Properties and physiological function of Ca2+-dependent K+ currents in uniglomerular olfactory projection neurons

Ca²⁺-activated potassium currents [I_K(Ca)] are an important link between the intracellular signaling system and the membrane potential, which shapes intrinsic electrophysiological properties. To better understand the ionic mechanisms that mediate intrinsic firing properties of olfactory uniglomerular projection neurons (uPNs), we used whole cell patch-clamp recordings in an intact adult brain preparation of the male cockroach Periplaneta americana to analyze I_K(Ca). In the insect brain, uPNs form the principal pathway from the antennal lobe to the protocerebrum, where centers for multimodal sensory processing and learning are located. In uPNs the activation of I_K(Ca) was clearly voltage and Ca²⁺ dependent. Thus under physiological conditions I_K(Ca) is strongly dependent on Ca²⁺ influx kinetics and on the membrane potential. The biophysical characterization suggests that I_K(Ca) is generated by big-conductance (BK) channels. A small-conductance (SK) channel-generated current could not be detected. I_K(Ca) was sensitive to charybdotoxin (CTX) and iberiotoxin (IbTX) but not to apamin. The functional role of I_K(Ca) was analyzed in occlusion experiments under current clamp, in which portions of I_K(Ca) were blocked by CTX or IbTX. Blockade of I_K(Ca) showed that I_K(Ca) contributes significantly to intrinsic electrophysiological properties such as the action potential waveform and membrane excitability.

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