Calcium-dependent Potassium Current Research Articles

A-current modifies the spike of C-type neurones in the rabbit nodose ganglion.

1. In the rabbit nodose ganglion, C-type fibre neurones (C neurones) can be divided into two subtypes according to their after-hyperpolarizing potential (AHP) i.e. those with a fast AHP only and those with a fast AHP and a subsequent slow AHP produced by a slow calcium-dependent potassium current. In addition we have shown that some C neurones can be divided into two groups according to the effect of membrane hyperpolarization on their spikes i.e. type 1 in which duration and amplitude do not change and type 2 in which duration and amplitude decrease significantly. 2. In the present report we studied the effect of A-current (IA) on spike duration, amplitude and slow AHP using intracellular recording techniques. 3. To detect the presence of IA, we first applied a series of increasing rectangular hyperpolarizing pulses to remove IA inactivation and then a short depolarizing pulse to trigger a spike. In type 1 C neurones the lag time of the spike in relation to hyperpolarization remains constant whereas in type 2 C neurones the spike only appears after IA inactivation and lag time in relation to hyperpolarization is lengthened. Thus, type 2 C neurones have an IA while type 1 C neurones do not. The fact that addition of cadmium did not change the lag time in type 2 C neurones shows that the IA is not calcium dependent. 4. Nodose neurones can be orthodromically activated by stimulation of the vagal peripheral process. In this way, after a hyperpolarizing pulse, IA can be fully activated by the orthodromic spike itself. Under these conditions it is possible to analyse the effects of IA on the spike. This was done by increasing either the hyperpolarizing potential, pulse duration, or the delay of the spike after the end of the pulse. We observed that maximum IA inactivation removal was always associated with the lowest duration and amplitude of the spike. 5. When IA inhibitors, 4-aminopyridine (4-AP) or catechol, were applied to type 2 C neurones, the delay of the spike after the hyperpolarization-depolarization test was no longer observed. In addition 4-AP abolished the shortening of the duration of the spike induced by steady hyperpolarization. 6. In type 2 C neurones with slow AHP, the IA-related decrease in spike duration was associated with a disappearance of the slow AHP. This indicates that IA decreases the calcium influx during the spike.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation of a high conductance calcium dependent potassium current in rat hippocampal neurons

Inactivating, high conductance BK-type currents have been recorded from inside-out patches (internal and external K + of 140 mM and 5 mM, respectively), obtained from cultured rat hippocampal neurons. The presence of prominent inactivation, not normally associated with BK channel activity, was dependent on two factors: a depolarizing step to 0 mV from a holding level of −80 mV and internal calcium at a concentration of 0.7 μM. Without the prior conditioning step to a negative potential, unitary currents were not evident at 0 mV; in addition, such currents were not elicited with the stimulus protocol if the internal Ca 2+ was reduced to a level of 0.3 μM. Concomitant with current inactivation was the finding of a delayed activation of BK currents following the depolarizing step. Higher internal calcium, at 100 μM, led to persistent and sustained channel activity at 0 mV which was not dependent on a prior step to −80 mV These results may be relevant to the complex nature of the repolarizing neuronal current Ic which is the macroscopic analogue of the unitary BK current.

Selective regulation of current densities underlies spontaneous changes in the activity of cultured neurons

We study the electrical activity patterns and the expression of conductances in adult stomatogastric ganglion (STG) neurons as a function of time in primary cell culture. When first plated in culture, these neurons had few active properties. After 1 d in culture they produced small action potentials that rapidly inactivated during maintained depolarization. After 2 d in culture they fired large action potentials tonically when depolarized, and their properties resembled very closely the properties of STG neurons pharmacologically isolated in the ganglion. After 3-4 d in culture, however, their electrical properties changed and they fired in bursts when depolarized. We characterized the currents expressed by these neurons in culture. They included two TTX-sensitive sodium currents, a calcium current, a delayed-rectifier-like current, a calcium-dependent potassium current, and two A-type currents. The changes in firing properties with time in culture were accompanied by an increase in inward and decrease in outward current densities. A single-compartment conductance-based model of an STG neuron was constructed by fitting the currents measured in the biological neurons. When the current densities in the model neuron were matched to those measured for the biological neurons in each activity state, the model neuron closely reproduced each state, indicating that the changes in current densities are sufficient to account for the changes in intrinsic properties. These data indicate that STG neurons isolated in culture change their intrinsic electrical properties by selectively adjusting the magnitudes of their ionic conductances.

