TARGET TISSUES AND INNERVATION REGULATE THE CHARACTERISTICS OF K-SITU( CURRENTS IN CHICK CILIARY GANGLION NEURONS DEVELOPING IN)

The expression of appropriate ensembles of ionic channels is necessary for the differentiation and normal function of vertebrate neurons. Ce ll-cell interactions may regulate the expression and properties of ion ic channels in embryonic neurons. Previous studies have shown that the expression of A-type K+ channels (I-A) and Ca2+-activated K+ channels (I-K[Ca]) is abnormal in chick ciliary ganglion neurons developing in vitro in the absence of normal cell-cell interactions. Other voltage- activated currents develop normally under these conditions. The presen t studies were designed to establish the role of the target tissues an d the preganglionic innervation in regulating the expression of these currents in embryonic chick ciliary ganglion neurons developing in sit u. Surgical manipulations were used to remove the developing optic ves icle, which contains the target tissues, the middorsal region of the m idbrain primordium, which contains the preganglionic nucleus, or both, all prior to the formation of the ciliary ganglion. I-A and I-K[Ca] w ere then examined in acutely isolated neurons that developed in ovo in the presence (OV+) or absence (OV-) of the normal target tissues, in the presence (MB(+)) or absence (MB(-)) of preganglionic innervation, and in the absence of both preganglionic innervation and target tissue s (OV-/MB(-)). The amplitude of I-A was unaffected by the operations. However, the activation and inactivation kinetics of I-A were two- to threefold faster in OV- or OV-/MB(-) cells compared to neurons isolate d from control OV+ ganglia at embryonic days 11-14 (E11-E14). There we re no changes in the voltage dependence of activation or steady-state inactivation, or in the time course of recovery from inactivation. By contrast, neurons isolated from MB-ganglia expressed an I-A with ampli tude, voltage dependence, and kinetics that were indistinguishable fro m those of control MB(+) and OV+ ganglia. Therefore, interactions with target tissues in the eye play a role in determining the characterist ics of I-A in developing ciliary ganglion neurons, whereas preganglion ic innervation does not. Furthermore, the amplitude of I-K[Ca] was red uced by 90-100 % in OV-, MB(-), and OV-/MB(-) neurons isolated at E12- E14 as compared to MB(+) and OV+ controls. Voltage-activated Ca2+ curr ents were present at normal amplitudes in all of these neurons. Thus, the expression of I-K[Ca] in chick ciliary ganglion neurons target tis sue interactions and preganglionic innervation. Therefore, cell-cell i nteractions are necessary for the expression of a normal ensemble of i onic channels in chick ciliary ganglion neurons developing in situ.