Opioidergic projections to sleep-active neurons in the ventrolateral preoptic nucleus

Although opioids are known to influence sleep–wake regulation, the neuroanatomic substrate(s) mediating these effects remain unresolved. We hypothesized that the influence of opiates on sleep may be mediated, at least in part, by the ventrolateral preoptic nucleus (VLPO), a key cell group for producing behavioral sleep. By combining in situ hybridization for kappa and mu receptor mRNA with immunostaining of Fos expressed by VLPO cells during sleep we show that > 85% of sleep-active VLPO neurons contain mRNA for either or both opioid receptors. Microinfusions of a kappa receptor agonist into the VLPO region increased NREM sleep by 51% during the subjective night, whereas a mu receptor agonist increased wakefulness by 60% during the subjective day. The sleep- and wake-promoting effects of the kappa and mu agonists were blocked by prior administration of their respective antagonist. Combining retrograde tracing from the VLPO with immunohistochemistry for dynorphin (Dyn, the endogenous kappa receptor agonist) or endomorphin 1 (EM1, the endogenous mu receptor agonist) we show that the central lateral parabrachial subnucleus (PBcl) provides Dyn inputs to the VLPO, whereas hypothalamic histaminergic neurons provide EM1 inputs to the VLPO. In summary, results from the present study suggest that central opioid inputs to the VLPO may play a role in sleep–wake regulation and that the VLPO likely mediates the hypnotic response to high levels of opioid analgesics.