Properties of inward and outward potassium currents in cultured mouse motoneurons

Inward rectifier potassium currents and calcium-dependent potassium currents have been studied in cultured embryonic mouse motoneurons. Sustained unitary inward rectifier potassium currents were recorded from cell-attached patches and the channel conductance was dependent on external K+ concentration with a value of 25 pS when external K+ was 140 mM. The channel open probability exhibited a sigmoidal dependence on potential with the largest values (near 0.7) at depolarizing patch potentials. Inactivating inward rectifier potassium currents were also recorded in some cell-attached patches following voltage steps to hyperpolarizing potentials with the rate of inactivation faster with larger hyperpolarizing steps. Whole-cell inward rectifier potassium currents increased from an initial level to a steady-state level with hyperpolarizing steps to -120 mV from a holding potential of -60 mV; with larger hyperpolarizing commands the peak currents decayed to the steady-state. The steady-state current-voltage relation exhibited a region of negative slope resistance. External Cs+ (0.5-1 mM) reduced the amplitudes of macroscopic currents and diminished the open times of unitary currents consistent with block of open rectifying channels with an estimated KD for channel block of 1 mM. A large conductance calcium-dependent potassium channel was isolated in inside-out patches with a conductance of 240 pS with symmetrical 140 mM K+ across the patches and a conductance of 110 pS when the external K+ was reduced to 5 mM. With symmetrical K+ the channel open probability exhibited a sigmoid dependence on potential with the largest values, in excess of 0.8, associated with patch depolarization. The dependence of open probability on potential was dependent on the concentrations of internal Ca2+ and external K+. Properties of inward rectifier and calcium-dependent K+ channels, such as the voltage dependence of open probability, are involved in the establishment of cellular excitability in motoneurons. Future studies will be useful to investigate whether channel properties of motoneurons are altered after cell treatment with neurotoxic agents including oxygen radicals or excitotoxic amino acids.

Activation of protein kinase C inhibits potassium currents in cultured endothelial cells.

The effect of protein kinase C on potassium channels in cultured endothelial cells was investigated by using whole-cell patch-clamp techniques. Activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), but not phorbol 12-monomyristate (PMM), an inactive analogue of phorbol esters, depressed an outward calcium-dependent potassium current. The inhibitory actions of PMA and PDBu could be reversed by the kinase inhibitor H-7. Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum calcium pump, and LP-805, a novel vasodilator which also releases endothelium-derived relaxing factors, activated the outward calcium-dependent potassium conductance. PMA and PDBu, but not PMM, reduced the outward conductance induced by cyclopiazonic acid and LP-805. These effects of PMA and PDBu on potassium currents may be mediated either by phosphorylation of ion channels, or by decreasing intracellular calcium concentration.

