Opening of large-conductance calcium-activated potassium channels by the substituted benzimidazolone NS004

1. We used electrophysiological techniques to examine the effects of 5-trifluoromethyl-1-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-2H-benzimidaz ole- 2-one (NS004) on large-conductance calcium-activated potassium (BK) channels. 2. We used recordings from excised membrane patches (cell-attached and inside-out single-channel configurations) and whole-cell patch-clamp recordings to examine the effects of NS004 on single BK channels and whole-cell outward currents, respectively, in rat GH3 clonal pituitary tumor cells. We also tested NS004 on voltage-clamped BK channels isolated from rat brain plasma membrane preparations and reconstituted into planar lipid bilayers. Finally, we used two-electrode voltage-clamp techniques to study the effects of NS004 on currents expressed in Xenopus laevis oocytes by the recently described Slo BK clone from Drosophila. 3. In GH3 cells and in Xenopus oocytes expressing the Slo gene product NS004 produced an increase in an iberiotoxin- or tetraethylammonium-sensitive whole-cell outward current, respectively. NS004 produced a significant increase in the activity of single GH3 cell BK channels and rat brain BK channels reconstituted into planar lipid bilayers. In both systems this was characterized by an increase in channel mean open time, a decrease in interburst interval, and an apparent increase in channel voltage/calcium sensitivity. 4. These data indicate that NS004 could be useful for investigating the biophysical and molecular properties of BK channels and for determining the functional consequences of the opening of BK channels.

A toxin from the venom of the predator snail Conus textile modulates ionic currents in Aplysia bursting pacemaker neuron

Conus textile crude venom and a peptide component ('King Kong' toxin) purified from this venom, alter membrane excitability of Aplysia neurons. Venom, applied to the medium bathing an abdominal ganglion, changes dramatically the electrical activity of bursting pacemaker neuron. The effects on bursting neuron R15 was examined in current-clamp and voltage-clamp modes. A dual phase effect of both the venom and the purified toxin were observed. The first phase starts immediately after venom or toxin application and is observed as an increase in membrane excitability, resulting in an enhancement of bursting. The second phase begins about 15 min later and consists of a long-lasting hyperpolarization. The dual phase effect of the venom and the toxin persists even when synaptic input is eliminated either by axotomy, or by recording from freshly dissociated neurons or from neurons in primary cell culture. The ionic currents affected are an inward current, INSR, which is activated upon depolarization and an anomalously rectifying potassium current, IR, which is activated upon hyperpolarization. In the first phase of toxin action INSR is increased. In the second phase both the venom and the toxin block INSR and increase IR. The toxin effects may be due to complex alteration of one or more second messenger cascades rather than a direct action on ion channels.

Properties and modulation of a calcium-activated potassium channel in rat olfactory bulb neurons

1. Single calcium-activated potassium channels (KCa channels) were recorded from membrane patches of rat olfactory bulb neurons in culture. Only one kind of KCa channel was seen, and it was present in approximately 50% of detached patches. 2. This channel, like maxi-KCa channels of other tissues, had a single-channel conductance of 270 pS, a reversal potential (Erev) of 0 mV in symmetrical K+, and was highly selective for K+ over Na+ and Cl-. 3. The KCa channel was blocked by d-tubocurarine (d-TC) on the cytoplasmic side, and charybdotoxin (CTX) on the extracellular side. This pharmacology is identical to that of one type of KCa channel from rat brain, observed previously in artificial bilayers and called the type 1 KCa channel. 4. The probability that the channel was in the open state (Po) increased with membrane depolarization. The position of the Po versus transmembrane voltage (Vm) curve was shifted by changes in [Ca2+]i so that the channel was open more often in higher [Ca2+]i. The gating kinetics resembled those of the type 1 KCa channel observed in bilayers. 5. Po was increased after superfusion of the cytoplasmic membrane surface with the active catalytic subunit of cyclic AMP-dependent protein kinase (PK-A), together with MgATP. Phosphorylation altered the distribution of channel closed times but had little effect on open times. The results suggest that phosphorylation is an important molecular mechanism in modulating the activity of this KCa channel from mammalian brain.

