Hypocretin/orexin peptide signaling in the ascending arousal system: elevation of intracellular calcium in the mouse dorsal raphe and laterodorsal tegmentum.
Dysfunction of the hypocretin/orexin (Hcrt/Orx) peptide system is closely linked to the sleep disorder narcolepsy, suggesting that it is also central to the normal regulation of sleep and wakefulness. Indeed, Hcrt/Orx peptides produce long-lasting excitation of arousal-related neurons, including those in the laterodorsal tegmentum (LDT) and the dorsal raphe (DR), although the mechanisms underlying these actions are not understood. Since Hcrt/Orx mobilizes intracellular calcium ([Ca(2+)](i)) in cells transfected with orexin receptors and since receptor-mediated Ca(2+) transients are ubiquitous signaling mechanisms, we investigated whether Hcrt/Orx regulates [Ca(2+)](i) in the LDT and DR. Changes in [Ca(2+)](i) were monitored by fluorescence changes of fura-2 AM loaded cells in young mouse brain slices. We found Hcrt/Orx (Orexin-A, 30-1,000 nM) evoked long-lasting increases in [Ca(2+)](i) with differing temporal profiles ranging from spiking to smooth plateaus. A fragment of Hcrt/Orx (16-33) failed to evoke changes in [Ca(2+)](i) and changes were not blocked by TTX or ionotropic glutamate receptor antagonists, suggesting they resulted from specific activation of postsynaptic orexin receptors. Unlike orexin receptor-transfected cells, Hcrt/Orx-responses were not attenuated by depletion of Ca(2+) stores with cyclopiazonic acid (CPA; 3-30 microM), thapsigargin (3 microM), or ryanodine (20 microM), although store-depletion by either CPA or ryanodine blocked Ca(2+) mobilization by the metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 30 microM). In contrast, Hcrt/Orx responses were strongly attenuated by lowering extracellular Ca(2+) ( approximately 20 microM) but were not inhibited by concentrations of KB-R7943 (10 microM) selective for blockade of sodium/calcium exchange. Nifedipine (10 microM), inhibited Hcrt/Orx responses but was more effective at abolishing spiking than plateau responses. Bay K 8644 (5-10 microM), an L-type calcium channel agonist, potentiated responses. Finally, responses were attenuated by inhibitors of protein kinase C (PKC) but not by inhibitors of adenylyl cyclase. Collectively, our findings indicate that Hcrt/Orx signaling in the reticular activating system involves elevation of [Ca(2+)](i) by a PKC-involved influx of Ca(2+) across the plasma membrane, in part, via L-type calcium channels. Thus the physiological release of Hcrt/Orx may help regulate Ca(2+)-dependent processes such as gene expression and NO production in the LDT and DR in relation with behavioral state. Accordingly, the loss of Hcrt/Orx signaling in narcolepsy would be expected to disrupt calcium-dependent processes in these and other target structures.
Kohlmeier
Kristi A
KA
Department of Physiology, New York Medical College, Valhalla, New York 10595, USA.
Inoue
Takafumi
T
Leonard
Christopher S
CS
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001143:Arousal / Q000187:drug effects / Q000502:physiology*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002352:Carrier Proteins / Q000494:pharmacology / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000187:drug effects / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008810:Mice, Inbred C57BL; D009479:Neuropeptides / Q000494:pharmacology / Q000502:physiology*; D000068797:Orexins; D011903:Raphe Nuclei / Q000187:drug effects / Q000502:physiology*; D013681:Tegmentum Mesencephali / Q000187:drug effects / Q000502:physiology*
D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D000068797:Orexins