Gain Modulation by Mouse Prefrontal Cortex Diminishes Fentanyl-Induced Respiratory Depression

Abstract

<b>Abstract ID 53000</b> <b>Poster Board 139</b> Breathing is generated by pontomedullary brain stem nuclei and modulated by limbic and cortical nuclei. Recent data show that neostigmine delivered into the prefrontal cortex (PFC) significantly modulates breathing of intact, behaving C57BL/6J (B6) mice (10.1016/j.resp.2022.103924). Although the PFC contains no respiratory neurons, it projects to brain stem neurons that regulate arousal and breathing. This study is evaluating the hypothesis that PFC neostigmine enhances gain modulation of breathing. Gain modulation is the process by which the responsiveness of a neural circuit or neuron is altered by activity of other neurons. These studies were approved by University of Tennessee IACUC and adhered to the ARRIVE guidelines. Mice (n = 7) were implanted with a 26-gauge guide cannula aimed at the medial PFC (histology pending). A week after surgical recovery, whole body plethysmography was used to measure breathing during wakefulness after three treatment conditions: 1) Intraperitoneal (ip) administration of saline (control) plus PFC microinjection of saline (black), 2) ip fentanyl (0.1 mg/kg) plus PFC saline (red), and 3) ip fentanyl (0.1 mg/kg) plus PFC neostigmine (0.016 nmol/50 nL) (green). As anticipated, fentanyl decreased respiratory frequency (Fig. 1A). PFC neostigmine caused an increase in tidal volume (Fig. 1B). PFC neostigmine also significantly (*P &lt; 0.05) diminished the depression in minute ventilation caused by fentanyl (Fig. 1C). These results were then quantified using a previous pharmacological model of gain modulation (10.1016/s1569-9048(02)00042-3). Gain was expressed as <i>F_o = (1 + F_tonick)F_i</i>. In this equation, <i>F_i</i> indicates pre- and <i>F_o</i> post-neostigmine measures of breathing, <i>F_tonic</i> represents baseline respiratory frequency, and (<i>k</i>) is a unitless multiplicative modulation coefficient. Fig. 1D shows time-dependent differences in respiratory frequency among three injection conditions via linear regression analysis. Minute 1 indicates recording onset after PFC injection. Trendline slopes (dashed lines) for frequency were used to determine treatment condition-dependent <i>k</i> values for condition 1 (-3.9), condition 2 (-7.2), and condition 3 (-4.8). Fig. 1E shows similar analyses applied to measures of respiratory duty cycle (inspiratory duration divided by duration of inspiration + expiration) considered to reflect respiratory effort. Fig. 1E shows that duty cycle was modulated logarithmically with multiplicative coefficient <i>k</i> = -0.030 when the PFC was injected with saline and <i>k</i> = -0.018 when the PFC was injected with neostigmine. This inverse exponential slope change reveals a 60% relative scalar increase in duty cycle due to PFC neostigmine administration. R² values show a high percent of variance in duty cycle modulation accounted for by treatment conditions 2 (77%) and 3 (68%). These results support the interpretation that PFC neostigmine administration influences breathing, in part, by enhancing the gain of PFC input to brain stem nuclei that regulate breathing (10.1152/jn.00017.2021). Support: The University of Tennessee