Afterhyperpolarization current in myenteric neurons of the guinea pig duodenum

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 omega -conotoxin GVIA (0.6-1 muM). Both events were markedly decreased by caffeine (2-5 mM) and by ryanodine (10-20 muM) 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 muM). Mean-variance analysis performed on the decay of the I-AHP revealed that the AHP K+ channels have a mean chord conductance of similar to 10 pS, and there are similar to 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 (less than or equal to0.5) within the first 2 s following an action potential. These results indicate that the AHP is largely a consequence of Ca2+ entry through omega -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.