REGULATION OF A-CURRENTS BY CELL-CELL INTERACTIONS AND NEUROTROPHIC FACTORS IN DEVELOPING CHICK PARASYMPATHETIC NEURONS

1. The developmental regulation of ion channel expression was studied in parasympathetic neurones isolated from the chick ciliary ganglion. Whole-cell patch clamp recordings were made from ciliary ganglion neur ones that were removed from the embryo on the ninth embryonic day (E9) and maintained in dissociated cell culture for an additional 4 days. Previous studies have shown that the expression of a transient voltage -activated K+ current (I-A) is regulated by unidentified environmental stimuli during these developmental stages. 2. The effect of interacti ons between neurones and target tissue on the expression of I-A was te sted by co-culturing ciliary ganglion neurones with chick striated mus cle cells. Neurones from the nerve-muscle co-cultures expressed normal amplitudes of I-A, but the neurones did not express normal levels of I-A when they were plated onto lysed muscle fibres. 3. The effect of i nteractions between ganglionic neurones and non-neuronal ganglionic ce lls was tested by culturing ganglia as explants rather than as dissoci ated cells. Neurones isolated from the explant cultures did not expres s normal levels of IA. Similarly, when dissociated ganglionic neurones were co-cultured with fibroblasts isolated from embryonic chick skin, they did not express normal amplitudes of I-A. 4. Chronic depolarizat ion caused by growing ciliary ganglion neurones in the presence of ele vated K+ concentrations did not allow for the normal expression of I-A , although it did promote the survival of these neurones in vitro. 5. Addition of 40 ng ml(-4) of recombinant human ciliary neurotrophic fac tor (CNTF) or basic fibroblast growth factor (bFGP) to the cell cultur e medium had no effect on I-A expression in developing chick ciliary g anglion neurones. However, 40 ng ml(-4) of acidic fibroblast growth fa ctor (aFGF) stimulated the expression of I-A. All trophic factors prom oted the growth and survival of ciliary ganglion neurones in vitro. 6. Dissociated ciliary ganglion neurones were maintained in a culture me dium containing an aqueous extract of the whole brain. Neurones develo ping under these conditions expressed normal levels of I-A. The stimul atory activity of the brain extract was destroyed by heating. 7. The e xpression of I-A in chick ciliary ganglion neurones developing in vitr o can be regulated by soluble growth factors and by interactions with certain other cell types. Similar interactions may regulate the expres sion of I-A in ciliary ganglion neurones developing in situ.