Diversity of mechanisms involved in aromatase regulation and estrogen action in the brain

Abstract

Background: The mechanisms through which estrogens modulate neuronal physiology, brain morphology, and behavior in recent years have proven to be far more complex than previously thought. For example, a second nuclear estrogen receptor has been identified, a new family of coregulatory proteins regulating steroid-dependent gene transcriptions was discovered and, finally, it has become clear that estrogens have surprisingly rapid effects based on their actions on cell membranes, which in turn result in the modulation of intracellular signaling cascades.

Scope of review: This paper presents a selective review of new findings in this area related to work in our laboratories, focusing on the role of estrogens in the activation of male sexual behavior. Two separate topics are considered. We first discuss functions of the steroid receptor coactivator-1 (SRC-1) that has emerged as a key limiting factor for behavioral effects of estradiol. Knocking-down its expression by antisense oligonucleotides drastically inhibits male-typical sexual behaviors. Secondly, we describe rapid regulations of brain estradiol production by calcium-dependent phosphorylations of the aromatase enzyme, themselves under the control of neurotransmitter activity.

Major conclusions: These rapid changes in estrogen bioavailability have clear behavioral consequences. Increases or decreases in estradiol concentrations respectively obtained by an acute injection of estradiol itself or of an aromatase inhibitor lead within 15-30 min to parallel changes in sexual behavior frequencies.

General significance: These new controls of estrogen action offer a vast array of possibilities for discrete local controls of estrogen action. They also represent a formidable challenge for neuroendocrinologists trying to obtain an integrated view of brain function in relation to behavior.

Figure 1

SRC-1 knock-down by antisense injections (AS) significantly inhibits testosterone–induced sexual behavior (A-B) and aromatase induction (C) in castrated male quail in comparison with control birds injected with scrambled antisense (Ctrl). Sexual behaviors are illustrated here by the estrogen-dependent cloacal contact movements and by the strictly androgen–dependent display, strutting (B). The SRC-1 knock-down also blocked the induction by testosterone of aromatase expression resulting in a smaller number of aromatase-immunoreactive cells delimiting a smaller surface of the medial preoptic area (C). See [42] for original data and their statistical analysis.

Figure 2

Expression of SRC-1 in the preoptic area-hypothalamus is significantly affected by the sex and endocrine condition of the subjects. The concentration of the SRC-1 protein (A, C) and of the corresponding mRNA (B, D) were measured by Western blot and quantitative real-time PCR respectively in sexually mature males (M) and females (F) (panels A, B) and in castrated males that had been treated with testosterone (CX+T) or not (CX) (panels C, D). SCR-1 is expressed at higher concentrations (both mRNA and protein) in males than in females and in testosterone-treated castrates than in castrates. Both the sex difference and the effect of testosterone were however not replicated in one other experiment (not shown here) for reasons that are not completely identified (see [54] for discussion). Redrawn from data in [54] where detail of statistical analyses can also be found.

Figure 3

Rapid changes in aromatase activity (AA) are observed in paired quail preoptic-hypothalamic explants maintained in vitro. A. One explant was exposed for 10 min [between 20 min (upward arrow) and 30 min (downward arrow)] to a K⁺-induced depolarization. Data are expressed as percentages of basal release, defined as the activity during the period preceding the experimental manipulation (15–20 min). B. Both explants were exposed for 10 min (between 20 and 30 min) to the glutamate agonist kainate (100 mM) but one explant had been pre-incubated with the non-NMDA glutamate antagonist NBQX. The gray area represents the control values, observed in the absence of any manipulation, derived from the experiment described in A. See [109] and [111] for detail of original data and their statistical analysis

Figure 4

Behavioral effects of rapid changes in estrogen availability following an acute injection of estradiol (A) or of the aromatase inhibitor Vorozole™ (B). A. A single injection of estradiol (E₂) performed 5, 15 or 30 min before the beginning of the behavioral test significantly increases the expression of sexual behavior measured here by the frequency of cloacal contact movements (CCM). B. Significant inhibition by a single injection of the aromatase inhibitor Vorozole™ performed 15, 30, 45 or 60 min before the beginning of the behavioral test of the expression of sexual behavior measured by the frequency of CCM. Control values for birds injected with the control vehicle solution are represented by the gray areas. Redrawn from data in [88] and [86]

Figure 5

Rapid changes in aromatase activity (AA) are observed in quail following copulation with a sexually mature female for 1, 5 or 15 min. A significant decrease is observed at the 5 min time point. Control birds were simply handled and returned to their home cage (Values in gray area). Brains were collected immediately after the end of the behavioral test and AA was quantified in the preoptic area-hypothalamus. Redrawn from data in [115] where detail of statistical analyses can be found.