Morphological and electrophysiological classification of hypothalamic supraoptic neurons

Changes in neuropeptide large dense core vesicle trafficking dynamics contribute to adaptive responses to a systemic homeostatic challenge

Neuropeptides are packed into large dense core vesicles (LDCVs) that are transported from the soma out into their processes. Limited information exists regarding mechanisms regulating LDCV trafficking, particularly during challenges to bodily homeostasis. Addressing this gap, we used 2-photon imaging in an ex vivo preparation to study LDCVs trafficking dynamics in vasopressin (VP) neurons, which traffic and release neuropeptide from their dendrites and axons. We report a dynamic bidirectional trafficking of VP-LDCVs with important differences in speed and directionality between axons and dendrites. Acute, short-lasting stimuli known to alter VP firing activity and axonal/dendritic release caused modest changes in VP-LDCVs trafficking dynamics. Conversely, chronic/sustained systemic osmotic challenges upregulated VP-LDCVs trafficking dynamic, with a larger effect in dendrites. These results support differential regulation of dendritic and axonal LDCV trafficking, and that changes in trafficking dynamics constitute a novel mechanism by which peptidergic neurons can efficiently adapt to conditions of increased hormonal demand.

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.

Long-range axonal projections of transplanted mouse embryonic stem cell-derived hypothalamic neurons into adult mouse brain

The hypothalamus is comprised of heterogenous cell populations and includes highly complex neural circuits that regulate the autonomic nerve system. Its dysfunction therefore results in severe endocrine disorders. Although recent experiments have been conducted for in vitro organogenesis of hypothalamic neurons from embryonic stem (ES) or induced pluripotent stem (iPS) cells, whether these stem cell-derived hypothalamic neurons can be useful for regenerative medicine remains unclear. We therefore performed orthotopic transplantation of mouse ES cell (mESC)-derived hypothalamic neurons into adult mouse brains. We generated electrophysiologically functional hypothalamic neurons from mESCs and transplanted them into the supraoptic nucleus of mice. Grafts extended their axons along hypothalamic nerve bundles in host brain, and some of them even projected into the posterior pituitary (PPit), which consists of distal axons of the magnocellular neurons located in hypothalamic supraoptic and paraventricular nuclei. The axonal projections to the PPit were not observed when the mESC-derived hypothalamic neurons were ectopically transplanted into the substantia nigra reticular part. These findings suggest that our stem cell-based orthotopic transplantation approach might contribute to the establishment of regenerative medicine for hypothalamic and pituitary disorders.

Norepinephrine innervation of the supraoptic nucleus contributes to increased copeptin and dilutional hyponatremia in male rats

Dilutional hyponatremia associated with liver cirrhosis is due to inappropriate release of arginine vasopressin (AVP). Elevated plasma AVP causes water retention resulting in a decrease in plasma osmolality. Cirrhosis, in this study caused by ligation of the common bile duct (BDL), leads to a decrease in central vascular blood volume and hypotension, stimuli for nonosmotic AVP release. The A1/A2 neurons stimulate the release of AVP from the supraoptic nucleus (SON) in response to nonosmotic stimuli. We hypothesize that the A1/A2 noradrenergic neurons support chronic release of AVP in cirrhosis leading to dilutional hyponatremia. Adult, male rats were anesthetized with 2-3% isoflurane (mixed with 95% O2/5% CO2) and injected in the SON with anti-dopamine β-hydroxylase (DBH) saporin (DSAP) or vehicle followed by either BDL or sham surgery. Plasma copeptin, osmolality, and hematocrit were measured. Brains were processed for ΔFosB, dopamine β-hydroxylase (DBH), and AVP immunohistochemistry. DSAP injection: 1) significantly reduced the number of DBH immunoreactive A1/A2 neurons (A1, P < 0.0001; A2, P = 0.0014), 2) significantly reduced the number of A1/A2 neurons immunoreactive to both DBH and ΔFosB positive neurons (A1, P = 0.0015; A2, P < 0.0001), 3) reduced the number of SON neurons immunoreactive to both AVP and ΔFosB (P < 0.0001), 4) prevented the increase in plasma copeptin observed in vehicle-injected BDL rats (P = 0.0011), and 5) normalized plasma osmolality and hematocrit (plasma osmolality, P = 0.0475; hematocrit, P = 0.0051) as compared with vehicle injection. Our data suggest that A1/A2 neurons contribute to increased plasma copeptin and hypoosmolality in male BDL rats.

