Visceral pain was induced in experiments on albino mice by i.p. injection of an acetic acid solution (acetate test, AT); under such conditions, we studied the involvement of the opioid cerebral system in analgesia induced by broad-range "noise" microwave irradiation (mode 1, frequency 30 to 300 GHz) and fixed-frequency irradiation (mode 2, frequency 61 ± 4 GHz) of the acupuncture point (AP) Е-36. Microwave irradiation of this AP in both modes used led to shorter durations of the AT-induced pain behavioral reaction. Such a decrease was more intense in mode 1 (by 35.7%, on average) than in mode 2 (by 20.3%). After injections of a blocker of opioid receptors, naloxone (1 or 5 mg/kg), the duration of the pain reaction under conditions of microwave irradiation of the AP in mode 1 was 49.3 and 99.8% greater, respectively, as compared with the values with no naloxone injections, while the respective naloxone-induced increments were 42.8 and 55.6% when using mode 2. These results support the existing concept on significant involvement of the opioid system in analgesia resulting from the action of low-intensity microwaves on the AP Е-36.
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