Opposing effects of aluminum on inward-rectifier potassium currents in bean root-tip protoplasts.

Inward currents in root cap protoplasts of the aluminum-tolerant cultivar, Dade, of Phaseolus vulgaris L. were investigated using the whole-cell patch-clamp technique. The properties of these currents were similar to those seen in inward rectifying K+ channels in other plant tissues. Replacing bath K+ with Na+ nearly abolished the observed currents. Higher bath K+ concentrations increased inward currents. AlCl3 in pH 4.7 bath solutions caused inward K+ currents to activate more rapidly and at more positive voltages when compared with AlCl3 free solutions. In 10 microM AlCl3 the activated inward K+ currents were significantly larger than in the AlCl3-free solution at all voltages except at the most negative voltage of -174 mV and the least negative of -74 mV. In contrast, in 80 microM Al3+, when hyperpolarizing voltages were most negative, the inward K+ currents were inhibited relative to the currents in 10 microM AlCl3. Enhancement of inward K+ currents by AlCl3 is consistent with Al3+ binding to the external surface of the root cap protoplast, decreasing the surface charge, thus causing the channels to sense a more negative membrane potential. Inhibition of inward K+ currents with higher AlCl3 concentrations and more negative voltages is consistent with Al3+ block of K+ channels.

Beta1-subunit of the Ca2+-activated K+ channel regulates contractile activity of mouse urinary bladder smooth muscle.

1. The large-conductance calcium-activated potassium (BK) channel plays an important role in controlling membrane potential and contractility of urinary bladder smooth muscle (UBSM). These channels are composed of a pore-forming alpha-subunit and an accessory, smooth muscle-specific, beta1-subunit. 2. Our aim was to determine the functional role of the beta1-subunit of the BK channel in controlling the contractions of UBSM by using BK channel beta1-subunit 'knock-out' (KO) mice. 3. The beta-galactosidase reporter (lacZ gene) was targeted to the beta1 locus, which provided the opportunity to examine the expression of the beta1-subunit in UBSM. Based on this approach, the beta1-subunit is highly expressed in UBSM. 4. BK channels lacking beta1-subunits have reduced activity, consistent with a shift in BK channel voltage/Ca2+ sensitivity. 5. Iberiotoxin, an inhibitor of BK channels, increased the amplitude and decreased the frequency of phasic contractions of UBSM strips from control mice. 6. The effects of the beta1-subunit deletion on contractions were similar to the effect of iberiotoxin on control mice. The UBSM strips from beta1-subunit KO mice had elevated phasic contraction amplitude and decreased frequency when compared to control UBSM strips. 7. Iberiotoxin increased the amplitude and frequency of phasic contractions, and UBSM tone of UBSM strips from beta1-subunit KO mice, suggesting that BK channels still regulate contractions in the absence of the beta1-subunit. 8. The results indicate that the beta1-subunit, by modulating BK channel activity, plays a significant role in the regulation of phasic contractions of the urinary bladder.

Calcium homeostasis in a clonal pituitary cell line of mouse corticotropes.

Calcium homeostasis was studied following a depolarization-induced transient increase in [Ca2+]i in single cells of the clonal pituitary cell line of corticotropes, AtT-20 cells. The KCl-induced increase in [Ca2+]i was blocked in (i) extracellular calcium-deficient solutions, (ii) external cobalt (2.0 mM), (iii) cadmium (200 microM), and (iv) nifedipine (2.0 microM). The mean increase in [Ca2+]i in single cells in the presence of an uncoupler of mitochondrial function [carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 1 microM] was 54 +/- 13 nM (n = 9). The increase in [Ca2+]i produced by FCCP was greater either during or following a KCl-induced [Ca2+]i load. However, FCCP did not significantly alter the clearance of calcium during a KCl-induced rise in [Ca2+]i. Fifty percent of the cells responded to caffeine (10 mM) with an increase in [Ca2+]i (191 +/- 24 nM; n = 21) above resting levels; this effect was blocked by ryanodine (10 microM). Thapsigargin (2 microM) and 2,5 di(-t-butyl)-1,4 hydroquinone (BuBHQ, 10 microM) produced increases in [Ca2+]i (47 +/- 11 nM, n = 6 and 22 +/- 4 nM, n = 8, respectively) that increased cell excitability. These results support a role for mitochondria and sarco-endoplasmic reticulum calcium stores in cytosolic [Ca2+]i regulation; however, none of these organelles are primarily responsible for the return of [Ca2+]i to resting levels following this KCl-induced [Ca2+]i load.

Low levels of K(ATP) channel activation decrease excitability and contractility of urinary bladder.

