The interplay between glutamatergic circuits and oxytocin neurons in the hypothalamus and its relevance to neurodevelopmental disorders

AgRP neurons mediate activity-dependent development of oxytocin connectivity and autonomic regulation

During postnatal life, the adipocyte-derived hormone leptin is required for proper targeting of neural inputs to the paraventricular nucleus of the hypothalamus (PVH) and impacts the activity of neurons containing agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus. Activity-dependent developmental mechanisms are known to play a defining role during postnatal organization of neural circuits, but whether leptin-mediated postnatal neuronal activity specifies neural projections to the PVH or impacts downstream connectivity is largely unexplored. Here, we blocked neuronal activity of AgRP neurons during a discrete postnatal period and evaluated development of AgRP inputs to defined regions in the PVH, as well as descending projections from PVH oxytocin neurons to the dorsal vagal complex (DVC) and assessed their dependence on leptin or postnatal AgRP neuronal activity. In leptin-deficient mice, AgRP inputs to PVH neurons were significantly reduced, as well as oxytocin-specific neuronal targeting by AgRP. Moreover, downstream oxytocin projections from the PVH to the DVC were also impaired, despite the lack of leptin receptors found on PVH oxytocin neurons. Blocking AgRP neuron activity specifically during early postnatal life reduced the density of AgRP inputs to the PVH, as well as the density of projections from PVH oxytocin neurons to the DVC, and these innervation deficits were associated with dysregulated autonomic function. These findings suggest that postnatal targeting of descending PVH oxytocin projections to the DVC requires leptin-mediated AgRP neuronal activity, and represents a novel activity-dependent mechanism for hypothalamic specification of metabolic circuitry, with consequences for autonomic regulation.

Significance statement

Hypothalamic neural circuits maintain homeostasis by coordinating endocrine signals with autonomic responses and behavioral outputs to ensure that physiological responses remain in tune with environmental demands. The paraventricular nucleus of the hypothalamus (PVH) plays a central role in metabolic regulation, and the architecture of its neural inputs and axonal projections is a defining feature of how it receives and conveys neuroendocrine information. In adults, leptin regulates multiple aspects of metabolic physiology, but it also functions during development to direct formation of circuits controlling homeostatic functions. Here we demonstrate that leptin acts to specify the input-output architecture of PVH circuits through an activity-dependent, transsynaptic mechanism, which represents a novel means of sculpting neuroendocrine circuitry, with lasting effects on how the brain controls energy balance.

Social Interaction Elicits Activity in Glutamatergic Neurons in the Posterior Intralaminar Complex of the Thalamus

BACKGROUND

The posterior intralaminar complex of the thalamus (PIL) is a multimodal nucleus that has been implicated in maternal behaviors and conspecific social behaviors in male and female rodents. Glutamatergic neurons are a major component of the PIL; however, their specific activity and role during social interactions has not yet been assessed.

METHODS

We used immunohistochemistry for the immediate early gene c-fos as a proxy for neuronal activity in the PIL of mice exposed to a novel social stimulus, a novel object stimulus, or no stimulus. We then used fiber photometry to record neural activity of glutamatergic neurons in the PIL in real time during social and nonsocial interactions. Finally, we used inhibitory DREADDs (designer receptors exclusively activated by designer drugs) in glutamatergic PIL neurons and tested social preference and social habituation-dishabituation.

RESULTS

We observed significantly more c-fos-positive cells in the PIL of mice exposed to a social stimulus versus an object stimulus or no stimulus. Neural activity of PIL glutamatergic neurons was increased when male and female mice were engaged in social interaction with a same-sex juvenile or opposite-sex adult, but not a toy mouse. Neural activity was positively correlated with social investigation bout length and negatively correlated with chronological order of bouts. Social preference was unaffected by inhibition; however, inhibiting activity of glutamatergic neurons in the PIL delayed the time that it took for female mice to form social habituation.

CONCLUSIONS

Together, these findings suggest that glutamatergic PIL neurons respond to social stimuli in both male and female mice and may regulate perceptual encoding of social information to facilitate recognition of social stimuli.

