When steroids, such as pregnenolone, progesterone and oestrogen, are synthesised de novo in neural tissues, they are more specifically referred to as neurosteroids. These neurosteroids bind specific receptors to promote essential brain functions. Pregnenolone supports cognition and protects mouse hippocampal cells against glutamate and amyloid peptide-induced cell death. Progesterone promotes myelination, spinogenesis, synaptogenesis, neuronal survival and dendritic growth. Allopregnanolone increases hippocampal neurogenesis, neuronal survival and cognitive functions. Oestrogens, such as oestradiol, regulate synaptic plasticity, reproductive behaviour, aggressive behaviour and learning. In addition, neurosteroids are neuroprotective in animal models of Alzheimer's disease, Parkinson's disease, brain injury and ageing. Using in situ hybridisation and/or immunohistochemistry, steroidogenic enzymes, including cytochrome P450 side-chain cleavage, 3b-hydroxysteroid dehydrogenase/D5-D4 isomerase, cytochrome P450arom, steroid 5a-reductase and 3a-hydroxysteroid dehydrogenase, have been detected in numerous brain regions, including the hippocampus, hypothalamus and cerebral cortex. In the present review, we summarise some of the studies related to the synthesis and function of oestrogens and progestagens in the central nervous system.

The adult brain shows remarkable neuroplasticity in response to hormones and the socioemotional modifications that they influence. In females with reproductive and maternal experience, this neuroplasticity includes the birth and death of new and existing cells in several forebrain regions involved in maternal caregiving and postpartum affective state. Such plasticity in midbrain sites critical for these processes has never been examined, though. Visualizing bromodeoxyuridine (BrdU) to label mitotic cells, l NeuroD for neuronal precursors, and TUNEL to identify dying cells, we found that the midbrain dorsal raphe nucleus (DR, the source of most ascending serotoninergic projections) exhibited significant neuroplasticity in response to motherhood. Specifically, BrdU analyses revealed that DR newborn cell survival (but not proliferation) was regulated by reproductive state, such that cells born early postpartum were less likely to survive compared to cells born during late pregnancy. Many of the surviving cells in the DR were NeuN immunoreactive, suggesting a neuronal phenotype. Similarly, late postpartum rats had fewer NeuroD-immunoreactive DR cells than early postpartum rats. Maternal experience contributed to the late postpartum reduction in DR newborn cell survival because removing the litter at parturition increased cell survival and reduced cell death. Unlike cytogenesis in the maternal hippocampus, which is reduced by circulating glucocorticoids, DR newborn cell survival was unaffected by postpartum adrenalectomy. These effects of reproductive state and motherhood on DR plasticity were associated with concurrent changes in DR levels of serotonin's precursor, 5-HTP, and its metabolite, 5-HIAA. Our results demonstrate for the first time that cytogenesis occurs in the midbrain DR of any adult mammal, that DR plasticity is influenced by female reproductive state and maternal experience, and that this plasticity is accompanied by changes in DR serotonergic function. Because serotonin is critical for postpartum caregiving behaviors and maternal affective state, neuroplastic changes in the DR may contribute to the neurochemical changes necessary for successful motherhood.

Early life stress (ELS) is a potent developmental disruptor and increases the risk for psychopathology. Various forms of ELS have been studied in both humans and rodents, and have been implicated in altered DNA methylation, gene transcription, stress hormone levels, and behavior. Although recent studies have focused on stress-induced epigenetic changes, the extent to which ELS alters HPA axis function and stress responsivity across generations, whether these effects are sex-specific, and how lineage interacts with upbringing to impact these effects, remain unclear. To address these points, two generations of rodents were utilized, with the first generation subjected to ELS via maternal separation, and the second to a balanced cross-fostering paradigm. We hypothesized that ELS would disrupt normative development in both generations, manifesting as altered methylation and expression of genes associated with stress signaling pathways (Nr3c1, Nr3c2, and Bdnf), blunted corticosterone (CORT), and anxiety-like behaviors. Additionally, we expected deficits in the second generation to be modulated by caretaking environment and for the pattern of results to differ between the sexes. Results suggest that direct exposure to ELS leads to sex-specific effects on gene regulation and HPA functioning in adulthood, with maternal separation leading to increases in Bdnf methylation in both sexes, decreases in Bdnf expression in females, and decreases in Nr3c1 methylation in males, as well as blunted CORT and less anxiety-like behavior in females. These alterations converged with caretaking to impart perturbations upon the subsequent generation. Across sex, ELS lineage led to decreased methylation of Nr3c1, and increased methylation of Bdnf. In fostered animals, upbringing by a previously stressed mother interacted with offspring lineage to impact methylation of Nr3c1 and Bdnf. Upbringing was also implicated in altered anxiety-like behavior in males, and baseline CORT levels in females. Such effects may correspond with observed alterations in maternal behavior across groups. In conclusion, ELS conferred enduring sex-specific alterations, both first-hand and trans -generationally via lineage and upbringing. Importantly, lineage of cross-fostered pups was sufficient to normalize or disturb maternal behavior of foster-dams, an observation requiring further elucidation. These results have implications for multi-generational effects of ELS in humans and may motivate early interventions.

