Antinociceptive and cardiovascular properties of esmolol following formalin injection in rats

Purpose: To assess the role of esmolol, a β₁ receptor blocker, in the modulation of pain in the absence of anesthesia. Methods: Rats were chronically instrumented to record mean arterial blood pressure (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 injected in the rat hind paw. Formalin-induced lifting, MAP and HR were recorded at five minute intervals for 35 min after formalin injection. Results: Formalin was associated with an early (Phase 1; 0-5 min) and late nociceptive response (Phase 2; 10-35 min). Esmolol did not affect Phase I. Although 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, high dose esmolol blunted the secondary increase in MAP. Both low and high doses of esmolol inhibited formalin-induced tachycardia during the first 30 min. Conclusion: Our data suggest that esmolol leads to analgesia and reduction of cardiovascular responses to pain.