No effect of hypergravity on adult rat ventral horn neuron size or SDH activity

Background; Spaceflights of short duration (similar to2 wk) result in adapt ations in the size and/or metabolic properties of a select population of mo toneurons located in the lumbosacral region of the rat spinal cord. A decre ase in succinate dehydrogenase (SDH, an oxidative marker enzyme) activity o f moderately sized (500-800 mum(2)) motoneurons in the retrodorsolateral re gion of the spinal cord (L-6) has been observed after a 14-d flight. Hypoth esis. Our hypothesis was that exposure to short-term hypergravity would res ult in adaptations in the opposite direction, reflecting a continuum of mor phological and biochemical responses in the spinal motoneurons from zero gr avity to hypergravity. Methods. Young, male rats were centrifuged at either 1.5 or 2.0 G for 2 wk. The size and SDH activity of a population of motone urons in the retrodorsolateral region of the spinal cord (L-5,) were determ ined and compared with age-matched rats maintained at 1.0 G. The absolute a nd relative (to body weight) masses of the soleus, gastrocnemius, adductor longus and tibialis anterior muscles were compared among the three groups. Results: There were no effects of either hypergravity intervention on the m otoneuron properties. Rats maintained under hypergravity conditions gained less body mass than rats kept at 1.0 G. For the 1.5 and 2.0 G groups, the m uscle absolute mass was smaller and relative mass similar to that observed in the 1.0 G rats, except for the adductor longus. The adductor longus abso lute mass was similar to and the relative mass larger in both hypergravity groups than in the 1.0 G group. Conclusions. Our hypothesis was rejected. T he findings suggest that rat motoneurons are more responsive to short-term chronic exposure to spaceflight than to hypergravity conditions.