Histamine H 3 ‐receptor antagonists inhibit gastroprotection by (R)‐α‐methylhistamine in the rat

(R)‐α‐methylhistamine, a selective agonist of histamine H 3 receptors, is capable of protecting the gastric mucosa against differently acting damaging agents. The objective of the present study was to determine whether H 3 receptors mediate its protective action in the rat. Gastric mucosal lesions were induced intragastrically (i.g.) by 0.6 N HCl, 1 ml rat −1 . (R)‐α‐methylhistamine, 100 mg kg −1 i.g., substantially reduced the severity of macroscopically and histologically assessed damage caused by concentrated acid. Prior treatment with highly selective H 3 ‐receptor antagonists, ciproxifan (0.3, 1 and 3 mg kg −1 i.g.) and clobenpropit (3, 10 and 30 mg kg −1 i.g.), dose‐dependently inhibited the protection exerted by (R)‐α‐methylhistamine up to a complete reversal. When given alone at high doses, both antagonists tended to worsen the HCl‐induced histologic damage. During basal conditions, (R)‐α‐methylhistamine, 100 mg kg −1 i.g., caused a significant increase in titratable acidity of the gastric juice. Prior treatment with ciproxifan (3 mg kg −1 i.g.) and clobenpropit (30 mg kg −1 i.g.) did not alter the secretory response to (R)‐α‐methylhistamine. Clobenpropit alone, but not ciproxifan, increased the volume of gastric juice, and both compounds alone had no effect on titratable acid. Present findings support evidence that H 3 receptors are actively involved in the maintenance of gastric mucosal integrity, with no apparent role in the regulation of basal gastric acid secretion.British Journal of Pharmacology (2000) 129 , 1597–1600; doi: 10.1038/sj.bjp.0703249