Explant cultures of adult amphibian sympathetic ganglia: electrophysiological and pharmacological investigation of neurotransmitter and nucleotide action

Experiments to test the role of phosphatidylinositol 4,5-bisphosphate in neurotransmitter-induced M-channel closure in bullfrog sympathetic neurons

Various neurotransmitters excite neurons by suppressing a ubiquitous, voltage-dependent, noninactivating K+ conductance called the M-conductance (gM). In bullfrog sympathetic ganglion neurons the suppression of gM by the P2Y agonist ATP involves phospholipase C (PLC). The present results are consistent with the involvement of the lipid and inositol phosphate cycles in the effects of both P2Y and muscarinic cholinergic agonists on gM. Impairment of resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP2) with the phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) slowed or blocked the recovery of agonist-induced gM suppression. This effect could not be attributed to an action of wortmannin on myosin light chain kinase or on phosphatidylinositol 3-kinase. Inhibition of PIP2 synthesis at an earlier point in the lipid cycle by the use of R59022 (40 microm) to inhibit diacylglycerol kinase also slowed the rate of recovery of successive ATP responses. This effect required several applications of agonist to deplete levels of various phospholipid intermediates in the lipid cycle. PIP2 antibodies attenuated the suppression of gM by agonists. Intracellular application of 20 microm PIP2 slowed the rundown of KCNQ2/3 currents expressed in COS-1 or tsA-201 cells, and 100 microm PIP2 produced a small potentiation of native M-current bullfrog sympathetic neurons. These are the results that might be expected if agonist-induced activation of PLC and the concomitant depletion of PIP2 contribute to the excitatory action of neurotransmitters that suppress gM.

ATP-inhibition of M current in frog sympathetic neurons involves phospholipase C but not Ins P(3), Ca(2+), PKC, or Ras

Suppression of the voltage-activated, noninactivating K(+) conductance (M conductance; g(M)) by muscarinic agonists, P(2Y) agonists or bradykinin increases neuronal excitability. All agonist effects are mediated, at least in part, via the Gq/(11) class of G protein. We found, using whole cell or perforated patch recording from bullfrog sympathetic B neurons that ATP-induced suppression of g(M) was attenuated by the phospholipase C (PLC) inhibitor, U73122 (IC(50) approximately 0.14 microM) but not by the inactive isomer, U73343. The ability of extracellularly applied U73122 to inhibit PLC was confirmed by its antagonism of ATP-induced elevation of intracellular Ca(2+) as measured by fura-2 photometry. ATP-induced g(M) suppression was not antagonized by the protein kinase C (PKC) inhibitor, chelerythrine (5 microM extracellular +10 microM intracellular), by the Ca(2+)-ATPase inhibitor, thapsigargin (5 microM), or by inositol trisphosphate (InsP(3)) receptor antagonists, heparin (approximaterly 300 microM) or xestospongin C (1.8 microM). The effect of ATP on g(M) was thus dependent on PLC yet independent of PKC and of InsP(3)-induced release of intracellular Ca(2+). We therefore tested the involvement of a PKC-independent action of diacylglycerol (DAG) that could occur via activation of Ras. This low-molecular-weight G protein is activated following DAG binding to Ras-GRP, a neuronal Ras-GTP exchange factor. However, impairment of Ras function by culturing neurons with isoprenylation inhibitors (perillic acid, 0.1 mM, or alpha-hydroxyfarnesyl-phosphonic acid, 10 microM) failed to affect ATP-induced g(M) suppression. Inhibition of MEK (mitogen-activated protein kinase), a downstream target of Ras, by using PD 98059 (10 microM) was also ineffective. The transduction mechanism used by ATP to suppress g(M) in frog sympathetic neurons therefore differs from the PLC-independent mechanism used by muscarine and from the PLC and Ca(2+)-dependent mechanism used by bradykinin and UTP in mammalian ganglia. The possibility remains that "lipid-signaling" mechanisms, perhaps involving PLC-induced depletion of phosphatidylinositol bisphosphate, are involved in PLC-mediated inhibition of g(M) by ATP in amphibian sympathetic neurons.

