Our observations demonstrate that most orexin neurons in the human hypothalamus produce prodynorphin mRNA and NARP protein. Moreover, in patients with narcolepsy, the number of neurons expressing these gene products is reduced in proportion to the loss of orexin neurons. These observations suggest that narcolepsy with cataplexy is caused by a loss of the orexin neurons themselves, rather than a failure to produce orexin.
Several lines of evidence now support this hypothesis. Disrupted orexin signaling is sufficient to produce the major symptoms of narcolepsy in several animal models 10, 12–14
. In mice lacking the orexin neurons 13
, the orexin field completely lacks prodynorphin mRNA 5
and NARP (R.E.; data not shown), further supporting the co-localization of these markers. We now show that within the orexin field of people with narcolepsy, the number of neurons containing prodynorphin mRNA, orexin, or NARP is reduced to 5–10% of normal. This cell loss is probably limited to the orexin neurons because we detected no loss of NARP or prodynorphin outside the orexin field. Although the orexin neurons might fail to produce multiple peptides, these observations make selective neurodegeneration or an autoimmune attack on the orexin neurons much more likely.
Three methodologic limitations warrant comment. First, we examined brains from only two individuals with narcolepsy, but our observations are consistent with a prior report showing a marked reduction in the number of orexin-IR neurons 2
. Our two patients have not been previously reported, and thus our neuropathologic observations contribute significantly to the six cases already published 1, 2, 15
. Second, our narcoleptic subjects were treated at some time with amphetamines, and methamphetamine can transiently increase prodynorphin mRNA in the striatum and hypothalamus of rats 16–18
. However, ectopic expression of dynorphin is unlikely to have occurred as the number of hypothalamic neurons producing prodynorphin and orexin were similarly reduced. Third, mRNA may have degraded in some brains with longer post mortem intervals. We detected prodynorphin mRNA in 80% of orexin neurons on average, but the brains with the shortest intervals had > 90% co-localization, suggesting that most or perhaps all orexin neurons contain dynorphin. Prodynorphin mRNA also may be expressed at low levels in a few other regions of the human hypothalamus 19
, and the more limited distribution of labeling in our study was probably also caused by a post mortem loss of mRNA.
Loss of dynorphin and NARP may contribute to the symptoms of narcolepsy. Though selective disruption of the orexin gene is sufficient to produce sleepiness and cataplexy 10, 14
, NARP may enable the orexin neurons to promote wakefulness by enhancing glutamate signaling 7, 20
. In addition, dynorphin and other products of the prodynorphin gene bind to opiate receptors, and opiates can improve the subjective sleepiness and cataplexy of narcolepsy 21–23
. Most importantly, mice simply lacking orexin have sleepiness and cataplexy, but mice lacking the orexin neurons have obesity as well 13, 24
, suggesting that loss of dynorphin or other signaling molecules contributes to the mild obesity of human narcolepsy 25, 26
. In the near future, narcolepsy may be treated with orexin agonists, but restoration of dynorphin and NARP signaling may also be necessary to correct all the symptoms of narcolepsy.