Prenatal androgens and the timing of seasonal reproductive transitions in sheep.

Both the onset of puberty in the lamb and the annual resumption of reproductive activity in adult male and female sheep are characterized by increased secretion of LH due to reduced responsiveness to steroid inhibition. However, the timing of puberty is sexually differentiated, for males undergo a reduction in sensitivity to steroid feedback at 10 wk of age, whereas females remain highly responsive to steroid inhibition until 30 wk. This sex difference is determined by androgens in utero. The present study was conducted to determine whether a sex difference exists in the timing of seasonal transitions in adult males and females. We compared serum LH in gonadectomized, estradiol-treated males (n = 7), females (n = 6), and androgenized females (n = 5) from blood samples collected twice weekly for one year. As determined by changes in the pattern of LH secretion, the onset and termination of the autumn breeding season were not different between males, females, and androgenized females (termination: 1 February +/- 4 days, mean +/- SE all groups; onset: males, 22 August +/- 4 days; females, 5 September +/- 18 days; androgenized females, 16 September +/- 10.5 days). However, there was a transient increase in LH (20 May to 23 June) in males, but not in females or androgenized females. Although no effects of prenatal testosterone were evident in the control of LH secretion in adult androgenized females, LH secretion in androgenized males was elevated throughout the nonbreeding season in 3 of 5 animals, indicating that exogenous testosterone may reduce seasonal increases in responsiveness to steroid inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sexual behaviour and LH secretion in spayed androgenized ewes after a s?ngle injection of testosterone or oestradiol-17beta

The behavioural and endocrine responses to single injections of 50 or 500 microgram oestradiol-17beta or 5 mg testosterone were recorded in spayed (control) ewes and in spayed ewes exposed to testosterone between Days 30 and 80 or Days 50 and 100 of prenatal life, The control ewes showed oestrus after injections on 17/18 occasions. The androgenized ewes showed poorer oestrous responses to each hormone although rams showed interest in the ewes. Masculine sexual and aggressive behaviour was shown by the androgenized ewes given either steroid. Both steroids caused a reduction in the plasma LH levels of all the ewes (negative feedback), followed by a preovulatory-type surge (positive feedback). The peak LH values were significantly lower (P is less than 0.05) in the Day 50-100 androgenized ewes than in the controls. It is concluded that prenatal androgenization causes a qualitative shift in the sexual behaviour of ewes from the female type to the male type and affects the sensitivity of the brain to "positive feedback" by steroids.     

Reduced intrafollicular androstenedione and estradiol levels in early-treated prenatally androgenized female rhesus monkeys receiving follicle-stimulating hormone therapy for in vitro fertilization

Five early-treated and four late-treated prenatally androgenized and five normal female rhesus monkeys were studied to determine whether prenatal testosterone propionate exposure beginning Gestational Days 40-44 (early-treated) or 100-115 (late-treated) affects follicular steroidogenesis during recombinant human FSH (rhFSH) treatment. All monkeys underwent rhFSH injections, without human chorionic gonadotropin administration, followed by oocyte retrieval. Serum FSH, LH, estradiol (E2), progesterone (P), 17alpha-hydroxyprogesterone (17 OHP), androstenedione (A4), testosterone, and dihydrotestosterone were measured basally during rhFSH therapy and at oocyte retrieval. Follicle fluid (FF) sex steroids, oocyte fertilization, and embryo development were analyzed. Circulating FSH, E2, 17 OHP, A4, and dihydrotestosterone levels increased similarly in all females. Serum LH levels decreased from basal levels in normal and late-treated prenatally androgenized females but were unchanged in early-treated prenatally androgenized females. Serum P levels at oocyte retrieval were comparable with those before FSH treatment in all females. All prenatally androgenized females showed reduced FF levels of A4 and E2 but not P or dihydrotestosterone. Intrafollicular T concentrations also were significantly lower in late-treated compared with early-treated prenatally androgenized females or normal females. In early-treated prenatally androgenized females, but not the other female groups, intrafollicular A4 and E2 levels were reduced in follicles containing oocytes that failed fertilization or produced zygotes with cleavage arrest before or at the five- to eight-cell embryo stage. Therefore, in monkeys receiving rhFSH therapy alone without human chorionic gonadotropin administration, early prenatal androgenization reduced FF concentrations of E2 and A4 in association with abnormal oocyte development, without having an effect on P, testosterone, or dihydrotestosterone concentrations.     

