Fentanyl-Induced Respiratory Depression and Locomotor Hyperactivity Are Mediated by μ-Opioid Receptors Expressed in Somatostatin-Negative Neurons

Opioid drugs are widely used as analgesics but cause respiratory depression, a potentially lethal side effect with overdose, by acting on μ-opioid receptors (MORs) expressed in brainstem regions involved in the control of breathing. Although many brainstem regions have been shown to regulate opioid-induced respiratory depression, the types of neurons involved have not been identified. Somatostatin is a major neuropeptide found in brainstem circuits regulating breathing, but it is unknown whether somatostatin-expressing circuits regulate respiratory depression by opioids. We examined the coexpression of Sst (gene encoding somatostatin) and Oprm1 (gene encoding MORs) mRNAs in brainstem regions involved in respiratory depression. Interestingly, Oprm1 mRNA expression was found in the majority (>50%) of Sst-expressing cells in the preBötzinger Complex, the nucleus tractus solitarius, the nucleus ambiguus, and the Kölliker–Fuse nucleus. We then compared respiratory responses to fentanyl between wild-type and Oprm1 full knock-out mice and found that the lack of MORs prevented respiratory rate depression from occurring. Next, using transgenic knock-out mice lacking functional MORs specifically in Sst-expressing cells, we compared respiratory responses to fentanyl between control and the conditional knock-out mice. We found that respiratory rate depression by fentanyl was preserved when MORs were deleted only in Sst-expressing cells. Our results show that despite coexpression of Sst and Oprm1 in respiratory circuits and the importance of somatostatin-expressing cells in the regulation of breathing, these cells do not mediate opioid-induced respiratory rate depression. Instead, MORs found in respiratory cell populations other than Sst-expressing cells likely contribute to the respiratory effects of fentanyl.