Abstract
We sought to determine the involvement of phosphatidyl inositol 3-kinase (PI3K) and AMP-activated protein kinase (AMPK) in the estrogenic antagonism of the cannabinoid regulation of energy homeostasis. Food intake and body weight were evaluated in ovariectomized female guinea pigs treated s.c. with estradiol benzoate (EB) or its sesame oil vehicle, or the CB1 receptor antagonist AM251 or its cremephor/ethanol/0.9% saline vehicle. AMPK catalytic subunit, PI3K p85α regulatory subunit and proopiomelanocortin (POMC) gene expression was assessed via quantitative RT-PCR in microdissected hypothalamic tissue. Whole-cell patch clamp recordings were performed in hypothalamic slices. Both EB and AM251 decreased food intake and weight gain, and increased AMPKα1, AMPKα2 and PI3K p85α gene expression in the mediobasal hypothalamus. 17β-Estradiol rapidly and markedly attenuated the decreases in glutamatergic miniature excitatory postsynaptic current (mEPSC) frequency caused by the cannabinoid receptor agonist WIN 55,212-2 in POMC neurons. This rapid estrogenic diminution of cannabinoid-induced decreases in mEPSC frequency was blocked by the estrogen receptor (ER) antagonist ICI 182,780 and the PI3K inhibitor PI 828, the latter of which also prevented the AM251-induced increase in mEPSC frequency. In addition, the AMPK activator metformin reversed the EB-induced decreases in food intake and weight gain and restored the ability of WIN 55,212-2 to reduce mEPSC frequency. These data reveal that estrogens physiologically antagonize cannabinoid-induced changes in appetite and POMC neuronal activity by activating PI3K and inhibiting AMPK. As such, they provide insight into the neuroanatomical substrates and signal transduction mechanisms upon which these counter-regulatory factors converge in the control of energy homeostasis.
Highlights
Ovarian estrogens are essential for the control of homeostasis
Estrogen rapidly diminishes the coupling of Gi/o-coupled receptors to their inwardly-rectifying K+ (GIRK) channels in anorexigenic, glucose-responsive guinea pig proopiomelanocortin (POMC) neurons by activating a phospholipase C (PLC)/protein kinase C (PKC)/protein kinase A (PKA) pathway that does not involve stimulation of ERα, ERβ, estrogen receptor (ER)-X or GPR30 [15,22,23,24]
Expression determined in arcuate nucleus (ARC) and ventromedial nucleus (VMN) tissue harvested after the feeding experiments. *, Values measured in estradiol benzoate (EB)- and AM251-treated animals that are significantly different (Student’s t-test; p < 0.05; n = 5−8) than those measured in vehicle-treated controls
Summary
Ovarian estrogens are essential for the control of homeostasis. For example, they regulate the reproductive axis via negative and positive feedback that allows for the proper timing of ovulation, and estrogen priming of the reproductive tract maximizes the likelihood of blastocyst implantation into the uterine endometrium [1,2]. It is becoming widely accepted that they can alter cell function on a much more rapid time scale than that accounted for by gene transcription alone [15]. Some of these rapid estrogenic actions are mediated via ERα tethered to the plasma membrane through palmitoylation and interactions with caveolin proteins [16,17,18]. ERα interacts with metabotropic glutamate receptors in a caveolin-dependent manner to modulate L-type calcium channel activity and phosphorylation of cAMP response element-binding protein in rat hippocampus, as well as in the limbic-hypothalamic circuit that controls rat sexual behavior [18,20,21]. Estrogen rapidly diminishes the coupling of Gi/o-coupled receptors to their inwardly-rectifying K+ (GIRK) channels in anorexigenic, glucose-responsive guinea pig proopiomelanocortin (POMC) neurons by activating a phospholipase C (PLC)/protein kinase C (PKC)/protein kinase A (PKA) pathway that does not involve stimulation of ERα, ERβ, ER-X or GPR30 [15,22,23,24]
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