Lactation reduces stress-caused dopaminergic activity and enhances GABAergic activity in the rat medial prefrontal cortex

Plasticity in auditory cortex during parenthood

Most animals display robust parental behaviors that support the survival and well-being of their offspring. The manifestation of parental behaviors is accompanied by physiological and hormonal changes, which affect both the body and the brain for better care giving. Rodents exhibit a behavior called pup retrieval - a stereotyped sequence of perception and action - used to identify and retrieve their newborn pups back to the nest. Pup retrieval consists of a significant auditory component, which depends on plasticity in the auditory cortex (ACx). We review the evidence of neural changes taking place in the ACx of rodents during the transition to parenthood. We discuss how the plastic changes both in and out of the ACx support the encoding of pup vocalizations. Key players in the mechanism of this plasticity are hormones and experience, both of which have a clear dynamic signature during the transition to parenthood. Mothers, co caring females, and fathers have been used as models to understand parental plasticity at disparate levels of organization. Yet, common principles of cortical plasticity and the biological mechanisms underlying its involvement in parental behavior are just beginning to be unpacked.

GABA System Modifications During Periods of Hormonal Flux Across the Female Lifespan

The female lifespan is marked by periods of dramatic hormonal fluctuation. Changes in the ovarian hormones estradiol and progesterone, in addition to the progesterone metabolite allopregnanolone, are among the most significant and have been shown to have widespread effects on the brain. This review summarizes current understanding of alterations that occur within the GABA system during the major hormonal transition periods of puberty, the ovarian cycle, pregnancy and the postpartum period, as well as reproductive aging. The functional impacts of altered inhibitory activity during these times are also discussed. Lastly, avenues for future research are identified, which, if pursued, can broaden understanding of the GABA system in the female brain and potentially lead to better treatments for women experiencing changes in brain function at each of these hormonal transition periods.

Postpartum State, but Not Maternal Caregiving or Level of Anxiety, Increases Medial Prefrontal Cortex GAD65 and vGAT in Female Rats

Upregulation of the inhibitory neurotransmitter, GABA, is involved in many of the behavioral differences between postpartum and nulliparous female rodents. This is evidenced by studies showing that pharmacological blockade of GABAergic activity impairs maternal caregiving and postpartum affective behaviors. However, the influence of motherhood on the capacity for GABA synthesis or release in the medial prefrontal cortex (mPFC; brain region involved in many social and affective behaviors) is not well-understood. Western blotting was used to compare postpartum and nulliparous rats in protein levels of the 65-kD isoform of glutamic acid decarboxylase (GAD65; synthesizes most GABA released from terminals) and vesicular GABA transporter (vGAT; accumulates GABA into synaptic vesicles for release) in the mPFC. We found that postpartum mothers had higher GAD65 and vGAT compared to virgins, but such differences were not found between maternally sensitized and non-sensitized virgins, indicating that reproduction rather than just the display of maternal caregiving is required. To test whether GAD65 and vGAT levels in the mPFC were more specifically related to anxiety-related behavior within postpartum mothers, we selected 8 low-anxiety and 8 high-anxiety dams based on their time spent in the open arms of an elevated plus maze on postpartum day 7. There were no significant differences between the anxiety groups in either GAD65 or vGAT levels. These data further indicate that frontal cortical GABA is affected by female reproduction and more likely contributes to differences in the display of socioemotional behaviors across, but not within, female reproductive state.

GABA in the medial prefrontal cortex regulates anxiety-like behavior during the postpartum period

The postpartum period is commonly accompanied by emotional changes, which for many new mothers includes a reduction in anxiety. Previous research in rodents has shown that the postpartum attenuation in anxiety is dependent on offspring contact and has further implicated enhanced GABAergic neurotransmission as an underlying mechanism. However, the specific brain regions where GABA acts to regulate the offspring-induced reduction in postpartum anxiety requires further investigation. Here, we test the hypothesis that offspring interactions suppress anxiety-like behavior in postpartum female rats via GABA signaling in the medial prefrontal cortex (mPFC). Our results show a postpartum reduction in anxiety-like behavior, an effect which was abolished by localized infusion of the GABAA receptor antagonist bicuculline in the mPFC. We also show that activation of GABAA receptors in the mPFC by the agonist muscimol was effective in restoring anxiolyisis in mothers separated from their pups. Lastly, we show that heightened anxiety-like behavior in pup-separated mothers was accompanied by a lower number and percentage of activated GABAergic neurons within the mPFC. Together, these results suggest that mother-offspring interactions reduce anxiety-like behavior in postpartum females via GABAA neurotransmission in the mPFC and in doing so provide insight into mechanisms that may become dysfunctional in mothers who experience high postpartum anxiety.

Adaptive Modifications of Maternal Hypothalamic-Pituitary-Adrenal Axis Activity during Lactation and Salsolinol as a New Player in this Phenomenon

Both basal and stress-induced secretory activities of the hypothalamic-pituitary-adrenal (HPA) axis are distinctly modified in lactating females. On the one hand, it aims to meet the physiological demands of the mother, and on the other hand, the appropriate and stable plasma cortisol level is one of the essential factors for the proper offspring development. Specific adaptations of HPA axis activity to lactation have been extensively studied in several animal species and humans, providing interesting data on the HPA axis plasticity mechanism. However, most of the data related to this phenomenon are derived from studies in rats. The purpose of this review is to highlight these adaptations, with a particular emphasis on stress reaction and differences that occur between species. Existing data on breastfeeding women are also included in several aspects. Finally, data from the experiments in sheep are presented, indicating a new regulatory factor of the HPA axis-salsolinol-which typical role was revealed in lactation. It is suggested that this dopamine derivative is involved in both maintaining basal and suppressing stress-induced HPA axis activities in lactating dams.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

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.

Using animal models to study post-partum psychiatric disorders

The post-partum period represents a time during which all maternal organisms undergo substantial plasticity in a wide variety of systems in order to ensure the well-being of the offspring. Although this time is generally associated with increased calmness and decreased stress responses, for a substantial subset of mothers, this period represents a time of particular risk for the onset of psychiatric disorders. Thus, post-partum anxiety, depression and, to a lesser extent, psychosis may develop, and not only affect the well-being of the mother but also place at risk the long-term health of the infant. Although the risk factors for these disorders, as well as normal peripartum-associated adaptations, are well known, the underlying aetiology of post-partum psychiatric disorders remains poorly understood. However, there have been a number of attempts to model these disorders in basic research, which aim to reveal their underlying mechanisms. In the following review, we first discuss known peripartum adaptations and then describe post-partum mood and anxiety disorders, including their risk factors, prevalence and symptoms. Thereafter, we discuss the animal models that have been designed in order to study them and what they have revealed about their aetiology to date. Overall, these studies show that it is feasible to study such complex disorders in animal models, but that more needs to be done in order to increase our knowledge of these severe and debilitating mood and anxiety disorders.