Association of increased pain threshold by noise with central opioid neurons

Several studies indicated that stress would induce analgesia. Noise, one of the stressors, was assumed to be one of the elements to enhance the threshold of pain tolerance. Since noise might affect human's daily life, it is important to know the mechanism underlying this phenomenon. The objective of this study was to explore the possible mechanism which was trying to explain how the noise affects central nervous system and the possible relationship between this effect and the involvement of opioid neurons. In the preliminary study, the analgesic effect was corroborated in ICR mice in a formalin study. The results are as follows: [1] Naloxone (a micro-opioid receptor antagonist; 1 mg/kg, i.p.), beta-FNA (a delta-opioid receptor antagonist; 5, 10 mg, i.c.v.) and naltrindole (a delta-opioid receptor antagonist; 1, 5 mg/kg, i.p.) were found to reduce antinociceptive effect. [2] nor-BNI (a kappa-antagonist; 1 microg, i.c.v.) had much effect on noise induced analgesic. In conclusion, this study suggests that noise stress enhanced the threshold of analgesia, which might be related to micro- and delta-opioid receptors in the central nervous system.