Long-term ionic plasticity of GABAergic signalling in the hypothalamus

Spatial transcriptomics reveal basal sex differences in supraoptic nucleus gene expression of adult rats related to cell signaling and ribosomal pathways

BACKGROUND

The supraoptic nucleus (SON) of the hypothalamus contains magnocellular neurosecretory cells that secrete the hormones vasopressin and oxytocin. Sex differences in SON gene expression have been relatively unexplored. Our study used spatially resolved transcriptomics to visualize gene expression profiles in the SON of adult male (n = 4) and female (n = 4) Sprague-Dawley rats using Visium Spatial Gene Expression (10x Genomics).

METHODS

Briefly, 10-μm coronal sections (~ 4 × 4 mm) containing the SON were collected from each rat and processed using Visium slides and recommended protocols. Data were analyzed using 10x Genomics' Space Ranger and Loupe Browser applications and other bioinformatic tools. Two unique differential expression (DE) analysis methods, Loupe Browser and DESeq2, were used.

RESULTS

Loupe Browser DE analysis of the SON identified 116 significant differentially expressed genes (DEGs) common to both sexes (e.g., Avp and Oxt), 31 significant DEGs unique to the males, and 73 significant DEGs unique to the females. DESeq2 analysis revealed 183 significant DEGs between the two groups. Gene Ontology (GO) enrichment and pathway analyses using significant genes identified via Loupe Browser revealed GO terms and pathways related to (1) neurohypophyseal hormone activity, regulation of peptide hormone secretion, and regulation of ion transport for the significant genes common to both males and females, (2) Gi signaling/G-protein mediated events for the significant genes unique to males, and (3) potassium ion transport/voltage-gated potassium channels for the significant genes unique to females, as some examples. GO/pathway analyses using significant genes identified via DESeq2 comparing female vs. male groups revealed GO terms/pathways related to ribosomal structure/function. Ingenuity Pathway Analysis (IPA) identified additional sex differences in canonical pathways (e.g., 'Mitochondrial Dysfunction', 'Oxidative Phosphorylation') and upstream regulators (e.g., CSF3, NFKB complex, TNF, GRIN3A).

CONCLUSION

There was little overlap in the IPA results for the two different DE methods. These results suggest sex differences in SON gene expression that are associated with cell signaling and ribosomal pathways.

Effects of bile duct ligation on the inhibitory control of supraoptic vasopressin neurons

Dilutional hyponatremia due to increased plasma arginine vasopressin (AVP) is associated with liver cirrhosis. However, plasma AVP remains elevated despite progressive hypoosmolality. This study investigated changes to inhibitory control of supraoptic nucleus (SON) AVP neurons during liver cirrhosis. Experiments were conducted with adult male Sprague-Dawley rats. Bile duct ligation was used as a model of chronic liver cirrhosis. An adeno-associated virus containing a construct with an AVP promoter and either green fluorescent protein (GFP) or a ratiometric chloride indicator, ClopHensorN, was bilaterally injected into the SON of rats. After 2 weeks, rats received either BDL or sham surgery, and liver cirrhosis was allowed to develop for 4 weeks. In vitro, loose patch recordings of action potentials were obtained from GFP-labeled and unlabeled SON neurons in response to a brief focal application of the GABAA agonist muscimol (100 μM). Changes to intracellular chloride ([Cl]i) following muscimol application were determined by changes to the fluorescence ratio of ClopHensorN. The contribution of cation chloride cotransporters NKCC1 and KCC2 to changes in intracellular chloride was investigated using their respective antagonists, bumetanide (BU, 10 μM) and VU0240551 (10 μM). Plasma osmolality and hematocrit were measured as a marker of dilutional hyponatremia. The results showed reduced or absent GABAA -mediated inhibition in a greater proportion of AVP neurons from BDL rats as compared to sham rats (100% inhibition in sham vs. 47% in BDL, p = .001). Muscimol application was associated with increased [Cl]i in most cells from BDL as compared to cells from sham rats (χ2  = 30.24, p < .001). NKCC1 contributed to the impaired inhibition observed in BDL rats (p < .001 BDL - BU vs. BDL + BU). The results show that impaired inhibition of SON AVP neurons and increased intracellular chloride contribute to the sustained dilutional hyponatremia in liver cirrhosis.

Arterial Blood Pressure, Neuronal Excitability, Mineral Metabolism and Cell Volume Regulation Mechanisms Revealed by Xenopus laevis oocytes

Xenopus laevis oocytes have been an invaluable tool to discover and explore the molecular mechanisms and characteristics of many proteins, in particular integral membrane proteins. The oocytes were fundamental in many projects designed to identify the cDNA encoding a diversity of membrane proteins including receptors, transporters, channels and pores. In addition to being a powerful tool for cloning, oocytes were later used to experiment with the functional characterization of many of the identified proteins. In this review I present an overview of my personal 30-year experience using Xenopus laevis oocytes and the impact this had on a variety of fields such as arterial blood pressure, neuronal excitability, mineral metabolism and cell volume regulation.

The electrophysiologic properties of gonadotropin-releasing hormone neurons

For about two decades, recordings of identified gonadotropin-releasing hormone (GnRH) neurons have provided a wealth of information on their properties. We describe areas of consensus and debate the intrinsic electrophysiologic properties of these cells, their response to fast synaptic and neuromodulatory input, Ca2+ imaging correlates of action potential firing, and signaling pathways regulating these aspects. How steroid feedback and development change these properties, functions of GnRH neuron subcompartments and local networks, as revealed by chemo- and optogenetic approaches, are also considered.

