Both the cholinergic and non-cholinergic components of airway excitation are inhibited by morphine in the guinea-pig

Inhaled fentanyl citrate improves exercise endurance during high-intensity constant work rate cycle exercise in chronic obstructive pulmonary disease

CONTEXT

Activity limitation and dyspnea are the dominant symptoms of chronic obstructive pulmonary disease (COPD). Traditionally, efforts to alleviate these symptoms have focused on improving ventilatory mechanics, reducing ventilatory demand, or both of these in combination. Nevertheless, many patients with COPD remain incapacitated by dyspnea and exercise intolerance despite optimal therapy.

OBJECTIVES

To determine the effect of single-dose inhalation of nebulized fentanyl citrate (a μ-opioid agonist drug) on exercise tolerance and dyspnea in COPD.

METHODS

In a randomized, double-blind, placebo-controlled, crossover study, 12 stable patients with COPD (mean ± standard error of the mean post-β(2)-agonist forced expiratory volume in one second [FEV(1)] and FEV(1) to forced vital capacity ratio of 69% ± 4% predicted and 49% ± 3%, respectively) received either nebulized fentanyl citrate (50 mcg) or placebo on two separate days. After each treatment, patients performed pulmonary function tests and a symptom-limited constant work rate cycle exercise test at 75% of their maximum incremental work rate.

RESULTS

There were no significant postdose differences in spirometric parameters or plethysmographic lung volumes. Neither the intensity nor the unpleasantness of perceived dyspnea was, on average, significantly different at isotime (5.0 ± 0.6 minutes) or at peak exercise after treatment with fentanyl citrate vs. placebo. Compared with placebo, fentanyl citrate was associated with 1) increased exercise endurance time by 1.30 ± 0.43 minutes or 25% ± 8% (P=0.01); 2) small but consistent increases in dynamic inspiratory capacity by ∼0.10 L at isotime and at peak exercise (both P≤0.03); and 3) no concomitant change in ventilatory demand, breathing pattern, pulmonary gas exchange, and/or cardiometabolic function during exercise. The mean rate of increase in dyspnea intensity (1.2 ± 0.3 vs. 2.9 ± 0.8 Borg units/minute, P=0.03) and unpleasantness ratings (0.5 ± 0.2 vs. 2.9 ± 1.3 Borg units/minute, P=0.06) between isotime and peak exercise was less after treatment with fentanyl citrate vs. placebo.

CONCLUSION

Single-dose inhalation of fentanyl citrate was associated with significant and potentially clinically important improvements in exercise tolerance in COPD. These improvements were accompanied by a delay in the onset of intolerable dyspnea during exercise near the limits of tolerance.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

Endogenous opioids as mediators of asthma

Changes in airway innervation are believed to play a key role in the pathophysiology of asthma. A group of regulatory peptides which act as neuroregulators is resembled by the opioids. Their localization to neurons projecting into airways suggested a possible role as regulators of neurogenic inflammation, bronchoconstriction and mucus secretion. They mainly act through modification of tachykinergic and cholinergic impulses and their ability to inhibit bronchoconstriction prompted discussion of their potential value in asthma therapy. Apart from the presence of the classical opioids and their receptors in the lung and their functional role, a new group of peptides such as nociceptin and endomorphins have been characterized in the airways. Whereas at least endomorphin-1 acts via the classical OP(3) (mu) receptor, nociceptin binds to a new receptor termed opioid receptor-like-receptor (ORL(1)) and inhibits tachykinergic constriction. Contrary to these promising modulatory effects on airway smooth muscle tone, effective therapeutic strategies have not been developed yet. In conclusion, opioids resemble a group of regulatory peptides which are present within airway-innervating nerve fibres and influence a multitude of airway functions via modification of neural transmission.

