Progesterone rapidly alters the use of place and response memory during spatial navigation in female rats

Progesterone and allopregnanolone facilitate excitatory synaptic transmission in the infralimbic cortex via activation of membrane progesterone receptors

Estrogens and progesterone can have rapid effects on neuronal function and can modify the use of spatial navigation strategies dependent upon the prefrontal cortex, striatum, and hippocampus. Here, we assessed the effects of 17β-estradiol (E2), progesterone, and its metabolite allopregnanolone, on evoked excitatory postsynaptic potentials in the infralimbic region of the female rat prefrontal cortex. Field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of layer I were first characterized by recording responses at multiple depths between the cortical surface and the underlying white matter. Current source density analysis showed that the short latency negative component was generated by activation of synaptic currents within layer I, and that putative polysynaptic responses were generated in layers III to V. The amplitude of evoked field EPSPs in layer I was not significantly affected by 20 min application of 17β-estradiol (10 nM), but both 100 nM progesterone and 1 µM allopregnanolone caused lasting increases in field EPSP amplitude. The effects of progesterone were not blocked by the nuclear progesterone receptor antagonist RU486 (1 µM). Both progesterone and allopregnanolone are known to activate membrane progesterone receptors, and we found that the membrane progesterone receptor agonist Org OD 02-0 facilitated EPSPs, and also occluded further increases induced by either progesterone or allopregnanolone. These results provide evidence that both progesterone and allopregnanolone facilitate synaptic responses in layer I of the infralimbic cortex by activating membrane progesterone receptors.

Sex and estradiol effects in the rodent dorsal striatum

17β-Estradiol (E2) is a sex hormone that acts on many brain regions to produce changes in neuronal activity and learning. A key brain region sensitive to E2 is the dorsal striatum (also called caudate-putamen), which controls motor behaviour, goal-directed learning and habit learning. In adult rodents, oestrogen receptors (ERs) in the dorsal striatum are localized to the plasma membrane and include ERα, ERβ and G protein-coupled ER (GPER). E2, either naturally produced or exogenously applied, may influence neuronal excitability, basal synaptic transmission and long-term synaptic potentiation. These effects may be due to direct action on signalling pathways or may be due to changes in dopamine availability. In particular, estradiol influences dopamine release, dopamine receptor expression and dopamine transporter expression. We review the cellular effects that E2 has in the dorsal striatum, distinguishing between exogenously applied E2 and the oestrous cycle, as well as its influence on dorsal striatal-dependent motor and learning behaviour.

17β-Estradiol reduces inhibitory synaptic currents in entorhinal cortex neurons through G protein-coupled estrogen receptor-1 activation of extracellular signal-regulated kinase

Estrogens are believed to modulate cognitive functions in part through the modulation of synaptic transmission in the cortex and hippocampus. Administration of 17β-estradiol (E2) can rapidly enhance excitatory synaptic transmission in the hippocampus and facilitate excitatory synaptic transmission in rat lateral entorhinal cortex via activation of the G protein-coupled estrogen receptor-1 (GPER1). To assess the mechanisms through which GPER1 activation facilitates synaptic transmission, we assessed the effects of acute 10 nM E2 administration on pharmacologically isolated evoked excitatory and inhibitory synaptic currents in layer II/III entorhinal neurons. Female Long-Evans rats were ovariectomized between postnatal day (PD) 63 and 74 and implanted with a subdermal E2 capsule to maintain continuous low levels of E2. Electrophysiological recordings were obtained between 7 and 20 days after ovariectomy. Application of E2 for 20 min did not significantly affect AMPA or NMDA receptor-mediated excitatory synaptic currents. However, GABA receptor-mediated inhibitory synaptic currents (IPSCs) were markedly reduced by E2 and returned towards baseline levels during the 20-min washout period. The inhibition of GABA-mediated IPSCs was blocked in the presence of the GPER1 receptor antagonist G15. GPER1 can modulate protein kinase A (PKA), but blocking PKA with intracellular KT5720 did not prevent the E2-induced reduction in IPSCs. GPER1 can also stimulate extracellular signal-regulated kinase (ERK), a negative modulator of GABAA receptors, and blocking activation of ERK with PD90859 prevented the E2-induced reduction of IPSCs. E2 can therefore result in a rapid GPER1 and ERK signaling-mediated reduction in GABA-mediated IPSCs. This provides a novel mechanism through which E2 can rapidly modulate synaptic excitability in entorhinal layer II/III neurons and may also contribute to E2 and ERK-dependent alterations in synaptic transmission in other brain areas.

