Casavant, Reema H., Costa M. Colbert, and Stuart E. Dryer. A-current expression is regulated by activity but not by target tissues in developing lumbar motoneurons of the chick embryo. J Neurophysiol 92: 2644 -2651, 2004. First published May 26, 2004 10.1152/ jn.00307.2004. The functional expression of A-type K ϩ channels (I A ) was examined in chick lumbar motoneurons (LMNs) at embryonic days 6 and 11 (E6 and E11). We observed a threefold increase in I A density between E6 and E11 in spinal cord slices and acutely dissociated LMNs. There was no change in current density, kinetics, or voltage dependence of I A in E11 homozygous limbless mutants or in E11 embryos in which hindlimbs were surgically removed at E6. Moreover, chronic in ovo administration of D-tubocurarine, which causes an increase in motoneuron branching on the surface of target muscles, had no effect on I A . Electrical activity played an important role in I A regulation in LMNs in vitro and in ovo. Blocking spontaneous electrical activity of LMNs by chronic in ovo application of mecamylamine or muscimol reduced I A by 80%. LMNs cultured in the presence of TTX also failed to express normal densities of I A , even when the cultures also contained target tissues. The portion of I A that remained after in ovo or in vitro blockade of activity inactivated more quickly than the I A of LMNs that were allowed to discharge spikes. The developmental expression of LMN I A increases significantly during development, and this increase is activity dependent but does not require interactions with target tissues. Ongoing activity also seems to regulate the kinetics of I A inactivation. I N T R O D U C T I O NNeuronal populations can be distinguished by their morphological, physiological, and biochemical characteristics. The intrinsic electrophysiological properties of neurons represent an especially important phenotypic trait, because they control the resting level of excitability, the transmission of impulses, and the integration of synaptic inputs. These intrinsic properties are determined by the functional expression of a specific ensemble of ionic channels.The mature complement of neuronal ion channels is not present at the stage of terminal mitosis. Rather, channel expression is a dynamic and developmentally regulated process. The developmental expression of some types of channels proceeds according to an internal developmental program (Dourado and Dryer 1992; Spitzer and Lamborghini 1976), whereas expression of others appears to require neurotrophic interactions with other cell types, ongoing electrical activity, or both (reviewed in Dryer et al. 2003). Although the precise spatiotemporal pattern of channel expression depends on the cell population being studied, the developmental expression of some channels, including Ca 2ϩ -activated K ϩ channels (I K(Ca) ) and A-type K ϩ channels (I A ), requires cell-cell interactions in several neuronal populations (Cameron et al. , 1999(Cameron et al. , 2001 Dryer 1992, 1994;Dourado et al. 1994; Martin-Caraballo and ...

Application of arachidonic acid evoked robust activation of large-conductance K ϩ channels in cell-attached and excised inside-out patches from acutely isolated chick ciliary ganglion neurons. A similar effect was produced by 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analogue of arachidonic acid. The unitary conductance of fatty acid-activated channels was 35-40 pS at ϩ20 mV with physiological gradients of K ϩ and 165 pS at ϩ20 mV with an extracellular K ϩ concentration of 37.5 mM and an intracellular K ϩ concentration of 150 mM. Gating of these channels in cell-attached patches was potentiated by membrane stretch. Channel gating evoked by both lipids was concentration-dependent, with detectable activation apparent at 4 M in the majority of patches and maximal activation occurring between 32 and 64 M. Gating was relatively voltage-independent. Large-conductance K ϩ channels were also activated in inside-out patches by the monounsaturated fatty acid 11-cis-eicosenoic acid but not by the fully saturated fatty acid arachidic acid. Application of 100 M H 2 O 2 , an agent that activates cytosolic phospholipase A 2 , also caused activation of large-conductance K ϩ channels in intact neurons. The stimulatory effects of H 2 O 2 were blocked by pretreatment with 20 M 4-bromophenacyl bromide, an irreversible inhibitor of phospholipase A 2 . Therefore, mobilization of endogenous fatty acids can cause activation of large-conductance K ϩ channels in autonomic neurons.