Hypocretin1/orexinA-immunoreactive axons form few synaptic contacts on rat ventral tegmental area neurons that project to the medial prefrontal cortex

Abstract

BackgroundHypocretins/orexins (Hcrt/Ox) are hypothalamic neuropeptides involved in sleep-wakefulness regulation. Deficiency in Hcrt/Ox neurotransmission results in the sleep disorder narcolepsy, which is characterized by an inability to maintain wakefulness. The Hcrt/Ox neurons are maximally active during wakefulness and project widely to the ventral tegmental area (VTA). A dopamine-containing nucleus projecting extensively to the cerebral cortex, the VTA enhances wakefulness. In the present study, we used retrograde tracing from the medial prefrontal cortex (mPFC) to examine whether Hcrt1/OxA neurons target VTA neurons that could sustain behavioral wakefulness through their projections to mPFC.ResultsThe retrograde tracer Fluorogold (FG) was injected into mPFC and, after an optimal survival period, sections through the VTA were processed for dual immunolabeling of anti-FG and either anti-Hcrt1/OxA or anti-TH antisera. Most VTA neurons projecting to the mPFC were located in the parabrachial nucleus of the ipsilateral VTA and were non-dopaminergic. Only axonal profiles showed Hcrt1/OxA-immunoreactivity in VTA. Hcrt1/OxA reactivity was observed in axonal boutons and many unmyelinated axons. The Hcrt1/OxA immunoreactivity was found filling axons but it was also observed in parts of the cytoplasm and dense-core vesicles. Hcrt1/OxA-labeled boutons frequently apposed FG-immunolabeled dendrites. However, Hcrt1/OxA-labeled boutons rarely established synapses, which, when they were established, were mainly asymmetric (excitatory-type), with either FG-labeled or unlabeled dendrites.ConclusionsOur results provide ultrastructural evidence that Hcrt1/OxA neurons may exert a direct synaptic influence on mesocortical neurons that would facilitate arousal and wakefulness. The paucity of synapses, however, suggest that the activity of VTA neurons with cortical projections might also be modulated by Hcrt1/OxA non-synaptic actions. In addition, Hcrt1/OxA could modulate the postsynaptic excitatory responses of VTA neurons with cortical projections to a co-released excitatory transmitter from Hcrt1/OxA axons. Our observation of Hcrt1/OxA targeting of mesocortical neurons supports Hcrt1/OxA wakefulness enhancement in the VTA and could help explain the characteristic hypersomnia present in narcoleptic patients.

Recursive Partitioning Analysis of Fractional Low-Frequency Fluctuations in Narcolepsy With Cataplexy

“…In both the adult and juvenile participants, narcolepsy patients showed decreased fALFF in bilateral SFGmed, bilateral supra-marginal gyrus and bilateral IPL compared with healthy controls, while narcolepsy patients showed increased fALFF in bilateral SMC and bilateral middle temporal gyrus compared with healthy controls. Both the medial frontal cortex, supra-marginal gyrus, and parietal lobe are abundant in hypocretin projection ( 30 , 31 ), which can explain the reduced fALFF value in these regions among narcolepsy due to hypocretin deficiency, consistent with two previous positron emission tomography studies ( 7 , 8 ). Increased fALFF in bilateral SMC, extending to bilateral paracentral lobule (Figure 1 ), may be a compensation of hypocretin deficiency in motor cortex among narcolepsy, although a contradictory result has been reported hypo-activity in sensorimotor cortex in narcolepsy by transcranial magnetic stimulation (TMS) in a previous study ( 32 ).…”

In vivo functional diversity of midbrain dopamine neurons within identified axonal projections

“…The unique mismatch between the extent of retrograde labelling and the difficulty of identifying and recording mesocortical DA neurons in vivo indicated that this DA subpopulation might either be particularly vulnerable to our labelling protocol or mostly silent under isoflurane anesthesia. The latter possibility is not unlikely given the evidence for orexinergic modulation of mesocortical DA neurons (Del Cid-Pellitero and Garzón, 2014). Furthermore, our study focused on rostral striatal areas not covering the caudal ‘tail’ region, which is a projection area of DA neurons located in the SN pars lateralis.…”

Orexinergic bouton density is lower in the cerebral cortex of cetaceans compared to artiodactyls

“…Orexin released from non-synaptic orexinergic axon varicosities in the dorsal raphe diffused through the extracellular space and activated distant high affinity receptors, providing a longer sustained activation than orexin released through chemical synaptic transmission (Del Cid-Pellitero and Garzón, 2011). Thus, it appears that the excitatory action of orexin in the cerebral cortex might be initiated through two different communication channels.…”