Hubert V. Forster

Here we determined whether any of the deleterious physiological effects of high-dose fentanyl could be countered by naloxone and/or d-cysteine ethyl ester (d-CYSee) in adult goats. Fentanyl induced hypoventilation and sustained increases in respiratory and airway muscle activity and hypoxemia. Naloxone reversed all fentanyl effects except tonic muscle activation, and d-CYSee reversed fentanyl-induced hypoventilation. These data suggest some effects of fentanyl are naloxone-insensitive and that d-CYSee may be a valuable countermeasure for OIRD.

Here, we defined the time course of integrated physiologic effects of high doses of fentanyl in adult goats. High-dose fentanyl caused hypoventilation and sustained increases in tonic respiratory-related chest wall and airway muscle activity, increased the alveolar-to-arterial O ₂ gradient, and caused dose-dependent increases in acute withdrawal-like behaviors. These data suggest that the integrated physiologic responses to fentanyl in adult goats closely resemble those in humans.

Chronic hypercapnia elicited time- and region-dependent tissue-level transcriptomic responses in brainstem regions involved in cardiorespiratory control. At 3 h, differentially expressed genes (DEGs) were enriched in vascular- and endothelial-associated GO terms. At 24 h, mitochondria-associated transcripts were downregulated within the NTS/DMV and VRC. By 7 days, few DEGs were detected. These findings reveal novel, temporally distinct transcriptional responses to CH and identify vascular- and mitochondria-related pathways for future studies of physiological acclimatization to chronic hypercapnia.

Background/Objectives: Opioids inhibit breathing that can lead to fatal overdose, highlighting the need for testing effective countermeasure agents and potential mechanisms of action. Here we examined the role muscarinic cholinergic receptors play in the ability of L-cysteine ethyl ester (L-CYSee) to overcome the deleterious effects of morphine on ventilatory parameters in male Sprague Dawley rats and the ventilatory responses during a subsequent hypoxic-hypercapnic (HH) challenge. Methods: Ventilatory parameters were measured by whole body plethysmography. Results: The injection of the muscarinic receptor antagonist, atropine (1.0 mg/kg, IV), elicited an array of ventilatory responses (e.g., an increase in frequency of breathing coupled with a fall in tidal volume). Injection of morphine (10 mg/kg, IV) to vehicle-treated rats elicited a depression of breathing, including sustained decreases in tidal volume, minute ventilation, peak inspiratory flow, and peak inspiratory and expiratory drives, which were associated with marked increases in end inspiratory pause (EIP) and end expiratory pause (EEP), expiratory flow at 50% expired tidal volume (EF₅₀), and rate of achieving peak expiratory flow (Rpef). Most effects of morphine (10 mg/kg, IV) were not altered in atropine-treated rats, except that increases in EIP, EEP and Rpef were reduced. Subsequent injections of L-CYSee (2 × 500 μmol/kg, IV given 15 min apart) overcame the adverse actions of morphine on ventilatory parameters in vehicle-treated rats. The effects of L-CYSee, such as reversal of the effects of morphine on frequency of breathing, tidal volume and minute ventilation, were markedly reduced in atropine-treated rats. The ability of L-CYSee to reverse the adverse effects of morphine to a HH gas challenge was markedly diminished in atropine-treated rats. Conclusions: These findings demonstrate that muscarinic receptors play a vital role in the ability of L-CYSee to overcome the deleterious effects of morphine.

Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Moderate levels of chronic hypercapnia (CH; ∼70 mmHg) in healthy adult female goats elicited similar steady-state physiological adaptations compared with mild CH (∼55 mmHg). However, unlike mild CH, moderate CH chronically suppressed acute CO₂/[H⁺] chemosensitivity and reversed the arterial to mixed expired CO₂ gradient. These findings suggest that moderate CH suppresses vital mechanisms of ventilatory control and recruits additional physiological systems (i.e., gastric CO₂ release) to help buffer excess CO₂.

In healthy adult goats, steady-state ventilation and most physiological measures return to control within 24 h after termination of chronic hypercapnia (CH). However, the acute [H⁺] chemoreflex is increased, and measured ventilation exceeds predicted ventilation. At 24 h of recovery, excitatory neuromodulators are above control, but other measured markers of neuroplasticity are unchanged from control. Our data suggest that CH elicits persistent physiological and neurochemical changes for up to 24 h after termination of CH.

Despite the prevalence of CO ₂ retention in human disease, little is known about the adaptive neurobiological effects of chronic hypercapnia. We have recently shown 30‐d exposure to increased inspired CO ₂ (InCO ₂ ) leads to a steady‐state ventilation that exceeds the level predicted by the sustained acidosis and the acute CO ₂ /H ⁺ chemoreflex, suggesting plasticity within respiratory control centers. Based on data showing brainstem changes in aminergic and inflammatory signaling during carotid body denervation‐induced hypercapnia, we hypothesized chronic hypercapnia per se will lead to similar changes. We found that: 1 ) increased InCO ₂ increased IL‐1β in the medullary raphe (MR), ventral respiratory column, and cuneate nucleus after 24 h, but not after 30 d of hypercapnia; 2 ) the number of serotonergic and total neurons were reduced within the MR and ventrolateral medulla following 30 d of increased InCO ₂ ; 3 ) markers of tryptophan metabolism were altered following 24 h, but not 30 d of InCO ₂ ; and 4 ) there were few changes in brainstem amine levels following 24 h or 30 d of increased InCO ₂ . We conclude that these changes may contribute to initiating or maintaining respiratory neuroplasticity during chronic hypercapnia but alone do not account for ventilatory acclimatization to chronic increased InCO ₂ .—Burgraff, N. J., Neumueller, S. E., Buchholz, K. J., LeClaire, J., Hodges, M. R., Pan, L., Forster, H. V. Brainstem serotonergic, catecholaminergic, and inflammatory adaptations during chronic hypercapnia in goats. FASEB J. 33, 14491‐14505 (2019). www.fasebj.org