Dopaminergic transmission in Lymnaea: physiology, pharmacology, and plasticity

Bibliography: p. 278-29

Ca(2+)-dependent regulation of a non-selective cation channel from Aplysia bag cell neurones

Ca(2+)-activated, non-selective cation channels feature prominently in the regulation of neuronal excitability, yet the mechanism of their Ca(2+) activation is poorly defined. In the bag cell neurones of Aplysia californica, opening of a voltage-gated, non-selective cation channel initiates a long-lasting afterdischarge that induces egg-laying behaviour. The present study used single-channel recording to investigate Ca(2+) activation in this cation channel. Perfusion of Ca(2+) onto the cytoplasmic face of channels in excised, inside-out patches yielded a Ca(2+) activation EC(50) of 10 μm with a Hill coefficient of 0.66. Increasing Ca(2+) from 100 nm to 10 μm caused an apparent hyperpolarizing shift in the open probability (P(o)) versus voltage curve. Beyond 10 μm Ca(2+), additional changes in voltage dependence were not evident. Perfusion of Ba(2+) onto the cytoplasmic face did not alter P(o); moreover, in outside-out recordings, P(o) was decreased by replacing external Ca(2+) with Ba(2+) as a charge carrier, suggesting Ca(2+) influx through the channel may provide positive feedback. The lack of Ba(2+) sensitivity implicated calmodulin in Ca(2+) activation. Consistent with this, the application to the cytoplasmic face of calmodulin antagonists, calmidazolium and calmodulin-binding domain, reduced P(o), whereas exogenous calmodulin increased P(o). Overall, the data indicated that the cation channel is activated by Ca(2+) through closely associated calmodulin. Bag cell neurone intracellular Ca(2+) rises markedly at the onset of the afterdischarge, which would enhance channel opening and promote bursting to elicit reproduction. Cation channels are essential to nervous system function in many organisms, and closely associated calmodulin may represent a widespread mechanism for their Ca(2+) sensitivity