THE EFFECTS OF PERILYMPHATIC TONICITY ON ENDOLYMPH COMPOSITION AND SYNAPTIC ACTIVITY AT THE FROG SEMICIRCULAR CANAL

The effects of changes in perilymphatic tonicity on the semicircular c anal were investigated by combining the measurements of transepithelia l potential and endolymphatic ionic composition in the isolated frog p osterior canal with the electrophysiological assessment of synaptic ac tivity and sensory spike firing at the posterior canal in the isolated intact labyrinth. In the isolated posterior canal, the endolymph was replaced by an endolymph-like solution of known composition, in the pr esence of basolateral perilymph-like solutions of normal (230 mosmol/k g), reduced (105 mosmol/kg, low NaCl) or increased osmolarity (550 mos mol/kg, Na-Gluconate added). Altered perilymphatic tonicity did not pr oduce significant changes in endolymphatic ionic concentrations during up to 5 min. In the presence of hypotonic perilymph, decreased osmola lity, K and Cl concentrations were observed at 10 min. In the presence of hypertonic perilymph, the endolymphatic osmolality began to increa se at 5 min and by 10 min Na concentration had also significantly incr eased. On decreasing the tonicity of the external solution an immediat e decline was observed in transepithelial potential, whereas hypertoni city produced the opposite effect. In the intact frog labyrinth, mEPSP s and spike potentials were recorded from single fibers of the posteri or nerve in normal Ringer's (240 mosmol/kg) as well as in solutions wi th modified tonicity. Hypotonic solutions consistently decreased and h ypertonic solutions consistently increased mEPSPs and spike frequencie s, independent of the species whose concentration was altered. These e ffects ensued within 1-2 min after the start of perfusion with the tes t solutions. In particular, when the tonicity was changed by varying N a concentration the mean mEPSP rate was directly related to osmolality . Size histograms of synaptic potentials were well described by single log-normal distribution functions under all experimental conditions. Hypotonic solutions (105 mosmol/kg) markedly shifted the histograms to the left. Hypertonic solutions (380-550 mosmol/kg, NaCl or Na-Glucona te added) shifted the histograms to the right. Hypertonic solutions ob tained by adding sucrose to normal Ringer's solution (final osmolality 550 mosmol/kg) increased mEPSP and spike rates, but did not display a ppreciable effects an mEPSP size. All effects on spike discharge and o n mEPSP rate and size were rapidly reversible. In Ca-free, 10 mM EGTA, Ringer's solution, the sensory discharge was completely abolished and did not recover on making the solution hypertonic. These results indi cate that perilymphatic solutions with altered tonicity produce small and slowly ensuing changes in the transepithelial parameters which may indirectly affect the sensory discharge rate, whereas relevant, early and reversible effects occur at the cytoneural junction. In particula r, the modulation of mEPSP amplitude appears to be postsynaptic; the p resynaptic effect on mEPSP rate of occurrence is presumably linked to local calcium levels, in agreement with previous results indicating th at calcium inflow is required to sustain basal transmitter release in this preparation. (C) 1998 Elsevier Science B.V. All rights reserved.