Ca²⁺-Induced Ca²⁺Release Activates Spontaneous Miniature Outward Currents (SMOCs) in Parasympathetic Cardiac Neurons

Abstract

Mudpuppy parasympathetic cardiac neurons exhibit spontaneous miniature outward currents (SMOCs) that are thought to be due to the activation of clusters of large conductance Ca(2+)-activated K(+) channels (BK channels) by localized release of Ca(2+) from internal stores close to the plasma membrane. Perforated-patch whole cell recordings were used to determine whether Ca(2+)-induced Ca(2+) release (CICR) is involved in SMOC generation. We confirmed that BK channels are involved by showing that SMOCs are inhibited by 100 nM iberiotoxin or 500 microM tetraethylammonium (TEA), but not by 100 nM apamin. SMOC frequency is decreased in solutions that contain 0 Ca(2+)/3.6 mM Mg(2+), and also in the presence of 1 microM nifedipine and 3 microM omega-conotoxin GVIA, suggesting that SMOC activation is dependent on calcium influx. However, Ca(2+) influx alone is not sufficient; SMOC activation is also dependent on Ca(2+) release from the caffeine- and ryanodine-sensitive Ca(2+) store, because exposure to 2 mM caffeine consistently caused an increase in SMOC frequency, and 10-100 microM ryanodine altered the configuration of SMOCs and eventually inhibited SMOC activity. Depletion of intracellular Ca(2+) stores by the Ca-ATPase inhibitor cyclopiazonic acid (10 microM) inhibited SMOC activity, even when Ca(2+) influx was not compromised. We also tested the effects of the membrane-permeable Ca(2+) chelators, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid-AM (BAPTA-AM) and EGTA-AM. EGTA-AM (10 microM) caused no inhibition of SMOC activation, whereas 10 microM BAPTA-AM consistently inhibited SMOCs. After SMOCs were completely inhibited by BAPTA, 3 mM caffeine caused SMOC activity to resume. This effect was reversible on removal of caffeine and suggests that the source of Ca(2+) that triggers the internal Ca(2+) release channel is different from the source of Ca(2+) that activates clusters of BK channels. We propose that influx of Ca(2+) through voltage-dependent Ca(2+) channels is required for SMOC generation, but that the influx of Ca(2+) triggers CICR from intracellular stores, which then activates the BK channels responsible for SMOC generation.

Presynaptic Ryanodine Receptors Are Required for Normal Quantal Size at theCaenorhabditis elegansNeuromuscular Junction

“…However, unc-68 mutations reduced mPSC frequency by Ͼ70%, (Verkhratsky and Shmigol, 1996). CICR has been demonstrated in a variety of cell types, including neurons (Narita et al, 1998;Merriam et al, 1999;Sandler and Barbara, 1999). The ER is a continuous network that extends to all parts of the neuron (Berridge, 1998;Rolls et al, 2002).…”

Spontaneous Opening of T-Type Ca2+Channels Contributes to the Irregular Firing of Dopamine Neurons in Neonatal Rats

“…The frequency of SMOCs was highest at approximately Ϫ50 mV in DA neurons, whereas it peaked at Ϫ20 mV or more depolarized potentials in other types of cells (Satin and Adams, 1987;Fletcher and Chiappinelli, 1992;Merriam et al, 1999). This difference can be accounted for by the selective coupling of low-threshold T-type Ca 2ϩ channels with SMOCs in DA neurons but not in other cells (Merriam et al, 1999). The activation threshold for T-type channels is Ϫ70 to Ϫ60 mV, whereas they become fully activated at Ϫ40 to Ϫ30 mV (Randall and Tsien, 1997;PerezReyes, 2003).…”

“…SMOCs-STOCs are mediated by the activation of Ca 2ϩ -sensitive K ϩ channels. Focal release of Ca 2ϩ from internal stores is generally thought to provide the source of Ca 2ϩ (Satin and Adams, 1987;Berridge, 1998;Merriam et al, 1999 (Womack and Khodakhah, 2004).…”

“…Indeed, this mechanism has been used to explain the generation of SMOCs in mudpuppy parasympathetic cardiac neurons (Merriam et al, 1999) and rat Meynert neurons (Arima et al, 2001). One unique property of SMOCs in DA neurons lies in the voltage dependence.…”

Understanding calcium waves and sparks in central neurons

“…These currents result from the activation of BK channels and lead to muscle relaxation. In a few neurons, similar currents called spontaneous miniature outward currents (SMOCs) have been detected that activate either SK channels (in rat medial preoptic neurons 129 ) or BK channels (in parasympathetic cardiac neurons 130 ). These studies did not determine in which part of the neuron the currents were activated nor did they suggest a clear consequence of their generation.…”