Quantitative autoradiographic analysis of Mu and delta opioid binding sites in the rat hippocampal formation

The distributions of mu and delta opioid binding sites were studied in rat hippocampal formation by using quantitative in vitro autoradiography. Mu binding sites, labeled with ¹²⁵I‐FK‐33824, showed a highly organized laminar distribution. Binding was greatest at the foot of the obliterated hippocampal fissure in stratum lacunosum‐moleculare of CA3. Stratum pyramidale and stratum lacunosum‐moleculare of CA2 and stratum pyramidale of CA3 were next highest in mu binding, followed by stratum oriens and stratum radiatum of CA2, stratum oriens of CA3, and stratum pyramidale of CA1. The distribution of delta binding sites, labeled with ¹²⁵.I‐D‐ala² ‐Dleu⁵‐enkephalin in the presence of the unlabeled mu receptor ligand PL‐032, was similar to the distribution of mu binding in that binding within each region was greatest in a band centered over stratum pyramidale and in stratum lacunosum‐moleculare. Over all, delta binding was greatest in CA2 followed by CA3 and then CA1. Compared to mu binding, delta binding was relatively enriched in stratum moleculare of the dentate gyrus. These laminar distributions correlate reasonably well with the distribution of enkephalin immuno‐reactivity in hippocampal formation, although binding was surprisingly low in stratum lucidum, an area rich in dynorphin and enkephalin immunoreactivity.