The role of sensory nerve endings in nerve growth factor-induced airway hyperresponsiveness to histamine in guinea-pigs

Nerve growth factor induces an airway hyperresponsiveness in vivo in guinea-pigs, as we have shown previously. Since antagonizing the neurokinin-1 (NK1) receptor can prevent this NGF-induced airway hyperresponsiveness and since sensory nerves release tachykinins, we investigated the role of sensory nerves in the NGF-induced airway hyperresponsiveness. We used isolated tracheal rings from guinea-pigs to measure tracheal contractility. In these rings sensory nerve endings are present, but these endings lack any contact with their cell bodies. In this in vitro system, NGF dose-dependently induced a tracheal hyperresponsiveness to histamine. The NK1 receptor antagonist SR140333 could block the induction of tracheal hyperresponsiveness. To further investigate the involvement of sensory nerve endings we used the cannabinoid receptor 1 (CB1) agonist R-methanandamide to inhibit excitatory events at the nerve terminal. The CB1 receptor agonist was capable of blocking the tracheal hyperresponsiveness to NGF in the isolated system, as well as the airway hyperresponsiveness to NGF in vivo. This indicates that NGF can induce an increase in airway responsiveness in the absence of sensory nerve cell bodies. NGF may act by increasing substance P release from sensory nerve endings, without upregulation of substance P in the neurons. Substance P in its turn is responsible for the induction of the NGF-induced airway hyperresponsiveness. Keywords: Nerve growth factor, airway hyperresponsiveness, guinea-pig, sensory nerve ending, substance P, SR140333, methanandamide Introduction The neurotrophin NGF is a newly studied mediator in relation to allergic diseases (Braun et al., 1998; de vries et al., 1999; Virchow et al., 1998). Circulating NGF levels are increased in humans with rhinoconjunctivitis, urticaria-angioedema or asthma; NGF serum levels were particularly high in patients with allergic asthma (Bonini et al., 1996). Moreover, an increase in the neurotrophins NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) in bronchoalveolar lavage fluids after allergen challenge is reported in patients with asthma (Virchow et al., 1998). In patients with allergic rhinitis a very fast increase in NGF in the nasal lavage fluids is found 10 min after allergen challenge (Sanico et al., 2000). Furthermore, allergen challenge induced an increase in NGF levels in bronchoalveolar lavage fluid of sensitized mice (Braun et al., 1998). Inflammatory mediators, including interleukin-1, interleukin-4, interleukin-5, tumour necrosis factor α, and interferon-γ have been shown to induce the release of NGF (Brodie, 1996; Brodie et al., 1998; Hattori et al., 1994; Yoshida et al., 1992). In addition to neurons, non-neuronal cells such as mast cells (Leon et al., 1994), fibroblasts (Hattori et al., 1994), T-cells (Lambiase et al., 1997; Moalem et al., 2000), eosinophils (Solomon et al., 1998) and lymphocytes (Barouch et al., 2000) are able to synthesize NGF. NGF shows various pathologic properties in inflammatory models, which could be interesting in relation to allergic asthma as well (reviewed in Braun et al., 1999; 2000). NGF affects immune cell activity, as it promotes inflammatory mediator release from basophils (Burgi et al., 1996), mast cells (Matsuda et al., 1988), T- and B-cells (Lambiase et al., 1997; Otten et al., 1989) and macrophages (Susaki et al., 1996). Furthermore, NGF induces antibody synthesis and secretion from B cells (Otten et al., 1989), attracts mast cells (Sawada et al., 2000), and induces differentiation of mast cells (Welker et al., 2000) as well as of monocytes (Ehrhard et al., 1993). Moreover, NGF induces a shift to the production of Th2 type cytokines in a model for multiple sclerosis (Villoslada et al., 2000), as well as in a mouse model for allergic asthma (Braun et al., 1998). Furthermore, NGF is able to sensitize neurons and it induces an enhanced production of substance P and other tachykinins in sensory nerves (Lindsay & Harmar, 1989). Tachykinins play a role in neurogenic inflammation and in the development of airway hyperresponsiveness and asthma (Lundberg, 1995). We showed that administration of NGF induced an airway hyperresponsiveness in guinea-pigs (de vries et al., 1999). The use of a NK1 receptor antagonist could prevent this hyperresponsiveness. This points to a role for substance P, the preferred ligand of the NK1 receptor (de vries et al., 1999). Furthermore, Hunter et al. (2000) showed that NGF induces an increase in substance P in guinea-pig airway sensory neurons. The present study focuses on the role of sensory nerves in NGF-induced airway hyperresponsiveness. Isolated tracheal rings containing sensory nerve endings, without any contact with their cell bodies, were used to elucidate the involvement of sensory nerve endings as opposed to the neuronal cell body.