Morphophysiology of Primary Vestibular Afferents Recorded from an in vitro Preparation of Mouse Inner Ear

Lee, Heung Youp; Chae, Sayong; Kang, Jun Myung; Bang, Choong Ill; Brichta, A M

Abstract

BACKGROUND AND OBJECTIVES
We are developing an in vitro preparation of the mouse inner ear so as to study morphophysiologic character of primary vestibular afferents and synaptic transmission within the vestibular epithelium. MATERIALS AND METHOD: We have intra-axonally recorded from over 300 ampullary fibers, close to the base of their respective anterior and lateral crista (<500 micrometer from hair cell/afferent nerve synapse), and labelled as a sub-set of these with biocytin (n=71). Discharge activity can be classified as regular or irregular based on the variation of the interspike interval (coefficient of variation). Using a micropusher to indent exposed windows of membranous labyrinth, we have characterized the response properties of both anterior and horizontal canal afferents. We studied afferent activity in response to sinusoidal indentations of anterior and horizontal membranous canal. RESULTS: The majority of labelled units were dimorphic (56 out of 71), having both calyx and bouton terminals and there was no labelled bouton terminal. Whether action potentials (Aps) were spontaneous or elicited with current, a heterogeneity of discharge activity was observed and these were similar to those previously reported in in vivo recordings from other mammalian species. In recordings over a range of frequencies from 0.01 to 10.0Hz, afferents responded with sinusoidal changes at discharge rates and modulation of membrane potential in a predictable manner. The phase response of the afferent discharge was characterized by frequency-dependent shifts in peak activity. The peak activity of anterior canal was in advance of the maximum indentation (180dgrees out of phase), with largest phase leads at 0.01 Hz (59.2+/-14.1dgrees) and the smallest phase leads occurring at 1.0 Hz (13.4+/-9.3dgrees), while maximum indentation was in advance of the peak activity at 10.0 Hz (-17.6+/-9.1dgrees). These phase shifts were similar to those reported in in vivo recordings from mammals, despite our use of artificial rather than natural rotational stimuli. CONCLUSION: We developed an in-vitro mouse model to study morphophysiologic characteristics of primary vestibular afferent nerve and synaptic transmission.