Sex Differences in Biophysical Signatures across Molecularly Defined Medial Amygdala Neuronal Subpopulations

Development and function of the medial amygdala

Across studied vertebrates, the medial amygdala (MeA) is a central hub for relaying sensory information with social and/or survival relevance to downstream nuclei such as the bed nucleus of stria terminalis (BNST) and the hypothalamus. MeA-driven behaviors, such as mating, aggression, parenting, and predator avoidance are processed by different molecularly defined inhibitory and excitatory neuronal output populations. Work over the past two decades has deciphered how diverse MeA neurons arise from embryonic development, revealing contributions from multiple telencephalic and diencephalic progenitor domains. Here, we first provide a brief overview of current findings regarding the role of the MeA in social behaviors, followed by a deeper dive into current knowledge of how this complex structure is specified during development. We outline a conceptual model of MeA formation that has emerged based on these findings. We further postulate how embryonic developmental programming of the MeA may inform later emergence of stereotypical circuitry governing hardwired behaviors.

A hypothalamic-amygdala circuit underlying sexually dimorphic aggression

Male animals often display higher levels of aggression than females. However, the neural circuitry mechanisms underlying this sexually dimorphic aggression remain elusive. Here, we identify a hypothalamic-amygdala circuit that mediates male-biased aggression in mice. Specifically, the ventrolateral part of the ventromedial hypothalamus (VMHvl), a sexually dimorphic region associated with eliciting male-biased aggression, projects densely to the posterior substantia innominata (pSI), an area that promotes similar levels of attack in both sexes of mice. Although the VMHvl innervates the pSI unidirectionally through both excitatory and inhibitory connections, it is the excitatory VMHvl-pSI projections that are strengthened in males to promote aggression, whereas the inhibitory connections that reduce aggressive behavior are strengthened in females. Consequently, the convergent hypothalamic input onto the pSI leads to heightened pSI activity in males, resulting in male-biased aggression. Our findings reveal a sexually distinct excitation-inhibition balance of a hypothalamic-amygdala circuit that underlies sexually dimorphic aggression.

Transcriptionally defined amygdala subpopulations play distinct roles in innate social behaviors

Social behaviors are innate and supported by dedicated neural circuits, but the molecular identities of these circuits and how they are established developmentally and shaped by experience remain unclear. Here we show that medial amygdala (MeA) cells originating from two embryonically parcellated developmental lineages have distinct response patterns and functions in social behavior in male mice. MeA cells expressing the transcription factor Foxp2 (MeAFoxp2) are specialized for processing male conspecific cues and are essential for adult inter-male aggression. By contrast, MeA cells derived from the Dbx1 lineage (MeADbx1) respond broadly to social cues, respond strongly during ejaculation and are not essential for male aggression. Furthermore, MeAFoxp2 and MeADbx1 cells show differential anatomical and functional connectivity. Altogether, our results suggest a developmentally hardwired aggression circuit at the MeA level and a lineage-based circuit organization by which a cell's embryonic transcription factor profile determines its social information representation and behavioral relevance during adulthood.

Sexual dimorphism in synaptic inputs to the mouse amygdala and orbital cortex

The medial amygdala (MeA) is a sexually dimorphic brain region that regulates fear responses, emotional memories, and social behaviors. It is known to be larger and contains more cells in males. The MeA integrates information through input connections from olfactory regions, bed nucleus of the stria terminalis, ventral hippocampus, and thalamic and hypothalamic structures. We hypothesize that in addition to the size differences, there are differences in regional connectivity between the sexes. In this study, we utilized G-deleted rabies monosynaptic retrograde tracing to compare amygdala presynaptic cells in male and female whole mouse brains. We report differences in connection patterns to the amygdala, with higher overall connectivity (presynaptic per starter) in males and a larger fraction of inputs originating from the bed nucleus of the stria terminalis, lateral septum, and medial preoptic area. Furthermore, we examined input connections to the orbital cortex (ORB), a brain region shown to be larger in volume in females, and found the opposite trend, where females had more total inputs. Together, our findings extend the evidence for sexual dimorphism in the brain to the neuronal wiring pattern, with likely impacts on behavior and disease susceptibility.

