Homeostatic plasticity of the nociceptive system after priming the motor cortex: a pilot study

Background and aims: Homeostatic plasticity (HP) stabilizes neuronal activity within a physiological range and may be important in pain-related neuroplasticity. HP has been demonstrated by a reversal of the cortical motor excitability after two blocks of non-invasive brain stimulation. In chronic and experimental pain, HP is impaired in the motor cortex (M1). The aim is to investigate if the nociceptive system is regulated by HP.

Methods: A randomized placebo-controlled pilot study was conducted with 9 healthy individuals. HP was induced in the left M1 by applying anodal tDCS at C3 and FP2 (return electrode). A capsaicin (pain) or placebo (control) patch was applied to the right hand dorsum. To assess the nociceptive response to HP induction, 60 trials of nociceptive evoked potentials using a concentric pin electrode on the right volar forearm were recorded at 62 channels. Four-time points were measured: baseline, 30min post-patch, post-HP, and 20min post-HP. The negative-positive complex (N-P) was extracted from Cz.

Results: After 30min post-patch, the mean capsaicin-induced pain remained above 5±1.75, while for placebo it remained 0±0.16. A two-way-mixed ANOVA of the N-P revealed a significant effect of time (p=0.001) and interaction (p=0.006), but not of groups (p=0.947). For placebo, N-P was increased post-HP (P=0.052) and decreased beyond baseline post 20min (P=0.011). The capsaicin group decreased at every timepoint (P<0.002), except between patch and post (P=0.76).

Conclusions: Based on these pilot findings, no clear indication of an HP response is observed from the nociceptive system. Possibly suggesting that the assumed HP in M1 may be an isolated act.