Muscarinic agonists depress calcium-dependent gK in bullfrog sympathetic neurons

Nicotinic and muscarinic synaptic transmission in canine intracardiac ganglion cells innervating the sinoatrial node

Nicotinic and muscarinic mediated synaptic mechanisms were investigated in isolated, canine intracardiac ganglia taken from the right atrial fat pad. Using conventional intracellular microelectrode recording techniques on 216 neurons, fast and slow synaptic potentials were evoked by single or trains of stimulation of presynaptic fibers in interganglionic nerves. By varying the stimulus intensity, single or multiple fast excitatory postsynaptic potentials (f-EPSPs) were evoked, indicating the convergence of synaptic inputs on these cells. These f-EPSPs often reached the action potential threshold, were enhanced by the acetylcholinesterase inhibitor physostigmine and were blocked by the nicotinic antagonist hexamethonium. The f-EPSPs were accompanied by a decreased input resistance and had an extrapolated reversal potential of -7.1 mV, suggesting increased conductances to more than one cation. Repetitive presynaptic stimulation evoked slow excitatory postsynaptic potentials (s-EPSPs) in 41% of the cells while slow inhibitory postsynaptic potentials (s-IPSPs) or s-IPSPs followed by s-EPSPs were evoked in 19% of the cells. All slow potentials were abolished by atropine and low Ca2+/high Mg2+ solutions and enhanced by physostigmine. Hexamethonium and adrenergic receptor antagonists had no effects on s-EPSP and s-IPSP. The M1 receptor antagonist pirenzepine reversibly blocked the s-EPSP but not the s-IPSP. On the other hand, the M2 receptor blocker 4-diphenyl-acetoxy-N-methyl piperidine methiodide (4-DAMP) had no effects on the s-EPSP. These observations suggest that s-EPSPs and s-EPSPs are mediated by distinct muscarinic receptors. The amplitude of the s-EPSP and the depolarization evoked by the muscarinic agonist, bethanechol were accompanied by increased input resistance. These responses were decreased in amplitude by membrane hyperpolarization and either reversed polarity or declined to zero amplitude at about -80 mV, suggesting the inhibition of a potassium conductance.

Slow frequency-dependence of action potential afterhyperpolarization in bullfrog sympathetic ganglion neurones

The after hyperpolarizatin (AHP) which follows the action potential (AP) in bullfrog sympathetic ganglion B-cells involves activation of Ca(2+)-sensitive K+ conductances following Ca2+ influx via Ca2+ channels. The duration of AHPs evoked at 2-s stimulus intervals were 70.05 +/- 3.76% of those evoked at 90-s stimulus intervals (n = 35). Since there was no consistent effect of ryanodine (5 microM), ruthenium red, (300 microM) or dantrolene Na (35 microM) on this frequency dependence, it is unlikely to result from release of Ca2+ from intracellular stores. Ca2+ currents (ICa), studied by means of the whole-cell patch-clamp technique, exhibited a slow frequency dependence as a result of a slow inactivation process which was independent of Ca(2+)-induced ICa inactivation and ICa run-down. There was excellent correlation (r = 0.964) between the estimated changes in Ca2+ influx and the expected activation of the Ca(2+)-sensitive K+ current, IAHP. This result is consistent with the hypothesis that the frequency dependence of the AHP is a consequence of the slow inactivation of ICa.

Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

M1 and M2 muscarinic receptors mediate excitation and inhibition of guinea-pig intracardiac neurones in culture

1. The effects of muscarine upon intracardiac neurones cultured from ganglia within the atria and interatrial septum of the newborn guinea-pig heart were studied using intracellular recording techniques. 2. Muscarine applied to the neuronal soma typically produced a biphasic change in membrane potential which consisted of a small hyperpolarization followed by a depolarization. In addition, muscarine (0.01-10 microM) inhibited the calcium-dependent, after-hyperpolarization (AHP) and greatly increased the number of action potentials that could be evoked by a given depolarizing current. 3. The hyperpolarization was associated with a decrease in input resistance and it reversed to become a depolarization at a potential of -86.5 mV. This response was antagonized by 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP; 100 nM) and AF-DX 116 (500 nM), but was unaffected by pirenzepine (0.1-5 microM). 4. Two types of slow depolarization were observed in the presence of muscarine. The most common was associated with an increase in input resistance in the potential range -70 to -40 mV. Pirenzepine (100 nM) selectively antagonized this response, 4-DAMP (100 nM) similarly antagonized the response, but was non-selective. AF-DX 116 (0.5-5 microM) showed no antagonist effect. The less common depolarization (5% of cells) had a long latency and was associated with a decrease in input resistance. 5. Muscarine reduced the duration of the action potential and inhibited the AHP. Cadmium chloride (100 microM) mimicked these actions of muscarine. Application of muscarine immediately following a train of action potentials did not inhibit the AHP, suggesting that muscarine did not directly inhibit the calcium-activated potassium current (IK(Ca)). Muscarine-induced depression of the slow AHP was antagonized by 4-DAMP (100 nM) but was not antagonized by either pirenzepine (0.1-0.5 microM) or AF-DX 116 (0.5-5 microM). 6. It is concluded that the muscarine-induced depolarization of guinea-pig intracardiac neurones results from reduction of a potassium conductance similar to the M-conductance, through activation of M1 muscarinic receptors. The hyperpolarization results from an increase in potassium conductance, through activation of M2 muscarinic receptors.

Muscarinic suppression of the M-current in the rat sympathetic ganglion is mediated by receptors of the M1-subtype

1. Under voltage-clamp dissociated adult and foetal rat superior cervical ganglion (s.c.g.) cells exhibited a non-inactivating voltage- and time-dependent component of K+ current termed the M-current (IM). IM was detected and measured from the current decay during hyperpolarizing voltage steps applied from potentials where IM was pre-activated. 2. Neither the resting membrane current nor the amplitude of these current decay relaxations were reduced by omitting Ca from the bathing fluid, showing that the M-current was not a 'Ca-activated' K-current dependent on a primary Ca-influx. Concentrations of (+)-tubocurarine sufficient to block the slow Ca-activated K-current IAHP did not inhibit IM or antagonize the effect of muscarinic agonists on IM, showing that IM was not contaminated by IAHP. Tetraethylammonium (1 mM), which blocks the fast Ca-activated K-current IC, produced a small inhibition of IM. This was not due to contamination of IM by IC since muscarinic agonists did not consistently block IC. 3. The muscarinic agonists muscarine, oxotremorine, McN-A-343 and methacholine reversibly suppressed IM, resulting in an inward (depolarizing) current. The rank order of potency was: oxotremorine greater than or equal to muscarine greater than McN-A-343 greater than methacholine. 4. The suppression of IM by muscarine was similar in cultured cells derived from adult and foetal tissue to that seen in the intact ganglia. 5. IM-suppression by muscarine was inhibited by pirenzepine (Pz) and AF-DX 116 with mean pKB values of 7.53 +/- 0.13 (n = 3) and 6.02 +/- 0.13 (n = 4) respectively. 6. The suppression of IM by muscarinic agonists was not affected by gallamine (10-30 microM). 4-Diphenylacetoxy-N-methylpiperidine methiodide inhibited the response at 300 nM. 7. Pirenzepine inhibited the contractions of the guinea-pig isolated ileum produced by muscarine with a mean pKB of 6.37 +/- 0.03 (n = 8). 8. These results suggest that the receptors mediating suppression of the M-current accord with those designated pharmacologically as M1 and that these receptors reach maturity at a very early stage in the development of the rat s.c.g.