Vasopressin and oxytocin expression in hypothalamic supraoptic nucleus and plasma electrolytes changes in water-deprived male Meriones libycus

In mammals, plasmatic osmolality needs to be stable, and it is highly related to the hydric state of the animals which depends on the activity of the hypothalamic neurohypophysial system and more particularly by vasopressin secretion. Meriones, a desert rodent, can survive even without drinking for more than one month. The mechanism(s) by which they survive under these conditions remains poorly understood. In this study, we examine the water’s deprivation consequences on the: (1) anatomy, morphology, and physiology of the hypothalamic supraoptic nucleus, (2) body mass and plasma electrolytes changes in male desert rodents ‘Meriones libycus’ subjected to water deprivation for 30 days. The effect of water deprivation was evaluated on the structural and cellular organization of the supraoptic nucleus by morphological observations and immunohistochemical approaches, allowing the labeling of AVP but also oxytocin. Our finding demonstrated that upon water deprivation (1) the body weight decreased and reached a plateau after a month of water restriction. (2) The plasmatic osmolality began to decrease and return to values similar to control animals at day 30. (3) The SON, both in hydrated and water-deprived animals, is highly developed.(4) The AVP labeling in the SON increased upon dehydration at variance with OT. These changes observed in body mass and plasma osmolality reveal an important adaptive process of male Meriones in response to prolonged water deprivation. Overall, this animal represents an interesting model for the study of water body homeostasis and the mechanisms underlying the survival of desert rodents to xeric environments.

Protective effect of HCN2-induced SON sensitization on chronic visceral hypersensitivity in neonatal-CRD rat model

Abnormal brain-gut interactions contribute to the development of chronic visceral hypersensitivity (CVH), which is the pivotal feature of irritable bowel syndrome (IBS). Despite the consensus with respect to the vital role of hyperpolarization-activated cyclic nucleotide-gated 2 (HCN2) channels in promoting painful symptoms in the peripheral nervous system, we identified that the upregulation of HCN2 in supraoptic nucleus (SON) was involved in the modulation of CVH in rat model of neonatal colorectal distention (n-CRD). Specifically, colorectal distention (CRD) upregulated the expression of c-Fos in SON in adult CVH rats, indicating the involvement of SON sensitazation in visceral sensation. Moreover, the administration of ZD7288 (the pan-HCN channel inhibitor) rather than 8-Br-cAMP (the non-specific HCN channel agonist) aggravated the CVH symptoms and reduced the phosphorylation level of CaMKII-CREB cascade. Together, the findings indicated that the upregulation of supraoptic HCN2 contributed to the sensitization of SON, which had protective effects on the modulation of CVH with the involvement of CaMKII-CREB cascade in n-CRD rat model.

