Comparison of excitatory currents activated by different transmitters on crustacean muscle. I. Acetylcholine-activated channels

The channel-blocking action of methonium compounds on rat submandibular ganglion cells. 1983

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium).

All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at − 80 mV being fairly similar (5.9 × 10⁶ to 18.1 × 10⁶M⁻¹ s⁻¹) for all of the compounds except C4, which had no effect on the e.s.c. decay.

All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine.

Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to − 40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist.

When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked.

Even at 1 Mm, C6 does not prevent acetylcholine from opening ionic channels.

It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

Anomalous voltage dependence of channel blockade at a crustacean glutamate-mediated synapse

1. The voltage dependence and concentration dependence of blockade of glutamate-activated currents by the diquaternary amine, chlorisondamine, were examined in a marine crustacean muscle. 2. Chlorisondamine results in the splitting of focally recorded synaptic current decays into two exponential components. The fast component becomes faster with increases in drug concentration and with hyperpolarization. The slow decay rate is unchanged or faster with hyperpolarization and the relative amplitude of the slow component is increased with hyperpolarization. 3. The alteration of synaptic current decay rates by chlorisondamine over the range of 5 to 100 microM and -80 to -140 mV is quantitatively consistent with a simple channel blockade model with a zero-voltage blocking rate of 6 x 10(5) M-1 s-1 at 12 degrees C with a voltage dependence of about 40 mV per e-fold change. The unblocking rate is about 5 s-1 at 0 mV and increases with hyperpolarization with a voltage dependence of about 30 mV per e-fold change. 4. The dose dependence and voltage dependence of blockade of ionophoretically activated glutamate currents by chlorisondamine are qualitatively consistent with the kinetic estimates. 5. The anomalous voltage dependence of the unblocking process is considered in terms of the possibility that the relief from blockade by chlorisondamine occurs by transit of chlorisondamine through the ion channel opened by glutamate.

The channel-blocking action of methonium compounds on rat submandibular ganglion cells

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium). All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at -80 mV being fairly similar (5.9 X 10(6) to 18.1 X 10(6)M-1S-1) for all of the compounds except C4, which had no effect on the e.s.c. decay. All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine. Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to -40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist. When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked. Even at 1 mM, C6 does not prevent acetylcholine from opening ionic channels. It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

Comparison of excitatory currents activated by different transmitters on crustacean muscle. II. Glutamate-activated currents and comparison with acetylcholine currents present on the same muscle

The properties of glutamate-activated excitatory currents on the gm6 muscle from the foregut of the spiny lobsters Panulirus argus and interruptus and the crab Cancer borealis were examined using either noise analysis, analysis of synaptic current decays, or slow iontophoretic currents. The properties of acetylcholine currents activated in nonjunctional regions of the gm6 muscle were also examined. At 12 degrees C and -80 mV, the predominant time constant of power spectra from glutamate-activated current noise was approximately 7 ms and the elementary conductance was approximately 34 pS. At 12 degrees C and -80 mV, the predominant time constant of acetylcholine-activated channels was approximately 11 ms with a conductance of approximately 12 pS. Focally recorded glutamatergic extracellular synaptic currents on the gm6 muscle decayed with time constants of approximately 7-8 ms at 12 degrees C and -80 mV. The decay time constant was prolonged e-fold about every 225-mV hyperpolarization in membrane potential. The Q10 of the time constant of the synaptic current decay was approximately 2.6. The voltage dependence of the steady-state conductance increase activated by iontophoretic application of glutamate has the opposite direction of the steady-state conductance activated by cholinergic agonists when compared on the gm6 muscles. The glutamate-activated conductance increase is diminished with hyperpolarization. The properties of the marine crustacean glutamate channels are discussed in relation to glutamate channels in other organisms and to the acetylcholine channels found on the gm6 muscle and the gm1 muscle of the decapod foregut (Lingle and Auerbach, 1983).