DIFFERENTIAL DEVELOPMENT OF BETA-ENDORPHIN AND MU-OPIOID BINDING-SITES IN MOUSE-BRAIN

Mouse brains of various ages from embryonal day 14 (E14) to adult were analyzed for opioid receptor binding using the enkephalin analog Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAMGE) and the opiate alkaloid dihydromorphine (DHM) as mu-selective radioligands. Binding parameters were estimated from homologous and heterologous competition binding curves. During the postnatal period, K(d) values for [H-3]DAMGE did not change but B(max) values (fmol/mg protein) increased 2.7 fold from postnatal day 3 (P3) to P7. Miner receptor density fluctuations were evident from P7 to adult. Similar results were obtained with [H-3]DHM. In contrast, estimation of total mu-binding sites (fmol/brain) revealed a continuous rise from P3 to the adult. The postnatal developmental profile of total mu-binding sites was comparable to the weight gain of mouse brain and the increase in protein content. In contrast, during the same period beta-endorphin immunoreactivity (IR) levels undergo an increase that is inversely proportional to mu-opioid receptor B(max) values. [H-3]DAMGE binding to E14 membrane preparations was inhibited to a greater extent by Gpp(NH)p than that to P1 or adult. Additional characterization of mu-receptors was accomplished by heterologous competition binding assays. IC50 values for beta-endorphin in competition with [H-3]DHM and [H-3]DAMGE were age dependent and differed for the two radiohgands. These results suggest that mu-receptor selectivity for mu-specific peptide and alkaloid ligands changes as a function of age.