Facilitation of calcium-dependent potassium current

The activation of Ca-dependent K+ current, Ic, was studied in macropatches on the cell bodies of molluscan neurons. When a depolarizing voltage-clamp pulse was applied repeatedly, Ic facilitated in a manner that resembled the facilitation of synaptic transmitter release. Facilitation was characterized by an increase in Ic amplitude, a progressive increase in instantaneous outward current, and a decrease in utilization time. Experiments were done to investigate the mechanism responsible for Ic facilitation. Facilitation was reduced by microinjection of an exogenous Ca2+ buffer into the cytoplasm, indicating that facilitation is a Ca(2+)-dependent process. It was also reduced at elevated temperatures. Conversely, facilitation was greatly potentiated by blocking the Na/Ca exchange mechanism. It is concluded that the facilitation of Ca-dependent K+ current results from the accumulation of Ca2+ at the inner face of the membrane during the repeated activation of Ca2+ channels by depolarization. The Ca2+ indicator fluo-3 was used in fluorescence imaging experiments to measure changes in [Ca]i near the cell membrane during repeated depolarizing pulses and the interpretation of these results was aided by numerical simulations of Ca2+ accumulation, diffusion, and buffering in the peripheral cytoplasm. These experiments showed that the time course of Ic facilitation matches the time course of Ca2+ accumulation at the membrane. It was found that the strength of Ic facilitation varies among patches on the same neuron, suggesting that the accumulation of Ca2+ is not uniform along the inner surface of the membrane and that gradients in [Ca]i develop and are maintained during trains of depolarizing pulses. Potential mechanisms that may lead to local differences in Ca2+ accumulation and Ic facilitation are discussed.

A major role for calcium-dependent potassium current in action potential repolarization in adrenal chromaffin cells

To determine the extent which Ca dependent K current (I KCa) contributes during an action potential (AP), bovine chromaffin cells were voltage-clamped using a pre-recorded AP as the command voltage waveform. Based on (1) differential sensitivity of I KCa and Ca-independent K current ( I K ) to tetraethylammonium; (2) measurements of AP currents under conditions where Ca activation of I KCa had been abolished; and (3) blockade by charybdotoxin, I KCa comprised 70–90% of the outward K current during AP repolarization. In addition, observations are made concerning the form of AP-evoked Ca current.

GABAB and adenosine receptors mediate enhancement of the K+ current, IAHP, by reducing adenylyl cyclase activity in rat CA3 hippocampal neurons.

1. Gamma-aminobuturic acid-B (GABAB) and adenosine A1 receptors, which are expressed in hippocampal pyramidal cells, are linked to pertussis toxin-sensitive G-proteins known to be coupled negatively to the enzyme adenylyl cyclase. This study investigates the electrophysiological consequences of adenylyl cyclase inhibition in response to stimulation of these receptors. 2. Single-electrode voltage-clamp recordings were obtained from CA3 pyramidal cells in rat hippocampal slice cultures in presence of tetrodotoxin. The calcium-dependent potassium current (IAHP), which is very sensitive to intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP), was used as an electrophysiological indicator of adenylyl cyclase activity. 3. Application of baclofen (10 microM), a selective agonist at GABAB receptors, or adenosine (50 microM) each resulted in a transient decrease followed by a significant enhancement in the amplitude of evoked IAHP. The initial reduction in amplitude of IAHP probably reflects inadequacies in voltage clamp of electronically distant dendritic sites, due to the shunting caused by concomitant activation of potassium conductance by baclofen/adenosine. Comparable increases in membrane conductance in response to the GABAA agonist, muscimol, caused a similar reduction in IAHP. The enhancement of IAHP is consistent with an inhibition of constitutively active adenylyl cyclase. 4. The receptor mediating the responses to adenosine was identified as belonging to the A1 subtype on the basis of its sensitivity to the selective antagonist 8-cyclopentyl-1,3-dipropylxanthine.(ABSTRACT TRUNCATED AT 250 WORDS)

The mechanism of noradrenergic alpha 1 excitatory modulation of pontine reticular formation neurons

