Insulin-like growth factor-1 regulation of alpha(1)-adrenergic receptor signaling is estradiol dependent in the preoptic area and hypothalamus of female rats

Membrane-initiated actions of sex steroids and reproductive behavior: A historical account

It was assumed for a long time that sex steroids are activating reproductive behaviors by the same mechanisms that produce their morphological and physiological effects in the periphery. However during the last few decades an increasing number of examples were identified where behavioral effects of steroids were just too fast to be mediated via changes in DNA transcription. This progressively forced behavioral neuroendocrinologists to recognize that part of the effects of steroids on behavior are mediated by membrane-initiated events. In this review we present a selection of these early data that changed the conceptual landscape and we provide a summary the different types of membrane-associated receptors (estrogens, androgens and progestagens receptors) that are playing the most important role in the control of reproductive behaviors. Then we finally describe in more detail three separate behavioral systems in which membrane-initiated events have clearly been established to contribute to behavior control.

Effects of adipokine administration to the hypothalamic preoptic area on body temperature in rats

The effects of adipokine administration to the hypothalamic preoptic area (POA), which is one of the body temperature (BT) regulation centers in the central nervous system, on BT were investigated in male Wistar rats. BT was measured in conscious rats using telemetry. Insulin-like growth factor-1 (IGF-1), interleukin-1β (IL-1β), monocyte chemoattractant protein-1 and lipocalin-2 produced hyperthermia, and the effects induced by IL-1β (25 ng) and IGF-1 (5 μg) were sustainable and remarkable. IL-6 did not show any significant effect. The IGF-1-induced effect was inhibited by pretreatment with IGF binding protein 3 (IGFBP3) or NVP-AEW541 (NVP, a selective inhibitor of type 1 IGF receptor tyrosine kinase, IGF1R TK). NVP-induced inhibition was observed only in the early phase of IGF-1-induced hyperthermia. In addition, IGF-1 increased the IL-1β concentration in the microdialysate of POA perfusion, but did not increase the IL-1β concentration in the plasma or the PGE₂ concentration in the microdialysate. These findings suggested that IGF-1 produced hyperthermia, which was mediated, at least a part, through an increased IL-1β concentration after activation of IGF1R TK in the POA, and the IGF-IGFBP system possibly participates in BT homeostasis in the POA.

IGF-1 in the Brain as a Regulator of Reproductive Neuroendocrine Function

Given the close relationship among neuroendocrine systems, it Is likely that there may be common signals that coordinate the acquisition of adult reproductive function with other homeo-static processes. In this review, we focus on central nervous system insulin-like growth factor-1 (IGF-1) as a signal controlling reproductive function, with possible links to somatic growth, particularly during puberty. In vertebrates, the appropriate neurosecretion of the decapeptide gonadotropin-releas-ing hormone (GnRH) plays a critical role in the progression of puberty. Gonadotropin-releasing hormone is released in pulses from neuroterminals in the median eminence (ME), and each GnRH pulse triggers the production of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These pituitary hormones in turn stimulate the synthesis and release of sex steroids by the gonads. Any factor that affects GnRH or gonadotropin pulsatility is important for puberty and reproductive function and, among these factors, the neurotrophic factor IGF-1 is a strong candidate. Although IGF-1 is most commonly studied as the tertiary peripheral hormone in the somatotropic axis via its synthesis in the liver, IGF-1 Is also synthesIzed in the brain, within neurons and glia. In neuroendocrine brain regions, central IGF-1 plays roles in the regulation of neuroendocrine functions, including direct actions on GnRH neurons. Moreover, GnRH neurons themselves co-express IGF-1 and the IGF-1 receptor, and this expression is developmentally regulated. Here, we examine the role of IGF-1 acting in the hypothalamus as a critical link between reproductive and other neuroendocrine functions.