Neuroendocrine consequences of prenatal androgen exposure in the female rat: absence of luteinizing hormone surges, suppression of progesterone receptor gene expression, and acceleration of the gonadotropin-releasing hormone pulse generator

Preovulatory GnRH and LH surges depend on activation of estrogen (E2)-inducible progesterone receptors (PGRs) in the preoptic area (POA). Surges do not occur in males, or in perinatally androgenized females. We sought to determine whether prenatal androgen exposure suppresses basal or E2-induced Pgr mRNA expression or E2-induced LH surges (or both) in adulthood, and whether any such effects may be mediated by androgen receptor activation. We also assessed whether prenatal androgens alter subsequent GnRH pulsatility. Pregnant rats received testosterone or vehicle daily on Embryonic Days 16-19. POA-hypothalamic tissues were obtained in adulthood for PgrA and PgrB (PgrA+B) mRNA analysis. Females that had prenatal exposure to testosterone (pT) displayed reduced PgrA+B mRNA levels (P < 0.01) compared with those that had prenatal exposure to vehicle (pV). Additional pregnant animals were treated with vehicle or testosterone, or with 5alpha-dihydrotestosterone (DHT). In adult ovariectomized offspring, estradiol benzoate produced a 2-fold increase (P < 0.05) in PgrA+B expression in the POA of pV females, but not in pT females or those that had prenatal exposure to DHT (pDHT). Prenatal testosterone and DHT exposure also prevented estradiol benzoate-induced LH surges observed in pV rats. Blood sampling of ovariectomized rats revealed increased LH pulse frequency in pDHT versus pV females (P < 0.05). Our findings support the hypothesis that prenatal androgen receptor activation can contribute to the permanent defeminization of the GnRH neurosecretory system, rendering it incapable of initiating GnRH surges, while accelerating basal GnRH pulse generator activity in adulthood. We propose that the effects of prenatal androgen receptor activation on GnRH neurosecretion are mediated in part via permanent impairment of E2-induced PgrA+B gene expression in the POA.     

Exposure to female pheromones stimulates a specific type of neuronal population in the male but not female magnocellular division of the medial preoptic nucleus (MPN mag) of the Syrian hamster

The magnocellular division of the medial preoptic area (MPN mag) integrates pheromonal and hormonal signals to play a critical role in the expression of male typical sex behavior. The MPN mag contains two morphologically distinct neuronal populations; the percentage of each type within the nucleus is sex specific. Males have more neurons with a single nucleolus whereas females have more with multiple nucleoli. To determine which neuronal subtype mediates pheromonal induction of copulation, tissue from male and female hamsters exposed to female pheromones was immunolabeled for the immediate early protein (EGR-1). Subsequently the tissue was counterstained and the number of ERG-1 neurons with one or two nuclei was determined. The results indicate that pheromones stimulate neurons with single nucleoli in males but fail to stimulate either neuronal subtype in females suggesting that synaptic input to the MPN mag is sexually differentiated.     

Androgenic Effects on Hypothalamic Asymmetry in a Sexually Differentiated Nucleus Related to Vocal Behavior in Mongolian Gerbils

