The adult peripheral taste system is capable of extensive functional plasticity after injury. Sectioning the chorda tympani (CT), a primary sensory afferent nerve, elicits transient changes in the uninjured, contralateral population of taste receptor cells. Remarkably, the deficits are specific to the sodium transduction pathway. Normal function is quickly restored in the intact nerve, in parallel with an influx of macrophages to both the denervated and uninjured sides of the tongue. However, changing the dietary environment by restricting sodium blocks the macrophage response and prolongs functional alterations. Since the functional deficits occur before macrophages are present in the peripheral taste system, we hypothesized that neutrophils play a role in modulating neural responses in the intact CT. First, the dynamics of the neutrophil response to nerve injury were analyzed in control-fed and sodium-deficient rats. Nerve sectioning briefly increased the number of neutrophils on both the denervated and uninjured sides of the tongue. The low-sodium diet amplified and extended the bilateral neutrophil response to injury, in parallel with the persistent changes in sodium taste function. To test the impact of neutrophils on taste function, we depleted these cells prior to nerve sectioning and recorded neural responses from the intact CT. This treatment restored normal sodium responses in the uninjured nerve. Moreover, recruiting neutrophils to the tongue induced deficits in sodium taste function in both CT nerves. Neutrophils play a critical role in ongoing inflammatory responses in the oral cavity, and may induce changes in taste perception. We also suggest that balanced neutrophil and macrophage responses enable normal neural responses after neural injury.

a b s t r a c tDuring Drosophila embryogenesis, establishment of ventral and lateral cell fates requires spatial regulation of an extracellular serine protease cascade composed of Nudel, Gastrulation Defective (GD), Snake, and Easter. Pipe, a sulfotransferase expressed ventrally during oogenesis, sulfates secreted targets that somehow confer positive spatial input to this cascade. Nudel and GD activation are pipe-independent, while Easter activation requires pipe. The effect of pipe on Snake activation has been unknown. Here we show that Snake activation is cascade-dependent but pipe-independent. These findings support a conclusion that Snake's activation of Easter is the first spatially regulated step in the dorsoventral protease cascade.