Topography and characteristics of specific binding sites for non-opioid γ-type endorphins in the rat brain as studied by autoradiography with [35S]Met-desenkephalin-γ-endorphin

An in vitro autoradiographic study was performed to characterize specific rat brain binding sites for non-opioid neuroleptic-like gamma-type endorphins, using [S-35]Met-des-enkephalin-gamma-endorphin ([S-35]Met-DEgammaE; [35]S-beta-endorphin(5-17)) with high specific activity as radioligand. The binding sites appeared to be confined to rat forebrain regions, e.g., orbital cortex, frontal cortex, cingulate cortex, piriform cortex, nucleus accumbens, amygdala, mediodorsal nucleus of the thalamus and arcuate and periventricular nuclei of the hypothalamus. These regions are part of the mesocorticolimbic feedback circuit. Densitometric analysis of the autoradiographs revealed that the density of the binding sites was highest in the mediodorsal nucleus of the thalamus and the amygdala. Concentration-dependent displacement of [S-35]Met-DEgammaE (500 pM) with DEgammaE yielded an IC50 of 0.6 nM whereas DEalphaE (beta-endorphin(6-16)) had an IC50 of 210 nM. Various endorphins, sharing the gamma-endorphin C terminus, displaced [S-35]Met-DEgammaE to the same extent as non-labelled DEgammaE (at 10(-6)M) whereas non-endorphin peptides did not show displacing capacity. Possible relationships of the binding sites with opioid receptors were investigated. DAMGO (mu) and DPDPE (delta) displaced [S-35]Met-DEgammaE to some extent at 10(-6)M whereas U69,593 (K) was inactive, suggesting that the binding sites for gamma-type endorphins may resemble mu- and delta-opioid receptors in some aspects. Similarly, relationships with dopamine receptors were investigated. Haloperidol partially displaced [S-35]Met-DEgammaE whereas sulpiride, SKF38,393 and 3-PPP at 10(-6) M did not induce significant displacement. Thus, binding sites are distinct from dopamine receptors. Finally, the monoclonal anti-DEgammaE anti-idiotypic antibody CR14.1 appeared to be a potent competitor of [S-35]Met-DEgammaE. The results obtained indicate that [S-35]Met-DEgammaE labels a specific class of binding sites in the brain. These binding sites are selective for gamma-type endorphins, and are distinct from opioid and dopamine receptors. In view of their topography and binding characteristics, the binding sites for gamma-type endorphins may be of relevance for the neuroleptic-like activity of these peptides.