Low concentrations of muscarine potentiate M-current in bullfrog sympathetic B-neurones

The concentration-dependence of the effect of muscarine on M-current (IM) and the underlying M-conductance (gM) in B-cells of bullfrog sympathetic ganglion was examined using whole-cell recording techniques. High concentrations of muscarine (> or = 200 nM) produced the classical suppression and over-recovery of steady-state IM at -30 mV. By contrast, low concentrations of muscarine (< or = 30 nM) shifted the gM activation curve to more negative potentials, increased the activation time constant (tau a) and increased steady-state IM. This effect may reflect muscarine-induced changes in submembrane Ca2+ concentration.

Structure-activity relationship of a pyrimidine receptor in the rat isolated superior cervical ganglion

1. The effects of pyrimidines and purines on the d.c. potential of the rat isolated superior cervical ganglion (SCG) have been examined by a grease-gap technique to determine the structure-activity requirements of the receptor activated by pyrimidines, i.e. a pyrimidinoceptor. 2. 5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranosyl (ZTP), the pyrimidines, cytidine 5'-triphosphate (CTP), uridine 5'-triphosphate (UTP) and thymidine 5'-triphosphate (TTP) and the purines, adenosine 5'-triphosphate (ATP; in the presence of an A1-purinoceptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1 microM)), adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), guanosine 5'-triphosphate (GTP), inosine 5'-triphosphate (1TP) depolarized ganglia in a concentration-dependent manner. The relative order of ZTP and purine 5'-triphosphates in depolarizing ganglia was ZTP > or = ATP gamma S > > ATP > or = ITP = GTP, and for the pyrimidine 5'-triphosphates UTP > TTP > or = CTP. Depolarizations evoked by ATP gamma S were followed by concentration-dependent hyperpolarizations at 100 and 1000 microM. 3. At concentrations of between 0.1 microM and 1 mM, uridine 5'-diphosphate (UDP), uridine 5'-diphosphoglucose (UDPG) and uridine 5'-diphosphoglucuronic acid (UDPGA) evoked significant and concentration-dependent depolarizations, whereas uridine 5'-monophosphate (UMP), uridine and uracil were inactive or produced small (< 45 microV) depolarizations. The relative order of potency of uridine analogues in depolarizing ganglia was UDP > or = UTP > UDPG > UDPGA > > uracil > or = UMP = pseudouridine > or = uridine. At 3 and 10 mM, uridine produced concentration-dependent hyperpolarizations. Nikkomycin Z, a nucleoside resembling UTP (viz. the triphosphate chain at the 5'-position on the ribose moiety being replaced by a peptide), was inactive between 1 microM and 1 mM. Generally, a concentration of 10 mM was required before thymidine, 6-azathymine, 6-azauracil or 6-azauridine depolarized ganglia. 4. Suramin (300 microM), a P2-purinoceptor antagonist, significantly depressed depolarizations evoked by alpha, beta-methylene-ATP (alpha, beta-MeATP; 100 microM), ATP gamma S (100 microM), CTP (1 mM), GTP (1 mM), ZTP (30 microM) and ATP (300 microM) in the presence of DPCPX (1 microM). Suramin reversed a small depolarization evoked by UMP (1 mM) into a small hyperpolarization. In contrast depolarizations evoked by UDP, UTP, UDPG (all at 100 microM) and TTP (300 microM) were unaltered or enhanced by suramin. 5. It is concluded that the rat SCG contains distinct nucleotide receptors including a P2-purinoceptor (activated by alpha, beta-MeATP, ATP, GTP, ITP and ZTP) and a pyrimidinoceptor (activated by UTP, UDP, UDPG, UDPGA and TTP). The pyrimidinoceptor on rat SCG neurones had specific structure activity requirements with the di- and triphosphates of uridine being the most effective depolarizing agonists examined.

