Characterization of inward currents and channels underlying burst activity in motoneurons of crab cardiac ganglion.

Large cell motoneurons in the Cancer borealis cardiac ganglion generate rhythmic bursts of action potentials responsible for cardiac contractions. While it is well known that these burst potentials are dependent on coordinated interactions among depolarizing and hyperpolarizing conductances, the depolarizing currents present in these cells, and their biophysical characteristics, have not been thoroughly described. In this study we used a combined molecular biology and electrophysiology approach to look at channel identity, expression, localization, and biophysical properties for two distinct high-voltage-activated calcium currents present in these cells: a slow calcium current (ICaS) and a transient calcium current (ICaT). Our data indicate that CbCaV1 is a putative voltage-gated calcium channel subunit in part responsible for an L-type current, while CbCaV2 (formerly cacophony) is a subunit in part responsible for a P/Q-type current. These channels appear to be localized primarily to the somata of the motoneurons. A third calcium channel gene (CbCaV3) was identified that encodes a putative T-type calcium channel subunit and is expressed in these cells, but electrophysiological studies failed to detect this current in motoneuron somata. In addition, we identify and characterize for the first time in these cells a calcium-activated nonselective cationic current (ICAN), as well as a largely noninactivating TTX-sensitive current reminiscent of a persistent sodium current. The identification and further characterization of these currents allow both biological and modeling studies to move forward with more attention to the complexity of interactions among these distinct components underlying generation of bursting output in motoneurons.