Membrane estrogen receptor regulation of hypothalamic function

Over the decades, our understanding of estrogen receptor (ER) function has evolved. Today we are confronted by at least two nuclear ERs, ERα and ERβ, and a number of putative membrane ERs, including ERα, ERβ, ER-X, GPR30 and Gq-mER. These receptors all bind estrogens or at least estrogenic compounds and activate intracellular signaling pathways. In some cases, a well-defined pharmacology and physiology has been discovered. In other cases, the identity or the function remains to be elucidated. This mini-review attempts to synthesize our understanding of 17β-estradiol membrane signaling within hypothalamic circuits involved in homeostatic functions, focusing on reproduction and energy balance.

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats

Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female rats primed with E2 and progesterone or vehicle. In young females, blockade of IGF-I receptors attenuated the steroid hormone-induced LH surge, reduced the percent of GnRH neurons expressing c-fos on the day of the LH surge, and decreased the total number of neurons expressing c-fos in the preoptic area. Middle-aged females had fewer GnRH neurons expressing c-fos during the LH surge than young females, and the LH surge amplitude was attenuated. Infusion of an IGF-I dose previously shown to increase LH surge amplitude did not increase the percent of GnRH neurons expressing c-fos in middle-aged females. Brain IGF-I receptor blockade did not modify E2 induction of progestin receptor-immunoreactive neurons in the preoptic area, arcuate, or ventromedial hypothalamus of young rats. These findings indicate that brain IGF-I receptors are required for E2 activation of GnRH neurons in young rats and for robust GnRH release from axon terminals in middle-aged females. IGF-I likely exerts its effects by actions on E2-sensitive neurons that are upstream of GnRH neurons and terminals.

Physiological consequences of membrane-initiated estrogen signaling in the brain

Many of the actions of 17beta-estradiol (E2) in the central nervous system (CNS) are mediated via the classical nuclear steroid receptors, ER(alpha) and ERbeta, which interact with the estrogen response element to modulate gene expression. In addition to the nuclear-initiated estrogen signaling, E2 signaling in the brain can occur rapidly within minutes prior to any sufficient effects on transcription of relevant genes. These rapid, membrane-initiated E2 signaling mechanisms have now been characterized in many brain regions, most importantly in neurons of the hypothalamus and hippocampus. Furthermore, our understanding of the physiological effects of membrane-initiated pathways is now a major field of interest in the hypothalamic control of reproduction, energy balance, thermoregulation and other homeostatic functions as well as the effects of E2 on physiological and pathophysiological functions of the hippocampus. Membrane signaling pathways impact neuronal excitability, signal transduction, cell death, neurotransmitter release and gene expression. This review will summarize recent findings on membrane-initiated E2 signaling in the hypothalamus and hippocampus and its contribution to the control of physiological and behavioral functions.

Estrogen signaling in hypothalamic circuits controlling reproduction

It is well known that many of the actions of 17β-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. Yet, it is not well understood how estrogen signals via membrane receptors and how these signals impact not only membrane excitability but also gene transcription in neurons that modulate GnRH neuronal excitability. Indeed, it has been known for some time that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in hypothalamic neurons critical for reproductive function.

Hypothalamic insulin-like growth factor-I receptors are necessary for hormone-dependent luteinizing hormone surges: implications for female reproductive aging

Brain IGF-I receptors are required for maintenance of estrous cycles in young adult female rats. Circulating and hypothalamic IGF-I levels decrease with aging, suggesting a role for IGF-I in the onset of reproductive senescence. Therefore, the present study investigated potential mechanisms of action of brain IGF-I receptors in the regulation of LH surges in young adult and middle-aged rats. We continuously infused IGF-I, the selective IGF-I receptor antagonist JB-1, or vehicle into the third ventricle of ovariectomized young adult and middle-aged female rats primed with estradiol and progesterone. Pharmacological blockade of IGF-I receptors attenuated and delayed the LH surge in young adult rats, reminiscent of the LH surge pattern that heralds the onset of reproductive senescence in middle-aged female rats. Infusion of IGF-I alone had no effect on the LH surge but reversed JB-1 attenuation of the surge in young females. In middle-aged rats, infusion of low doses of IGF-I partially restored LH surge amplitude, and infusion of JB-1 completely obliterated the surge. Intraventricular infusion of IGF-I or JB-1 did not modify pituitary sensitivity to exogenous GnRH or GnRH peptide content in the anterior or mediobasal hypothalamus in either young or middle-aged rats. These findings support the hypothesis that brain IGF-I receptor signaling is necessary for GnRH neuron activation under estrogen-positive feedback conditions and that decreased brain IGF-I signaling in middle-aged females contributes, in part, to LH surge dysfunction by disrupting estradiol-sensitive processes that affect GnRH neuron activation and/or GnRH release.

