Parameters of neuroendocrine aromatization and estrogen receptor occupation in the male rat

New evidence for the selective, long-lasting central effects of the brain-targeted estradiol, Estredox

The present study examined the dose- and time-dependent central effects of an estradiol chemical delivery system cyclodextrin complex (E(2)-CDS-CD) on the reestablishment of copulatory behavior of castrated male and ovariectomized female rats with concomitant determination of the blood luteinizing hormone (LH) and E(2) levels. In orchidectomized males, Estredox, after single doses of 0.3 and 3.0 mg/kg iv, reestablished the mounting and intromission up to 4 weeks. The LH suppressive effect lasted to Day 7 and 28, respectively. After repeated administration for 10 days at a dose of 0.01mg/kg iv, significant effect was obtained by Day 14. Ovariectomized females were treated iv daily for 5 days either with E(2)-CDS-CD, estradiol benzoate (EB) or vehicle, and the lordosis quotient was determined. At a dose of 0.03 mg/kg the duration of EB's effect was 10 days shorter and only one-third of that of E(2)-CDS-CD. The LH suppression lasted to Day 18. On the other hand, after EB treatment there was no significant decrease in LH levels. The low plasma E(2) levels indicated fast rate of peripheral elimination in both males and females. The brain-targeting E(2) indicates better efficacy and increased safety in replacement therapies because of the reduced peripheral side effects.

Testosterone and its metabolites affect afterdischarge thresholds and the development of amygdala kindled seizures

In boys with epilepsy, pubertal increases in seizure frequency may be associated with rising androgen levels. The present study tested the hypothesis that testosterone (T) and/or its metabolites might affect amygdala seizure thresholds and the development of secondary generalization from amygdala foci (kindling). Afterdischarge thresholds and kindling rate were measured in gonadectomized (GDX) male rats, with or without T replacement therapy. Drugs that block either androgen or estradiol (E(2)) receptor-mediated responses were also tested.

METHODS

Kindling electrodes were implanted in the basolateral amygdala of adult male Wistar rats. In Experiment 1, subjects were GDX and implanted with a silastic capsule containing either: cholesterol (control); T; 5% E(2) in cholesterol; or 5alpha-dihydrotestosterone (DHT). In Experiment 2, intact subjects were treated with daily injections of vehicle (control); daily injections of flutamide (an androgen receptor antagonist); or Silastic implants containing 1,4,9-androstatriene 3,17-dione (ATD; an aromatase inhibitor).

RESULTS

In Experiment 1, initial afterdischarge (AD) thresholds were significantly lowered by E(2) treatment, as compared to cholesterol controls, and remained low throughout the kindling paradigm. In T replaced males, AD threshold significantly decreased over the kindling period, a response that was not observed in DHT treated rats. Rates of kindling were significantly faster as a result of T, E(2) and DHT treatment, as compared to cholesterol controls. E(2) treated males kindled the fastest of all 3 groups. In Experiment 2, initial AD thresholds were significantly lowered by flutamide treatment, as compared to cholesterol controls, and remained low throughout the kindling paradigm. AD threshold significantly decreased over the kindling period in intact males, a response that was blocked by ATD treatment. Both flutamide and ATD significantly slowed the rate of kindling, as compared to intact controls. ATD had the most dramatic inhibitory effect on kindling rate.

CONCLUSIONS

In males, T and its two metabolites, E(2) and DHT, all appear to enhance the development of amygdala-kindled seizures. E(2) has the most potent epileptogenic effect. Antagonism of E(2) mediated effects in the brain may have potential therapeutic value for males with epilepsy.

