Progestin receptor cells in mouse cerebral cortex during early postnatal development: a comparison with preoptic area and central hypothalamus using autoradiography with [125I]progestin

Cell-Genotype Specific Effects of Mecp2 Mutation on Spontaneous and Nicotinic Acetylcholine Receptor-Evoked Currents in Medial Prefrontal Cortical Pyramidal Neurons in Female Rett Model Mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutation in the X-linked MECP2 gene. Random X-inactivation produces a mosaic of mutant (MT) and wild-type (WT) neurons in female Mecp2+/- (het) mice. Many RTT symptoms are alleviated by increasing activity in medial prefrontal cortex (mPFC) in RTT model mice (Howell et al., 2017). Using a GFP-MeCP2 fusion protein to distinguish WT from MT pyramidal neurons in mPFC we found cell autonomous (cell genotype specific) and non-autonomous effects of MeCP2 deficiency on spontaneous excitatory/inhibitory balance, nicotinic acetylcholine receptor (nAChR) currents and evoked activity. MT Layer 5 and 6 (L5, L6) neurons of male nulls, and MT L6 of het mice had reduced spontaneous excitatory synaptic input compared to WT in wild-type male (WTm), female (WTf) and het mice. Inhibitory synaptic charge in MT L6 equaled WT in 2-4-month hets. At 6-7 months inhibitory charge in WT in het slices was increased compared to both MT in het and WT in WTf; however, in hets the excitatory/inhibitory charge ratio was still greater in WT compared to MT. nAChR currents were reduced in L6 of nulls and MT L6 in het slices compared to WT neurons of het, WTm and WTf. At 2-4 months, ACh perfusion increased frequency of inhibitory currents to L6 neurons equally in all genotypes but increased excitatory inputs to MT and WT in hets less than WT in WTfs. Unexpectedly ACh perfusion evoked greater sustained IPSC and EPSC input to L5 neurons of nulls compared to WTm.

Does puberty mark a transition in sensitive periods for plasticity in the associative neocortex?

Postnatal brain development is studded with sensitive periods during which experience dependent plasticity is enhanced. This enables rapid learning from environmental inputs and reorganization of cortical circuits that matches behavior with environmental contingencies. Significant headway has been achieved in characterizing and understanding sensitive period biology in primary sensory cortices, but relatively little is known about sensitive period biology in associative neocortex. One possible mediator is the onset of puberty, which marks the transition to adolescence, when animals shift their behavior toward gaining independence and exploring their social world. Puberty onset correlates with reduced behavioral plasticity in some domains and enhanced plasticity in others, and therefore may drive the transition from juvenile to adolescent brain function. Pubertal onset is also occurring earlier in developed nations, particularly in unserved populations, and earlier puberty is associated with vulnerability for substance use, depression and anxiety. In the present article we review the evidence that supports a causal role for puberty in developmental changes in the function and neurobiology of the associative neocortex. We also propose a model for how pubertal hormones may regulate sensitive period plasticity in associative neocortex. We conclude that the evidence suggests puberty onset may play a causal role in some aspects of associative neocortical development, but that further research that manipulates puberty and measures gonadal hormones is required. We argue that further work of this kind is urgently needed to determine how earlier puberty may negatively impact human health and learning potential. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Differential effects of synthetic progestagens on neuron survival and estrogen neuroprotection in cultured neurons

Progesterone and other progestagens are used in combination with estrogens for clinical purposes, including contraception and postmenopausal hormone therapy. Progesterone and estrogens have interactive effects in brain, however interactions between synthetic progestagens and 17β-estradiol (E2) in neurons are not well understood. In this study, we investigated the effects of seven clinically relevant progestagens on estrogen receptor (ER) mRNA expression, E2-induced neuroprotection, and E2-induced BDNF mRNA expression. We found that medroxyprogesterone acetate decreased both ERα and ERβ expression and blocked E2-mediated neuroprotection and BDNF expression. Conversely, levonorgestrel and nesterone increased ERα and or ERβ expression, were neuroprotective, and failed to attenuate E2-mediated increases in neuron survival and BDNF expression. Other progestagens tested, including norethindrone, norethindrone acetate, norethynodrel, and norgestimate, had variable effects on the measured endpoints. Our results demonstrate a range of qualitatively different actions of progestagens in cultured neurons, suggesting significant variability in the neural effects of clinically utilized progestagens.

Developmental sex differences in nicotinic currents of prefrontal layer VI neurons in mice and rats

Background

There is a large sex difference in the prevalence of attention deficit disorder; yet, relatively little is known about sex differences in the development of prefrontal attention circuitry. In male rats, nicotinic acetylcholine receptors excite corticothalamic neurons in layer VI, which are thought to play an important role in attention by gating the sensitivity of thalamic neurons to incoming stimuli. These nicotinic currents in male rats are significantly larger during the first postnatal month when prefrontal circuitry is maturing. The present study was undertaken to investigate whether there are sex differences in the nicotinic currents in prefrontal layer VI neurons during development.

