The temperature dependence of high-threshold calcium channel currents recorded from adult rat dorsal raphe neurones.

The temperature dependence of HVA calcium channel currents, using barium as the charge carrier, was studied in acutely isolated adult rat dorsal raphe neurones. Current amplitude was found to be highly sensitive with a Q10 of between 1.7 and 2.5. The most sensitive component of current is that that is activated from hyperpolarized holding potentials, and inactivates during a 200 msec test pulse. The least sensitive is the more sustained current elicited from depolarized potentials. Increases in temperature were also found to cause an irreversible shift in the current-voltage relationship in the hyperpolarizing direction. By far the most temperature-sensitive property was the activation time constant with an extraordinary Q10 of between 10 and 12. This was not significantly affected by holding potential, though the time constant itself is dependent on the test potential. Increases in temperature to 25 degrees C or above revealed a fast inactivating component, not seen at lower temperatures. These findings suggest that there are at least three components of HVA current in dorsal raphe neurones. In addition, the remarkably high Q10 for activation kinetics suggests that the processes underlying calcium channel current activation are multifaceted and complex. The following paper puts forward a new hypothesis which attempts to explain the way in which neurotransmitters modulate the activation kinetics of HVA calcium channel currents.