Aging Enhances Synaptic Efficacy in a Latent Motor Pathway Following Spinal Cord Hemisection in Adult Rats

Abstract

The effect of aging on the development of the neural circuitry mediating the crossed phrenic phenomenon (CPP) was investigated in anesthetized, vagotomized, artificially ventilated young adult (9-10 weeks old) and older adult (9-10 months old) rats. Cervical spinal cord hemisection rostral to the phrenic nucleus abolishes respiratory activity in the ipsilateral phrenic nerve. The respiratory activity can be restored by subjecting the animal to respiratory stress after spinal cord injury. The stress activates a normally latent respiratory motor pathway which mediates the functional recovery in the phrenic nerve ipsilateral to hemisection. This is the CPP and the recovered activity in the phrenic nerve is designated, "crossed phrenic activity." In the present study, blood pressure and end-tidal CO 2 concentration were monitored. Crossed phrenic activity was induced by asphyxia (turning off the ventilator within 30 min following a left C 2 spinal cord hemisection) and recorded from the left phrenic nerve. The mean area under the three largest successive integrated waveforms of the phrenic nerve bursts served as our quantitative assessment of the CPP. The results showed that the mean integrated area in the young rat group was 10.79 ± 1.58 mm 2. The mean integrated area in the older adult rats was significant greater ( P < 0.05) at 41.58 ± 10.05 mm 2. Thus, there is almost a fourfold enhancement of crossed phrenic nerve activity that can be generated in older adult rats as compared to young adult rats. In addition, the CPP persists for significantly longer periods during progressive asphyxial hypoxia in the older adult rats than in the young adult rats. Based on these observations we conclude: as rats age from young maturity to older maturity, there is a gradual reorganization of the neural circuitry which enables older animals to more strongly express the crossed phrenic phenomenon. We hypothesize that periodic respiratory stresses throughout the life of the animal may be responsible for the reorganization of the circuitry. The increased duration of the CPP in the older rats compared to that in the younger rats may be due to a larger residual volume of air in the lungs of the older animals.