The Ca2+-sensitive K+-currents underlying the slow afterhyperpolarization of bullfrog sympathetic neurones

Functional triads consisting of ryanodine receptors, Ca(2+) channels, and Ca(2+)-activated K(+) channels in bullfrog sympathetic neurons. Plastic modulation of action potential

Fluorescent ryanodine revealed the distribution of ryanodine receptors in the submembrane cytoplasm (less than a few micrometers) of cultured bullfrog sympathetic ganglion cells. Rises in cytosolic Ca(2+) ([Ca(2+)](i)) elicited by single or repetitive action potentials (APs) propagated at a high speed (150 microm/s) in constant amplitude and rate of rise in the cytoplasm bearing ryanodine receptors, and then in the slower, waning manner in the deeper region. Ryanodine (10 microM), a ryanodine receptor blocker (and/or a half opener), or thapsigargin (1-2 microM), a Ca(2+)-pump blocker, or omega-conotoxin GVIA (omega-CgTx, 1 microM), a N-type Ca(2+) channel blocker, blocked the fast propagation, but did not affect the slower spread. Ca(2+) entry thus triggered the regenerative activation of Ca(2+)-induced Ca(2+) release (CICR) in the submembrane region, followed by buffered Ca(2+) diffusion in the deeper cytoplasm. Computer simulation assuming Ca(2+) release in the submembrane region reproduced the Ca(2+) dynamics. Ryanodine or thapsigargin decreased the rate of spike repolarization of an AP to 80%, but not in the presence of iberiotoxin (IbTx, 100 nM), a BK-type Ca(2+)-activated K(+) channel blocker, or omega-CgTx, both of which decreased the rate to 50%. The spike repolarization rate and the amplitude of a single AP-induced rise in [Ca(2+)](i) gradually decreased to a plateau during repetition of APs at 50 Hz, but reduced less in the presence of ryanodine or thapsigargin. The amplitude of each of the [Ca(2+)](i) rise correlated well with the reduction in the IbTx-sensitive component of spike repolarization. The apamin-sensitive SK-type Ca(2+)-activated K(+) current, underlying the afterhyperpolarization of APs, increased during repetitive APs, decayed faster than the accompanying rise in [Ca(2+)](i), and was suppressed by CICR blockers. Thus, ryanodine receptors form a functional triad with N-type Ca(2+) channels and BK channels, and a loose coupling with SK channels in bullfrog sympathetic neurons, plastically modulating AP.

Evidence for the calcium-dependent potentiation of M-current obtained by the ratiometric measurement of the fura-2 fluorescence in bullfrog sympathetic neurons

Intracellular Ca2+ concentration ([Ca]i) was measured following the activation of an inward Ca2+ current and subsequent potentiation of an M-type K+ current (IM) in bullfrog sympathetic neurons. Fura-2 was used as an indicator for [Ca]i. The fluorescence ratio at 340 and 380 nm (F340/F380) was elevated from 0.36 to 1.22 when IM was potentiated by 68% following the Ca2+ current. Based on the in vivo calibration curve obtained from cells permeabilized with digitonin (20 microM), the F340/F380 value of 1.22 was equivalent to a [Ca]i of 0.97 microM. We therefore propose that a rise in [Ca]i into the micromolar range can lead to the potentiation of IM in amphibian autonomic neurons.

