FUNCTIONAL EXPRESSION OF A-CURRENTS IN EMBRYONIC CHICK SYMPATHETIC NEURONS DURING DEVELOPMENT IN-SITU AND IN-VITRO

1. The functional expression of transient voltage-activated K+ current s (I-A) was examined using whole-cell recording techniques in embryoni c chick sympathetic ganglion neurones that developed in situ and under various growth conditions in vitro. 2. The density of I-A increased d ramatically during development in sympathetic neurones isolated acutel y between embryonic days 7 and 20 (E7-E20). The time course of I-A ina ctivation became significantly faster between E7 and E13, With these p rotocols, neuronal differentiation and development occurred entirely i n situ. 3. Sympathetic neurones isolated at E9 and maintained in vitro for 4 days did not express a normal I-A compared to neurones isolated acutely at E13. Those neurones that were in physical contact with oth er neurones expressed normal densities of I-A, but the resulting inact ivation kinetics were abnormally slow. Sympathetic neurones that were cultured on the membrane fragments of lysed neurones expressed normal densities of I-A even when they failed to make visible connections wit h other viable neurones, but the resulting inactivation kinetics were abnormally slow. Those cultured neurones that were not in physical con tact with other cells or their membranes had markedly reduced densitie s of I-A with abnormally slow inactivation kinetics. 4. Application of 5-100 ng m1(-1) 2.5 S nerve growth factor by itself did not promote n ormal I-A density or kinetics in E9 sympathetic neurones cultured for 4 days. 5. Sympathetic neurones that developed in vitro in physical co ntact with ventral spinal cord explants, cardiac myocytes or aortic sm ooth muscle cells expressed normal densities of I-A, but the inactivat ion kinetics were abnormally slow. Cell culture media conditioned by t hese tissues failed to promote normal I-A expression. Sympathetic neur ones cultured as explants or maintained under depolarizing conditions did not express a normal I-A. 6. Embryonic chick sympathetic neurones exhibit developmental changes in the density and kinetics of I-A that can be regulated independently by extrinsic environmental factors incl uding interactions with insoluble components of the plasma membranes o f some cells.