OPRM1 and respiratory depression: Because most Tac1 preBötC cells co-expressed μ-opioid receptors (encoded by the gene Oprm1) in the preBötC, their stimulation reversed the effects of opioid drugs on breathing, suggesting that Tac1 preBötC neurons constitute a robust excitatory neuronal population promoting breathing during calm states and that can overcome respiratory depression with narcotics.