ACUTE EFFECT OF NORADRENERGIC MODULATION ON MOTOR OUTPUT AND CORTICOSPINAL EXCITABILITY

INTRODUCTION: Despite an extensive literature in animals on the role of the noradrenergic system in the control of many higher functions (arousal, attention, sensory integration, etc. [1]) and in modulation of spinal excitability [2], very little is known on noradrenergic actions on the motor system and the functional consequences in human. Therefore, the aim of present study was to investigate the role of noradrenergic modulation in the control of motor output, by comparing the acute effect of Reboxetine (REB), a noradrenaline reuptake inhibitor, to a placebo (PLA) on knee extensors performance and corticospinal and spinal excitability. METHODS: Eleven young males took part in two randomized experiments during which they received either 8 mg of REB or a PLA. The torque produced during a maximal voluntary contraction (MVC) and its variability, quantified as the coefficient of variation for torque, during submaximal contractions ranging from 5 to 50% MVC were measured. Paired electrical (PES) and transcranial magnetic stimulation (TMS) were used to assess changes in voluntary activation during MVC. Electrical stimulation and TMS were also used to assess spinal and corticospinal excitability by recording the Hoffman reflex (H reflex) and TMS-induced motor evoked potential (MEP) recruitment curves during muscle activation at 20% MVC. RESULTS: MVC torque and torque steadiness increased (P<0.001) respectively by 9.5 and 24% (mean change for 5-50% MVC) in REB compared with PLA condition. Voluntary activation tested by TMS and PES was greater (~3%; P<0.05) in REB than PLA condition. The maximal amplitudes of H reflex and MEP and, the slope of their recruitment curves were significantly enhanced by REB (P<0.05). The ratio between TMS-induced EMG silent period and the corresponding MEP (SP/MEP) was reduced in REB condition (P<0.01). CONCLUSION: The present findings indicate that intake of a noradrenaline reuptake inhibitor increases voluntary activation during MVC and improves the accuracy in force control. The motor performance improvements were accompanied by a greater corticospinal and spinal excitability. In conclusion, our results support a significant role for the noradrenergic system in adjusting the motor output during maximal and steady contractions. References: 1. Berridge CW, Waterhouse BD (2003). Brain Research Reviews. 42(1):33-84. 2. Heckman CJ, Mottram C, Quinlan K, Theiss R, Schuster J (2009). Clin Neurophysiol. 120(12):2040-54.