Phorbol ester-induced M-current suppression in bull-frog sympathetic ganglion cells: insensitivity to kinase inhibitors

1. The effects of 1-oleoyl-2-acetyl-sn-glycerol (OAG), phorbol 12-myristate 13-acetate (PMA), 4-alpha-phorbol and muscarine on B-neurones from bull-frog sympathetic ganglion were studied by means of whole-cell patch-clamp recording. With the exception of 4-alpha-phorbol, all of these agonists reduced the steady-state outward current recorded at -30 mV as a result of suppression of a voltage-dependent, non-inactivating K(+)-current, the M-current, (IM). 2. Of the cells tested, 34% displayed bona fide responses to OAG (20 microM). The chance of recording a response was not decreased when the protein kinase inhibitor, 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7; 50 or 75 microM) was included simultaneously in the extracellular solution and in the pipette solution. 3. The presence of 50 microM H-7 on both sides of the membrane or 500 nM staurosporine in the pipette solution did not prevent responses to brief (1-2 min) or prolonged (> 20 min) applications of PMA. 4. Brief (1-2 min) extracellular application of H-7 (300 microM) suppressed IM by about 29%. 5. The most likely explanation of these data is that PMA and OAG modulate IM via a mechanism that is independent of protein kinase C (PKC). The availability of such a mechanism poses new questions as to the mechanism of muscarine-induced IM suppression.

Action of purine and pyrimidine nucleotides on the rat superior cervical ganglion

1. Using a grease-gap technique, we have investigated the effects of purine and pyrimidine nucleotides on the d.c. potential of the rat isolated superior cervical ganglion (SCG). 2. Of the purines tested, adenosine, adenosine 5'-triphosphate (ATP), beta,gamma-methylene-adenosine 5'-triphosphate (beta,gamma-MeATP) at up to 300 microM produced concentration-dependent hyperpolarizations, whereas 2-methyl-thio-ATP (2-Me.S.ATP) and alpha,beta-methylene-ATP (alpha,beta-MeATP) depolarized ganglia. Of the pyrimidines tested, uridine 5'-triphosphate (UTP) produced concentration-dependent depolarizations and cytosine 5'-triphosphate (CTP) at 1000 microM produced considerably smaller but significant depolarizations. In contrast uridine 5'-monophosphate (UMP) at 1000 microM hyperpolarized ganglia. The relative order of potency of purines and pyrimidines to depolarize ganglia was: UTP > alpha,beta-MeATP >> CTP > 2-Me.S.ATP and to hyperpolarize ganglia was: adenosine = beta,gamma-MeATP > ATP > UMP. 3. The ability of purines and pyrimidines to alter the depolarizing response caused by muscarine and of purines to alter depolarization induced by gamma-aminobutyric acid (GABA) was determined. The relative order of potency of nucleotides in depressing submaximal depolarization caused by muscarine (100 nM) was: adenosine = ATP > beta,gamma-MeATP whereas 2-Me.S.ATP, alpha,beta-MeATP and UTP did not significantly alter depolarization caused by muscarine. At 100 microM beta,gamma-MeATP and adenosine but not ATP potentiated GABA-induced depolarizations. 4. Hyperpolarizations caused by adenosine, ATP, beta,gamma-MeATP and UMP and depolarizations caused by alpha,beta-MeATP were enhanced in medium containing reduced concentrations of calcium (0.1 mM) and potassium (2 mM). In this medium 8-phenyltheophylline abolished hyperpolarizations caused by adenosine and reversed hyperpolarizations caused by ATP into depolarizations. Suramin (300 microM), a P2-purinoceptor antagonist, significantly reduced the depolarizing response caused by alpha,beta-MeATP and significantly increased hyperpolarizations caused by ATP and Beta,gamma-MeATP. Suramin (300 microM) did not significantly alter depolarizations caused by l,l-dimethyl-4-phenylpiperazinium (10 microM), potassium(3 mM) or muscarine (100 nM) and significantly potentiated depolarizations caused by UTP (100 microM).5.It is concluded that the rat SCG contains PI-purinoceptors that hyperpolarize the ganglion and diminish sensitivity to muscarine, and P2X-purinoceptors that depolarize the SCG. There is also some evidence to suggest the presence of receptors for UTP, i.e., pyrimidinoceptors, which depolarize SCG neurones.

