Expression of Kir3.3 potassium channel subunits in supraependymal axons

The serotonergic system of the brainstem raphe is involved in mood control, the sleep-wake cycle, autonomic function, and stress response. The axons of certain dorsal raphe neurons form a dense serotonergic supraependymal plexus lining the brain ventricles, likely regulating ependymal metabolism and activity including ciliary movements and glucose homeostasis. In raphe neurons, serotonin exerts its function partly via 5-HT autoreceptors and G protein-gated inwardly rectifying potassium channels (Kir3/GIRK). To consider a similar mechanism in supraependymal fibres we examined immunocytochemically the distribution of Kir3 potassium channel subunits on supraependymal axons. In the present study, we show that the Kir3.3 subunit protein is expressed in raphe-derived axons at the light and electron microscopic level, but none of the other Kir3 subfamily members or the K(ATP) channel subunits Kir6.1 and Kir6.2. Thus, Kir3.3 containing potassium channels may be of functional importance in autoregulation and excitability of supraependymal fibres and the complex serotonergic regulation along the parenchyma/CSF border.