Activation of ATP-sensitive potassium (K(ATP)) channels can regulate smooth muscle function through membrane potential hyperpolarization. A critical issue in understanding the role of K(ATP) channels is the relationship between channel activation and the effect on tissue function. Here, we explored this relationship in urinary bladder smooth muscle (UBSM) from the detrusor by activating K(ATP) channels with the synthetic compounds N-(4-benzoylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropionamide (ZD-6169) and levcromakalim. The effects of ZD-6169 and levcromakalim on K(ATP) channel currents in isolated UBSM cells, on action potentials, and on related phasic contractions of isolated UBSM strips were examined. ZD-6169 and levcromakalim at 1.02 and 2.63 microM, respectively, caused half-maximal activation (K1/2) of K(ATP) currents in single UBSM cells (see Heppner TJ, Bonev A, Li JH, Kau ST, and Nelson MT. Pharmacology 53: 170-179, 1996). In contrast, much lower concentrations (K(1/2) = 47 nM for ZD-6169 and K1/2 = 38 nM for levcromakalim) caused inhibition of action potentials and phasic contractions of UBSM. The results suggest that activation of <1% of K(ATP) channels is sufficient to inhibit significantly action potentials and the related phasic contractions.

Voltage dependence of the coupling of Ca(2+) sparks to BK(Ca) channels in urinary bladder smooth muscle.

Large-conductance Ca(2+)-dependent K(+) (BK(Ca)) channels play a critical role in regulating urinary bladder smooth muscle (UBSM) excitability and contractility. Measurements of BK(Ca) currents and intracellular Ca(2+) revealed that BK(Ca) currents are activated by Ca(2+) release events (Ca(2+) sparks) from ryanodine receptors (RyRs) in the sarcoplasmic reticulum. The goals of this project were to characterize Ca(2+) sparks and BK(Ca) currents and to determine the voltage dependence of the coupling of RyRs (Ca(2+) sparks) to BK(Ca) channels in UBSM. Ca(2+) sparks in UBSM had properties similar to those described in arterial smooth muscle. Most Ca(2+) sparks caused BK(Ca) currents at all voltages tested, consistent with the BK(Ca) channels sensing approximately 10 microM Ca(2+). Membrane potential depolarization from -50 to -20 mV increased Ca(2+) spark and BK(Ca) current frequency threefold. However, membrane depolarization over this range had a differential effect on spark and current amplitude, with Ca(2+) spark amplitude increasing by only 30% and BK(Ca) current amplitude increasing 16-fold. A major component of the amplitude modulation of spark-activated BK(Ca) current was quantitatively explained by the known voltage dependence of the Ca(2+) sensitivity of BK(Ca) channels. We, therefore, propose that membrane potential, or any other agent that modulates the Ca(2+) sensitivity of BK(Ca) channels, profoundly alters the coupling strength of Ca(2+) sparks to BK(Ca) channels.

Regulation of urinary bladder smooth muscle contractions by ryanodine receptors and BK and SK channels.

This study examines the roles of voltage-dependent Ca(2+) channels (VDCC), ryanodine receptors (RyRs), large-conductance Ca(2+)-activated K(+) (BK) channels, and small-conductance Ca(2+)-activated K(+) (SK) channels in the regulation of phasic contractions of guinea pig urinary bladder smooth muscle (UBSM). Nisoldipine (100 nM), a dihydropyridine inhibitor of VDCC, abolished spontaneous UBSM contractions. Ryanodine (10 microM) increased contraction frequency and thereby integrated force and, in the presence of the SK blocker apamin, had a greater effect on integrated force than ryanodine alone. Blocking BK (iberiotoxin, 100 nM) or SK (apamin, 100 nM) channels increased contraction amplitude and duration but decreased frequency. The contractile response to iberiotoxin was more pronounced than to apamin. The increases in contraction amplitude and duration to apamin were substantially augmented with ryanodine pretreatment. These results indicate that BK and SK channels have prominent roles as negative feedback elements to limit UBSM contraction amplitude and duration. RyRs also appear to play a significant role as a negative feedback regulator of contraction frequency and duration, and this role is influenced by the activity of SK channels.

Kir2.1 encodes the inward rectifier potassium channel in rat arterial smooth muscle cells.

