Amine and Amino Acid Receptors in Gastropod Neurons

102

SUMMARY1. A voltage-clamp study was made of some properties of the non-synaptic hyperpolarization-activated C1-conductance recently described in Aplysia neurones loaded with Cl-ions (Chesnoy-Marchais, 1982). The experiments were performed on an identified family of neurones, which present cholinergic responses allowing an easy measurement of the equilibrium potentials of Cl-(EC1) and K+ ions (EK).2. The Cl-selectivity ofthe hyperpolarization-activated conductance was deduced from four observations: (1) the extrapolated reversal potential of the hyperpolarization-activated current, Er, was close to the reversal potential ofthe cholinergic Cl-response, which is the equilibrium potential for Cl-ions, EC1. 3. The steady-state Cl-conductance (ass) increases steeply with hyperpolarization.The kinetics of activation and deactivation are exponential and are characterized by the same voltage-dependent time constant (r), of the order of a few seconds or fractions of seconds. The curves g.8(V) and r( V) can both be fitted by a two-state model in which the rate constants are exponential functions of the membrane potential (e-fold change for 12-16 mV).4. The Cl-current is much more affected by changes of the intracellular Clconcentration than predicted simply from the change in Cl-driving force. Both the conductance and the time constant of activation are strongly modified. Modifications of the extracellular Cl-concentration do not always alter the amplitude of the hyperpolarization-activated Cl-current, but systematically affect its kinetics.5. The hyperpolarization-activated current is abolished after prolonged exposure of the cell to an artificial sea water where N03-ions replace Cl-ions, as well as after intracellular injections of N03-ions.6. Increasing the external pH shifts the g.(V) and r(V) curves to the left.Lowering the external pH has reverse but less pronounced effects. 7. In cells which were not loaded with Cl-ions and did not present the hyperpolarization-activated Cl-current, this current could be detected if the D. CHESNO Y-MARCHAIS hyperpolarizing jump was preceded by short depolarizing pulses. In cells which were loaded with Cl-ions, the Cl-current became larger after a short depolarizing pulse.In the presence of extracellular C02+ ions, depolarizing pulses no longer increased the Cl-current.8. The Cl-current is not affected by extracellularly applied DIDS (4,4'-diisothiocyano-2,2'-disulphonic acid stilbene), but is markedly reduced by intracellular injection of DIDS.9. Extracellular Cs+ ions, which have been reported to block some cationic hyperpolarization-activated inward currents, do not reduce the hyperpolarizationactivated Cl-current. High concentrations of Cs+ produce complex effects which are probably due to an increased synaptic activity, but the hyperpolarization-activatedCl-current persists after complete substitution of the extracellular and intracellular monovalent cations by Cs+.

Section: Effect Of External C8+ Ionssupporting

confidence: 60%

Characterization of a chloride conductance activated by hyperpolarization in Aplysia neurones.

Chesnoy-Marchais¹

1983

102

“…The corresponding changes in membrane potential are inverted when the cell is hyperpolarized beyond about -60 mV for the response generated by chloride, and -80 mV for that generated by potassium; these values have been found sufficiently reproducible to allow the inversion level to identify the ion concerned. It has also been found that if the carbachol is applied to the soma by iontophoresis from a sufficiently distant point only the potassium response is seen (Ascher & Kehoe, 1975). Fig.…”

Section: Resultsmentioning

confidence: 92%

Voltage‐current relationship of a carbachol‐induced potassium‐ion pathway in Aplysia neurones.

Ginsborg¹,

Kado²

1975

SUMMARY1. The electrical characteristics of a potassium ion selective pathway produced by the action of carbachol on Aplysia neurones (Kehoe, 1972b) has been studied.2. The relationship between current and voltage has been found to be non-linear, the conductance increasing with depolarization and decreasing with hyperpolarization. The degree of rectification was reduced when the external potassium was raised to 50 mm from its normal value of 10 mm.3. The direction of the rectification and the effect of increased potassium are as predicted by the 'constant field' theory, but the degree of rectification is somewhat larger.

“…Reversal of the response The development above -10 mV of large conductances linked to the generation of action potentials has prevented so far the observation of a clear reversal of the excitatory response to ACh in molluscs (see Ascher & Kehoe, 1975;Ascher et al 1978a). However, after replacement of Na by Mg, Sr, or sucrose, we have been able to observe the inversion of the response.…”

Section: Elementary Current In LI and Sr Sea Watermentioning

confidence: 99%

Interaction of permeant ions with channels activated by acetylcholine in Aplysia neurones.

Marchais¹,

Marty²

1979

SUMMARY1. Aplysia neurones with an excitatory response to acetylcholine (ACh) were voltage-clamped, and the ACh-induced currents were studied using noise and relaxation techniques. The mean channel open time, T, and the amplitude of the elementary current, ie1, were determined from these experiments, and the variation of these parameters with the ionic content of the extracellular solution was analysed. The goal of this work was to test whether permeant ions may bind in a voltagedependent manner to channel sites and thereby hinder channel closing, as has been proposed before (Ascher, Marty & Neild, 1978a).2. The relation between T and the membrane potential V has a similar shape in normal sea water and after total replacement of Na ions with Li or Cs. In contrast, the shape of the r(V) relation is modified if Na is replaced by Mg, Sr, or Ba. 5. Experiments were performed in solutions containing Na and sucrose, or Mg and mannitol. In both cases r was smaller than in an isotonic Na or Mg solution.6. None of the above observations can be accounted for on the sole basis of outer surface potential changes.7. A quantitative model of the interaction between permeant ions and AChsensitive channels is proposed. The possible relevance of this model for the interpretation of r( V) curves in other systems is discussed.