Target‐derived factors regulate the expression of Ca(2+)‐activated K+ currents in developing chick sympathetic neurones.

1. The functional expression of Ca(2+)‐activated K+ currents (IK(Ca)) and voltage‐activated Ca2+ currents (ICa) was examined using whole‐cell recordings from chick lumbar sympathetic neurones developing in situ and under various conditions in vitro. 2. Macroscopic IK(Ca) was expressed at low current density (< 0.01 mA cm‐2) in neurones isolated at embryonic days 9‐16 (E9‐16). IK(Ca) was expressed at high densities (> 0.04 mA cm‐2) at E17‐19. By contrast, there was no significant difference in ICa density between sympathetic neurones isolated at E13 and E18. 3. When sympathetic neurones were isolated at E13 and maintained in vitro for 5 days, IK(Ca) was expressed at a significantly lower density (< 0.01 mA cm‐2) than in neurones isolated acutely at E18 (> 0.04 mA cm‐2). There was no difference in ICa density between neurones that developed in vitro and in situ. 4. When E13 sympathetic neurones were cultured for 5 days in the presence of a confluent layer of ventricular myocytes, they expressed IK(Ca) at a high density (> 0.04 mA cm‐2), similar to that of E18 neurones that developed entirely in situ. Cardiac cell‐conditioned medium produced similar effects. However, co‐culture of sympathetic neurones with spinal cord explants did not allow for normal IK(Ca) expression in vitro. 5. Culturing sympathetic neurones in the presence of 5 ng ml‐1 nerve growth factor (NGF) caused a significant increase in IK(Ca) density but this effect was only seen in 50% of cells examined. 6. The largest developmental changes in macroscopic IK(Ca) occur several days after other K+ currents and ICa are expressed at maximal density. The normal developmental expression of IK(Ca) is dependent upon extrinsic factors, including target‐derived differentiation factors.