Firing Patterns of Type II Spiral Ganglion NeuronsIn Vitro

Abstract

Type I and type II spiral ganglion neurons convey auditory information from the sensory receptors in the cochlea to the CNS. The numerous type I neurons have been extensively characterized, but the small population of type II neurons with their unmyelinated axons are undetectable with most recording methods. Despite the paucity of information about the type II neurons, it is clear that they must have a significant role in sound processing because they innervate the large number of outer hair cells that are critical for maintaining normal responses to stimuli. To elucidate the function of type II neurons, we have developed an approach for studying their electrophysiological features in vitro.Type II neurons obtained from postnatal day 6 -7 mice displayed distinctly different firing properties than type I neurons. They showed slower accommodation, lower action potential thresholds, and more prolonged responses to depolarizing current injection than the type I neurons. These differences were most evident in neurons from the basal, high-frequency region of the cochlea. The basal type I neurons displayed uniformly fast firing features, whereas the basal type II neurons showed particularly slow accommodation and responses to depolarization. Interestingly, neurons from the apical, low-frequency region of the cochlea showed the opposite trend. These data suggest that the type I and type II neurons have specialized electrophysiological characteristics tailored to their different roles in auditory signal processing. In particular, the type II neuron properties are consistent with cells in other sensory systems that receive convergent synaptic input for high-sensitivity stimulus detection.

Complex Regulation of Spiral Ganglion Neuron Firing Patterns by Neurotrophin-3

“…Because NT-3 has been shown to promote survival of spiral ganglion neurons in vivo and in vitro (Avila et al, 1993;Mou et al, 1997;Hansen et al, 2001;Fritzsch et al, 2004), one mechanism could be that NT-3 selectively promotes the survival of a subpopulation of spiral ganglion neurons. Our laboratory has shown previously that putative type II spiral ganglion neurons differ in the base from their type I counterparts by demonstrating slow accommodation, longer latencies, and prolonged membrane time constants (Reid et al, 2004), similar to neurons exposed to the appropriate concentrations of NT-3 in the present study. Therefore, it is possible that NT-3 may simply enhance the survival of type II neurons in our cultures, thus increasing the preponderance of this electrophysiological phenotype.…”

“…Although in previous studies (Adamson et al, 2002a,b;Reid et al, 2004) we had not observed significant differences in threshold between apical and basal neurons or between neurons with and without added neurotrophins (BDNF or NT-3), in the present study there was a tendency for threshold voltage to differ in a manner similar to other electrophysiological parameters (Fig. 4c).…”

Reciprocal Regulation of Presynaptic and Postsynaptic Proteins in Bipolar Spiral Ganglion Neurons by Neurotrophins

“…Presynaptic and postsynaptic proteins are differentially distributed within the spiral ganglion Previous studies have shown that although 95% of the neurons within the spiral ganglion are of one class, they display clear differences in their ion channel composition and resulting firing patterns (Mo and Davis, 1997;Adamson et al, 2002b;Reid et al, 2004;Zhou et al, 2005). Neurons that innervate high frequency sensory receptors at the basal end of the cochlea possess voltagegated ion channel types that mediate abbreviated and rapidly accommodating responses to prolonged step depolarizations.…”

“…The first was a neuronal culture in which the apical and basal one-fifth of the spiral ganglion were isolated from postnatal day 7 (P7) CBA/CaJ mice and plated in separate culture dishes; then used after 6 d in vitro (div). This preparation was used to compare new data directly with previous investigations in which electrophysiological parameters have been fully characterized along with extensive immunocytochemical, pharmacological, and molecular studies of the underlying voltage-gated ion channel contributions (Adamson et al, 2002a,b;Reid et al, 2004;Zhou et al, 2005;Chen and Davis, 2006). In all cultures, cells were maintained in growth medium: DMEM (Sigma D6171) supplemented with 10% fetal bovine serum, 4 mM L-glutamine, and 0.1% penicillin-streptomycin.…”

Inner ear pathology and loss of hearing in estrogen receptor-β deficient mice

“…Double staining of ER-a and ER-b in 12-month-old WT mice was performed as described by Reid et al (2004). For staining of ER-a, a monoclonal rabbit anti-human ER-a antibody (RM-9101, Neomarkers) was used, and for ER-b a chicken anti-rat/human antibody (anti-ER-b IgY 503, raised in our lab; see above).…”