Spontaneous Miniature Hyperpolarizations Affect Threshold for Action Potential Generation in Mudpuppy Cardiac Neurons

Abstract

Mudpuppy parasympathetic neurons exhibit spontaneous miniature hyperpolarizations (SMHs) that are generated by potassium currents, which are spontaneous miniature outward currents (SMOCs), flowing through clusters of large conductance voltage- and calcium (Ca2+)-activated potassium (BK) channels. The underlying SMOCs are initiated by a Ca2+-induced Ca2+ release (CICR) mechanism. Perforated-patch whole cell voltage recordings were used to determine whether activation of SMHs contributed to action potential (AP) repolarization or affected the latency to AP generation. Blockade of BK channels by iberiotoxin (IBX, 100 nM) slowed AP repolarization and increased AP duration. Treatment with ω-conotoxin GVIA (3 μM) or nifedipine (10 μM) to inhibit Ca2+ influx through N- or L-type voltage-dependent calcium channels (VDCCs), respectively, also decreased the rate of AP repolarization and increased AP duration. Elimination of CICR by treatment with either thapsigargin (1 μM) or ryanodine (10 μM) produced no significant change in AP repolarization or duration. Blockade of BK channels with IBX and inhibition of N-type VDCCs with ω-conotoxin GVIA, but not inhibition of L-type VDCCs with nifedipine, decreased the latency of AP generation. A decrease in latency to AP generation occurred with elimination of SMHs by inhibition of CICR following treatment with thapsigargin. Ryanodine treatment decreased AP latency in three of six cells. Apamin (100 nM) had no affect on AP repolarization, duration, or latency to AP generation, but did decrease the hyperpolarizing afterpotential (HAP). Inhibition of L-type VDCCs by nifedipine also decreased HAP amplitude. Inhibition of CICR by either thapsigargin or ryanodine treatment increased the number of APs generated with long depolarizing current pulses, whereas exposure to IBX or ω-conotoxin GVIA depressed excitability. We conclude that CICR, the process responsible for SMH generation, represents a unique mechanism to modulate the response to subthreshold depolarizing currents that drive the membrane potential toward the threshold for AP initiation but does not contribute to AP repolarization. Subthreshold depolarizations would not activate sufficient numbers of VDCCs to allow Ca2+ influx to elevate [Ca2+]i to the extent needed to directly activate nearby BK channels. However, the elevation in [Ca2+]i is sufficient to trigger CICR from ryanodine-sensitive Ca2+ stores. Thus CICR acts as an amplification mechanism to trigger a local elevation of [Ca2+]i near a cluster of BK channels to activate these channels at negative levels of membrane potential.