Publications

BACKGROUND AND PURPOSE: Clinical studies have demonstrated significant reductions in allergen-induced nasal symptoms of atopic rhinitis subjects by CysLT1 antagonists, including neuronally mediated symptoms such as sneeze, itch and reflex hypersecretion. Here, we test the hypothesis that cysteinyl leukotrienes activate and/or alter the activity of nasal nociceptive (capsaicin-sensitive) sensory neurones. EXPERIMENTAL APPROACH: Using retrograde tracer (DiI), we labelled guinea-pig trigeminal sensory neurones that projected fibres to the nasal mucosa. Single-neurone reverse transcriptase (RT)-PCR was used to evaluate CysLT receptor gene expression. The effect of cysteinyl leukotrienes on individual nasal sensory nerve activity was assessed in Ca2+ assays and whole-cell gramicidin-perforated patch-clamp studies. KEY RESULTS: Nasal C-fibre neurones express CysLT1 but not CysLT2 mRNA. LTD4 and LTC4 increased intracellular [Ca2+]free in a population of capsaicin-sensitive trigeminal nerves, an effect blocked by the CysLT1 antagonist ICI198615. In current clamp mode, LTD4 had no effect on resting membrane potential. However, LTD4 significantly increased electrical excitability (action potential discharge during current pulses) threefold in capsaicin-sensitive nasal neurones, which was inhibited by CysLT1 antagonists ICI198615 and montelukast. LTD4 had no effect on electrical excitability in capsaicin-insensitive neurones. Finally, LTD4 significantly augmented histamine-induced responses in capsaicin-sensitive neurones as measured by increased action potential discharge, peak frequency and membrane depolarization. CONCLUSIONS AND IMPLICATIONS: LTD4, likely through CysLT1 receptors, directly increases the excitability of capsaicin-sensitive guinea-pig nasal trigeminal neurones, demonstrating a novel mechanism for the actions of cysteinyl leukotrienes and potentially explains the effectiveness of CysLT1 antagonists in treating nasal allergen-induced neuronal symptoms.

Transient receptor potential (TRP) A1 channels are cation channels found preferentially on nociceptive sensory neurones, including capsaicin-sensitive TRPV1-expressing vagal bronchopulmonary C-fibres, and are activated by electrophilic compounds such as mustard oil and cinnamaldehyde. Oxidative stress, a pathological feature of many respiratory diseases, causes the endogenous formation of a number of reactive electrophilic alkenals via lipid peroxidation. One such alkenal, 4-hydroxynonenal (4HNE), activates TRPA1 in cultured sensory neurones. However, our data demonstrate that 100 microm 4HNE was unable to evoke significant action potential discharge or tachykinin release from bronchopulmonary C-fibre terminals. Instead, another endogenously produced alkenal, 4-oxononenal (4ONE, 10 microm), which is far more electrophilic than 4HNE, caused substantial action potential discharge and tachykinin release from bronchopulmonary C-fibre terminals. The activation of mouse bronchopulmonary C-fibre terminals by 4ONE (10-100 microm) was mediated entirely by TRPA1 channels, based on the absence of responses in C-fibre terminals from TRPA1 knockout mice. Interestingly, although the robust increases in calcium caused by 4ONE (0.1-10 microm) in dissociated vagal neurones were essentially abolished in TRPA1 knockout mice, at 100 microm 4ONE caused a large TRPV1-dependent response. Furthermore, 4ONE (100 microm) was shown to activate TRPV1 channel-expressing HEK cells. In conclusion, the data support the hypothesis that 4-ONE is a relevant endogenous activator of vagal C-fibres via an interaction with TRPA1, and at less relevant concentrations, it may activate nerves via TRPV1.

Transient receptor potential (TRP) A1 and TRPM8 are ion channels that have been localized to afferent nociceptive nerves. These TRP channels may be of particular relevance to respiratory nociceptors in that they can be activated by various inhaled irritants and/or cold air. We addressed the hypothesis that mouse vagal sensory nerves projecting to the airways express TRPA1 and TRPM8 and that they can be activated via these receptors. Single cell RT-PCR analysis revealed that TRPA1 mRNA, but not TRPM8, is uniformly expressed in lung-labelled TRPV1-expressing vagal sensory neurons. Neither TRPA1 nor TRPM8 mRNA was expressed in TRPV1-negative neurons. Capsaicin-sensitive, but not capsaicin-insensitive, lung-specific neurons responded to cinnamaldehyde, a TRPA1 agonist, with increases in intracellular calcium. Menthol, a TRPM8 agonist, was ineffective at increasing cellular calcium in lung-specific vagal sensory neurons. Cinnamaldehyde also induced TRPA1-like inward currents (as measured by means of whole cell patch clamp recordings) in capsaicin-sensitive neurons. In an ex vivo vagal innervated mouse lung preparation, cinnamaldehyde evoked action potential discharge in mouse vagal C-fibres with a peak frequency similar to that observed with capsaicin. Cinnamaldehyde inhalation in vivo mimicked capsaicin in eliciting strong central-reflex changes in breathing pattern. Taken together, our results support the hypothesis that TRPA1, but not TRPM8, is expressed in vagal sensory nerves innervating the airways. TRPA1 activation provides a mechanism by which certain environmental stimuli may elicit action potential discharge in airway afferent C-fibres and the consequent nocifensor reflexes.

The induction of action potentials in airway sensory nerves relies on events leading to the opening of cation channels in the nerve terminal membrane and subsequent membrane depolarization. If the membrane depolarization is of sufficient rate and amplitude, action potential initiation will occur. The action potentials are then conducted to the central nervous system, leading to the initiation of various sensations and cardiorespiratory reflexes. Triggering events in airway sensory nerves include mechanical perturbation, inflammatory mediators, pH, temperature, and osmolarity acting through a variety of ionotropic and metabotropic receptors. Action potential initiation can be modulated (positively or negatively) through independent mechanisms caused mainly by autacoids and other metabotropic receptor ligands. Finally, gene expression of sensory nerves can be altered in adult mammals. This neuroplasticity can change the function of sensory nerves and likely involve both neurotrophin and use-dependent mechanisms. Here we provide a brief overview of some of the transduction mechanisms underlying these events.

BACKGROUND: Inflammation of the nasal mucosa leads to sneezing, nasal itch, rhinorrhea, and nasal blockage. Many of these symptoms are likely the result of nasal trigeminal sensory nerve stimulation by inflammatory mediators. Nasal challenge with the C-fiber stimulant capsaicin causes a different set of symptoms than those evoked by histamine, suggesting that these 2 stimuli may activate separate subpopulations of nasal sensory nerves. OBJECTIVE: To investigate the trigeminal sensory nerves innervating the guinea pig nasal mucosa and to address specifically the hypothesis that histamine and capsaicin activate distinct subgroups of these nerves. METHODS: Guinea pig trigeminal neurons (retrogradely labeled from the nasal mucosa) were assessed for their responses to histamine and capsaicin by studying changes in the intracellular free calcium concentration, and assessed for substance P immunoreactivity. RESULTS: Only 60% of the nasal-specific trigeminal sensory neurons were found to be capsaicin-sensitive. Histamine stimulated only a subset (