ONTOGENY OF CELL-SURFACE MU-OPIOID ([H-3]DAGO) BINDING-SITES IN RAT HYPOTHALAMUS AND EXVIVO DETERMINATION OF BLOOD-BRAIN-BARRIER PENETRATION BY OPIOID PEPTIDE FK-33-824

Previous studies have demonstrated that the LH response to naloxone changes during development but the reason(s) for this are unknown. In the present work we have investigated the possibility that variations in cell surface opioid receptor levels (determined in tissue slice/punches) or changes in the ability of opioids to enter the CNS might be responsible. Opioid binding data indicate that both [3H]naloxone and [3H]DAGO-labelled binding sites remain at low levels until 10 days of age after which there is a progressive rise to adult levels at 15 days ([3H]DAGO) and 21 days ([3H]naloxone). Although several peaks and nadirs were observed in this detailed profile of receptor ontogeny, no exact correlation with the time course of LH response to opioid drugs was found. In an adaption of the slice binding assay we are able to quantify drug penetration into the brain (ex vivo binding). Ex vivo binding studies of blood-brain barrier (BBB) ontogeny indicate that there are changes with age in the ability of opioid peptides, injected subcutaneously, to inhibit binding at the μ-receptor. FK 33-824 induced a reduction in [3H]DAGO binding in the mediobasal hypothalamus until 15 days of age. FK in older rats had no effect on [3H]DAGO binding suggesting that formation of the BBB is complete at this age. In contrast, FK injection reduced binding in the median eminence-arcuate nucleus area (outside BBB) until 30 days of age. Surprisingly, this area also became refractory to FK injection after this age. We are unable to account for this in terms of BBB function but note that this phenomenon is coincident with the ineffectiveness of FK to reduce LH secretion. These results provide a better understanding of the LH response to opioid drugs during ontogeny and the regulation of this opioid effect through formation of the BBB. © 1990.