STIMULATION OF A NICOTINIC ACH RECEPTOR CAUSES DEPOLARIZATION AND ACTIVATION OF L-TYPE CA2+ CHANNELS IN RAT PINEALOCYTES

1. Membrane voltage (V-m) recordings were obtained from isolated rat p inealocytes using the patch-clamp technique. In parallel to the electr ophysiological experiments, intracellular Ca2+ measurements were perfo rmed using fura-2. 2. The resting V-m averaged -43 mV and replacement of extracellular NaCl by KCl completely depolarized the cells. This in dicates that the resting V-m is dominated by a Ii conductance. Single- channel recordings revealed the presence of a large conductance Ca2+-a ctivated charybdotoxin-sensitise K+ channel. 3. Application of ACh (10 0 mu M) depolarized the pinealocytes on average by 16 mV. The depolari zing effect of ACh was mimicked by nicotine (50 mu M) and was prevente d by tubocurarine (100 mu M). 4. The ACh-induced depolarization was la rgely abolished in the absence of extracellular Na+ but was not signif icantly affected by extracellular Ca2+ removal. 5. Application of ACh (100 mu M) caused an increase in [Ca2+](i). This increase was complete ly dependent on the presence of extracellular Ca2+ and was largely red uced after extracellular Naf removal. Nifedipine (1 mu M) reduced the ACh-induced increase in [Ca2+](i) by about 50%. 6. Our findings indica te that in rat pinealocytes stimulation of a nicotinic ACh receptor (n AChR) induces depolarization mainly by Nat influx via the nAChR. The d epolarization then activates L-type Ca2+ channels, which are responsib le for the nifedipine-sensitive portion of the intracellular Ca2+ incr ease. Ca2+ influx via the nAChR probably also contributes to the obser ved rise in [Ca2+](i).