Refinement: The impact of peri-operative analgesia on neuropharmacological outcomes in rodent models of chronic pain

Why did we fund this project?

This award aims to refine rodent models of chronic pain by establishing if analgesia used to treat acute surgical pain affects the subsequent experimental endpoints assessed in the chronic pain phase.

Chronic pain is persistent pain that continues long after an injury has happened, or without any obvious tissue damage. A number of animal models, including mice, rats, guinea pigs, and ferrets are used to investigate underlying mechanisms and identify novel targets for therapeutics. Studies primarily involve nerve injury for example by ligating a spinal nerve, or other surgical procedures. Pain is then measured using pain-related behaviours, such as mechanical allodynia and electrophysiology studies. Historically, analgesia has not been used for the acute pain in the days immediately after surgery due to concerns that it can confound the study of chronic pain that occurs weeks later in the experimental models used. However, preliminary data from Dr Kirsty Banner indicates that this may not be the case and that there may be an opportunity to refine the studies using analgesia.

The student will expand on the pilot data using animal models of neuropathic pain and cancer-induced bone pain. Analgesia will be given in the acute phase and subsequent behavioural and electrophysiological measures will be taken in the acute and chronic pain phases. Chronic pain is associated with a dysregulation in descending pain modulation and by comparing the data with that from control experiments without analgesia it will be possible to determine whether pain relief can be used without confounding studies. The student will develop skills in surgery, electrophysiology, behavioural analyses, immunohistochemistry and confocal microscopy. 

Chronic pain remains an area of considerable unmet clinical need, affecting approximately 20% of the population. Multiple pathophysiological mechanisms contribute to the pain of a single disease and the majority of currently used analgesics reach only 30% efficacy. Animal research is vital for expanding our knowledge of the pathophysiological processes that contribute to chronic pain conditions as varied as neuropathy, osteoarthritis (OA) and cancer-induced bone pain (CIBP); identifying novel underlying molecular mechanisms that can be targeted pharmacotherapeutically in order to reduce the level of pain experienced by the patient is key.

When performing chronic pain model surgical procedures on rodents there is an ethical need to provide analgesia. However, it is vital to maintain measurable pain-related behaviours for the neuropharmacological research questions that scientists want to answer. Unfortunately, this means that largely, animals are not given peri-operative analgesia and so pain arises not only from the nerve ligation (the surgical procedure for neuropathy) for example, but also from the tissue damage/inflammation associated with the surgical incisions themselves.

Dr Bannister wants to refine the current standard practice of administering no peri-operative analgesia to rodents undergoing such surgeries. Pharmacological intervention in the period immediately preceding or proceeding surgery is often believed to impact outcome in terms of behavioural/neuronal phenotype. Dr Bannister has produced pilot data that challenges this belief. In spinal nerve ligated rats, a single dose of pregabalin (3mg/kg) administered 15 minutes prior to surgery:

1) does not significantly change baseline spinal neuronal responses 14-16 days post-surgery

2) enhances the inhibitory effect of subsequent administration of a single experimental dose of systemic pregabalin (3mg/kg) on spinal neuronal responses

3) does not impact the inhibitory effect of subsequent administration of a single dose of systemic morphine (3mg/kg) on spinal neuronal responses.

There are several gaps in our knowledge. Does peri-operative analgesia impact pain measurement outcomes in rodent models of chronic pain all instances? Does perioperative analgesia produce a lasting effect on pain behaviours that persist after tissue (i.e. surgical incision) healing, and does this relate to long-lasting changes in central nervous system neuronal activity? Is the inhibitory effect of subsequent administration of a single dose of analgesic on central neuronal responses significantly altered (enhanced or reduced)?

We will dissect the influence of perioperative pregabalin or buprenorphine on pain behaviours. Hypothetically, pain behaviours (ordinarily observed from day 1 for each pain model) will be reduced in the early post-operative time frame as the animals suffer less pain. Whether or not perioperative analgesia has a long-term neurological effect in terms of experimental outcome according to the neuropharmacology in question will be analysed. If post-operative pain behaviours can be reduced in the acute period, while neurological activity is unchanged in the chronic period, researchers should be willing to re-address the importance of perioperative analgesia in rodent models of chronic pain. We will maximise the data obtained from every animal: following the electrophysiology experiments brain and spinal cord tissue will be dissected and the anatomy of a unique descending inhibitory pathway, known to be dysfunctional in all 3 chronic pain rodent models will be defined.

In total our proposal will deliver the following impacts:

(a) Establish a culture of perioperative analgesic administration in rodents undergoing surgery to induce neuropathy, OA and CIBP

(b) Broaden investigation of impact of a range of perioperative analgesics on neurological activity in rodent chronic pain models

(c) Elucidate novel brain-spinal cord neurobiological plasticity that underlies the neuropathic/OA/CIBP pain state

1. Kucharczyk MW, Di Domenico F and Bannister K (2022). Distinct brainstem to spinal cord noradrenergic pathways inversely regulate spinal neuronal activity. Brain 145(7): 2293–300. doi: 10.1093/brain/awac085