A large animal neuropathic pain model in sheep: a strategy for improving the predictability of preclinical models for therapeutic development

Abstract

BackgroundEvaluation of analgesics in large animals is a necessary step in the development of better pain medications or gene therapy prior to clinical trials. However, chronic neuropathic pain models in large animals are limited. To address this deficiency, we developed a neuropathic pain model in sheep, which shares many anatomical similarities in spine dimensions and cerebrospinal fluid volume as humans.MethodsA neuropathic pain state was induced in sheep by tight ligation and axotomy of the common peroneal nerve. The analgesic effect of intrathecal (IT) morphine was investigated. Interspecies comparison was conducted by analyzing the ceiling doses of IT morphine for humans, sheep, and rats.ResultsPeroneal nerve injury (PNI) produced an 86% decrease in von-Frey filament-evoked withdrawal threshold on postsurgery day 3 and the decrease lasted for the 8-week test period. Compared to the pre-injury, sham, and contralateral hindlimb, the IT morphine dose that produces 50% of maximum analgesia (ED50) for injured PNI hindlimb was 1.8-fold larger and Emax, the dose that produces maximal analgesia, was 6.1-fold lower. The sheep model closely predicts human IT morphine ceiling dose by allometric scaling. This is in contrast to the approximately 10-fold lower morphine ceiling dose predicted by the rat spinal nerve ligated or spared nerve injury models.ConclusionPNI sheep model has a fast onset and shows stable and long-lasting pain behavioral characteristics. Since the antinociceptive properties of IT morphine are similar to those observed in humans, the PNI sheep model will be a useful tool for the development of analgesics. Its large size and consistent chronic pain behavior will facilitate the development and evaluation of surgical intervention and gene therapy. The PNI sheep pain model provides us with the opportunity for multi-species testing, which will improve the success of clinical trials.

Pain assessment in animal models: do we need further studies?

“…Wilkes et al used a stiff von Frey filament tip connected to an anesthesiometer on the hind limb of the animal, exerting increasing pressure until it elicited a withdrawal response 30. Stubsjøen et al inflated a tourniquet to a pressure of 300 mmHg (or until sheep showed signs of aversion) and Ong et al used four different electrical stimuli to produce a response 31,32. Mather et al measured the pain threshold with a pneumo-mechanical pressure device on the animal’s foreleg 33…”

Lost but making progress—Where will new analgesic drugs come from?

“…The reverse, however, is often not true. Preclinical investigations all too frequently identify pathways that modulate pain in animal assays, but molecules that target these pathways fail in clinical trials (62–65). Examples include the substance P neurokinin-1 (NK-1) receptor antagonists (66–68), the fatty acid amino hydrolase inhibitors that elevate endocannabinoid tone (69), and peripherally restricted cannabinoid CB1 receptor agonists (70).…”

Repeated electrical stimulations as a tool to evoke temporal summation of nociceptive inputs in healthy, non-medicated experimental sheep

“…c o m / l o c a t e / p h b thresholds remained lower than in healthy animals even after successful treatment of the inflammatory process. In addition, surgically induced peroneal nerve injury leading to neuropathic pain and thus to a significant persistent increase in skin sensitivity has been promoted as a useful model for evaluating analgesics in sheep [7]. Unfortunately, both models are invasive and non-reversible and therefore not appropriate to be applied in refinement studies aiming at improving analgesia in sheep undergoing experimental orthopaedic surgery as animal models.…”