Orexins (also called hypocretins) are peptide neurotransmitters expressed in neurons of the lateral hypothalamic area (LHA). Mice lacking the orexin peptides develop narcolepsy-like symptoms, whereas mice with a selective loss of the orexin neurons develop hypophagia and severe obesity in addition to the narcolepsy phenotype. These different phenotypes suggest that orexin neurons may contain neurotransmitters besides orexin that regulate feeding and energy balance. Dynorphin neurons are common in the LHA, and dynorphin has been shown to influence feeding; hence, we studied whether dynorphin and orexin are colocalized. In rats, double-label in situ hybridization revealed that nearly all (94%) neurons expressing prepro-orexin mRNA also expressed prodynorphin mRNA. The converse was also true: 96% of neurons in the LHA containing prodynorphin mRNA also expressed prepro-orexin mRNA. Double-label immunohistochemistry confirmed that orexin-A and dynorphin-A peptides were highly colocalized in the LHA. Wild-type mice and orexin knock-out mice showed abundant prodynorphin mRNA-expressing neurons in the LHA, but orexin/ataxin-3 mice with a selective loss of the orexin neurons completely lacked prodynorphin mRNA in this area, further confirming that within the LHA, dynorphin expression is restricted to the orexin neurons. These findings suggest that dynorphin-A may play an important role in the function of the orexin neurons.

Modafinil is an increasingly popular wake-promoting drug used for the treatment of narcolepsy, but its precise mechanism of action is unknown. To determine potential pathways via which modafinil acts, we administered a range of doses of modafinil to rats, recorded sleep/wake activity, and studied the pattern of neuronal activation using Fos immunohistochemistry. To contrast modafinil-induced wakefulness with spontaneous wakefulness, we administered modafinil at midnight, during the normal waking period of rats. To determine the influence of circadian phase or ambient light, we also injected modafinil at noon on a normal light/dark cycle or in constant darkness. We found that 75 mg/kg modafinil increased Fos immunoreactivity in the tuberomammillary nucleus (TMN) and in orexin (hypocretin) neurons of the perifornical area, two cell groups implicated in the regulation of wakefulness. This low dose of modafinil also increased the number of Fos-immunoreactive (Fos-IR) neurons in the lateral subdivision of the central nucleus of the amygdala. Higher doses increased the number of Fos-IR neurons in the striatum and cingulate cortex. In contrast to previous studies, modafinil did not produce statistically significant increases in Fos expression in either the suprachiasmatic nucleus or the anterior hypothalamic area. These observations suggest that modafinil may promote waking via activation of TMN and orexin neurons, two regions implicated in the promotion of normal wakefulness. Selective pharmacological activation of these hypothalamic regions may represent a novel approach to inducing wakefulness.

Idiopathic narcolepsy usually results from a loss of the hypothalamic neuropeptide orexin (hypocretin), but the cause of secondary narcolepsy resulting from focal brain lesions is unknown. The authors describe a young man who developed narcolepsy after a large hypothalamic stroke. His lesion included much of the hypothalamic region in which orexin is produced, and his CSF concentration of orexin was low. The authors hypothesize that a loss of orexin neurons or their relevant targets may be the specific neuropathology causing this and many other cases of secondary narcolepsy.