Mu-opioid receptor system modulates responses to vocal bonding and distress signals in humans

Laughter is a contagious prosocial signal that conveys bonding motivation; adult crying conversely communicates desire for social proximity by signalling distress. Endogenous mu-opioid receptors (MORs) modulate sociability in humans and non-human primates. In this combined PET-fMRI study (n=17) we tested whether central MOR tone is associated with regional brain responses to social signals of laughter and crying sounds. MOR availability was measured with positron emission tomography using high-affinity agonist radioligand [ 11 C]carfentanil. Haemodynamic responses to social laughter and crying sounds were measured using functional magnetic resonance imaging (fMRI). Social laughter evoked activation in the auditory cortex, insula, cingulate cortex, amygdala, primary and secondary somatosensory cortex, primary and secondary motor cortex; crying sounds led to more restricted activation in auditory cortex and nearby areas. MOR availability was negatively correlated with the haemodynamic responses to social laughter in primary and secondary somatosensory cortex, primary and secondary motor cortex, posterior insula, posterior cingulate cortex, precuneus, cuneus, temporal gyri, and lingual gyrus. For crying-evoked activations, MOR availability was negatively correlated with medial and lateral prefrontal haemodynamic responses. Altogether our findings highlight the role of MOR system in modulating acute brain responses to both positive and negative social signals.