Papers of the Week

Social environment influences the trajectory of developing opioid use disorder (OUD). Thus, the present study tested the hypothesis that sociability levels will affect the responses to opioids. Mice were tested for their baseline sociability, anxiety levels, pain sensitivities, and their acute locomotor response to 5 mg/kg opioids. Then, they were administered repeatedly with saline, hydrocodone, or morphine (20 mg/kg for 5 days, and then 40 mg/kg for 5 days). Subsequently, they were examined for the expression of locomotor sensitization and retested for the effects of opioids on their sociability, anxiety levels, and pain sensitivity. On the basis of their baseline sociability level, mice were divided into socially avoiding and socially exploring. Socially avoiding and socially exploring mice did not differ in their baseline weight and anxiety sensitivities. Socially avoiding mice had slightly higher baseline heat sensitivity than those in socially exploring mice. Repeated administration of opioids had differential effects in socially avoiding and socially exploring mice. In both social groups, repeated morphine administration had overall stronger effects compared with hydrocodone. Morphine-treated socially exploring mice developed greater locomotor sensitization than those in morphine-treated socially avoiding mice. Morphine-treated socially avoiding mice, but not socially exploring mice, spent more time in the center zone of the open-field test and in the light zone of light/dark boxes, and developed heat hyperalgesia. This study suggests that socially exploring animals are more sensitive to the sensitizing effects of opioids. In contrast, opioids have greater effects on the stress and pain systems of socially avoiding animals. Thus, the underlying mechanisms for developing OUD might differ in individuals with various sociability levels.