Modulating motor cortical neuroplasticity with priming paired associative stimulation in young and old adults

Objective: To examine the effect of priming paired associative stimulation (PAS) on the modulation of motor cortex (M1) plasticity in young and old adults. Methods: Fifteen young (20-27 yrs) and 15 old (61-79 yrs) subjects participated in 3 experimental sessions, with each session involving two consecutive PAS protocols separated by 10 mins. The first (priming) protocol was either PAS(LTP) (ISI = N20 latency + 2 ms), PASLTD (ISI = N20 latency - 10 ms), or PAS(Control) (ISI = 100 ms), whereas the second (test) protocol was always PASLTP. Changes in M1 excitability were assessed from motor evoked potentials (MEPs) in a hand muscle. Results: In young subjects, MEPs were larger after PAS(LTP) + PAS(LTP) than PAS(LTD) + PAS(LTP) (P < 0.0001) and PAS(Control) + PAS(LTP) (P = 0.0008), whereas the response to PAS(Control) + PAS(LTP) was not different to PAS(LTD) + PAS(LTP) (P = 0.3). In old subjects, MEPs were smaller after PAS(LTP) + PAS(LTP) compared with PAS(Control) + PAS(LTP) (P = 0.02), whereas PAS(LTD) + PAS(LTP) was similar to PAS(Control) + PAS(LTP) (P = 0.08). Age-related comparisons within each priming condition showed that the response to PAS(LTP) + PAS(LTP) was significantly greater in young subjects (P = 0.03). Conclusion: Data show that priming with PASLTP was effective in young but not old subjects. Significance: These findings suggest a limited utility of priming PAS for augmenting plasticity induction in old adults. (C) 2017 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.