Opioid-Induced Respiratory Depression in C57BL/6J Mice: The Role of Prefrontal Cortex Cholinergic Transmission in the Wakefulness Stimulus for Breathing

Opioid-induced respiratory depression (OIRD) is the primary cause of death from opioid overdose. Opioids depress breathing by diminishing the wakefulness stimulus for breathing in humans and mice. Cholinergic transmission in the prefrontal cortex (PFC) promotes cortical EEG activation, increases wakefulness, and stimulates breathing. However, no previous studies have tested whether increasing cholinergic transmission in the PFC can mitigate respiratory depression caused by systemically administered fentanyl in mice. The series of studies comprising this honors thesis is split into two phases. The first phase included a concentration response study evaluating the effects of fentanyl on breathing in C57BL/6J mice breathing room air. Adult male mice (n = 12) received intraperitoneal (IP) injections of saline and fentanyl in increasing half log unit doses. The second phase tested the hypothesis that microinjection of the acetylcholinesterase inhibitor neostigmine into the PFC attenuates OIRD caused by systemic fentanyl administration in male B6 mice. Adult male mice (n = 4) were surgically implanted with microinjection guide tubes aimed at the medial prefrontal cortex. Subjects were given a systemic injection of either saline or fentanyl, followed by a microinjection of saline or neostigmine. Preliminary data show that neostigmine mitigated the effects of fentanyl on breathing and significantly slowed the onset of OIRD. Histology pending, these results support the interpretation that cholinergic transmission in the PFC contributes to the wakefulness stimulus for breathing, extend previous data showing that breathing is stimulated by PFC neostigmine, and demonstrate that enhancing PFC cholinergic transmission offsets OIRD caused by systemically administered fentanyl.

This thesis was submitted in fulfillment of the Psychology Honors Concentration at UTK and supervised by Dr. Ralph Lydic and Dr. Helen Baghdoyan.