Rectifying Potassium Conductance Research Articles

Effects of phospholipase A2 inhibitors on coupling of alpha 2-adrenoceptors to inwardly rectifying potassium currents in guinea-pig submucosal neurones.

1. Noradrenaline hyperpolarizes guinea-pig submucosal neurones by opening inwardly rectifying potassium channels. Intracellular recordings were made from submucosal neurones and the possible involvement of the phospholipase A2 pathway in this response was examined. 2. The non-specific phospholipase A2 inhibitors, quinacrine (10 microM) and 4-bromophenacyl bromide (4-BPB, 10 microM) inhibited nerve-evoked inhibitory synaptic potentials (i.p.s.ps) and hyperpolarizations to somatostatin and UK 14304. Quinacrine and 4-BPB also blocked the inward rectification present in current-voltage curves in the absence of somatostatin or UK 14304. 3. The more selective phospholipase A2 inhibitor, cyclosporin A (10 microM) and the lipoxygenase and cyclo-oxygenase inhibitor, eicosatetraynoic acid (ETYA, 20 microM) and nordihydroguairetic acid (NDGA, 20 microM) did not alter i.p.s.ps or hyperpolarizations to UK 14304. 4. Exogenously applied arachidonic acid (1-300 microM) did not mimic the i.p.s.p. or the hyperpolarization to UK 14304. 5. We conclude that arachidonic acid or its eicosanoid metabolites produced by phospholipase A2 stimulation are unlikely to be involved in the receptor G-protein coupled activation of potassium currents in submucosal neurones. The inhibition of the noradrenaline-induced hyperpolarization by quinacrine and 4-BPB is most likely due primarily to blockade of the basal inwardly rectifying potassium conductance present in these neurones.

Control of resting membrane potential by delayed rectifier potassium currents in ferret airway smooth muscle cells.

1. In order to determine the physiological role of specific potassium currents in airway smooth muscle, potassium currents were measured in freshly dissociated ferret trachealis cells using the nystatin-permeabilized, whole-cell method, at 35 degrees C. 2. The magnitude of the outward currents was markedly increased as bath temperature was increased from 22 to 35 degrees C. This increase was primarily due to the increase in maximum potassium conductance (gK,max), although there was also a small leftward shift in the relationship between gK and voltage at higher temperatures. The maximum conductance and the kinetics of current activation and inactivation were also temperature dependent. At 35 degrees C, gating of the current was steeply voltage dependent between -40 and 0 mV. Current activation was well fitted by fourth-order kinetics; the mean time constants of activation (30 mV clamp step) were 1.09 +/- 0.17 and 1.96 +/- 0.27 ms at 35 and 22 degrees C, respectively. 3. Outward currents using the nystatin method were qualitatively similar to delayed rectifier currents recorded in dialysed cells with high calcium buffering capacity solutions. 4-Aminopyridine (4-AP; 2 mM), a specific blocker of delayed rectifier potassium channels in this tissue, inhibited over 80% of the outward current evoked by voltage-clamp steps to between -10 and +20 mV (n = 6). Less than 5% of the outward current was blocked over the same voltage range by charybdotoxin (100 nM; n = 15), a specific antagonist of large-conductance, calcium-activated potassium channels in this tissue. 4. The degree to which delayed rectifier and calcium-activated potassium conductances control resting membrane potential was examined in current-clamp experiments. The resting membrane potential of current clamped cells was -33.6 +/- 1.0 mV (n = 62). Application of 4-AP (2 mM) resulted in a 14.4 +/- 1.0 mV depolarization (n = 8) and an increase in input resistance. Charybdotoxin (100 nM) had no effect on resting membrane potential (n = 6). 5. Force measurements were made in isolated strips of trachealis muscle to determine the effect of pharmacological blockade of individual potassium conductances on resting tone. In the presence of tetrodotoxin (1 microM) and atropine (1 microM), 4-AP increased baseline tension in a dose-dependent manner, with an EC50 of 1.8 mM (n = 13); application of 5 mM 4-AP increased tone to 86.8 +/- 8.1% of that produced by 1 microM methacholine, and this tone was almost completely inhibited by nifedipine (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Fast and slow photoreceptors ? a comparative study of the functional diversity of coding and conductances in the Diptera

