the activity of one or two preganglionic inputs with high quantal contents. BI neurones may include vasoconstrictor neurones, whereas the other types include secretomotor, pilomotor and other neurones projecting to targets in the head.

Effects of a cognition-enhancer, linopirdine (DuP 996), on M-type potassium currents (IK(M)) and some other voltage- and ligand-gated membrane currents in rat sympathetic neurons

Linopirdine is a cognition enhancer which augments depolarization-induced transmitter release in the cortex and which is under consideration for potential treatment of Alzheimer's disease. It has previously been reported to inhibit M-type K+ currents in rat hippocampal neurons. In the present experiments we have tested its effect on whole-cell M-currents and single M-channels, and on a range of other membrane currents, in dissociated rat superior cervical sympathetic ganglion cells. Linopirdine inhibited the whole-cell M-current with an IC50 of 3.4 microM and blocked M-channels recorded in excised outside-out membrane patches but not in inside-out patches. This suggests that linopirdine directly blocks M-channels from the outside. It was much less effective in inhibiting other voltage-gated potassium currents [delayed rectifier (IK(V)), IC50 63 microM; transient (IA) current, IC50 69 microM] and produced no detectable inhibition of the fast and slow Ca(2+)-activated K+ currents IC and IAHP or of a hyperpolarization-activated cation current (IQ/Ih) at 10-30 microM. However, it reduced acetylcholine-activated nicotinic currents and GABA-activated Cl- currents with IC50 values of 7.6 and 26 microM respectively. It is concluded that linopirdine shows some 20-fold selectivity for M-channels among different K+ channels but can also block some transmitter-gated channels. The relationship between M-channel block and the central actions of linopirdine are discussed.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Inhibition of an inward rectifier potassium channel (Kir2.3) by G-protein betagamma subunits

The molecular basis of G-protein inhibition of inward rectifier K+ currents was examined by co-expression of G-proteins and cloned Kir2 channel subunits in Xenopus oocytes. Channels encoded by Kir2.3 (HRK1/HIR/BIRK2/BIR11) were completely suppressed by co-expression with G-protein betagamma subunits, whereas channels encoded by Kir2. 1 (IRK1), which shares 60% amino acid identity with Kir2.3, were unaffected. Co-expression of Galphai1 and Galphaq subunits also partially suppressed Kir2.3 currents, but Galphat, Galphas, and a constitutively active mutant of Galphail (Q204L) were ineffective. Gbetagamma and Kir2.3 subunits were co-immunoprecipitated using an anti-Kir2.3 antibody. Direct binding of G-protein betagamma subunits to fusion proteins containing Kir2.3 N terminus, but not to fusion proteins containing Kir2.1 N terminus, was also demonstrated. The results are consistent with suppression of Kir2.3 currents resulting from a direct protein-protein interaction between the channel and G-protein betagamma subunits. When Kir2.1 and Kir2.3 subunits were coexpressed, the G-protein inhibitory phenotype of Kir2.3 was dominant, suggesting that co-expression of Kir2.3 with other Kir subunits might give rise to novel G-protein-inhibitable inward rectifier currents.