Methodology/Principal Findings

Using whole cell recording in prefrontal brain slice, we examined the inward currents elicited by nicotinic stimulation in male and female rats and two strains of mice. We found a prominent sex difference in the currents during the first postnatal month when males had significantly greater nicotinic currents in layer VI neurons compared to females. These differences were apparent with three agonists: acetylcholine, carbachol, and nicotine. Furthermore, the developmental sex difference in nicotinic currents occurred despite male and female rodents displaying a similar pattern and proportion of layer VI neurons possessing a key nicotinic receptor subunit.

Conclusions/Significance

This is the first illustration at a cellular level that prefrontal attention circuitry is differently affected by nicotinic receptor stimulation in males and females during development. This transient sex difference may help to define the cellular and circuit mechanisms that underlie vulnerability to attention deficit disorder.

Progestin receptor is transiently expressed perinatally in neurons of the rat isocortex

Steroid hormones influence the development of numerous brain regions, including some that are not classically considered steroid-sensitive. For example, nuclear receptors for both androgen and estrogen have been detected in neonatal cortical cells. High levels of progestin binding and progestin receptor (PR) mRNA have also been reported in early perinatal isocortex. PR expression coincides with high levels of de novo progesterone produced within the cortex, suggesting that PR and its ligand influence the important developmental cortical processes occurring shortly after birth. In order to better understand the role PR plays in cortical development, we used the cellular-level resolution of immunohistochemistry and in situ hybridization (ISH) to characterize changes in perinatal PR expression within specific cortical lamina. PR immunoreactivity (PR-ir) was examined at embryonic days (E) 18, 20, 21, 22, and postnatal days (P) 1, 3, 6, 9, 13, and 27. We find that PR-ir is transiently expressed in specific lamina of frontal, parietal, temporal, and occipital cortex. PR-ir was observed in subplate cells on E18, in increasingly superficial lamina (primarily lamina V, then II/III) during early postnatal development, and was absent by P27. Double-labeling immunohistochemistry indicated that PR-ir colocalizes with the neuronal marker, microtubule associated protein-2, but not with the glial marker, nestin, nor with gamma-aminobutyric acid. These results suggest that specific subpopulations of cortical neurons may be transiently sensitive to progesterone, and that progesterone and its receptor may play a critical role in the fundamental mechanisms underlying normal cortical development.

Progesterone receptors and neural development: a gap between bench and bedside?

Despite a recent increase in the clinical use of progesterone in pregnant women and premature neonates, very little is understood about the potential role of this hormone and its receptors in neural development. Findings from rodent models indicate that the brain is indeed sensitive to progesterone during critical periods of development and maturation. Dramatic sex differences in progesterone receptor (PR) expression, in which males express higher levels of PR than females in specific regions, suggest that PR may play an important role in the sexual differentiation of brain and behavior and that the expression of PR may be one mechanism by which testicular hormones masculinize the brain. PR is also transiently expressed during fetal and neonatal development in areas of the brain associated with cognitive behaviors. PR protein and mRNA are expressed in pyramidal cell layers of perinatal cortex in an anatomically and developmentally specific manner, generating the intriguing hypothesis that progesterone is essential for normal cortical development. Basic research elucidating a potential role for progesterone and PR in developing brain is reviewed in light of the clinical use of this hormone. The necessity for future research integrating findings from the bench and the bedside is evident.

The many faces of progesterone: a role in adult and developing male brain

In addition to its well documented action in female-typical behaviors, progesterone exerts an influence on the brain and behavior of males. This review will discuss the role of progesterone and its receptor in male-typical reproductive behaviors in adulthood and the role of progesterone and its receptor in neural development, in both sexual differentiation of the brain as well as in the development of "non-reproductive" functions. The seemingly inconsistent and contradictory results on progesterone in males that exist in the literature illustrate the complexity of progesterone's actions and illuminate the need for further research in this area. As progestin-containing contraceptives in men are currently being tested and progesterone administration to pregnant women and premature newborns increases, a better understanding of the role of this hormone in behavior and brain development becomes essential.