The alpha 1 adrenergic receptor occurs in all major divisions of the CNS and is thought to play a role in all behaviors influenced by norepinephrine (NE). In the medial pontine reticular formation (mPRF), the proposed site of adrenergic enhancement of startle responses (Davis, 1984), alpha 1 agonists excite most neurons (Gerber et al., 1990). We here report that alpha 1 excitation results from a reduction of a voltage- and calcium-dependent potassium current, not previously recognized as ligand-modulated. The calcium sensitivity is suggested by its antagonism with Mg2+, Cd2+, Ba2+, low concentrations of tetraethylammonium, and charybdotoxin. The voltage sensitivity of this conductance falls within the membrane potential range critical to action potential generation. Based on this voltage sensitivity, the change in repetitive firing characteristics may be predicted according to a mathematical model of the mPRF neuronal electrophysiology. The predicted response to a 50% decrease in the phenylephrine (PE)-sensitive conductance is similar to the observed responses, with respect to both the current response under voltage-clamp conditions and alterations of the AHP and frequency/current curve. In contrast, modeling a reduction of a voltage-insensitive leak current predicts none of these changes. Thus, the noradrenergic reduction of this current depolarizes the membrane, increases the likelihood of an initial response to depolarizing input, and increases firing rate during sustained depolarization in a manner consistent with an NE role as an excitatory neuromodulator of the mPRF.

Serotonin has different effects on two classes of Betz cells from the cat.

1. Intracellular recording from cat Betz cells in vitro revealed a strong correlation between the dominant effect of serotonin (5-HT) and the Betz cell subtype in which it occurred. In large Betz cells that show posthyperpolarization excitation (termed PHE cells), 5-HT evoked a long-lasting membrane depolarization, whereas 5-HT evoked an initial hyperpolarization of variable duration in smaller Betz cells that show posthyperpolorization inhibition (termed PHI cells). 2. Voltage-clamp studies revealed that 5-HT caused a depolarizing shift of activation of the cation current Ih, which resulted in the depolarization in PHE cells, whereas the hyperpolarization in PHI cells is caused by an increase in a resting potassium conductance. 3. The effect of 5-HT on firing properties during constant current stimulation also differed consistently in the two types of Betz cells. In PHE cells the initial firing rate increased after 5-HT application, but the steady firing was unaffected. The depolarizing shift of Ih activation caused the increase of initial firing rate. 4. In PHI cells 5-HT caused a decrease in spike frequency adaptation. The decrease in adaptation was caused by a combination of two conductance changes. First, 5-HT caused a slow afterdepolarization in PHI cells that could trigger repetitive firing in the absence of further stimulation. The sADP depended on calcium entry through voltage-gated channels and was associated with a decrease in membrane conductance. Second, 5-HT caused reduction of a slow calcium-dependent potassium current that normally contributes to slow adaptation. 5. In conclusion, the effect of 5-HT on excitability differs systematically in Betz cell subtypes in part because they have different dominant ionic mechanisms that are modulated. If we assume that PHE cells and PHI cells represent fast and slow pyramidal tract (PT) neurons respectively, 5-HT will cause early recruitment of fast PT cells and delay recruitment of slow PT cells during low levels of synaptic excitation.

Afterpotentials of penicillin-induced epileptiform neuronal discharges in the motor cortex of the rat in vivo

Interictal spikes and sharp waves in the EEG are followed by intervals in which the excitability of the brain seems to be normal or decreased. Often interictal spikes even appear in rhythmical patterns with intervals in the order of 0.5–2 s. These observations suggest that intrinsic and synaptic inhibitory and excitatory processes are activated which outlast the duration of the interictal discharge. In the present study such afterpotentials were analyzed in penicillin foci of the rat motor cortex in vivo using intracellular recording techniques. Paroxysmal depolarizations (PDS) of neurons within the focus were followed by afterpotentials comprising several components. Fast afterpotentials with a duration of 640 ms were associated with a sevenfold increase in membrane conductance. The fast afterpotentials were depolarizing in the majority of recordings and had an average equilibrium potential of − 62 mV. This equilibrium potential was Cl −- dependent and was not affected by intracellular EGTA or Cs +. It is suggested that these afterpotentials represent GABA A responses. In 38% of the neurons slow afterhyperpolarizations with a twofold increase in membrane conductance and a duration of 2 s were observed. These afterhyperpolarizations had a reversal potential of − 79 mV, were blocked by intracellular Cs +, were reduced in duration and amplitude by intracellular EGTA, and are suggested to present a combination of a GABA B response and a calcium-dependent potassium current. In addition, slow afterdepolarizations with a duration of about 1900 ms were registered in 16% of the recordings. It is concluded that afterpotentials with several intrinsic and synaptic components follow penicillin-induced PDS. Among these are giant Cl −-dependent potentials which probably represent GABA A responses, GABA B responses and a slow calcium- dependent potasssium current. It is suggested that the depolarizing equilibrium potential of the Cl −-dependent component is due to intracellular Cl − accumulation which might favor transition to ictal discharges.

