NMDA-receptor agonist reveals LTP-like properties of 10-Hz rTMS in the human motor cortex

Abstract

Repetitive Transcranial Magnetic Stimulation (rTMS) is a safe and effective treatment for a growing number of neuropsychiatric disorders. While the utilization of clinical rTMS is advancing rapidly, it is happening without a clear mechanistic understanding of how TMS changes the brain. Improved mechanistic understanding could ultimately improve clinical treatments through targeted navigation of the infinite rTMS parameter space and potential pharmacological augmentation. The early hypothesis that high-frequency (≥5-Hz) rTMS works through long-term potentiation (LTP) at the cellular level was based on limited evidence, and competing hypotheses are gaining attention. However, LTP-like effects have been observed with 10-Hz magnetic stimulation in mouse hippocampal slices, including increased dendritic spine size, increased GluA1-receptor composition, and n-methyl-d-aspartate (NMDA)-receptor-dependent potentiation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptor-mediated post-synaptic currents [[1]Vlachos A. Muller-Dahlhaus F. Rosskopp J. Lenz M. Ziemann U. Deller T. Repetitive magnetic stimulation induces functional and structural plasticity of excitatory postsynapses in mouse organotypic hippocampal slice cultures.J Neurosci. 2012; 32: 17514-17523Crossref PubMed Scopus (115) Google Scholar]. In the human motor cortex, various plasticity-promoting rTMS protocols depend on NMDA-receptors [2Huang Y.Z. Chen R.S. Rothwell J.C. Wen H.Y. The after-effect of human theta burst stimulation is NMDA receptor dependent.Clin Neurophysiol. 2007; 118: 1028-1032Crossref PubMed Scopus (361) Google Scholar, 3Suppa A. Biasiotta A. Belvisi D. Marsili L. La Cesa S. Truini A. Cruccu G. Berardelli A. Heat-evoked experimental pain induces long-term potentiation-like plasticity in human primary motor cortex.Cerebr Cortex. 2013; 23: 1942-1951Crossref PubMed Scopus (36) Google Scholar, 4Stefan K. Kunesch E. Benecke R. Cohen L.G. Classen J. Mechanisms of enhancement of human motor cortex excitability induced by interventional paired associative stimulation.J Physiol. 2002; 543: 699-708Crossref PubMed Scopus (482) Google Scholar]. However, specificity is lacking, as it is unclear if NMDA receptor activation is sufficient to enhance rTMS-induced facilitation. For example, NMDA-receptor activation with d-cycloserine (DCS) was insufficient to enhance facilitation induced by intermittent theta burst stimulation (iTBS) in two independent studies [[5]Teo J.T.H. Swayne O.B. Rothwell J.C. Further evidence for NMDA-dependence of the after-effects of human theta burst stimulation.Clin Neurophysiol. 2007; 118Crossref Scopus (72) Google Scholar,[6]Selby B. MacMaster F.P. Kirton A. McGirr A. D-Cycloserine blunts motor cortex facilitation after intermittent theta burst transcranial magnetic stimulation a double-blind randomized placebo-controlled crossover study.Brain Stimul. 2019; 12 (2019): 1063-1065Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar]. By contrast, DCS was sufficient to enhance 10-Hz rTMS-mediated facilitation over 1-hour in a crossover design, again supporting an LTP-like mechanistic hypothesis [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. Combining pharmacology and rTMS with mechanism-revealing neurophysiology protocols is an unexplored approach. Paired-pulse protocols including intracortical inhibition and intracortical facilitation (ICI and ICF) are well-established. They capitalize on differential stimulation intensity sensitivities of GABAergic and glutamatergic neurons, respectively [[8]Ziemann U. Rothwell J.C. Ridding M.C. Interaction between intracortical inhibition and facilitation in human motor cortex.J Physiol. 1996; 496: 873-881Crossref PubMed Scopus (876) Google Scholar]. We hypothesized that ICF and ICI would reveal LTP-like features underlying previously published 10 Hz rTMS-induced facilitation with DCS from the same cohort [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. Ten healthy adults (4 female) aged 26–37 years who met TMS safety requirements participated in the study. All participants provided informed consent. This study was approved by the Medical University of South Carolina Institutional Review Board. Participants were randomly assigned to receive 100 mg DCS or identical microcrystalline cellulose capsules (Tidewater pharmacy, Mt. Pleasant, SC) in a double-blinded, randomized, crossover study. rTMS was administered at the time of peak drug bioavailability, which is 2 hours after oral ingestion (Fig. 1A). Neuronavigated determination of primary motor cortex (M1) hotspot and resting motor threshold (rMT) was previously described [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. Paired-pulse measures were obtained before and after rTMS, or approximately 1 hour, and 3 hours after drug disbursement, respectively [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. Paired pulses were separated by inter-stimulus interval (1) of 3 ms for ICI, and 15 ms for ICF [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. The conditioning stimulus (CS) was subthreshold intensity (80% rMT), and the testing stimulus (TS) was