1. The molecular nature of the strong inward rectifier K+ channel in vascular smooth muscle was explored by using isolated cell RT-PCR, cDNA cloning and expression techniques. 2. RT-PCR of RNA from single smooth muscle cells of rat cerebral (basilar), coronary and mesenteric arteries revealed transcripts for Kir2.1. Transcripts for Kir2.2 and Kir2.3 were not found. 3. Quantitative PCR analysis revealed significant differences in transcript levels of Kir2.1 between the different vascular preparations (n = 3; P < 0.05). A two-fold difference was detected between Kir2.1 mRNA and beta-actin mRNA in coronary arteries when compared with relative levels measured in mesenteric and basilar preparations. 4. Kir2.1 was cloned from rat mesenteric vascular smooth muscle cells and expressed in Xenopus oocytes. Currents were strongly inwardly rectifying and selective for K+. 5. The effect of extracellular Ba2+, Ca2+, Mg2+ and Cs2+ ions on cloned Kir2.1 channels expressed in Xenopus oocytes was examined. Ba2+ and Cs+ block were steeply voltage dependent, whereas block by external Ca2+ and Mg2+ exhibited little voltage dependence. The apparent half-block constants and voltage dependences for Ba2+, Cs+, Ca2+ and Mg2+ were very similar for inward rectifier K+ currents from native cells and cloned Kir2.1 channels expressed in oocytes. 6. Molecular studies demonstrate that Kir2.1 is the only member of the Kir2 channel subfamily present in vascular arterial smooth muscle cells. Expression of cloned Kir2.1 in Xenopus oocytes resulted in inward rectifier K+ currents that strongly resemble those that are observed in native vascular arterial smooth muscle cells. We conclude that Kir2.1 encodes for inward rectifier K+ channels in arterial smooth muscle.

Frequency modulation of Ca2+ sparks is involved in regulation of arterial diameter by cyclic nucleotides.

Forskolin, which elevates cAMP levels, and sodium nitroprusside (SNP) and nicorandil, which elevate cGMP levels, increased, by two- to threefold, the frequency of subcellular Ca2+ release ("Ca2+ sparks") through ryanodine-sensitive Ca2+ release (RyR) channels in the sarcoplasmic reticulum (SR) of myocytes isolated from cerebral and coronary arteries of rats. Forskolin, SNP, nicorandil, dibutyryl-cAMP, and adenosine increased the frequency of Ca(2+)-sensitive K+ (KCa) currents ["spontaneous transient outward currents" (STOCs)] by two- to threefold, consistent with Ca2+ sparks activating STOCs. These agents also increased the mean amplitude of STOCs by 1.3-fold, an effect that could be explained by activation of KCa channels, independent of effects on Ca2+ sparks. To test the hypothesis that cAMP could act to dilate arteries through activation of the Ca2+ spark-->KCa channel pathway, the effects of blockers of KCa channels (iberiotoxin) and of Ca2+ sparks (ryanodine) on forskolin-induced dilations of pressurized cerebral arteries were examined. Forskolin-induced dilations were partially inhibited by iberiotoxin and ryanodine (with no additive effects) and were entirely prevented by elevating external K+. Forskolin lowered average Ca2+ in pressurized arteries while increasing ryanodine-sensitive, caffeine-induced Ca2+ transients. These experiments suggest a new mechanism for cyclic nucleotide-mediated dilations through an increase in Ca2+ spark frequency, caused by effects on SR Ca2+ load and possibly on the RyR channel, which leads to increased STOC frequency, membrane potential hyperpolarization, closure of voltage-dependent Ca2+ channels, decrease in arterial wall Ca2+, and, ultimately, vasodilation.

Ca2+ channels, ryanodine receptors and Ca(2+)-activated K+ channels: a functional unit for regulating arterial tone.

Local calcium transients ('Ca2+ sparks') are thought to be elementary Ca2+ signals in heart, skeletal and smooth muscle cells. Ca2+ sparks result from the opening of a single, or the coordinated opening of many, tightly clustered ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). In arterial smooth muscle, Ca2+ sparks appear to be involved in opposing the tonic contraction of the blood vessel. Intravascular pressure causes a graded membrane potential depolarization to approximately -40 mV, an elevation of arterial wall [Ca2+]i and contraction ('myogenic tone') of arteries. Ca2+ sparks activate calcium-sensitive K+ (KCa) channels in the sarcolemmal membrane to cause membrane hyperpolarization, which opposes the pressure induced depolarization. Thus, inhibition of Ca2+ sparks by ryanodine, or of KCa channels by iberiotoxin, leads to membrane depolarization, activation of L-type voltage-gated Ca2+ channels, and vasoconstriction. Conversely, activation of Ca2+ sparks can lead to vasodilation through activation of KCa channels. Our recent work is aimed at studying the properties and roles of Ca2+ sparks in the regulation of arterial smooth muscle function. The modulation of Ca2+ spark frequency and amplitude by membrane potential, cyclic nucleotides and protein kinase C will be explored. The role of local Ca2+ entry through voltage-dependent Ca2+ channels in the regulation of Ca2+ spark properties will also be examined. Finally, using functional evidence from cardiac myocytes, and histological evidence from smooth muscle, we shall explore whether Ca2+ channels, RyR channels, and KCa channels function as a coupled unit, through Ca2+ and voltage, to regulate arterial smooth muscle membrane potential and vascular tone.

