Associative plasticity in intracortical inhibitory circuits in human motor cortex.
OBJECTIVE
Paired associative stimulation (PAS) is a transcranial magnetic stimulation technique inducing Hebbian-like synaptic plasticity in the human motor cortex (M1). PAS is produced by repetitive pairing of a peripheral nerve shock and a transcranial magnetic stimulus (TMS). Its effect is assessed by a change in size of a motor evoked response (MEP). MEP size results from excitatory and inhibitory influences exerted on cortical pyramidal cells, but no robust effects on inhibitory networks have been demonstrated so far.


METHODS
In 38 healthy volunteers, we assessed whether a PAS intervention influences three intracortical inhibitory circuits: short (SICI) and long (LICI) intracortical inhibitions reflecting activity of GABA(A) and GABA(B) interneurons, respectively, and long afferent inhibition (LAI) reflecting activity of somatosensory inputs.


RESULTS
After PAS, MEP sizes, LICI and LAI levels were significantly changed while changes of SICI were inconsistent. The changes in LICI and LAI lasted 45 min after PAS. Their direction depended on the delay between the arrival time of the afferent volley at the cortex and the TMS-induced cortical activation during the PAS.


CONCLUSIONS
PAS influences inhibitory circuits in M1.


CONCLUSIONS
PAS paradigms can demonstrate Hebbian-like plasticity at selected inhibitory networks as well as excitatory networks.
Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, USA. Heike.Russmann@chuv.ch
Heike
H
Russmann
Jean-Charles
JC
Lamy
Ejaz A
EA
Shamim
Sabine
S
Meunier
Mark
M
Hallett
D000328:Adult; D000368:Aged; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D005385:Fingers / Q000294:innervation / Q000502:physiology; D006801:Humans; D007395:Interneurons / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009415:Nerve Net / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology*; D018079:Receptors, GABA / Q000502:physiology; D013569:Synapses / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation
D018079:Receptors, GABA