Analysis of whole‐cell currents by patch clamp of guinea‐pig myenteric neurones in intact ganglia
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Whole-cell patch-clamp recordings taken from guinea-pig duodenal myenteric neurones within intact ganglia were used to determine the properties of S and AH neurones. Major currents that determine the states of AH neurones were identified and quantified. S neurones had resting potentials of -47 +/- 6 mV and input resistances (R(in)) of 713 +/- 49 MOmega at voltages ranging from -90 to -40 mV. At more negative levels, activation of a time-independent, caesium-sensitive, inward-rectifier current (I(Kir)) decreased R(in) to 103 +/- 10 MOmega. AH neurones had resting potentials of -57 +/- 4 mV and R(in) was 502 +/- 27 MOmega. R(in) fell to 194 +/- 16 MOmega upon hyperpolarization. This decrease was attributable mainly to the activation of a cationic h current, I(h), and to I(Kir). Resting potential and R(in) exhibited a low sensitivity to changes in [K(+)](o) in both AH and S neurones. This indicates that both cells have a low background K(+) permeability. The cationic current, I(h), contributed about 20 % to the resting conductance of AH neurones. It had a half-activation voltage of -72 +/- 2 mV, and a voltage sensitivity of 8.2 +/- 0.7 mV per e-fold change. I(h) has relatively fast, voltage-dependent kinetics, with on and off time constants in the range of 50-350 ms. AH neurones had a previously undescribed, low threshold, slowly inactivating, sodium-dependent current that was poorly sensitive to TTX. In AH neurones, the post-action-potential slow hyperpolarizing current, I(AHP), displayed large variation from cell to cell. I(AHP) appeared to be highly Ca(2+) sensitive, since its activation with either membrane depolarization or caffeine (1 mM) was not prevented by perfusing the cell with 10 mM BAPTA. We determined the identity of the Ca(2+) channels linked to I(AHP). Action potentials of AH neurones that were elongated by TEA (10 mM) were similarly shortened and I(AHP) was suppressed with each of the three omega-conotoxins GVIA, MVIIA and MVIIC (0.3-0.5 microM), but not with omega-agatoxin IVA (0.2 microM). There was no additivity between the effects of the three conotoxins, which indicates the presence of N- but not of P/Q-type Ca(2+) channels. A residual Ca(2+) current, resistant to all toxins, but blocked by 0.5 mM Cd(2+), could not generate I(AHP). This patch-clamp study, performed on intact ganglia, demonstrates that the AH neurones of the guinea-pig duodenum are under the control of four major currents, I(AHP), I(h), an N-type Ca(2+) current and a slowly inactivating Na(+) current.
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