Exposure to reduced gravity impairs junctional transmission at the semicircular canal in the frog labyrinth

Abstract

The effects of microgravity on frog semicircular canals have been studied by electrophysiological and morphological approaches. Reduced gravity (microG) was simulated by a random positioning machine (RPM), which continually and randomly modified the orientation in space of the anesthetized animal. As this procedure stimulates the semicircular canals, the effect of altered gravity was isolated by comparing microG-treatment with an identical rotary stimulation in the presence of normal gravity (normoG). Electrophysiological experiments were performed in the isolated labyrinth, extracted from the animals after the treatment, and mounted on a turntable. Junctional activity was measured by recording quantal events (mEPSPs) and spikes from the afferent fibers close to the junction, at rest and during rotational stimulation. MicroGtreated animals displayed a marked decrease in the frequency of resting and evoked mEPSP discharge, vs. both control and normoG (mean decrease ϳ50%). Spike discharge was also depressed: 57% of microG-treated frogs displayed no spikes at rest and during rotation at 0.1 Hz, vs. 23-31% of control or normoG frogs. Among the firing units, during one cycle of sinusoidal rotation at 0.1 Hz microG-treated units emitted an average of 41.8 Ϯ 8.06 spikes, vs. 77.2 Ϯ 8.19 in controls. Patch-clamp analysis on dissociated hair cells revealed altered Ca 2ϩ handling, after microG, consistent with and supportive of the specificity of microG effects. Marked morphological signs of cellular suffering were observed after microG, mainly in the central part of the sensory epithelium. Functional changes due to microgravity were reversible within a few days. resting and evoked sensory discharge; quantal analysis; hair cells; ionic currents; reperfusion; multiple organ damage INFORMATION ON HEAD POSITION and movement is elaborated in the central nervous system by integrating the input from labyrinthine organs with visual and somesthesic information. Most vestibular functions are directly or indirectly affected by gravity; thus, changes in gravity conditions are bound to interfere with sensation, perception, and central elaboration of vestibular information. Many common and evident disturbances of this system arise during central processing and integration of sensory information, due to incoherence between visual and gravity (vestibular, neck muscle tension, etc.) sensory inputs. However, little is known about possible primary alterations of vestibular function under altered gravity, due to significant and measurable interference with transduction processes at the vestibular organs and with neural encoding of the static and dynamic stimuli the head is exposed to.A series of structural (4, 5, 13, 14, 19), biochemical (12), genetic (for a review see ref. 11), behavioral (2), and developmental (3) modifications have been reported to occur in several cellular systems following exposure to conditions of altered gravity. Functional data are scarce: an increased response sensitivity of toadfish utricular afferents has been de...

Acute effects of gentamicin on the ionic currents of semicircular canal hair cells in the frog

“…Both actions of gentamicin e cell repolarization due to transduction channel block, and the decrease in Ca 2þ inflow due to block of the outer and inner mouth of the Ca 2þ channels e converge in depressing transmitter release at the cytoneural junction. Gentamicin would thus clamp the hair cell in a silent status: the cell becomes insensitive to physiological stimulation and the Ca-dependent afferent transmitter release, which is substantial even at rest (Rossi et al, 1994(Rossi et al, , 2010, is remarkably reduced.…”

Sensory transduction at the frog semicircular canal: how hair cell membrane potential controls junctional transmission

“…Often the mEPSP frequencies were very high in the experiments here reported (several hundreds/s), and simple Wiener filtering, after deleting the spikes, was insufficient to accurately measure the rate of occurrence of mEPSPs from the noisy recording. We therefore employed a recently developed refinement of the procedure (Rossi et al, 2010 ), which combines the above mentioned Wiener filtering routine for event detection with least-square-errors optimization of fit and identification of possibly neglected events; this procedure yields the best fitting set of parameters (time of occurrence, t 0 , size factor, h , and waveform parameters β and γ for each event); events with particularly small amplitude or aberrant waveform can be excluded as artifacts.…”

“…Analysis of mEPSP rates at times immediately preceding or following the spikes, the observation that momentary rates up to 800 mEPSPs/s could be occasionally estimated in spike-free portions of the recordings, and the consideration that the coincidence of several mEPSPs must occur to generate a spike, led us to estimate that 4–6 mEPSPs were missed for each subtracted spike. Therefore, mEPSP counts were corrected by adding five mEPSPs in the 3 ms interval corresponding to each subtracted spike (Rossi et al, 2010 ).…”

“…The effect of tricaine was completely and rapidly reversible upon returning to the normal Ringer solution. This property was also used to demonstrate the reliability of the mEPSP automated counting (and spike subtraction) procedure before and after tricaine (Rossi et al, 2010 ).…”

“…The complex interplay of ionic currents at the isolated hair cell has also been described in detail (Housley et al, 1989 ; Lang and Correia, 1989 ; Masetto et al, 1994 ; Rennie and Correia, 1994 ; Brichta et al, 2002 ; Catacuzzeno et al, 2003 ; Martini et al, 2009 ), and a number of categories of sensory units have been discriminated, based on electrophysiological properties and spike firing patterns (Wilson and Melvill Jones, 1979 ; Goldberg, 1991 ; Goldberg and Brichta, 2002 ; Pfanzelt et al, 2008 ). As regards the cellular and junctional mechanisms in the intact vestibular apparatus, direct information on hair cell properties has been obtained through intracellular recording from amphibian saccular hair cells (Bracho and Budelli, 1978 ) at rest, whereas quantal emission and spike discharge in single afferent units, both at rest and during mechanical stimulation, have been thoroughly characterized by recording from the frog posterior semicircular canal, close to the cytoneural junction (Housley et al, 1989 ; Rossi et al, 1989 , 1994 , 2010 ). Only type II hair cells are present in this preparation, and they are in the physiological milieu, with the apical membrane in contact with the endolymph and the basolateral membrane surrounded by a perilymph-like solution.…”