Chloride current observed as calcium-gated tail current in trigeminal root ganglion neurons of the marine catfish, Plotosus

Abstract

Isolated trigeminal ganglion (TRG) neurons of Plotosus in primary culture were studied with patch electrodes in a whole-cell recording configuration. When Ca currents were isolated using electrodes filled with CsCl and Ca buffer in Na- and K-free saline, a large tail current was induced. Both Ca and tail currents were blocked by 2 mM Co 2+ in the bath. Reversal potential of the tail current was close to the equilibrium potential for Cl −. The reversal potential was altered by substitution of external Cl − with Br − or methanesulphonate − (MSA −). The anion permeability ratios were estimated as P Br/P Cl=1.95 and P MSA/P Cl<0.05. These results suggested that the tail current was the Ca-gated Cl current. In Plotosus neurons, two types of Ca current, high- and low-voltage activated (HVA and LVA), have been described. Both types of Ca currents were able to induce a tail current. Ba currents through both HVA and LVA channels proved to be ineffective. Upon repetitive activation, the tail current progressively increased in the presence of a nearly constant peak Ca current, and showed a more prolonged complex decay phase. When the electrode was filled with 20 mM EGTA, no tail current was induced, not even after repetitive stimulation. Therefore, the tail current seemed to be regulated by local Ca activity in the vicinity of the membrane. The Ca-gated Cl tail current may regulate the membrane excitability of TRG neurons during repetitive firing.