Effects of caffeine on intracellular calcium, calcium current and calcium-dependent potassium current in anterior pituitary GH3 cells.

Caffeine elicits physiological responses in a variety of cell types by triggering the mobilization of Ca2+ from intracellular organelles. Here we investigate the effects of caffeine on intracellular Ca2+ concentration ([Ca2+]i) and ionic currents in anterior pituitary cells (GH3) cells. Caffeine has a biphasic effect on Ca(2+)-activated K+ current [IK(Ca)]: it induces a transient increase superimposed upon a sustained inhibition. While the transient increase coincides with a rise in [Ca2+]i, the sustained inhibition of IK(Ca) is correlated with a sustained inhibition of the L-type Ca2+ current. The L-type Ca2+ current is also inhibited by other agents that mobilize intracellular Ca2+, including thyrotropin releasing hormone (TRH) and ryanodine, but in a matter distinct from caffeine. Unlike the caffeine effect, the TRH-induced inhibition "washes-out" under whole-cell patch-clamp conditions and is eliminated by intracellular Ca2+ chelators. Likewise, the ryanodine-induced inhibition desensitizes while the caffeine-induced inhibition does not. Simultaneous [Ca2+]i and Ca2+ current measurements show that caffeine can inhibit Ca2+ current without changing [Ca2+]i. Single-channel recordings show that caffeine reduces mean open time without affecting single-channel conductance of L-type channels. Hence the effects of caffeine on ion channels in GH3 cells are attributable both to mobilization of intracellular Ca2+ and to a direct effect on the gating of L-type Ca2+ channels.

Bradykinin modulates potassium and calcium currents in neuroblastoma hybrid cells via different pertussis toxin-insensitive pathways

In NG 108-15 cells, bradykinin (BK) activates a potassium current (I K,BK) and inhibits the voltage-dependent calcium current (I Ca,V). BK also stimulates a phosphatidylinositol-specific phospholipase C (PI-PLC). The subsequent release of inositol 1,4,5-trisphosphate and increase in intracellular calcium contribute to I K,BK, through activation of a calcium-dependent potassium current. In membranes from these cells, stimulation of PI-PLC by BK is mediated by G q and/or G 11 two homologous, pertussis toxin-insensitive G proteins. Here, we have investigated the role of G q/l1 in the electrical responses to BK. GTPγS mimicked and occluded both actions of BK, and both effects were insensitive to pertussis toxin. Perfusion of an anti-G q/11α antibody into the pipette suppressed I K,BK, but not the inhibition of I Ca,V by BK. Thus, BK couples to I K,BK via G q/11, but coupling to I Ca,V is most likely via a different, pertussis toxin-insensitive G protein.

Cholecystokinin-gated currents in neurons of the rat solitary complex in vitro.

