H 3 receptor-mediated inhibition of intestinal acetylcholine release: pharmacological characterization of signal transduction pathways

Abstract

The present study investigates the mechanisms through which prejunctional histamine H3 receptors modulate intestinal cholinergic neurotransmission. The experiments were performed on longitudinal muscle-myenteric plexus preparations of guinea pig ileum, preincubated with [3H]choline, superfused with physiological salt solution containing hemicholinium-3, and subjected to electrical field stimulation. The stimulation-induced outflow of radioactivity was taken as an index of endogenous acetylcholine release. The electrically induced [3H]acetylcholine release was inhibited by histamine (EC50)=33.5 nM) or the H3 receptor agonist R-alpha-methylhistamine (EC50=41.6 nM), whereas it was not affected by pyridylethylamine (H1 agonist), impromidine (H2 agonist), pyrilamine (H1 antagonist), cimetidine (H2 antagonist), thioperamide or clobenpropit (H3 antagonists). The inhibitory effects of histamine or R-alpha-methylhistamine were antagonized by thioperamide (pKd= 8.31 and 8.53, respectively) or clobenpropit (pKd=9.44 and 9.32, respectively), but not by pyrilamine or cimetidine. The modulatory action of histamine on the evoked tritium outflow was attenuated by pertussis toxin and abolished by N-ethylmaleimide, two selective blockers of Gi/Go proteins. Tetraethylammonium or 4-aminopyridine, acting as inhibitors of voltage-dependent K+ channels, enhanced the evoked tritium outflow when tested alone, and apparently counteracted the inhibitory effect of histamine. However, the blocking actions of tetraethylammonium and 4-aminopyridine were no longer evident when their enhancing actions were compensated by appropriate reductions of Ca2+ concentration in the superfusion medium. Histamine-induced inhibition of evoked tritium output was enhanced by omega-conotoxin, a selective blocker of N-type Ca2+ channels, or low Ca2+ concentration, whereas it was not modified by nifedipine, an antagonist of L-type Ca2+ channels. In addition, the inhibitory effect of histamine was not significantly affected by forskolin (activator of adenylyl cyclase), 8-bromo-cyclic AMP (a stable analog of cyclic AMP), rolipram (a selective blocker of type IV phosphodiesterase), phorbol myristate acetate (activator of protein kinase C), H-89 (N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide, inhibitor of protein kinase A), Ro-31-8220 (2-(1-[3-(amidinothio)propyl]-1H-indol-3-yl)-3-(1-methylindol-3-yl)-maleimide, inhibitor of protein kinase C), KT5823 (N-methyl-(8R*,9S*,11S*)-(-)-9-methoxy-9-methoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo [a,g]cycloocta[c,d,e]-trinden-1-one, inhibitor of protein kinase G), or lavendustin A (inhibitor of tyrosine kinase). The present results indicate that histamine inhibits intestinal cholinergic neurotransmission through presynaptic H3 receptors coupled to Gi/Go proteins. It is suggested that adenylyl cyclase, serine-threonine protein kinase and tyrosine kinase pathways are not implicated in this regulatory action, and that Gi/Go proteins modulate the activity of N-type Ca2+ chann...

Cholinesterase-like organocatalysis by imidazole and imidazole-bearing molecules

“…Millimolar concentrations of histamine can be reached in inflammatory districts4546 and probably in the gastrointestinal tract after ingestion of histamine rich-foods or foods triggering histamine release from the body. Although interactions between histamine and Ach have been already described both at peripheral and central levels474849, findings of the present study may offer a novel point of view for the comprehension of the complex relationship between histaminergic and cholinergic systems. Noteworthy, while histamine metabolism leads to the loss of its biological activity, it does not alter the catalytic potential of its imidazole ring, at least until the complete removal from the internal milieu or the cell cytoplasm.…”

Histamine Receptors and Antihistamines: From Discovery to Clinical Applications

“…The same group reported the synthesis of selective ligands for H 3 R, including the agonist (R)-α-methylhistamine and the antagonist thioperamide [44] . Using selective H 3 R agonists and antagonists it was shown that signal transduction by the H 3 R is mediated by pertussis toxin-sensitive G i /G o proteins leading to a decrease in intracellular cAMP concentrations [45,46] and to a decrease in extracellular Ca 2+ influx in neurons [47] .…”

Presynaptic H3Autoreceptors Modulate Histamine Synthesis through cAMP Pathway

“…Schlicker et al (1994) indicated that H 3 receptormediated inhibition of noradrenaline release was dependent of Ca 2ϩ influx. Also, Blandizzi et al (2001) suggested that H 3 receptor-inhibition of intestinal acetylcholine release could be mediated by modulation of N-type calcium channels. A direct coupling of G protein ␤␥ complexes to Ca 2ϩ channels might produce these effects of H 3 receptors on voltage-dependent calcium channels (Diverse-Pierluissi et al, 1997 and 2000).…”