Potassium currents in adult rat intracardiac neurones

1. Properties of K+ currents were studied in isolated adult rat parasympathetic intracardiac neurones with the use of single-electrode voltage-clamp techniques. 2. A hyperpolarization-activated inward rectifier current was revealed when the membrane was clamped close to the resting level (-60 mV). The slowly developing inward relaxation had a mean amplitude of 450 pA at -150 mV, an activation threshold of -60 to -70 mV and a relaxation time constant of 41 ms at -120 mV. The current was reversibly blocked by Cs+ (1 mM) and became smaller with reduced [K+]o and [Na+]o, indicating that this inward rectifier current probably is a time- and voltage-dependent Na(+)-K+ current. 3. Step depolarizations from the holding potential of -80 mV evoked a transient (< 100 ms at -40 mV) outward K+ current (IA) which was blocked by 4-aminopyridine (4-AP, 1 mM). The time constants for IA inactivation were 20 ms at -50 mV and 16 ms at -20 mV. The steady-state activation and (removal of) inactivation curve showed a small overlap between -70 and -40 mV; the reversal potential of IA was close to EK. 4. Step hyperpolarizations from the depolarized potentials, i.e. -30 mV, revealed a slow inward relaxation associated with the deactivation of a time- and voltage-dependent current. The inward relaxation became faster at more hyperpolarized potentials and reversed at -85 and -53 mV in 4.7 and 15 mM [K+]o. This current was blocked by muscarine (20 microM) and Ba2+ (1 mM) but not affected by Cs+ (1 mM); this current may correspond to the M-current (IM). 5. Depolarization-activated outward K+ currents were evoked by holding the membrane close to the resting potential in the presence of tetrodotoxin (TTX, 3 microM), 4-AP (1 mM) and Ba2+ (1 mM). The amplitude of the outward relaxation and the tail current became smaller as the [K+]o was elevated. The outward tail current was reduced in a Ca(2+)-free solution and the residual current was eliminated by the addition of tetraethylammonium (TEA, 10 mM); the reversal potential was shifted in a direction predicted by the Nernst equation. These findings suggest the presence of delayed rectifier K+ current and Ca(2+)-activated K+ current. 6. Superfusion of TEA, Ba2+ and 4-AP, but not Cs+, induced rhythmic discharges in some of the otherwise quiescent intracardiac neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

The slow Ca(2+)-activated K+ current, IAHP, in the rat sympathetic neurone

1. Adult and intact sympathetic neurones of the rat superior cervical ganglion maintained in vitro at 37 degrees C were analysed using the two-electrode voltage-clamp technique in order to investigate the slow component of the Ca(2+)-dependent K+ current, IAHP. 2. The relationship between the after-hyperpolarization (AHP) conductance, gAHP, and estimated Ca2+ influx resulting from short-duration calcium currents evoked at various voltages proved to be linear over a wide range of injected Ca2+ charge. An inflow of about 1.7 x 10(7) Ca2+ ions was required before significant activation of gAHP occurred. After priming, the gAHP sensitivity was about 0.3 nS pC-1 of Ca2+ inward charge. 3. IAHP was repeatedly measured at different membrane potentials; its amplitude decreased linearly with membrane hyperpolarization and was mostly abolished close to the K+ reversal potential, EK (-93 mV). The monoexponential decay rate of IAHP was a linear function of total Ca2+ entry and was not significantly altered by membrane potential in the -40 to -80 mV range. 4. Voltage-clamp tracings of IAHP could be modelled as a difference between two exponentials with tau on approximately 5 ms and tau off = 50-250 ms. 5. Sympathetic neurones discharged only once at the onset of a long-lasting depolarizing step. If IAHP was selectively blocked by apamin or D-tubocurarine treatments, accommodation was abolished and an unusual repetitive firing appeared. 6. Summation of IAHP was demonstrated under voltage-clamp conditions when the depolarizing steps were repeated sufficiently close to one another. Under current-clamp conditions the threshold depolarizing charge for action potential discharge significantly increased with progressive pulse numbers in the train, suggesting that an opposing conductance was accumulating with repetitive firing. This frequency-dependent spike firing ability was eliminated by pharmacological inhibition of the slow IAHP. 7. The IAHP was significantly activated by a single action potential; it was turned on cumulatively by Ca2+ load during successive action potential discharge and acted to further limit cell excitability.

