Excitotoxic degeneration of hypothalamic orexin neurons in slice culture

Several lines of evidence indicate that narcolepsy, a sleep disorder, results from the loss of hypothalamic orexin (hypocretin)-containing neurons, but the mechanisms responsible for selective elimination of this neuronal population are unknown. Using organotypic rat hypothalamic slice cultures, we investigated vulnerability of orexin neurons to excitotoxic insults. Twenty-four hours of incubation with N-methyl-D-aspartate (NMDA) followed by a recovery period of 72 h resulted in a marked decrease in the number of orexin-immunoreactive neurons, whereas melanin-concentrating hormone (MCH)-immunoreactive neurons in the same cultures were relatively spared. In contrast, orexin neurons were more resistant to kainic acid cytotoxicity than MCH neurons. Examinations of the effects of several endogenous glutamate receptor agonists as well as a glutamate transporter blocker highlighted quinolinic acid as an endogenous excitotoxin that could cause selective loss of orexin neurons as compared to MCH neurons by activating NMDA receptors. In addition, quinolinic acid-induced decrease of orexin neurons was prevented by an inhibitor of poly(ADP-ribose) polymerases. These results provide the first evidence concerning cytotoxic consequences onto orexin neurons, and indicate that NMDA receptor-mediated injury may contribute to the selective loss of these neurons in the hypothalamus, a prominent neuropathological feature found in narcolepsy patients.