The role of gonadal steroid receptor activation in the restoration of sociosexual behavior in adult male rats

This work tested the hypothesis that gonadal steroid receptor activation was necessary for the restoration of several sociosexual behaviors (such as copulatory behavior, partner preference, 50-kHz vocalizations, and scent marking) in testosterone-treated gonadectomized male rats. Gonadal steroid receptors were blocked by systemic administration of the antiandrogen hydroxyflutamide, the antiestrogen RU 58668, or both antagonists simultaneously in a restoration paradigm. Inhibiting androgen receptors with hydroxyflutamide blocked the restoration of male copulatory behavior, partner preference (time spent with a sexually receptive female over a nonreceptive female), 50-kHz ultrasonic vocalizations, and scent marking. On the other hand, we did not find that blocking estrogen receptors with RU 58668 inhibited the restoration of copulatory behavior or partner preference in testosterone-treated gonadectomized male rats, even though the level of brain nuclear estrogen receptor occupation was significantly reduced to the level found in gonadectomized males. However, the restoration of scent marking and 50-kHz vocalizations were impaired by RU 58668. Blocking both nuclear androgen and estrogen receptors with the two antagonists simultaneously did not have a greater inhibitory effect than treatment with each antagonist alone. Therefore, the activation of nuclear estrogen receptors is necessary for the restoration of some, but not all, sociosexual behaviors, which are also androgen receptor-dependent. Besides nuclear estrogen receptors, there are additional, but unknown, targets of estradiol that play a role in mediating copulatory behavior in adult male rats. Moreover, the signals from multiple gonadal steroid signaling pathways converge in the regulation of some sociosexual behaviors in adult male rats.

Aromatase activity in the rat brain: hormonal regulation and sex differences

The intracellular conversion of testosterone to estradiol by the aromatase enzyme complex is an important step in many of the central actions of testosterone. In rats, estrogen given alone, or in combination with dihydrotestosterone, mimics most of the behavioral effects of testosterone, whereas treatment with antiestrogens or aromatase inhibitors block facilitation of copulatory behavior by testosterone. We used a highly sensitive in vitro radiometric assay to analyze the distribution and regulation of brain aromatase activity. Studies using micropunch dissections revealed that the highest levels of aromatase activity are found in an interconnected group of sexually dimorphic nuclei which constitutes a neural circuit important in the control of male sexual behavior. Androgen regulated aromatase activity in many diencephalic nuclei, including the medial preoptic nucleus, but not in the medial and cortical nuclei of the amygdala. Additional genetic evidence for both androgen-dependent and -independent control of brain AA was obtained by studies of androgen-insensitive testicular-feminized rats. These observations suggest that critical differences in enzyme responsiveness are present in different brain areas. Within several nuclei, sex differences in aromatase induction correlated with differences in nuclear androgen receptor concentrations suggesting that neural responsiveness to testosterone is sexually differentiated. Estradiol and dihydrotestosterone acted synergistically to regulate aromatase activity in the preoptic area. In addition, time-course studies showed that estrogen treatment increased the duration of nuclear androgen receptor occupation in the preoptic area of male rats treated with dihydrotestosterone. These results suggest possible ways that estrogens and androgens may interact at the cellular level to regulate neural function and behavior.

Time-courses of the appearance/disappearance of nuclear androgen + receptor complexes in the brain and adenohypophysis following testosterone administration/withdrawal to castrated male rats: relationships with gonadotropin secretion

We characterized the temporal dynamics of brain and pituitary cell nuclear androgen receptor binding and serum androgen and gonadotropin levels associated with the implantation and removal of testosterone (T)-filled Silastic capsules into performed s.c. flank pouches of castrated, awake male rats. These capsules produced serum T levels in the physiologic range. The number of cell nuclear androgen + receptor complexes, as measured in an exchange assay using [3H]R1881, increased 15-fold at 0.5 h after capsule insertion in the HPAS (combined hypothalamus, preoptic area, amygdala and septum) and anterior pituitary gland, but then showed a second progressive rise within the next 8 h. This pattern suggests that T exerts an initial action in the tissues to alter the affinity and/or number of available androgen receptors. There was a lag time of 2-4 h to the first indication of negative feedback suppression of LH secretion. Serum LH levels declined only slightly at 4 h after capsule insertion but continued to fall thereafter, reaching undetectable values by 24 h. In contrast, serum FSH levels declined only slightly after 24 h of T exposure. After removal of the T capsules, serum T levels declined to castrate values within 2 h at which time the level of androgen + receptor complexes had fallen to 60% in the brain and pituitary. Serum LH and FSH concentrations were unchanged at 2 h after capsule removal, but rose significantly within the next 2 h. The data indicate that the occupation of androgen receptors rapidly changes in response to variations in circulating T in a fashion that implicates their involvement in the expression of this steroid's negative feedback actions on gonadotropin secretion.

