Blockade and spontaneous recovery of ganglionic transmission after treatment by irreversible inhibitors of cholinesterase

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.

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.

Action of an irreversible acetylcholine esterase inhibitor, soman, on muscarinic hyperpolarization in cat bladder parasympathetic ganglia

1. Intracellular recording techniques were used to examine the action of an irreversible acetylcholine esterase (AChE) inhibitor, soman, on the hyperpolarizations mediated through muscarinic cholinoceptors in cat bladder parasympathetic neurones. 2. Soman (0.1-10 microM) depressed the amplitude and prolonged the duration of the muscarinic slow inhibitory postsynaptic potential (s-i.p.s.p.) elicited by a preganglionic tetanus (40 Hz for 1 s) in the presence of mecamylamine (20 microM), phentolamine (1 microM) and caffeine (1 mM), in a dose-dependent manner. The effect of soman on the amplitude of the s-i.p.s.p. was partially reversible, while the effect on the duration was irreversible. 3. Soman hyperpolarized the membrane and decreased input resistance, but this effect could not account for soman-induced inhibition of the s-i.p.s.p. 4. Soman depressed the amplitude and prolonged the duration of a muscarinic hyperpolarization induced by pressure application of acetylcholine (ACh) in the presence of mecamylamine, phentolamine and caffeine. The time course of this effect paralleled that on the synaptically-evoked muscarinic s-i.p.s.p. 5. A reversible AChE inhibitor, pyridostigmine (10-100 microM), also depressed the amplitude and prolonged the duration of a muscarinic hyperpolarization induced by either preganglionic stimulation or ACh pressure application. These actions were reversible, and not accompanied by a significant change in membrane potential or input resistance. 6. The inhibitory action of soman (1 microM) on the muscarinic hyperpolarization was prevented by pyridostigmine (10 microM), but not by atropine (1 microM). 7. These results demonstrate that soman prolongs not only the muscarinic hyperpolarization, but also inhibits its amplitude through a postsynaptic action, probably through AChE inhibition, in cat bladder parasympathetic neurones.

[Frequency potentiation of neuronal synaptic reactions in the medial tegmentum of the medulla oblongata to microstimulation of the inhibitory point of the pons Varolii]

Synaptic responses of medial medullary neurons to single (2 pps) and repetitive (30-50 pps) stimuli delivered to the pontine inhibitory point were recorded in decerebrated cats. Firing index, inhibitory to the less extent, excitatory postsynaptic potentials usually increased when repetitive stimulation was applied. Suppression of the background impulse activity was observed in some neurons. Frequency potentiation makes a substantial contribution to functional effect of stimulation of the inhibitory point (termination of elicited locomotion).

Noncholinesterase actions of an irreversible acetylcholinesterase inhibitor on synaptic transmission and membrane properties in autonomic ganglia

Superfusion of the organophosphorous acetylcholinesterase inhibitor soman (pinacolyl methylphosphonofluoridate; 0.01-25 microM) produced a dose-dependent reduction of extracellularly and intracellularly recorded synaptic responses in the isolated rat superior cervical ganglia at frequencies of orthodromic stimulation that do not normally produce synaptic depression. The magnitude of depression was dependent upon the frequency of stimulation (0.02-1 Hz), was maintained after the removal of soman from the superfusion solution, and recovered by over 65% during periods of inactivity. The depression of synaptic transmission produced by soman was not dependent upon the inhibition of acetylcholinesterase (AChE) activity by this agent. Transmission was increasingly depressed by doses of soman greater than those needed to inactivate all measurable ganglionic AChE activity. Dose-dependent depression of synaptic transmission in soman also occurred after pretreatment with the irreversible AChE inhibitor diisopropylphosphofluoridate (DFP; 100 microM), which inhibited greater than 98% of the AChE activity in the ganglia. Soman produced a decline in the input resistance, resting potential, spike amplitude, and spike threshold and a reduction in the hyperpolarizing afterpotential. Soman-induced depression of synaptic transmission was not due primarily to a blockade of postsynaptic nicotinic receptors. At concentrations of soman which produced significant depression in transmission, ganglionic depolarization produced by bath-applied carbamylcholine (carbachol) was either slightly depressed or facilitated. In the presence of soman, repetitive focal application of acetylcholine or carbachol did not reveal use-dependent desensitization. Muscarinic antagonists, atropine and pirenzepine, protected against the use-dependent depression of synaptic transmission induced by soman. These results suggest that a principal site of action for soman is at the presynaptic terminal and that this site is sensitive to muscarinic receptor blockade.