Control of CNS neuronal excitability by estrogens via membrane-initiated signaling

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system (CNS) are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane-associated steroid receptors for E2 in hypothalamic and other brain neurons. Indeed, we are just beginning to understand how E2 signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. We know that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane-delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. This review will concentrate on rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus and hippocampus, the nature of receptors involved and how they contribute to CNS functions.

Membrane-initiated estrogen signaling in hypothalamic neurons

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. But it is not well understood how estrogen signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Distribution and localization patterns of estrogen receptor-beta and insulin-like growth factor-1 receptors in neurons and glial cells of the female rat substantia nigra: localization of ERbeta and IGF-1R in substantia nigra

Although several studies have focused on the neuroprotective effects of estrogen (E2) on stroke, there have been tantalizing reports on the potential neuroprotective role of E2 in degenerative neuronal diseases such as Alzheimer's and Parkinson's (PD). In animal models of PD, E2 protects the nigrostriatal dopaminergic (DA) system against neurotoxins. However, little is known about the cellular and molecular mechanism(s) involved by which E2 elicits its neuroprotective effects on the nigrostriatal DA system. A preferred mechanism for neuroprotection is the interaction of E2 with specific neuroprotective growth factors and receptors. One such neuroprotective factor/receptor system is insulin-like growth factor-1 (IGF-1). E2 neuroprotective effects in the substantia nigra (SN) DA system have been shown to be dependent on IGF-1. To determine whether E2 also interacts with the IGF-1 receptor (IGF-1R) and to determine the cellular localization of estrogen receptor (ER) and IGF-1R, we compared the distribution of ER and IGF-1R in the SN. Stereological measurements revealed that 40% of the subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir) SN pars compacta (SNpc) DA neurons are immunoreactive for estrogen receptor-beta (ERbeta). No immunolabeling for ERalpha was observed. In situ hybridization and immunocytochemistry studies confirmed the expression of IGF-1R mRNA and revealed that almost all TH-ir SNpc DA neurons were immunoreactive for IGF-1R, respectively. Moreover, one-third of glial fibrillary acidic protein (GFAP-ir) cells in the SN were ERbeta-ir, and 67% of GFAP-ir cells expressed IGF-1R-ir. Therefore, the localization of ERbeta and IGF-1R on SNpc DA neurons and astrocytes suggests a modulatory role of E2 on IGF-1R, and this modulation may affect neuroprotection.

Insulin-like growth factor-I receptor and estrogen receptor crosstalk mediates hormone-induced neurite outgrowth in PC12 cells

Estradiol (E(2)) and insulin-like growth factor-I (IGF-I) can act independently or in concert to promote neurite outgrowth in vivo and in cultured neurons. This study examined the role of crosstalk between estrogen receptor (ER)alpha and the IGF-I receptor as a critical mediator of hormone- and growth factor-dependent neurite outgrowth in a homogenous cell system. We used control PC12 cells and PC12 cells stably transfected with ER alpha, both of which express IGF-I receptor. Cells were treated for 1 week with vehicle, 1 nM E(2) or 100 ng/ml IGF-I alone or with E(2) or IGF-I in the presence of either the IGF-I receptor antagonist JB1 or the ER antagonist ICI 182,780. IGF-I significantly increased neurite outgrowth, as measured by the percentage of process-bearing cells, and absolute neurite length per cell in both control and ER alpha-transfected PC12 cells. In contrast, E(2) increased process formation and extension only in PC12 cells that were stably transfected with ER alpha. ICI 182,780 and JB1 blocked the IGF-I-induced increases in neurite length in both cell types. The efficacy of ICI 182,780 in control PC12 cells may have been due to the upregulation of ER alpha in these cells by the 7-day treatment with IGF-I. The ER and IGF-I receptor antagonists similarly blocked the E(2)-induced increase in neurite lengths in ER alpha-transfected cells. Immunofluorescent analysis of the cellular distribution of an axonal marker, phospho-neurofilament, verified that the processes extended by PC12 cells were neurites. These data suggest that receptor crosstalk between IGF-I receptors and ER alpha has an important role in neurite formation and extension even in a single-cell system.