Ca(2+)-activated K+ channels regulate action potential repolarization in urinary bladder smooth muscle.

The goal of this study was to examine the role of large conductance Ca(2+)-activated K+ channels in the regulation of cell excitability in urinary bladder smooth muscle from the guinea pig. Ca(2+)-activated K+ channels were studied with single-channel recording techniques and found to be intracellular Ca2+ and voltage dependent and sensitive to external tetraethylammonium and blocked by nanomolar concentrations of iberiotoxin (apparent dissociation constant of 4 nM). Spontaneous action potentials recorded from intact tissue strips depended on external Ca2+ and were inhibited by Ca2+ channel blockers. Iberiotoxin (100 nM) significantly altered the configuration of the action potential by increasing the duration and peak amplitude of the action potential and decreasing the rate of decay. Iberiotoxin also increased the action potential frequency from 0.11 to 0.29 Hz. This study suggests that Ca(2+)-activated K+ channels play a significant role in the repolarization of the action potential and in the maintenance of the resting membrane potential of the urinary bladder smooth muscle.

Zeneca ZD6169 activates ATP-sensitive K+ channels in the urinary bladder of the guinea pig.

The effects of Zeneca ZD6169, a tertiary carbinol, and levcromakalim were examined on the membrane potential of intact smooth muscle cells, and on ATP-sensitive K+ (KATP) channel currents in isolated smooth muscle cells from the guinea pig urinary bladder. ZD6169 and levcromakalim induced a glibenclamide-sensitive hyperpolarization of the membrane potential. The ZD6169- and levcromakalim-induced KATP currents were half-maximal at 1.02 and 2.63 mumol/l, respectively, with Hill coefficients of 1.46 and 1.62, respectively. The ZD6169-induced KATP currents were inhibited by internal ATP (3.0 mmol/l), reduced 34% by activators of protein kinase C, and decreased 35% when the external pH was lowered to 6.4. This study provides the first characterization of ZD6169 on KATP currents and indicates that ZD6169 is a potent opener of KATP channels in the smooth muscle from the urinary bladder.

Studies of the K(ATP) channel opening activity of the new dihydropyridine compound 9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridined ione in bladder detrusor in vitro.

The potassium (K+) channel opening activity of ZM244085 (9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridined ione, CAS 149398-59-4), a novel dihydropyridine (DHP), was ascertained. In a set of functional assays, its mechanoinhibitory effect on myogenic activity of guinea pig bladder detrusor muscles, either mildly or highly depolarized with 15 or 80 mmol/l KCl, was measured. ZM244085 had negligible effect on the tone of the detrusor contracted with 80 mmol/l KCl but reduced the myogenic activity induced with 15 mmol/l KCl (IC50=4.2 +/- 0.4 mumol/l). Glibenclamide, an ATP-sensitive K+ (KATP) channel blocker, competitively antagonized this action of ZM244085 with a pA2 value of 7.6. This functional profile of ZM244085 is similar to that of the prototypic K+ channel opener cromakalim but stands in contrast to that of typical DHP Ca2+ channel blockers such as nifedipine and nimodipine. The membrane potential of the guinea pig detrusor, recorded with intracellular microelectrodes, was hyperpolarized 6.8 +/- 3.1 mV by ZM244085 (10 mumol/l). This hyperpolarization was completely blocked by glibenclamide but not affected by apamin (10 mumol/l), a toxin blocking specifically small conductance and Ca2+ dependent K+ (SKCa) channels. ZM244085 (10 mumol/l) increased the whole cell KATP current in isolated guinea pig detrusor cells by 8.8 +/- 2.5 pA, but failed to activate large conductance and Ca2+ dependent K+ (BKCa) channels in excised inside-out membrane patches from those cells. The results from these studies showed that ZM244085 is a K+ channel opener which activates predominantly KATP channels in vitro to relax bladder detrusors.

VX enhances neuronal excitability and alters membrane properties of Rana catesbeiana sympathetic ganglion neurons.

