[Effect of hyperbaric air on biorhythms of the pain sensitivity and analgesic effect of morphine].

The circadian and high-frequency (2-2.5 h) pain sensitivity rhythms were observed in male mongrel rats under natural illumination conditions (LD 24, January-February). A minimum pain sensitivity threshold with respect to subcutaneous electrode stimulation was observed in the dark phase of the LD 24 cycle. Morphine injections (5 mg/kg, i.p.) led to inversion of the circadian rhythm and expansion of the high-frequency spectrum. The rhythm synchronism was broken under the hyperbaria conditions, whereby the circadian acrophase shifted to exhibit a maximum within the day light phase. The concurrent morphine administration on the hyperbaria background induced the back inversion of the circadian pain sensitivity rhythm. The high-frequency rhythm was not significantly affected by increased pressure--neither alone nor in combination with morphine injections.