Muscarinic agonists and potassium currents in guinea-pig myenteric neurones

1. Intracellular electrophysiological recordings were obtained from single neurones of the guinea-pig myenteric plexus in vitro. Using single electrode voltage clamp techniques, four distinct potassium currents were described and the effects of muscarinic agonists on these currents were studied. 2. A calcium-dependent potassium current (gKCa) was present in AH neurones at rest, and was much increased following a brief depolarization (50 ms, to 0 mV). Muscarinic agonists reduced both the resting current and the current evoked by depolarization. Pirenzepine competitively antagonized the suppression by muscarine of the calcium-dependent potassium current (or after-hyperpolarization) following an action potential. The dissociation equilibrium constant for pirenzepine was about 10 nM. 3. The conductance of AH neurones increased two to three fold when they were hyperpolarized negative to -90 mV. This inward rectification was blocked by extracellular caesium (2 mM) or rubidium (2 mM), but not by tetraethylammonium (TEA, 40 mM), 4-aminopyridine (100 microM) or cobalt (2 mM). The inward rectification was unaffected by muscarinic agonists. 4. When AH neurones were depolarized from very negative holding potentials (less than -80 mV) a brief outward current was recorded with a duration of about 200 ms. This transient or A current was completely blocked by 4-aminopyridine (100 microM) but was not affected by tetrodotoxin (300 nM), TEA (40 mM) or cobalt (2 mM). Muscarinic agonists did not affect the A current. 5. In S neurones, and in AH neurones in calcium-free solutions, the potassium conductance (in TEA and caesium) behaved according to constant field assumptions. This background conductance was suppressed by muscarinic agonists. 6. It is concluded that the depolarization by muscarinic agonists of myenteric AH neurones is due to a suppression of both a calcium-dependent potassium conductance and a background potassium conductance. Muscarinic depolarization of S neurones results only from suppression of the background potassium conductance. Effects on both conductances result from M1-receptor activation. Inward rectifying and transient outward (A) potassium currents are unaffected.

Clindamycin-induced alteration of ganglionic function. I. Direct effects on ganglion cell properties

The influence of the lincosamide antibiotic, clindamycin, on the properties of bullfrog sympathetic ganglion B cells has been determined in vitro using conventional voltage recording methods or single microelectrode voltage-clamp recording techniques. Individual neurons were depolarized with both bath application or local perfusion of clindamycin. The amplitude of the depolarization was not altered by pretreatment with 50 microM (+)-tubocurarine, 10-microM atropine, or 1.5 microM tetrodotoxin (TTX), indicating that the clindamycin-induced depolarization does not result from either the activation of (1) nicotinic receptors, (2) muscarinic receptors, or (3) voltage-gated sodium channels. Clindamycin partially inhibited IM, an action which accounts for part of the clindamycin-induced depolarization. The duration of the hyperpolarizing afterpotential (HAP) following the action potential was decreased in the presence of clindamycin. Clindamycin decreased the amplitude and maximum rate of rise (MRR) of TTX-insensitive action potentials. As calcium influx is thought to contribute to the depolarizing phase of the TTX-insensitive spikes, we suggest that the decrease in HAP duration by clindamycin results from a decrease in the somal calcium current. Further, it is suggested that a decrease in IM and HAP duration may be responsible for the increased excitability exhibited during exposure to clindamycin.

Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 microM-1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2-20 mV). Under voltage-clamp conditions, Ad (100 microM) produced fast, slow and mixed types of inward currents (AdIs) with amplitude of 2.9 +/- 1.3 nA. beta-Adrenoceptors may be responsible for the generation of these AdDs. The slow AdI which lasted for 1-5 min was associated with a decreased membrane conductance. The slow AdI decreased at hyperpolarized potential level and eventually nullified at -70 mV. No reversal of the slow AdI polarity was observed in the Ringer solution. Injection of Cs2+ into the ganglion cells produced a marked depression of the amplitude of the slow AdI. The slow AdI was blocked by bath-applied Ba2+ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range -35 to -55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast AdI decreased and reversed its polarity at -36 +/- 8.3 mV. Removal of Cl ion from superfusing solution depressed the fast AdI, suggesting that activation of Cl- conductances may be involved in the generation of the fast AdI. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.

