Chapter 6 GABAA Receptor Function and Gene Expression During Pregnancy and Postpartum

GABA system as the cause and effect in early development

GABA is the primary inhibitory neurotransmitter in the adult brain and through its actions on GABAARs, it protects against excitotoxicity and seizure activity, ensures temporal fidelity of neurotransmission, and regulates concerted rhythmic activity of neuronal populations. In the developing brain, the development of GABAergic neurons precedes that of glutamatergic neurons and the GABA system serves as a guide and framework for the development of other brain systems. Despite this early start, the maturation of the GABA system also continues well into the early postnatal period. In this review, we organize evidence around two scenarios based on the essential and protracted nature of GABA system development: 1) disruptions in the development of the GABA system can lead to large scale disruptions in other developmental processes (i.e., GABA as the cause), 2) protracted maturation of this system makes it vulnerable to the effects of developmental insults (i.e., GABA as the effect). While ample evidence supports the importance of GABA/GABAAR system in both scenarios, large gaps in existing knowledge prevent strong mechanistic conclusions.

Mood disorders and hormonal status across women's life: a narrative review

Depressive disorders and anxiety states represent one of the most frequent psychiatric pathologies occurring transiently in vulnerable women throughout their life, from puberty to menopause. It is now known that sex hormones play a key role on the nervous system, interfering with neuronal plasticity and enhancing the processes of learning, memory, cognition, and mood. Numerous mechanisms are at the base of these processes, displaying interactions between estrogen and serotoninergic, dopaminergic, and GABAergic receptors at the central level. Therefore, given the sexual steroids fluctuations throughout the entire female lifespan, and considering the role played by sex hormones at the central level, it is not surprising to observe the onset of mood or neurodegenerative disorders over time. This is especially true for women in hormonal transition phase, such as puberty, postpartum and the menopausal transition. Moreover, all these conditions are characterized by hormone withdrawal, imbalance, or modifications due to menopausal hormone therapies or contraceptives which could prompt to a deterioration of mood and cognition impairment or to an improvement in the quality of life. More studies are needed to better understand the hormone-related effects on the nervous system, and the underlying pathways involved in transitional or chronic mood disorders, to promote new patient-specific therapeutic strategies more effective than the current ones and tailored according to the individual need and women's life period.

Neurobiology of peripartum mental illness

At least one in seven pregnant or recently postpartum women will experience a mental illness such as an anxiety disorder, depressive disorder, or substance use disorder. These mental illnesses have detrimental effects on the health of the mother, child, and family, but little is known about the hypothalamic and other neural correlates of maternal mental health concerns. The transition to parenthood alone is a time of remarkable neural plasticity, so it is perhaps not surprising that current research is showing that maternal mental illness has unique neural profiles. Furthermore, the neural systems affected by peripartum mental illness overlap and interact with the systems involved in maternal caregiving behaviors, and mother-infant interactions are, therefore, highly susceptible to disruption. This review discusses what we know about the unique neural changes occurring during peripartum mental illness and the role of the hypothalamus in these illnesses. With an improved understanding of the neural correlates of maternal mental health and disease, we will be better equipped to predict risk, develop effective treatments, and ultimately prevent suffering for millions of parents during this critical time in life.

Adaptations in autonomic nervous system regulation in normal and hypertensive pregnancy

