Sheikholeslami, B.; Hamidi, M.; Lavasani, H.; Sharifzadeh, M.; Rouini, M. Lack of Evidence for Involvement of P-Glycoprotein in Brain Uptake of the Centrally Acting Analgesic, Tramadol in the Rat. J Pharm Pharm Sci 2012, 15, 606-615.