RESPONSES OF VESTIBULAR AFFERENTS TO MECHANICAL STIMULATION AND DRUG APPLICATION IN MOUSE IN VITRO LABYRINTH

A.J. Camp, H-Y. Lee, R.J. Callister, A.M. Brichta, School of Biomedical Sciences, Faculty of Health, University of Newcastle, Callaghan, NSW 2308.

We have recorded intra-axonally from anterior and horizontal primary afferents from an in vitro preparation of the mouse labyrinth. Despite the trauma associated with isolation and lower recording temperatures (23 to 34 °C), bony labyrinths retain their ability to transduce mechanical stimulation into afferent activity. Using a micropusher to indent the membranous labyrinth we have characterised activity of both anterior and horizontal canal afferents in response to sinusoidal indentations. In recordings from 97 afferents over a range of frequencies from 0.01 to 10 Hz, afferents responded with sinusoidal changes in discharge rates in a predictable manner. Phase response of afferent discharge was characterised by frequency-dependent shifts in peak activity and was usually in advance of maximum indentation, with large phase leads at low frequencies (106 ± 28.1° for 0.01 Hz; mean ± s.d.). The smallest phase leads occurred around 1 Hz; 15 ± 29.3°. These phase shifts are similar to those reported in in vivo recordings from mammals, despite our use of artificial rather than natural stimuli. Calculation of sensitivity was problematic, because indentation partially collapsed the canal. This made it difficult to provide consistent stimuli. Consequently while sensitivity could be calculated for individual units these results could not be pooled across animals. This in vitro preparation also allows application of drugs and we have begun to study the effects of various neuromodulators on afferent discharge. Our preliminary results suggest that TTX (1 µM) abolishes afferent discharge by acting directly on axons because its effect occurs within 30 seconds or less. CNQX (10 µM) can take up to 6 minutes to abolish background afferent activity suggesting that it probably acts at the hair cell / primary afferent synapse. In conclusion, isolated mouse in vitro labyrinths are viable preparations for studying transduction and synaptic mechanisms in the mammalian peripheral vestibular apparatus.

Supported by National Health and Medical Research Council of Australia, Garnett Passe and Rodney Williams Foundation, Hunter Medical Research Institute to AMB and RJC.