Lung opioid receptors: pharmacology and possible target for nebulized morphine in dyspnea

Opioid receptors are located throughout the respiratory tract. Yet, these have received relatively scant attention compared to other opioid receptors. The most abundant sites within the respiratory tract appear localized within the alveolar walls, other sites appear to line the smooth muscle within the trachea and main bronchi near the lumen. There is about 100-times greater [3H]morphine binding density within the bronchioles and lobes than in the main bronchi or trachea. In addition to the usual mu, delta and kappa types of opioid receptors, 'non-conventional' opioid binding sites have been suggested, although the function of these or of the other opioid receptors in the pulmonary tract is not known. However, they might explain the otherwise counterintuitive apparent utility of morphine treatment of dyspnea. Dyspnea is a common and distressing symptom in terminally-ill cancer patients and patients with chronic lung disease. It results from multiple causes, is difficult to treat and is a significant precipitating factor for late-stage hospital or hospice admissions. Nebulized morphine or other opioids have been reported to have beneficial effect, but the mechanism by which opioids might produce this seemingly contradictory effect is not clear. We review here lung opioid receptor distribution, pharmacology and possible clinical relevance in the treatment of dyspnea.

Nociceptin effects in the airways

The opioid-like heptadecapeptide nociceptin (NC) has the following effects in the airways (investigated in isolated tracheae and bronchi from guinea pig or rat): the electric field stimulation (EFS)-induces release of acetylcholine (ACh), the tachykinin substance P (SP) and calcitonin gene-related peptide (CGRP) is reduced after pretreatment with NC, and EFS-induced tachykinergic nonadrenergic-noncholinergic (NANC) bronchoconstriction is inhibited by NC. Both the NC-mediated inhibition of neurotransmission and of smooth muscle contraction occurred in a concentration-dependent manner. Because these effects were naloxone-insensitive, were blocked by the NC receptor antagonist [F/G]NC(1-13)NH(2), and could be mimicked by the NC analogs, NCNH(2) and NC(1-13)NH(2), it is thought that they are distinct from the classic opioid receptors. That these pharmacological actions of NC are of relevance for airway physiology is highly probable given the presence of NC-immunoreactivity in the nerve fibers of the airways and of opioid-like receptor (ORL-1) transcripts in the jugular ganglia, from where the tachykinin-containing afferents arise.

Abnormal modulation of cholinergic neurotransmission by opioid in hyperresponsive bronchus of rats

1. The electrical field stimulation (EFS)-induced bronchoconstriction in vitro in rats challenged by DNP-Ascaris antigen was significantly greater than that in normal rats. 2. Morphine inhibited the EFS-induced bronchoconstriction in normal rats. Whereas the inhibition of EFS-induced bronchoconstriction by the opioid was little, if any, in the DNP-Ascaris-challenged rats. 3. These findings suggest that dysfunction of presynaptic inhibitory modulation through the opioid receptor may take place in the airways of DNP-Ascaris-challenged rats.

Lower oesophageal sphincter hypersensitivity to opioid receptor stimulation in patients with idiopathic achalasia

Impairment of non-cholinergic innervation of the lower oesophageal sphincter has been suggested in idiopathic achalasia. As opioid nerves are present in the lower oesophageal sphincter and opioid peptides affect lower oesophageal sphincter motility, the effect of an opioid agonist, morphine (100 micrograms/kg iv), and an opioid blocker, naloxone (80 micrograms/kg iv), on lower oesophageal sphincter motor function was assessed in 10 healthy subjects and in 10 patients with untreated idiopathic achalasia on separate days and in randomised order. In addition, in six of the patients, naloxone 0.8 mg iv was injected 60 minutes after morphine and recordings continued for a further five minutes. Lower oesophageal sphincter pressure was monitored by a sleeve device. In the healthy subjects morphine decreased (p < 0.01) resting lower oesophageal sphincter pressure by 4 (1) mm Hg (23 (8)%). In the achalasia patients the effect was more marked, lower oesophageal sphincter pressure being reduced (p < 0.01) by 11 (2) mm Hg (46 (8)%). Naloxone reversed lower oesophageal sphincter pressure to basal. Both absolute and percentage decreases after morphine were significantly greater (p < 0.05) in the achalasia patients than in the healthy subjects. Swallow induced lower oesophageal sphincter relaxation was significantly decreased (p < 0.05) by morphine in the healthy subjects but not in the achalasia patients. Naloxone had no effect on resting lower oesophageal sphincter pressure or swallow induced relaxation in either healthy subjects or achalasia patients. In conclusion achalasia patients are hypersensitive to the effect of morphine on resting lower oesophageal sphincter pressure. This finding is unlikely to be the result of a denervation process involving opioid nerves.

