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+ currents (I-A) was examined using whole-cell recording techniques in embryonic chick sympathetic ganglion neurones that developed in situ and under various growth conditions in vitro. 2. The density of I-A increased dramatically during development in sympathetic neurones isolated acutely between embryonic days 7 and 20 (E7-E20). The time course of I-A inactivation became significantly faster between E7 and E13, With these protocols, neuronal differentiation and development occurred entirely in 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 other neurones expressed normal densities of I-A, but the resulting inactivation 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 with other viable neurones, but the resulting inactivation kinetics were abnormally slow. Those cultured neurones that were not in physical contact with other cells or their membranes had markedly reduced densities 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 normal I-A density or kinetics in E9 sympathetic neurones cultured for 4 days. 5. Sympathetic neurones that developed in vitro in physical contact with ventral spinal cord explants, cardiac myocytes or aortic smooth muscle cells expressed normal densities of I-A, but the inactivation kinetics were abnormally slow. Cell culture media conditioned by these tissues failed to promote normal I-A expression. Sympathetic neurones 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 including interactions with insoluble components of the plasma membranes of some cells.