Pharmacological and physiological properties of the after-hyperpolarization current of bullfrog ganglion neurones

1. The slowly decaying, calcium-dependent after-hyperpolarization (a.h.p.) that follows action potentials in bullfrog ganglion B cells has previously been shown to be generated by a potassium current called IAHP. We have recorded IAHP using a switched, single-electrode hybrid clamp where current-clamp mode was changed to voltage-clamp mode immediately after repolarization of a spike or the last spike of a train. 2. Reduction of extracellular calcium reduced the decay time of IAHP following a single spike. At all levels of extracellular calcium tested (0.5-4 mM), the decay time of IAHP was longer following a train of action potentials than following a single action potential. Thus, the time course of IAHP evoked by action potentials is a function of the calcium load induced by the action potentials. Conversely, agents that reduce the amount of IAHP activated without affecting its rate of decay, probably do not affect calcium influx. 3. Muscarine (2 or 10 microM) inhibits IAHP following an action potential by at most 30% and has no effect on decay rate of IAHP. These results suggest that muscarine has little or no effect on either calcium influx or sequestration. Decay of the a.h.p. is accelerated by muscarine but this effect is due to an increased leak conductance. 4. Charybdotoxin (CTX) between 4 and 20 nM, prolongs action potential duration in a manner consistent with blockade of the voltage- and calcium-dependent potassium current (Ic) involved in spike repolarization in these cells. This action is consistent with its reported action on analogous channels in other systems. However, CTX also reduces IAHP. Thus, in bullfrog ganglion neurones, two distinct calcium-dependent potassium currents exhibit a comparable sensitivity to CTX. This cannot be due to a decreased influx of calcium because the decay rate of IAHP following an action potential is unchanged. The action of CTX was observed with both crude and purified preparations of CTX. 5. Apamin (25 nM) and (+)-tubocurarine (concentration giving 50% of maximal inhibition = 20 microM) block IAHP without affecting action potential duration. The action of (+)-tubocurarine is more readily reversible than apamin. Approximately 20% of IAHP is resistant to blockade by either apamin or (+)-tubocurarine. 6. Muscarine was used to block the M-current (IM) selectively and (+)-tubocurarine was used to inhibit IAHP selectively. Both currents were shown to contribute to spike frequency adaptation. Inhibition of both IM and IAHP has a synergistic action to increase repetitive firing.

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

Ca2+-sensitive K+ currents involved in the slow afterhyperpolarization (a.h.p.) of an action potential of bullfrog sympathetic neurones were studied with a single-electrode voltage clamp method. The outward tail current (IAH) generated after the end of a depolarizing command pulse (from the holding potential of -60 mV to 0 mV, 5-20 ms in duration), mimicking an action potential, was separated into at least two exponential components (IAHf and IAHs). They were identified as K+ currents, since their reversal potentials were close to the K+ equilibrium potential and they were sensitive to external K+. The time constant of IAHf (tf; 44 ms at -60 mV) was decreased by membrane hyperpolarization from -40 to -80 mV, while that of IAHs (ts; 213 ms) remained constant. Removal of external Ca2+ or addition of Cd2+ significantly decreased the IAHs amplitude (As) and tf without a change in ts and the IAHf amplitude (Af). On the other hand, increasing Ca2+ influx by applying repetitive command pulses enhanced both Af and As with negligible effects on tf and ts, and produced a much slower component. Intracellular injection of EGTA reduced Af with no effect on tf, and increased As with a decreased ts. Both muscarine and (+/-)-tubocurarine, which reduced IAHs, hardly affected IAHf. These results indicate that a.h.p. is induced by the activation of two distinct Ca2+-dependent K+ channels, which differ in voltage sensitivity, Ca2+-dependence and pharmacology.