Quisqualate and ACPD are agonists for a glutamate-activated current in identified Aplysia neurons

1. Glutamate is an important neurotransmitter in both vertebrates and invertebrates, yet the characterization of molluscan glutamate receptors and their relationships to vertebrate receptors is incomplete. This study uses two-electrode voltage clamp to characterize glutamate-evoked currents in cultured neurons from the marine gastropod mollusk Aplysia californica. 2. The identified buccal ganglion neurons, B1 and B2, display two pharmacologically distinct current responses to pressure-applied glutamate. One response is a desensitizing chloride current similar to that found in other large buccal ganglion neurons. The other is a nondesensitizing potassium current that is strongly outwardly rectifying. 3. The potassium current response has a higher sensitivity to glutamate than the chloride response. 4. Individual neurons, isolated in primary cell culture, exhibit different relative proportions of the two currents. 5. The glutamate agonists quisqualate and (1S,3R)-aminocyclopentane-1,3-dicarboxylic acid specifically evoke the potassium response but not the chloride response. The glutamate agonist (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid does not evoke any appreciable response. Kainate and N-methyl-D-aspartate also have no effect on these cells. Another glutamate analogue, ibotenate, evokes the transient chloride current but not the potassium current. 6. These results indicate that the glutamate receptor mediating the outward potassium response does not conform in its pharmacological profile to any of the known vertebrate glutamate receptor types.

A peptide derived from the Shaker B K+ channel produces short and long blocks of reconstituted Ca(2+)-dependent K+ channels

A 20 amino acid synthetic peptide, corresponding to the amino-terminal region of the Shaker B (ShB) K+ channel and responsible for its fast inactivation, can block large conductance Ca(2+)-dependent K+ channels from rat brain and muscle. The ShB inactivation peptide produces two kinetically distinct blocking events in these channels. At lower concentrations, it produces short blocks, and at higher concentrations long-lived blocks also appear. The L7E mutant peptide produces only infrequent short blocks (no long-lived blocks) at a much higher concentration. Internal tetraethylammonium competes with the peptide for the short block, which is also relieved by K+ influx. These results suggest that the peptide induces the short block by binding within the pore of Ca(2+)-dependent K+ channels. The long block is not affected by increased K+ influx, indicating that the binding site mediating this block may be different from that involved in the short block. The short block of Ca(2+)-dependent K+ channels and the inactivation of Shaker exhibit similar characteristics with respect to blocking affinity and open pore blockade. This suggests a conserved binding region for the peptide in the pore regions of these very different classes of K+ channel.

Modulation of growth of Aplysia neurons by an endogenous lectin

We have purified and characterized a galactose-binding lectin from the gonads of the mollusk Aplysia californica that modulates neurite outgrowth from cultured Aplysia neurons. Agglutination of sheep red blood cells (RBC) by this lectin, termed Aplysia gonad lectin (AGL), is inhibited strongly by galactose and to a lesser extent by fucose. On SDS-PAGE, AGL appears as a single species with a molecular weight of 34 kD under reducing conditions, and 65 kD under nonreducing conditions. This suggests that AGL is a disulfide-linked dimer in its native state. Amino terminal sequence analysis of purified AGL indicates a similarity to another galactose-binding lectin, phytohemagglutinin-E (E-PHA), found in red kidney beans. By using polyclonal antibodies prepared against AGL, we have found that the lectin is present in the gonads and eggs but not in other tissues of adult Aplysia californica. We have examined biological actions of AGL on Aplysia neurons growing in primary cell culture. AGL affects several properties of these neurons. The addition of 100 nM AGL to cultured neurons enhances neurite outgrowth from the cell soma, resulting in a greater number of primary processes. In addition, AGL acts as a neurotrophic agent, increasing neurite viability in vitro. This trophic effect is not seen with concanavalin A (con A), another lectin known to affect several properties of cultured Aplysia neurons. The results are consistent with the suggestion that AGL may play a role in neuronal differentiation and/or maintenance of viability.

Na(+)-activated K+ channels are widely distributed in rat CNS and in Xenopus oocytes

We recorded the activity of K+ channels activated by sodium (KNa channels) in two widely used preparations, primary cell cultures prepared from neocortex, cerebellum, midbrain, brainstem and spinal cord, and Xenopus oocytes. KNa channels from all regions shared an absolute dependence on [Na+], had conductances of 140-170 pS in symmetrical 150 mM K+ and exhibited characteristic substates. The role of this channel must now be considered in terms of its widespread distribution.

