Analysis of pre- and postsynaptic activity in the frog semicircular canal following ototoxic insult: differential recovery of background and evoked afferent activity

Frogs were treated with a single dose of gentamicin administered intraotically to produce severe degeneration of posterior semicircular canal hair cells and to evaluate the time course of functional damage and recovery both at pre- and postsynaptic level. In isolated canal preparations the endoampullar potential, which reflects the summed receptor potentials of crista hair cells, was progressively reduced in amplitude and completely abolished 6 days after gentamicin treatment. At this time the crista epithelium was devoid of hair cells. The recovery of the endoampullar potential began around 9 days after the ototoxic insult and its amplitude progressively increased to reach, after 20 days, values close to those observed in control experiments. The endoampullar potential amplitude was related to the degree of hair cell regeneration in the crista epithelium. Consistent with the presynaptic damage, the slow generator potential (representing the summed miniature excitatory postsynaptic potential [mEPSP] activity of all posterior nerve fibres) and the resting and evoked spike discharge recorded from the whole ampullar nerve were abolished 6 days after gentamicin treatment. The recovery of the background and evoked afferent activity showed different behaviours. Background spike activity became detectable around 8 days after the ototoxic insult, but was not modulated by canal stimulation at this time, and no generator potential was detected. Moreover, the resting spike frequency fully recovered and reached control values around 15 days after gentamicin treatment, whereas the evoked activity attained normal values only 20 days after the ototoxic insult. These results were confirmed by intracellular recordings from single afferent fibres of the ampullar nerve in intact labyrinth preparations. Absence of any resting and evoked discharge was the most common pattern observed in the early period from 7 to 8 days after gentamicin treatment. Fifty-five percent of impaled afferents were silent while the others showed low resting frequencies of mEPSPs and spikes, and were unresponsive to canal rotation. In the intermediate period from 14 to 15 days after gentamicin treatment, background mEPSP and spike frequencies approached those evaluated in control experiments, but the frequencies of the evoked mEPSPs and spikes were clearly lower than in controls. In the late period, from 18 to 20 days after the ototoxic insult, the impaled afferents showed normal evoked mEPSP and spike frequencies. The present data indicate that the frog semicircular canal completely recovers its pre- and postsynaptic activity following severe ototoxic insult. During the regeneration process, the cytoneural junction regains function and the resting discharge reappears before recovery of mechanoelectrical transduction.