Cutaneous magnetic stimulation reduces rat chronic pain via activation of the supra-spinal descending pathway

Abstract

Recent studies have demonstrated that magnetic stimulation (MS) can induce cellular responses such as Ca(2+) influx into the cultured neurons and glia, leading to increased intracellular phosphorylation. We have demonstrated previously that MS reduces rat neuropathic pain associated with the prevention of neuronal degeneration. Thus, we aimed to elucidate the actions of MS in relation to modulation of spinal neuron-glia and the descending inhibitory system in chronic pain. The male SD rats intrathecally implanted with catheters were subjected to sciatic nerve ligation (CCI). MS is a low power apparatus characterized by two different frequencies, 2 KHz and 83 MHz. Rats were given MS to the skin (injured sciatic nerve) for 10 min from the seventh day after CCI. The paw withdrawal latency (PWL) evoked by thermal stimuli was measured for 14 days after CCI. Immunohistochemistry for Iba-1 or GFAP was performed after 4% paraformaldehyde fixation (microscopic analysis). We employed microdialysis for measuring CSF 5-HIAA as a reflection of 5-HT release by MS stimulation. Following CCI, rats showed a decrease in PWL after CCI, and the decrease continued until the 14th day. With MS treatment, the decrease in PWL was reduced during the 10-14 day after CCI. Injection of JNK-1 inhibitors on the 14th day antagonized the analgesic effect of MS. MS also eliminated the CCI-induced decrease in GFAP immunoreactivity. Moreover, MS evoked spinal 5-HT release reflected by increase in spinal 5-HIAA level. Thus, we demonstrate that a novel magnetic stimulator used cutaneously can ameliorate chronic pain by not only preventing abnormal spinal neuron-glia interaction, but also through the activation of the supra-spinal descending inhibitory system.

Fig. 1

Schematic drawing of experimental design. Rats were intrathecally implanted with either a PE-10 catheter or microdialysis probe depending on the protocol. Magnetic stimulation (MS) was subsequently given to the rat’s skin at the location of the injured sciatic nerve

Fig. 2

Microphotograph of magnetic stimulator. a Generator attached to control system with computer. b Probe for cutaneous MS

Fig. 3

Time course of paw withdrawal latency (PWL) after chronic constriction injury (CCI). Chronic treatment with MS reduced the decrease of PWL assessed by response to thermal stimulation to the injured hind paw. Values are indicated as mean ± SEM. Filled circle Sham-operated (n = 6), filled square untreated (n = 8), filled triangle MS (n = 6), *P < 0.05 versus Sham-operated, ^# P < 0.05 versus untreated

Fig. 4

Modulation of analgesic effects of MS by injecting either anti-BDNF or MAPKs inhibitors after CCI. The analgesic effect of MS was significantly antagonized by p38-MAPK and JNK-1 inhibitors, suggesting MS induces BDNF from glia and mediates intracellular phosphorylation. Values are indicated as mean ± SEM. *P < 0.05 versus untreated, ^# P < 0.05 versus MS + medication

Fig. 5

Representative microphotograph for astrocytes stained by GFAP antibody. Arrows indicate cells with positive staining for the antibody

Fig. 6

Changes in immunoreactivity for staining of astrocytes (GFAP) during the chronic stage of CCI. The number of activated astrocytes expressed by GFAP on the ipsilateral side of spinal cord significantly decreased on day 14 in the untreated group (P < 0.05). In contrast, chronic treatment with MS reduced the decrease of activated astrocytes immunoreactivity. Values are indicated as mean ± SEM. *P < 0.05 versus Sham-operated, ^# P < 0.05 versus MS

Fig. 7

Modulation of the analgesic effect of MS by injecting local or systemic naloxone. The analgesic effect of MS was significantly antagonized by naloxone administration, indicating that MS evokes descending enkephalinergic inhibitory system. Values are indicated as mean ± SEM. *P < 0.05 versus untreated, ^# P < 0.05 versus MS + medication

Fig. 8

Time course of CSF 5-HIAA levels evoked by cutaneous magnetic stimulation. A transient increase of CSF 5-HIAA was found at 10 min collection after the start of MS, indicating that MS can activate the bulb spinal descending serotoninergic inhibitory system. Values are indicated as mean ± SEM. *P < 0.05 versus control