1. From a comparison of the photoresponses and membrane properties of photoreceptors from 20 species of Diptera, we conclude that coding in the time domain is matched to the dictates of visual ecology. This matching involves the dynamics of phototransduction and the use of an appropriate mix of potassium conductances to tune the photoreceptor membrane. 2. Rapidly flying, manoeuvrable diurnal Diptera from several families have fast photoreceptors, with corner frequencies (the frequency at which signal power falls by a half) of between 50 and 107 Hz. The ponderous and predominantly nocturnal tipulids have slow photoreceptors with fully light adapted corner frequencies of 16 to 19 Hz. 3. Dark adapted fast photoreceptors have a lower gain (as indicated by lower noise levels), a lower sensitivity, and light adapt more rapidly than dark adapted slow photoreceptors. Fast cells also have much lower input resistances and shorter time constants. 4. Fast photoreceptors rectify more strongly in the steady state because of a weakly inactivating delayed rectifier potassium conductance with fast and slow components of activation. Slow photoreceptors rectify less strongly in the steady state because their membrane properties are dominated by strongly inactivating outward currents with reversal potentials in the range — 80 to -90 mV. 5. The differences between potassium conductances match the differing functional requirements of fast and slow photoreceptors. The non-inactivating delayed rectifier promotes the rapid response of fast cells by reducing the membrane time constant. This is an expensive strategy, involving large conductances and currents. Slowly flying nocturnal insects do not require a high speed of response. The potassium conductances in their slow photoreceptors inactivate to avoid costly and unnecessary ion fluxes. 8. Both the dynamics of the photoresponse and photoreceptor membrane properties exhibit sexual dimorphism. Light adapted photoreceptors in the enlarged male dorsal eye of Bibio markii have a corner frequency of 42 Hz, compared with 27 Hz for cells in the smaller female eye. This difference in frequency response correlates with the male's higher spatial acuity and is accompanied by consistent differences in potassium conductance activation rate. We conclude that the divison between fast and slow cells is the product of cellular constraints, metabolic costs and the requirements of coding efficiency at different light levels and retinal image velocities.

Common ionic mechanisms of excitation by substance P and other transmitters in guinea-pig submucosal neurones.

1. Intracellular recordings were made from submucosal neurones and single-electrode voltage-clamp methods were used to record membrane currents. The actions of substance P (SP), 5-hydroxytryptamine (5-HT), muscarine, vasoactive intestinal polypeptide (VIP), forskolin and nerve stimulation were studied. 2. Substance P, 5-HT (in the presence of 5-HT3 receptor antagonists), muscarine, VIP, forskolin and slow excitatory synaptic transmission all produced identical responses: an inward current associated with a membrane conductance decrease at the resting potential. The actions of any one occluded the actions of any other and all responses were pertussis-toxin insensitive. 3. These agonists produced a voltage-independent decrease in a 'leak' potassium conductance between -40 and -120 mV in 14% of neurones. 4. These agonists decreased a voltage-dependent, calcium-activated potassium conductance between -40 and -80 mV in all other (86%) neurones. The agonists still evoked an inward current without apparent conductance change at potentials between -90 and -130 mV. 5. In a low calcium solution containing cobalt or cadmium, the agonists produced an inward current associated with a conductance increase from -40 to -120 mV. Ion replacement studies indicated this current was due to an increase in a cation-selective (mainly sodium) conductance. 6. The agonists also reduced the inwardly rectifying potassium current that is activated by somatostatin and alpha 2-adrenoceptor agonists in these neurones. The agonists did not alter the inwardly rectifying potassium current that is present in these neurones in the absence of somatostatin or alpha 2-agonists. 7. Thus, SP, 5-HT, muscarine, VIP and the release of slow excitatory transmitters all appear to act through a common intracellular transduction pathway, an increase in adenylate cyclase. This results in an activation of a sodium-selective cation current and an inhibition of three distinct potassium conductances: the background potassium conductance, the calcium-activated potassium conductance and the inwardly rectifying potassium conductance activated by somatostatin and alpha 2-adrenoceptor agonists.

