The orexin/hypocretin peptides are produced in neurons of the lateral hypothalamus which send widespread projections throughout the brain. Orexins are implicated in the maintenance of wakefulness and disruption of their function in animal models leads to narcolepsy (Hungs and Mignot, 2001). Recent in vitro experiments suggested that orexin neurons have intrinsic electrical pacemaker activity (Eggermann et al. 2003). Because orexin neurons are active principally during wakefulness, this intrinsic firing may be critical for the normal sleep-wake cycle. It is therefore of interest to elucidate the biophysical and molecular mechanisms responsible for the pacemaker activity of orexin neurons. Studies of other intrinsically firing neurons show that pacemaker activity arises from a complex interplay between plasmalemmal ionic conductances. Thus our aim was to characterise the molecular and biophysical determinants of ionic currents expressed by orexin neurons.

We performed standard whole-cell patch-clamp recordings in living brainslices of humanely killed mice (C57BL/6, male, P12-20) to elucidate the biophysical identity of these currents, by measuring the voltage-dependence of activation and the time-and voltage-dependence of inactivation, and their sensitivity to pharmacological agents. We also used multi-label immunocytochemistry combined with confocal microscopy to determine the molecular identity of the ion channel subunits which co-localise with orexin in lateral hypothalamic neurons. Together, these data allowed identification of currents which determine the firing properties of orexin neurons.

*The first two authors contributed equally to this work. This work is supported by the Wellcome Trust.