Directional control of neurite outgrowth from cultured hippocampal neurons is modulated by the lectin concanavalin A

Cell surface carbohydrates play an important role in the regulation of neurite outgrowth during neuronal development. We have investigated the actions of the plant lectin concanavalin A (Con A), a carbohydrate-binding protein, on neurite outgrowth from hippocampal pyramidal neurons in primary cell culture. Neurons plated in culture medium containing nanomolar concentrations of Con A have a larger number of primary neurites arising directly from the cell soma than do neurons plated in culture medium alone. Furthermore, Con A causes counterclockwise turning of neurites in over 70% of the cultured neurons. Both of these effects of Con A are blocked by the hapten sugar alpha-methyl-D-mannopyranoside, suggesting that they result from the interaction of Con A with a cell surface carbohydrate. Another lectin with a different sugar specificity, wheat germ agglutinin, does not modulate neurite outgrowth. Analysis of neurite outgrowth using video-enhanced microscopy reveals that the counterclockwise turning is accompanied by directionally biased extension of filopodia from the growth cones of growing neurites. Treatment of the neurons with cytochalasin, which disrupts actin polymerization, eliminates the neurite turning induced by Con A, suggesting that actin microfilaments are involved in directional control of neurite outgrowth.

Properties and rundown of sodium-activated potassium channels in rat olfactory bulb neurons

We have used single-channel recording techniques to investigate the properties of sodium-activated potassium channels (KNa channels) in cultured rat olfactory bulb neurons, and in large neurons in the mitral cell layer of thin slices of olfactory bulb. Ion channels highly selective for potassium over sodium and chloride, and requiring 10-180 mM internal sodium (Nai) for their activation, were present in approximately 75% of inside-out membrane patches detached from cultured olfactory bulb neurons. Most of these patches contained several KNa channels. KNa channels were seen in cell-attached patches only when Nai was raised by including veratridine in the extracellular medium. Preincubation of the cell in TTX or removal of extracellular sodium prevented this effect of veratridine, confirming that the channels observed under these conditions were indeed KNa channels. Lithium did not substitute for Nai in activating these channels. With 150 mM potassium on both sides of the membrane, KNa channels had a single-channel conductance of 172 pS, and at least two subconducting states were observed in addition to this fully open state. Under these ionic conditions, the channels exhibited linear fully open channel current-voltage curves over the potential range of -100 to 0 mV. At voltages more positive than the potassium equilibrium potential, the single-channel currents exhibited inward rectification as a result of sodium block of outward potassium current. The channels opened in bursts, during which they fluctuated between the fully open and closed states, and the substates. Between bursts they sometimes entered a long-lived inactive state that could last for up to several minutes. In addition, KNa channels in the detached patches exhibited rundown, a progressive irreversible loss in activity, over a time course that varied from less than 1 min to longer than 1 hr. Rundown of KNa channel activity in cell-attached patches (in the presence of veratridine) did not occur, suggesting that some intracellular factor necessary for KNa channel activity is lost when the membrane patch is detached from the cell.

Protein kinase activity closely associated with a reconstituted calcium-activated potassium channel

Modulation of the activity of potassium and other ion channels is an essential feature of nervous system function. The open probability of a large conductance Ca(2+)-activated K+ channel from rat brain, incorporated into planar lipid bilayers, is increased by the addition of adenosine triphosphate (ATP) to the cytoplasmic side of the channel. This modulation takes place without the addition of protein kinase, requires Mg2+, and is mimicked by an ATP analog that serves as a substrate for protein kinases but not by a nonhydrolyzable ATP analog. Addition of protein phosphatase 1 reverses the modulation by MgATP. Thus, there may be an endogenous protein kinase activity firmly associated with this K+ channel. Some ion channels may exist in a complex that contains regulatory protein kinases and phosphatases.

Concanavalin A: a tool to investigate neuronal plasticity

Neuronal plasticity is the ability of neurons to alter their cellular properties in response to changes in their environment. These changes are typically triggered by the binding of specific ligands, such as neurotransmitters, growth factors or other neuromodulators, to receptors on the neuronal membrane surface. Since the extracellular domains of many of these receptors are glycosylated, they can also be bound by lectins--proteins with high affinity binding sites for polysaccharides. Different lectins have different affinities for various sugar residues. This feature has made lectins useful in the investigation of the regional localization and relative mobility of different classes of glycosylated membrane receptors, and in the subsequent purification of the receptors. This article reviews some of the different kinds of neuronal plasticity produced by the plant lectin concanavalin A (Con A), such as enhancement of neurite outgrowth, modulation of neurotransmitter responses, and alteration in the specificity and strength of synaptic connections.

