Ca2+-dependent and Na+-dependent K+ conductances contribute to a slow AHP in thalamic paraventricular nucleus neurons: a novel target for orexin receptors.

Thalamic paraventricular nucleus (PVT) neurons exhibit a postburst apamin-resistant slow afterhyperpolarization (sAHP) that is unique to midline thalamus, displays activity dependence, and is abolished in tetrodotoxin. Analysis of the underlying sI(AHP) confirmed a requirement for Ca(2+) influx with contributions from P/Q-, N-, L-, and R subtype channels, a reversal potential near E(K)(+) and a significant reduction by UCL-2077, barium or TEA, consistent with a role for K(Ca) channels. sI(AHP) was significantly reduced by activation of either the cAMP or the protein kinase C (PKC) signaling pathway. Further analysis of the sAHP revealed an activity-dependent but Ca(2+)-independent component that was reduced in high K(+) and blockable after Na(+) substitution with Li(+) or in the presence of quinidine, suggesting a role for K(Na) channels. The Ca(2+)-independent sAHP component was selectively reduced by activation of the PKC signaling pathway. The sAHP contributed to spike frequency adaptation, which was sensitive to activation of either cAMP or PKC signaling pathways and, near the peak of membrane hyperpolarization, was sufficient to cause de-inactivation of low threshold T-Type Ca(2+) channels, thus promoting burst firing. PVT neurons are densely innervated by orexin-immunoreactive fibers, and depolarized by exogenously applied orexins. We now report that orexin A significantly reduced both Ca(2+)-dependent and -independent sI(AHP), and spike frequency adaptation. Furthermore orexin A-induced sI(AHP) inhibition was mediated through activation of PKC but not PKA. Collectively, these observations suggest that K(Ca) and K(Na) channels have a role in a sAHP that contributes to spike frequency adaptation and neuronal excitability in PVT neurons and that the sAHP is a novel target for modulation by the arousal- and feeding-promoting orexin neuropeptides.