Figure 3. Up-regulation of GABA_B2 receptor expression in superficial laminae of the spinal cord after chronic oxycodone treatment.

A–C: Immunofluorescent staining showing GABA_B2 receptor labelling in the dorsal horn in (A) vincristine-saline, (B) vincristine-morphine, and (C) vincristine-oxycodone treated animals. D: Analysis of the fluorescence intensity profiles of GABA_B2 receptor immunoreactivity from superficial to deeper layers in control/vincristine treated animals chronically treated with saline, morphine or oxycodone. The selected zone was a vertical area in laminae I through IV (white selection in A). The insert shows the mean fluorescent intensity ± SEM in the three groups of vincristine-treated rats as mean ± SEM. For ease of reading, it was not possible to show the SEM of all groups. E: Mean fluorescent intensity of GABA_B2 receptor immonoreactivity at the level of the first 50 μm of the spinal cord dorsal horn. Vincristine treatment did not alter the intensity of GABA_B2 receptor staining as compared to saline-treated animals. However, oxycodone (but not morphine) significantly increased the intensity of GABA_B2 receptor staining in the superficial lamina. (n = 4 vincristine-morphine treated rats; n = 4 saline-morphine treated rats), (n = 4 vincristine-oxycodone treated rats; n = 4 saline-oxycodone treated rats), and (n = 4 vincristine-saline treated rats; n = 4 saline-saline treated rats) *p<0.05 and **p<0.01 vincristine-oxycodone vs. vincristine-morphine. Data are expressed as mean ± SEM.