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Department of Biological Science, Florida State University, Tallahassee 32306-4075, USA.

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.

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