Role of the cation-chloride-cotransporters in the circadian system

The circadian system plays an immense role in controlling physiological processes in our body. The suprachiasmatic nucleus (SCN) supervises this system, regulating and harmonising the circadian rhythms in our body. Most neurons present in the SCN are GABAergic neurons. Although GABA is considered the main inhibitory neurotransmitter of the CNS, recent studies have shown that excitatory responses were recorded in this area. These responses are enabled by an increase in intracellular chloride ions [Cl^-]_i levels. The chloride (Cl^-) levels in GABAergic neurons are controlled by two solute carrier 12 (SLC12) cation-chloride-cotransporters (CCCs): Na⁺/K⁺/Cl^- co-transporter (NKCC1) and K⁺/Cl^- co-transporter (KCC2), that respectively cause an influx and efflux of Cl^-. Recent works have found altered expression and/or activity of either of these co-transporters in SCN neurons and have been associated with circadian rhythms. In this review, we summarize and discuss the role of CCCs in circadian rhythms, and highlight these recent advances which attest to CCC's growing potential as strong research and therapeutic targets.

Role of angiotensin II in chronic intermittent hypoxia-induced hypertension and cognitive decline

Sleep apnea is characterized by momentary interruptions in normal respiration and leads to periods of decreased oxygen, or intermittent hypoxia. Chronic intermittent hypoxia is a model of the hypoxemia associated with sleep apnea and results in a sustained hypertension that is maintained during normoxia. Adaptations of the carotid body and activation of the renin-angiotensin system may contribute to the development of hypertension associated with chronic intermittent hypoxia. The subsequent activation of the brain renin-angiotensin system may produce changes in sympathetic regulatory neural networks that support the maintenance of the hypertension associated with intermittent hypoxia. Hypertension and sleep apnea not only increase risk for cardiovascular disease but are also risk factors for cognitive decline and Alzheimer's disease. Activation of the angiotensin system could be a common mechanism that links these disorders.

Prenatal infection with Mycoplasma pulmonis in rats exaggerates the angiotensin II pressor response in adult offspring

Exposure to different stressors in utero is linked to adult diseases such as obesity and hypertension. In this study, the impact of prenatal infection (PNI) on adult body weight and cardiovascular function was evaluated using a naturally occurring rodent pathogen, Mycoplasma pulmonis (MP). Pregnant Sprague-Dawley rats were infected with MP on gestational day 14 and gave birth naturally. Adult PNI offspring weighed more than controls, but resting mean arterial pressure (MAP) was unchanged. Subcutaneous injection of angiotensin II (10 μg/kg) elicited a rise in MAP that was greater in both male and female PNI offspring compared with controls (P < 0.03). The accompanying reflex bradycardia was similar to the controls, suggesting that PNI induced baroreflex dysfunction. Subcutaneous nicotine administration, a potent cardiorespiratory stimulus, also elicited a transient rise in MAP that was generally greater in the PNI group, but the change in MAP from baseline was only significant in the PNI females compared with controls (P < 0.03). Elevated body weight and cardiovascular reactivity in the PNI offspring was associated with an increase in the ratio of hypothalamic corticotrophin-releasing hormone receptors type 1 to type 2 gene expression in both sexes compared with controls. These findings support previous studies demonstrating that PNI induces alterations in cardiovascular function and body weight. Yet, unlike previous studies utilizing other models of PNI (e.g., endotoxin), MP PNI did not induce resting hypertension. Thus, our study provides a foundation for future studies evaluating the cardiovascular risks of offspring exposed to microbial challenges in utero.

Brain-Derived Neurotrophic Factor and Supraoptic Vasopressin Neurons in Hyponatremia

Hyponatremia due to elevated arginine vasopressin (AVP) secretion increases mortality in liver failure patients. The mechanisms causing dysregulation of AVP secretion are unknown. Our hypothesis is that inappropriate AVP release associated with liver failure is due to increased brain-derived neurotrophic factor (BDNF) in the supraoptic nucleus (SON). BDNF diminishes GABAA inhibition in SON AVP neurons by increasing intracellular chloride through tyrosine receptor kinase B (TrkB) activation and downregulation of K+/Cl- cotransporter 2 (KCC2). This loss of inhibition could increase AVP secretion. This hypothesis was tested using shRNA against BDNF (shBDNF) in the SON in bile duct ligated (BDL) male rats. All BDL rats had significantly increased liver weight (p < 0.05; 6-9) compared to shams. BDL rats with control -shRNA injections (BDL scrambled [SCR]) developed hyponatremia with increased plasma AVP and copeptin (CPP; all p < 0.05; 6-9) compared to sham groups. This is the first study to show that phosphorylation of TrkB is significantly increased along with significant decrease in phosphorylation of KCC2 in BDL SCR rats compared to the sham rats (p < 0.05;6-8). Knockdown of BDNF in the SON of BDL rats (BDL shBDNF) significantly increased plasma osmolality and hematocrit compared to BDL SCR rats (p < 0.05; 6-9). The BDL shBDNF rats had significant (p < 0.05; 6-9) decreases in plasma AVP and CPP concentration compared to BDL SCR rats. The BDNF knockdown also significantly blocked the increase in TrkB phosphorylation and decrease in KCC2 phosphorylation (p < 0.05; 6-8). The results indicate that BDNF produced in the SON contributes to increased AVP secretion and hyponatremia during liver failure.