Compelling evidence links the recently discovered hypothalamic peptides Hypocretin/Orexin (Hcrt/Orx) to rapid eye movement sleep (REM) control and the sleep disorder narcolepsy, yet how they influence sleep-related systems is not well understood. We investigated the action of Hcrt/Orx on mesopontine cholinergic (MPCh) neurons of the laterodorsal tegmental nucleus (LDT), a target group whose function is altered in canine narcolepsy and appears pivotal for normal REM and wakefulness. Extracellular recordings from mouse brainstem slices revealed that Hcrt/Orx evoked prolonged firing of LDT neurons. Whole-cell recordings revealed that Hcrt/Orx had actions on both presynaptic neurons and at postsynaptic sites. Hcrt/Orx produced an increase in frequency and amplitude of spontaneous EPSCs without equivalent effect on IPSCs, by triggering action potentials and enhancing spike-evoked synaptic transmission in glutamatergic afferents. Postsynaptically, Hcrt/Orx produced an inward current and an increase in membrane current noise, which were accompanied by a conductance increase. These persisted in TTX, ionotropic glutamate receptor antagonists, and low extracellular calcium. Both presynaptic and postsynaptic actions were specific because they were not mimicked by an Hcrt/Orx fragment, and both actions were observed for cholinergic and noncholinergic LDT neurons. Finally, extracellular recordings during postsynaptic potential blockade demonstrated that postsynaptic actions of Hcrt/Orx alone could evoke prolonged firing. In the context of other recent work, our findings suggest that Hcrt/Orx neurons may coordinate the activity of the entire reticular activating system during waking. Moreover, these findings address specific hypotheses regarding the cellular mechanisms underlying REM disregulation in narcolepsy.
Burlet, S., Tyler, C. J., & Leonard, C. S. (2002). Direct and indirect excitation of laterodorsal tegmental neurons by hypocretin/orexin peptides: implications for wakefulness and narcolepsy. Journal of Neuroscience, 22(7), 2862–2872. Retrieved from http://www.jneurosci.org/content/22/7/2862