Sex differences in progesterone receptor immunoreactivity in neonatal mouse brain depend on estrogen receptor alpha expression

Around the time of birth, male rats express higher levels of progesterone receptors in the medial preoptic nucleus (MPN) than female rats, suggesting that the MPN may be differentially sensitive to maternal hormones in developing males and females. Preliminary evidence suggests that this sex difference depends on the activation of estrogen receptors around birth. To test whether estrogen receptor alpha (ER alpha) is involved, we compared progesterone receptor immunoreactivity (PRir) in the brains of male and female neonatal mice that lacked a functional ER alpha gene or were wild type for the disrupted gene. We demonstrate that males express much higher levels of PRir in the MPN and the ventromedial nucleus of the neonatal mouse brain than females, and that PRir expression is dependent on the expression of ER alpha in these regions. In contrast, PRir levels in neocortex are not altered by ER alpha gene disruption. The results of this study suggest that the induction of PR via ER alpha may render specific regions of the developing male brain more sensitive to progesterone than the developing female brain, and may thereby underlie sexual differentiation of these regions.

Receptor localization of steroid hormones and drugs: discoveries through the use of thaw-mount and dry-mount autoradiography

The history of receptor autoradiography, its development and applications, testify to the utility of this histochemical technique for localizing radiolabeled hormones and drugs at cellular and subcellular sites of action in intact tissues. Localization of diffusible compounds has been a challenge that was met through the introduction of the "thaw-mount" and "dry-mount" autoradiographic techniques thirty years ago. With this cellular receptor autoradiography, used alone or combined with other histochemical techniques, sites of specific binding and deposition in vivo and in vitro have been characterized. Numerous discoveries, some reviewed in this article, provided information that led to new concepts and opened new areas of research. As an example, in recent years more than fifty target tissues for vitamin D have been specified, challenging the conventional view about the main biological role of vitamin D. The functions of most of these vitamin D target tissues are unrelated to the regulation of systemic calcium homeostasis, but pertain to the (seasonal) regulation of endo- and exocrine secretion, cell proliferation, reproduction, neural, immune and cardiovascular responses, and adaptation to stress. Receptor autoradiography with cellular resolution has become an indispensable tool in drug research and development, since information can be obtained that is difficult or impossible to gain otherwise.

Intracellular progesterone receptors are differentially regulated by sex steroid hormones in the hypothalamus and the cerebral cortex of the rabbit

The aim of this study was to examine the role of sex steroid hormones in the regulation of intracellular progesterone receptors (PR) in the rabbit central nervous system. We determined PR concentration in cytosol preparations from the hypothalamus, the frontal, tempo-parietal and occipital cortex, by using the specific binding of the synthetic progestin [3H]ORG 2058. PR concentration was higher in the hypothalamus of intact adult females than in that of adult males and prepubertal females, whereas no significant differences were observed in the cerebral cortex of these animals. PR concentration was similar in the three cortical regions analyzed, indicating a homogeneous distribution of PR in the cerebral cortex. The administration of estradiol to ovariectomized animals increased PR concentration in the hypothalamus but not in the cortex. The administration of progesterone to ovariectomized rabbits did not modify PR concentration in any region, however when progesterone was administered after estradiol, it induced a significant diminution in hypothalamic PR concentration without effects in the cortex. These findings suggest that in the rabbit, PR are estrogen regulated in the hypothalamus but not in the cerebral cortex. In the latter, PR are not regulated by progesterone, whereas in the former the estrogen-induced PR are down-regulated by progesterone. Interestingly, hypothalamic PR constitutively expressed in ovariectomized animals are progesterone-insensitive.

Molecular aspects of sexual differentiation of the rodent brain

The sexual differentiation of the brain is orchestrated by gonadal steroids during a restricted developmental period and results in permanent changes in the neural substrate including the capacity to support ovulation and expression of sex-specific reproductive behaviors. Sry gene-induced development of testes constitutes a binary switch directing all subsequent differentiation. Androgen produced by the testes of the embryonic male differentiates secondary sex characteristics but also acts in the brain to "masculinize" the neural substrate, many of the latter are the result of aromatization of testosterone to estrogen. Molecular characteristics of aromatase and 5 alpha-reductase enzymes are reviewed. It is assumed that estrogen binds to its receptor which then binds to DNA, inducing transcription of specific genes. Mutations in steroid receptor genes can markedly alter hormone-mediated differentiation. Two questions are addressed: (1), is the assumption correct that estrogen's effects on sexual differentiation are via the classic genomic action of steroids?; and (2) if so, what genes are transcribed as a result of the activated estrogen receptor complex? We have used antisense oligodeoxynucleotides designed to hybridize with and block the translation of mRNA for the estrogen receptor. Administration of antisense into brain of 3-day-old pups had permanent effects on estrogen-induced differentiation as indicated by behavioral and brain morphology differences in adulthood. These results demonstrate the effectiveness of antisense oligonucleotides if administered during a critical period and further confirm the widely accepted tenet that estrogen acts on the brain via its receptor. Subsequent experiments can now address the question of what genes are being activated by the estrogen receptor during development.