The relationship between courtship ultrasound emission rates and the volume of a discrete, sex-related, hypothalamic nucleus, the sexually dimorphic area, pars compacta (SDApc), in male and neonatally androgenized female gerbils is lateralized. Unbiased stereological estimates of neuron number and nuclear and neuropil volume are also laterally asymmetric in male SDApcs. In this study sexual differentiation and lateral asymmetry of stereologically assessed cytoarchitectural SDApc components, and their relationship to male-typical behaviors, including vocal emission, were examined in masculinized females. Female neonates received a single injection of testosterone propionate (TP) or control vehicle (Control) and were then implanted with silastic cannulae of testosterone at 65 days of age. Total SDApc volume, neuron number, nuclear volume, and neuropil volume had significantly greater values in TP compared to Control females. Neuron number was laterally asymmetric in TP females, since the left SDApc contained a greater number of smaller neurons, possibly interneurons, than the right. Courtship vocal emission and two other behaviors were masculinized in TP females. Left SDApc total volume and, most significantly, left neuron number, were correlated with vocal rates. No other lateralized correlations between behaviors and stereological estimates were found. It was concluded that various stereological parameters and the lateralization of vocal behavior and brain asymmetry depend on the early sexually differentiating effects of androgens. It is suggested that in gerbils, androgens have a role in the survival of interneurons in a laterally asymmetric hypothalamic nucleus which is an index of vocal control.     

Early androgenization and aggression pheromone in inbred mice

Three experiments were conducted to investigate the effects of early androgenization on the social interactions in mice. Results of Expt 1 indicated that neonatally testosterone propionate (TP) treated females, when mature, were attacked faster, more frequently, and for longer durations by trained fighters than females treated neonatally with oil. Experiment 2 showed that male castrates coated with urine from neonatally TP-treated females were attacked more often and for longer durations than castrates coated with urine from neonatally oil-treated females. The results implicated a urinary pathway of pheromonal output. Experiment 3 was conducted to investigate the relatively long-term social interactions between a naive male and a female treated at birth with TP or between a naive male and a female treated at birth with oil. The androgenized females showed a greater number of wounds after 5 days of pair housing than the control females. Five out of seven androgenized females were killed. Six control females gave birth, and none of the remaining two androgenized females did.     

The role of endogenous gonadotropin-releasing hormone in the control of luteinizing hormone and testosterone secretion in the juvenile male monkey, Macaca fascicularis

To determine what changes occur in the activity of gonadotropin-releasing hormone (GnRH) neurons during pubertal development in primate species we tested the hypotheses that there are morphologic differences between GnRH-containing neurons in juvenile versus adult monkeys, and the low activity of the reproductive axis is governed by hypothalamic GnRH release in monkeys prior to puberty. We removed the brains from 5 juvenile and 5 adult male monkeys (Macaca fascicularis) and blocked, sectioned, and prepared each hypothalamus for light microscopic immunocytochemistry for GnRH-containing cells. The distribution and number of GnRH-containing neurons were similar in adult and juvenile brains; however, GnRH-containing perikarya in adult brains were significantly larger in total cross-sectional area (200 +/- 12 vs. 169 +/- 8 micron 2, P less than 0.05) and in cross-sectional area of the cytoplasm (139 +/- 2 vs. 88 +/- 6 micron 2, P less than 0.05) than in juvenile brains. In another group of 10 juvenile male macaques, we administered an antiserum to GnRH (Fraser #94; 2 ml/kg, i.v.) and monitored the effects on plasma luteinizing hormone (LH) and testosterone concentrations. The percentage of plasma samples with detectable LH levels decreased significantly (from 26.67 +/- 8.3% to 5.3 +/- 3.4%, P less than 0.05) after GnRH antiserum administration; however, plasma testosterone concentrations (0.08 +/- 0.02 ng/ml) remained unchanged. We conclude that during pubertal maturation in primate species there is increased synthesis and release of GnRH from a population of GnRH neurons that are active prior to puberty.     

Sexual dimorphism of blood pressure in spontaneously hypertensive rats is androgen dependent

