Ultra-low dose naltrexone attenuates morphine-induced astrocyte hypertrophy. (A) Representative three dimensional images of astrocytes 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). Morphine treatment produced a significant increase in astrocytic cell volume as compared with saline-treated and naltrexone-treated controls. Co-administration of ultra-low dose naltrexone with morphine attenuated this hypertrophy; astrocytes have significantly smaller volumes as compared to morphine only treatment (### = p < 0.001). Data represent means ± s.e.m. for n = 12-24 cells per rat from n = 5 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, *** = p < 0.001. Scale bar, 30 μm.