“…Due to similar changes in f R and V T , each dose of fentanyl produced qualitatively similar changes in V E consisting of rapid and large decreases followed by a recovery period and then a rise to about 50% above pre-injection values. While the ability of opioids, such as morphine, to depress breathing and the mechanisms by which this occurs have been extensively studied ( Lalley, 2008 ; Pattinson, 2008 ; Dahan et al, 2010 ; Pattinson and Wise, 2016 ; Baby et al, 2018 ; Dahan et al, 2018 ; Bateman et al, 2021 ; Baby et al, 2021a ; Baby et al, 2021b ; Ramirez et al, 2021 ), the sites/mechanisms of action by which fentanyl exerts its cardiorespiratory and antinociceptive effects are less well understood ( Laubie et al, 1977 ; Fone and Wilson, 1986 ; Mayer et al, 1989 ; Yamakura et al, 1999 ; Lalley, 2003 ; Maegawa and Tonussi, 2003 ; Griffioen et al, 2004 ; Mastronicola et al, 2004 ; Nikolaishvili et al, 2004 ; Hajiha et al, 2009 ; Tschirhart et al, 2019 ; Webster and Rauck, 2021 ; Ramos-Matos et al, 2022 ). Mechanisms of action of fentanyl involve activation of μ-ORs in brainstem nuclei controlling cardiorespiratory and nociceptive functions ( Laubie et al, 1977 ; Fone and Wilson, 1986 ; Lalley, 2003 ; Griffioen et al, 2004 ; Nikolaishvili, et al, 2004 ; Hajiha, et al, 2009 ; Webster and Rauck, 2021 ; Ramos-Matos et al, 2022 ), and in peripheral structures, including the carotid body ( Mayer et al, 1989 ; Henderson et al, 2014 ; Tschirhart et al, 2019 ; Ramos-Matos et al, 2022 ).…”