Differential inhibitory effects of opioids on cigarette smoke, capsaicin and electrically-induced goblet cell secretion in guinea-pig trachea

1. Goblet cell secretion in guinea-pig airways is under neural control. Opioids have previously been shown to inhibit neurogenic plasma exudation and bronchoconstriction in guinea-pig airways. We have now examined the effects of morphine and opioid peptides on tracheal goblet cell secretion induced by either electrical stimulation of the cervical vagus nerves, exogenous capsaicin, or acute inhalation of cigarette smoke. The degree of goblet cell secretion was determined by a morphometric method and expressed as a mucus score which is inversely related to mucus discharge. 2. Morphine, 1 mg kg-1, completely blocked goblet cell secretion induced by electrical stimulation of the vagus nerves. Morphine also inhibited the response to cigarette smoke given either at a low dose (10 breaths of 1:10 diluted in air), which principally activates cholinergic nerves, or at a high dose (20 breaths of undiluted), which activates capsaicin-sensitive sensory nerves, by 100% and 73% respectively. In contrast, morphine had no significant inhibitory effect on capsaicin-induced goblet cell secretion. The inhibitory effect of morphine was reversed by naloxone. 3. Selective mu- or delta-opioid receptor agonists, [D-Ala2, NMePhe4, Glyol5]enkephalin (DAMGO) or [D-Pen2, D-Pen5]enkephalin (DPDPE) respectively, caused a dose-related inhibition of low dose cigarette smoke-induced goblet cell discharge, with DPDPE more potent than DAMGO. A kappa-receptor agonist, trans-3,4-dichloro-N-methyl-N-(2-(1-pyrollidinyl)cyclohexyl) benzeneacetamine (U-50,488H), had no inhibitory effect. DPDPE had no inhibitory effect on goblet cell secretion induced by exogenous methacholine. 4. DAMGO dose-dependently blocked the response to high dose cigarette smoke with a maximal inhibition of 95% at 2 x 10(-7) mol kg-1. Neither DPDPE nor U-50,488H had any significant inhibitory effect. The increase in goblet cell secretion induced by exogenous substance P was not affected by DAMGO.5. We conclude that opioids inhibit neurally-mediated goblet cell secretion via actions at prejunctional delta and mu-receptors on cholinergic nerves and at mu-receptors on sensory nerve endings, and that capsaicin activation of sensory nerves is via a different mechanism from that of electrical or cigarette smoke activation.

The modulatory effects of prostaglandins on both excitatory and inhibitory non-adrenergic non-cholinergic neurotransmission in guinea-pig airways

Guinea-pig trachea and bronchi were used to investigate the effects of indomethacin and prostaglandin E2 (PGE2) on non-adrenergic non-cholinergic excitatory (e-NANC) and inhibitory (i-NANC) neurotransmission evoked by electrical field stimulation. Indomethacin potentiated e-NANC responses in bronchi with intact epithelium but had no effect on epithelium-denuded preparations. Inhibitory NANC responses were increased by indomethacin independent of the epithelium. Both i-NANC and e-NANC neurotransmission were suppressed by PGE2 in a dose-dependent manner. These results indicate that endogenous prostaglandins (e.g. PGE2) generated from the epithelium have an inhibitory effect on i-NANC and e-NANC nerve responses in airways. The epithelium is presumably not the only source for generation of prostaglandins that are involved in i-NANC neurotransmission.