Activation of potassium channels in renal epithelioid cells (MDCK) by extracellular ATP

Extracellular ATP has been shown to stimulate transepithelial chloride transport in confluent Madin-Darby canine kidney (MDCK) cell layers and to enhance potassium conductance in subconfluent MDCK cells. The present study has been performed to test for the effect of extracellular ATP on channel activity in patches from subconfluent MDCK cells. Within 8 s, addition of extracellular ATP (10 mumol/l) leads to a sustained, but fully reversible, appearance of potassium-selective channels in cell-attached patches [increase of open probability from 0.03 +/- 0.02 (n = 10) to 0.50 +/- 0.07 (n = 6)]. With the use of pipettes filled with 145 mmol/l KCl, inwardly rectifying property of the channels is disclosed with a single-channel conductance of 65.7 +/- 3.1 pS (n = 9) at zero potential difference between pipette and bath and with a reversal potential of 75.4 +/- 2.0 mV (n = 5; pipette negative vs. reference in the bath). The open probability of the channels is not significantly modified by altering pipette potential from -50 mV, pipette positive, to 50 mV, pipette negative. At extracellular calcium activities of less than 10 nmol/l, ATP leads to a transient activation of channels. In conclusion, extracellular ATP activates inwardly rectifying potassium channels in the cell membrane of subconfluent MDCK cells. A sustained activation of the channels requires the presence of extracellular calcium and is probably mediated by increases in intracellular calcium.

Amphibian sympathetic ganglia as a model system for investigating regeneration in the vertebrate peripheral nervous system

1. The paravertebral sympathetic ganglion of the bullfrog serves as an excellent experimental system in which to study the response of vertebrate neurones to axotomy and the mechanisms associated with regeneration. 2. Various types of lesions to the axons (axotomy) of these neurones promote distinct and reproducible changes in the electrophysiological properties of the cell bodies which are not a consequence of changes in cell body morphology. 3. The axotomy-induced increase in spike width and decrease in the amplitude of the action potential after-hyperpolarization may allow an increase in Ca2+ influx and thereby promote regrowth. 4. The axotomy-induced decrease in after-hyperpolarization duration may reflect the disconnection of the neurone with its target and the loss of available nerve growth factor (NGF) from the target. 5. Experiments with NGF antibodies provide evidence that an NGF-like substances serves to maintain the normal electrophysiological characteristics of amphibian sympathetic neurones.

Spontaneous miniature outward currents in cultured bullfrog neurons

Spontaneous miniature hyperpolarizations were observed in cultured bullfrog neurons. Depolarization increased the frequency and amplitude of the events. Under voltage-clamp, these events were manifested as spontaneous miniature outward currents of SMOCs which were usually less than 2 nA, had a rapid rising phase and a slower voltage-dependent exponential decay. Analysis of inter-event intervals suggested that SMOCs occurred randomly, while analysis of their amplitudes yielded exponential amplitude distributions. Mean SMOC amplitudes and SMOC frequency increased with depolarization, even with 100 microM CdCl2 present. Time constants of SMOC decay resembled time constants obtained from voltage-jump experiments on Ca2+-loaded cells, and together with the sensitivity of SMOCs to tetraethyl ammonium (TEA), suggested that SMOCs are due to activation of fast Ca2+-gated potassium channels. We propose that a SMOC occurs when 10-5000 of these channels are activated by punctate intracellular Ca2+ release.

