Zucker obese rats are insensitive to the CRH-increasing effect of oleoyl-estrone

Adult female Zucker lean and obese rats were treated for 14 days with 3.5 nm/kg oleoyl-estrone (OE) in liposomes (Merlin-2) through continuous i.v. injection with osmotic minipumps. Rat wt. and food intake were measured daily. On days 0, 3, 6, 10, and 14, groups of rats were killed and their hypothalamic nuclei [lateral preoptic (LPO), median preoptic (MPO), paraventricular (PVN), ventromedial (VMH), and arcuate (ARC)] were dissected, homogenized, and used for the measurement of corticosterone-releasing hormone (CRH) by radioimmunoassay. The OE treatment decreased food intake by 67.4% in lean and 62.6% in obese rats (means for 14 days). Body wt. decreased steadily in lean and obese rats, the gap between controls and treated rats becoming 11.5% of initial body wt. in the lean and 12.4% in the obese. The levels of CRH in the ARC nucleus were at least 10-fold higher than in the other nuclei. No changes in CRH were observed in any of the nuclei of obese rats, with levels up to day 6 similar to those of lean rats. In the lean rats, the LPO and ARC nuclei showed peaks on day 10, while the MPO showed no changes and the PVN and VMH nuclei showed a progressive increase, to a maximum at the end of the study (day 14). This contrasted with the peak of plasma adrenocorticotropic hormone (ACTH) and corticosterone (day 6 in lean and day 14 in obese rats). There was a definite lack of correlation between the plasma levels of these two hormones and the levels of CRH in the hypothalamic nuclei, and between the latter and the decreases in appetite in the rats. The loss of appetite induced by OE is not necessarily mediated by CRH, because the obese rats show an intense decrease in voluntary food intake but their hypothalamic nuclei CRH levels do not change at all. Hypothalamic nuclei CRH does not, necessarily, mediate the rise in glucocorticoids induced by OE treatment, because this is observed in lean and obese rats, lean rats increases being mismatched with those of hypothalamic CRH. The OE induced changes in hypothalamic CRH require a fully functional leptinergic pathway, because it is not observed in Zucker fa/fa rats lacking a working leptin receptor. This–indirectly–shows that leptin is needed for its synthesis or modulation.