Mechanical allodynia and spinal up-regulation of P2X4 receptor in experimental autoimmune neuritis rats.

Experimental autoimmune neuritis (EAN) is the animal model of acute inflammatory demyelinating polyradiculoneuropathy (AIDP) that is the most common subtype of Guillain-Barre syndrome (GBS). While neuropathic pain is a common symptom of GBS, its underlying mechanisms remain elusive. Central sensitization, particularly spinal glia (microglia and astrocytes) activation, is important for the initiation and maintenance of neuropathic pain. P2X(4) receptor (P2X(4)R) is an ATP-gated ion channel and its spinal up-regulation has been found to be crucial for the development of neuropathic pain following peripheral nerve injury. The initiation of mechanical allodynia in rat EAN was observed at day 9 before the onset of neurological signs. Maximal level of mechanical allodynia was observed from days 17-19 and then a slow recovery, long after the cessation of typical neurological signs of EAN, until day 37 was observed. Expression of P2X(4)R in lumbar spinal cords was studied by immunohistochemistry. P2X(4)R immunoreactivity (IR) was mainly seen in gray matter, particularly in the dorsal horn. Accumulation of P2X(4)R(+) cells in the lumbar dorsal horn was observed at day 9, reached the maximal level at day 17 and remained elevated until day 37 after immunization. Furthermore, a negative correlation between the density of P2X(4)R(+) cells in the lumbar dorsal horn with mean hind-paw withdrawal threshold in EAN rats was seen, indicating that P2X(4)R might contribute to EAN mechanical allodynia. Double staining revealed that almost all P2X(4)R(+) cells co-expressed CD68, a marker for reactive microglia, but not the astrocyte marker, glial fibrillary acidic protein (GFAP). Our data demonstrate that EAN induces mechanical allodynia and P2X(4)R expression in spinal microglia, suggesting that EAN is a good animal model for neuropathic pain in polyneuropathy and spinal microglia activation might participate in EAN-induced neuropathic pain.