Neural response in vestibular organ of Helix aspersa to centrifugation and re-adaptation to normal gravity

Abstract

Gravity plays a key role in shaping the vestibular sensitivity (VS) of terrestrial organisms. We studied VS changes in the statocyst of the gastropod Helix aspersa immediately after 4-, 16-, and 32-day exposures to a 1.4G hypergravic field or following a 7-day recovery period. In the same animals we measured latencies of behavioral "negative gravitaxis" responses to a head-down pitch before and after centrifugation and found significant delays after 16- and 32-day runs. In an isolated neural preparation we recorded the electrophysiological responses of the statocyst nerve to static tilt (±19°) and sinusoids (±12°; 0.1 Hz). Spike sorting software was used to separate individual sensory cells' patterns out of a common trace. In correspondence with behavior we observed a VS decrease in animals after 16- (p < 0.05) and 32-day (p < 0.01) centrifugations. These findings reveal the capability of statoreceptors to adjust their sensitivity in response to a prolonged change in the force of gravity. Interestingly, background discharge rate increased after 16 and 32 days in hypergravity and continued to rise through the recovery period. This result indicates that adaptive mechanisms to novel gravity levels were long lasting, and re-adaptation from hypergravity is a more complex process than just "return to normal".

Adaptive Changes in the Vestibular System of Land Snail to a 30-Day Spaceflight and Readaptation on Return to Earth

“…For Foton-M3 data (with comparable snail size used), all available latencies (up to T2) are shorter than in preflight controls, thus in agreement with the hypothesis that gravitaxis temporal parameters will change in an opposite fashion after μG than after hypergravity. No differences in T4 duration between μG PF and control groups of snails were found in the data of Bion-M1, also in agreement with the previous study of hypergravity influences and the assumption that the T4 phase may reflect more motor than vestibular function in snails (Popova and Boyle, 2015 ).…”

“…Snails belonging to the same genus of Helix ( H. aspersa ), the common garden snail, were exposed to increased gravitation by centrifugation for 16 and 32 days and showed an increased latency or slowing of T3 phase of gravitaxis (Popova and Boyle, 2015 ). In the data of Bion-M1 project opposite effects in PF snails to the control snails were not seen.…”

“…Vestibular sensitivity in the statocyst of the garden snail was recently studied following exposure to hyper-gravity conditions using centrifugation (Popova and Boyle, 2015 ). A significant decrease in response sensitivity to head-down tilt was detected after 16- and 32-day (but not 4-day) exposures to a 1.4G [resultant = √1G (Earth) 2 + 1G (centripetal) 2 ].…”

“…Kreidl replaced the sand with reduced ferrous oxide and manipulated the crayfish's posture using a magnet, thereby simulating the action of the normal gravitational field with a magnetic one. Despite the simplicity of the gastropod organ of balance, there are many functional similarities between the snail's statocyst and the more highly evolved otolith organs of vertebrate species (Popova and Boyle, 2015 ). Statoreceptors, like hair cells in the vertebrate utricle and saccule, are mechanoreceptors that depend on a weight-lending mass to provide a shearing deformation of the underlying ciliary processes to sense position and acceleration with respect to gravity.…”

“…The land snail Helix lucorum Linnaeus ( Pulmonata, Gastropoda ) also orients to the vertical using its bilateral statocysts and visual receptors, and can actually move against the force of gravity using a sticky excretion. We selected the land snail for a month-long study on an unmanned orbiting satellite for many reasons, such as their small size permitting a sufficient sample size given our volume limit, snails are hardy and can remain metabolically active under confined conditions for the duration of the mission, snails do not require elaborate life support systems, recordings can be made directly from the sensory receptors themselves (Balaban et al, 2011 ; Popova and Boyle, 2015 ), and importantly certain snail behaviors and their underlying cellular circuits are well described (Balaban, 2002 ). In the present study snails spent 30 days under conditions of weightlessness and darkness, thereby removing its normal cues of the vertical.…”