Intact male and female spontaneously hypertensive rats showed a progressive increase in blood pressure with growth; male attained systolic blood pressure levels of 244 +/- 6 mmHg, and females 205 +/- 3 mmHg at age 22 weeks. Orchidectomy at age 4 weeks significantly attenuated the systolic blood pressure elevation in the male (195 +/- 4 mmHg at age 22 weeks), but ovariectomy at age 4 weeks had no effect on the development of hypertension in the female. The pattern of development of hypertension in orchidectomized males was the same as that in intact and ovariectomized females. Administration of testosterone propionate to gonadectomized rats of both sexes conferred a male pattern of blood pressure development. These results indicate that the sexually dimorphic pattern of hypertension in the spontaneously hypertensive rat is androgen dependent, rather than estrogen dependent. Plasma norepinephrine levels did not differ between the sexes, nor were they altered by gonadectomy or testosterone replacement, suggesting that the higher blood pressures in the intact male and androgen treated male and female SHR are not dependent on increased sympathetic outflow in the established phase of hypertension. Stores of norepinephrine in the posterior hypothalamic region were significantly greater in intact male rats and testosterone treated rats of both sexes than in intact or ovariectomized females, and were higher in the pons of intact female rats than in all other groups. These alterations in central catecholamine stores were not correlated with blood pressure. Further study is needed to assess the functional significance of these androgen mediated alterations in posterior hypothalamic neurons as a determinant of the androgen mediated sexual dimorphism of blood pressure in the spontaneously hypertensive rat.     

Emerging methodologies for the study of hypothalamic gonadotropin-releasing-hormone (GnRH) neurons

Gonadotropin-releasing-hormone (GnRH) neurons form part of a central neural oscillator that controls sexual reproduction through intermittent release of the GnRH peptide. Activity of GnRH neurons, and by extension release of GnRH, has been proposed to reflect intrinsic properties and synaptic input of GnRH neurons. To study GnRH neurons, we used traditional electrophysiology and computational methods. These emerging methodologies enhance the elucidation of processing in GnRH neurons. We used dynamic current-clamping to understand how living GnRH somata process input from glutamate and GABA, two key neurotransmitters in the neuroendocrine hypothalamus. In order to study the impact of synaptic integration in dendrites and neuronal morphology, we have developed full-morphology models of GnRH neurons. Using dynamic clamping, we have demonstrated that small-amplitude glutamatergic currents can drive repetitive firing in GnRH neurons. Furthermore, application of simulated GABAergic synapses with a depolarized reversal potential have revealed two functional subpopulations of GnRH neurons: one population in which GABA chronically depolarizes membrane potential (without inducing action potentials) and a second population in which GABAergic excitation results in slow spiking. Finally, when AMPA-type and GABA-type simulated inputs are applied together, action potentials occur when the AMPA-type conductance occurs during the descending phase of GABAergic excitation and at the nadir of GABAergic inhibition. Compartmental computer models have shown that excitatory synapses at >300 microns from somtata are unable to drive spiking with purely passive dendrites. In models with active dendrites, distal synapses are more efficient at driving spiking than somatic inputs. We then used our models to extend the results from dynamic current clamping at GnRH somata to distribute synaptic inputs along the dendrite. We show that propagation delays for dendritic synapses alter synaptic integration in GnRH neurons by widening the temporal window of interaction for the generation of action potentials. Finally, we have shown that changes in dendrite morphology can modulate the output of GnRH neurons by altering the efficacy of action potential generation in response to after-depolarization potentials (ADPs). Taken together, the methodologies of dynamic current clamping and multi-compartmental modeling can make major contributions to the study of synaptic integration and structure-function relationships in hypothalamic GnRH neurons. Use of these methodological approaches will continue to provide keen insights leading to conceptual advances in our understanding of reproductive hormone secretion in normal and pathological physiology and open the door to understanding whether the mechanisms of pulsatile GnRH release are conserved across species.     

Effects of a gonadotrophin-releasing hormone antagonist on gonadotrophin secretion and gonadal development in neonatal pigs