Theophylline affects three different potassium currents in dissociated rat cortical neurones

1. The effects of theophylline in pyramidal neurones acutely dissociated from the rat frontal cortex were investigated in the whole-cell configuration, using the nystatin-perforated patch-clamp technique. 2. Ten millimolar theophylline evoked triphasic responses: a small slow outward current (Iso), then a large transient outward current (Ito) and finally a slow sustained inward current (Isi). The reversal potentials of the three current components shifted 56-58 mV for a 10-fold change in extracellular K+ concentration, thereby indicating that all these current components were predominantly carried by K+. 3. Iso had no voltage dependence, whereas Ito showed a steep outward rectification. Iso was relatively resistant to tetraethylammonium (TEA) with an IC50 of 10 mM. Ito was susceptible to submillimolar TEA with an IC50 of 0.8 mM. 4. Isi was a net inward current mainly resulting from suppression of the M-current (IM). 5. These three current components had a distinct concentration dependence; in particular, Isi was evoked at a relatively lower concentration range. 6. Ito was not observed when the intracellular Ca2+ was chelated by 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) of 10 mM, using the conventional whole-cell recording configuration, whereas both Iso and Isi were retained but gradually diminished. 7. In Ca(2+)-free external solution, these responses were fully elicited by the first application of theophylline. However, Ito disappeared during successive applications and Iso, but not Isi, also decreased. Similar results were obtained in the presence of ryanodine. 8. Theophylline apparently affects three different kinds of K+ currents in rat cortical neurones. Both Iso and Ito depend on internal calcium mobilized from an intracellular Ca2+ store by theophylline, while Isi was not primarily mediated by a change in [Ca2+]i.

Muscarinic modulation of conductances underlying the afterhyperpolarization in neurons of the rat basolateral amygdala

The excitability level of pyramidal neurons in the basolateral amygdala (BLA) is greatly increased following muscarinic receptor activation, an effect associated with an increased rate of action potential firing and reduction of the afterhyperpolarization (AHP). We impaled BLA pyramidal neurons in slices of rat ventral forebrain with a single microelectrode to examine the currents underlying the AHP and spike frequency accommodation and determine their sensitivities to muscarinic modulation. In voltage-clamp, depolarizing steps were followed by biphasic outward tail currents, consisting of rapidly decaying (IFast) and slowly decaying (ISlow) current components. These corresponded temporally with the medium and slow portions of the AHP, respectively. The reversal potential for the IFast component of the AHP tail current shifted in the depolarizing direction with increases in the extracellular K+ concentration. The amplitude of IFast was reduced during perfusion of 0-Ca2+ medium or by superfusion of TEA (1-5 mM) or carbachol (10-40 microM). It is suggested that IFast was produced by the rapidly decaying Ca(2+)-activated K+ current (IC) and the muscarinic-sensitive M-current (IM). The ISlow tail current component reversed at the estimated values for EK in medium containing either normal or elevated K+ levels. This component was eliminated by perfusion of 0-Ca2+ medium or inclusion of cyclic-AMP in the recording electrode. It was not blocked by TEA (5 mM) or apamin (50-500 nM), but was reduced by carbachol in a dose-dependent manner (IC50 = 0.5 microM). Electrical stimulation of cholinergic afferent pathways to the BLA produced inhibition of ISlow, an effect which was enhanced by eserine and prevented by atropine. Loss of the ISlow component was always accompanied by similar reductions in accommodation and the slow AHP. It was concluded that this tail current component resulted from the slowly decaying Ca(2+)-activated K+ current, IAHP. Thus, the muscarinic inhibition of IAHP contributes to the enhanced excitability exhibited by BLA pyramidal neurons following cholinergic stimulation.

Intracellular calcium dynamics in response to action potentials in bullfrog sympathetic ganglion cells

