Map activation of various brain regions using different frequencies of electroacupuncture ST36, utilizing the Fos-CreER strategy

Objective: Electroacupuncture (EA) is an alternative treatment option for pain. Different frequencies of EA have different pain-relieving effects; however, the central mechanism is still not well understood. Methods: The FosTRAP:Ai9 mice were divided into three groups (sham, 2 Hz, and 100 Hz). The mice were intraperitoneally injected with 4-hydroxytamoxifen (4-OHT) immediately after EA at Zusanli (ST36) for 30 min to record the activated neurons. One week later, the mice were sacrificed, and the number of TRAP-treated neurons activated by EA in the thalamus, amygdala, cortex, and hypothalamus was determined. Results: In the cortex, 2 Hz EA activated more TRAP-treated neurons than 100 Hz EA did in the cingulate cortex area 1 (Cg1) and primary somatosensory cortex (S1), and 2 Hz and 100 Hz EAs did not differ from sham EA. TRAP-treated neurons activated by 2 Hz EA were upregulated in the insular cortex (IC) and secondary somatosensory cortex (S2) compared with those activated by 100 Hz and sham EA. In the thalamus, the number of TRAP-treated neurons activated by 2 Hz EA was elevated in the paraventricular thalamic nucleus (PV) compared with those activated by sham EA. In the ventrolateral thalamic nucleus (VL), the number of TRAP-treated neurons activated by 2 Hz EA was significantly upregulated compared with those activated by 100 Hz EA, and sham EA showed no difference compared to 2 Hz EA or 100 Hz EA. TRAP-treated neurons were more frequently activated in the ventral posterolateral thalamic nucleus (VPL) by 2 Hz EA than by 100 Hz or sham EA. Conclusion: Low-frequency EA ST36 effectively activates neurons in the Cg1, S1, S2, IC, VPL, PV, and VL. The enhanced excitability of the aforementioned nuclei induced by low-frequency EA may be related to its superior efficacy in the treatment of neuropathological pain.