NANC responses--role in control of airway tone

Nebulized Morphine for the Treatment of Dyspnea in Cancer Patients

Objective:

To review the literature concerning the use of nebulized opioids to treat dyspnea in terminally ill cancer patients.

Data Sources:

English-language journal articles were obtained by a MEDLINE search (1966-August 1997).

Study Selection:

All clinical trials and case reports involving nebulized opioids. Representative studies discussing neurogenic inflammation and pulmonary opioid receptors were also reviewed.

Data Extraction:

Studies were selected for review on the basis of study design and use of nebulized opioids. Case reports were selected from the palliative care literature.

Data Synthesis:

High-density, low-affinity opioid receptors have been identified in lung tissue and may play a role in the alleviation of dyspnea in some patients. The activation of these receptors in the lung may attenuate the release of inflammatory mediators and neuropeptides such as substance P. However, because nebulized opioids are absorbed buccally or through the airway mucosa, the relief observed may be due to a central rather than peripheral effect.

Conclusions:

Several case reports in the palliative care literature have described favorable results when nebulized opioids are used to treat dyspnea. However, data from controlled trials in populations other than patients with cancer do not substantiate the use of nebulized opioids. Unfortunately, controlled clinical trials have not been conducted in cancer patients; thus, extrapolation is difficult. Well-designed clinical trials in patients with cancer are necessary to determine the efficacy of nebulized morphine in treating dyspnea.

Nitric oxide and the non-adrenergic non-cholinergic neurotransmission

In the early 1960s, the first evidence was reported demonstrating neurally mediated responses in the presence of adrenergic and cholinergic antagonists, leading to the introduction of the concept of non-adrenergic non-cholinergic neurotransmission. The inhibitory component of this part of the autonomic nervous system has been illustrated in numerous organ systems mediating a wide range of physiological events. Since the discovery of these nerves, several substances have been proposed as putative neurotransmitter, with ATP and vasoactive intestinal polypeptide as main candidates. Finally, the ongoing research on the nature of the substance released by these nerves has generated the nitrergic theory proposing nitric oxide as putative neurotransmitter. By now, increasing evidence is reported to support the idea that inhibitory neurons release more neurotransmitters, interacting with each other at pre- and/or postsynaptic levels.

Neural mechanisms and axon reflexes in asthma. Where are we?

For many years, asthma has been classified as a "neural" disease, with an imbalance between constrictor and dilator nerves being responsible for the symptomatology. Although, nowadays, asthma is recognized as an inflammatory disorder of the airways, neural mechanisms remain very important; axon reflexes, in particular, have received a lot of attention in recent years. In this commentary, an overview is given on the innervation of the airways and its relevance in asthma, and potential new insights in airways innervation are discussed. In a second part, the role of axon reflexes is highlighted. Although neuropeptides such as substance P and neurokinin A are present in human airways, where they produce many of the features characteristic of asthma, and although there is an elevation of their content in induced sputum from asthmatics, there is still no clear direct evidence for the existence of operational axon reflexes in human airways. Most of the research focused on this subject is performed in guinea pigs, where such an axon reflex clearly operates in the airways. In these animals, different receptors have been identified on C-fiber endings, which, upon stimulation, cause inhibition of neuropeptide release. Some of these receptors have also been identified on human airway nerves. Therefore, it has been suggested that modulation of axon reflexes could be of potential benefit in asthma treatment. Indeed, some drugs (e.g. sodium cromoglycate, nedocromil sodium, and ketotifen), which have been demonstrated to partially inhibit neuropeptide release in guinea pig airways, have anti-inflammatory effects on neuropeptide release in guinea pig airways, do not seem to have any anti-inflammatory effects in human asthma. Other drugs, however, such as beta2-mimetics, which have a much more pronounced inhibitory effect in asthma. In conclusion, although there is a lot of indirect evidence for the existence of axon reflex mechanisms in human airways, most of the data now available are derived from animal studies. The key question of whether axon reflexes are operational in human airways remains unanswered. Hopefully, the near future will bring a solution to this enigma with the introduction of very potent tachykinin antagonists for the treatment of human asthma.