Effects of chronic morphine infusion on the expression of µ-opioid receptor at the dorsal reticular nucleus. Representative photomicrographs of µ-opioid receptor–immunoreactive cells at the dorsal reticular nucleus of saline- (A) or morphine- (B) infused animals. Typical µ-opioid receptor immunolabeling is marked by arrows. Scale bar in panel B, 100 µm (A is at the same magnification). The numbers of immunoreactive cells in morphine-infused were significantly higher compared with saline-infused animals (n = 6 per group; t₁₀ = 3.6, P = 0.005; C). No differences were found in µ-opioid receptor messenger RNA levels between saline- and morphine-infused animals (3 samples per group and n = 3 animals each sample; t₄ = 1.7; P = 0.167; D). The data in D are presented as relative differences to saline-infused animals. Data in C and D are presented as mean ± SD.