Antinociceptive effects of some α-sympathomimetic agents

Abstract

Xylazine (3–10 mg/kg), clonidine (1–3 mg/kg) and L.D.2855, three α-sympathomimetic agents with marked central effects, were found to be potent antinociceptive agents on the hot plate test in mice and on the test of electrical stimulation of rat's tail when administered intraperitoneally. In contrast, classical α-sympathomimetic agents, naphazoline, tetryzoline and oxymetazoline, were devoid of any antinociceptive action by parenteral route. Intracerebroventricular administration of xylazine, clonidine and L.D. 2855 induced an antinociceptive action at smaller doses than by intraparenteral route of administration. These results indicate a predominant central site of action. Intracerebroventricular administration of naphazoline in rats and of naphazoline, tetryzoline and oxymetazoline in mice, induced a strong antinociceptive effect. These results indicate that the failure to cross the blood-brain barrier was the cause of the ineffectiveness of classical α-sympathomimetic agents to cause analgesia. In conclusion, α-sympathomimetic mechanisms seem to induce antinociceptive action in rodents.