Male (N = 8) and female (N = 8) pigs were assigned to receive saline or a potent GnRH antagonist ([Ac-D2Nal1,D4-Cl-Phe2,D-Trp3,D-Arg6, D-Ala10]- GnRH*HOAc; 1 mg/kg body weight) at 14 days of age. The GnRH antagonist caused LH to decline (P less than 0.01) from 1.7 ng/ml at 0 h to less than 0.5 ng/ml during 4-32 h in males and females. Concentrations of FSH in gilts declined slowly from 75 +/- 8 to 56 +/- 5 ng/ml (P less than 0.05) at 32 h. In males FSH was low (5.7 +/- 0.5 ng/ml) at 0 h and did not change significantly. To observe the effect of long-term treatment with GnRH antagonist, 10 male and 10 female pigs, 3 days of age, were treated with saline or 1 mg GnRH antagonist per kg body weight every 36 h for 21 days. Concentrations of LH were reduced (P less than 0.01) to 0.2-0.4 ng/ml throughout the experimental period in male and female piglets treated with GnRH antagonist. Plasma FSH increased in control females, but remained suppressed (P less than 0.001) in females treated with GnRH antagonist. Treatment with the GnRH antagonist suppressed FSH levels in males on Days 8 and 16 (P less than 0.05), but not on Day 24. Treatment of females with the GnRH antagonist did not influence (P greater than 0.10) oestradiol-17 beta concentrations. Administration of GnRH antagonist to males suppressed testosterone and oestradiol-17 beta values (P less than 0.01) and reduced testicular weight (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex differences in the motor nucleus of cranial nerve IX-X in Xenopus laevis: a quantitative Golgi study

In the clawed frog (Xenopus laevis), motor neurons in cranial nerve nucleus IX-X control contraction of laryngeal muscles responsible for sexually dimorphic vocal behaviors. We examined sex differences in dendritic arbors of n.IX-X cells using the Golgi-Cox method. Three morphological classes of somal types (ovoid, triangular, and elongate) are present in similar frequencies in n.IX-X of both males and females. The male n.IX-X neuron is a more complex and hypertrophied version of the female n.IX-X cell. The number of primary dendrites is the same for both sexes, but males have more total dendritic segments. The overall dendritic length of male n.IX-X neurons is two to three times that of the female. Males have longer dendritic segments between all branch points. Male and female frogs differ in levels of circulating androgens; neurons of n.IX-X are targets for androgenic steroids. To determine if androgen can affect dendritic morphology in adult females, we examined Golgi-impregnated cells in n.IX-X from ovariectomized females treated with testosterone for 1 month. The total number of dendritic segments was reduced by androgen treatment due to reduction in the number of higher order dendritic segments; the number of primary dendritic segments was unchanged. Androgen treatment may induce resorption of higher order dendritic branches. The overall dendritic length of androgen-treated female n.IX-X neurons was unchanged, and dendritic segments were longer. Thus, although androgen can alter dendrites of n.IX-X cells in adult females, this short-term treatment does not produce a masculine dendritic architecture.     

Sex-specific effects of gonadal steroids on conspecific odor preference in the rat

Effects of gonadal steroids on conspecific odor preference for either (1) sexually active male or active female, (2) sexually active or gonadectomized (gdx) males, (3) sexually active or gdx females, and (4) gdx males or gdx females were determined in male and female rats in a three-chamber apparatus. For the first test, gdx females were made sexually active by treatments with estradiol benzoate (EB) and progesterone (P), and sexually active males were selected by prior screening. Sexually active males and females preferred opposite-sex odor over same-sex odor. Odor of sexually active opposite-sex conspecifics was preferred over that of inactive ones. Immediately after the completion of the first test, sexually active males were gdx and females were left without hormonal treatment. Second and third tests were carried out 2 and 5 weeks after the first test. In the second test, gdx males preferred odor of sexually active males rather than that of receptive females (male-directed preference); in the third test, both males and females showed no preference when tested with four stimulus pairs. The final tests were carried out in gdx males with EB and P, and gdx females with 2-week exposure to testosterone (T). Males with EB and P showed a male-directed preference again, whereas T-treated females kept their own female preference. Injection of EB alone to gdx males did not induce any preference. The present study clearly demonstrated sex difference in conspecific odor preference. Although both male and female preferences depend on their circulating sex steroids, the direction of male preference is more susceptible to their hormonal states, compared to that of females.     

Reversal of reproductive deficiency in the hpg male mouse by neonatal androgenization.

