Tyrosine Hydroxylase Expression in Type II Cochlear Afferents in Mice

Abstract

Acoustic information propagates from the ear to the brain via spiral ganglion neurons that innervate hair cells in the cochlea. These afferents include unmyelinated type II fibers that constitute 5 % of the total, the majority being myelinated type I neurons. Lack of specific genetic markers of type II afferents in the cochlea has been a roadblock in studying their functional role. Unexpectedly, type II afferents were visualized by reporter proteins induced by tyrosine hydroxylase (TH)-driven Cre recombinase. The present study was designed to determine whether TH-driven Cre recombinase (TH-2A-CreER) provides a selective and reliable tool for identification and genetic manipulation of type II rather than type I cochlear afferents. The BTH-2A-CreER neurons^radiated from the spiral lamina, crossed the tunnel of Corti, turned towards the base of the cochlea, and traveled beneath the rows of outer hair cells. Neither the processes nor the somata of TH-2A-CreER neurons were labeled by antibodies that specifically labeled type I afferents and medial efferents.TH-2A-CreER-positive processes partially co-labeled with antibodies to peripherin, a known marker of type II afferents. Individual TH-2A-CreER neurons gave off short branches contacting 7-25 outer hair cells (OHCs). Only a fraction of TH-2A-CreER boutons were associated with CtBP2-immunopositive ribbons. These results show that TH-2A-CreER provides a selective marker for type II versus type I afferents and can be used to describe the morphology and arborization pattern of type II cochlear afferents in the mouse cochlea.

Hair Cell Mechanotransduction Regulates Spontaneous Activity and Spiral Ganglion Subtype Specification in the Auditory System

“…Previous studies had identified several genes that are expressed at different levels in mature type I and type II SGNs (Barclay et al, 2011; Hafidi, 1998; Lallemend et al, 2007; Mou et al, 1998; Nishimura et al, 2017; Vyas et al, 2017), and we have identified here many more. The functional consequences of these differences need to be explored, but one gene, Ngfr , provides an interesting example.…”

“…Mature type I SGNs can be distinguished from type II SGNs by high levels of Tubb3 expression while mature type II SGNs strongly express Prph (Barclay et al, 2011; Hafidi, 1998; Lallemend et al, 2007; Mou et al, 1998; Nishimura et al, 2017; Vyas et al, 2017). Unbiased clustering revealed groups of neurons that were separable on a t-Distributed Stochastic Neighbor Embedding (t-SNE) plot of all cells (Fig.…”

Opposing expression gradients of calcitonin‐related polypeptide alpha (Calca/Cgrpα) and tyrosine hydroxylase (Th) in type II afferent neurons of the mouse cochlea

“…Recent studies suggest that type II afferents may be cochlear nociceptors, and can be excited by ATP released during tissue damage, by analogy to somatic pain‐sensing C‐fibers. The present work compares the expression patterns among cochlear type II afferents of two genes found in C‐fibers: calcitonin‐related polypeptide alpha ( Calca/Cgrpα ), specific to pain‐sensing C‐fibers, and tyrosine hydroxylase ( Th ), specific to low‐threshold mechanoreceptive C‐fibers, which was shown previously to be a selective biomarker of type II versus type I cochlear afferents (Vyas et al, ). Whole‐mount cochlear preparations from 3‐week‐ to 2‐month‐old CGRPα‐EGFP (GENSAT) mice showed expression of Cgrp α in a subset of SGNs with type II‐like peripheral dendrites extending beneath OHCs.…”

Dynamic Expression of Sox2, Gata3, and Prox1 during Primary Auditory Neuron Development in the Mammalian Cochlea

“…Cells at the apical turn with strong Gata3 expression should be type II neurons since they also strongly expressed NF200 and weakly expressed TuJ1, which is another characteristic of type II neurons [18, 64, 65]. More recently, it was demonstrated that some tyrosine hydroxylase positive neurons with type II morphology did not express Peripherin [21] and type II PANs existed in Peripherin KO mice [23], both of which support our finding that some type II PANs were Peripherin negative.…”

“…Type II PANs make up the remaining 5% of all PANs and have pseudo-unipolar neurites that connect multiple outer hair cells (OHCs) with the CN [11, 12]. The biological differences between type I and type II neurons have been reported [13–21] (also see review on type II PANs by Zhang and Coate [22]). More recently, it was argued that type II PANs should be nociceptors mediating auditory pain and do not drive the olivocochlear reflex [23, 24]; however, it remains unknown how they develop from a common neuroblast.…”