Central implants of dilute estradiol enhance the satiety effect of CCK-8

Using the cerebrospinal fluid to understand ingestive behavior

The cerebrospinal fluid (CSF) offers a window into the workings of the brain and blood-brain barrier (BBB). Molecules that enter into the central nervous system (CNS) by passive diffusion or receptor-mediated transport through the choroid plexus often appear in the CSF prior to acting within the brain. Other molecules enter the CNS by passing through the BBB into the brain's interstitial fluid prior to appearing in the CSF. This pattern is also often observed for molecules synthesized by neurons or glia within the CNS. The CSF is therefore an important conduit for the entry and clearance of molecules into/from the CNS and thereby constitutes an important window onto brain activity and barrier function. Assessing the CSF basally, under experimental conditions, or in the context of challenges or metabolic diseases can provide powerful insights about brain function. Here, we review important findings made by our labs, as influenced by the late Randall Sakai, by interrogating the CSF.

Activation of central, but not peripheral, estrogen receptors is necessary for estradiol's anorexigenic effect in ovariectomized rats

Estradiol appears to exert its anorexigenic effect by activating nuclear estrogen receptors (ERs), which are expressed widely in peripheral tissues and in the brain. Here, we used ICI-182,780 (ICI), a pure antiestrogen with limited ability to cross the blood-brain barrier, to assess the relative involvement of peripheral vs. central ERs to estradiol's anorexigenic effect. Food intake was measured after peripheral (sc) administration of ICI or vehicle in ovariectomized rats treated with acute injections of estradiol benzoate and sesame oil over a 2-wk period. Uterine weight was assessed as a biological assay of peripheral ER activation. In a second experiment, food intake was measured after central (lateral ventricular) administration of ICI or vehicle in ovariectomized rats receiving acute injections of estradiol benzoate and oil over a period of 10 d. In order to assess the possible spread of ICI from the brain to the periphery, vaginal cytology samples were examined as a biological assay of peripheral ER activation. Peripherally administered ICI failed to attenuate estradiol's anorexigenic effect at a dose that was sufficient to block estradiol's uterotrophic effect. This suggests that peripheral activation of ERs is not necessary for estradiol's anorexigenic effect. Although central infusion of 4 nm ICI blocked estradiol's anorexigenic effect, it did not attenuate estradiol's ability to increase the presence of cornified cells in vaginal cytology samples, suggesting that ICI did not leak into the periphery. We conclude that activation of central, but not peripheral, ERs is necessary for estradiol's anorexigenic effect.

Estrogens inhibit food intake in CCK-1 receptor-deficient rats

In human and many other animals, estrogens inhibit food intake and increases spontaneous activity. Previous studies hypothesized that the anorexigenic effect of estrogens is mediated by the cholecystokinin (CCK)-induced satiety effect. In the present study, we investigated whether estrogens-induced anorexigenic and hyper-active effects are present in Otsuka-Long-Evans-Tokushima-Fatty (OLETF) rat, which is deficient in the CCK1 receptor. In OLETF rats with a regular 4-day estrous cycle, food intake decreased and spontaneous activity increased significantly more during estrus than diestrus as compared to control Long-Evans-Tokushima-Otsuka (LETO) rats. Subcutaneous injection of estradiol benzoate into ovariectomized OLETF rats significantly decreased feeding and increased spontaneous activity to the same extent as in LETO rats. These results suggest that the anorexigenic and hyper-active effects of estrogen can be mediated via pathways other than CCK-CCK1 receptor signaling pathway in CCK1 receptor-deficient rats.

Hindbrain administration of estradiol inhibits feeding and activates estrogen receptor-alpha-expressing cells in the nucleus tractus solitarius of ovariectomized rats

17beta-estradiol (E2), acting via estrogen receptor (ER)-alpha, inhibits feeding in animals. One mechanism apparently involves an increase in the satiating potency of cholecystokinin (CCK) released from the small intestine by ingested food. For example, the satiating potency of intraduodenal lipid infusions is increased by E2 in ovariectomized rats; this increased satiation is dependent on CCK, and it is accompanied by increases in the numbers of ERalpha-positive cells that express c-Fos in a subregion of the caudal nucleus tractus solitarius (cNTS) that receives abdominal vagal afferent projections. To test whether direct administration of E2 to this area of the hindbrain is sufficient to inhibit food intake, we first implanted 0.2 microg estradiol benzoate (EB) in cholesterol or cholesterol alone either sc or onto the surface of the hindbrain over the cNTS. Food intake was significantly reduced after hindbrain EB implants but not after sc EB implants. Next we verified that equimolar hindbrain implants of E2 and EB had similar feeding-inhibitory effects and determined that only small amounts of E2 reached brain areas outside the dorsal caudal hindbrain after hindbrain implants of (3)H-labeled E2. Neither plasma estradiol concentration nor plasma inflammatory cytokine concentration was increased by either hindbrain or sc EB implants. Finally, hindbrain EB implants, but not sc implants, increased c-Fos in ERalpha-positive cells in the cNTS after ip injection of 4 microg/kg CCK-8. We conclude that E2, acting via ERalpha in cNTS neurons, including neurons stimulated by ip CCK, is sufficient to inhibit feeding.

Changes in hypothalamic estrogen receptor-containing cell numbers in response to feed restriction in the female lamb

The mechanism whereby undernutrition enhances the ability of estradiol (E) to inhibit reproductive activity is unknown. This study aimed to determine the effect of feed restriction on E receptor (ER)-containing cell numbers in the female sheep hypothalamus. Ovariectomized lambs at 7 months of age received either ad libitum (AL; n = 5) or restricted (FR; n = 10) levels of feed intake. Lambs were weighted weekly and FR lambs fed to lose approximately 15% of their initial body weights over 7 weeks, at the end of which jugular blood samples were collected at 10-min intervals for 5 h to assess the patterns of LH release. After blood collection, lambs were euthanized and hypothalami collected for immunocytochemical detection of ER. Based on LH secretory profiles, FR lambs were subdivided into two groups. The first group (FR + LH; n = 5) exhibited patterns of LH release similar to AL controls. LH secretion in the second group (FR-LH; n = 5) was obviously suppressed. Numbers of ER-containing cells did not differ significantly (p > 0.10) among treatment groups in the bed nucleus stria terminalis, anterior hypothalamic area and arcuate nucleus. ER-containing cell numbers were greater (p < 0.05) in the preoptic area (POA) but less (p < 0.05) in the ventromedial/ventrolateral hypothalamus (VMH/VLH) for FR-LH lambs compared to AL animals. Notably, for both the POA and VMH/VLH, ER-containing cell numbers in the FR + LH animals were intermediate and did not differ (p > 0.10) from either FR-LH or AL lambs. These results suggest that feed restriction differentially alters ER-containing cell numbers in specific regions of the ovine hypothalamus (numbers increased in the POA but decreased in the VMH/VLH). These changes may, at least in part, represent a mechanism whereby undernutrition enhances the ability of E to inhibit reproduction.