Autoradiographic comparison of [H-3]DPDPE and [H-3]DSLET binding: Evidence for distinct delta(1) and delta(2) opioid receptor populations in rat brain

The delta opioid ligands, [H-3]DPDPE (delta(1)) and [H-3]DSLET (delta(2)) were used in quantitative autoradiographic experiments to ascertain whether separate populations of delta opioid subtypes could be identified in rat brain. Densitometric image analysis showed a general similarity in delta(1) and delta(2) distributions. However, statistically significant differences in binding levels were observed in anatomically discrete regions. Examples of these regions and their delta(2)/delta(1) ratio(s) are: dorsomedial hypothalamus (9.3), ventromedial hypothalamus (4.9), superior colliculis (2.7), medial division of bed nucleus stria terminalis (1.6-3.0), external cortex of the inferior colliculis (2.1), amygdaloid nuclei (1.5-2.1), cingulate cortex (1.8), CA1, CA2, and CA3 regions of Ammon's horn (1.6-2.0), dentate gyrus (1.7), laminar VI of the frontal, forelimb, hindlimb and parietal cortices (1.3-1.8), nucleus accumbens (1.4) and caudate/putamen (1.3). These findings provide evidence supporting the existence of distinct delta(1) and delta(2) opioid receptors.