Introduction
The respiratory tract can be divided into upper airway including the nasal cavities, the pharynx and larynx, and the lower airway comprising the tracheobronchial tract and the respiratory portion of the lungs. The main function of the nose includes conditioning (warming and humidifying) of inhaled air and protection (filtering, trigeminal function and olfaction) of the respiratory part of the lower airway against inhalation of exogenous particles and airborne irritants. The submucosal blood vessels, the muco-ciliary transport system and the local immune mechanisms are the main tissue components involved in these functions. The function of these elements is influenced, at least in part, by biologically active agents released from both sensory and efferent sympathetic and parasympathetic autonomic nerves. A very dense sympathetic innervation is present around both resistance and capacitance vessels of the nasal mucosa. In contrast, parasympathetic fibres are very dense around nasal mucus glands. Noradrenaline (NA) is considered as the classical transmitter released from postganglionic sympathetic nerves. Neuropeptide Y (NPY) is a 36 amino acid peptide first discovered in the porcine brain. The co-localization of NPY with NA has been observed in many peripheral sympathetic nerve fibres. In the nasal mucosa, co-existence of NA and NPY immunoreactivity was observed in sympathetic nerves present around both resistance and capacitance vessels of most species including mankind. Presence of NPY was also demonstrated in adrenergic nerves found in both lower air- way and vascular smooth muscle of the respiratory tract in several different species. Somatostatin was also observed in association with NA and NPY in nasal sympathetic nerves. Further, NPY has also been found in parasympathetic nerves in the airways in association with vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) in humans as well as in various other species.
NPY effect on airway mucus secretions
Exogenous NPY does not reduce glandular secretions in the human nasal mucus. However, a randomized double-blind, three ways, crossover placebo-controlled study has shown that pretreatment with exogenous NPY significantly reduces both nasal obstruction and mucus secretion induced by allergen challenge.
NPY receptors
The multiple biological effects of NPY are exerted through G-protein-coupled receptors. The NPY receptor family includes the Y1 receptor first characterized as a post-synaptic receptor, the Y2 receptor, known as a presynaptic receptor and the Y3 receptor considered as the NPY-preferring receptor. The Y4 receptor was characterized as a pancreatic polypeptide receptor. The Y5 is involved in feeding behavior and the Y6 receptor was recently cloned but its function remains unknown. The predominant NPY receptor type in nasal mucosa blood vessels is of the Y1 type. However, the presence of Y2 receptors is strongly suggested by in vivo studies with Y2 agonists.
Endogenous release of NPY
After SNS, NPY was detectable in the nasal venous effluent only after high frequency stimulation whereas NA was secreted already upon low frequency stimulation. Co-release of NPY and catecholamines has been demonstrated in man during intense physical exercise. Interestingly, physical exercise has been shown to induce significant reduction of subjective and objective nasal airway resistance, most likely secondary to vasoconstriction of nasal capacitance vessels, in both control and rhinitis patients. Variations of plasma NPY concentrations over time correlated better with post-exercise nasal vasoconstriction than NA levels.
Modulator effects of NPY
There is evidence suggesting that NPY released from adrenergic nerves modulates the effector response to transmitters originating from several neuronal pathways. In the lower airways, parasympathetic nerve activity can be inhibited by NPY. In anaesthetized cats, sympathetic nerve stimulation induced significant and prolonged attenuation of the vasodilator response to subsequent parasympathetic stimulation. Exogenous NPY mimicked the effect of sympathetic stimulation in attenuating subsequent parasympathetic evoked vasodilation. Similar observations were made in anaesthetized dogs. Attenuation of parasympathetic-evoked vasodilation and mucus secretion could be mimicked by the NPY analog N-acetyl (Leu28, Leu31) NPY 24-36, a Y2-receptor agonist. In contrast, the NPY analog (Leu31, Pro34), a potent Y1 receptor agonist did not have any effect on subsequent vasodilation and mucus secretion induced by parasympathetic stimulation. All together, these observations suggest that nasal sympathetic nerve stimulation attenuate parasympathetic vasodilation via NPY release acting on pre-junctional Y2 receptors. Observations obtained from in vitro experiments using electrical field stimulation on guinea pig tracheal smooth muscle strongly suggest that NPY modulates cholinergic neurotransmission via prejunctional mechanisms. Further studies have shown that NPY has an inhibitory action on tachykinin release such as substance P (SP) from peripheral endings of capsaicin-sensitive air- way sensory nerves.
In vitro, exogenous NPY modulates the contractile response of guinea pig tracheal smooth muscle induced by VIP, NA, SP and 5-hydroxytryptamine at both the pre- and postjunctional level. Intranasal or intrabronchial pretreatment with TASP-V, a potent NPY Y2 receptor agonist, reduces both nasal obstruction and bronchoconstriction produced by histamine challenge in the pig. In healthy human volunteers TASP-V significantly lessens the nasal airway resistance increase induced by local application of histamine. In the airways, blood flow, tracheal and bronchial smooth muscle tone, mucus and fluid production can be influenced by NPY. These functional effects can be direct or indirect via modulation of other neurotransmitters. The neuromodulatory role of NPY has been well documented on both parasympathetic activity and neurogenic inflammation, i.e., plasma extravasation elicited by sensory C-fibres stimulation. Most experiments suggest that NPY modulates neurotransmitter release by prejunctional mechanisms.
NPY metabolism
Some proteases have been shown to be involved in the metabolism of biologically active peptides in various tissues. The enzymes neutral endopeptidase (NEP) and dipeptidylpeptidase IV (DPPIV) are involved in NPY degradation. The activity of NEP can be inhibited by phosphoramidon. The inhibitory effect of NPY on the contraction of human bronchial segments in vitro could be enhanced in the presence of phosphoramidon. This observation suggests that NEP modulates the effect of NPY in the human airway by an inactivation mechanism. Similar observations were made regarding DPPIV. In patients suffering from chronic rhinosinusitis or bronchitis, the activity of both NEP and DPPIV is significantly reduced. This suggests that the catabolism of NPY and other neuropeptides could also be involved in upper airway homeostasis. Neutral endopeptidase activity and concentration of sensory neuropeptides in the human nasal mucosa varies proportionally to patient symptoms. These observations could explain the significant increase of NPY as well as SP and VIP in airway diseases.
Reference:
Zukowska, Z., & Feuerstein, G. Z. (Eds.). (2006). The NPY Family of Peptides in Immune Disorders, Inflammation, Angiogenesis, and Cancer. Springer Science & Business Media.