Levonorgestrel maintains goal-directed behavior in habit-trained intact female rats

Hormonal contraceptives are utilized by millions of women worldwide. However, it remains unclear if these powerful endocrine modulators may alter cognitive function. Habit formation involves the progression of instrumental learning as it goes from being a conscious goal-directed process to a cue-driven automatic habitual motor response. Dysregulated goal and/or habit is implicated in numerous psychopathologies, underscoring the relevance of examining the effect of hormonal contraceptives on goal-directed and habitual behavior. This study examined the effect of levonorgestrel (LNG), a widely used progestin-type contraceptive, on the development of habit in intact female rats. Rats were implanted with subcutaneous capsules that slowly released LNG over the course of the experiment or cholesterol-filled capsules. All female rats underwent operant training followed by reward devaluation to test for habit. One group of females was trained at a level that is sub-threshold to habit, while another group of females was trained to a level well over the habit threshold observed in intact females. The results reveal that all sub-threshold trained rats remained goal-directed irrespective of LGN treatment, suggesting LNG is not advancing habit formation in female rats at this level of reinforcement. However, in rats that were overtrained well above the threshold, cholesterol females showed habitual behavior, thus replicating a portion of our original studies. In contrast, LNG-treated habit-trained rats remained goal-directed, indicating that LNG impedes the development and/or expression of habit following this level of supra-threshold to habit training. Thus, LNG may offset habit formation by sustaining attentional or motivational processes during learning in intact female rats. These results may be clinically relevant to women using this type of hormonal contraceptive as well as in other progestin-based hormone therapies.

A mouse model of oral contraceptive exposure: Depression, motivation, and the stress response

Hormonal contraceptives, including oral contraceptives (OCs), regulate hormonal cycles and broadly affect physiological processes, including stress responsivity. Whereas many users describe overall improved mood, up to 10 % of OC users experience adverse effects, including depression and anxiety. Given the link between regulation of hypothalamic-pituitary-adrenal (HPA) axis, stress exposure, and risk for depression, it is likely that OC-effects on stress mediate increased risk or increased resilience to these disorders. In this study, we developed and characterized a tractable mouse model of OC exposure with which to identify the mechanisms underlying OC modulation of brain, behavior, and mood. Specifically, we aimed to determine whether translationally relevant doses of OC-hormones in mice mimic changes in stress responsivity observed in humans taking OCs and describe behavioral changes during OC exposure. Young adult female C57Bl/6 N mice received daily ethinyl estradiol (EE) and levonorgestrel (LVNG) in 10 % sucrose, EE and drospirenone (DRSP) in 10 % sucrose, or 10 % sucrose alone. Translationally relevant doses of EE + LVNG-exposure, but not EE + DRSP, suppressed the acute stress response, consistent with effects observed in human OC users. EE + LVNG caused a specific anhedonia-like effect, without broad changes in stress-coping behavior, other depression-like behaviors, or anxiety-like behaviors. The suppression of regular estrous cycling, together with the blunting of the corticosterone response to acute stress, demonstrate the utility of this model for future studies to identify the mechanisms underlying OC interactions with stress, motivation, and risk for depression.

Sex-dependent differences in animal cognition

The differences in cognitive processes driven by biological sex are the issues that have gotten growing attention recently. Considering the increasing population suffering from various cognitive impairments and the development of therapeutic strategies, it is essential that we recognize the mechanisms responsible for discrepancies observed in male and female learning and memory functions. In this review, we discuss recent reports from preclinical studies on rodents regarding selected cognitive domains to explore the state of knowledge on sex-dependent differences and point to challenges encountered during such research. We focus on spatial, recognition, and emotional memory, as well as on executive functions, such as attention, cognitive flexibility, and working memory. This review will help to acknowledge sex-related differences in cognition and indicate some fields that lack sufficient data.