Muscarinic receptor subtypes: Modulation of ion channels

The discovery of five muscarinic receptor subtypes by molecular genetic techniques has resulted in new approaches to understanding their function. This involves the expression of the individual genes encoding each receptor subtype in isolation, such that their effects and mechanisms of action can be studied. The coupling of the receptors with G-proteins and ion channels is the subject of this review and emphasis is placed upon the assignment of genetically defined receptor subtypes with a given physiological function. Activation of inwardly rectifying potassium conductances by m2 and m4 and inhibition by m1, as well as stimulation of calcium-dependent conductances by m1, m3 and m5 are discussed.

Galanin activates an inwardly rectifying potassium conductance in mudpuppy atrial myocytes.

Galanin- and bethanechol-activated K+ currents have been studied in mudpuppy atrial myocytes. The galanin and bethanechol K+ currents were time-dependent and inwardly rectifying. In GTP gamma S, the galanin and bethanechol currents were reduced progressively as G-protein gated K+ channels became activated. GDP beta S inhibited agonist-induced outward currents. We conclude that galanin and bethanechol activate the same or a very similar inwardly rectifying K+ conductance and that activation of a G protein is required.

On the Mechanism of β-adrenergic Activation of Cardiac Outward Currents : Chloride and Delayed Rectifier Potassium Conductance : Mechanism of intracellular regulation of cardiac ion channels and its abnormality

On the Mechanism of β-adrenergic Activation of Cardiac Outward Currents : Chloride and Delayed Rectifier Potassium Conductance : Mechanism of intracellular regulation of cardiac ion channels and its abnormality

GABAb receptors are coupled to a barium-insensitive outward rectifying potassium conductance in premotor respiratory neurons

1. Labeled premotor respiratory neurons from neonatal rats in culture were used to study the effects of (+/-)baclofen, a selective gamma-aminobutyric acid (GABA)b receptor agonist known to inhibit rhythmic breathing movements in mammals. Bath application of (+/-)baclofen-activated outward currents in cell-attached patches, suggesting that a second messenger system linked the (+/-)baclofen-activated conductance (GBac) to the GABAb receptor. 2.GBac channels exhibited outward rectification and were insensitive to blockade by Ba2+ and Cs+. The single-channel conductance was 100 pS in symmetrical K+ solutions and decreased as [K+]o was reduced. The reversal potential for the GBac channel shifted 45 mV/decade when [K+]o was changed indicating that it was predominantly selective for K+ ions. These properties were similar to those of the S-channel in Aplysia sensory neurons. 3. The properties of GBac channels were distinct from those associated with the GABAb mediated slow inhibitory postsynaptic potential (IPSP), indicating that GABAb receptors can be associated with more than one type of K+ channel. We propose that GBac channels modulate the repetitive firing activity of premotor respiratory neurons and may also participate in presynaptic inhibition.

M4 muscarinic receptor subtype activates an inwardly rectifying potassium conductance in AtT20 cells

The modulation of an inwardly rectifying potassium conductance by muscarinic receptor stimulation was studied in the AtT-20 pituitary cell line, using the whole-cell patch-clamp technique. Only m4 mRNA was detected in these cells, thus, it is assumed that the actions of muscarinic receptor stimulation are mediated by the m4 receptor. AtT-20 cells express a slowly activating inwardly rectifying potassium conductance. Application of acetylcholine (ACh), resulted in an atropine sensitive, reversible increase in inwardly rectifying current. The ACh-induced current differed from the current recorded in control, in that it was fast activating, while the control current was slowly activating. Inclusion of GTPγS in the patch pipette activated an inward current with characteristics similar to the ACh-induced current, and the ACh-induced current response could be inhibited by pre-incubation with pertussis toxin (PTX). It is concluded that the m4 muscarinic receptor is coupled to an inwardly rectifying potassium conductance via a PTX sensitive G-protein.