1. Ionic conductances controlled by type A and type B cholecystokinin (CCK) receptors were studied in neurons of the rat nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMNV), using intracellular and whole-cell patch clamp recordings in current or voltage clamp configuration during bath application of agonists (CCK8, CCK4, BC 264) and antagonists. 2. CCKA receptor-related inhibition was associated with a membrane hyperpolarization and a decrease in input resistance that developed 2-6 min after the arrival of drug into the extracellular medium. These effects were induced by 5 nM CCK8 but not BC 264 and they were blocked by the CCKA antagonist, L-364,718, but not by the CCKB antagonist, L-365,260. 3. CCKA receptor-related inhibition was generated by a potassium current that reversed at a reversal potential E(rev) of -73 +/- 1 (mean +/- SE) mV with bathing potassium concentration [K+]o = 6 mM and at -88 +/- 1 with [K+]o = 3 mM, in agreement with the Nernst equation for potassium ions. 4. CCKB receptor-related excitation was associated with a membrane depolarization and an increase of the input resistance induced by the following agonists at threshold concentrations: CCK8 (0.2 nM) > or = BC 264 (0.4 nM) > CCK4 (10.9 nM). The increase of input resistance was abolished by L-365,260 and was maintained after blockade of the CCKA current by L-364,718. 5. CCKB receptor-related excitation, in the neurons (30% of cases) in which clear response reversal was observed, appeared to be generated by a decrease of a potassium conductance. Responses showed a reversal potential E(rev) of -68 +/- 4 mV with [K+]o = 6 mM and -89 +/- 1 mV with [K+]o = 3 mM, verifying predictions from the Nernst equation applied to potassium ions. However, in 70% of cases, clear reversal was not observed at membrane potentials negative to the theoretical potassium equilibrium potential EK. 6. In voltage clamp studies, CCK8 induced a 181 +/- 17 pA inward current associated with a 26 +/- 4% decrease in the instantaneous current (I(ins)) generated by hyperpolarizing voltage steps. This effect on I(ins) was demonstrated in the absence of effects on the outward noninactivating potassium current (IM) and on the inward noninactivating cationic current (IQ). 7. CCKB receptor-mediated excitation was not suppressed by cobalt, a blocker of calcium currents, and was not associated with a change of the calcium-dependent potassium current (IK(Ca)).(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of a slow synaptic response by a metabotropic glutamate receptor antagonist in hippocampal CA3 pyramidal cells.

The effects of a novel antagonist of metabotropic glutamate receptors were investigated in CA3 pyramidal cells in hippocampal slice cultures of the rat. Earlier experiments showed that selective activation of metabotropic glutamate receptors with low concentrations of an agonist, 1S, 3R-1-amino-cyclopentane-1,3-dicarboxylic acid (ACPD), induced an inward current associated with a decrease in membrane conductance and inhibition of the slow calcium-dependent potassium current. These responses were strongly and reversibly reduced by the antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG, 0.5-1 mM). In the presence of antagonists of ionotropic glutamate receptors, stimulation of the afferent mossy fibres evoked postsynaptic responses in CA3 pyramidal cells which paralleled those observed with exogenously applied metabotropic glutamate receptor agonists, i.e. a slow inward current and a reduction of calcium-dependent potassium current. Both responses were greatly reduced by bath-applied MCPG (1 mM). These results show that MCPG acts as an effective antagonist at metabotropic glutamate receptors coupled to potassium conductances in the hippocampus. Furthermore, they confirm that glutamate release from presynaptic terminals can modulate postsynaptic properties by activation of metabotropic glutamate receptors.

Potassium currents in type II vestibular hair cells isolated from the guinea-pig's crista ampullaris.

Type II vestibular hair cells were isolated from cristae ampullares of guinea-pig and maintained in vitro for 2-3 h. Outward membrane currents were studied under whole-cell voltage-clamp conditions. Type II hair cells had resting potentials of about -45 mV. Depolarizing voltage steps from a holding potential of -80 or -90 mV induced time- and voltage-dependent outward currents which slowly decayed to a sustained level. Tail currents reversed at about -70 mV, indicating that the outward currents were mainly carried by potassium ions. The currents had an activation threshold around -50 mV. The transient component was completely removed by a depolarizing pre-pulse positive to -10 mV. While bath application of 4-aminopyridine (5 mM) reduced both components, extracellular tetraethylammonium (10 mM) or zero calcium preferentially diminished the sustained current. We conclude that at least two potassium conductances are present, a delayed rectifier with a relatively fast inactivation and a calcium-dependent potassium current. Depolarizing current injections induced an electrical resonance in the voltage responses, with a frequency of 25-100 Hz, larger currents causing higher frequencies.

