Together, opioid receptors in the PNEC–C nerve fiber network within the tracheobronchial epithelium seem to be quite likely targets for inhaled treatment. How intrabronchial opioids might work locally depends mostly on the roles played by PNECs and afferent nerves within the respiratory system.
Pulmonary neuroendocrine cells as potential main sensors in the human lung
The PNEC network consists of solitary PNECs, widely distributed within the mucosa of the tracheobronchial tree, and their clusters, known as neuroepithelial bodies (NEBs), which are localized, at least in animals, primarily in the intrapulmonary airways (for review see [17▪▪]).
PNECs and NEBs have been implicated in the regulation of lung function, including airway oxygen sensing, control of bronchial tone and pulmonary blood flow, modulation of immune responses, and maintenance of a stem cell niche [18▪,19]. In early lung development, PNECs and NEBs modulate fetal lung growth and differentiation, whereas in the perinatal period they serve as polymodal airway sensors that monitor changes in airway gas concentration and respond to hypoxia, hypercapnia, and acidosis. Whether adult human lungs can also directly sense oxygen concentration in inhaled air remains uncertain and requires further studies. One of the main difficulties in the interpretation of experiments involving PNECs and NEBs in humans is because of the fact that most studies have been performed on animals. The PNEC network differs in the morphology, localization, and function between species. For example, human PNECs are smaller than the PNECs in rat, have an interdigitating morphology, are located within epithelium (but not in the subepithelial layer), and, of note, are found in the trachea and major bronchi, but not in alveolar tissue [20▪▪]. The concept of PNECs as airway sensors is supported by the presence of their cellular digits projecting toward the apical surface (Fig. 2c and d)  and direct innervation by the afferent neuronal fibers [20▪▪,21]. Recent data suggest that human adult solitary PNECs serve mainly as airway chemosensory neuroendocrine sentinels in epithelium. They may give rise to neural and paracrine and endocrine reactions through the release of serotonin (5-HT), calcitonin gene-related peptide (CGRP), and other mediators which subsequently modify the smooth muscle tone of bronchial and pulmonary vessels, modulate inflammatory responses, and initiate sensory neural transmission to the central nervous system (CNS) mainly via vagal afferent fibers [20▪▪]. It should be noted that PNECs possess release mechanisms tailored to different stimuli. Hypoxia is detected by the NADPH oxidase coupled to a variety of O2-sensitive voltage-dependent K+ channel proteins [18▪]. The subsequent closure of K+ channels causes membrane depolarization, activation of voltage-gated Ca2+ channels that in turn leads to the influx of Ca2+, and triggers the exocytosis of 5-HT and neuropeptides. 5-HT activates both postsynaptic receptors on nerve endings and paracrine and autocrine receptors (including 5-HT-3 found in human PNECs and NEBs proposed to act as autoreceptors in positive feedback modulation) . Together, 5-HT release initiates hypoxia chemotransmission to CNS to modulate breathing (via vagal afferents) and local effects, leading to the increase of blood flow in better ventilated portions of the lung.
Mechanical stimulation of PNECs and NEBs, in contrast, leads to the opening of the transient receptor potential canonical-5 (TRPC5) channel with subsequent calcium-dependent exocytosis of ATP [23▪,24]. ATP activates the purinergic (P2X2/3) receptors on vagal afferents allowing fast conduction to the CNS and on Clara-like cells within the epithelium as paracrine signaling [23▪]. Less is known on the chemosensory function of PNECs. However, recently adult human PNECs have been shown to express different olfactory receptors and release of 5-HT and CGRP under the stimulation with volatile chemicals [20▪▪]. Although sensory function of PNECs remains to be investigated, it is possible that the specific pathways of PNECs activation may be a therapeutic target.
The potential role of PNECs in the pathophysiology of dyspnea is at present speculative. Interestingly, plasma concentration of chromogranin A, a neurosecretion marker of PNECs, has recently been demonstrated as a strong and independent predictor of all-cause mortality at 1 year in patients with acute dyspnea in an emergency setting . What is more, different pulmonary disorders leading to dyspnea, including pulmonary hypertension, cystic fibrosis, small-cell lung carcinoma, chronic bronchitis, or emphysema, are associated with PNEC and NEB dysplasia or dysfunction . The higher expression of PNECs in the lungs of COPD patients, along with a changed distribution of 5-HT and CGRP receptors, suggests an increased PNEC-dependent chemoresponsiveness in this disease [20▪▪].
Together, these data indicate that adult human PNECs might represent a cellular target for the management of breathlessness. The potential for opioids to act at PNECs had been suggested by old experiments, such as the inhibition of 5-HT secretion from PNEC-derived cancer cell culture by a DOR agonist  or opioid-induced relief of breathlessness and bronchoconstriction provoked by ozone, known as a strong stimulus for PNEC activation . The demonstration of the presence of MOR, DOR, and KOR on PNECs in human bronchial epithelium opened a new gateway to research on potential opioid agonist involvement in the neurohormonal regulation of pulmonary vascular or bronchial responses, as well as breathlessness perception.