Effects of phorbol dibutyrate on M currents and M current inhibition in bullfrog sympathetic neurons

1. Effects of bath-applied phorbol dibutyrate (PDBu) on M currents (IM) and on the inhibition of IM by muscarine and luteinizing hormone-releasing hormone (LHRH) were recorded in voltage-clamped bullfrog lumbar sympathetic ganglion cells. 2. PDBu (0.1-30 microM) produced a slowly developing, irreversible and partial (less than or equal to 60%) inhibition of IM. This effect was not replicated by 4-alpha-phorbol or by vehicle. 3. After treatment with PDBu, residual IM showed a reduced sensitivity to inhibition by muscarine or LHRH but not by Ba2+. The reduced response to muscarine appeared to result from a 10-fold shift in the concentration dependence for inhibition. 4. PDBu did not clearly reproduce the ability of muscarine to inhibit the slow, Ca-activated K current IAHP or to increase the leak conductance at hyperpolarized potentials. The latter effect of muscarine was enhanced, rather than inhibited, by PDBu. 5. IM and IAHP were not inhibited by 1 mM dibutyryl cyclic AMP or by 20 microM forskolin. 6. It is concluded that activation of protein kinase C, but not protein kinase A, partly replicates the effect of muscarine on frog sympathetic neurons.

Evidence for epinephrine-induced depolarization in neurons of bullfrog sympathetic ganglia

The response to epinephrine (EP) was determined for neurons in bullfrog sympathetic ganglia by intracellular and voltage-clamp recording techniques. EP (5 microM-1 mM) produced a concentration-dependent depolarization mediated through beta-adrenoceptors. The EP-induced depolarization (EPD) was associated with a decrease in the membrane conductance. The EP-induced current (EP1) was decreased at hyperpolarizing potential levels and nullified at -70 mV. No reversal of the EPI polarity was seen. It is concluded that the EPD is generated by the suppression of a voltage-dependent gK, probably the M-channel.

Activation of M2 muscarinic receptors causes an alteration of action potentials by modulation of Ca entry in isolated sympathetic neurons of rabbits

Activation of muscarinic receptors caused an alteration of the configuration of action potentials in the isolated sympathetic ganglion cells of rabbits, e.g. elimination of the shoulder in the falling phase and depression of the afterspike-hyperpolarization and after-depolarization. The responsible receptors appeared to be of the M2-type since the above changes by the low concentration (greater than or equal to 10 nM) of muscarinic agonists were potently antagonized by AF-DX 116. Similar changes were induced by removing the external Ca2+ and no more changes were produced by further application of muscarinic agonists in the absence of external Ca2+. The action potential in sympathetic neurons is thus shown to be modulated via the M2 receptor possibly by regulating the Ca entry.

Tubocurarine suppresses slow calcium-dependent after-hyperpolarization in guinea-pig inferior mesenteric ganglion cells

Intracellular recordings were made from neurones of the isolated guinea-pig inferior mesenteric ganglia. Single-spike potentials evoked by either depolarizing current pulses applied through the recording micro-electrode or stimulation of the hypogastric nerves were followed by an after-hyperpolarization (a.h.). The spike a.h. in 40% of the neurones, referred to herein as type I, had a relatively short duration (less than 50 ms) and exhibited a monophasic decay with a mean time constant (tau) of 11.4 ms. In the remaining cells (type II), the spike was followed by a long a.h. (greater than 100 ms) having a double-exponential decay; the fast and slow components of the a.h. are termed a.h.f and a.h.s, respectively, and they had mean tau values of 11.4 and 74 ms, respectively. A.h.f and a.h.s of type II neurones were reduced by membrane hyperpolarization and reversed their polarities between -80 and -90 mV. The reversal potentials shifted in a manner closely predicted by the Nernst equation as external K+ concentration was increased. Superfusion of low-Ca2+ high-Mg2+ solution to type II neurones reduced the a.h.f and a.h.s by 32 and 82%, respectively, indicating that a.h.s is largely Ca2+-dependent. Application of (+)-tubocurarine (10-100 microM) reversibly suppressed the a.h.s without affecting a.h.f in a concentration-dependent manner. Following a short train of action potentials evoked from type II neurones, the post-tetanic hyperpolarization (p.t.h.) was similarly depressed by (+)-tubocurarine in a dose-dependent manner. (+)-tubocurarine did not significantly change the amplitude of Ca2+-dependent spike potentials evoked in neurones bathed in Na+-free high-Ca2+ plus tetraethylammonium (5-10 mM) solution. The results indicate that (+)-tubocurarine selectively suppresses a.h.s, a slow Ca2+-dependent a.h., the consequence of which is a facilitation of repetitive discharges of the cells.