Hardwired to attack: Transcriptionally defined amygdala subpopulations play distinct roles in innate social behaviors

Social behaviors are innate and supported by dedicated neural circuits, but it remains unclear whether these circuits are developmentally hardwired or established through social experience. Here, we revealed distinct response patterns and functions in social behavior of medial amygdala (MeA) cells originating from two embryonically parcellated developmental lineages. MeA cells in male mice that express the transcription factor Foxp2 (MeAFoxp2) are specialized for processing male conspecific cues even before puberty and are essential for adult inter-male aggression. In contrast, MeA cells derived from the Dbx1-lineage (MeADbx1) respond broadly to social cues and are non-essential for male aggression. Furthermore, MeAFoxp2 and MeADbx1 cells show differential anatomical and functional connectivity. Altogether, our results support a developmentally hardwired aggression circuit at the level of the MeA and we propose a lineage-based circuit organization by which a cell's embryonic transcription factor profile determines its social information representation and behavior relevance during adulthood.

The molecular phenotype of kisspeptin neurons in the medial amygdala of female mice

Reproduction is regulated through the hypothalamic-pituitary-gonadal (HPG) axis, largely via the action of kisspeptin neurons in the hypothalamus. Importantly, Kiss1 neurons have been identified in other brain regions, including the medial amygdala (MeA). Though the MeA is implicated in regulating aspects of both reproductive physiology and behavior, as well as non-reproductive processes, the functional roles of MeA Kiss1 neurons are largely unknown. Additionally, besides their stimulation by estrogen, little is known about how MeA Kiss1 neurons are regulated. Using a RiboTag mouse model in conjunction with RNA-seq, we examined the molecular profile of MeA Kiss1 neurons to identify transcripts that are co-expressed in MeA Kiss1 neurons of female mice and whether these transcripts are modulated by estradiol (E2) treatment. RNA-seq identified >13,800 gene transcripts co-expressed in female MeA Kiss1 neurons, including genes for neuropeptides and receptors implicated in reproduction, metabolism, and other neuroendocrine functions. Of the >13,800 genes co-expressed in MeA Kiss1 neurons, only 45 genes demonstrated significantly different expression levels due to E2 treatment. Gene transcripts such as Kiss1, Gal, and Oxtr increased in response to E2 treatment, while fewer transcripts, such as Esr1 and Cyp26b1, were downregulated by E2. Dual RNAscope and immunohistochemistry was performed to validate co-expression of MeA Kiss1 with Cck and Cartpt. These results are the first to establish a profile of genes actively expressed by MeA Kiss1 neurons, including a subset of genes regulated by E2, which provides a useful foundation for future investigations into the regulation and function of MeA Kiss1 neurons.

Sex Differences in the Amygdaloid Projections to the Ventrolateral Periaqueductal Gray and Their Activation During Inflammatory Pain in the Rat

Preclinical and clinical studies have reported sex differences in pain and analgesia. These differences may be linked to anatomical structures of the central nervous system pain modulatory circuitry, and/or hormonal milieu. The midbrain periaqueductal gray is a critical brain region for descending inhibition of pain. The PAG projects to the rostral ventromedial medulla (RVM), which projects bilaterally to the spinal cord to inhibit pain. In addition to pain, this descending circuit (or pathway) can be engaged by endogenous opioids (i.e., endorphins) or exogenous opioids (i.e., morphine), and we have previously reported sex differences in the activation of this circuit during pain and analgesia. Forebrain structures, including the amygdala, project to and engage the PAG-RVM circuit during persistent inflammatory pain. However, there are limited studies in females detailing this amygdalar-PAG pathway and its involvement during persistent inflammatory pain. The objective of the present study was to delineate the neural projections from the amygdala to the PAG in male and female rats to determine if they are sexually distinct in their anatomical organization. We also examined the activation of this pathway by inflammatory pain and the co-localization of receptors for estrogen. Injection of the retrograde tracer fluorogold (FG) into the ventrolateral PAG (vlPAG) resulted in dense retrograde labeling in both the central amygdala (CeA) and medial amygdala (MeA). While the number of CeA-vlPAG neurons were comparable between the sexes, there were more MeA-vlPAG neurons in females. Inflammatory pain resulted in greater activation of the amygdala in males; however, females displayed higher Fos expression within CeA-vlPAG projection neurons. Females expressed higher ERα in the MeA and CeA and the same was true of the projection neurons. Together, these data indicate that although the MeA-vlPAG projections are denser in females, inflammatory pain does not significantly activate these projections. In contrast, inflammatory pain resulted in a greater activation of the CeA-vlPAG pathway in females. As females experience a greater number of chronic pain syndromes, the CeA-vlPAG pathway may play a facilitatory (and not inhibitory) role in pain modulation.