at a predetermined rMT for 1 mV (∼120% rMT). We collected single- and paired-pulse MEPs with Magstim 2002 and BiStim capacitors (Magstim, UK) in bins of 20 pulses, jittered 4–7 sec apart. We analyzed MEPs with Spike2 software (Cambridge Electronic Devices, UK). We amplified and filtered the raw signal with CED 1902 and 1401 microprocessors (Cambridge Electronic Devices, UK). We administered 10-Hz rTMS over the left M1 with MagPro R30 with figure-8 B65 cooled coil (MagVenture, Denmark) because the MagStim device could not deliver rTMS. We delivered 300 pulses over 20 minutes with a 1.5 sec-on/58.5 sec-off duty cycle at 80% rMT (rMT obtained separately for MagVenture system), as previously described [[7]Brown J.C. DeVries W.H. Korte J.E. Sahlem G.L. Bonilha L. Short E.B. et al.NMDA receptor partial agonist, d-cycloserine, enhances 10 Hz rTMS-induced motor plasticity, suggesting long-term potentiation (LTP) as underlying mechanism.Brain Stimul. 2020; 13: 530-532Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar]. We calculated the average MEP peak-to-peak amplitude for each bin to create a paired-pulse/single-pulse ratio (MEPPP/MEPSP) to yield ICF and ICI measures in respective protocols, then calculated change in ICF and ICI associated with rTMS (i.e., Δ = post-rTMS value minus pre-rTMS value) to generate ΔICF and ΔICI. Wilcoxon signed-rank tests compared ΔICF and ΔICI between DCS and PBO conditions within subjects (e.g., ΔICFDCS vs. ΔICFPBO) to assess within-subject relative change. Two-sided P value < 0.05 was considered statistically significant. Mean ΔICFDCS was −0.48, compared to 0.36 for ΔICFPBO (Fig. 1B, p = 0.037, Cohen’s d = 1.13). Mean ΔICIDCS was −0.26 with DCS, compared to 0.25 for ΔICIPBO (Fig. 1D, p = 0.027, Cohen’s d = 1.16). At face value, these results were paradoxical. In order to better understand them, we examined the raw MEP values before ratios were calculated (Fig. 1C and E, MEP values in legends). The PBO condition produced modest facilitation before rTMS, which then became robust (steeper slope) after rTMS (Fig. 1C). This is consistent with the hypothesis that ‘excitatory’ rTMS would enhance glutamatergic tone in an LTP-like manner. DCS, by contrast, produced robust facilitation (Mean (PBO) ICF: 1.69 vs. 1.25) before rTMS; consistent with a glutamatergic (NMDA receptor) agonist effect. We attribute the modest ICF and lack of facilitation after 10 Hz in the PBO group to the inherent MEP variability from a single time point (consider [[2]Huang Y.Z. Chen R.S. Rothwell J.C. Wen H.Y. The after-effect of human theta burst stimulation is NMDA receptor dependent.Clin Neurophysiol. 2007; 118: 1028-1032Crossref PubMed Scopus (361) Google Scholar]), which can be mitigated through same-subject comparisons enabled by a crossover design. Intriguigingly, after rTMS, which creates a high baseline MEPSP (Mean ± SE, 1.55 ± 0.28), the subsequent facilitation is quite modest, suggesting a saturation or ‘occlusion’ effect - a classic finding in LTP studies [[9]Whitlock J.R. Heynen A.J. Shuler M.G. Bear M.F. Learning induces long-term potentiation in the Hippocampus.Science. 2006; 313: 1093-1097Crossref PubMed Scopus (1308) Google Scholar]. Interestingly, observations of numerous human MEP studies suggest there may be a physiologic upper-limit near 75% above baseline. In contrast to ICF, ICI was mitigated by rTMS alone (i.e., PBO; Fig. 1E), again consistent with the expected effect following an ‘excitatory’ stimulation protocol. DCS, on the other hand, expectedly blunted ICI before rTMS, but unexpectedly, produced the strongest inhibition after rTMS. We speculate that this effect may be the result of concurrent homeostatic depression superimposed on a short-term plasticity ICI protocol [[10]Turrigiano G. Homeostatic synaptic plasticity: local and global mechanisms for stabilizing neuronal function.Cold Spring Harb Perspect Biol. 2012; 4a005736Crossref PubMed Scopus (535) Google Scholar]. Animal studies are needed to verify this hypothesis, but we find it remarkable that these results occur in conjunction with the ‘occlusion’ observed with ICF. Moreover, all MEPSP data and PBO data are consistent with what we might expect from ‘excitatory’ rTMS and NMDA receptor agonist activity. These results suggest that 10-Hz rTMS produces an excitatory effect through glutamatergic activity; and that NMDA receptor partial agonist DCS in combination with 10-Hz rTMS may produce occlusion of ICF, and homeostatic depression unmasked by ICI. In summary, these results suggest LTP-like mechanisms underlying 10-Hz rTMS in the human motor cortex; though our small sample size warrants caution to avoid type I error. Replication is warranted. The authors declare no conflict of interests. This work was supported by the DART Training Grant at MUSC, NIDA Grant number: R25DA020537 , and the National Institute Of General Medical Sciences of the National Institutes of Health under Award Number P20GM130452 , Center for Biomedical Research Excellence , Center for Neuromodulation . Dr. Yuan’s effort was supported by NIMH grant R25MH101076 . The following is the Supplementary data to this article: Download .pdf (.21 MB) Help with pdf files Multimedia component 1

Topics & Concepts