Electrophysiological properties of sympathetic preganglionic neurons in the cat spinal cord in vitro

Intracellular recordings were obtained from sympathetic preganglionic neurons of the intermedio-lateral nucleus of the adult cat in slices of upper thoracic spinal cord maintained in vitro. The neurons were identified by their antidromic responses to stimulation of various ipsilateral sites. Sites from which antidromic responses could be evoked were the white ramus, the ventral root, the ventral root exit zone, the white matter between the latter and the outer edge of the tip of the ventral horn, the lateral edge of the ventral horn. Resting membrane potential was --61.3 +/- 1.6 mV (mean +/- SEM), input resistance 67.5 +/- 3.7 M omega, time constant 11.5 +/- 1.2 ms. The amplitude of the action potential generated by antidromic or direct stimulation was 77.4 +/- 2.3 mV. Threshold for direct spikes was 18.2 +/- 1.8 mV. The action potential had an average duration of 3.03 +/- 0.16 ms. It showed a prominent "hump" on the falling phase. The action potential had a tetrodotoxin (TTX)-sensitive and a TTX-resistant component. The latter was abolished by cobalt. Tetraethylammonium, cesium and barium prolonged the action potential duration which acquired a plateau-shape. A prolonged after-hyperpolarization (AHP) followed the sympathetic preganglionic neuron spike. Following a single spike, AHP duration and peak amplitude were 2.8 +/- 0.3 s and 16.6 +/- 0.7 mV, respectively. The AHP was abolished by cesium or barium, but enhanced by tetraethylammonium. An AHP followed the TTX-resistant spike. EPSPs and IPSPs could be generated by focal stimulation. The EPSP triggered spikes when threshold (15.0 +/- 2.0 mV) was reached.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous muscarinic suppression of the Ca-activated K-current in bullfrog sympathetic neurons

Neurons in bullfrog sympathetic ganglia were voltage-clamped using a single microelectrode. A prolonged outward current which was identified as the Ca-activated K-current secondary to a transient Ca entry through voltage-operated channels was shortened by oxotremorine. An inward Ca-current was not significantly depressed by oxotremorine. It was suggested that muscarinic agonists accelerate the re-closure of K-channels either directly or secondarily via their effects on an intracellular sequestration process of Ca ions. It was also suggested that a small amount of acetylcholine only sufficient to cause a miniature synaptic current via nicotinic receptors could shorten the Ca-activated K-current via muscarinic receptors.

