An Analgesic Evaluation of Isoxsuprine in Horses

Abstract

Isoxsuprine is used clinically to treat navicular disease and laminitis in horses. Although it is thought to increase digital and laminar blood flow, isoxsuprine's mechanism of action remains controversial, and analgesia has been suggested recently as such possible mechanism. This research investigated the analgesic potential of isoxsuprine in healthy horses submitted to a mechanical nociceptive test. Isoxsuprine (1.2 mg/kg), xylazine (1.1 mg/kg), distilled water : ethanol 95% (2 : 1, v/v, 20 ml) and saline (0.9%, 20 ml) were injected intravenously, and nociceptive thresholds were measured over 90 min. Only xylazine significantly increased nociceptive thresholds, confirming that alpha(2)-adrenoceptor agonists produce analgesia in horses. Our results do not support an analgesic mechanism of action for isoxsuprine in horses, suggesting that other mechanisms might account for the clinical efficacy of this drug or that mechanical nociceptive testing may not be sufficiently sensitive to demonstrate an analgesic effect for this drug.

A comparative study of xylazine-induced mechanical hypoalgesia in donkeys and horses

“…Direct comparisons between donkeys and horses were possible due to the fact that similar forces were needed to elicit withdrawal of the left foreleg in both species before any drugs were given. Comparable threshold values in horses were obtained in another study (Lizarraga et al. 2004a).…”

“…This technique detects analgesia induced by α 2 ‐adrenergic agonists and discriminates between analgesia and sedation in horses (Chambers et al. 1993, 1994; Lizarraga et al. 2004a).…”

“…This methodology has been validated and widely used to evaluate nociception and the analgesic effect of drugs in horses (Chambers et al. 1993, 1994; Lizarraga et al. 2004a; Love et al.…”

“…The analgesic effects of a 2 -adrenergic agonists are well characterized in horses (Chambers et al 1994;Moens et al 2003;Lizarraga et al 2004a;Rohrbach et al 2009). However, the scarce information on donkeys is of limited value since analgesia has only been assessed in non-controlled studies by means of pinprick methods (Mostafa et al 1995;Varshney et al 1996;El-Maghraby & Atta 1997;Joubert et al 1999).…”

“…In the experiments described here, mechanical nociceptive thresholds from left forelimbs were measured to assess the hypoalgesic action of different doses of xylazine in parallel studies on both donkeys and horses. This methodology has been validated and widely used to evaluate nociception and the analgesic effect of drugs in horses (Chambers et al 1993(Chambers et al , 1994Lizarraga et al 2004a;Love et al 2011).…”