OPRD1 and Anxiety: Further studies have identified these AN loci as opioid delta 1 receptor (OPRD1), which acts as a receptor for endogenous opioids, and serotonin 1D (HTR1D), which is a 5‐HT receptor and acts on central nervous system and affects anxiety as well as locomotion (Bergen et al., 2003; Brown et al., 2007).