1. Dynamic changes in the intracellular free Ca2+ concentration ([Ca2+]i) following electrical membrane activity, were recorded from the neurone soma of the excised bullfrog sympathetic ganglion, using Fura-2 fluorescence and compared with the accompanying Ca(2+)-dependent electrical membrane responses. 2. The resting [Ca2+]i was about 100 nM, a value little changed by penetration with an intracellular electrode. 3. A net rise in fluorescence at a wavelength of 340 nm (Ca2+ transient) induced by a single action potential in Ringer solution rose almost in parallel with the initial decay phase of a slow Ca(2+)-dependent after-hyperpolarization; decayed in parallel with the late phase; and increased in amplitude and duration in the presence of tetraethylammonium (20 mM). 4. A Ca2+ transient induced by repetitive action potentials was increased asymptotically in amplitude and progressively in duration by increasing the number of spikes, and was slower in time course than the associated Ca(2+)-dependent K+ current. 5. Scanning a single horizontal line across the cytoplasm with an ultraviolet argon ion laser (351 nm) and recording Indo-1 fluorescence with a confocal microscope demonstrated an inward spread of a rise in [Ca2+]i following a tetanus. 6. Both single spike- and tetanus-induced Ca2+ transients were abolished in a Ca(2+)-free solution, while single or repetitive transient rises in [Ca2+]i induced by caffeine (5-10 mM) were generated under the same conditions. 7. Ryanodine (10-50 microM) did not affect tetanus-induced Ca2+ transients, whereas it blocked completely the caffeine-induced oscillation of [Ca2+]i. 8. Ca2+ transients induced by a tetanus in Ringer solution were independent of the interval from the preceding tetanus. The amplitude of Ca2+ transients induced by a tetanus in the presence of caffeine (5 mM) was equal to, or greater than, that generated in Ringer solution in any of the phases of [Ca2+]i oscillation. 9. It is suggested that under the physiological conditions here, the induction of action potentials does not cause the release of Ca2+ in the cells of the freshly excised bullfrog sympathetic ganglion, and that Ca(2+)-buffering systems contribute not only to lowering a transient rise in [Ca2+]i but also to sustaining an increased [Ca2+]i after a large Ca2+ load into the cell.

Compound 48/80 blocks transmission and increases the excitability of ganglion neurons

Compound 48/80 (5.0-50 micrograms/ml) significantly and reversibly decreased (1) the amplitude, but not the shape of the compound action potential, (2) the amplitude and duration of the acetylcholine potential and (3) the residual fast excitatory postsynaptic potential recorded from neurons of the 9th and 10th paravertebral ganglia of the bullfrog Rana catesbeiana. The excitability of B-type ganglion neurons in the presence of nicotinic and muscarinic receptor antagonists was increased by compound 48/80 without altering the input resistance or membrane potential. In addition, compound 48/80 (10-50 micrograms/ml) significantly decreased the duration of the spike afterhyperpolarization (AHP). The amplitude but not the decay rate of the current underlying the slow component of the spike AHP was decreased by compound 48/80. Compound 48/80 did not, however, alter either the amplitude or the duration of calcium-dependent spikes. Intracellular recordings from dissociated sympathetic neurons also demonstrated a compound 48/80-induced increase in neuronal excitability. These results suggest that compound 48/80 interacts with the nicotinic receptor/channel complex to decrease ganglionic transmission, and also has a direct action to increase neuronal excitability by blocking potassium channels mediating the duration of the spike AHP.

Blockade by local anaesthetics of the single Ca(2+)-activated K+ channel in rat hippocampal neurones

1. Effects of local anaesthetics on single Ca(2+)-activated K+ channels were investigated using the inside-out configuration of the patch-clamp technique in single pyramidal neurones, which were freshly dissociated from rat hippocampus by use of proteolytic enzymes. 2. No significant effect was observed when 2 mM benzocaine was applied on either side of the membrane patch, or when 2 mM lignocaine or QX-314 was applied to the external surface of the membrane. 3. Lignocaine 1 mM, applied to the internal surface, slightly reduced the amplitude of the single K+ channel current. When applied to the internal surface QX-314 reduced the amplitude of the K+ channel current, accompanied by an increase in noise in the open channel current, suggesting a fast flickering block. The blocking effect of QX-314 on the outward current increased with depolarization, suggesting a binding site for the drug at an electrical distance of about 0.5 across the membrane field. 4. The open time histogram showed one exponential component and the closed time histogram showed at least two components. The mean open time of the outward current was increased when the amplitude was reduced by the drugs. 5. The ionized form of the local anaesthetics had a similar action on the Ca(2+)-activated K+ channels to that on Na+ channels, that is, they enter into the channel from the cytoplasmic side to induce open channel block. The blocking kinetics, however, might be so fast that they were beyond the frequency response of our recording apparatus, thus the recorded current amplitude was decreased. In contrast the K+ channel was not accessible via hydrophobic pathways for the neutral form, which is also known to block the sodium channel.

