Role of astrocyte activation in midbrain periaqueductal gray in a rat model of neuropathic pain

Objective To investigate the role of astrocyte activation in the midbrain periaqueductal gray (PAG) in a rat model of neuropathic pain. Methods A total of 72 male Sprague-Dawley rats, aged 9 weeks, weighing 200-250 g, were randomly divided into 5 groups using a random number table: normal control group (group C, n=8), sham operation group (group S, n=8), neuropathic pain group (group NP, n=32), normal saline control group (group NS, n=12), and fluorocitrate (an astrocyte inhibitor) group (group FC, n=12). Neuropathic pain was induced by chronic constriction injury (CCI) to the sciatic nerve in NP, NS and FC groups.In group FC, 10 pmol fluorocitrate 0.5 μl was injected into the nuclei of the ventrolateral PAG on day 14 after CCI, and the equal volume of normal saline was given instead in group NS.Eight rats in C and S groups, 8 rats before CCI, on days 3, 7, 14 and 21 after CCI in group NP, and 8 rats in NS and FC groups were selected, and the mechanical pain threshold was measured before CCI, on day 14 after CCI and at 30, 45, 60, 75, 90 and 105 min after administration.After measurement of the mechanical pain threshold, the rats were sacrificed, and the midbrain PAG was isolated for determination of the expression of astrocyte marker glial fibrillary acidic protein by immunofluorescence. Results Compared with group C, the mechanical pain threshold was significantly decreased on days 3, 7, 14 and 21 after CCI, the number of activated astrocytes in the PAG was significantly increased on days 7, 14 and 21 after CCI in group NP (P 0.05). Compared with group NS, the mechanical pain threshold at each time point after adiministration was significantly increased, and the number of activated astrocytes in the PAG was significantly decreased in group FC (P<0.01). Conclusion Astrocyte activation in the midbrain PAG is involved in the maintenance of neuropathic pain in rats. Key words: Astrocytes; Cerebral aqueduct; Periaqueductal gray; Neuralgia