Influence of estradiol on insulin-like growth factor-1-induced luteinizing hormone secretion

Several studies suggest an interrelationship between estradiol (E2) and insulin-like growth factor-1 (IGF-1) at the hypothalamic level. The present study was designed to discern if the capability of IGF-1 to release LH and influence the timing of female puberty is influenced by E2. Twenty-eight-day-old female rats were ovariectomized (OVEX), then implanted with a third ventricular (3V) cannula. Two weeks later, these animals received subcutaneous (s.c.) injection of oil, or either one or two injections of E2 in the form of estradiol benzoate (1 microg). Forty-eight hours later, four basal blood samples were drawn then the animals received IGF-1 (200 ng) or saline via the 3V and four more blood samples were taken. Results indicated that E2 replacement lowered basal LH levels and IGF-1 induced a significant LH release in only animals that had E2 levels above 20 pg/ml. These levels of E2 were also associated with increases (p<0.05) in the expression of both IGF-1 receptor (IGF-1R) mRNA and protein. In order to further support the hypothesis that the action of IGF-1 at the time of puberty is influenced by E2, 24-day-old intact female rats received s.c. injection of sesame oil or 0.1 microg of E2. The next day, the E2-treated animals also received twice daily s.c. injections of either IGF-1 (500 ng) or saline until vaginal opening (VO) occurred. The animals that received E2 plus IGF-1 showed VO at 31.1 days, which was 2.5 days earlier (p<0.01) than E2-treated animals and 4 days earlier (p<0.001) than IGF-1-treated and saline control animals. Taken together, these results indicate that the hypothalamic action of IGF-1 to stimulate LH release and advance female pubertal development is dependent upon the influence of E2.

Ovarian Steroid and Growth Factor Regulation of Female Reproductive Function Involves Modification of Hypothalamic α1-Adrenoceptor Signaling

The ovarian steroids estradiol (E(2)) and progesterone (P) act on target neurons in the hypothalamus and preoptic area to coordinate the expression of female reproductive behaviors with the timing of the preovulatory luteinizing hormone (LH) surge. This chapter will summarize evidence that E(2) and P facilitation of the receptive component of female reproductive behavior, lordosis, involves changes in both the expression of and intracellular signal transduction pathways engaged by alpha(1)-adrenergic receptors in the hypothalamus and preoptic area. The alpha(1)-adrenoceptors are thought to mediate the facilitatory effects of the catecholamine neurotransmitter norepinephrine on both lordosis behavior and LH release. E(2) first induces the expression of the alpha(1B)-adrenergic receptor subtype in the hypothalamus and preoptic area. P then acts in an E(2)-dependent manner to promote linkage of hypothalamic alpha(1)-adrenoceptors to an intracellular signaling pathway involving nitric oxide and cyclic GMP. This chapter will also describe recent findings that implicate brain insulin-like growth factor-I (IGF-I) receptors as obligatory co-mediators of hormonal regulation of hypothalamic alpha(1)-adrenoceptors and female neuroendocrine function. Additional studies suggest that E(2) and IGF-I facilitate lordosis behavior by activating kinases traditionally associated with growth factor signal transduction (mitogen-activated protein kinases and phosphatidlyinositol-3-kinases). These molecular events are proposed to help coordinate the timing of ovulation with the expression of sexual receptivity, thereby maximizing reproductive success.

