The Journal of Neurophysiology Vol. 85 No. 5 May 2001, pp. 1941-1951
Copyright ©2001 by the American Physiological Society

Afterhyperpolarization Current in Myenteric Neurons of the Guinea Pig Duodenum

Department of Anatomy and Cell Biology, University of Melbourne, Parkville, Victoria 3010, Australia

Vogalis, Fivos, John B. Furness, and Wolf A. A. Kunze. Afterhyperpolarization Current in Myenteric Neurons of the Guinea Pig Duodenum. J. Neurophysiol. 85: 1941-1951, 2001. Whole cell patch and cell-attached recordings were obtained from neurons in intact ganglia of the myenteric plexus of the guinea pig duodenum. Two classes of neuron were identified electrophysiologically: phasically firing AH neurons that had a pronounced slow afterhyperpolarization (AHP) and tonically firing S neurons that lacked a slow AHP. We investigated the properties of the slow AHP and the underlying current (I_AHP) to address the roles of Ca²⁺ entry and Ca²⁺ release in the AHP and the characteristics of the K⁺ channels that are activated. AH neurons had a resting potential of 54 mV and the AHP, which followed a volley of three suprathreshold depolarizing current pulses delivered at 50 Hz through the pipette, averaged 11 mV at its peak, which occurred 0.5-1 s following the stimulus. The duration of these AHPs averaged 7 s. Under voltage-clamp conditions, I_AHP's were recorded at holding potentials of 50 to 65 mV, following brief depolarization of AH neurons (20-100 ms) to positive potentials (+35 to +50 mV). The null potential of the I_AHP at its peak was 89 mV. The AHP and I_AHP were largely blocked by -conotoxin GVIA (0.6-1 µM). Both events were markedly decreased by caffeine (2-5 mM) and by ryanodine (10-20 µM) added to the bathing solution. Pharmacological suppression of the I_AHP with TEA (20 mM) or charybdotoxin (50-100 nM) unmasked an early transient inward current at 55 mV following step depolarization that reversed at 34 mV and was inhibited by niflumic acid (50-100 µM). Mean-variance analysis performed on the decay of the I_AHP revealed that the AHP K⁺ channels have a mean chord conductance of ~10 pS, and there are ~4,000 per AH neuron. Spectral analysis showed that the AHP channels have a mean open dwell time of 2.8 ms. Cell-attached patch recordings from AH neurons confirmed that the channels that open following action currents have a small unitary conductance (10-17 pS) and open with a high probability (0.5) within the first 2 s following an action potential. These results indicate that the AHP is largely a consequence of Ca²⁺ entry through -conotoxin GVIA-sensitive Ca²⁺ channels during the action potential, Ca²⁺-triggered Ca²⁺ release from caffeine-sensitive stores and the opening of Ca²⁺-sensitive small-conductance K⁺ channels.