Oxytocin neurons mediate stress-induced social memory impairment

Oxytocin has long been thought to play a substantial role in social behaviors, such as social attachment and parenting behavior. However, how oxytocin neurons respond to social and non-social stimuli is largely unknown, especially in high temporal resolution. Here, we recorded the in vivo real-time responses of oxytocin neurons in the paraventricular nucleus of the hypothalamus (PVN) in freely behaving mice. Our results revealed that oxytocin neurons were activated more significantly by stressors than social stimuli. The activation of oxytocin neurons was precisely correlated with struggling behavior during stress. Furthermore, we found that oxytocin mediated stress-induced social memory impairment. Our results reveal an important role of PVN oxytocin neurons in stress-induced social amnesia.

Brain proteomic atlas of alcohol use disorder in adult males

Alcohol use disorder (AUD) affects transcriptomic, epigenetic and proteomic expression in several organs, including the brain. There has not been a comprehensive analysis of altered protein abundance focusing on the multiple brain regions that undergo neuroadaptations occurring in AUD. We performed a quantitative proteomic analysis using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of human postmortem tissue from brain regions that play key roles in the development and maintenance of AUD, the amygdala (AMG), hippocampus (HIPP), hypothalamus (HYP), nucleus accumbens (NAc), prefrontal cortex (PFC) and ventral tegmental area (VTA). Brain tissues were from adult males with AUD (n = 11) and matched controls (n = 16). Across the two groups, there were >6000 proteins quantified with differential protein abundance in AUD compared to controls in each of the six brain regions. The region with the greatest number of differentially expressed proteins was the AMG, followed by the HYP. Pathways associated with differentially expressed proteins between groups (fold change > 1.5 and LIMMA p < 0.01) were analyzed by Ingenuity Pathway Analysis (IPA). In the AMG, adrenergic, opioid, oxytocin, GABA receptor and cytokine pathways were among the most enriched. In the HYP, dopaminergic signaling pathways were the most enriched. Proteins with differential abundance in AUD highlight potential therapeutic targets such as oxytocin, CSNK1D (PF-670462), GABAB receptor and opioid receptors and may lead to the identification of other potential targets. These results improve our understanding of the molecular alterations of AUD across brain regions that are associated with the development and maintenance of AUD. Proteomic data from this study is publicly available at www.lmdomics.org/AUDBrainProteomeAtlas/ .

Developmental Changes in Hypothalamic and Serum Oxytocin Levels in Prenatally Normally Nourished and Undernourished Rats

Changes in the activities of some metabolic factors have been suggested to increase the risk of conditions associated with the Developmental Origins of Health and Disease (DOHaD). We examined changes in oxytocin (OT), a metabolic factor, and OT receptor (OTR) mRNA levels throughout the developmental period in rats of intrauterine undernutrition. Pregnant rats were divided into two groups: a maternal normal nutrition (mNN) and maternal undernutrition (mUN) group. Serum OT concentrations and hypothalamic mRNA levels of OT and OTR were measured in both offspring at various postnatal stages. Both offspring showed significant increases in serum OT concentrations during the neonatal period, significant reductions around the pubertal period, and significant increases in adulthood. Hypothalamic OT mRNA expression levels gradually increased from the neonatal to pubertal period and decreased in adulthood in both offspring. In the pre-weaning period, hypothalamic OT mRNA expression levels were significantly lower in the mUN offspring than in the mNN offspring. In the mUN offspring, hypothalamic OTR mRNA expression levels transiently increased during the neonatal period, decreased around the pubertal period, and increased again in adulthood, whereas transient changes were not detected in mNN offspring. These changes could affect nutritional and metabolic regulation systems in later life and play a role in the mechanisms underlying DOHaD.

