Localization of hypocretin-like immunoreactivity in the brain of the diurnal rodent, Arvicanthis niloticus

The neuropeptide hypocretin (HCRT, also called orexin) acts in the brain to increase arousal and inhibit REM sleep. There is also substantial evidence that disruption of the hypocretin system results in narcolepsy. The distribution of HCRT + fibers in nocturnal animals is consistent with its role in arousal; fibers are concentrated in brain areas important in arousal and the inhibition of REM sleep. The distribution of HCRT-like immunoreactive (HCRT +) cells and fibers has been described in nocturnal but not diurnal rodents. We therefore examined the anatomical distribution of HCRT + cells and fibers in the diurnal murid rodent Arvicanthis niloticus (unstriped Nile grass rat). Arvicanthis niloticus were perfused and brain sections were collected through the forebrain and midbrain and processed for HCRT immunocytochemistry. Hypocretin-like immunopositive cell bodies were located in the lateral hypothalamus, dorsomedial hypothalamus, and perifornical area. The densest staining for HCRT + neuronal fibers was seen in the paraventricular thalamic nucleus, the locus coeruleus, and the raphe nuclei. The distribution of HCRT + cells and fibers is consistent with that found in other rodents such as rats and Syrian hamsters. Although the pattern of HCRT-like immunostaining for cells and fibers is similar in nocturnal rodents and diurnal A. niloticus, it will be important to compare the pattern of HCRT release, as well as activity of HCRT cells, between nocturnal and diurnal species.