Sex-specific divergent maturational trajectories in the postnatal rat basolateral amygdala

In rodents and humans, the basolateral amygdala (BLA), essential for emotional behaviors, is profoundly reorganized during adolescence. We compared in both sexes the morphology, neuronal, and synaptic properties of BLA neurons in rats at puberty and adulthood. BLA neurons were more excitable in males than in females at adulthood. At pubescence, male action potentials were smaller and shorter than females’ while fast afterhyperpolarizations were larger in males. During postnatal maturation, spine length increased and decreased in females and males, respectively, while there was a reduction in spine head size in females. Excitatory synaptic properties, estimated from stimuli-response relationships, spontaneous post-synaptic currents, and AMPA/NMDA ratio also displayed sex-specific maturational differences. Finally, the developmental courses of long-term potentiation and depression were sexually dimorphic. These data reveal divergent maturational trajectories in the BLA of male and female rats and suggest sex-specific substrates to the BLA linked behaviors at adolescence and adulthood.

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The BLA is immature at puberty and its development toward adulthood is sex-specific

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At adulthood, neuronal excitability is lower in females than in males

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The maturation of spine morphology is more pronounced in females

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The developmental courses of LTP and LTD are sexually divergent

Sex-Specific Social Behavior and Amygdala Proteomic Deficits in Foxp2 +/- Mutant Mice

In humans, mutations in the transcription factor encoding gene, FOXP2, are associated with language and Autism Spectrum Disorders (ASD), the latter characterized by deficits in social interactions. However, little is known regarding the function of Foxp2 in male or female social behavior. Our previous studies in mice revealed high expression of Foxp2 within the medial subnucleus of the amygdala (MeA), a limbic brain region highly implicated in innate social behaviors such as mating, aggression, and parental care. Here, using a comprehensive panel of behavioral tests in male and female Foxp2 +/- heterozygous mice, we investigated the role Foxp2 plays in MeA-linked innate social behaviors. We reveal significant deficits in olfactory processing, social interaction, mating, aggressive, and parental behaviors. Interestingly, some of these deficits are displayed in a sex-specific manner. To examine the consequences of Foxp2 loss of function specifically in the MeA, we conducted a proteomic analysis of microdissected MeA tissue. This analyses revealed putative sex differences expression of a host of proteins implicated in neuronal communication, connectivity, and dopamine signaling. Consistent with this, we discovered that MeA Foxp2-lineage cells were responsive to dopamine with differences between males and females. Thus, our findings reveal a central and sex-specific role for Foxp2 in social behavior and MeA function.

Organization of neural circuits underlying social behavior: A consideration of the medial amygdala

The medial amygdala (MeA) is critical for the expression of a broad range of social behaviors, and is also connected to many other brain regions that mediate those same behaviors. Here, we summarize recent advances toward elucidating mechanisms that enable the MeA to regulate a diversity of social behaviors, and also consider what role the MeA plays within the broader network of regions that orchestrate social sensorimotor transformations. We outline the molecular, anatomical, and electrophysiological features of the MeA that segregate distinct social behaviors, propose experimental strategies to disambiguate sensory representations from behavioral function in the context of a social interaction, and consider to what extent MeA function may overlap with other regions mediating similar behaviors.

Sexual dimorphism in pre-clinical studies of depression

Although there is a sex bias in the pathological mechanisms exhibited by brain disorders, investigation of the female brain in biomedical science has long been neglected. Use of the male model has generally been the preferred option as the female animal model exhibits both biological variability and hormonal fluctuations. Existing studies that compare behavioral and/or molecular alterations in animal models of brain diseases are generally underrepresented, and most utilize the male model. Nevertheless, in recent years there has been a trend toward the increased inclusion of females in brain studies. However, current knowledge regarding sex-based differences in depression and stress-related disorders is limited. This can be improved by reviewing preclinical studies that highlight sex differences in depression. This paper therefore presents a review of sex-based preclinical studies of depression. These shed light on the discrepancies between males and females regarding the biological mechanisms that underpin mechanistic alterations in the diseased brain. This review also highlights the conclusions drawn by preclinical studies to advance our understanding of mood disorders, encouraging researchers to promote ways of investigating and managing sexually dimorphic disorders.