Enhanced responsiveness of rat's dorsal spinocerebellar tract neurons under simulated weightlessness

Abstract

Objective To examine the extracellular discharge of dorsal spinocerebellar tract (DSCT) neurons in Clarke's nucleus in segment L1-2 of rat's spinal cord in order to verify the effects of simulated weightlessness on the electrophysiological characteristics of DSCT neurons activated by gastrocnemius-soleus (GS) nerve and ramp-and-hold stretch stimuli applied to Achille tendon.Methods Forty female Sprague-Dawley rats were randomly and averagely divided into control group and 14 d simulated weightlessness group.Weightlessness was simulated by tail suspension of rats.DSCT neuron activity was recorded extracellularly for both groups. Results DSCT neurons showed low frequency discharges without any stretch being applied to the muscle.The ipsilateral GS nerve stimulation resulted in synaptic activation of DSCT neurons that was characterized by short latency (1-3 ms) and long latency (6-15 ms) spike discharges.The latency of DSCT neuron with short latency (mDSCT) neurons and DSCT neuron with long latency (pDSCT) neurons was (1.69± 1.15) ms and (10.04±3.15) ms respectively.The spontaneous discharge rate of mDSCT and pDSCT neurons was (15.78±10.36) spikes/s and (20.71 ± 12.32) spikes/s respectively.The stimuli threshold of mDSCT and pDSCT neurons was (0.39+0.35) mA and (1.32±0.70) mA respectively.After 14 d tail suspension,the latency of mDSCT and pDSCT neurons was (4.18± 1.06) ms and (12.24 ± 3.30)ms respectively.The spontaneous discharge rate of mDSCT and pDSCT neurons was ( 13.58± 11.40)spikes/s and (24.71 ±10.27) spikes/s respectively.The stimuli threshold of mDSCT and pDSCT neurons was (0.24 ± 0.16) mA and (1.03 ± 0.42) mA,respectively.Though the spontaneous discharge rate of mDSCT and pDSCT neurons was not significantly altered,the latency of mDSCT and pDSCT neurons was obviously longer (t=2.905,3.886,P＜0.05,respectively) and the stimuli threshold was significantly lower (t=3.768,2.097,P＜0.05,respectively) than those of the control rats. Conclusions Our findings suggest that DSCT neurons become more responsive to electrical stimulation of peripheral nerve with the quantity and pattern changes of muscle spindle input information in simulated weightlessness.These confirm further that the central progressing of muscle spindle afferent would change during the simulated weightlessness. Key words: Weightlessness simulation; Rats, Sprague-Dawley; Hindlimb suspension; Spinocerebellar tract; Spinal cord; Muscle spindles