The effects of soman on the electrical properties and excitability of bullfrog sympathetic ganglion neurones

1. The effects of soman (0.1-10 microM), an irreversible inhibitor of acetylcholinesterase (AChE), were examined on the electrical properties of ganglion neurones of the paravertebral sympathetic chain of the bullfrog, Rana catesbeiana. 2. Soman (10 microM) depolarized 29 of 35 (83%) ganglion neurones studied by 6.4 +/- 0.65 mV within 10 min of application and reduced the cell input resistance in 9 of 11 neurones examined (82%) to 55 +/- 5.3% of control. 3. Soman (10 microM) significantly reduced the maximum amplitude and the maximum rate of rise of the action potential and the duration, but not the amplitude, of the after-hyperpolarization (AHP) following the action potential elicited by either direct or antidromic stimulation. The maximum rate of fall and the duration of the action potential were not significantly affected by soman. These actions of soman were independent of the agent-induced depolarization. When examined by a single microelectrode voltage clamp, soman reduced the amplitude and the time constant of the current underlying the slow AHP, IAHs. 4. Soman (1-10 microM) produced an increase in neuronal excitability which was evidenced as either an increase in the number of action potentials or a decrease in the interspike interval in response to constant-current depolarizing pulses. The soman-induced increase in excitability occurred independently of both the agent-induced depolarization and the decrease in input resistance, was reversible with washing, was not caused by an inhibition of the M-current and was also recorded in dissociated sympathetic ganglion neurones.5. The effects of soman on the membrane potential, input resistance and the duration of the AHP but not cell excitability were blocked by pretreatment with atropine (10 microM). Pretreatment with dihydro-/J-erythroidine (DHbetalE) (10 microM) was ineffective in blocking or reversing the effects of soman. These results suggest that the direct actions of soman on the electrical properties of these neurones are mediated by activation of muscarinic receptors.

A five-conductance model of the action potential in the rat sympathetic neurone

The origin of the action potential in neurones has yet to be answered satisfactorily for most cells. We present here a five-conductance model of the somatic membrane of the mature and intact sympathetic neurone studied in situ in the isolated rat superior cervical ganglion under two-electrode voltage-clamp conditions. The neural membrane hosts five separate types of voltage-dependent ionic conductances, which have been isolated at 37 degrees C by using simple manipulations such as conditioning-test protocols and external ionic pharmacological treatments. The total current could be separated into two distinct inward components: (1) the sodium current, INa, and (2) the calcium current, ICa; and three outward components: (1) the delayed rectifier, IKV, (2) the transient IA, and (3) the calcium-dependent IKCa. Each current has been kinetically characterized in the framework of the Hodgkin-Huxley scheme used for the squid giant axon. Continuous mathematical functions are now available for the activation and inactivation (where present) gating mechanisms of each current which, together with the maximum conductance values measured in the experiments, allow for a satisfactory reconstruction of the individual current tracings over a wide range of membrane voltage. The results obtained are integrated in a full mathematical model which, by describing the electrical behaviour of the neurone under current-clamp conditions, leads to a quantitative understanding of the physiological firing pattern. While, as expected, the fast inward current carried by Na+ contributes to the depolarizing phase of the action potential, the spike falling phase is more complex than previous explanations. IKCa, with a minor contribution from IKV, repolarizes the neurone only under conditions of low cell internal negativity. Their role becomes less pronounced in the voltage range negative to -60 mV, where membrane repolarization allows IA to deinactivate. In the spike arising from these voltage levels the membrane repolarization is mainly sustained by IA, which proves to be the only current sufficiently fast and large enough to recharge the membrane capacitor at the speed observed during activity. Different modes of firing coexist in the same neurone and the switching from one to another is fast and governed by the membrane potential level, which makes the selection between the different voltage-dependent channel systems. The neurone thus seems to be prepared to operate within a wide voltage range; the results presented indicate the basic factors underlying the different discrete behaviours.