Some aspects of reproductive function in the GnRH-deficient hypogonadal (hpg) mutant mouse can be restored by transplanting normal fetal brain tissue containing GnRH cells into the central nervous system of adult hpg mice. However, hpg males showing physiological response to the graft fail to display sexual behavior and are infertile. We hypothesized that the reproductive deficit of these males is due to insufficient perinatal exposure to testicular androgens as a consequence of the GnRH deficiency. To test this hypothesis we androgenized hpg males by giving them neonatal injections of testosterone propionate (TP). Controls consisted of hpg males not androgenized neonatally and of normal males. All three groups received a TP implant in adulthood, and their copulatory behavior and reproductive capability were recorded. In addition, other hpg males, not androgenized neonatally, received fetal brain transplants containing GnRH neurons and were also tested for copulatory behavior and reproductive capability before and after receiving a TP implant. Three of 8 neonatally androgenized hpg males expressed the full repertoire of male sexual behavior, including intromission and ejaculation, and sired several litters. Three of 7 control hpg males that were not androgenized neonatally but received TP implants in adulthood also displayed mounting and intromission, but there was no evidence of ejaculation, and these males failed to impregnate normal females. Of the 8 hpg males that responded to a fetal transplant with testicular growth, only 1 displayed mounting behavior. However, when given a TP implant, 4 of 8 hpg males with grafts displayed mounting and intromissions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Attractivity of male and female rats which are hormonally manipulated during early development and in adulthood

Attractivity is one aspect of female sexuality relevant for the understanding of male-female sexual interactions. In a previous study, it was shown that intact males were equally attracted to early androgenized, gonadally intact females as to normally developed, estrous females. The present study was designed to investigate in what way hormones given in adulthood might influence attractivity of early androgenized females in adulthood. Specifically, we compared the attractivity of neonatally androgenized females (NeoTP) to the attractivity of normally developed females (NeoOIL), neonatally castrated males (NeoCASTR), and neonatally sham-castrated males (NeoSHAM) when different groups received either OIL, estradiol benzoate (EB) or testosterone propionate (TP) in adulthood. The male's preference to stay in the vicinity of one incentive in favor of the other was taken as an index of attractivity. The results show that, under the present hormonal conditions, NeoTP-females are generally less attractive than NeoOIL-females, more attractive than NeoSHAM-males, and equally attractive as NeoCASTR-males. TP-treated androgenized females were found to be equally attractive as TP-treated NeoSHAM-males. It is concluded that, relative to normally developed females, androgenized females become less attractive when the endogenous secretion of sex steroids is artifically controlled by gonadectomy and/or by administration of fixed amounts of sex steroids.     

Fetal programming: prenatal androgen disrupts positive feedback actions of estradiol but does not affect timing of puberty in female sheep

We studied the impact of prenatal androgen exposure on the timing of onset of puberty, maintenance of cyclicity in the first breeding season, and the LH surge mechanism in female sheep. Pregnant sheep were injected with testosterone propionate (100 mg i.m.) twice each week from Day 30 to Day 90 (D30-90) or from Day 60 to Day 90 (D60-90) of gestation (term = 147 days). Concentrations of plasma progesterone and gonadotropins were measured in blood samples collected twice each week from control (n = 10), D60-90 (n = 13), and D30-90 (n = 3) animals. Rate of weight gain and initiation of estrous behavior were also monitored. After the first breeding season, when the animals entered anestrus, competency of the gonadotropin surge system to respond to estradiol positive feedback was tested in the absence or presence of progesterone priming for 12 days. Prenatally androgenized females had similar body weight gain and achieved puberty (start of first progestogenic cycle) at the same time as controls. Duration of the breeding season and the number of cycles that occurred during the first breeding season were similar between control and prenatally androgenized sheep. In contrast, prenatal exposure to androgens compromised the positive feedback effects of estradiol. Onset of LH/FSH surges following the estradiol stimulus was delayed in both groups of androgenized ewes compared with the controls in both the absence and presence of progesterone priming. In addition, the magnitude of LH and FSH surges in the two animals that surged in the D30-90 group were only one third and one half, respectively, of the magnitudes observed in the control and D60-90 groups. The present findings indicate that disruption of the surge system can account for the fertility problems that occur during adulthood in prenatally androgenized sheep.     