Modulation of calcium-activated potassium channels from rat brain by protein kinase A and phosphatase 2A

By incorporating plasma membrane vesicles into planar lipid bilayers, we previously characterized a family of four types of Ca(2+)-activated K+ channels from rat brain (Reinhart et al., 1989). Two of these are "large-conductance" or "maxi"-K+ channels, which differ in their gating kinetics and toxin sensitivity and are henceforth referred to as "type 1" and "type 2" channels. Here we show that the gating of these two channel types can be modulated by phosphorylation and dephosphorylation. The effects of cAMP-dependent protein kinase catalytic subunit (PK-A) on type 1 maxi-K+ channels are complex in that, while half of these channels are upregulated by the kinase, about one out of seven channels is downregulated. Thus, there may be several distinct channels within the type 1 category. Type 2 maxi-K+ channels are consistently downregulated by PK-A. The effects of PK-A on both channel types are reversed by the catalytic subunit of protein phosphatase 2A (PP-2A), but not by protein phosphatase 1 (PP-1). Furthermore, some of the type 1 maxi-K+ channels can be modulated by PP-2A, even without any prior PK-A treatment, indicating they are in a phosphorylated state when they are incorporated into the bilayer. The results demonstrate that (1) type 1 and type 2 maxi-K+ channels are substrates for PK-A; (2) phosphorylation can shift the open probability of channels in either direction, by a mechanism involving multiple phosphorylation sites; (3) phosphorylation alters the Ca2+/voltage sensitivity of these channels; and (4) dephosphorylation of type 1 and type 2 channels is catalyzed by specific phosphatases.

Role of arachidonic acid in depolarization-induced modulation of ion currents in Aplysia giant neurons

1. The effects of membrane depolarization on inward currents subsequently elicited by hyperpolarization were studied with the use of two-electrode, voltage-clamp techniques in the giant neurons LP1 and R2 of Aplysia. 2. Several successive sets of brief depolarizing pulses, or bursts, were used to depolarize the giant neurons. Two distinct inward currents elicited by hyperpolarization were found to be altered after these sets of depolarizing pulses. These currents were distinguished by their voltage dependence, reversal potential, and sensitivity to 1 mM BaCl2. One of the inward currents was increased after depolarization. It was outwardly rectifying, reversed at -50 mV, and not blocked by Ba2+, suggesting it was a chloride current (ICl). The other inward current, which was decreased after depolarization, was inwardly rectifying, reversed at -70 mV, and completely inhibited by Ba2+. These are characteristics of the inwardly rectifying potassium current (IR), a current previously described to be inhibited after depolarization in Aplysia neuron R 15. Depolarization typically increased the putative ICl and decreased IR for minutes, with the decrease in IR consistently outlasting the increase in an initial brief net increase in inward current followed by a long-lasting decrease. 3. Several criteria suggest arachidonic acid (AA) may mediate depolarization-induced modulation of IR. Previously, free AA has been shown to constitutively inhibit IR in the resting state. Also, depolarization has been reported to stimulate liberation of AA from storage in Aplysia ganglia. Consistent with previous results in neuron R 15, depolarization-induced modulation of IR in giant neurons was dependent on external calcium. Indomethacin and 4-bromophenacylbromide (BPB), pharmacologic agents that activate IR through inhibition of AA turnover, altered the effect of depolarization on IR. In contrast serotonin (5HT), which activates IR through adenosine 3',5'-cyclic monophosphate (cAMP), did not alter the effect of depolarization. Also, extended perfusion with bovine serum albumin (BSA), which strips AA from lipid storage in neurons, decreased the depolarization-induced modulation of IR. We conclude that the calcium influx accompanying depolarization activates the phospholipase responsible for liberation of AA from phospholipid, and the liberated AA then acts to inhibit IR. The molecular mechanism of this AA-mediated inhibition remains to be determined. 4. Depolarization-induced modulation of ICl was also dependent on external calcium but was not affected by BPB and only slightly decreased with indomethacin. This suggested AA was probably not involved in this modulation. However, 5HT opposed the modulation of IC1 induced by previous depolarization, suggesting cAMP may be involved in this effect of depolarization.