The behavioral modifications associated with early motherhood, which include high aggression, caring for the young, and low anxiety, are all affected by acute pharmacological manipulation of serotonin signaling. However, the effects on all these behaviors of permanently disrupting serotonin signaling from one of its primary sources, the dorsal raphe nucleus (DR), have not been examined in detail. To address this, serotonin-specific lesions centered on the dorsomedial DR (DRdm; DR subregion strongly implicated in emotional behaviors) were induced at mid-pregnancy (day 15) or early postpartum (day 2) in rats using a saporin-conjugated neurotoxin targeting the serotonin transporter (Anti-SERT-SAP). Prepartum or postpartum Anti-SERT-SAP reduced DRdm serotonin immunoreactivity by ∼40-65%, and postpartum Anti-SERT-SAP also reduced it in the ventromedial and lateral wings of the DR, as well as in the median raphe. Serotonin-immunoreactive fibers were significantly reduced in the anterior hypothalamus, but not medial preoptic area, of lesioned dams. Pre- or postpartum lesions both greatly reduced maternal aggression, but while prepartum lesions did not affect later undisturbed maternal caregiving, the larger postpartum lesions prevented the postpartum decline in kyphotic nursing and reduced pup licking. Serotonin lesions did not affect pup retrieval, but the prepartum lesions temporarily increased maternal hovering over and licking the pups observed immediately after the disruptive retrieval tests. Dams' anxiety-like behaviors and litter weight gains were unaffected by the lesions. These findings suggest that DRdm serotonin projecting to the AH is particularly critical for maternal aggression, but that more widespread disruption of midbrain raphe serotonin is necessary to greatly impair maternal caregiving. Postpartum anxiety may rely more on other neurochemical systems or different midbrain serotonergic cell populations.

Rationale Maternal behavior in laboratory rats requires a network of brain structures including the ventral bed nucleus of the stria terminalis (BSTv) and medial preoptic area (mPOA). Neurotransmitter systems in the BSTv and mPOA influencing maternal behaviors are not well understood, although norepinephrine is an excellent candidate because the BSTv contains the densest noradrenergic fiber plexus in the forebrain and norepinephrine in the mPOA is known to influence other female reproductive functions. Objectives We hypothesized that downregulated noradrenergic activity in the BSTv and mPOA is necessary for mothering. Methods Postpartum mother-litter interactions were observed after BSTv infusion of yohimbine (an α2 autoreceptor antagonist that increases norepinephrine release), and after BSTv or mPOA infusion of the more selective α2 autoreceptor antagonist idazoxan. Lastly, noradrenergic input to the BSTv/mPOA was selectively lesioned in nulliparous rats with anti-DBH-saporin to determine if this would facilitate mothering. Results BSTv yohimbine almost abolished retrieval of pups but did not significantly affect dams’ ability to initiate contact, lick or nurse them. BSTv idazoxan disrupted retrieval somewhat less than yohimbine, but significantly reduced nursing. mPOA idazoxan impaired retrieval more severely than that found after BSTv infusion. Anti-DBH-saporin almost eliminated noradrenergic terminals in the BSTv and reduced them by over 60% in the mPOA, but did not promote maternal responding. It also did not affect females’ anxiety-related behavior. Conclusions Downregulated noradrenergic activity in the BSTv and mPOA is necessary for postpartum maternal behavior in rats, but eliminating this system alone is insufficient to promote maternal behaviors in nulliparous females.