Inwardly rectifying potassium conductances in AtT-20 clonal pituitary cells.

We have detected two inwardly rectifying potassium conductances in AtT-20 clonal corticotrophs, a cell line derived from the mouse pituitary gland. An agonist-independent potassium conductance was activated by voltage steps negative to the reversal potential for potassium (VK) and was completely blocked by 1 mM barium in the bathing solution. The conductance was transient and inactivated completely with a time constant of about 80 ms. Reducing the external sodium concentration from 140 mM to 14 mM attenuated inactivation. In the presence of 100 nM somatostatin an inwardly rectifying conductance, which reversed at potentials close to VK, was also elicited. This conductance exhibited a maximal slope conductance that increased with increasing extracellular potassium. Rectification depends on both voltage and extracellular potassium concentration (Vm-VK). The inward current induced by somatostatin during voltage steps negative to VK was completely blocked by 1 mM extracellular barium, whereas the outward somatostatin-induced current activated at the holding current, which was about 30 mV positive to VK, was unaffected by 1 mM extracellular barium. The muscarinic agonist carbachol (10 microM) also induces an inwardly rectifying conductance of similar magnitude to that induced by somatostatin. Since the agonist-independent potassium current exhibits sodium-dependent inactivation, whereas the hormone-induced current does not inactivate, these currents are probably carried by different populations of potassium channels.

Muscarine increases cation conductance and decreases potassium conductance in rat locus coeruleus neurones.

1. Whole-cell patch-clamp recordings were made from rat locus coeruleus neurones in slices of brain tissue in vitro. Muscarine (30 microM) caused an inward current of about 100 pA in neurones voltage clamped at -60 mV. 2. In about 75% of cells, the current elicited by muscarine was independent of potential in the range -60 to -120 mV and had no associated conductance change. 3. In about 25% of cells, the current became smaller with hyperpolarization, was associated with a decreased conductance, and reversed polarity between -100 and -140 mV. The reversal potential changed with the logarithm of the extracellular potassium concentration. Barium and caesium blocked inward rectification and also prevented reversal of the muscarine current. 4. When potassium ions of the intracellular and extracellular solutions were replaced by caesium, the current evoked by muscarine became smaller with depolarization at reversed polarity at +9 mV. This current was associated with an increase in conductance, and was greatly reduced when the extracellular sodium concentration was reduced to 20 mM. 5. The results could be quantitatively accounted for by a model in which muscarine both increases a voltage-independent cation conductance and decreases the inward rectifier potassium conductance.

Muscarinic agonists activate an inwardly rectifying potassium conductance in medial pontine reticular formation neurons of the rat in vitro

Intracellular recordings were obtained from neurons in pontine reticular formation slices of the rat to characterize a cholinergic-gated increase in conductance. The conductance increase was associated with a hyperpolarization of the membrane potential and with an outward current under voltage-clamp conditions. Current-voltage relations and potassium substitution experiments indicated mediation by a change in permeability, primarily to potassium. This potassium conductance exhibited inward rectification at membrane potentials negative to resting potential, a novel finding for cholinergic actions in CNS neurons. Further characterization of this inwardly rectifying potassium conductance revealed marked sensitivity to low concentrations of barium. Cholinergically evoked currents were relatively unaffected by the presence of extracellular cesium. Cholinergic effects persisted in TTX. The outward currents elicited by carbachol or methacholine were blocked only by high concentrations of pirenzepine, a selective antagonist of the M1 muscarinic receptor. The interaction between these agents is quantitatively consistent with cholinergic action at postsynaptic muscarinic receptors of the non-M1 subtype.

Calcium-activated potassium conductance in presynaptic terminals at the crayfish neuromuscular junction.