1. The effects of VX (10 microM) were examined on sympathetic ganglion neurons from the bullfrog using intracellular recording techniques. 2. VX significantly increased the amplitude of the residual EPSP from 4.8 +/- 0.86 mV (n = 4) to 13.7 +/- 1.23 mV (n = 4). 3. VX significantly decreased the membrane potential 5.2 +/- 0.75 mV (n = 6). The input resistance and the duration of the spike afterhyperpolarization (AHP) were also reduced 69.8% and 69.6% of control, respectively. 4. VX increased neuronal excitability greater than 200% (n = 5) of control. 5. The VX-induced neuronal excitability may result from a reduction in the duration of the AHP and contribute to the CNS toxicity.

Comparison of atropine pre- and post-treatment in ganglion neurons exposed to soman.

The prevention and reversal of the effects of soman (pinacoloxymethylphosporyl fluoride) have been examined on the electrical properties of sympathetic ganglion neurons in vitro from the adult bullfrog Rana catesbeiana. Atropine (10 microM) pre-treatment (before soman) blocked the soman-induced decrease in the membrane potential, membrane resistance, and afterhyperpolarization (AHP) duration. Atropine post-treatment (after soman exposure) restored the soman-induced decrease in the membrane potential but was ineffective in reversing either the membrane resistance or the duration of the AHP. These results demonstrate: 1) that the effects of soman on the electrical properties of these neurons are mediated by the activation of muscarinic receptors, 2) that following receptor activation different cellular mechanisms may be involved in the regulation of the electrical properties of the neuron, and 3) pre-treatment rather than post-treatment with atropine is more effective in blocking these direct effects of soman. These results are discussed in reference to the increased effectiveness of atropine pretreatment in the protocols for soman-induced neurotoxicity.

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.

Soman reversibly decreases the duration of Ca2+ and Ba2+ action potentials in bullfrog sympathetic neurons.

Soman, (pinacoloxymethyl-phosphoryl fluoride) (0.1-10 microM) an irreversible cholinesterase inhibitor, reversibly reduced the duration of calcium (Ca2+)- and barium (Ba2+) spikes without significantly affecting spike amplitude in sympathetic postganglionic neurons of the adult bullfrog (Rana catesbeiana). The soman-induced shortening of the spike duration was not prevented by pretreatment with either (+)-tubocurarine (100 microM) or hexamethonium (100 microM) and atropine (10 microM) and was also recorded from acutely-dissociated sympathetic neurons. These results suggest that soman has a direct action to decrease calcium entry through voltage-dependent channels activated during a spike. This effect may contribute to both the decrease in the duration of the spike after-hyperpolarization (AHP) and the enhanced neuronal excitability produced by soman in these neurons.

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.

The effects of irreversible acetylcholinesterase inhibitors on transmission through sympathetic ganglia of the bullfrog.

The effects of soman, sarin and VX were examined on ganglionic transmission through paravertebral chain ganglia of the bullfrog, Rana catesbeiana. Low frequency (0.1 Hz), short (2 sec) and long (10 sec) trains of preganglionic stimulation, after exposure to the agents, induced repetitive activity in the extracellularly recorded compound action potential. An irreversible transient depression was observed after exposure to the agents during the first second of short and long stimulus trains. Long stimulus trains of high frequency were required to produce a rundown in the amplitude of the compound action potential, whether recorded in the presence of each agent (10 microM) or following a wash with agent-free solution. The rundown of the compound action potential was use-dependent and not blocked or reversed by atropine (10 microM). Intracellular recordings, in the presence of either soman or VX, demonstrated (1) an increase in the amplitude of the residual excitatory postsynaptic potential or current evoked by synaptic stimulation, (2) an increase in the amplitude and duration of the acetylcholine-induced potential, (3) no increase in either the amplitude or duration of the carbachol-induced potential, (4) repetitive firing with orthodromic but not antidromic stimulation and (5) a concentration- and frequency-dependent depolarization of individual ganglion neurons with orthodromic stimulation which resulted in a decrease in the generation of action potentials. These results suggest that the agent-induced decrease in the compound action potential occurred as a consequence of activity-dependent depolarization of ganglion neurons, which occurs after inhibition of cholinesterase.

Effects of paraoxon on spike initiation and conduction block in the giant interneurons of the American cockroach, Periplaneta americana.

1. The effects of paraoxon were studied on spike initiation and conduction in the giant interneurons (GIs) of the American cockroach, using electrophysiological techniques. 2. Paraoxon treatment induced high-frequency bursts in GI axons. During these bursts, overshooting spikes recorded in the sixth abdominal ganglion were replaced, in phase, by small, decremental potentials. 3. These small potentials were not EPSPs since current injection could modulate their frequency. 4. An analysis of anteriorly conducted spikes indicates that the site of spike initiation is located near the dendritic region of the GI and is unchanged by paraoxon treatment.