Neurogenic inflammation in the airways
Irritation of the sensory C nerve endings by 5-HT or CGRP released from PNECs initiates afferent signals, as well as stimulates the synthesis, antidromic transport, and peripheral release of neuropeptides (such as substance P, neurokinin A, or CGRP; Fig. 3a).
This last phenomenon leads to vasodilatation, recruitment, and activation of immune cells and stimulation of mucus secretion, all of which contribute to the neurogenic inflammation [28▪].
The major role in this process seems to be played by the transient receptor potential vanilloid type 1 (TRPV1) receptor, a nonselective cation channel expressed in the sensory neurons and various nonneuronal cells including human bronchial epithelial cells [29▪,30].
The activation of TRPV1 receptors by chemical irritants, inflammatory mediators, and tissue damaging stimuli leads to the release of neuropeptides [31,32]. TRPV1-mediated proinflammatory pathways may be responsible for whetting the inflammation and in consequence worsening of the disease, as it has been recently suggested by TRPV1 overexpression observed in patients with steroid-refractory asthma . Interestingly, it has been shown that opioids inhibit the activity of TRPV1 and this inhibition is MOR specific, mediated via Gi/o proteins and the cyclic AMP and protein kinase A (cAMP–PKA) pathway .
The expression of opioid receptors on sensory C nerve fibers and the opioid-related inhibition of neuropeptide release appear to be a very attractive hypothesis regarding the potential anti-inflammatory effects of opioids. Such mechanisms have been proven to function in other organ systems, making the hypothesis even more attractive.
Opioid-modulated PNEC-C nerve fibers-CNS pathway
Summarizing all the mechanisms described above, it can be hypothesized that opioids administered into the tracheobronchial region inhibit PNEC and C nerve fibers activity and, consequently, both signal transmission to CNS and neurogenic inflammation in the periphery (Fig. 3b).
POTENTIAL UNIQUE ROLE OF MORPHINE IN THE LUNG
The efficacy of nebulized opioids in breathlessness might depend not only on the method of inhalation, but also on the choice of opioid. The potential impact of these two factors was recently demonstrated . Morphine was administered with the Bronchial Control Treatment System-Sidestream (BCTS-S), a new dosimetric pneumatic method allowing the preferential deposition of opioid in the trachea and major bronchi . With this delivery system, morphine underwent a different kind of metabolism than when administered orally or intravenously . The Tmax for the metabolites of morphine after nebulization was shorter (20–30 min) and the proportion of its metabolite, morphine-6-glucuronide (M6G), was higher when compared with systemic administration. It might be hypothesized that glucuronidase UGT 2B7, which was shown to be present in the bronchial epithelium , preferentially produced M6G at a lower concentration of morphine . Together, these facts suggest that nebulized morphine is metabolized locally in the lungs. M6G is a stronger opioid receptor agonist than the parent drug and acts predominantly peripherally because of the limited blood–brain barrier permeability . It is possible that a small dose of morphine administered into the tracheobronchial region evokes predominantly peripheral effects via locally produced M6G acting on the opioid receptors expressed on PNECs–C nerve fibers network.
Opioid receptors are present in the human lung on structures directly communicating with, or localized close to, airway lumens, which play the role of chemosensitive sentinels and are crucial for the regulation of breathing and dyspnea perception. These structures are responsible for the transduction and transmission (both to and from CNS) of information needed to optimize gas exchange and to modulate respiration. In pathophysiology, these pathways participate in the generation of breathlessness or cough. It is well known that systemic opioids acting on the CNS can influence the functioning of this circuit. However, recent evidence has shown peripheral pathways are well organized and can be a target for the therapeutic interventions. In order to achieve direct peripheral effects for breathlessness relief, opioids should probably be administered in close proximity to their receptors, located in the PNECs and sensory C-fibers of the bronchial epithelium. The choice of opioids is important as they may have different modes of action and efficacy within the respiratory system. To give an example, topical morphine action might at least partially depend on the local generation of M6G, which is mainly peripherally acting and a more potent opioid agonist than its parent drug.
Altogether, the peripheral opioid receptors in PNECs and C nerve fibers within the bronchial epithelium may be utilized as a target for therapeutic efforts.
Recent data indicate that PNECs and bronchial sensory C-fibers play crucial roles in many processes, including breathlessness, cough, inflammation, and wound healing. More studies are needed to clarify to what extent topical or systemic administration of opioids might modify their function.
Funding sources: none.
Conflicts of interest
The authors declare that they do not have any potential conflicts of interest directly relating to the work presented in this article. All the authors have seen, reviewed, and approved this article. All authors have completed and signed the copyright transfer forms, and these have been included with the article submission.
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Keywords:© 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
C nerve fibers; neurogenic inflammation; opioid receptors; pulmonary neuroendocrine cells