There is an increase in basal sympathetic nerve activity (SNA) during normal pregnancy; this counteracts profound primary vasodilation. However, pregnancy also impairs baroreflex control of heart rate and SNA, contributing to increased mortality secondary to peripartum hemorrhage. Pregnancy-induced hypertensive disorders evoke even greater elevations in SNA, which likely contribute to the hypertension. Information concerning mechanisms is limited. In normal pregnancy, increased angiotensin II acts centrally to support elevated SNA. Hypothalamic sites, including the subfornical organ, paraventricular nucleus, and arcuate nucleus, are likely (but unproven) targets. Moreover, no definitive mechanisms for exaggerated sympathoexcitation in hypertensive pregnancy have been identified. In addition, normal pregnancy increases gamma aminobutyric acid inhibition of the rostral ventrolateral medulla (RVLM), a key brainstem site that transmits excitatory inputs to spinal sympathetic preganglionic neurons. Accumulated evidence supports a major role for locally increased production and actions of the neurosteroid allopregnanolone as one mechanism. A consequence is suppression of baroreflex function, but increased basal SNA indicates that excitatory influences predominate in the RVLM. However, many questions remain regarding other sites and factors that support increased SNA during normal pregnancy and, more importantly, the mechanisms underlying excessive sympathoexcitation in life-threatening hypertensive pregnancy disorders such as preeclampsia.

Brexanolone, a neurosteroid antidepressant, vindicates the GABAergic deficit hypothesis of depression and may foster resilience

The GABAergic deficit hypothesis of depression states that a deficit of GABAergic transmission in defined neural circuits is causal for depression. Conversely, an enhancement of GABA transmission, including that triggered by selective serotonin reuptake inhibitors or ketamine, has antidepressant effects. Brexanolone, an intravenous formulation of the endogenous neurosteroid allopregnanolone, showed clinically significant antidepressant activity in postpartum depression. By allosterically enhancing GABA _A receptor function, the antidepressant activity of allopregnanolone is attributed to an increase in GABAergic inhibition. In addition, allopregnanolone may stabilize normal mood by decreasing the activity of stress-responsive dentate granule cells and thereby sustain resilience behavior. Therefore, allopregnanolone may augment and extend its antidepressant activity by fostering resilience. The recent structural resolution of the neurosteroid binding domain of GABA _A receptors will expedite the development of more selective ligands as a potential new class of central nervous system drugs.

Glial cells as mediators of protective actions of prolactin (PRL) in the CNS

Prolactin (PRL) is a hormone with multiple actions in the central nervous system (CNS) spanning from physiology to pathology. PRL exerts different actions through its receptors that can be found in both neurons and glial cells (astrocytes, microglia and oligodendrocytes) of the brain. Even though its effects during pregnancy and lactation, stress, anxiety, and depression are well studied, recent work on this hormone has brought to light a new role of PRL: that of a protective agent against brain damage and, consequently, against neurodegeneration. The mechanisms through which this protection takes place have not been fully elucidated; however, neurogenesis and anti-apoptosis are some of the plausible mechanisms that could mediate this effect. There is substantial information that implies the involvement of glial activation in this PRL effect, as shown in various models of brain damage. Taking into account glial cell dynamics and actions in various pathological conditions, combined with the neuroprotective effect of PRL, we consider of importance the revision of all the information about the interaction between these two cell types, as it will provide comprehensive knowledge about this new target of PRL against neuropathology.

Progesterone for smoking relapse prevention following delivery: A pilot, randomized, double-blind study

INTRODUCTION

Close to half of women who were smokers prior to conception quit smoking in pregnancy, when endogenous progesterone levels are high. However, at least half resume pre-pregnancy smoking levels within weeks after delivery and when progesterone levels drop. The current pilot study tested the feasibility and preliminary efficacy of postpartum progesterone replacement in preventing relapse to smoking in postpartum women with a history of pre-pregnancy smoking.

METHODS

This was an 8-week, double-blind, parallel, randomized, placebo-controlled pilot trial of 41 women with a history of pre-pregnancy smoking who achieved abstinence by 32 weeks of gestation. Immediately following delivery women were randomized to oral micronized progesterone (200mg twice daily) or placebo via computerized urn randomization program. The main outcome measures were descriptions of study feasibility: recruitment and retention. Secondary outcomes were 7-day point prevalence of abstinence at week 8, time to relapse and smoking cravings.

