Norepinephrine causes a biphasic change in mammalian pinealocye membrane potential: role of alpha1B-adrenoreceptors, phospholipase C, and Ca2+.

Perforated patch clamp recording was used to study the control of membrane potential (V(m)) and spontaneous electrical activity in the rat pinealocyte by norepinephrine. Norepinephrine did not alter spiking frequency. However, it was found to act through α(1B)-adrenoreceptors in a concentration-dependent manner (0.1-10 μM) to produce a biphasic change in V(m). The initial response was a hyperpolarization (∼13 mV from a resting potential of -46 mV) due to a transient (∼5 sec) outward K(+) current (∼50 pA). This current appears to be triggered by Ca(2+) released from intracellular stores, based on the observation that it was also seen in cells bathed in Ca(2+)-deficient medium. In addition, pharmacological studies indicate that this current was dependent on phospholipase C (PLC) activation and was in part mediated by bicuculline methiodide and apamin-sensitive Ca(2+)-controlled K(+) channels. The initial transient hyperpolarization was followed by a sustained depolarization (∼4 mV) due to an inward current (∼10 pA). This response was dependent on PLC-dependent activation of Na(+)/Ca(2+) influx but did not involve nifedipine-sensitive voltage-gated Ca(2+) channels. Together, these results indicate for the first time that activation of α(1B)-adrenoreceptors initiates a PLC-dependent biphasic change in pinealocyte V(m) characterized by an initial transient hyperpolarization mediated by a mixture of Ca(2+)-activated K(+) channels followed by a sustained depolarization mediated by a Ca(2+)-conducting nonselective cation channel. These observations indicate that both continuous elevation of intracellular Ca(2+) and sustained depolarization at approximately -40 mV are associated with and are likely to be required for activation of the pinealocyte.