Fentanyl, but not Morphine, Increases Cortical Electroencephalographic (EEG) Delta Power during Wakefulness in C57BL/6J (B6) Mice

Cortical EEG signals comprise differing oscillations that represent lower-level traits characteristic of higher-level behavioral phenotypes such as sleep and wakefulness (Physiology 32:60, 2017). Opioid-induced changes in neurobehavioral function also are reflected in altered EEG frequency and amplitude. These EEG features have the potential to provide insights regarding the desired actions and undesired side effects of opioids. In rat, morphine (Anesthesiology 103:779, 2005) and fentanyl (Sci Rep 9:14122, 2019) increase EEG slow waves. Studies of mice provide the additional opportunity to use mutated or transgenic animals with features potentially relevant to respiratory depression or addiction. This study is testing the hypothesis that subcutaneous injection of fentanyl and morphine causes dose-dependent EEG changes in wild-type mice. Experiments were conducted in accordance with the FASEB Statement of Principles for the use of Animals in Research and Education. Adult male B6 mice (n = 4) were implanted with telemeters (DSI HD-X02) for wireless recording of cortical EEG and neck muscle electromyogram. The recordings were used to quantify the effects of opioids on sleep and wakefulness. These studies demonstrated dose-dependent sleep disruption by both opioids (see companion abstract by Zebadua Unzaga et al.). EEG data were digitized by DSI Ponemah software and analyzed using public domain software for creating multi-tapered EEG spectrograms (J Vis Exp 153:e60333, 2019). Analyses were based on EEG data from the first 30 min after opioid injection, when all mice were awake. EEG power was expressed as percent change relative to saline (vehicle control) for six frequency bands: delta (0.5 to 4 Hz), theta (4 to 8 Hz), alpha (8 to 13 Hz), sigma (12 to 15 Hz), beta (13 to 30 Hz), and gamma (30 to 60 Hz). Increasing half log doses (mg/kg) ranged from 0.001 to 3 for fentanyl and 0.01 to 30 for morphine. Repeated measures ANOVA with the Geisser-Greenhouse correction showed a significant dose main-effect of fentanyl on EEG delta power (F(1.857, 5.571) = 7.454, p = 0.0276) and alpha power (F(1.705, 5.116) = 6.316, p = 0.0439). For fentanyl (3 mg/kg) compared to saline, Cohen's d = 2.9 for delta power. By contrast, for morphine there was a significant main effect of dose only for EEG sigma power (F(2.162, 6.486) = 6.557, p = 0.0265). Comparing morphine (30 mg/kg) to saline, the Cohen's d = 1.1 for sigma power. Although both fentanyl and morphine are full mu-opioid receptor agonists, they significantly altered EEG power at different frequency bands. Fentanyl, but not morphine, blocks alpha-1 adrenergic receptor subtypes and vesicular reuptake of several monoamine transmitters (J Pharmacol Exp Ther 343:376, 2020). These results support the interpretation that the differential effects of fentanyl and morphine on EEG power may be due, in part, to non-mu-opioid receptor of fentanyl.