Naloxone increases conditioned fear responses during social buffering in male rats.

Social buffering is the phenomenon in which the presence of an affiliative conspecific mitigates stress responses. We previously demonstrated that social buffering completely ameliorates conditioned fear responses in rats. However, the neuromodulators involved in social buffering are poorly understood. Given that opioids, dopamine, oxytocin and vasopressin play an important role in affiliative behaviour, here, we assessed the effects of the most well-known antagonists, naloxone (opioid receptor antagonist), haloperidol (dopamine D2 receptor antagonist), atosiban (oxytocin receptor antagonist) and SR49059 (vasopressin V1a receptor antagonist), on social buffering. In Experiment 1, fear-conditioned male subjects were intraperitoneally administered one of the four antagonists 25 min prior to exposure to a conditioned stimulus with an unfamiliar non-conditioned rat. Naloxone, but not the other three antagonists, increased freezing and decreased walking and investigation as compared with saline administration. In Experiment 2, identical naloxone administration did not affect locomotor activity, anxiety-like behaviour or freezing in an open-field test. In Experiment 3, after confirming that the same naloxone administration again increased conditioned fear responses, as done in Experiment 1, we measured Fos expression in 16 brain regions. Compared with saline, naloxone increased Fos expression in the paraventricular nucleus of the hypothalamus and decreased Fos expression in the nucleus accumbens shell, anterior cingulate cortex and insular cortex and tended to decrease Fos expression in the nucleus accumbens core. Based on these results, we suggest that naloxone blocks social buffering of conditioned fear responses in male rats.