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Decrease of the number of opioid receptors and of the responsiveness to morphineduring neuronal differentiation
R Maggi1, Z Q Ma, F Pimpinelli
1Department of Endocrinology, Institute of Pharmacological Sciences, University of Milan, Italy. maggir@imiucca.csi.unimi.it
Abstract:
Estrogens modulate the density of opioid receptors in selected brain areas; however, it is not clear whether they exert such an effect directly on the cells which express the opioid receptors. Therefore, we analyzed the binding of [3H]-diprenorphine in human neuroblastoma cells stably transfected with the estrogen receptor cDNA (SK-ER3 cell line). A 16-hour exposure of these cells with 1 nM 17beta-estradiol induces a progressive morphological differentiation which appears clearly established 6 days after the suspension of the treatment. The binding of [3H]-diprenorphine was then measured immediately after the exposure to 17beta-estradiol (16 h) as well as 6 days later. The results shows that the number of opioid receptors in SK-ER3 cells is unaffected at 16 h but appears significantly reduced at 6 days. This effect is blocked by the estrogen antagonist ICI-182780, and is coincident to a decrease of the inhibitory effect of morphine on cyclic AMP accumulation. Binding experiments performed using selective ligands suggest that the micro subclass of opioid receptors is down-regulated by estradiol in SK-ER3 cells.
Insights
Estrogen exposure reduces opioid receptor density in human neuroblastoma cells, specifically down-regulating micro-opioid receptors. This effect on opioid receptors is delayed and blocked by estrogen antagonists.
Area of Science:
- Neuroendocrinology
- Molecular Pharmacology
Background:
- Estrogens are known to modulate opioid receptor density in the brain.
- Direct estrogenic effects on opioid receptor-expressing cells remain unclear.
Purpose of the Study:
- To investigate whether estrogens directly affect opioid receptor density in cells expressing estrogen receptors.
- To analyze the impact of 17beta-estradiol on opioid receptor binding in a human neuroblastoma cell line.
Main Methods:
- Utilized human neuroblastoma cells stably transfected with estrogen receptor cDNA (SK-ER3 cell line).
- Exposed cells to 17beta-estradiol and measured [3H]-diprenorphine binding at different time points (16 hours and 6 days post-treatment).
- Assessed morphological differentiation and cyclic AMP accumulation in response to morphine.
Main Results:
- 17beta-estradiol induced morphological differentiation in SK-ER3 cells.
- Opioid receptor density was unaffected immediately after 16-hour estradiol exposure but significantly reduced 6 days later.
- The reduction in opioid receptors was blocked by the estrogen antagonist ICI-182780.
- Estradiol treatment led to a decreased inhibitory effect of morphine on cyclic AMP accumulation.
- Binding experiments indicated down-regulation of the micro subclass of opioid receptors.
Conclusions:
- Estrogens can directly down-regulate opioid receptor density in cells expressing estrogen receptors.
- The effect of estradiol on opioid receptor number is delayed and occurs after cellular differentiation.
- Micro-opioid receptor down-regulation by estradiol may impact cellular responses to opioids.