The localization of the motor neurons innervating the cricothyroid muscle in the adult dog by the fluorescent retrograde axonal labeling technique

Noradrenergic Mechanisms in Fentanyl-Mediated Rapid Death Explain Failure of Naloxone in the Opioid Crisis

In December 2018, the Centers for Disease Control declared fentanyl the deadliest drug in America. Opioid overdose is the single greatest cause of death in the United States adult population (ages 18-50), and fentanyl and its analogs [fentanyl/fentanyl analogs (F/FAs)] are currently involved in >50% of these deaths. Anesthesiologists in the United States were introduced to fentanyl in the early 1970s when it revolutionized surgical anesthesia by combining profound analgesia with hemodynamic stability. However, they quickly had to master its unique side effect. F/FAs can produce profound rigidity in the diaphragm, chest wall and upper airway within an extremely narrow dosing range. This clinical effect was called wooden chest syndrome (WCS) by anesthesiologists and is not commonly known outside of anesthesiology or to clinicians or researchers in addiction research/medicine. WCS is almost routinely fatal without expert airway management. This review provides relevant clinical human pharmacology and animal data demonstrating that the significant increase in the number of F/FA-induced deaths may involve α-adrenergic and cholinergic receptor-mediated mechanical failure of the respiratory and cardiovascular systems with rapid development of rigidity and airway closure. Although morphine and its prodrug, heroin, can cause mild rigidity in abdominal muscles at high doses, neither presents with the distinct and rapid respiratory failure seen with F/FA-induced WCS, separating F/FA overdose from the slower onset of respiratory depression caused by morphine-derived alkaloids. This distinction has significant consequences for the design and implementation of new pharmacologic strategies to effectively prevent F/FA-induced death. SIGNIFICANCE STATEMENT: Deaths from fentanyl and F/FAs are increasing in spite of availability and awareness of the opioid reversal drug naloxone. This article reviews literature suggesting that naloxone may be ineffective against centrally mediated noradrenergic and cholinergic effects of F/FAs, which clinically manifest as severe muscle rigidity and airway compromise (e.g., wooden chest syndrome) that is rapid and distinct from respiratory depression seen with morphine-derived alkaloids. A physiologic model is proposed and implications for new drug development and treatment are discussed.

Expression of Sex Steroid Hormone Receptors in Vagal Motor Neurons Innervating the Trachea and Esophagus in Mouse

The medullary vagal motor nuclei, the nucleus ambiguus (NA) and dorsal motor nucleus of the vagus (DMV), innervate the respiratory and gastrointestinal tracts. We conducted immunohistochemical analysis of expression of the androgen receptor (AR) and estrogen receptor α (ERα), in relation to innervation of the trachea and esophagus via vagal motor nuclei in mice. AR and ERα were expressed in the rostral NA and in part of the DMV. Tracing experiments using cholera toxin B subunit demonstrated that neurons of vagal motor nuclei that innervate the trachea and esophagus express AR and ERα. There was no difference in expression of sex steroid hormone receptors between trachea- and esophagus-innervating neurons. These results suggest that sex steroid hormones may act on vagal motor nuclei via their receptors, thereby regulating functions of the trachea and esophagus.

The localization of motoneurons innervating the canine pharyngeal constrictor muscles in the posterior larynx by the fluorescent double-labeling technique

The localization of motoneurons innervating the canine pharyngeal constrictor muscles in the posterior larynx, i.e., the hyopharyngeal muscle (HP), the thyropharyngeal muscle (TP) and the cricopharyngeal muscle (CP), was investigated by the fluorescent labeling technique. Labeled cells were found in the rostral portion of the ipsilateral nucleus ambiguus. The rostral ends of these cell columns were located at nearly the same level. The caudal ends of the HP cell column and the TP cell column were also located at nearly the same level. However, the CP cell column was elongated a little more caudally. The HP cells and the TP cells were intermingled in the dorsal part of the nucleus. The CP cells were mainly located in the ventral part of the nucleus. No double-labeled cells were detected.

Nucleus ambiguus motoneurons innervating the canine intrinsic laryngeal muscles by the fluorescent labeling technique

The localization of motoneurons innervating the canine intrinsic laryngeal muscles was investigated by the fluorescent labeling technique. Labeled cells were found in the ipsilateral nucleus ambiguus. The most rostral labeled neurons for the cricothyroid muscle, the posterior cricoarytenoid muscle, the thyroarytenoid muscle, and the lateral cricoarytenoid muscle were found at progressively more caudal levels, respectively, within the nucleus ambiguus. The rostral tip of the arytenoid muscle cell column was at the same level as the lateral cricoarytenoid muscle cell column. The cells labeled from the cricothyroid muscle occupied the ventral part of the nucleus at the rostral level of the nucleus. At the middle level of the nucleus, the cells from the posterior cricoarytenoid muscle occupied the ventral part of the nucleus and the cells from the thyroarytenoid muscle, the lateral cricoarytenoid muscle and the arytenoid muscle occupied the dorsal part of the nucleus. The existence of double-labeled cells which innervated both thyroarytenoid muscle and lateral cricoarytenoid muscle was detected.

Localization of motoneurons innervating the levator veli palatini muscle in the cat

Recent investigations of the nucleus ambiguus (NA) have attempted to identify motoneurons associated with the branchiomeric muscles of the larynx and pharynx. However, relatively little attention has been directed to the levator veli palatini muscle (LVP) which is critical in respiration, deglutition and eustachian tube function. Although the consensus is that cranial nerve X (vagus) innervates this muscle, some investigators have suggested that the LVP is innervated by either cranial nerve VII (facial) or IX (glossopharyngeal). The present study was designed to identify the specific location of LVP motoneurons within the brainstem. Horseradish peroxidase (HRP) was injected into the LVP of 18 cats. Following a survival period of 24-48 hours, animals were sacrificed and tissue processed according to Mesulam's TMB procedure. HRP labeled cells were located in the rostral NA both ipsilateral and contralateral to the side of injection and in the ipsilateral retrofacial nucleus (RFN). There were no labeled cells in the facial nucleus. Innervation of the LVP by cranial nerve VII would thus be excluded. This is the first report to definitively localize LVP motoneurons. Although the innervation of LVP by cranial nerve X is generally agreed upon in basic anatomy textbooks, identification of LVP motoneurons within the NA does not exclude innervation by cranial nerve IX.