The Counteraction of Opioid-Induced Ventilatory Depression by the Serotonin 1A-Agonist 8-OH-DPAT Does Not Antagonize Antinociception in Rats In Situ and In Vivo

Spontaneous breathing during mechanical ventilation is gaining increasing importance during intensive care but is depressed by narcotics, such as opioids. Serotonin 1A-receptor (5-HT(1A)-R) agonists have been shown to antagonize opioid-induced ventilatory depression, but both enhancement and attenuation of nociceptive reflexes have been found with different experimental models. To clarify contradictory findings, we simultaneously determined dose-response functions of the standard 5-HT(1A)-R-agonist 8-OH-DPAT and two different opioids for spontaneous ventilation and nociception. Two hypotheses were tested: 1) 8-OH-DPAT at a dose to stimulate spontaneous breathing does not activate nociceptive reflexes. 2) 8-OH-DPAT does not diminish opioid-induced antinociception.(A) A dose-response relationship of 8-OH-DPAT, spontaneous phrenic nerve activity and a nociceptive C-fiber reflex (CFR) were established simultaneously in an in situ perfused, nonanesthetized, rat brainstem-spinal cord preparation. (B) Fentanyl was administered in situ to investigate the interaction with 8-OH-DPAT on phrenic nerve activity and nociceptive CFR. Additional experiments involved the selective 5-HT(1A)-R-antagonist WAY 100 635 to exclude effects of receptors other than 5-HT(1A)-R. (C) The effects of 8-OH-DPAT on spontaneous ventilation and nociceptive tail-flick reflex with and without morphine were verified in in vivo anesthetized rats.Low-dose 8-OH-DPAT (0.001 and 0.01 microM in situ, 0.1 microg/kg in vivo) enhanced nociceptive reflexes but did not activate spontaneous ventilation. On the contrary, high doses of 8-OH-DPAT (1 microM in situ and 10-100 microg/kg in vivo) stimulated ventilation, whereas nociceptive CFR amplitude in situ returned to baseline and tail-flick reflex was depressed in vivo. Opioid-induced ventilatory depression was antagonized by 8-OH-DPAT (1 microM in situ, and 10 microg/kg in vivo), whereas antinociception sustained. Selective 5-HT(1A)-R-antagonist WAY 100 635 (1 microM) prevented the effects of 8-OH-DPAT in situ.5-HT(1A)-R-agonist 8-OH-DPAT activates spontaneous breathing without diminishing opioid-induced antinociception in rats.