Characterisation of the hyperpolarisation activated chloride current in rat superior cervical ganglion neurons

Although the cation currents that are found in rat superior cervical ganglia (SCG) neurons have been extensively studied and characterised, the chloride conductances in these neurons have not. The aim of the experiments presented here was to characterise a number of features of the hyperpolarisation activated current that is present in rat SCG neurons. The current was found to activate at potentials hyperpolarised to -20mV in a time and voltage dependent manner. Current activation was inwardly rectifying. Inactivation was not observed and the activation kinetics were best described by two exponential functions. The current was unaffected by changes in the external potassium and/or sodium concentrations and was not blocked by either 10mM barium or caesium. The reversal potential, however, was approximately 0mV which agreed with the chloride equilibrium potential. The hyperpolarisation activated current was therefore carried by chloride ions and was designated IC1IR The halide selectivity sequence of IC1IR was Cl > Br > I. IC1IR was blocked in a voltage dependant manner by cadmium, zinc and the organic chloride channel blockers; DIDS, 9AC and NPPB. IC1IR was not, however, affected by the application of SITS or niflumic acid. Acidification and alkalisation of the external medium resulted in a voltage dependent increase and decrease in the amplitude of IC1JR respectively. In addition, was found to be sensitive to external hypotonicity. The properties of Ic1IR described are almost identical to those of ClC-2 and mRNA for ClC-2 was found to be expressed in rat SCG neurons by RT-PCR. It therefore seems very likely that the ClC-2 chloride channel underlies IC1IR in these cells.