Medial prefrontal cortex receives input from dorsal raphe nucleus neurons targeted by hypocretin1/orexinA-containing axons.

The medial prefrontal cortex (mPFC) is strongly involved in cognition and behavior. It receives input from brainstem nuclei implicated in behavioral wakefulness and electrographic cortical activation, such as the dorsal raphe nucleus (DRN). Moreover, the hypocretinergic/orexinergic (Hcrt/Ox) hypothalamic neurons innervate DRN, thus modulating its activity and presumably allowing transitions between sleep-wakefulness cycle states. Dysfunction in this system is associated with narcolepsy. In this study we aimed to determine the precise location of DRN neurons projecting to mPFC and the extent to which they contain serotonin (5-hydroxytryptamine); we have also assessed whether Hcrt1/OxA neurons innervate DRN neurons that could sustain behavioral wakefulness through their projections to mPFC. The retrograde tracer Fluorogold was injected into mPFC and DRN sections were processed for double immunolabeling of anti-Fluorogold and either anti-5-hydroxytryptamine or anti-Hcrt1/OxA antisera. Most DRN neurons projecting to mPFC were located in the ventral sector of the rostral and intermediate DRN, and around half of them were serotonergic. Hcrt1/OxA-immunoreactivity in DRN was observed in unmyelinated axons and axon boutons (varicosities or axon terminals). Hcrt1/OxA immunoreactivity was observed within the cytoplasm and in dense-cored vesicles of these axons. Hcrt1/OxA-labeled boutons established both asymmetric synapses (n=30) and appositional contacts (n=102) with Fluorogold-labeled dendrites belonging to DRN neurons projecting to mPFC. Our results show that Hcrt1/OxA neurons may exert a direct synaptic influence on DRN neurons that could facilitate wakefulness, although other non-synaptic actions through volume transmission are also suggested.