Effects of ATD on male sexual behavior and androgen receptor binding: a reexamination of the aromatization hypothesis

The aromatization hypothesis asserts that testosterone (T) must be aromatized to estradiol (E2) to activate copulatory behavior in the male rat. In support of this hypothesis, the aromatization inhibitor, ATD, has been found to suppress male sexual behavior in T-treated rats. In our experiment, we first replicated this finding by peripherally injecting ATD (15 mg/day) or propylene glycol into T-treated (two 10-mm Silastic capsules) or control castrated male rats. In a second experiment, we bilaterally implanted either ATD-filled or blank cannulae into the medial preoptic area (MPOA) of either T-treated or control castrated male rats. With this more local distribution of ATD, a lesser decline in sexual behavior was found, suggesting that other brain areas are involved in the neurohormonal activation of copulatory behavior in the male rat. To determine whether in vivo ATD interacts with androgen or estrogen receptors, we conducted cell nuclear androgen and estrogen receptor binding assays of hypothalamus, preoptic area, amygdala, and septum following treatment with the combinations of systemic T alone. ATD plus T, ATD alone, and blank control. In all four brain areas binding of T to androgen receptors was significantly decreased in the presence of ATD, suggesting that ATD may act both as an androgen receptor blocker and as an aromatization inhibitor. Competitive binding studies indicated that ATD competes in vitro for cytosol androgen receptors, thus substantiating the in vivo antiandrogenic effects of ATD. Cell nuclear estrogen receptor binding was not significantly increased by exposure to T in the physiological range. No agonistic properties of ATD were observed either behaviorally or biochemically. Thus, an alternative explanation for the inhibitory effects of ATD on male sexual behavior is that ATD prevents T from binding to androgen receptors.

Estrogen-concentrating hypothalamic and limbic neurons project to the medial preoptic nucleus

Estrogen-concentrating neurons that project to the medial preoptic nucleus of the male rat were found to be numerous in limbic and hypothalamic cell groups including the ventral part of the lateral septum, bed nucleus of the stria terminalis, medial amygdaloid nucleus, the ventromedial nucleus, and the amygdalohippocampal zone. This steroid-sensitive circuitry is implicated in the activation of reproductive processes in the male.

Estrogen formation in the mammalian brain: possible role of aromatase in sexual differentiation of the hippocampus and neocortex

Recent studies suggest that sex differences in cognitive function may involve effects of circulating androgens on the developing cerebral cortex and hippocampus. The mechanism of these effects is not understood. In rhesus monkeys, aromatase activity is present in the hippocampus and several areas of the cerebral cortex during late fetal and early postnatal life. Similarly, work in rats and mice indicates that the hippocampus and cerebral cortex may be capable of estrogen biosynthesis during early development. These results are consistent with the hypothesis that the actions of androgens on the developing cerebral cortex and hippocampus may involve local estrogen-mediated effects similar to those responsible for differentiation of the hypothalamic mechanisms controlling reproductive function.

Metabolism and binding of androgens in the spinal cord of the rat

The binding of [3H]androgens and estrogens, and the metabolism of [3H]androgens, were studied in the spinal cord of the adult rat. High-affinity, specific binding sites for [3H]testosterone and [3H]estradiol were detected in cytosol fractions from the spinal cords of castrate animals. Equilibrium dissociation constants for reaction of these sites with their respective ligands were similar to those of androgen and estrogen receptors from other regions of the central nervous system. Nuclear binding of [3H]estradiol was observed in the spinal cord 1 h after intravenous administration of the isotope. Likewise, exchange assay demonstrated the presence of high-affinity androgen binding sites in spinal cord nuclei from orchidectomized, testosterone propionate treated animals. 5 alpha-Reductase activity in homogenates of the spinal cord was relatively high, approximately 3 times that in the pooled hypothalamus, preoptic area, septum and amygdala. However, in contrast to the latter brain regions, estrogen formation was not detectable in spinal cord tissue. No sex differences were observed in the metabolism of [3H]testosterone by spinal cord homogenates. These results confirm the presence of androgen and estrogen receptors in the rat spinal cord. The lack of detectable aromatase activity in the spinal cord is consistent with the hypothesis that the effects of circulating testosterone on spinal reflex function are mediated primarily through the androgen receptor system.