Membrane potential changes that typically evoke transmitter release were studied by recording intracellularly from the excitor axon near presynaptic terminals of the crayfish opener neuromuscular junction. Depolarization of the presynaptic terminal with intracellular current pulses activated a conductance that caused a decrease in depolarization during the constant current pulse. This conductance was identified as a calcium-activated potassium conductance, gK(Ca), by its disappearance in a zero-calcium/EGTA medium and its block by cadmium, barium, tetraethylammonium ions, and charybdotoxin. In addition to gK(Ca), a delayed rectifier potassium conductance (gK) is present in or near the presynaptic terminal. Both these potassium conductances are involved in the repolarization of the membrane during a presynaptic action potential.

Ca(2+)-insensitive modulation of a K+ conductance by inositol polyphosphates

Macrophages derived from phorbol ester-induced human leukemic (HL-60) cells exhibit a voltage-activated inward rectifying potassium conductance which was modulated by macrophage colony-stimulating factor (Wieland, S. J., Chou, R. H., and Gong, Q. H. (1990) J. Cell. Physiol. 142, 643-651). Roles of intracellular messengers in this regulatory mechanism were investigated. Intracellular dialysis with inositol 1,3,4,5-tetrakisphosphate (IP4) or inositol 1,4,5-trisphosphate during tight-seal whole cell recording produced a rapid increase in the inward rectifying conductance. Changes in intracellular Ca2+ levels alone did not reproduce the stimulatory effect of these modulators. Intracellular dialysis with guanosine 5'-O-(thiotriphosphate) (GTP gamma S) resulted in profound inhibition of this conductance. These data suggest a novel cellular function for inositol polyphosphates, particularly IP4, and show antagonistic modulation with GTP gamma S on a human macrophage inward rectifier.

Voltage‐activated potassium channels in blowfly photoreceptors and their role in light adaptation.

1. The membrane properties of the photoreceptors of the blowfly (Calliphora vicina) were investigated in situ by making intracellular recordings in the intact retina, using discontinuous single-electrode current and voltage clamp techniques. Single channels were investigated using inside-out patches from dissociated photoreceptors. 2. Photoreceptors have a resting potential in darkness of -60.4 +/- 6.6 mV (mean +/- S.D.; n = 43), a resting input resistance of 32 +/- 3 M omega (n = 11) and membrane time constant of 4.1 +/- 1 ms (n = 9). These values give a total cell capacitance of 0.13 nF and an effective membrane area of 1.3 x 10(-4) cm2. 3. Single-electrode voltage clamp reveals a voltage-sensitive outward current with an activation threshold at approximately -75 mV. This conductance has two kinetic components, the slower component activating at more depolarized levels. On the basis of its kinetics, a reversal potential of -85 +/- 6 mV (n = 6), sensitivity to intracellularly injected tetraethylammonium chloride (TEA), and its slow and partial inactivation (approximately 25%) this mechanism is classified as a delayed rectifier potassium conductance. 4. Voltage-sensitive potassium channels showing similar properties were found in excised inside-out patches from dissociated photoreceptors. Single-channel conductances are ca 20 pS for both fast and slow kinetic components, indicating a channel density in the intact cell of ca 2 microns -2. The reversal potential follows the Nernst slope for potassium ions. 5. The voltage dependence of the conductance was determined in patches containing channels of predominantly one or the other kinetic component. The midpoint of the activation curve is -65 mV for the fast and -50 mV for the slow component. Activation time constants (measured from a holding potential of -100 mV) are voltage dependent, and in the range 1-10 ms for the fast and 5-40 ms for the slow component. Both kinetic components are blocked by TEA (greater than 2.5 mM). The slow component is more sensitive to quinidine (greater than 200 microM), and the fast component to 4-aminopyridine (4-AP; greater than 200 microM). 6. In the intact preparation the outward current shows no dependence on light stimulation in the studied ranges of voltage (up to -25 mV) and intensity (up to 5.5 x 10(4) effective photons). Ensemble averages of channel openings in perfused inside-out patches show no dependence on calcium concentration in the range 10 nM-1.8 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Models of membrane resonance in pigeon semicircular canal type II hair cells