The calcium-dependent potassium conductance in rat sympathetic neurones

1. Adult and intact sympathetic neurones of isolated rat superior cervical ganglia were subjected to a two-electrode voltage-clamp analysis at 37 degrees C in order to investigate the Ca2(+)-dependent K+ conductance. 2. At each potential a Ca2(+)-dependent K+ current, IKCa, was determined as the difference between the current that could be attributed to the voltage-dependent K+ current, IKV, following Ca2+ channel blockade by Cd2+ and the total current generated. The final IKCa curves were obtained after correcting the experimental tracings for the underlying ICa current component. 3. IKCa became detectable during commands to -30 mV. About 3.6 x 10(5) Ca2+ ions are required to enter the cell before IKCa is initiated. The current was modelled on the basis of a 0.4-0.6 ms delay followed by an exponential activation of a fast component, IKCaf, simultaneously with a much slower exponential activation, IKCas. Experiments indicate a sigmoidal activation curve for the fast conductance, gKCf, with half-maximal activation at -13.0 mV and a slope factor of 4.7 mV (for 5 mM-Ca2+ in the bath). The associated time constant, tau kcf, ranged from 0.8 to 2.0 ms. The slow conductance exhibited a similar steady-state activation curve but an activation time constant in the 48-280 ms range. The maximum mean gKC was 0.32 microS per neurone for either the fast or slow component. 4. Excess K+ ions accumulate in the perineuronal space during K+ current flow giving rise to rapidly occurring, large K+ reversal potential (EK) modifications (up to -45 mV for the largest currents). The kinetics of K+ extracellular load can be described satisfactorily by a simple exponential function (tau = 0.9-2.8 ms). The characteristics of K+ wash-out appear similar to those of accumulation. 5. The immediate effect of such an extracellular K+ build-up is to make the apparent IKCa activation kinetics faster and to reduce (up to 50%) the true value of the K+ conductance. We simulated the predictions of a K+ diffusion model and generated new functions describing the IKCa steady-state activation, activation rate and maximum conductance values which satisfactorily reconstruct the IKCa current tracings together with the K+ accumulation process near the membrane. 6. A small component of the Ca2(+)-dependent K+ current, IAHP, was observed which survived at membrane potential levels negative to -40 mV. Increasing Ca2+ influx by applying longer pulses enhanced IAHP, which on the other hand was also activated by depolarizations of short duration.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular calcium ions are required for muscarine-sensitive potassium current in bullfrog sympathetic neurons

Cultured bullfrog sympathetic neurons were voltage-clamped in the whole-cell configuration. The extracellular medium contained tetrodotoxin (3 microM) and cesium (1 mM) to block and inward sodium current and a hyperpolarization-activated cation current Attempts were made to separate the M-current from four other potassium currents. Tetraethylammonium (30 mM) was used to block a classical delayed rectifier current (IK) and a fast calcium-activated current (IC). Apamin (30 nM) was used to block a slow calcium-activated current (IAHP). 4-Aminopyridine (1 mM) was used to reduce the amplitude of a transient current (IA). In these conditions, the maximum M-conductance near 0 mV was reduced by as much as 90% when divalent cations such as cobalt (1 mM) were added to the superfusate. The maximum M-conductance was also reduced by as much as 60% when calcium ions were removed from the superfusate. The half-activation voltage in the steady-state activation curve and the reversal potential of the M-current were not significantly changed in the calcium-free solution. It is suggested that the presence of calcium ions in the extracellular space is required for the M-current activation.

Cyclic AMP regulates an inward rectifying sodium-potassium current in dissociated bull-frog sympathetic neurones

