Asphalt exposure enhances neuropeptide levels in sensory neurons projecting to the rat nasal epithelium

Ozone exposure initiates a sequential signaling cascade in airways involving interleukin-1beta release, nerve growth factor secretion, and substance P upregulation

Previous studies demonstrated that interleukin-1β (IL-1β) and nerve growth factor (NGF) increase synthesis of substance P (SP) in airway neurons both after ozone (O3) exposure and by direct application. It was postulated that NGF mediates O3-induced IL-1β effects on SP. The current study specifically focused on the influence of O3 on IL-1β, NGF, and SP levels in mice bronchoalveolar lavage fluid (BALF) and whether these mediators may be linked in an inflammatory-neuronal cascade in vivo. The findings showed that in vivo O3 exposure induced an increase of all three proteins in mouse BALF and that O3-induced elevations in both NGF and SP are mediated by the inflammatory cytokine IL-1β. Further, inhibition of NGF reduced O3 induced increases of SP in both the lung BALF and lung tissue, demonstrating NGF serves as a mediator of IL-1β effects on SP. These data indicate that IL-1β is an early mediator of O3-induced rise in NGF and subsequent SP release in mice in vivo.

Substance p regulates environmental tobacco smoke-enhanced tracheal smooth muscle responsiveness in mice

Environmental tobacco smoke (ETS) is an environmental trigger that leads to airway inflammation and airway hyperresponsiveness (AHR) in susceptible individuals and animals, but the underlying mechanism is not fully understood. Substance P (SP) release from sensory nerve fibers has been linked to AHR. The present experiments characterize the role of SP in tracheal smooth muscle on ETS-increased airway responses. The mice were exposed to either sidestream tobacco smoke (SS), a surrogate to ETS, or filtered air (FA) for 1 day or 5 consecutive days. Contractions of tracheal smooth muscle to SP and electrical field stimulation (EFS) were not significantly altered in 1 of day SS-exposed mice. However, 5 of days SS exposure significantly increased airway smooth muscle contractions to SP and EFS. Administration of CP-99994, an antagonist of the neurokinin (NK)1 receptor, attenuates the SS exposure-enhanced tracheal smooth muscle responses to EFS. Furthermore, the immunohistochemistry showed that SP nerve fibers were increased in tracheal smooth muscle after 5 of days SS exposure. These results suggest that the increased SP production may contribute to SS-enhanced smooth muscle responsiveness in mice trachea.

TRPA1 receptors mediate environmental irritant-induced meningeal vasodilatation

The TRPA1 receptor is a member of the transient receptor potential (TRP) family of ion channels expressed in nociceptive neurons. TRPA1 receptors are targeted by pungent compounds from mustard and garlic and environmental irritants such as formaldehyde and acrolein. Ingestion or inhalation of these chemical agents causes irritation and burning in the nasal and oral mucosa and respiratory lining. Headaches have been widely reported to be induced by inhalation of environmental irritants, but it is unclear how these agents produce headache. Stimulation of trigeminal neurons releases CGRP and substance P and induces neurogenic inflammation associated with the pain of migraine. Here we test the hypothesis that activation of TRPA1 receptors is the mechanistic link between environmental irritants and peptide-mediated neurogenic inflammation. Known TRPA1 agonists and environmental irritants stimulate CGRP release from dissociated rat trigeminal ganglia neurons and this release is blocked by a selective TRPA1 antagonist, HC-030031. Further, TRPA1 agonists and environmental irritants increase meningeal blood flow following intranasal administration. Prior dural application of the CGRP antagonist, CGRP(8-37), or intranasal or dural administration of HC-030031, blocks the increases in blood flow elicited by environmental irritants. Together these results demonstrate that TRPA1 receptor activation by environmental irritants stimulates CGRP release and increases cerebral blood flow. We suggest that these events contribute to headache associated with environmental irritants.

Sensory neural responses to ozone exposure during early postnatal development in rat airways

Airway infections or irritant exposures during early postnatal periods may contribute to the onset of childhood asthma. The purpose of this study was to examine critical periods of postnatal airway development during which ozone (O(3)) exposure leads to heightened neural responses. Rats were exposed to O(3) (2 ppm) or filtered air for 1 hour on specific postnatal days (PDs) between PD1 and PD29, and killed 24 hours after exposure. In a second experiment, rats were exposed to O(3) on PD2-PD6, inside a proposed critical period of development, or on PD19-PD23, outside the critical period. Both groups were re-exposed to O(3) on PD28, and killed 24 hours later. Airways were removed, fixed, and prepared for substance P (SP) immunocytochemistry. SP nerve fiber density (NFD) in control extrapulmonary (EXP) epithelium/lamina propria (EPLP) increased threefold, from 1% to 3.3% from PD1-PD3 through PD13-PD15, and maintained through PD29. Upon O(3) exposure, SP-NFD in EXP-smooth muscle (SM) and intrapulmonary (INT)-SM increased at least twofold at PD1-PD3 through PD13-PD15 in comparison to air exposure. No change was observed at PD21-PD22 or PD28-PD29. In critical period studies, SP-NFD in the INT-SM and EXP-SM of the PD2-PD6 O(3) group re-exposed to O(3) on PD28 was significantly higher than that of the group exposed at PD19-PD23 and re-exposed at PD28. These findings suggest that O(3)-mediated changes in sensory innervation of SM are more responsive during earlier postnatal development. Enhanced responsiveness of airway sensory nerves may be a contributing mechanism of increased susceptibility to environmental exposures observed in human infants and children.

Nerve growth factor and substance P regulation in nasal sensory neurons after toluene diisocyanate exposure

Toluene diisocyanate (TDI) exposure produces rhinitis and nasal irritation, and increases the synthesis and release of substance P (SP) from airway sensory nerves. The mechanism leading to enhanced SP production following irritant inhalation remains unclear, but may involve actions of nerve growth factor (NGF). NGF binds trkA receptors located on sensory nerve terminals. Activation of trkA receptors initiates kinase-signaling cascades, which ultimately may increase SP. However, the effects of inhaled irritants on NGF release are not known. In this study, NGF levels in nasal lavages were examined following instillation of 10% TDI into both nasal cavities. NGF was significantly increased 2, 6, 12, and 24 h after TDI exposure compared with controls. The increase in NGF preceded the neuronal and mucosal increases in SP. Pretreatment with K252a, a nonselective tyrosine-kinase inhibitor, prevented the increase in SP-immunoreactivity in TG neurons and epithelial nerve fibers and the inflammatory response to TDI exposure. Because NGF binds to trkA tyrosine-kinase receptors, the NGF released during TDI exposure may mediate SP upregulation in airway sensory neurons, innervating the nasal cavity.