Estradiol valerate (EV) treatment in the rat induces a lesion of the hypothalamic arcuate nucleus, resulting in significant decreases in hypothalamic β-endorphin. In addition, the EV treatment causes a selective increase in µ-opioid binding in the medial preoptic area (MPOA). Since β-endorphin neurons located in the arcuate nucleus project extensively to the MPOA, we have hypothesized that the EV-induced loss of these afferents induces a compensatory upregulation of µ-opioid receptors in opioid target neurons. In order to test this hypothesis, we have utilized monosodium glutamate (MSG) treated animals as a model of β-endorphin cell loss and hence of β-endorphin deafferentation of the MPOA. Neonatal MSG treatment has been shown to result in the destruction of 80-90% of arcuate neurons accompanied by pronounced decreases in β-endorphin concentrations in both arcuate nucleus and MPOA. µ-Opioid binding sites were radioautographically labeled in sections from the MPOA of sham- and MSG-injected animals using the methionine enkephalin analogue 125I-FK 33-824 and quantitated by computer-assisted densitometry. The remainder of the hypothalamus of these same animals was utilized for the determination of the β-endorphin concentration. The hypothalami of rats treated with MSG exhibited 62% (p < 0.01) less β-endorphin than saline-injected controls. In addition, the mean µ-opioid-binding densities in the MPOA were 24% (p < 0.05) above controls in the MSG-treated group. Linear regression analysis of hypothalamic β-endorphin concentrations and µ-opioid-binding densities within the same animals yielded an inverse proportional relationship with a coefficient of correlation of-0.85 and a goodness of fit of 0.7. These results substantiate the hypothesis that EV-induced destruction of β-endorphin neurons in the arcuate nucleus may result in chronic µ-opioid receptor upregulation in the MPOA and further suggest that β-endorphin may regulate µ-opioid receptor density in this area.

Keywords:
β-Endorphin, Opioids, Receptors, Hypothalamus, Radioautography, Arcuate nucleus
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© 1992 S. Karger AG, Basel
1992
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