Developmental changes of potassium currents of embryonic leech ganglion cells in primary culture

The expression of calcium-activated potassium currents (I-K(Ca)), delayed outward rectifier potassium currents (I-K(slow)), and transient outward currents (I-A) was studied during the development of the nervous system of the leech using the whole-cell patch-clamp recording technique. Dissociated cells were isolated from leech embryos between stage E7 and E16 and maintained in primary culture, K+ currents were recorded at E7, when only few anterior ganglia had formed beneath the primordial mouth, I-K(slow) was present in all cells tested, while I-K(Ca) was expressed in only 67% of the cells studied. Even as early as E7, different types of I-K(Ca) have been found. Neither frequency of occurrence nor the charge density of I-K(Ca) showed significant changes between E7 and E16. The density of I-K(slow), however, increased by a factor of two between E7 and E8, which resulted in a significant increase in the total K+ current of these cells. This rise in potassium outward current developed in parallel with the appearance of Na+ and Ca2+ inward currents (Schirrmacher and Deitmer: J Exp Biol 155:435-453, 1991) during early development, shaping the electrical excitability in embryonic leech neurones. I-A could be separated by its voltage-dependence and pharmacological properties. The current was detected at stage E9, when all 32 ganglia are formed in the embryo. The frequency of occurrence of I-A increased from 16% at E9 to 70% at E15. The channel density, steady state inactivation, and kinetics showed no significant changes during development. (C) 1997 Wiley-Liss, Inc.