Constant turnover of arachidonic acid and inhibition of a potassium current in Aplysia giant neurons

Steady-state currents at hyperpolarized membrane potentials were studied in the homologous giant neurons, LP1 and R2, of Aplysia using two-electrode voltage clamp. Nearly half of the steady-state current at voltages more hyperpolarized than -70 mV had characteristics similar to the inwardly rectifying potassium current (IR) described previously in Aplysia neurons. The pharmacological agents 4-bromophenacylbromide, indomethacin, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate were found to modulate IR. IR was stimulated with BPB and indomethacin and inhibited with TPA. These agents altered IR by a mechanism independent of cAMP, which can also modulate IR. The effects of these modulators are consistent with their actions on arachidonic acid (AA) metabolism in Aplysia nervous system, suggesting AA may constitutively inhibit IR. When ganglia were perfused for 12 hr with medium containing BSA to absorb extracellular fatty acids, IR was increased nearly twofold. This increase was partially inhibited by addition of AA to the perfusion medium, and completely inhibited by pretreatment of ganglia with BPB. Although no direct effect of short-term exposure to exogenous AA was observed, long term exposure to exogenous AA and several other unsaturated fatty acids was accompanied by a decrease in IR.

Regulation of intracellular free arachidonic acid in Aplysia nervous system

We have studied the regulation of arachidonic acid (AA) uptake, metabolism, and release in Aplysia nervous system. Following uptake of [3H]AA, the distribution of radioactivity in intracellular and extracellular lipid pools was measured as a function of time in the presence or absence of exogenous AA. The greatest amount of AA was esterified into phosphatidylinositol (relative to pool size). We found that the intracellular free AA pool underwent rapid turnover, and that radioactive free AA and eicosanoids were released at a rapid rate into the extracellular medium, both in the presence and absence of exogenous AA. Most of the released radioactivity originated from phosphatidylinositol. Two pharmacological agents were found to modulate AA metabolism in Aplysia ganglia. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate, stimulated liberation of AA from phosphatidylinositol and phosphatidylcholine. This resulted in an increase in free internal and secreted AA, an increase in conversion of AA to eicosanoids, and an increase in esterification of AA into triacylglycerol. The half maximal dose for TPA-stimulated AA turnover was 15 nM, and the stimulation was dependent on the presence of extracellular calcium. 4-bromophenacylbromide inhibited the redistribution of radioactivity from phospholipid into triacylglycerol, indicating BPB was acting as a phospholipase inhibitor in Aplysia as it does in other systems. These pharmacological agents, in addition to providing information about the regulation of AA metabolism and release, are useful tools for investigating the physiological function of the rapid turnover of AA in Aplysia nervous system.

Activity-dependent neuromodulation in Aplysia neuron R15: intracellular calcium antagonizes neurotransmitter responses mediated by cAMP

1. The effect of electrical activity on the response to the neuromodulators serotonin (5-HT) and the neuropeptide egg-laying hormone (ELH) was studied in the Aplysia bursting pacemaker neuron R15. 2. Previous work has shown that 5-HT and ELH augment R15s bursting activity by enhancing two ionic currents, an inwardly rectifying K+ current (IR) and a voltage-gated Ca2+ current (ICa), and that the enhancement of the currents is mediated by the intracellular second-messenger adenosine 3',5'-cyclic monophosphate (cAMP). Here we show that both spontaneous action potentials and voltage-clamp depolarizations suppress the modulation by 5-HT and ELH of these currents. Both spontaneous and evoked depolarizations decrease the magnitude and dramatically speed the decay of the modulation of IR and ICa. 3. The depolarization-induced suppression is blocked by intracellular ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N',-tetraacetic acid (EGTA), indicating that the suppression is Ca-dependent. The suppression is specific for responses mediated by cAMP; a non-cyclic AMP-mediated response to acetylcholine is not affected by depolarizing pulses. 4. The Ca-dependent suppression of IR modulation differs from the Ca-dependent suppression of ICa modulation. Ca2+ influx decreases the sensitivity of IR to neuromodulators without reducing the maximal response elicited by high concentrations of neuromodulators. In contrast, Ca2+ not only decreases the sensitivity of ICa but also reduces the maximal effect elicited by high concentrations of neuromodulators. We have shown previously that intracellular Ca2+ also inactivates the basal IR and ICa in neuron R15 by distinct mechanisms. The inactivation of IR is due to an antagonistic action of Ca2+ on cAMP metabolism, whereas the inactivation of the basal ICa is due primarily to a more direct action of Ca2+, perhaps on the Ca channels themselves. 5. We also studied the interaction between action potentials and neuromodulator released onto R15 from an endogenous source: bag cell neurons, which release large amounts of ELH during an intense "afterdischarge." IR and ICa become greatly enhanced during the afterdischarge, even though R15 continually fires action potentials. In addition, Ca-dependent inactivation of IR is suppressed during the afterdischarge. We suggest that the bag cells release an amount of ELH sufficient to temporarily saturate the cAMP-mediated enhancement of IR and that this temporarily prevents the suppressive effects of Ca2+ on IR. 6. The activity-dependent suppression of neuromodulation in neuron R15 is an example of neuronal plasticity that results from interactions between intracellular messengers.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective formation and modulation of electrical synapses between cultured Aplysia neurons

