Does elimination of afferent input modify the changes in rat motoneurone properties that occur following chronic spinal cord transection?
Résumé
The purpose of this study was to determine the effects of 6-8 weeks of chronic spinal cord isolation (SI, removal of descending, ascending and afferent inputs), compared to the same duration of spinal cord transection (ST, removal of descending input only) on hindlimb motoneurone biophysical properties. Adult female Sprague-Dawley rats were placed into three groups: 1) Con (no removal of descending or afferent inputs), 2) ST, and 3) SI. The electrophysiological properties from sciatic nerve motoneurones were recorded from deeply anesthetized rats. Motoneurones in SI rats had significantly (p < 0.01) lower rheobase currents and higher spike afterhyperpolarization amplitudes and input resistances compared to motoneurones in Con rats. A higher percentage (Chi-square, p = 0.01) of motoneurones in SI than Con rats demonstrated frequency-current (f-I) relationships consistent with activation of persistent inward currents. Motoneurone steady state f-I slopes determined by increasing steps of 500-ms current pulses were significantly lower (p < 0.02) in SI than Con rats. Motoneurone spike frequency adaptation measured using 30-s square-wave current injections (1.5 - 3.0 nA above the estimated rhythmic firing threshold), was similar for Con and SI motoneurones. Changes in motoneurone properties following SI did not differ from ST. These findings indicate that the removal of afferent and ascending inputs along with descending inputs has little additional affect on motoneurone properties than removal of descending inputs alone. This study is the first to demonstrate that intact ascending and afferent input does not modify the effects of spinal transection on basic and rhythmic firing properties of rat hindlimb motoneurones.