RESULTS

The trial was feasible with adequate randomization, 64% (41/64) of eligible women, and trial retention, 78% (32/41) completed the trial. Women taking progesterone were 1.8 times more likely to be abstinent during week 8 and took longer to relapse (10 vs. 4 weeks) compared to the placebo group, although these differences did not reach statistical significance. After adjusting for age and pre-quit smoking level, the number needed to treat was 7. There was a 10% greater decline per week in craving ratings in the progesterone group compared to placebo (β=-0.10, 95% CI: -0.15, -0.04, p<0.01). No serious adverse events occurred during the trial.

CONCLUSIONS

These preliminary findings support the promise of progesterone treatment in postpartum smokers and could constitute a therapeutic breakthrough.If these preliminary findings can be evaluated and replicated in a larger study with sufficient power, this may constitute an acceptable and safe smoking relapse prevention strategy for use during lactation.

Postpartum estrogen withdrawal impairs GABAergic inhibition and LTD induction in basolateral amygdala complex via down-regulation of GPR30

Postpartum estrogen (E2) withdrawal is known to be a particularly vulnerable time for depressive symptoms. In this study, ovariectomized (OVX) mice were treated with co-administration of estradiol benzoate and progesterone (E2/P4) followed by administration of E2 alone (E2) and a subsequent E2 withdrawal (EW) to mimic the hormonal changes during pregnancy and postpartum. The objective of this study was to investigate the influence of E2 withdrawal after hormone-simulated pregnancy on synaptic function and plasticity in basolateral amygdala complex (BLA). In comparison to control mice, EW mice spent less time in the central portion of open-field test and open arms of elevated plus-maze. Excitatory postsynaptic potentials (EPSPs) slopes at external capsule BLA synapse were reduced in E2/P4-mice, recovered in E2-mice, and increased in EW-mice. EW-mice showed a significant increase in duration of EPSPs and paired-pulse inhibition (PPI) with multi-spike responses of EPSPs and impairment of long-term depression (LTD) induction, which were corrected by GABA_AR agonist muscimol. Levels of estrogen receptor (ER) GPR30, ERα and ERβ expression in BLA of EW-mice were lower than those in control mice. The bath-application of GPR30 agonist G-1 in BLA of EW-mice recovered the GABA_AR-mediated inhibition and LTD indication, but ERβ agonist DPN or ERα agonist PPT could not. A single BLA-injection of G-1 rather than DPN or PPT in EW-mice could partially relieve the anxiety-like behaviors. The results indicate that postpartum E2 withdrawal causes dysfunction of GABA_AR-mediated inhibition in the BLA through reducing GPR30 expression, which impairs LTD induction and causes anxiety-like behaviors.

Lower allopregnanolone during pregnancy predicts postpartum depression: An exploratory study

Current evidence is mixed on the role of progesterone and its metabolites in perinatal mood and anxiety disorders. We measured second and third trimester (T2 and T3) progesterone (PROG) and allopregnanolone (ALLO) levels by ELISA and postpartum depression (PPD) by clinician interview (DSM-IV criteria) in 60 pregnant women with a prior diagnosis of a mood disorder. Methods included multivariate and logistic regression with general linear mixed effect models. We found that, after adjustment, every additional ng/mL of T2 ALLO resulted in a 63% (95% CI 13% to 84%, p=0.022) reduction in the risk of developing PPD. Our findings extend previous work connecting ALLO and depression within pregnancy, and indicate that the relationship between pregnancy ALLO and PPD is worth further exploration in a larger sample.

Structural and Functional Plasticity in the Maternal Brain Circuitry

Parenting recruits a distributed network of brain structures (and neuromodulators) that coordinates caregiving responses attuned to the young's affect, needs, and developmental stage. Many of these structures and connections undergo significant structural and functional plasticity, mediated by the interplay between maternal hormones and social experience while the reciprocal relationship between the mother and her infant forms and develops. These alterations account for the remarkable behavioral plasticity of mothers. This review will examine the molecular and neurobiological modulation and plasticity through which parenting develops and adjusts in new mothers, primarily discussing recent findings in nonhuman animals. A better understanding of how parenting impacts the brain at the molecular, cellular, systems/network, and behavioral levels is likely to significantly contribute to novel strategies for treating postpartum neuropsychiatric disorders in new mothers, and critical for both the mother's physiological and mental health and the development and well-being of her young.

