The increased satiating potency of CCK-8 by estradiol is not mediated by upregulation of NTS CCK receptors

Estradiol benzoate (EB) increases the satiating effect of CCK-8 in ovariectomized rats. It is possible that this effect of EB is due to upregulation of CCKA receptors in the terminals of vagal afferent fibers because these receptors have been implicated in the mediation of the satiating effect of intraperitoneally injected CCK-8. To test this hypothesis, we used in vitro quantitative autoradiography to measure the effects of EB on the binding characteristics of CCK receptors in the nucleus tractus solitarius (NTS), a region that contains central terminal projections of abdominal vagal afferent fibers. As additional measures of EB's effects on CCK receptors, we also characterized EB's effects on CCK-8 binding in the area postrema (AP), a brain region rich in CCKA receptors, the ventromedial hypothalamus (VMH), a region rich in CCKB receptors, and in the pancreas, a gland rich in CCKA receptors. Saturation binding experiments were run using [I-125]CCK-8 (similar to 40 pM, 2200 Ci/mmol) and 0.1-100 nM unlabelled CCK-8. EB did not change the number (B-max) or affinity (K-d) of CCK receptors in the NTS. Furthermore, competition experiments with 500 nM of the selective CCKA receptor antagonist devazepide or the selective CCKB antagonist L365,260 demonstrated that EB failed to affect CCK receptor subtype number in the medial and lateral divisions of the NTS. EB also did nor affect binding in the AP or VMH. These results do not confirm our hypothesis. The lack of effect of EB on vagal CCKA receptors in the NTS was not due to inappropriate conditions of tissue sampling or autoradiographic technique because EB increased the number, but not the affinity, of CCK( )A receptors in the pancreas significantly.