Pharmacological Modulation of Respiratory Circuits

ME ([Met⁵]-enkephalin acetate salt), bestatin, DL-thiorphan, strychnine, picrotoxin, DNQX, 4-aminopyridine (4AP), and MK801 were from Sigma-Aldrich (St Louis, MO). Tetrodotoxin and ML-297 was from Tocris Bio-Techne (Minneapolis, MN). All drugs were applied by bath perfusion. bestatin (10 µM) and thiorphan (1 µM) were included with ME to prevent degradation. ME is an endogenous opioid peptide agonist for mu and delta opioid receptors. Delta opioid receptors are not expressed in KF or preBötC neurons (Varga et al., 2020 (link)) and do not cause opioid-induced respiratory depression (Dahan et al., 2001 (link)). An EC80 concentration of ME (1–3 µM) was used to ensure robust and reliable responses but avoid acute receptor desensitization that occurs with higher concentrations (Levitt and Williams, 2018 (link)).