Neomycin and gadolinium applied to an L5 spinal nerve lesion prevent mechanical allodynia-like behaviour in rats.

In male Wistar rats, the left ventral ramus of the L5 spinal nerve (RvL5) was cut, and animals were tested for allodynia-like behaviour in response to mechanical stimuli which were applied with von Frey hairs (4.3-205 mN) to the plantar skin of the hindpaw supplied by the intact L4 spinal nerve. After surgery, allodynia-like behaviour was evoked prominently on the operated (left) side and weakly on the contralateral (right) side. Eleven sham-operated rats displayed low levels of allodynia-like behaviour on the operated side comparable to that seen contralateral to the lesion in nerve-transected animals. In 14 rats, allodynic behaviour to mechanical stimuli on either side was dose dependently and permanently reduced, after 4-32 mg of the antibiotic Nebacetin had been applied locally at the transection site immediately after cutting the nerve. The same effect was observed when one of the active compounds of Nebacetin, neomycin, but not the other, bacitracin, was applied by continuous local infusion. Allodynia-like behaviour to stimuli between 4.3 mN and 124 mN was also prevented when 1-8 mg gadolinium acetate was applied at the transection site, whereas allodynic behaviour to stimuli of 205 mN was not affected at least for the first 7 days after the lesion. Both gadolinium and Nebacetin failed to prevent allodynic behaviour when applied later than 7 h after the nerve lesion. Neither substance interfered with the development of tactile allodynia when applied to the nerve rostral to the lesion site. The results suggest that Nebacetin and gadolinium interfere with the mechanisms at the RvL5 transection site crucial for the initiation of sensitisation of dorsal horn neurones and the development of mechanical allodynia.