Considering the role of Tac1-expressing cells in rhythmic breathing and knowing that the preBötC plays a major part in respiratory depression by opioid drugs (Montandon et al., 2011; Stucke et al., 2015), we determined whether Tac1 mRNA is co-expressed with Oprm1 (gene for MORs) mRNA in the preBötC. Here, OPRM1 is linked to respiratory depression.