1. Bull-frog sympathetic neurones in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 2. A hyperpolarization-activated sodium-potassium current (H-current: IH) was separated from other membrane currents in a nominally calcium-free solution containing cobalt (2 mM), magnesium (4 mM), barium (2 mM), tetraethylammonium (20 mM), tetrodotoxin (3 microM), apamin (30 nM) and 4-aminopyridine (1 mM). IH was selectively blocked by caesium (10-300 microM). 3. The steady-state activation of IH occurred between -60 and -130 mV. The H-conductance was 4.1-6.6 nS at the half-activation voltage of -90 mV. With the concentrations of potassium and sodium ions in the superfusate at 20 and 70 mM, respectively, the reversal potential of IH was about -20 mV. IH was activated with a time constant of 2.8 s at -90 mV and 22 degrees C. The Q10 between 16 and 26 degrees C was 4.3. 4. A non-hydrolysable ATP analogue in the pipette solution did not support IH activation. Intracellular 'loading' of GTP-gamma-S (30-500 microM) led to a progressive activation of IH. 5. Forskolin (10 microM) increased the maximum conductance of IH by 70%. This was associated with a depolarizing shift in the half-activation voltage (5-10 mV) and in the voltage dependence of the activation/deactivation time constant of IH. 6. Essentially the same results as with forskolin were obtained by intracellular 'loading' with cyclic AMP (3-10 microM) or bath application of 8-bromo cyclic AMP (0.1-1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (0.1-1 mM). 7. The protein kinase inhibitor H-8 (1-10 microM) decreased the peak amplitude of IH. Phorbol 12-myristate 13-acetate (10 microM), a protein kinase C activator, was without effect. 8. It is concluded that a voltage-dependent cation current can be regulated by the basal activity of adenylate cyclase, presumably through protein kinase A, in vertebrate sympathetic neurones.

Effects of ryanodine on the spike after-hyperpolarization in sympathetic neurones of the rat superior cervical ganglion

Effects of ryanodine on sympathetic neurones of the rat superior cervical ganglion were investigated by means of intracellular recording. Ryanodine (1 microM) significantly shortened the after-hyperpolarization (AH) following the spike evoked by current injection or pre-ganglionic stimulation without affecting the configuration of the spikes. The shortening of AH caused by ryanodine was dose-dependent at concentrations between 0.1 and 1 microM and was slowly recovered by washing the tissue over 1 h. A partial inhibition of the apamin-sensitive slow component of AH was the maximal effect obtained at 1 microM. Although the input membrane resistance was not changed, ryanodine evoked repetitive discharges at long intervals in response to long depolarizing current pulses applied across the cell membrane. Ryanodine (5 microM) did not depress the Ca-spike but shortened the following AH in a lesser degree than that following the normal spike. Spontaneous small fluctuations of the resting membrane potential were occasionally observed under normal conditions. They were facilitated by caffeine and abolished by ryanodine. Caffeine also enhanced the slow component of the AH but did not affect it in the presence of ryanodine. These results suggest that ryanodine inhibits Ca release from intracellular store sites. The released Ca may contribute to generating the long-lasting AH and to regulating the excitability of rat sympathetic neurones.

Calcium-dependent potassium conductance in neurons of rabbit vesical pelvic ganglia

Intracellular recordings were made from neurons of vesical pelvic (parasympathetic) ganglia (VPG) isolated from the rabbit urinary bladder. Spontaneous hyperpolarizations (SH), occurring at intervals of 30 s to 5 min, could be recorded from 53% of VPG neurons in Krebs solution. The action potential was associated with inward sodium and calcium currents and was followed by fast and slow afterhyperpolarizations (AHPs). The action potential also evoked an additional hyperpolarization which was identical to the SH. The SH and the AHPs were associated with a decrease in the input resistance and reversed their polarity close to the potassium equilibrium potential. Intracellular cesium ions blocked the AHPs and the SH. Superfusing the preparation with a calcium-free solution produced a depolarization associated with an increased input resistance. The outward rectification activated at the resting membrane potential was depressed in the calcium-free solution. The removal of extracellular calcium ions also depressed both the SH and the spike AHPs. Bath-application of caffeine (1-3 mM) increased the frequency of the appearance of the SH. Injection of EGTA into VPG neurons caused a depolarization due to a blockade of the outward rectification. EGTA also depressed the slow AHP and the SH. These results suggest that the neuronal membrane of the rabbit VPG is endowed with a calcium-dependent potassium conductance (gKCa). Apamin (0.3-5 nM) and (+)-tubocurarine (30-300 microM) blocked the slow AHP and the SH without affecting the fast AHP and the resting membrane potential. Tetraethylammonium (TEA, 0.3-5 mM) suppressed the fast AHP and the SH without affecting the outward rectification. TEA augmented the slow AHP. Barium ions (0.1-1 mM) depressed the AHPs, the SH and the outward rectification. These pharmacological properties imply that at least 3 kinds of gKCa systems underlie the generation of the outward rectification, the spike AHPs and the SH.