Ventrolateral preoptic nucleus contains sleep-active, galaninergic neurons in multiple mammalian species

The ventrolateral preoptic nucleus (VLPO) is a group of sleep-active neurons that has been identified in the hypothalamus of rats and is thought to inhibit the major ascending monoaminergic arousal systems during sleep; lesions of the VLPO cause insomnia. Identification of the VLPO in other species has been complicated by the lack of a marker for this cell population, other than the expression of Fos during sleep. We now report that a high percentage of the sleep-active (Fos-expressing) VLPO neurons express mRNA for the inhibitory neuropeptide, galanin, in nocturnal rodents (mice and rats), diurnal rodents (degus), and cats. A homologous (i.e. galanin mRNA-containing cell group) is clearly distinguishable in the ventrolateral region of the preoptic area in diurnal and nocturnal monkeys, as well as in humans. Galanin expression may serve to identify sleep-active neurons in the ventrolateral preoptic area of the mammalian brain. The VLPO appears to be a critical component of sleep circuitry across multiple species, and we hypothesize that shrinkage of the VLPO with advancing age may explain sleep deficits in elderly humans.