Orexins cause depolarization via nonselective cationic and K+ channels in isolated locus coeruleus neurons

The locus coeruleus (LC) contains noradrenergic neurons that are innervated by orexin (ORX)-like immunoreactive axons and express both orexin receptor-1 and -2. We studied effects of ORX-A and -B (ORX-A/B) on dissociated LC neurons by using whole-cell patch clamp techniques. In current-clamp mode, LC neurons were depolarized by application of ORX-A (10−7 M) [53% of neurons tested; 9.0 ± 0.2 mV (n = 5)], or ORX-B (10−7 M) [38% of neurons tested; 4.0 ± 0.1 mV (n = 5)]. Firing frequencies of action potentials increased during application [1.1 ± 0.2 Hz (n = 5) in ORX-A; 0.8 ± 0.2 Hz (n = 5) in ORX-B] and returned to the control level [0.2 ± 0.1 Hz (n = 5)] after removal. The ORX-A/B-induced depolarization was well maintained in the presence of TTX (3 × 10−7 M), CNQX (10−6 M) and AP5 (10−5 M). In voltage-clamp mode, removal of external Na+ suppressed both ORX-A/B-induced currents and shifted their reversal potentials from approximately −45 mV to −60 mV. In addition, ORX-A/B inhibited sustained K+ currents. These results suggest that ORX-A/B increase the firing frequency of LC neurons through the depolarization probably produced by both augmentation of the nonselective cationic conductance and inhibition of the sustained K+ conductance.