Effect of cysteamine on vocalization responses by arterial algogenics in guinea pigs

Effects of cysteamine (2-mercaptoethylamine) on vocalization responses, a parameter of nociceptive response, was studied in conscious guinea pigs. Intra-arterial injection of bradykinin (3 μg), acetylcholine (300 μg), capsaicin (3 μg) and vanillyl n-nonoylamide (3 μg, VNA) induced severe vocalization responses. Three hours after an administration of cysteamine (300mg/kg, s.c.), a significant suppressive effect was observed for bradykinin and acetylcholine-evoked vocalization. A weak suppressive effect appeared in capsaicin and VNA-evoked vocalization, but there were not statistically significant changes on vocalization counts when compared with the value of the saline-treated animals. However, consecutive pretreatment of guinea pigs with VNA led to a complete suppression of capsaicinoids-evoked vocalization only. Cysteamine completely suppressed bradykinin, acetylcholine, capsaicin and VNA-evoked vocalization responses in VNA desensitized guinea pigs. These findings suggest that the mechanisms of bradykinin and acetylcholine-evoked vocalization response differ from that of capsaicinoids.