Chronic Intermittent Ethanol Regulates Hippocampal GABA(A) Receptor Delta Subunit Gene Expression

Chronic ethanol consumption causes structural and functional reorganization in the hippocampus and induces alterations in the gene expression of gamma-aminobutyric acid type A receptors (GABA_ARs). Distinct forced intermittent exposure models have been used previously to investigate changes in GABA_AR expression, with contrasting results. Here, we used repeated cycles of a Chronic Intermittent Ethanol paradigm to examine the relationship between voluntary, dependence-associated ethanol consumption, and GABA_AR gene expression in mouse hippocampus. Adult male C57BL/6J mice were exposed to four 16-h ethanol vapor (or air) cycles in inhalation chambers alternated with limited-access two-bottle choice between ethanol (15%) and water consumption. The mice exposed to ethanol vapor showed significant increases in ethanol consumption compared to their air-matched controls. GABA_AR alpha4 and delta subunit gene expression were measured by qRT-PCR at different stages. There were significant changes in GABA_AR delta subunit transcript levels at different time points in ethanol-vapor exposed mice, while the alpha4 subunit levels remained unchanged. Correlated concurrent blood ethanol concentrations suggested that GABA_AR delta subunit mRNA levels fluctuate depending on ethanol intoxication, dependence, and withdrawal state. Using a vapor-based Chronic Intermittent Ethanol procedure with combined two-bottle choice consumption, we corroborated previous evidences showing that discontinuous ethanol exposure affects GABA_AR delta subunit expression but we did not observe changes in alpha4 subunit. These findings indicate that hippocampal GABA_AR delta subunit expression changes transiently over the course of a Chronic Intermittent Ethanol paradigm associated with voluntary intake, in response to ethanol-mediated disturbance of GABAergic neurotransmission.

The Contribution of Normal Pregnancy to Eclampsia

Eclampsia, clinically defined as unexplained seizure in a woman with preeclampsia, is a life threatening complication unique to the pregnant state. However, a subpopulation of women with seemingly uncomplicated pregnancies experience de novo seizure without preeclamptic signs or symptoms, suggesting pregnancy alone may predispose the brain to seizure. Here, we hypothesized that normal pregnancy lowers seizure threshold and investigated mechanisms by which pregnancy may affect seizure susceptibility, including neuroinflammation and plasticity of gamma-aminobutyric acid type A receptor (GABA_AR) subunit expression. Seizure threshold was determined by quantifying the amount of pentylenetetrazole (PTZ) required to elicit electrical seizure in Sprague Dawley rats that were either nonpregnant (Nonpreg, n = 7) or pregnant (Preg; d20, n = 6). Seizure-induced vasogenic edema was also measured. Further, activation of microglia, a measure of neuroinflammation (n = 6-8/group), and GABA_AR δ- and γ2-subunit protein expression in the cerebral cortex and hippocampus (n = 6/group) was determined. Seizure threshold was lower in Preg compared to Nonpreg rats (36.7±9.6 vs. 65.0±14.5 mg/kg PTZ; p<0.01) that was associated with greater vasogenic edema formation (78.55±0.11 vs. 78.04±0.19% water; p<0.05). The % of active microglia was similar between groups; however, pregnancy was associated with downregulation of cortical GABA_AR-δ and hippocampal GABA_AR-γ2 expression. Overall, pregnancy appears to be a state of increased seizure susceptibility that is not due to neuroinflammation, but rather is associated with reduced expression of GABA_AR subunits and greater edema. Understanding neurophysiological changes occurring in normal pregnancy could allow for better prevention and management of de novo seizure, including pathologic states such as eclampsia.