High estradiol reduces adult neurogenesis but strengthens functional connectivity within the hippocampus during spatial pattern separation in adult female rats

Adult neurogenesis in the dentate gyrus plays an important role for pattern separation, the process of separating similar inputs and forming distinct neural representations. Estradiol modulates neurogenesis and hippocampus function, but to date no examination of estradiol's effects on pattern separation have been conducted. Here, we examined estrogenic regulation of adult neurogenesis and functional connectivity in the hippocampus after the spatial pattern separation task in female rats. Ovariectomized Sprague-Dawley rats received daily injections of vehicle, 0.32 μg (Low) or 5 μg (High) of estradiol benzoate until the end of experiment. A single bromodeoxyuridine (BrdU) was injected one day after initiation of hormone or vehicle treatment and rats were tested in the delayed nonmatching to position spatial pattern separation task in the 8-arm radial maze for 12 days beginning two weeks after BrdU injection. Rats were perfused 90 min after the final trial and brain sections were immunohistochemically stained for BrdU/neuronal nuclei (NeuN) (new neurons), Ki67 (cell proliferation), and the immediate early gene, zif268 (activation). Results showed that high, but not low, estradiol reduced the density of BrdU/NeuN-ir cells and had significant inter-regional correlations of zif268-ir cell density in the hippocampus following pattern separation. Estradiol treatment did not influence pattern separation performance or strategy use. These results show that higher doses of estradiol can reduce neurogenesis but at the same time increases correlations of activity of neurons within the hippocampus during spatial pattern separation.

The role of progesterone in memory bias during spatial navigation in females

Rats can use several memory systems to navigate a maze toward a reward. Two of these are place memory and response memory and female rats can be biased to predominantly use one over another. Both progesterone and estrogens have been shown to alter memory bias. Although the effects of estrogens have been well documented, the effects of progesterone remain somewhat unexplored. Mechanisms through which progesterone may be acting to exert its effects are reviewed here. Converging evidence suggests that the actions of progesterone differ depending on the presence of estrogens, frequently acting in opposition to estrogens when administered together. The hippocampus, dorsal striatum, and prefrontal cortex are likely involved, as is the progesterone metabolite, allopregnanolone. There is a need for more research on progesterone and memory bias, especially considering current formulations of hormonal contraceptives include progestins.

Combined oral contraceptives and mental health: Are adolescence and the gut-brain axis the missing links?

Combined oral contraceptives (containing synthetic forms of estradiol and progestins) are one of the most commonly used drugs among females. However, their effects on the gut-brain axis have not been investigated to a great extent despite clear evidence that suggest bi-directional interactions between the gut microbiome and endogenous sex hormones. Moreover, oral contraceptives are prescribed during adolescence, a critical period of development during which several brain structures and systems, such as hypothalamic-pituitary-gonadal axis, undergo maturation. Considering that oral contraceptives could impact the developing adolescent brain and that these effects may be mediated by the gut-brain axis, further research investigating the effects of oral contraceptives on the gut-brain axis is imperative. This article briefly reviews evidence from animal and human studies on the effects of combined oral contraceptives on the brain and the gut microbiota particularly during adolescence.

Combined effects of the contraceptive hormones, ethinyl estradiol and levonorgestrel, on the use of place and response memory in gonadally-intact female rats

During maze navigation rats can rely on hippocampus-mediated place memory or striatum-mediated response memory. Ovarian hormones bias whether females use place or response memory to reach a reward. Here, we investigated the impact of the contraceptive hormones, ethinyl estradiol (EE) and levonorgestrel (LNG), on memory bias. A total of 63 gonadally-intact female rats were treated with either 10 μg/kg of EE alone, 20 μg/kg of LNG alone, both 10 μg/kg of EE and 20 μg/kg of LNG together, or a sesame oil injection with 5% ethanol as a vehicle control. Rats in the control condition were tested during the diestrus phase of the estrous cycle in order to control for the low circulating levels of gonadotropin and ovarian hormones that occur with oral contraceptive administration. Rats treated with LNG alone had a bias towards the use of place memory compared to diestrus phase control rats. This bias was not observed if LNG was administered in combination with EE. Rats treated with EE or EE+LNG did not have a statistically significant difference in memory bias compared to rats in the control group. These data show that synthetic hormones contained in oral contraceptives administered to females influence which cognitive strategy is predominantly used during navigation.