Participation of growth factor signal transduction pathways in estradiol facilitation of female reproductive behavior

Estradiol (E(2)) regulates female reproductive behavior (lordosis) by acting on estrogen-sensitive neurons. We recently showed that E(2) facilitation of lordosis behavior requires concurrent activation of brain IGF-I receptors. The present study confirmed this finding and sought to identify the downstream signaling pathways involved in estrogen/IGF-I priming of lordosis. Intracerebroventricular infusions of a selective IGF-I receptor antagonist were administered to ovariectomized rats every 12 h beginning 1 h before the first of two daily E(2) injections. IGF-I receptor blockade partially inhibits lordosis if the antagonist is infused throughout the 2-d estrogen treatment period but not if it is administered only during the first or last 12 h of estrogen treatment. Because E(2) and IGF-I can activate phosphatidylinositol-3-kinase (PI3K) and MAPK, we infused agents that block PI3K and/or MAPK activity as described above. Both PI3K inhibitors (wortmannin and LY294002) and MAPK inhibitors (PD98059 and U0126) partially attenuate lordosis when administered during estrogen priming. None of these drugs modifies lordosis if they are infused only once, during the last 12 h of estrogen treatment. When both wortmannin and PD98059 are infused during E(2) priming, lordosis behavior is completely abolished. These data suggest that activation of both PI3K and MAPK by E(2) and IGF-I mediates hormonal facilitation of lordosis behavior.

An antagonist of estrogen receptors blocks the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus of adult female rat

Interdependence between estradiol and insulin-like growth factor-I has been documented for different neural events, including neuronal differentiation, synaptic plasticity, neuroendocrine regulation and neuroprotection. In the present study we have assessed whether both factors interact in the regulation of neurogenesis in the adult rat dentate gyrus. Wistar albino female rats were bilaterally ovariectomized and treated with estradiol, insulin-like growth factor-I and/or the estrogen receptor antagonist ICI 182,780. Estradiol was administered in a subcutaneous silastic capsule. Insulin-like growth factor-I and ICI 182,780 were delivered in the lateral cerebral ventricle. Animals received six daily injections of 5-bromo-2-deoxyuridine and were killed 24 h after the last injection. The total number of 5-bromo-2-deoxyuridine-positive neurons was significantly increased in animals treated with insulin-like growth factor-I, compared with rats treated with vehicles, while rats treated with both insulin-like growth factor-I and estradiol showed a higher number of 5-bromo-2-deoxyuridine-positive neurons than rats treated with insulin-like growth factor-I or estradiol alone. The antiestrogen ICI 182,780 blocked the effect of insulin-like growth factor-I on the number of 5-bromo-2-deoxyuridine neurons with independence of whether the animals were treated or not with estradiol. These findings suggest that estrogen receptors are involved in the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus, and that estradiol and insulin-like growth factor-I have a cooperative interaction to promote neurogenesis. The interaction between insulin-like growth factor-I and estradiol may participate in changes in the rate of neurogenesis during different endocrine and physiological conditions, and may be related to the decline in neurogenesis with ageing.

Interactions of estrogen and insulin-like growth factor-I in the brain: molecular mechanisms and functional implications

In the brain, as in other tissues, estradiol interacts with growth factors. One of the growth factors that is involved in the neural actions of estradiol is insulin-like growth factor-I (IGF-I). Estradiol and IGF-I cooperate in the central nervous system to regulate neuronal development, neural plasticity, neuroendocrine events and the response of neural tissue to injury. The precise molecular mechanisms involved in these interactions are still not well understood. In the central nervous system there is abundant co-expression of estrogen receptors (ERs) and IGF-I receptors (IGF-IRs) in the same cells. Furthermore, the expression of estrogen receptors and IGF-I receptors in the brain is cross-regulated. In addition, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB) it has been shown that estradiol affects IGF-I signaling pathways in the brain. Estradiol treatment results in a dose-dependent increase in the phosphorylation of ERK and Akt/PKB in the brain of adult ovariectomized rats. In addition, estradiol and IGF-I have a synergistic effects on the activation of Akt/PKB in the adult rat brain. These findings suggest that estrogen effects in the brain may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.

