Values are luminescence intensity ± SEM from 3 to 5 independent experiments at 18 h post-infection (*p < 0.005 vs. un-infected cells; $p < 0.05 vs. R5 HIV-1; #p < 0.05 vs. R5 + morphine (M); ¶p < 0.05 vs. R5 + maraviroc (MVC); §p < 0.05 vs. R5 + M + MVC; ¥p < 0.05 vs. R5 + M + MVC + naltrexone). (a–b) Modified and reprinted with permission from Arnatt et al. (2016). (c–e) Modified and reprinted with permission from El-Hage et al. (2013)