Figure 2. Delta opioid receptors in the central and peripheral nervous systems as potential targets for neurologic and psychiatric disorders.

A. Schematic representation of delta opioid receptor binding sites. Delta opioid receptors are particularly abundant (black squares) in the olfactory bulb, cortex, amygdala and striatum (caudate putamen and nucleus accumbens). Delta opioid receptors are also expressed at moderate level in the interpeduncular and pontine nuclei, hippocampus as well as spinal cord and dorsal root ganglia (grey squares), and at a much lower level in hypothalamus, thalamus, mesencephalon, and brain stem (open squares) (Adapted from [3]). Brain areas of high delta opioid receptor expression are involved in several neural processes whose dysfunction may lead to neurological or psychiatric conditions, including pain (pain processing and awareness), anxiety and depression (emotional processing), addictive and impulse disorders (motivation and reward, learning and memory, inhibitory controls). There is a high expression of delta opioid receptors in dopaminergic terminals (striatum), and the role of delta opioid receptors in reward and motivation is complex. B. Delta opioid receptor binding sites visualized by ligand autoradiography ([³H]deltorphin I, sagittal section left, courtesy of Ian Kitchen) or in knockin fluorescent delta opioid receptor reporter mice (coronal section, [21]). Abbreviations: Amg: amygdala; Cpu: Caudate putamen; Cx, cortex; DRG: dorsal root ganglia; FC: frontal cortex; Hip: hippocampus; Hyp: hypothalamus; IP, interpeduncular nucleus; LC: locus coeruleus; NAc: nucleus accumbens; OB: olfactory bulb; PN, pontine nucleus; PAG: periaqueductal gray; RN: raphe nucleus; SC: spinal cord; Ssc: somatosensorial cortex; Th: thalamus; Tu: olfactory tubercle; VTA: ventral tegmental area.