Spontaneous miniature outward currents in mechanically dissociated rat Meynert neurons

1. Spontaneous miniature outward currents (SMOCs) were observed in mechanically dissociated rat Meynert neurons using nystatin perforated patch recordings under voltage-clamp conditions. 2. SMOCs were blocked by apamin, a selective blocker of small conductance Ca2+-activated Bf (XK) channels, but not by blockers for other types of Ca2+-activated K+ channel. 3. Ryanodine (10-100 muM) reduced both the amplitude and frequency of SMOCs. Caffeine (1 mM) increased the SMOC frequency. Blockers of the sarco/endoplasmic reticulum Ca2+-ATPase completely abolished SMOCs, indicating a requirement for functioning sarco/endoplasmic reticulum (SR/ER) Ca2+ stores. 4. Both Cd2+-containing and Ca2+-free solutions partially inhibited SMOC frequency, a result which suggests that Ca2+ influx contributes to, but is not essential for, SMOC generation. 5. Thus, SMOCs are XK currents linked to ryanodine- and caffeine-sensitive SR/ER Ca2+ stores, and are only indirectly influenced by extracellular Ca2+ influx. The development of this new, minimally invasive mechanical dissociation method has revealed that SMOCs are common in native CNS neurons.