Muscarinic receptor blockade has differential effects on the excitability of intracortical circuits in the human motor cortex.

The aim of the present study was to investigate whether muscarinic receptor blockade with scopolamine modifies the excitability of specific cortical networks of the human motor cortex as tested with transcranial magnetic stimulation. The effects of scopolamine on the excitability of human motor cortex were investigated in four healthy subjects using transcranial magnetic stimulation before and after an intravenous dose of scopolamine (0.006 mg/kg). We measured the threshold for motor responses, amplitude of motor responses, the duration of the cortical silent period, intracortical inhibition and facilitation, and short-latency inhibition produced by somatosensory input from the hand. In addition, we evaluated the amplitude of motor responses evoked by electrical anodal stimulation, since these responses originate from direct activation of corticospinal axons in the white matter and are not sensitive to changes in cortical excitability. Scopolamine decreased the threshold to magnetic stimuli and increased the amplitude of motor responses evoked by magnetic stimulation. In contrast, motor responses evoked by electrical stimulation were unaffected by administration of scopolamine. Scopolamine also led to a highly significant reduction in the amount of short-latency inhibition produced by somatosensory input from the hand. In contrast, short-latency intracortical inhibition and facilitation were not modified by scopolamine. The differential effect of scopolamine on motor responses evoked by magnetic and electrical stimulation of the motor cortex and the selective effect on somatosensory inhibition demonstrate that muscarinic blockade modifies the excitability of specific cortical networks in the human motor cortex.