Centering the Needs of Transgender, Nonbinary, and Gender-Diverse Populations in Neuroendocrine Models of Gender-Affirming Hormone Therapy

Most studies attempting to address the health care needs of the millions of transgender, nonbinary, and/or gender-diverse (TNG) individuals rely on human subjects, overlooking the benefits of translational research in animal models. Researchers have identified many ways in which gonadal steroid hormones regulate neuronal gene expression, connectivity, activity, and function across the brain to control behavior. However, these discoveries primarily benefit cisgender populations. Research into the effects of exogenous hormones such as estradiol, testosterone, and progesterone has a direct translational benefit for TNG individuals on gender-affirming hormone therapies (GAHTs). Despite this potential, endocrinological health care for TNG individuals remains largely unimproved. Here, we outline important areas of translational research that could address the unique health care needs of TNG individuals on GAHT. We highlight key biomedical questions regarding GAHT that can be investigated using animal models. We discuss how contemporary research fails to address the needs of GAHT users and identify equitable practices for cisgender scientists engaging with this work. We conclude that if necessary and important steps are taken to address these issues, translational research on GAHTs will greatly benefit the health care outcomes of TNG people.

What is there to know about the effects of progestins on the human brain and cognition?

Progestins are an important component of hormonal contraceptives (HCs) and hormone replacement therapies (HRTs). Despite an increasing number of studies elucidating the effects of HCs and HRTs, little is known about the effects of different types of progestins included in these medications on the brain. Animal studies suggest that various progestins interact differently with sex steroid, mineralocorticoid and glucocorticoid receptors and have specific modulatory effects on neurotransmitter systems and on the expression of neuropeptides, suggesting differential impacts on cognition and behavior. This review focuses on the currently available knowledge from human behavioral and neuroimaging studies pooled with evidence from animal research regarding the effects of progestins on the brain. The reviewed information is highly relevant for improving women's mental health and making informed choices regarding specific types of contraception or treatment.

Modeling hormonal contraception in female rats: A framework for studies in behavioral neurobiology

Research on hormonal contraceptives (HC) in animal models is lacking, and as a result, so is our understanding of the impact of HC on the brain and behavior. Here, we provide a review of the pharmacology of HC, as well as the methodology and best practices for designing a model of HC in female rats. We outline specific methodological considerations regarding dosing, route of administration, exposure time/timing, and selecting a control group. We also provide a framework outlining important levels of analysis for thinking about the impact of HC on behavioral and neurobiological outcomes. The purpose of this review is to equip researchers with foundational knowledge, and some basic elements of experimental design for future studies investigating the impact of HC on the brain and behavior of female rats.

Hormonal contraceptives, stress, and the brain: The critical need for animal models

Hormonal contraceptives are among the most important health and economic developments in the 20thCentury, providing unprecedented reproductive control and a range of health benefits including decreased premenstrual symptoms and protections against various cancers. Hormonal contraceptives modulate neural function and stress responsivity. These changes are usually innocuous or even beneficial, including their effects onmood. However, in approximately 4-10% of users, or up to 30 million people at any given time, hormonal contraceptives trigger depression or anxiety symptoms. How hormonal contraceptives contribute to these responses and who is at risk for adverse outcomes remain unknown. In this paper, we discussstudies of hormonal contraceptive use in humans and describe the ways in which laboratory animal models of contraceptive hormone exposure will be an essential tool for expanding findings to understand the precise mechanisms by which hormonal contraceptives influence the brain, stress responses, and depression risk.