We used ketamine anaesthesia for our sciatic nerve lesions (partial ligation of 1/2-1/3 of the sciatic nerve), and this had no effect on hyperalgesia (which developed normally post-injury).

We've used isoflurane & buprenorphine for doing the Bennet and Xie model with good success, but I too doubt that Ketamine/Xylazine would be a problem since the presence of the chromic gut ligatures cause a chronic response. We've used the Bennet and Xie model with 4 loose ligatures of chromic gut suture. The ligatures don't need to be tight--they need to be just right. The presence of the chromic gut recruits likely macrophages which damage and demylinate the nerve. Using the right kind of suture is important. The ligatures aren't supposed to constrict or damage the nerve directly. If you are getting hypoalgesia, then you are probably directly damaging the nerve. They should still bear weight on the paw after the procedure. Another thing to consider is how you are testing hyperalgesia. The thermal latency test by Hargreaves et al. can detect more subtle effects than the hotplate method.

Bennett, G. J., & Xie, Y. K. (1988). A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain, 33(1), 87-107.

Hargreaves, K., Dubner, R., Brown, F., Flores, C., & Joris, J. (1988). A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia. Pain, 32(1), 77-88.

I totally agree with Thomas. I will also recommend to do both thermal and mechanical nociceptive stimulation.

Could it be the diet that the animals have been receiving?

**Dietary fat and protein interact in suppressing neuropathic pain-related disorders following a partial sciatic ligation injury in rats.

Pérez J, Ware MA, Chevalier S, Gougeon R, Bennett GJ, Shir Y.

Pain. 2004 Oct;111(3):297-305.

Also, our lab recently found out that dietary soy also prevented the progression of back pain in our mouse model. Something to consider...

I too have done nerve injuries under ketamine anesthesia and still obtained hyperalgesia. It is important to note that the location for stimulus application may vary depending on which of the branches you are transecting or ligating. Each of the branches of the sciatic nerve has a different innervation territory on the paw of the animal. Which branch(es) are you attempting to transact or ligate and where exactly are you applying the stimulus?

Thank you all for your replies. It would seem that ketamine/xylazine is unlikely to be the problem.

Thomas, we have also used isoflurane on occasion, mostly to top up anesthesia during surgery. I have been unable to obtain chromic gut (or even normal gut) sutures in Argentina, which is currently experiencing difficulties with imports, so we have been using silk sutures for the ligation. As this was not a satisfactory alternative, we went to the spared nerve injury model with transection and ligation. If the chromic gut is essential, I will have to make arrangements.

Maral, I was unaware of the dietary effect and I will certainly look into that. As Argentina is a major soy producer, it is quite likely that our chow is soy-based.

Renee, we have principally used partial injuries (crush axonotmesis and different ligation protocols) to the sciatic nerve at mid-thigh level, after exit of the pelvic nerve and before trifurcation into the lower limb branches. We also tried transection of the peroneal and tibial branches, leaving the sural branch intact. The test is on a hot/cold plate, so the stimulus is on the base of the paw. While the operated animals do alter their stance, they do respond to both heat and cold - just not more quickly than the control animals.

We're finally getting hyperalgesia! I can't say which was the important variable as we changed chow, acquired chromic catgut, made 4 loose ligatures our standard and looked at stance during the plate tests with both hot and cold - so thanks to all for the help.