Multiple targets of chemosensitive signaling in locus coeruleus neurons: Role of K⁺ and Ca²⁺ channels

We studied chemosensitive signaling in locus coeruleus (LC) neurons using both perforated and whole cell patch techniques. Upon inhibition of fast Na⁺ spikes by tetrodotoxin (TTX), hypercapnic acidosis [HA; 15% CO₂, extracellular pH (pH_o) 6.8] induced small, slow spikes. These spikes were inhibited by Co²⁺ or nifedipine and were attributed to activation of L-type Ca²⁺ channels by HA. Upon inhibition of both Na⁺ and Ca²⁺ spikes, HA resulted in a membrane depolarization of 3.52 ± 0.61 mV (n = 17) that was reduced by tetraethylammonium (TEA) (1.49 ± 0.70 mV, n = 7; P < 0.05) and absent (-0.97 ± 0.73 mV, n = 7; P < 0.001) upon exposure to isohydric hypercapnia (IH; 15% CO₂, 77 mM HCO₃^-, pH_o 7.45). Either HA or IH, but not 50 mM Na-propionate, activated Ca²⁺ channels. Inhibition of L-type Ca²⁺ channels by nifedipine reduced HA-induced increased firing rate and eliminated IH-induced increased firing rate. We conclude that chemosensitive signals (e.g., HA or IH) have multiple targets in LC neurons, including TEA-sensitive K⁺ channels and TWIK-related acid-sensitive K⁺ (TASK) channels. Furthermore, HA and IH activate L-type Ca²⁺ channels, and this activation is part of chemosensitive signaling in LC neurons.