Pain
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Chronic pain is often accompanied by anxiety and depression disorders. Amygdala nuclei play important roles in emotional responses, fear, depression, anxiety, and pain modulation. The exact mechanism of how amygdala neurons are involved in pain and anxiety is not completely understood. ⋯ Optogenetic activation of SOM+ neurons slightly reduced the mechanical hyperalgesia in the pain model but did not change the mechanical sensitivity in naïve mice. Instead, it induced anxiety-like behavior. Our results suggest that the PKCδ+ and SOM+ neurons in the central amygdala exert different functions in regulating pain-like and anxiety-like behaviors in mice.
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Opioids are the frontline analgesics in pain management. However, chronic use of opioid analgesics causes paradoxical pain that contributes to the decrease of their efficacy in pain control and the escalation of dose in long-term management of pain. The underling pathogenic mechanism is not well understood. ⋯ Surprisingly, ablating microglia using these specific and effective approaches did not cause detectable impairment in the expression of hyperalgesia induced by morphine. We confirmed this conclusion with a behavioral test of mechanical and thermal hyperalgesia, in male and female mice, and with different species (mouse and rat). These findings raise caution about the widely assumed contribution of microglia to the development of opioid-induced hyperalgesia.