Assessment of neurotransmitter release in human iPSC-derived neuronal/glial cells: a missing in vitro assay for regulatory developmental neurotoxicity testing

Human induced pluripotent stem cell (hiPSC)-derived neural stem cells (NSCs) and their differentiated neuronal/glial derivatives have been recently considered suitable to assess in vitro developmental neurotoxicity (DNT) triggered by exposure to environmental chemicals. The use of human-relevant test systems combined with in vitro assays specific for different neurodevelopmental events, enables a mechanistic understanding of the possible impact of environmental chemicals on the developing brain, avoiding extrapolation uncertainties associated with in vivo studies. Currently proposed in vitro battery for regulatory DNT testing accounts for several assays suitable to study key neurodevelopmental processes, including NSC proliferation and apoptosis, differentiation into neurons and glia, neuronal migration, synaptogenesis, and neuronal network formation. However, assays suitable to measure interference of compounds with neurotransmitter release or clearance are at present not included, which represents a clear gap of the biological applicability domain of such a testing battery. Here we applied a HPLC-based methodology to measure the release of neurotransmitters in a previously characterized hiPSC-derived NSC model undergoing differentiation towards neurons and glia. Glutamate release was assessed in control cultures and upon depolarization, as well as in cultures repeatedly exposed to some known neurotoxicants (BDE47 and lead) and chemical mixtures. Obtained data indicate that these cells have the ability to release glutamate in a vesicular manner, and that both glutamate clearance and vesicular release concur in the maintenance of extracellular glutamate levels. In conclusion, analysis of neurotransmitter release is a sensitive readout that should be included in the envisioned battery of in vitro assays for DNT testing.

Dynamic modulation of pulsatile activities of oxytocin neurons in lactating wild-type mice

Breastfeeding, which is essential for the survival of mammalian infants, is critically mediated by pulsatile secretion of the pituitary hormone oxytocin from the central oxytocin neurons located in the paraventricular and supraoptic hypothalamic nuclei of mothers. Despite its importance, the molecular and neural circuit mechanisms of the milk ejection reflex remain poorly understood, in part because a mouse model to study lactation was only recently established. In our previous study, we successfully introduced fiber photometry-based chronic imaging of the pulsatile activities of oxytocin neurons during lactation. However, the necessity of Cre recombinase-based double knock-in mice substantially compromised the use of various Cre-dependent neuroscience toolkits. To overcome this obstacle, we developed a simple Cre-free method for monitoring oxytocin neurons by an adeno-associated virus vector driving GCaMP6s under a 2.6 kb mouse oxytocin mini-promoter. Using this method, we monitored calcium ion transients of oxytocin neurons in the paraventricular nucleus in wild-type C57BL/6N and ICR mothers without genetic crossing. By combining this method with video recordings of mothers and pups, we found that the pulsatile activities of oxytocin neurons require physical mother-pup contact for the milk ejection reflex. Notably, the frequencies of photometric signals were dynamically modulated by mother-pup reunions after isolation and during natural weaning stages. Collectively, the present study illuminates the temporal dynamics of pulsatile activities of oxytocin neurons in wild-type mice and provides a tool to characterize maternal oxytocin functions.

Activation of Calcium-Sensing Receptor in the Area Postrema Inhibits Food Intake via Glutamatergic and GABAergic Signaling Pathways

SCOPE

A high-protein diet has become a popular way to lose weight. Calcium-sensing receptor (CaSR) is activated by amino acids in addition to calcium ions. CaSR shows dense expression in the area postrema (AP), which participates in feeding regulation. The effect of CaSR in the AP on food intake and the potential mechanism involved is investigated.

METHODS AND RESULTS

Male C57BL/6 mice are used to observe the effect of R568 (agonist of CaSR) on food intake. Enzyme-linked immunosorbent assay, immunofluorescence staining, and chemogenetics are used to explore the neural signaling involved. CaSR activation in the AP inhibited acute feeding; R568 increases the content of glutamate and γ-aminobutyric acid (GABA) in the AP, whereas only glutamatergic neurons mediate the effect of R568. GABA-A receptor and ionic glutamate receptor (N-methyl-D-aspartate receptor [NMDAR]) in the paraventricular nucleus of hypothalamus (PVN) are involved in the effect of R568. Promotion of oxytocin (OT) synthesis in the PVN also participates in the effect of R568, and this mechanism is mediated by NMDAR in the PVN.

CONCLUSION

CaSR activation in the AP suppresses feeding, and AP-PVN glutamatergic and GABAergic signaling pathways are involved.