prior to actual physical actions. By expressing these receptor-coding regions in HEK-293, the authors were able to identify an elevated affinity of the G77 allele for β-endorphin (~3.5 fold), but not exogenous ligands, similarly to original in vitro work in the A118G SNP (Bond et al., 1998). Subsequent studies have shown an increase in attachment behavior in G77 allele-carrying infants, who displayed increased distress vocalization during protracted periods of mother-infant separation and increased maternal contact during mother-infant reunion (Barr et al., 2008), possibly reflecting increased attachment reward through enhanced β-endorphin function. However, in light of the recent reports of increased social pain associated with the G118 allele (Way et al., 2009), an alternate explanation could be that the infants carrying the G118 allele had a greater sensitivity to maternal rejection, possibly through reduced MOPR function in the neocortex.
