title: Carbon monoxide and nitric oxide: interacting messengers in muscarinic signaling to the brain's circadian clock.
abstract: Within the central nervous system, acetylcholine (ACh) functions as a state-dependent modulator at a range of sites, but its signaling mechanisms are yet unclear. Cholinergic projections from the brain stem and basal forebrain innervate the suprachiasmatic nucleus (SCN), the master circadian clock in mammals, and cholinergic stimuli adjust clock timing. Cholinergic effects on clock state require muscarinic receptor-mediated activation of guanylyl cyclase and cGMP synthesis, although the effect is indirect. Here we evaluate the roles of carbon monoxide (CO) and nitric oxide (NO), major activators of cGMP synthesis. Both heme oxygenase 2 (HO-2) and neuronal nitric oxide synthase (nNOS), enzymes that synthesize CO and NO, respectively, are expressed in rat SCN, with HO-2 localized to the central core of the SCN, whereas nNOS is a punctate plexus. Hemin, an activator of HO-2, but not the NO donor, SNAP, mimicked cholinergic effects on circadian timing. Selective inhibitors of HO fully blocked cholinergic clock resetting, whereas NOS inhibition partially attenuated this effect. Hemoglobin, an extracellular scavenger of both NO and CO, blocked cholinergic stimulation of cGMP synthesis, whereas l-NAME, a specific inhibitor of NOS, had no effect on cholinergic stimulation of cGMP, but decreased the cGMP basal level. We conclude that basal NO production generates cGMP tone that primes the clock for cholinergic signaling, whereas HO/CO transmit muscarinic receptor activation to the cGMP-signaling pathway that modulates clock state. In light of the recently reported inhibitory interaction between HO-2/CO and amyloid-beta, a marker of Alzheimer's disease (AD), we speculate that HO-2/CO signaling may be a defective component of cholinergic neurotransmission in the pathophysiology of AD, whose manifestations include disintegration of circadian timing.
authors.affiliation: Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: L R
authors.initials: LR
authors.lastname: Artinian
authors.forename: J M
authors.initials: JM
authors.lastname: Ding
authors.forename: M U
authors.initials: MU
authors.lastname: Gillette
mesh_terms: D000818:Animals; D001921:Brain / Q000502:physiology*; D002248:Carbon Monoxide / Q000502:physiology*; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology*; D006152:Cyclic GMP / Q000378:metabolism; D006419:Heme Oxygenase (Decyclizing) / Q000378:metabolism; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009569:Nitric Oxide / Q000502:physiology*; D020030:Nitric Oxide Donors / Q000494:pharmacology; D019001:Nitric Oxide Synthase / Q000378:metabolism; D052248:Nitric Oxide Synthase Type I; D051381:Rats; D020318:Rats, Long-Evans; D011950:Receptors, Cholinergic / Q000502:physiology; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D026423:S-Nitroso-N-Acetylpenicillamine / Q000494:pharmacology; D015398:Signal Transduction; D013493:Suprachiasmatic Nucleus / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology
chemical_list: D020030:Nitric Oxide Donors; D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D009569:Nitric Oxide; D026423:S-Nitroso-N-Acetylpenicillamine; D002248:Carbon Monoxide; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; C496322:Nos1 protein, rat; D006419:Heme Oxygenase (Decyclizing); C105245:heme oxygenase-2; D006152:Cyclic GMP; D019331:NG-Nitroarginine Methyl Ester