Abstract

The current investigations on social stress primarily point to the negative health consequences of being in a stressful social hierarchy. The repetitive nature of such stressors seems to affect behavioral response to pain both in rodents and humans. Moreover, a large discrepancy in the possibility of social stresses affecting pain perception in the two genders exists. The present study examined the effect of chronic social stress on nociceptive responses of both sexes by implementing of food deprivation, food intake inequality and unstable social status (cage-mate change every 3 days) for a period of 14 days in 96 Balb/c mice. In this regard we injected 20 µl formalin 2% into the plantar surface of hind paw at the end of stress period and scored pain behaviors of all subjects, then serum concentrations of proinflammatory cytokines were measured. Our results showed that there was significant difference in chronic phase of formalin test following implementation of food deprivation and inequality (P<0.05) as compared to control group, so that pain perception was decreased considerably and this decline in inequality exposed subjects was well above isolated ones (P<0.05); whereas unstable social situation did not affect pain perception. Moreover, IL-1 and IL-6 concentrations in serum of stressed mice of both genders were well above control group (p<0.05). Finally, despite chronic pain perception in control and unstable male subjects was larger than females; the decrease of chronic pain perception in male stressed animals (poverty and inequality experienced subjects) was much more than stressed females. These results revealed that although food deprivation and social inequality can induce hypoalgesia, some socioeconomic situations like social instability don't affect pain sensation, whereas there were similar increases of proinflammatory cytokines level in all socially stressed subjects. In addition, males display larger hypoalgesic responses to inequality as compared with females.

Highlights

  • Health is not just the outcome of genetic or biological processes but is influenced by the social and economic conditions in which we live

  • A first peak of Licking behavior during the first 6 minute block reflects the behavioral response to acute pain, whereas the second part of the curve represents persistent pain. This biphasic response was not observed in animals which underwent food deprivation merely or coincided with food inequality (Figures 2– 5); according to these findings pain behavior had no increase at the second phase of formalin test in food deprived and inequality experienced animals which showed a significant effect of these social stressors on creation of hypoalgesia (p,0.05)

  • In animals which just experienced cage-mate change a biphasic curve after injection of formalin was created as same as control groups; representing that chronic pain perception was not affected by this type of social stressor (Figure 6)

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Summary

Introduction

Health is not just the outcome of genetic or biological processes but is influenced by the social and economic conditions in which we live. These influences have become known as the ‘social determinants of health’. Other studies have reported that repeated exposure to such stressors can potentiate acute pain perception in humans and animals [10,11,12,13,14]. It has been shown both short-term and intermittent food deprivation diminishes acute nociception in laboratory rodents. Numerous animal models exist for the exploration of mechanism(s) and mediators of persistent pain in particular [16] and such studies in rodents addressing the link between the pain and social stresses are likely to be relevant in humans [17]

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