When dissociated neurons from the mollusc, Aplysia californica, are placed in primary cell culture, they form electrical synapses in a specific, yet alterable, manner. Pairs of neurons from the same ganglion ("homoganglionic" pairs) form electrical synapses with high coupling coefficients. This is due to relatively high macroscopic junctional conductance as determined directly by voltage clamping both neurons of each pair. By contrast, synapses between pairs of neurons from different ganglia ("heteroganglionic" pairs) exhibit lower coupling coefficients as a result of lower macroscopic junctional conductance. Both types of junction are nonrectifying, not gated by voltage, and resistant to uncoupling by octanol and heptanol. This dichotomy of synaptic efficacy is altered upon exposure of the neurons to the lectin, conacanavalin A (Con A). Acute treatment of heteroganglionic cell pairs with Con A increases their junctional conductance to the higher level characteristic of homoganglionic pairs within several hours. However, the higher junctional conductance of homoganglionic pairs is not modulated by Con A. The results presented here suggest that synaptic specificity among these regenerating neurons may be mediated at least in part by ganglion-specific cell-recognition molecules. Furthermore, these molecules may be, or may be linked to, lectin receptors that regulate gap junction channels.

Concanavalin A modulates a potassium channel in cultured Aplysia neurons

A novel 100 pS K(+)-selective ion channel is frequently observed in cell-attached membrane patches from cultured Aplysia neurons. The activity of this channel is moderately voltage-dependent, but channel openings are rare and brief even when the patch is strongly depolarized. However, the activity of the channel is increased dramatically by the addition of the lectin concanavalin A (Con A), to the patch pipette. The channel is also activated by Con A in the bathing medium, suggesting that the lectin's action is via an as yet unidentified intracellular second messenger. In the one single-channel patch studied, Con A had no effect on the channel mean open time; rather it decreased the average duration of the long closed times between bursts of openings. Thus Con A increases either the open probability of single channels, the number of functional channels in the patch, or both. The functional significance of the Con A-induced modulation of K+ channel activity remains to be determined.

A family of calcium-dependent potassium channels from rat brain

By incorporating rat brain plasma membrane vesicles into planar lipid bilayers, we have found and characterized four types of Ca2(+)-activated K+ channels. The unitary conductances of these channels are 242 +/- 14 pS, 236 +/- 16 pS, 135 +/- 10 pS, and 76 +/- 6 pS in symmetrical 150 mM KCI buffers. These channels share a number of properties. They are all activated by depolarizing voltages, activated by micromolar concentrations of internal Ca2+ with a Hill coefficient for Ca2+ activation of between 2 and 3, noninactivating under our assay conditions, blocked by low millimolar concentrations of TEA from the outside, apamin-insensitive, and very selective for K+ over Na+ and Cl-. Three of the four channels are also blocked by nanomolar concentrations of charybdotoxin. One of the high conductance Ca2(+)-activated K+ channels is novel in that it is not blocked by charybdotoxin and exhibits gating kinetics highlighted by long closed times and long open times. This family of closely related Ca2(+)-activated K+ channels may share structural domains underlying particular functions.

