Inhibiting social support from massage-like stroking increases morphine dependence.

Our previous studies showed that altering solely the drug experience of the cage mates with which rodents are housed affects the development of morphine dependence. In this study, we used designer receptors exclusively activated by designer drugs to artificially increase or decrease the activity of peripheral dorsal root ganglia sensory neurons expressing the G-protein-coupled receptor MRGPRB4. This is because sensory MRGPRB4-expressing neurons were shown to specifically detect the sensation of massage-like stroking resulting from social grooming, which is an important affiliative social behavior in the rodent. Blocking the sensation of social grooming in morphine-treated mice housed with drug-naive mice (i.e. morphine cage mates) significantly increased the display of jumping behavior in morphine-withdrawn animals. Activating the sensation of social grooming in morphine-treated animals housed solely with other morphine-treated animals (i.e. morphine only) did not significantly alter the display of jumping behavior in morphine-withdrawn animals. Repetitive jumping behaviors have been shown to correlate with morphine dependence. Thus, this study showed a role of social grooming in the protective effect of being housed with drug-naive mice on the development of morphine dependence. It further confirms a role of social support in the development of substance use problems.