Serotonin unmasks functional NK-2 receptors in vagal sensory neurones of the guinea-pig.
1. The regulation of substance P (SP) responsiveness in acutely isolated nodose neurones from adult guinea-pigs was investigated using standard intracellular recording techniques. 2. In control neurones, SP produced no measurable electrophysiological effects. However, following incubation with serotonin (5-HT, 10 microM), 64% of neurones were depolarized by 10 +/- 0.6 mV (n = 84 of 132 neurones) by SP (100 nM). 5-HT-induced SP responses were inhibited by SR48968 (100 nM, n = 6), a neurokinin 2 (NK-2) receptor antagonist, but were unaffected by CP99,994 and SR142801, NK-1 and NK-3 receptor antagonists (n = 3 each), respectively. 3. 5-HT-induced unmasking of SP responses was maximal within 5 min. Increasing the 5-HT incubation time up to 120 min did not increase the mean response amplitude or the percentage of SP responsive neurones (P = 0.611 and 0.867, respectively). 4. 5-HT-induced unmasking of SP responses was dose dependent (EC50 = 14 nM). A 5-HT3 receptor agonist CPBG (1 microM), mimicked the unmasking effects of 5-HT (n = 10 of 19 neurones), while 5-CT (10 microM), a non-selective 5-HT agonist devoid of action at 5-HT3 receptors, did not (n = 18). ICS205-930 (1 microM), a 5-HT3 receptor antagonist, completely blocked the 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 5. In 68% of the neurones tested, bath-applied 5-HT (10 microM) evoked a 178 +/- 29.5 nM increase in [Ca2+]i (n = 16), which was blocked by nominally zero [Ca2+]o (n = 4) or by ICS205-930 (1 microM, n = 4). Nodose neurones incubated with 5-HT in the presence of nominally zero [Ca2+]o did not respond to SP (n = 12 of 13 neurones) in Locke solution containing normal [Ca2+]o, indicating that the 5-HT-mediated elevation of [Ca2+]i is required for unmasking of SP responses. Calmidazolium (100 nM), a calmodulin inhibitor, inhibited the unmasking effects of 5-HT (n = 5 of 5 neurones). 6. Incubating neurones with the nitric oxide (NO) donors papaNONOate (1 mM, 15-30 min) or SNAP (50 microM, 30-60 min) unmasked depolarizing SP responses in 71% and 45% of the neurones studied, respectively. L-NMMA (30 microM), a NO synthase inhibitor, blocked 5-HT-induced unmasking of SP responses (n = 10 of 10 neurones). 7. In sum, these results suggest that stimulation of 5-HT3 receptors activates an intracellular signalling cascade that couples calcium-calmodulin and NO activation to NK-2 receptor unmasking in sensory neurones.
Moore
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University of Maryland School of Medicine, Department of Pharmacology and Experimental Therapeutics, 685 W. Baltimore Street, Baltimore, MD 21201-1559,, USA.
Taylor
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Weinreich
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D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006168:Guinea Pigs; D006834:Hydrazines / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009475:Neurons, Afferent / Q000737:chemistry* / Q000187:drug effects / Q000502:physiology; D009569:Nitric Oxide / Q000378:metabolism / Q000494:pharmacology; D020030:Nitric Oxide Donors / Q000494:pharmacology; D009620:Nodose Ganglion / Q000737:chemistry* / Q000166:cytology; D010396:Penicillamine / Q000031:analogs & derivatives / Q000494:pharmacology; D010880:Piperidines / Q000494:pharmacology; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D018040:Receptors, Neurokinin-1 / Q000502:physiology; D018041:Receptors, Neurokinin-2 / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000502:physiology*; D018042:Receptors, Neurokinin-3 / Q000502:physiology; D011985:Receptors, Serotonin / Q000502:physiology; D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin / Q000494:pharmacology*; D012702:Serotonin Antagonists / Q000494:pharmacology; D013373:Substance P / Q000378:metabolism; D000077526:Tropisetron; D019323:omega-N-Methylarginine / Q000494:pharmacology
D001549:Benzamides; D004791:Enzyme Inhibitors; D006834:Hydrazines; D007093:Imidazoles; D007211:Indoles; D020030:Nitric Oxide Donors; C112653:PAPA NONOate; D010880:Piperidines; D011500:Protein Synthesis Inhibitors; D018040:Receptors, Neurokinin-1; D018041:Receptors, Neurokinin-2; D018042:Receptors, Neurokinin-3; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; C110881:S-nitro-N-acetylpenicillamine; D012702:Serotonin Antagonists; C080755:3-(2-methoxybenzylamino)-2-phenylpiperidine; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D012701:Serotonin; D013373:Substance P; C031938:calmidazolium; D000077526:Tropisetron; C073839:SR 48968; D003513:Cycloheximide; D010396:Penicillamine; C091554:SR 142801