Modulation of a subthreshold calcium current by the neuropeptide FMRFamide in Aplysia neuron R15

1. The effect of the endogenous neuropeptide FMRFamide (Phe-Met-Arg-Phe-amide) on the Aplysia bursting pacemaker neuron R15 was studied. Brief local applications of FMRFamide, both on R15 somata in situ, and on R15 somata that were isolated and maintained in primary cell culture, cause a hyperpolarization of the membrane potential and a suppression of spontaneous bursting or beating pacemaker activity. 2. Two-electrode voltage-clamp experiments revealed that FMRFamide decreases the amplitude of an inward current, which activates with depolarization starting at a membrane potential less depolarized than the threshold for action potentials. Previous studies have established that this subthreshold inward current is carried by calcium and is essential for the generation of bursting pacemaker activity in Aplysia neurons. The effect of FMRFamide on the subthreshold inward current of R15 is blocked by divalent cation calcium channel blockers, such as cobalt and manganese, and is unaffected by changing the external concentration of potassium or chloride ions, or addition of blockers of the calcium-activated potassium current, such as external tetraethylammonium or internal EGTA. 3. The subthreshold calcium current of R15 is also decreased by dopamine and by an unidentified synaptic neurotransmitter. These substances mimic and occlude the action of FMRFamide on the subthreshold calcium current, suggesting that all three transmitters converge to affect the same population of calcium channels in neuron R15. 4. The subthreshold calcium current is enhanced by neurotransmitters that elevate cyclic AMP in R15, including serotonin, and the Aplysia neuropeptide egg-laying hormone (ELH). Likewise, the effect of FMRFamide on the subthreshold calcium current is enhanced by serotonin, ELH, and a cyclic AMP analog, suggesting that FMRFamide and cyclic AMP have antagonistic actions on the same population of calcium channels in neuron R15. 5. We conclude that the suppression of spontaneous bursting or beating pacemaker activity in neuron R15 by FMRFamide is due to a decrease in the subthreshold calcium current. The subthreshold calcium current in R15 is a common target for modulation by many different transmitters, acting via several distinct molecular mechanisms.

Calcium-dependent inactivation of a potassium current in the Aplysia neuron R15

The endogenously bursting pacemaker neuron R15 of Aplysia exhibits an inwardly rectifying K+ current (IR) that was shown previously to be enhanced by various neurotransmitters via the intracellular second messenger, cyclic AMP (Drummond et al., 1980; Benson and Levitan, 1983; Levitan et al., 1987). Here we present evidence that Ca2+ influx, either caused by spontaneous bursting activity or elicited by depolarizing voltage-clamp pulses, causes a large, long-lasting inactivation of IR. The ionic current inactivated by bursts is identified as IR by several criteria: it activates steeply at membrane potentials more negative than the K+ equilibrium potential, has very fast kinetics, is reduced by lowering external K+ from 10 to 2 mM, and is blocked by adding 1 mM Ba2+, 10 mM Cs+, or 5 mM Rb+ to the bathing medium. The peak inactivation of IR is delayed following a single burst of spikes in R15, such that IR decreases maximally by about 20% after 60-90 sec, and then recovers gradually over more than 10 min. The inactivation caused by many bursts of spikes can reduce IR to less than 50% of its initial amplitude. The delay in onset and slow time course of recovery from inactivation of IR suggest that a complex biochemical mechanism underlies the effect of Ca2+ on IR. The effect of depolarization on IR is due specifically to the influx and intracellular accumulation of Ca2+. Depolarizing voltage-clamp pulses are maximally effective at reducing IR when they elicit a large influx of Ca2+, while pulses approaching the Ca2+ equilibrium potential have little effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of calcium-dependent inactivation of a potassium current in Aplysia neuron R15: interaction between calcium and cyclic AMP

In the preceding paper (Kramer and Levitan, 1988), we presented evidence that an inwardly rectifying K+ current (IR) is inactivated by Ca2+ influx accompanying spontaneous bursting activity in the Aplysia neuron R15. In this paper we examine the mechanism that enables Ca2+ to inactivate IR. Since IR is enhanced by cyclic AMP in neuron R15 (Drummond et al., 1980; Benson and Levitan, 1983), we examined the Ca2+-dependent inactivation of IR after application of either serotonin (5-HT), the adenylate cyclase activator forskolin, or a membrane-permeable cAMP analog, all agents that increase cAMP and hence the magnitude of IR. Even though more active IR channels are available under these conditions, less Ca2+-dependent inactivation is observed. This is contrasted with the Ca2+-dependent inactivation of the voltage-gated Ca2+ current (ICa). Elevating cAMP enhances ICa in R15 and also increases its Ca2+-dependent inactivation. Hence the mechanisms whereby Ca2+ inactivates IR and ICa appear to differ from each other. Elevating internal Ca2+ by repeatedly depolarizing the neuron suppresses the response of IR to brief applications of 5-HT, and speeds the relaxation of the response, suggesting that Ca2+ can interfere with the cAMP-dependent activation of IR. One biochemical site where Ca2+ can reduce cellular cAMP is by activating the Ca2+/calmodulin-sensitive form of phosphodiesterase. We have detected such enzyme activity in homogenates of Aplysia abdominal ganglia and extracts of single R15 somata. Inhibitors of the phosphodiesterase activity suppress the Ca2+-dependent inactivation of IR. Finally, we have used a radioimmunoassay to measure cAMP in individual R15 somata, and have found that R15 neurons hyperpolarized for prolonged periods contain more cAMP than do R15 neurons allowed to burst, consistent with the hypothesis that Ca2+ influx reduces cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrotonic synapses between Aplysia neurons in situ and in culture: aspects of regulation and measurements of permeability

