An anti-Parkinson drug ropinirole depletes orexin from rat hypothalamic slice culture.

Non-ergot-type dopamine receptor agonists such as ropinirole are used for the treatment of Parkinson disease, but they occasionally show serious side effects including sleep attacks and daytime sleepiness. These symptoms are reminiscent of narcolepsy, a major sleep disorder. Because narcolepsy is thought to result from deficiency of a hypothalamic neuropeptide orexin, we examined whether ropinirole affected the integrity of orexin-containing neurons, using organotypic slice culture of rat hypothalamus. Application of ropinirole induced a significant decrease in the number of orexin-immunoreactive neurons. The same treatment showed no significant effect on the number of melanin-concentrating hormone-immunoreactive neurons. The decrease of orexin-immunoreactive neurons was reversible after washout of ropinirole and was not accompanied by induction of cell death. Antagonism of dopamine D(2) receptors and of serotonin 5-HT(1A) receptors attenuated the effect of ropinirole, suggesting involvement of these receptors in depletion of orexin. On the other hand, a moderate concentration of N-methyl-d-aspartate that excited orexin neurons counteracted the effect of ropinirole on the number of orexin-immunoreactive neurons. These results suggest that ropinirole can cause deficiency of orexin by inhibiting excitatory activities of orexin neurons, which may be relevant to the adverse actions of this drug on sleep and wakefulness.