For studies of the contribution of primary afferent neurones to neuropathic pain states, an animal model that shows reproducible signs of neuropathic pain, namely mechanical allodynia, thermal hyperalgesia and spontaneous pain, is required. A commonly used model of spinal nerve injury involves tight ligation or cut of the entire L5 spinal nerve (SN) close to the dorsal root ganglion (DRG) (developed from that of Kim & Chung, 1992). However, resulting measures of pain-related behaviour are variable. Lodge et al. (1999) suggested that this might be due to (uncontrolled) damage to the L4 SN, which runs very close to the L5 SN. We have therefore tested whether a controlled injury to L4, in addition to L5 ligation and cut, leads to reduced variability and greater effectiveness of the model.

Under sodium pentobarbitone anaesthesia (40 mg kg^-1, I.P.) one of four surgical procedures was carried out on day 0 on female Wistar rats (140Ð150 g). (1) L5 + L4, n = 9: the left L5 SN was tightly ligated (silk suture 6-0, Ethicon) and cut proximal to its junction with L4 SN. A chromic cat-gut (5-0, Ethicon or Look¨) loop was then placed around the left L4 SN with a loop diameter at least 2 mm greater than that of the nerve. (2) L4, n = 8: the left L4 SN had a loose ligation as above but the left L5 SN was intact. (3) L5, n = 8: the left L5 SN was ligated and cut as described above. L4 SN was not ligated. (4) Sham, n = 8: the left L4 and L5 SNs were exposed but otherwise left intact. Behavioural tests were performed from day -3 pre-surgery (pre) to day 7 post-surgery (post). All experimental protocols were carried out under licence according to the UK Animals (Scientific Procedures) Act of 1986.

The L4 and sham groups showed no significant changes between ipsilateral and contralateral hindpaw in all tests. In both L5 + L4 and L5 groups, in all tests, the ipsilateral hindpaw became more sensitive (Friedman test). Means from pre to 7 days post for L5 and L5 + L4 groups, respectively, were as follows: mechanical allodynia (threshold to normally innocuous von Frey hairs) L5: 19.8 g (pre) to 6.9 g (post), P < 0.05; L5 + L4: 23.2 to 1.7g, P < 0.001; thermal hyperalgesia (withdrawal latency to noxious heat) L5: 16.5 to 9.8 s, P < 0.01; L5 + L4: 15.6 to 6.4 s, P < 0.001 and spontaneous pain-like behaviour (spontaneous foot liftings in a 5 min period): L5: 0 to 0.8 s, P > 0.05; L5 + L4: 0 to 9.7 s, P < 0.01. Thus for L5 + L4 the changes were larger, also the variability was smaller than in the L5 only group in all tests.

All animals were humanely killed at the end of the experiemnts.

This work was supported by a Wellcome Trust grant to S.N. Lawson; S. Koutsikou is supported by a Bristol University scholarship.

All procedures accord with current UK legislation.