Pigeon vestibular semicircular canal type II hair cells often exhibit voltage oscillations following current steps that depolarize the cell membrane from its resting potential. Currents active around the resting membrane potential and most likely responsible for the observed resonant behavior are the Ca(++)-insensitive, inactivating potassium conductance IA (A-current) and delayed rectifier potassium conductance IK. Several equivalent circuits are considered as representative of the hair cell membrane behavior, sufficient to explain and quantitatively fit the observed voltage oscillations. In addition to the membrane capacitance and frequency-independent parallel conductance, a third parallel element whose admittance function is of second order is necessary to describe and accurately predict all of the experimentally obtained current and voltage responses. Even though most voltage oscillations could be fitted by an equivalent circuit in which the second order admittance term is overdamped (i.e., represents a type of current with two time constants, one of activation and the other of inactivation), the sharpest quality resonance obtained with small current steps (around 20 pA) from the resting potential could be satisfactorily fit only by an underdamped term.

Opioids act at μ-receptors to hyperpolarize arcuate neurons via an inwardly rectifying potassium conductance

Intracellular recordings were made from 48 hypothalamic arcuate (ARC) neurons under current- and voltage-clamp in slices prepared from female guinea pigs which had been ovariectomized and pretreated with estradiol. Twenty ARC neurons were silent (RMP: -62 ± 2 mV) and 28 cells were spontaneously active (7.3 ± 1.1 Hz; threshold-57 ± 1 mV). The input resistance ( R in), determined in the potential range between −60 and −80 mV, was 358 ± 30 M μ ( n = 38) and ARC neurons showed inward rectification at potentials negative to the equilirbium potential for potassium. The selective μ-opioid agonist Tyr- D-Ala-Gly-MePhe-Gly-ol (DAGO) was applied by pressure pipette application at concentrations of 10 or 20 μM. DAGO decreased spontaneous firing and it hyperpolarized 26 of 31 neurons (9.6 ± 0.8 mV; range 3–21 mV). Concomitant with the hyperpolarization, DAGO caused a decrease in R in of 32±3, and the reversal potential, measured from current-voltage plots, was −94 ± 2 mV. These effects wre mimicked bny batgh concentrations of 0.5–1.0 μM DAGO. In voltage clamp, DAGO caused an outward current to flow at −60 mV (range 50–185 pA, n=6). This current reversal at −92±2 mV ( n = 6) and exhibited inward rectification. An additional 6 ARC neurons were tested with DAGO in varying extracellular concentrations of K + (2.5, 5 and 10 mM) and the reversal potential for the effect of DAGO shifted by 58 mV per decade change in extracellular K + concentration. DAGO decreased spontaneous postsynaptic potentials in some cells, but TTX (1 μM) had no effect on the ability of DAGO to hyperpolarize the membrane. The hyperpolarization and decrease in R in induced by DAGO were blocked by the opioid antagonist naloxone (100 nM−1 μM). DAGO responsive cells were unaffected by a κ-opioid agonist (trans-(±)-3,4-dichloro- N-methyl- N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide methanesulphonate; U50,488H), however, 2 of 5 cells also were hyperpolarized by a selective σ-receptor opioid agonist (Tyr- D-Pen-Gly-Phe- D-Pen; DPDPE). The effects of DPDPE, but not DAGO, were blocked by a σ-antagonist (ICI 174,864; 1 μM). The present results indicate that activation of ARC μ-receptors leads to an increase in an inwardly rectifying potassium conductance and a subsequent hyperopolarization of most ARC neurons. We suggest that this μ-receptor-induced hyperpolarization of ARC neurons may inderlie the opiod inhibition of reproductive events in the mammal.

Muscarine reduces inwardly rectifying potassium conductance in rat nucleus accumbens neurones.

