Pre- and postsynaptic recovery of semicircular canal activity in the frog following ototoxic insult

Aim: Inner ear hair cells of lower vertebrates regenerate following physical or chemical insults. In the present study we aimed to define the damage and recovery of the pre and postsynaptic activity in the frog crista ampullaris after ototoxic insult with the antibiotic, gentamicin. Methods: Frogs were treated with a single dose of gentamicin sulphate (GM-5mM) administered intraotically. The presynaptic activity was monitored by recording in isolated canal preparations the endoampullar (receptor) potential (Adc). The postsynaptic activity was monitored by recording the resting (R) and evoked spike activity (E) from the whole ampullar nerve and from single canal afferents. Results: We found that Adc was abolished 5 days after GM administration and that it progressively recovered close to control values 20 days after treatment. These results well fitted with the morphological damage and recovery of hair cells in the crista epithelium. Consistently with the presynaptic damage, the canal afferent activity was abolished 5 days after GM treatment, while the recovery of the background and the evoked afferent activity showed a different sequence. Background activity (R) was detected 7-8 days after GM treatment and it was not modulated by mechanical stimulation of the canal. In addition, R reached control values (14/15 days post-treatment), before those of the evoked activity (E-20 days post-treatment). Intracellular recordings confirmed the above mentioned results and revealed that the evoked EPSP discharge, together with the spike activity, reached normal values close to 20 days post-treatment. Conclusion: The present results show that frog semicircular canal rapidly and completely recovers its pre -and postsynaptic function following severe ototoxic insult and that during the recovery, the synaptic function of regenerating hair cells becomes functional before the mechano-transduction function.