Intensity of microglial labelling in the dorsal lumbar spinal cord of rats. (A) Representative photomicrographs acquired by confocal microscopy of spinal cord sections labelled for the microglial marker, CD3/CD11B (OX42). Spinal cord sections were collected from rats receiving intrathecal vehicle (saline; SAL) (i), morphine (15 μg; MS) (ii), morphine and naltrexone (5 ng; MS+NTX) (iii), or naltrexone (NTX) alone (iv). Photomicrographs were converted to gray scale and then analyzed to obtain mean gray values. Morphine treatment produced a significant increase in the amount of OX42 labelling as compared with saline control. Attenuation of increased OX42 immuno-labelling was observed in animals co-administered ultra-low dose naltrexone with morphine (### = P < 0.001 compared to morphine treatment). Naltrexone alone had no significant effect on OX42 immuno-labelling compared to saline control (P > 0.05). Data represent means ± s.e.m. for n = 6-8 sections per rat from n = 3-6 per group. Statistical analyses were performed by a one-way ANOVA followed by Tukey's post-hoc multiple comparison test. The asterisk denotes significant difference from saline-treated rats, * = P < 0.05.