Slow pulsatile movements of Schwann cells in vitro: a time-lapse cinemicrographic study

Slow pulsatile movements of Schwann cells in vitro were studied quantitatively by using time-lapse cinemicrography. Schwann cells from peripheral nerves of 3-day-old rats were cultured in serum-free medium. Most Schwann cells showed intermittent episodes of pulsatile movement; each episode consisted of one or several contractile pulses. About half of the episodes consisted of a single pulse, and episodes with more than four pulses were rare. The average episode of activity lasted 2.6 min, while the average duration of a single pulse was 1.5 min. The mean quiescent interval between episodes of activity was 3.7 min. Some cells showed no pulsatile activity. Active cells averaged 6.6 episodes/h. The fraction of time which a Schwann cell spent in pulsatile activity varied widely, with an average of 28%. Behavior of Schwann cells in HEPES-buffered Hanks saline was generally similar to that in the complete medium. Raising K+ to 40 mM or Ca++ to 10 mM did not markedly affect the time course of the pulsatile motility, although the contractions were more vigorous in the high Ca++. Pulsatile movement was reversibly inhibited by cytochalasin B and appeared to be potentiated by drugs that disrupt microtubules.

Single acetylcholine channel currents in sympathetic neurons

Single acetylcholine (ACh) channel currents were studied by the gigaohm patch-clamp technique in cultured sympathetic neurons of the bullfrog, Rana catesbeiana. Recordings were made at 22 degrees C on cell-attached and excised membrane patches. When ACh (0.5-1 microM) was present in the pipette, a single class of inward currents was observed with a chord conductance of 30 pS and a reversal potential of -2 mV. The mean channel open time was 11.6 ms at -65 mV and showed little or no voltage-dependence over the range -85 to -45 mV. These channels appear to mediate the fast nicotinic excitatory postsynaptic current.

Pharmacological inhibition of the M-current

1. The effects of muscarinic agonists, luteinizing hormone-releasing hormone (LHRH) analogues, uridine triphosphate (UTP) and divalent cations on K(+)-currents in voltage-clamped bullfrog sympathetic neurones have been studied.2. Muscarine (1-10 muM), D-ala(6) LHRH (1-5 muM), UTP (50-100 muM) and Ba(2+) (1-4 mM) selectively depressed the M-current (I(M)), without appreciable effect on the delayed rectifier, Ca(2+)-activated or transient outward currents (I(K), I(C) or I(A)).3. I(M)-inhibition was characterized by: (a) elimination of slow current relaxations accompanying voltage jumps in the membrane potential range -30 to -60 mV; (b) reduced voltage-dependent chord conductance over this range with no change in the voltage-independent chord conductance at more negative membrane potentials; (c) suppression of outward rectification in the steady-state current-voltage curve between -70 and -25 mV; and (d) development of an inward current which increased in amplitude between -70 and -20 mV in proportion to the decrease in steady-state I(M). The kinetics and voltage sensitivity of residual I(M) were unchanged.4. The magnitude of the inward current produced by muscarine or LHRH could be accounted for quantitatively by the reduction in steady-state I(M). No increase in leak current could be detected in the range -60 to -30 mV. In two cells muscarine (10 muM) increased the leak current and conductance at -70 to -100 mV, but not at more depolarized levels.5. I(M) was not modified by removing extracellular Ca(2+), adding a selective Ca(2+)-channel blocker (Cd(2+)), adding 1 mM-dibutyryl cyclic AMP or 8'Br cyclic GMP, or by intracellular ionophoresis of Ca(2+), 8'Br cyclic GMP, dibutyryl cyclic AMP, GTP-gamma-S or S-adenosylmethionine.6. It is concluded that the principal effects of these agents in unclamped neurones - depolarization, increased input resistance, reduced outward rectification and increased excitability - are due entirely to a selective inhibition of I(M). The intracellular transduction mechanism for I(M) inhibition is unknown.