Functional interactions between estrogen and insulin-like growth factor-I in the regulation of alpha 1B-adrenoceptors and female reproductive function

The ovarian hormone estradiol (E(2)) and insulin-like growth factor-I (IGF-I) interact in the CNS to regulate neuroendocrine function and synaptic remodeling. Previously, our laboratory showed that 2 d E(2) treatment induces alpha(1B)-adrenoceptor expression and promotes IGF-I enhancement of alpha(1)-adrenoceptor potentiation of cAMP accumulation in the preoptic area (POA) and hypothalamus (HYP). This study examined the hypothesis that E(2)-dependent aspects of female reproductive function, including alpha(1B)-adrenoceptor expression and function in the POA and HYP, are mediated by brain IGF-I receptors (IGF-IRs) in female rats. Ovariohysterectomized rats were implanted with a guide cannula aimed at the third ventricle and treated in vivo with vehicle or E(2) daily for 2 d before experimentation. Intracerebroventricular infusions of JB-1, a selective IGF-IR antagonist, were administered every 12 hr beginning 1 hr before the first E(2) injection. Administration of JB-1 during E(2) priming completely blocks hormone-induced luteinizing hormone release and partially inhibits hormone-dependent reproductive behavior. Reproductive behavior is restored by intracerebroventricular infusion of 8-bromo-cGMP, the second messenger implicated in alpha(1)-adrenergic facilitation of lordosis. In addition, blockade of IGF-IRs during E(2) priming prevents E(2)-induced increases in alpha(1B)-adenoceptor binding density and abolishes acute IGF-I enhancement of NE-stimulated cAMP accumulation in HYP and POA slices. These data document the existence of a novel mechanism by which IGF-I participates in the remodeling of noradrenergic receptor signaling in the HYP and POA after E(2) treatment. These events may help coordinate the timing of ovulation with the expression of sexual receptivity.

Mechanisms of ovarian steroid regulation of norepinephrine receptor-mediated signal transduction in the hypothalamus: implications for female reproductive physiology

In many mammalian species, the ovarian steroid hormones estradiol (E(2)) and progesterone (P) act in the hypothalamus and preoptic area to coordinate the timing of female sexual receptivity with ovulation. We study lordosis behavior, an important component of sexual receptivity in rats, and its regulation by E(2) and P as a model system for understanding how hormonal modulation of synaptic neurotransmission influences reproductive physiology and behavior. Our findings suggest that E(2) and P extensively regulate synaptic communication involving the catecholamine norepinephrine (NE) in the hypothalamus. Estrogen priming shifts the balance of postsynaptic NE receptor signaling in the hypothalamus and preoptic area away from beta-adrenergic activation of cAMP synthesis toward alpha(1)-adrenergic signaling pathways. Attenuation of beta-adrenergic signal transduction is achieved by receptor-G-protein uncoupling, apparently due to stable receptor phosphorylation. E(2) modification of alpha(1)-adrenergic signaling includes both increased expression of the alpha(1B)-adrenoceptor subtype and a dramatic, P-induced reconfiguration of the biochemical responses initiated by agonist activation of alpha(1)-adrenoceptors. Among these is the emergence of alpha(1)-adrenergic receptor coupling to cGMP synthesis. We also present evidence that estrogen promotes novel, functional interactions between insulin-like growth factor-1 (IGF-1) and alpha(1)-adrenergic receptor signaling in the hypothalamus and preoptic area. Thus, estrogen amplification of signaling mediated by alpha(1)-adrenoceptors is multifaceted, involving changes in gene expression (of the alpha(1B)-adrenoceptor), switching of receptor linkage to previously inactive intracellular pathways, and the promotion of cross talk between IGF-1 and NE receptors. We propose that this hormone-dependent remodeling of hypothalamic responses to NE maximizes reproductive success by coordinating the timing of the preovulatory release of gonadotropins with the period of behavioral receptivity in female rodents.