Electrophysiological properties and chemosensitivity of guinea pig nodose ganglion neurons in vitro

Conventional intracellular recording techniques were employed to obtain information on the electrophysiological and pharmacological characteristics of C-type neurons in the guinea pig nodose ganglia. Approximately 90% of the cell bodies gave rise to axons with conduction velocities consistent with C-fibers (0.9–1.1 m/s). The average resting membrane potential and input impedence was about −60 mV and 45 Mω, respectively. Orthodromic electrical stimulation of the vagus nerve 20–30 mm caudal to the ganglion produced overshooting action potentials in the nodose neurons. The falling phase of the action potential was followed by a transient (50–300 ms) fast hyperpolarization (AHP fast). In 20% of C-type neurons the AHP fast was followed by a slowly developing, long-lasting afterhyperpolarization (AHP slow) that limited the ability of the neuron to fire action potentials at high frequency. The AHP slow magnitude was dependent on the number of spikes, had a reversal potential of −87 mV, and was abolished by 100 μM cadmium chloride, suggesting that it is produced by a calcium-dependent potassium current. In about 30% of the nodose neurons, hyperpolarizing current steps from resting potential produced a time- and voltage-dependent anomalous rectification in the electrotonic potential. External cesium (1 mM), but not barium (100 μM) reversibly blocked this effect. Single-electrode voltage-clamp measurements revealed a slowly developing inward current in these neurons that grows in magnitude with step hyperpolarizations from resting potential, and has an estimated reversal potential of about −44 mV. These properties suggest that this current is analogous to I h observed in many peripheral and central neurons. Autacoids including serotonin, histamine, several prostanoids, peptidoleukotriene, and bradykinin, were examined for their ability to affect the excitability of the nodose neurons. Serotonin was the only autacoid capable of depolarizing the membrane potential to action potential firing threshold. The serotonin-induced membrane depolarization was associated with a significant increase in input conductance. Histamine depolarized the membrane potential of the C-type neurons in 28/30 neurons. Bradykinin, prostacyclin, and leukotriene C 4 were found to cause membrane depolarizations in a subset (73%, 31%, and 50%, respectively) of nodose neurons. The AHP slow was virtually abolished by bradykinin, prostacylin, and in a subset of neurons, leukotriene C 4. Inhibition of the AHP slow was accompanied by a change in the accommodative properties of the neurons, reflected by the increased frequency at which the neuron could successfully elicit repetitive action potentials. The results demonstrate that most autacoids do not overtly evoke action potentials in the somata of the guinea pig nodose ganglion C-type neurons, rather they influence their excitability by producing subthreshold membrane depolarizations and by modulating the rate of neuronal discharge.

Reconstruction of ionic currents in a molluscan photoreceptor

Two-microelectrode voltage-clamp measurements were made to determine the kinetics and voltage dependence of ionic currents across the soma membrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, IA and ICa(2+)-K+, the inward voltage-dependent calcium current, ICa2+ and the light-induced current, IIgt, were then described with Hodgkin-Huxley-type equations. The fast-activating and inactivating potassium current, IA, was described by the equation; IA(t) = gA(max)(ma infinity[1-exp(-t/tau ma)])3 x (ha infinity [1-exp(-t/tau ha)] + exp(-t/tau ha)) (Vm-EK), where the parameters ma infinity, ha infinity, tau ma, and tau ha are functions of membrane potential, Vm, and ma infinity and ha infinity are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, ICa(2+)-K+, was described by the equation, ICa(2+)-K+ (t) = gc(max)(mc infinity(VC)(1-exp[-t/tau mc (VC)]))pc (hc infinity(VC) [1-exp(-t/tau hc)] + exp(-t/tau hc(VC)])pc(VC-EK). In high external potassium, ICa(2+)-K+ could be measured in approximate isolation from other currents as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, ICa2+, activated rapidly, showed little inactivation, and was described by the equation: ICa2+ = gCa(max) [1 + exp](-Vm-5)/7]-1 (Vm-ECa), where gCa(max) was 0.5 microS. The light-induced current with both fast and slow phases was described by: IIgt(t) = IIgt1 + IIgt2 + IIgt3, IIgti = gIgti [1-exp(- ton/tau mi)] exp(-ton/tau hi)(Vm-EIgti) (i = 1, 2). For i = 3, /Igt(t) = gigt3m33h3(Vm - Eigt3)exp(-ton/Ton) x exp(-tfoff/t Off). Based on these reconstructions of ionic currents, learning-induced enhancement of the long lasting depolarization (LLD) of the photoreceptor'slight response was shown to arise from progressive inactivation of /A, lca2+ -K+, and lCa2+.

