Neuroplastic changes in vagal afferents inflicted by allergic
inflammation were examined in nodose ganglia (NG) removed from guinea pigs immunized to chick
ovalbumin. In control NG neurons,
substance P (SP; 0.1-10 microM) produces no discernable changes in membrane electrophysiological properties or [Ca2+]i. After exposing NG from immunized animals to the sensitizing
antigen in vitro, 83% of the neurons were depolarized by 100 nM SP. SP also produces an inward current, an increase in membrane conductance, and an elevation of [Ca2+]i. Buffering [Ca2+]i with
BAPTA blocked the [Ca2+]i rise and the SP depolarization, indicating that internal stores of Ca2+ are required. When
protein synthesis was inhibited >96% (as determined by [3H]
leucine incorporation),
antigen challenge still unmasked SP responses. The SP response was maximal 30 min after
antigen challenge, and it was evident for at least 8 hr in intact ganglia and for 3.5 d in isolated neurons. [beta-Ala8]
Neurokinin A ([beta-Ala8]NKA; 10 nM), an
NK-2 selective agonist, mimicked SP; selective NK-1 and NK-3 agonists were ineffective. The EC50 values for SP and [beta-Ala8]NKA membrane currents were 78 and 33 nM, respectively. Additionally,
SR48968, an
NK-2 receptor antagonist, blocked these responses. Thus,
antigen challenge appears to unmask an
NK-2 tachykinin receptor. These data further support the hypothesis that inflammatory mediators released during
immediate hypersensitivity (
allergic) reactions can produce profound effects on the excitability of sensory nerves. Unmasked
NK-2 receptors may serve an excitatory
autoreceptor function, provide a pathway for paracrine signaling between NG neurons, and contribute to ectopic sensory nerve activity.