The regulation of female sexual receptivity by estradiol is complex and requires both the classical nuclear receptors and membrane estrogen receptors. It is only through the combined actions of both of these types of receptors in the brain and peripheral tissues that the female is ready for sexual behavior. The nuclear receptors dimerize, bind to DNA and regulate transcription, and thereby the translation of new proteins. The membrane estrogen receptors exert their effects through another method. In the arcuate nucleus of the hypothalamus (ARH), hippocampus and striatum, estrogen receptor-α (ERα), the only estrogen receptor shown to be required for sexual receptivity, transactivates metabotropic glutamate receptors (mGluRs) in order to initiate G protein signaling. This leads to the phosphorylation of a diverse array of signaling molecules depending upon which mGluR is activated. In a membrane to nucleus signaling schema, membrane-initiated estradiol signaling can lead to the activation of cAMP response element binding protein (CREB), which also in turn affects transcription. For the activation of sexual receptivity, the association of ERα with mGluR1a, leading to the phosphorylation of PKCθ, is required in the ARH. In the present set of experiments, I show that three other events within the ARH are necessary for sexual receptivity: caveolin-1 (CAV1) mediated ERα trafficking, spinogenesis, and modulation of activity regulated cytoskeleton associated protein (Arc).
Estradiol regulates levels of membrane ERα modulating its own signaling, and CAV1 is a scaffold protein that traffics receptors to the membrane. In vitro, it has been observed to move ERα and the ERα-mGluR1 complex to the membrane. Without this protein, signaling in these neurons was significantly attenuated. Until now, no work had been done to examine whether CAV1-mediated ERα trafficking is involved in the activation of sexual receptivity in vivo. siRNA directed against CAV1 was used to knock down CAV1 protein in the ARH. This led to a significant reduction in membrane ERα, circuit activation and sexual receptivity.
Estradiol-mediated spinogenesis has been seen in several areas of the brain related to sexual receptivity including the ventromedial hypothalamus (VMH), which is considered the final common output from the limbic-hypothalamic lordosis behavior regulating module. Increases in dendritic spine density in the VMH and ARH have been proposed to regulate this behavior, but there has been no formal test of the idea. I induced the formation of spines in the ARH with estradiol and showed that this was mediated by membrane-initiated signaling requiring mGluR1a and the phosphorylation of cofilin. Deactivation of this natural actin depolymerizing agent allows the formation of new spines. I then used the β-actin polymerization inhibitor, cytochalasin D, to inhibit spinogenesis in the ARH. The loss of estradiol-induced dendritic spines markedly reduced lordosis behavior underlying the importance of morphological changes in the ARH for the estradiol-induction of sexual receptivity.
Mating experience has been shown to upregulate the immediate early genes in areas relevant to lordosis behavior like the VMH. Mating is a rich sensory experience involving tactile, olfactory, and auditory stimulation, which seems likely to modify the circuits that regulate it. I hypothesized that the activation of immediate early genes may modify future sexual behavior. To this end, I examined differences between sexual experienced females that were allowed a mating test after each injection of estradiol and sexually naïve females that were behaviorally tested once, but were given a similar number of estradiol injections as the experienced females. Unexpectedly, experienced rats were less receptive than the naïve females, even though they had more dendritic spines. The immediate early gene Arc was upregulated by the first sexual encounter and may play a role in the reduction of levels of membrane ERα in the ARH in experienced animals. When the Arc upregulation was inhibited with antisense oligodeoxynucleotides, receptivity in experienced females resembled that in naïve rats. These experiments indicate a vital role for Arc in the estradiol-regulation of sexual behavior.
In summary, the regulation of sexual behavior in females is complex and requires the coordination of a multitude of molecular targets. The trafficking of the ERα to the membrane by CAV1 allows for the initiation of estradiol-mediated spinogenesis. During sexual behavior, the circuit that regulates receptivity is further modified by Arc. When all of these molecular and morphological events are positively coordinated, a female will be sexual receptive.
