Voltage-dependent drug blockade of L-glutamate activated channels of the crayfish

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.

Acetylcholine responses of identified neurons in Helix pomatia--II. Pharmacological properties of acetylcholine responses

A pharmacological separation of depolarizing and hyperpolarizing mechanisms involved in the generation of acetylcholine (ACh) depolarizations was attempted in the identified neurons B1 and B3 of the buccal ganglia of Helix pomatia. The selectivity of the drugs employed was assayed in non-identified buccal neurons in which ACh increased a hyperpolarizing Cl- conductance. Voltage clamp techniques were used. Under control conditions the depolarizing ACh currents increased non-linearly with more negative membrane potentials. The hyperpolarizing ACh currents showed a linear potential dependence. The buffer substance Tris (5 mmol/l) depressed the depolarizing ACh currents. The effect was accentuated with more negative membrane potentials. Tris failed to affect hyperpolarizing ACh responses. HEPES (5 mmol/l) did not change depolarizing or hyperpolarizing ACh responses. d-Tubocurarine (0.02-0.2 mmol/l), hexamethonium (0.5-5.0 mmol/l) and atropine (0.1 mmol/l) blocked the depolarizing and hyperpolarizing ACh responses. Arecoline (0.1 mmol/l) had neither an agonistic nor an antagonistic effect on the identified and on the non-identified neurons. It displayed an anticholinesterase activity. Anthracene-9-carbonic acid (0.5 mmol/l) depressed selectively the hyperpolarizing ACh responses. In the neurons B1 and B3 no pharmacologically separable hyperpolarizing ACh responses were detected to be superimposed on the ACh depolarizations.