Plasticity of rat motoneuron rhythmic firing properties with varying levels of afferent and 1 descending inputs

33 Hindlimb motoneuron excitability was compared among exercise-trained (E), sedentary 34 (S), and spinal cord transected (T) Sprague-Dawley rats by examining the slope of the 35 frequency-current (F/I) relationship, using standard intracellular recording techniques in rats 36 anesthetized with ketamine/xylazine. The T group included spinal transected and spinal isolated 37 rats; the E animals were either spontaneously active (exercise wheel), or treadmill trained; and 38 rats in the S group were housed in pairs. An analysis of motoneuron initial (1 inter-spike 39 interval, ISI), early (mean of 1 three ISIs), and steady-state (mean of last three ISIs) discharge 40 rate slopes resulting from increasing and decreasing 500-ms injected square-wave depolarizing 41 current pulses was used to describe rhythmic motoneuron properties. The steepest slope 42 occurred in the S group (55.3 ± 22.2 Hz⋅nA), followed by the T group (35.5 ± 15.3 Hz⋅nA), 43 while the flattest slope was found in the E group (25.4 ± 10.9 Hz⋅nA). The steepest steady-state 44 slope occured in the S group, but was found to be similar between the T and E groups. 45 Furthermore, a spike-frequency adaptation index revealed a slower adaptation in motoneurons of 46 the E animals only (~40% lower). Finally, evidence for a secondary range of firing existed more 47 frequently in the T group (41%) compared to the S (12%) and E (31%) groups. The lower F/I 48 slope and lower SFA index of motoneurons for E rats may be a result of an increase in Na 49 conductance at the initial segment. The results show that motoneuronal rhythmic firing behavior 50 is plastic, depending on the volume of daily activation and on intact descending pathways. 51 52 Abbreviations: E, exercise; T, transected; S, sedentary; F/I, frequency-current; ISI, interspike 53 interval; ESFF, early-state firing frequency; SSFF, steady-state firing frequency; SFA, spike54 frequency adaptation; AHP, afterhyperpolarization; DCC, discontinuous current clamp 55