Effect of acetylcholine on membrane potential in toad dorsal root ganglion neurons and its underlying ionic basis

Summary

Intracellular recordings were made to investigate the responses of membrane potential to acetylcholine (ACh) on neurons in isolated toad dorsal root ganglion (DRG). In the 73 neurons examined, 67 were of type A, and the remaining 6 of type C cell. The resting membrane potential of these two types of cells was −67.5±1.3 mV (× ±SE). During the application of ACh (4 × 10^-4–6 × 10^-4 mol/L), the changes in membrane potential were as follows: 1) hyperpolarization, with amplitude of 9.1±3.0 mV (X ± SE; n = 23); 2) depolarization, with amplitude of 12.9 ±2.2 mV (X ±SE; n = 20); 3) biphasic response, i.e., hyperpolarization with amplitude of 8.0±2.4 mV (X±SE) followed by depolarization with amplitude of 10.9±2.1 mV (X±SE) (n=24); no effect (n=6).

The hyperpolarization induced by ACh was blocked by superfusion with atropine (1.3 × 10^-5 mol/L; n = 23), while ACh depolarization was blocked by the mixture of d-tubocurarine (1.4 × 10^-5 mol/L) and hexamethonium (1.4 ×10^-5 mol/L) (n = 18). When ACh caused hyperpolarization, the membrane conductance wascin reased by 13.8% and the reversal potential was about -96 mV (n=3). TEA (20 mmol/L) superfusion enhanced ACh depolarization amplitude by 48.2 ±3.2 % (× ± SE;n = 6), and depressed ACh hyperpolarization amplitude by 79.4 ±4.3 % (× ± SE; n= 8).MnCl₂ (4 mmol/l) superfusion decreased the amplitudes of ACh depolarization and hyperpolarization by 54.2 ±7.2 % (X ±SE; n= 5) and by 69.2±6.4 % (X±SE; n = 14) respectively. These results suggest that the depolarization and hyperpolarization induced by ACh are mediated by nicotinic and muscarinic receptors on the soma of toad DRG neurons separately, and it seems that ACh hyperpolarization involves activation of calcium-activated potassium conductance.