Effects of soma isolation on outward currents measured under voltage clamp in spiny lobster stomatogastric motor neurons.

1. Outward currents in identified cell types from the pyloric system of the stomatogastric ganglion (STG) of the spiny lobster, Panulirus marginatus, were studied under two-microelectrode voltage clamp. A comparison was made between data from intact cells and somata isolated by ligation of the primary neurite of these monopolar neurons. 2. Despite the elimination of current contributions from the extensive arborizations of STG neurons, few significant differences were found in the mean values of parameters for outward currents between populations of isolated somata and intact cells of a given type. Measurements that showed little difference included magnitude and activation threshold of a calcium-dependent outward current (IJ) and magnitude, activation threshold, voltage dependence, and inactivation time course of A current (IA). Although previous work has suggested that IJ might reside predominantly in the soma, IA is known to be distributed in poorly space-clamped neurite processes. The absence of obvious effects of isolation was thus unexpected. 3. To better understand the mechanisms involved, we used compartmental models derived from reconstructed neurons to simulate the effects of isolation. It was concluded that, for the particular conditions present in stomatogastric neurons, with a large, uniformly distributed outward current conductance activated, even though neurites and axon remain attached, most measured current flows through well-clamped soma membrane. 4. Factors contributing to this result included the outward sign of the current, the large specific conductance activated in these neurons (among the larger reported in somata), and the presence of only a single major process leaving the soma. The potential for serious errors in voltage-clamp measurements from intact cells remains if these conditions are not met.