Evidence for the reestablishment of copulatory behavior in castrated male rats with a brain-enhanced estradiol-chemical delivery system

We have developed a redox-chemical system for brain-enhanced drug delivery of estradiol based on an interconvertible dihydropyridine in equilibrium with pyridinium salt carrier. Estradiol, when combined with the carrier, readily crosses the blood-brain barrier and upon oxidation of the carrier is "locked" in the brain. The aim of this study was to evaluate the effects of an estradiol-chemical delivery system (E2-CDS) versus an equimolar dose of estradiol-17-valerate (E2-VAL) on copulatory behavior in orchidectomized rats. The data revealed that a single dose of E2-CDS was more efficacious than E2-VAL in stimulating mounting behavior (percent responding) and the effect was 100% through 5 weeks. E2-CDS increased intromission behavior more than E2-VAL through 28 days. Mount and intromission latencies were reduced by E2-CDS to a greater extent and for a longer time (28 days) than E2-VAL. Neither form of estradiol restored ejaculation parameters or penile reflexes. These data suggest that E2-CDS causes a potent and long-acting stimulation of proceptive and consummatory components of male sexual behavior, presumably acting through the local brain-release of estradiol.

Improved delivery through biological membranes XX: Nicotinamide----dihydronicotinamide based ester-linked redox carrier systems

The 1-carboxyalkyl nicotinamide----dihydronicotinamide redox pair is a new type of brain-enhanced chemical delivery system for drugs containing hydroxyl groups. The rate of oxidation of the dihydro carrier form and subsequent "lock-in" into the brain of the quaternary carrier-drug species, as well as the rate of the hydrolytic release of the drug, depend on both the length and branching of the alkyl chain separating the carrier ring system. Testosterone was used as a model drug, and it was successfully released in a sustained and enhanced manner to the brain in significantly higher concentrations than with the previously reported trigonelline----dihydrotrigonelline carrier system.

Nuclear and cytosolic androgen receptor levels in the limbic brain of neonatal male and female rats

Nuclear and cytosolic androgen receptors in the limbic brain were measured in neonatal male and female rat pups. There were no sex differences in cytosolic receptor concentrations during the neonatal period in any of the regions studied (hypothalamus, amygdala, preoptic area and septum). Receptor concentrations in all 4 regions increase gradually over the first 10 days of life, with no change in the affinity for 5-alpha-dihydrotestosterone. Nuclear receptor levels in intact pups, measured using an exchange assay, are highest between days 4 and 8 of life. In general, nuclear receptor levels are higher in males than in females; however, this sex difference is most consistently seen in the amygdala. These results are discussed in relation to sex differences in circulating testosterone levels and with respect to the contribution of androgens to the sexual differentiation of behavior.

Actions of estrogens and progestins on nerve cells

Estrogens and progestins alter electrical and chemical features of nerve cells, particularly in hypothalamus. Temporally, these events follow nuclear receptor occupation by these steroids, although not all effects have been proved to depend on translocation of receptors to the nucleus. Narrowing studies to focus on particular medial hypothalamic cells has been useful for understanding some of the actions of these steroids in brain. The variety of morphological, chemical, and electrical effects allow for a multiplicity in the cellular functions controlled by these hormones.

Neural gonadal steroid actions

Neurons sensitive to gonadal steroids are located strategically within neural circuits that mediate behaviors broadly related to the reproductive process. Some neuronal events and properties are regulated by these hormones. Variability in the occurrence and distribution of particular neural hormonal sensitivities across species may be related to variations in the hormonal requirements for sexual differentiation and for activation of reproductive behaviors.