Hypocretin1/OrexinA axon targeting of laterodorsal tegmental nucleus neurons projecting to the rat medial prefrontal cortex.

Cortical activation and goal-directed behaviors characterize wakefulness. One cortical region especially involved in these phenomena is the medial prefrontal cortex (mPFC), which receives many inputs from cholinergic-containing neurons in brain stem structures implicated in arousal and wakefulness, such as the laterodorsal tegmental nucleus (LDT). Hypocretins/orexins (Hcrt/Ox), whose dysfunction is linked to narcolepsy, maintains arousal and stabilizes sleep-wakefulness states. We aim to determine if Hcrt1/OxA axons (1) innervate LDT neurons projecting to the mPFC, a target that would allow them to sustain arousal and wakefulness, and (2) target preferentially cholinergic versus noncholinergic LDT neurons. The retrograde tracer Fluorogold (FG) was injected in the rat mPFC, and dual immunolabeling of anti-FG and either anti-choline acetyltransferase (ChAT) or anti-Hcrt1/OxA antisera was determined in LDT. Also, actual Hcrt1/OxA targeting of cholinergic LDT neurons was ascertained by dual anti-Hcrt1/OxA and anti-ChAT detection in additional noninjected animals. Many LDT FG-labeled neurons were cholinergic (52.05 ± 3.72%). Hcrt1/OxA immunoprecipitate was observed in cytoplasm and granular vesicles within axons. Some Hcrt1/OxA-containing axons established asymmetric excitatory-type synapses with either unlabeled (46/438) or FG-labeled (7/438) dendrites. One-third of the target neurons were ChAT labeled. Hcrt1/OxA excitatory input to LDT neurons projecting to mPFC probably contributes to the wakefulness-enhancing actions of Hcrt/Ox impaired in narcoleptics.