Female sexual behavior displayed by androgenized female rhesus macaques

Adult, prenatally androgenized female rhesus macaques (female pseudohermaphrodites) that had been ovariectomized were treated with estradiol benzoate (20 micrograms/day) and paired with males at weekly intervals for 4 weeks beginning on Day 12 of injection. Their sexual behavior was compared to that of a control group of females. The sexual behavior of the males toward the two groups of genetic females (control females with normal female genitalia and hermaphrodites with well-formed male genitalia) was also observed. Females and hermaphrodites were equally receptive to male invitations to copulate. Although there were some differences in the specific components of proceptive behavior displayed by the two groups, the overall differences were negligible. Earlier studies had shown that infant and juvenile hermaphrodites resemble males in patterns of play and sexual behavior. When treated with testosterone as adults and paired with receptive females, they displayed mounting patterns typical of males--one of seven hermaphrodites achieved intromission and ejaculated. It has now been demonstrated that when treated with estrogen and paired with males, they responded as females. Hence, the capacity to behave sexually as males is not incompatible with the capacity to respond sexually as females under certain hormonal and environmental conditions.     

Steroid-induced developmental plasticity in hypothalamic astrocytes: implications for synaptic patterning.

We have previously demonstrated that astrocytes in the developing arcuate nucleus of the rat hypothalamus exhibit a sexually dimorphic morphology as a result of differential exposure to gonadal steroids. Testosterone via its aromatized byproduct, estrogen, induces arcuate astrocytes to undergo differentiation during the first few days of life. These differentiated astrocytes exhibit a stellate morphology. Coincident with the steroid-induced increase in astrocyte differentiation is a reduction of dendritic spines on arcuate neurons. As a result, the arcuate nucleus of males has fewer axodendritic spine synapses than females and this dimorphism is retained throughout life. In the immediately adjacent ventromedial nucleus, neonatal astrocytes are immature and unresponsive to steroids. Neurons in this region show no change in dendritic spines in the first few days of life but do exhibit increased dendritic branching as a result of testosterone exposure. These findings illustrate the importance of distinct populations of astrocytes in restricted brain regions and their potential importance to the establishment of regionally specific synaptic patterning. Conflicting reports leave the site of steroid-mediated astrocyte responsiveness in the arcuate nucleus unresolved: Are gonadal steroids acting directly on astrocytes or are steroid-concentrating neurons mediating astrocytic responsiveness? In this review, we discuss the current understanding of astrocyte-neuron interactions and the possible mechanisms for steroid-mediated, astrocyte-directed synaptic patterning in the developing hypothalamus.     

Gonadal hormone regulation of neuronal-glial interactions in the developing neuroendocrine hypothalamus

Recent evidence indicates that, in addition to their well known effects on neurons, gonadal steroids may exert part of their neural effects through astroglia. In adult female rats astroglia participate in the phasic remodelling of synapses that takes place during the estrous cycle in the arcuate nucleus of the hypothalamus under the influence of estradiol. Astroglia also appear to be involved in the genesis of sex differences in synaptic connectivity. Gonadal steroids influence hypothalamic astroglia differentiation in vitro and in vivo. In monolayer mixed neuronal-glial cultures from fetal rat hypothalami, estradiol induces a progressive differentiation of astrocytes from a flattened epithelioid morphology to bipolar, radial and stellate shapes. This effect of estradiol on astroglia is dependent on the expression of specific molecules on the neuronal surface, such as the polysialic acid-rich form of the neural cell adhesion molecule. In the rat arcuate nucleus in situ, perinatal androgen influences astroglia gene expression and differentiation, resulting in a sex difference in astroglia organization by postnatal day 20. By this day, the amount of neuronal surface covered by astroglial processes is higher in males than in females. This difference in the coverage of neuronal surface by astroglia may be directly related to the reduced number of synaptic contacts that is established on the soma of male neurons compared to females.