Genetic identification of mouse trigeminal afferents responsible for mechanical allodynia.

The identities of primary afferents transducing mechanical allodynia following nerve injury remain unclear. We genetically label brushing-activated (BA) trigeminal ganglia (TG) neurons using FosCreER mice with trigeminal nerve injury (TNI). BA TG neurons are largely medium-sized. Many express neurofilament200 and Ntrk3, markers for low-threshold mechanoreceptors, with lower co-localization with nociceptor markers such as Calca or Trpv1. Chemogenetic inhibition of BA TG neurons reduces mechanical allodynia, whereas their chemogenetic activation increases spontaneous face wiping after TNI. Brushing-induced conditional knockdown (bcKD) of Piezo2 from BA TG afferents reduces punctate and dynamic mechanical allodynia. In vivo TG GCaMP Ca2+ imaging shows that Piezo2 bcKD reduces not only hypersensitivity to low-force mechanical stimulation, mostly among medium-sized neurons, but also, unexpectedly, TNI-induced spontaneous activity. Therefore, Fos is useful for genetic labeling and manipulation of BA TG neurons. Furthermore, innocuous mechanical stimuli activate multiple TG afferent subtypes after TNI, possibly accounting for the complexity of resulting painful symptoms.