Mohammad Hassan Karimfar et al. 2009. Low Dose Morphine Enhances Morphine Antinociception Effectsin the Animals Pretreated with Selective and Non-Selective Phosphodiestrase Inhibitors: Effects of phosphodiesterase inhibitors on morphine antinociception. Iranian Journal of Pharmaceutical Sciences. 5, 2 (Apr. 2009), 73–81. DOI:https://doi.org/10.22037/ijps.v5.41189.