Muscarinic and peptidergic excitation of bull-frog sympathetic neurones

The large B cells of bull-frog sympathetic ganglia are well known to be depolarized by slow synaptic transmission, muscarinic agonists, analogues of luteinizing hormone-releasing hormone (LHRH), and substance P. Voltage-clamp analysis shows that these actions result from two underlying mechanisms: inhibition of the M-current, a voltage-dependent potassium current; and in some cells, an inward current associated with an increase in conductance. The additional inward current appears as a voltage-insensitive change in the instantaneous conductance (i.e. apparent leak conductance). The additional inward current is typically slower in onset and offset than is M-current inhibition. It is typically seen for higher concentrations and longer durations of agonist application. In many cells, only a decrease in M-current can be demonstrated. Muscarine inhibits the M-current with 50% inhibition (I50) at 0.7 microM. At least 86% of the M-current is muscarine sensitive. At comparable concentrations, oxotremorine produces less M-current inhibition than does muscarine. Some analogues of teleost LHRH (T-LHRH) are more potent as M-current inhibitors than T-LHRH itself. Those peptides tend to act more slowly than T-LHRH. Substance P shows variable potency for M-current inhibition, with I50 s ranging from 2 nM to greater than 2 microM on different cells. The response to long applications of substance P desensitizes in some cells, which has not been observed for comparable applications of muscarinic or LHRH agonists. Other tachykinins (including substance K) inhibit the M-current. C-terminal fragments of substance P are ineffective, and M-current inhibition by substance P is not blocked by [D-Pro2,D-Trp7,9]- or [D-Arg1,D-Pro2, D-Trp7,9,Leu11] substance P. The slow muscarinic excitatory post-synaptic potential (e.p.s.p.) produces a graded inhibition of up to 90% of the M-current. Occasional cells show an additional inward current with an associated increase in conductance during the slow e.p.s.p. This effect is less marked than with exogenous muscarinic agonists. The late, slow e.p.s.p., which is produced by stimulation of high threshold C-fibre inputs and is resistant to cholinergic antagonists, also involves M-current inhibition. An additional inward current can be observed in some cells. M-current inhibition (by agonists or slow synaptic potentials) increases the number of spikes produced by a given depolarizing current, often allowing maintained firing. This action is not mimicked by equivalent depolarization, and is still seen when the cell is manually clamped to the original resting potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Two distinct Ca-dependent K currents in bullfrog sympathetic ganglion cells

Healthy bullfrog sympathetic ganglion cells often show a two-component afterhyperpolarization (AHP). Both components can be reduced or abolished by adding Ca-channel blockers or by removing external Ca. Application of a single electrode "hybrid clamp"--i.e., switching from current- to voltage-clamp at the peak of the AHP, reveals that the slow AHP component is generated by a small, slow, monotonically decaying outward current, which we call IAHP. IAHP is blocked by Ca-removal or by apamin and is a pure K current. It is slightly sensitive to muscarine and to tetraethylammonium ion but is much less so than muscarine-sensitive (IM) and fast Ca-dependent (IC) K currents. It also can be recorded in dual-electrode voltage-clamp experiments, where it is seen as a slow, small component of the outward tail current that follows brief depolarizations to 0 mV or beyond. IC is seen as an early, fast, large component of the same tail current. Both components are blocked by Ca removal, but only the IC component is blocked by low doses of tetraethylammonium ion. Thus, bullfrog ganglion cells exhibit two quite distinct Ca-dependent K currents, which differ in size, voltage-sensitivity, kinetics, and pharmacology. These two currents also play quite separate roles in shaping the action potential.

Calcium entry through acetylcholine-channels can activate potassium conductance in bullfrog sympathetic neurons

Fast B neurons in bullfrog sympathetic ganglia were voltage clamped with two microelectrodes. Acetylcholine (ACh) was applied onto the soma membrane by iontophoresis. A rapid nicotinic inward current was followed by a slow muscarinic inward current. After an addition of scopolamine to Ringer solution so as to block the muscarinic current, the nicotinic inward current was found to be followed by an outward current lasting for several hundred ms. It disappeared when the preceding nicotinic inward current was blocked by (+)-tubocurarine. The ACh-induced outward current was due to calcium entry through ACh-channels and subsequent opening of potassium channels. This may indicate that a rapid excitatory transmission leads to non-synaptic autoinhibition. The interaction between the calcium-dependent potassium conductance and the muscarinic action of ACh is proposed.