Distribution of hypocretin-containing neurons in the lateral hypothalamus and C-fos-immunoreactive neurons in the VLPO.

The present study investigated the distribution of neurons implicated in the regulation of sleep in three species generally used in sleep research, i.e., mice, rats and cats. We focused on sleep active neurons in the ventral lateral preoptic (VLPO) area and the hypocretin/orexin-containing neurons in the lateral hypothalamus. The latter groups of neurons were found recently to play an important role in the regulation of REM sleep. The expression of the transcription factor, c-Fos, was used to identify the VLPO. In mice and rats, in response to sleep, a discrete cluster of c-Fos positive cells was found in the VLPO. In mice, this cluster was located more medially compared to the rat, and as in the rat, galanin immunostained neurons were found in the VLPO. In the cat, c-Fos positive cells did not segregate to a specific location but were more diffusely represented in the preoptic area. In all three species, orexin/hypocretin-containing neurons were located only in the lateral hypothalamus with the distribution being more diffuse in the cat. The grouping of sleep-active cells in rodents makes it feasible to extract these cells for tissue culture and molecular analysis. Moreover, given that rodents have a distinct circadian distribution of sleep-wakefulness, the connectivity with the suprachiasmatic nucleus can also be determined.