Synaptic proteins are tonotopically graded in postnatal and adult type I and type II spiral ganglion neurons

Abstract

Inherent in the design of the mammalian auditory system is the precision necessary to transduce complex sounds and transmit the resulting electrical signals to higher neural centers. Unique specializations in the organ of Corti are required to make this conversion, such that mechanical and electrical properties of hair cell receptors are tailored to their specific role in signal coding. Electrophysiological and immunocytochemical characterizations have shown that this principle also applies to neurons of the spiral ganglion, as evidenced by distinctly different firing features and synaptic protein distributions of neurons that innervate high- and low-frequency regions of the cochlea. However, understanding the fine structure of how these properties are distributed along the cochlear partition and within the type I and type II classes of spiral ganglion neurons is necessary to appreciate their functional significance fully. To address this issue, we assessed the localization of the postsynaptic AMPA receptor subunits GluR2 and GluR3 and the presynaptic protein synaptophysin by using immunocytochemical labeling in both postnatal and adult tissue. We report that these presynaptic and postsynaptic proteins are distributed oppositely in relation to the tonotopic map and that they are equally distributed in each neuronal class, thus having an overall gradation from one end of the cochlea to the other. For synaptophysin, an additional layer of heterogeneity was superimposed orthogonal to the tonotopic axis. The highest anti-synaptophysin antibody levels were observed within neurons located close to the scala tympani compared with those located close to the scala vestibuli. Furthermore, we noted that the protein distribution patterns observed in postnatal preparations were largely retained in adult tissue sections, indicating that these features characterize spiral ganglion neurons in the fully developed ear.

Heterogeneous intrinsic excitability of murine spiral ganglion neurons is determined by Kv1 and HCN channels

“…The majority of changes in excitability stabilize by postnatal day 4 (P4) such that spiral ganglion neurons from P14 animals retain both regional as well as tonotopic characteristics of voltage thresholds presented herein (Crozier and Davis, 2013). In addition, evidence from our laboratory indicates that K v 1 channel distribution, among other important regulators of spiral ganglion neuron function such as the tonotopic gradient of neurotrophin-3, AMPA receptor distribution, and other potassium channels, are expressed with similar tonotopic trends in adult animals as postnatal animals when assessed with immunocytochemistry (Adamson et al, 2002b, Flores-Otero and Davis, 2011). Therefore, our results from pre-hearing animials may extend to the hearing animal, as well.…”

Effect of early embryonic deletion of huntingtin from pyramidal neurons on the development and long-term survival of neurons in cerebral cortex and striatum

“…Finally, huntingtin is reportedly important for BDNF expression levels, which is highly expressed in cortical pyramidal neurons (Zuccato and Cattaneo, 2009; Reiner et al, 2012b). Accordingly, we used immunolabeling to characterize BDNF in cortical pyramidal neurons, using the N20 antiserum from Santa Cruz (Cat # sc546), whose specificity is demonstrated in Flores-Otero and Davis (2011).…”

Tyrosine Hydroxylase Expression in Type II Cochlear Afferents in Mice

“…This somewhat surprising outcome merits further discussion, particularly given the observation that some TH-2A-CreER neurons appeared to be peripherin-negative. Many laboratories have described preferential labeling of type II afferents by peripherin antibodies, e.g., (Flores-Otero and Davis 2011;Hafidi 1998;Huang et al 2007a;McLean et al 2009;Wang and Green 2011). However, various factors can limit the use of peripherin as a type II biomarker.…”