Chronic exercise increases excitability of lamina X neurons through enhancement of persistent inward currents and dendritic development in mice

Chronic exercise has been shown to enhance excitability of spinal interneurons in rodents. However, the mechanisms underlying this enhancement remain unclear. In this study we investigated adaptability of lamina X neurons with 3-week treadmill exercise in mice of P21-P24. Whole-cell patch-clamp recording was performed on the interneurons from slices of T12-L4. The experimental results included the following. (1) Treadmill exercise reduced rheobase by 7.4 +/- 2.2 pA (control: 11.3 +/- 6.1 pA, n = 12; exercise: 3.8 +/- 4.6 pA, n = 13; P = 0.002) and hyperpolarized voltage threshold by 7.1 +/- 1.5 mV (control: -36.6 +/- 4.6 mV, exercise: -43.7 +/- 2.7 mV; P = 0.001). (2) Exercise enhanced persistent inward currents (PICs) with increase of amplitude (control: 140.6 +/- 56.3 pA, n = 25; exercise: 225.9 +/- 62.5 pA, n = 17; P = 0.001) and hyperpolarization of onset voltage (control: -50.3 +/- 3.6 mV, exercise: -56.5 +/- 5.5 mV; P = 0.001). (3) PICs consisted of dihydropyridine-sensitive calcium (Ca-PIC) and tetrodotoxin-sensitive sodium (Na-PIC) components. Exercise increased amplitude of both components but hyperpolarized onset voltage of Na-PIC only. (4) Exercise reduced derecruitment current of repetitive firing evoked by a current bi-ramp and prolonged firing in the falling phase of the bi-ramp. The derecruitment reduction was eliminated by bath application of 3 mu M riluzole or 25 mu M nimodipine, suggesting that both Na-PIC and Ca-PIC contributed to the exercise-prolonged hysteresis of firing. (5) Exercise facilitated dendritic development with significant increase in dendritic length by 285.1 +/- 113 mu m (control: 457.8 +/- 171.8 mu m, n = 12; exercise: 742.9 +/- 357 mu m, n = 14; P = 0.019). We concluded that 3-week treadmill exercise increased excitability of lamina X interneurons through enhancement of PICs and increase of dendritic length. This study provided insight into cellular and channel mechanisms underlying adaptation of the spinal motor system in exercise.

Automated summary

This study investigated the adaptability of lamina X neurons in mice after 3-week treadmill exercise and found that exercise enhanced excitability by increasing persistent inward currents and dendritic length. These findings provide insight into the cellular and channel mechanisms underlying the adaptation of the spinal motor system to exercise.