Properties of electrotonic synapses between L14 neurons in the abdominal ganglion of the marine mollusc Aplysia californica were examined in situ and between unidentified buccal neurons maintained in tissue culture. In culture, depolarizing postsynaptic potentials in response to a train of action potentials showed apparent facilitation with increasing spike number, which was attributable to the low-pass filter properties of electrotonic transmission via gap junctions and to network properties. Gap junctional conductance (gj), calculated from current-clamp data or measured directly under voltage clamp, indicated no significant dependence of gj on transjunctional or inside-outside potential in situ or in culture. Octanol, a local anesthetic agent that reduces gj in many other systems, had no effect on gj between Aplysia neurons. The effect of intracellular acidification, a treatment that rapidly and reversibly uncouples a variety of cell types, reduced gj between Aplysia neurons but did not completely abolish it. The relationship between intracellular pH (pHi), measured with ion-sensitive microelectrodes, and gj was steeper in cultured neurons than in situ and was maximally reduced by 70-80%, as compared to 50% or less in situ at the lowest pHi values tested. The coupling coefficient (k) was reduced less by low pHi than was gj, which could be explained by a simultaneous increase in nonjunctional membrane resistance. Permeability properties of Aplysia electrotonic synapses to a variety of tracer molecules were also examined between identified L 14 neurons in situ and in dissociated buccal, abdominal, and bag neurons in culture. The fluorescent dyes Lucifer yellow, 6-carboxyfluorescein, and dichlorofluorescein (1.2-1.4 nm maximal diameters) did not spread detectably from an injected neuron to its electrically coupled neighbors, regardless of the strength of electrotonic coupling. However, the smaller tetraalkylammonium ions TMA and TEA (diameters 0.66 and 0.8 nm, concentrations measured with ion-selective electrodes), could be detected in neighboring cells within minutes. In culture, transfer of the tetraalkylammonium ions was slow and not easily detectable in cell pairs where gj was low (less than 20 nS). The permeability was as high as 10(-10) cm3/sec in situ and 10(-12) cm3/sec in culture, and values were roughly correlated with simultaneously measured values of gj. Electrotonic synapses in the nervous system of Aplysia, therefore, have a quantitatively different spectrum of sensitivities than has been found for gap junctions of other systems and appear to possess reduced permeability to tracer molecules.

Serotonin acting via cyclic AMP enhances both the hyperpolarizing and depolarizing phases of bursting pacemaker activity in the Aplysia neuron R15

Bath application of 5-HT, at concentrations below 10 microM, enhances the amplitude of the interburst hyperpolarization in the Aplysia bursting pacemaker neuron R15. It is known that 5-HT acts via cyclic AMP to produce this effect by increasing the inwardly rectifying potassium current (IR). Here, we report that further elevating the concentration of 5-HT produces an enhancement of the depolarizing phase of the burst cycle that can eventually lead to tonic spiking activity. Voltage-clamp studies reveal that high concentrations of 5-HT continue to increase IR and, in addition enhance a voltage-gated inward current active near the action potential threshold. Pharmacological treatments and ion substitution experiments demonstrate that the inward current increased by high concentrations of 5-HT is a subthreshold calcium current (ICa). The 5-HT-induced increase in ICa is mimicked by bath application of the adenylate cyclase activator forskolin or injection of 8-bromo-cyclic AMP and is potentiated by the phosphodiesterase inhibitor isobutylmethylxanthine. It is concluded that 5-HT, acting via the second messenger cyclic AMP, can increase both potassium and calcium currents in neuron R15. It is also shown that the 5-HT-induced increase in these 2 opposing voltage-gated currents not only produces complex changes in bursting activity, but also dramatically alters R15's response to inhibitory and excitatory stimuli.