Exposure to reduced gravity impairs junctional transmission at the semicircular canal in the frog labyrinth.
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. MicroG-treated animals displayed a marked decrease in the frequency of resting and evoked mEPSP discharge, vs. both control and normoG (mean decrease approximately 50%). 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 + or - 8.06 spikes, vs. 77.2 + or - 8.19 in controls. Patch-clamp analysis on dissociated hair cells revealed altered Ca(2+) 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.
Rossi
Maria Lisa
ML
Department of Biology and Evolution, Section of Physiology and Biophysics, and National Institute of Neurosciences, Ferrara University, Ferrara, Italy. rsm@unife.it
Rubbini
Gemma
G
Gioglio
Luciana
L
Martini
Marta
M
Fesce
Riccardo
R
D000758:Anesthesia; D000818:Animals; D003655:Decerebrate State; D007758:Ear, Inner / Q000502:physiology*; D004594:Electrophysiology; D005071:Evoked Potentials / Q000502:physiology; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D054777:Hair Cells, Ampulla / Q000502:physiology; D066298:In Vitro Techniques; D018763:Muscle Fatigue / Q000502:physiology; D009469:Neuromuscular Junction / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology; D018408:Patch-Clamp Techniques; D010812:Physical Stimulation; D011893:Rana esculenta; D012399:Rotation; D012665:Semicircular Canals / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*; D014893:Weightlessness / Q000009:adverse effects*