Female assessment of male attractiveness and how preferred qualities impact reproductive success is central to the study of mate choice. Male attractiveness may depend on traits beneficial to the reproductive success (RS) of any female, termed ‘universal quality’, and/or on behavioral and biological interactions between potential mates that reflect ‘compatibility’. The steroid hormone testosterone (T) often underlies male attractiveness in rodents and is associated with enhanced paternal care in the monogamous and biparental California mouse (Peromyscus californicus). We hypothesized that (1) T-characteristics are universally attractive to female California mice and that (2) if reproductive success is higher for females mated with preferred males, then females mated with males preferred by other females will also have higher reproductive success. Alternatively, we speculated that pair compatibility, based on emergent pair qualities, is important for a species with coordinated offspring care. We assessed individual T-characteristics in three ways: (1) T-response to GnRH challenges (2) baseline T-level and (3) T-response to a female. Testosterone-response did not predict female preference, but females spent more time investigating males with higher baseline T (accounting for only 9.6% of the variation in investigation time). None of the T-measures was associated with RS. Females paired with males they preferred produced litters more quickly and had higher RS than females paired with their non-preferred males. Naïve females who did not undergo preference tests had equivalent RS regardless of whether their mate was preferred or non-preferred by another female. These data suggest that higher male T elicits investigation, but female preference in the California mouse is more strongly linked with compatibility because individual preference was a better predictor of RS than any T measure.

Most maternal caregiving behaviors change across lactation to match the developmental needs of the continuously aging offspring. However, it is mostly unknown whether the dams' postpartum stage or litter age is the primary driving force of these changes. In this study, postnatal day 1 and 8 litters were cross-fostered or in-fostered to postpartum day 1 or 8 dams. Five days later, undisturbed observations of maternal caregiving behaviors were performed on the subsequent two days. We found a main effect of dams' postpartum stage on the frequency that mothers spent with the pups and displayed erect postures over them (hovering over and kyphosis), although it was mostly driven by an interaction between postpartum stage and litter age: early-postpartum dams were in contact with younger litters and in erect postures more often with younger litters compared to later-postpartum dams with younger litters. Additionally, there was an interaction between postpartum stage and litter age on the litter weights because older litters living with later-postpartum dams were heavier than older litters living with early-postpartum dams. There was also an interaction between postpartum stage and litter age on the dams' bodyweight, with early-postpartum dams living with younger litters weighing the least and later-postpartum dams living with younger litters weighing the most. Because activity of the neuropeptide, orexin, within the medial preoptic area (mPOA) has been implicated in maternal nursing and other caregiving behaviors, we measured mPOA levels of orexin-A but it was not affected by postpartum stage or litter age (nor was there an interaction). However, high orexin-A was negatively associated with the frequency of contact with pups and the display of erect postures. These results indicate that changes in caregiving across lactation are driven by endogenous factors in the dams, age-related cues they receive from offspring, and interactions between these factors.

Gulf War Illness (GWI) is a persistent chronic neuroinflammatory illness exacerbated by external stressors and characterized by fatigue, musculoskeletal pain, cognitive, and neurological problems linked to underlying immunological dysfunction for which there is no known treatment. As the immune system and the brain communicate through several signaling pathways, including the hypothalamic–pituitary–adrenal (HPA) axis, it underlies many of the behavioral and physiological responses to stressors via blood-borne mediators, such as cytokines, chemokines, and hormones. Signaling by these molecules is mediated by the semipermeable blood–brain barrier (BBB) made up of a monocellular layer forming an integral part of the neuroimmune axis. BBB permeability can be altered and even diminished by both external factors (e.g., chemical agents) and internal conditions (e.g., acute or chronic stress, or cross-signaling from the hypothalamic–pituitary–gonadal (HPG) axis). Such a complex network of regulatory interactions that possess feed-forward and feedback connections can have multiple response dynamics that may include several stable homeostatic states beyond normal health. Here we compare immune and hormone measures in the blood of human clinical samples and mouse models of Gulf War Illness (GWI) subtyped by exposure to traumatic stress for subtyping this complex illness. We do this via constructing a detailed logic model of HPA–HPG–Immune regulatory behavior that also considers signaling pathways across the BBB to neuronal–glial interactions within the brain. We apply conditional interactions to model the effects of changes in BBB permeability. Several stable states are identified in the system beyond typical health. Following alignment of the human and mouse blood profiles in the context of the model, mouse brain sample measures were used to infer the neuroinflammatory state in human GWI and perform treatment simulations using a genetic algorithm to optimize the Monte Carlo simulations of the putative treatment strategies aimed at returning the ill system back to health. We identify several ideal multi-intervention strategies and potential drug candidates that may be used to treat chronic neuroinflammation in GWI.