Antinociceptive and cardiovascular properties of esmolol following formalin injection in rats

Purpose: To assess the role of esmolol, a beta (1) receptor blocker, in the modulation of pain in the absence of anesthesia. Methods: Rats were chronically instrumented to record mean arterial blood p ressure (MAP) and heart rate (HR). Animals were divided into three groups. Group I [esmolol high(EH) 150 mg.kg(-1).hr(-1); n=9], Group 2 [esmolol low (EL) 40 mg.kg(-1)hr(-1); n=7] and Group 3 saline (n = 9). Formalin 5% was i njected in the rat hind paw. Formalin-induced lifting, MAP and HR were reco rded at five minute intervals for 35 min after formalin injection. Results: Formalin was associated with an early(Phase 1; 0-5 min) and late n ociceptive response (Phase 2; 10-35 min). Esmolol did not affect Phase 1. A lthough low dose esmolol had minimum effects on nociceptive Phase 2, it was diminished with high dose esmolol. Formalin induced biphasic increases in MAP and HR. Although esmolol did not affect the initial increase in MAP, hi gh dose esmolol blunted the secondary increase in MAP Both low and high dos es of esmolol inhibited formalin-induced tachycardia during the first 30 mi n. Conclusion: Our data suggest that esmolol leads to analgesia and reduction of cardiovascular responses to pain.