Analgesic effect of environmental noise: a possible stress response in rats.

Environmental noise is a known stress, which induces alterations of various physiological responses in individuals exposed to it. Stress has been shown to cause changes in the perception of various sensations including pain and stress-induced analgesia has been observed following exposure to a diverse set of stimuli. To examine the algesic behavior of rats exposed to loud environmental noise, for long duration, we used an environment simulating chamber and conducted the tail flick test for the assessment of pain. The rats were divided into groups and subjected to loud noise for test sessions lasting 1 h, 2 h or 3 h in trials of 5 consecutive days. The noise was of two kinds--a continuous shrill noise (pure tone 92 dB & 98 dB) and an intermittent heavy artillery noise (white noise 102 dB). 15 min before and after each test session, tail flick latencies (TFL) were recorded at 5 min interval. The TFL recorded were normalised to an Index of Analgesia (IA) and the readings statistically analyzed using the F test (ANOVA), the significance being obtained by Tukey's test (at 5% level). The results revealed a significant increase in the TFL and the IA (P < 0.0001) in all the test groups demonstrating a significant analgesic response in rats subjected to noise stress. The analgesia was maximum immediately after noise exposure and declined with time. It was found to be directly related to the duration of exposure, the intensity and the characteristics of the noise with loud intermittent (white) noise and longer duration of exposure producing more analgesia.