Exploring parameter space in detailed single neuron models: simulations of the mitral and granule cells of the olfactory bulb.

1. Detailed compartmental computer simulations of single mitral and granule cells of the vertebrate olfactory bulb were constructed using previously published geometric data. Electrophysiological properties were determined by comparing model output to previously published experimental data, mainly current-clamp recordings. 2. The passive electrical properties of each model were explored by comparing model output with intracellular potential data from hyperpolarizing current injection experiments. The results suggest that membrane resistivity in both cells is nonuniform, with somatas having a substantially lower resistivity than the dendrites. 3. The active properties of these cells were explored by incorporating active ion channels into modeled compartments. On the basis of evidence from the literature, the mitral cell model included six channel types: fast sodium, fast delayed rectifier (Kfast), slow delayed rectifier (K), transient outward potassium current (KA), voltage- and calcium-dependent potassium current (KCa), and L-type calcium current. The granule cell model included four channel types: rat brain sodium, K, KA, and the non-inactivating muscarinic potassium current (KM). Modeled channels were based on the Hodgkin-Huxley formalism. 4. Representative kinetics for each of the channel classes above were obtained from the literature. The experimentally unknown spatial distributions of each included channel were obtained by systematic parameter searches. These were conducted in two ways: large-scale simulation series, in which each parameter was varied in turn, and an adaptation of a multidimensional conjugate gradient method. In each case, the simulated results were compared wtih experimental data using a curve-matching function evaluating mean squared differences of several aspects of the simulated and experimental voltage waveforms. 5. Systematic parameter variations revealed a single distinct region of parameter space in which the mitral cell model best fit the data. This region of parameter space was also very robust to parameter variations. Specifically, optimum performance was obtained when calcium and slow K channels were concentrated in the glomeruli, with a lower density in the soma and proximal secondary dendrites. The distribution of sodium and fast potassium channels, on the other hand, was highest at the soma and axon, with a much lighter distribution throughout the secondary dendrites. The KA and KCa channels were also concentrated near the soma. 6. The parameter search of the granule cell model was much less restrained by experimental data. Several parameter regimes were found that gave a good match to the data.(ABSTRACT TRUNCATED AT 400 WORDS)

Olfactory signalling in antennal receptor neurones of the locust ( Locusta migratoria)

The electrical properties of isolated antennal neurones from Locusta migratoria were investigated using patch clamp techniques. The neurones displayed a resting potential of −26±11 mV ( n=38) and an input resistance between 1–6 GΩ. Upon application of depolarizing voltage steps a non-inactivating non-specific inward current and a calcium-dependent outward potassium current were identified. Some neurones responded to stimulation by the grass odour hexenoic acid (1–100 μM) with an increase in ion channel activity in cell-attached recordings. This channel was identified as a calcium-activated potassium channel belonging to the maxi-K +-channel types. Odour-stimulation of antennal tissue and antennal cells using a fast quency device leads to a rapid generation of inositol 1,4,5-trisphosphate, supporting the idea that this substance acts as second messenger in insect olfaction. The odour-induced increase in inositol trisphosphate concentration may initiate a rise in intracellular calcium controlling the activity of calcium-dependent ion channels.