Emotion and mood adaptations in the peripartum female:complementary contributions of GABA and oxytocin

Peripartum hormones and sensory cues from young modify the maternal brain in ways that can render females either at risk for, or resilient to, elevated anxiety and depression. The neurochemical systems underlying these aspects of maternal emotional and mood states include the inhibitory neurotransmitter GABA and the neuropeptide oxytocin (OXT). Data from laboratory rodents indicate that increased activity at the GABA(A) receptor contributes to the postpartum suppression of anxiety-related behaviour that is mediated by physical contact with offspring, whereas dysregulation in GABAergic signalling results in deficits in maternal care, as well as anxiety- and depression-like behaviours during the postpartum period. Similarly, activation of the brain OXT system accompanied by increased OXT release within numerous brain sites in response to reproductive stimuli also reduces postpartum anxiety- and depression-like behaviours. Studies of peripartum women are consistent with these findings in rodents. Given the similar consequences of elevated central GABA and OXT activity on maternal anxiety and depression, balanced and partly reciprocal interactions between these two systems may be essential for their effects on maternal emotional and mood states, in addition to other aspects of postpartum behaviour and physiology.

Reduced stress responsiveness in pregnancy: relationship with pattern of forebrain c-fos mRNA expression

Stress-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis is known to be attenuated during late pregnancy and throughout lactation. To investigate the neural basis of this stress hyporesponsiveness we examined the changes in the restraint-induced HPA response and accompanying forebrain expression of c-fos mRNA that occur in rats between days 16 (D16) and 19 (D19) of gestation, times associated with declining levels of progesterone, a potential mediating factor. Compared to D16, the D19 group showed a significantly attenuated release of ACTH following 30min restraint. This reduced HPA response was accompanied by significantly lower levels of restraint-induced c-fos mRNA expression in the hypothalamic paraventricular nucleus. Other areas of the forebrain, including medial amygdala, piriform cortex, and ventrolateral septum, showed low c-fos mRNA expression in non-stressed (control) animals and a large increase following restraint, the magnitude of which was similar between D16 and D19 animals indicating no involvement in the differential HPA response to stress. However, a markedly different pattern of c-fos mRNA expression was observed in other brain areas, including barrel cortex and CA1 ventral and CA3 regions of the ventral hippocampus: D19 animals had low control expression which was increased by restraint, but D16 control animals had raised c-fos mRNA expression which was not further elevated by stress. These data demonstrate that region-specific changes in basal and stress-induced cellular activity occur during a period of late gestation coincident with attenuated HPA responsiveness. These changes in neuronal activity may contribute to the adaptive processes that prepare the mother for parturition and lactation.

Prolactin reduces the damaging effects of excitotoxicity in the dorsal hippocampus of the female rat independently of ovarian hormones

We reported previously that lactation prevents the cell damage induced by kainic acid (KA) excitotoxicity in the CA1, CA3, and CA4 areas of the dorsal hippocampus compared to rats in diestrus phase, and hypothesize that pronounced fluctuations of hormones, such as ovarian steroids and prolactin (PRL), have a role in the neuroprotection of the dorsal hippocampus during lactation. PRL is thought to be involved in modulating neural excitability and seizure activity. To investigate actions of prolactin that minimize KA-induced cell damage in the hippocampus, female intact and ovariectomized (OVX) rats were treated for 4 days with a daily dose of 100 microg of prolactin or vehicle. On the third day of prolactin treatment, rats received a systemic dose of 7.5 mg/kg of KA and were sacrificed 48 h later. Immunostaining for Neu-N revealed a significant decrease in cell number in the CA1, CA3 and CA4 areas of intact or OVX, vehicle-treated rats after KA, whereas prolactin treatment prevented cell loss in the CA3 area of intact, and in the CA1, CA3, and CA4 of OVX rats. Fluoro-Jade C staining confirmed these observations. Kainate-induced seizure behavior progressed further in OVX rats, but was attenuated in prolactin-treated rats, both intact and OVX, compared to vehicle-treated rats. These data indicate that prolactin diminishes the damaging actions of excitotoxicity in the kainate model of epilepsy.