BK-Type KCa channels in two parasympathetic cell types:: Differences in kinetic properties and developmental expression

The intrinsic electrical properties of identified choroid and ciliary neurons of the chick ciliary ganglion were examined by patch-clamp recording methods. These neurons are derived from a common pool of mesencephalic neural crest precursor cells but innervate different target tissues and have markedly different action potential waveforms and intrinsic patterns of repetitive spike discharge. Therefore it is important to determine whether these cell types express different types of plasma membrane ionic channels, and to ascertain the developmental stages at which these cell types begin to diverge. This study has focused on large-conductance Ca2+-activated K+ channels (K-Ca), which are known to regulate spike waveform and repetitive firing in many cell types. Both ciliary ganglion cell types, identified on the basis of size and somatostatin immunoreactivity, express a robust macroscopic K-Ca carried by a kinetically homogeneous population of large-conductance (BK-type) K-Ca channels. However, the kinetic properties of these channels are different in the two cell types. Steady-state fluctuation analyses of macroscopic K-Ca produced power spectra that could be fitted with a single Lorentzian curve in both cell types. However, the resulting corner frequency was significantly lower in choroid neurons than in ciliary neurons, suggesting that the underlying K-Ca channels have a longer mean open-time in choroid neurons. Consistent with fluctuation analyses, significantly slower gating of K-Ca channels in choroid neurons was also observed during macroscopic activation and deactivation at membrane potentials positive to -30 mV. Differences in the kinetic properties of K-Ca channels could also be observed directly in single-channel recordings from identified embryonic day 13 choroid and ciliary neurons. The mean open-time of large-conductance K-Ca channels was significantly greater in choroid neurons than in ciliary neurons in excised inside-out patches. The developmental expression of functional K-Ca channels appears to be regulated differently in the two cell types. Although both cell types acquire functional K-Ca at the same developmental stages (embryonic days 9-13), functional expression of these channels in ciliary neurons requires target-derived trophic factors. In contrast, expression of functional K-Ca channels proceeds normally in choroid neurons developing in vitro in the absence of target-derived trophic factors. Consistent with this, extracts of ciliary neuron target tissues (striated muscle of the iris/ciliary body) contain K-Ca stimulatory activity. However, K-Ca stimulatory activity cannot be detected in extracts of the smooth muscle targets of choroid neurons.