1. Intracellular recordings were made from neurones in the nucleus accumbens in slices from the rat brain maintained in vitro. 2. Muscarine (1-100 microM) depolarized 101 of 107 neurones; this was associated with an increase in the input resistance. The potential change reversed polarity with conditioning hyperpolarization and the reversal potential was linearly related to the logarithm of the extracellular potassium concentration. 3. The depolarization caused by muscarine was not changed by tetrodotoxin (1 microM) or by a solution that contained lower levels of calcium (0.24 instead of 2.4 mM), higher levels of magnesium (5 instead of 1.2 mM) and cobalt (2 mM). 4. Muscarine caused an inward current and a decrease in slope conductance when applied to neurones voltage clamped near their resting potential (-82 mV). The current caused by muscarine reversed polarity at the potassium equilibrium potential. The current-voltage relation of the neurones between -60 and -120 mV was well fitted by assuming a voltage-independent potassium conductance and an inward rectifier potassium conductance; muscarine reduced predominantly the inward rectifier conductance. 5. Phorbol-12,13-diacetate (3 microM) and 5-hydroxytryptamine mimicked the action of muscarine. The inward currents caused by muscarine or 5-hydroxytryptamine were occluded by the inward current evoked by the phorbol ester. 6. The depolarization caused by muscarine was competitively antagonized by pirenzepine; the dissociation constant of 11 nM suggested involvement of the M1 receptor. 7. It is concluded that muscarine acts at M1 receptors to reduce the membrane potassium conductance and that activation of protein kinase C may be an intermediate step.

5-Hydroxytryptamine acts at 5-HT2 receptors to decrease potassium conductance in rat nucleus accumbens neurones.

1. Intracellular recordings were made from neurones in the nucleus accumbens in slices from the rat brain maintained in vitro. 2. 5-Hydroxytryptamine (5-HT.1-100 microM) depolarized 170 of 203 (84%) neurones and caused them to discharge action potentials. The depolarization was associated with an increase in the input resistance, and was reversed in polarity by conditioning hyperpolarization; this reversal potential was linearly related to the logarithm of the extracellular potassium concentration. 3. Application of 5-HT to neurones voltage-clamped near their resting potential (typically about -80 mV) caused an inward current and a decrease in the slope conductance. The current caused by 5-HT reversed polarity at the potassium equilibrium potential. Analysis with an equivalent circuit model of the neurone at steady state indicated that 5-HT selectively reduced the inward rectifier potassium conductance. 4. The depolarization caused by 5-HT persisted in tetrodotoxin (1 microM). It was reduced but not abolished by a solution that contained lower levels of calcium (0.24 instead of 2.4 mM), higher levels of magnesium (5 instead of 1.2 mM), and cobalt (2 mM). 5. The depolarization caused by 5-HT was competitively antagonized by the 5-HT2 antagonists ketanserin and mianserin with dissociation equilibrium constants of 3 and 45 nM respectively: spiperone (300 nM) also blocked the action of 5-HT. The depolarization was not mimicked or blocked by a number of other agonists and antagonists selective for the 5-HT1 and 5-HT3 receptor types.

Effects of cromakalim (BRL 34915) on potassium conductances in CA3 neurons of the guinea-pig hippocampus in vitro.

The action of the potassium channel activator, cromakalim (BRL 34915), on membrane potential, input resistance and current-voltage-relationship of CA3 neurons in a slice preparation of the guinea-pig hippocampus was investigated by means of intracellular recordings. In the presence of tetrodotoxin, cromakalim (30-100 mumol/l) produced a hyperpolarization up to 4 mV associated with a decrease in input resistance up to 10 MOhms. Determination of the equilibrium potential of the cromakalim action revealed that the hyperpolarization is due to the activation of a potassium conductance. This cromakalim-activated potassium conductance was voltage-dependent, i.e. it increased with hyperpolarization. Among a number of potassium channel blockers tested, only Cs+ (2 mmol/l) and Ba2+ (0.5 mmol/l) were able to inhibit the cromakalim-induced effects. Simultaneously, both cations suppressed the hyperpolarizing inward rectification (anomalous rectification) in these neurons, indicating that cromakalim activated or potentiated an inwardly rectifying potassium conductance. In addition, cromakalim slightly enhanced both amplitude and duration of afterhyperpolarizations following single calcium-dependent action potentials, suggesting that cromakalim might have a weak facilitatory effect on calcium-dependent potassium conductances.