Role of different peripheral components in the expression of neuropathic pain syndrome

Peripheral nerve injury frequently leads to neuropathic pain like hyperalgesia, spontaneous pain, mechanical allodynia, thermal allodynia. It is uncertain where the neuropathic pain originates and how it is transmitted to the central nervous system. This study was performed in order to determine which peripheral component may lead to the symptoms of neuropathic pain. Under halothane anesthesia, male Sprague-Dawley rats were subjected to neuropathic surgery by tightly ligating and cutting the tibial and sural nerves and leaving the common peroneal nerve intact. Behavioral tests for mechanical allodynia, thermal allodynia, and spontaneous pain were performed for 2 weeks postoperatively. Subsequently, second operation was performed as follows: in experiment 1, the neuroma was removed; in experiment 2, the dorsal roots of the L4-L6 spinal segments were cut; in experiment 3, the dorsal roots of the L2-L6 spinal segments were cut. Behavioral tests were performed for 4 weeks after the second operation. Following the removal of the neuroma, neuropathic pain remained in experiment 1. After the cutting of the L4-L6 or L2-L6 dorsal roots, neuropathic pain was reduced in experiments 2 and 3. The most remarkable relief was seen after the cutting of the L2-L6 dorsal roots in experiment 3. According to the fact that the sciatic nerve is composed of the L4-L6 spinal nerves and the femoral nerve is composed of the L2-L4 spinal nerves, neuropathic pain is transmitted to the central nervous system via not only the injured nerves but also adjacent intact nerves. These results also suggest that the dorsal root ganglion is very important in the development of neuropathic pain syndrome.