Brain oxytocin: how puzzle stones from animal studies translate into psychiatry

The neuropeptide oxytocin has attracted great attention of the general public, basic neuroscience researchers, psychologists, and psychiatrists due to its profound pro-social, anxiolytic, and "anti-stress" behavioral and physiological effects, and its potential application for treatment of mental diseases associated with altered socio-emotional competence. During the last decade, substantial progress has been achieved in understanding the complex neurobiology of the oxytocin system, including oxytocinergic pathways, local release patterns, and oxytocin receptor distribution in the brain, as well as intraneuronal oxytocin receptor signaling. However, the picture of oxytocin actions remains far from being complete, and the central question remains: "How does a single neuropeptide exert such pleotropic actions?" Although this phenomenon, typical for many of about 100 identified neuropeptides, may emerge from the anatomical divergence of oxytocin neurons, their multiple central projections, distinct oxytocin-sensitive cell types in different brain regions, and multiple intraneuronal signaling pathways determining the specific cellular response, further basic studies are required. In conjunction, numerous reports on positive effects of intranasal application of oxytocin on human brain networks controlling socio-emotional behavior in health and disease require harmonic tandems of basic researchers and clinicians. During the COVID-19 crisis in 2020, oxytocin research seems central as question of social isolation-induced inactivation of the oxytocin system, and buffering effects of either activation of the endogenous system or intranasal application of synthetic oxytocin need to be thoroughly investigated.

Spexin-expressing neurons in the magnocellular nuclei of the human hypothalamus

Neuropeptides are involved in numerous brain activities being responsible for a wide spectrum of higher mental functions. The purpose of this concise, structural and qualitative investigation was to map the possible immunoreactivity of the novel neuropeptide spexin (SPX) within the human magnocellular hypothalamus. SPX is a newly identified peptide, a natural ligand for the galanin receptors (GALR) 2/3, with no molecular structure similarities to currently known regulatory factors. SPX seems to have multiple physiological functions, with an involvement in reproduction and food-intake regulation recently revealed in animal studies. For the first time we describe SPX expressing neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the human hypothalamus using immunohistochemical and fluorescent methods, key regions involved in the mechanisms of osmotic homeostasis, energy expenditure, consummatory behaviour, reproductive processes, social recognition and stress responses. The vast majority of neurons located in both examined neurosecretory nuclei show abundant SPX expression and this may indirectly implicate a potential contribution of SPX signalling to the hypothalamic physiology in the human brain.

A preliminary study of the effect of a high-salt diet on transcriptome dynamics in rat hypothalamic forebrain and brainstem cardiovascular control centers

BACKGROUND

High dietary salt intake is strongly correlated with cardiovascular (CV) diseases and it is regarded as a major risk factor associated with the pathogenesis of hypertension. The CV control centres in the brainstem (the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM)) and hypothalamic forebrain (the subfornical organ, SFO; the supraoptic nucleus, SON and the paraventricular nucleus, PVN) have critical roles in regulating CV autonomic motor outflows, and thus maintaining blood pressure (BP). Growing evidence has implicated autonomic regulatory networks in salt-sensitive HPN (SSH), but the genetic basis remains to be delineated. We hypothesized that the development and/ or maintenance of SSH is reliant on the change in the expression of genes in brain regions controlling the CV system.

METHODOLOGY

We used RNA-Sequencing (RNA-Seq) to describe the differential expression of genes in SFO, SON, PVN, NTS and RVLM of rats being chronically fed with high-salt (HS) diet. Subsequently, a selection of putatively regulated genes was validated with quantitative reverse transcription polymerase chain reaction (qRT-PCR) in both Spontaneously Hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats.

RESULTS

The findings enabled us to identify number of differentially expressed genes in SFO, SON, PVN, NTS and RVLM; that are either up-regulated in both strains of rats (SON- Caprin2, Sctr), down-regulated in both strains of rats (PVN- Orc, Gkap1), up-regulated only in SHRs (SFO- Apopt1, Lin52, AVP, OXT; SON- AVP, OXT; PVN- Caprin2, Sclt; RVLM- A4galt, Slc29a4, Cmc1) or down-regulated only in SHRs (SON- Ndufaf2, Kcnv1; PVN- Pi4k2a; NTS- Snrpd2l, Ankrd29, St6galnac6, Rnf157, Iglon5, Csrnp3, Rprd1a; RVLM- Ttr, Faim).

CONCLUSIONS

These findings demonstrated the adverse effects of HS diet on BP, which may be mediated via modulating the signaling systems in CV centers in the hypothalamic forebrain and brainstem.

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.

Unified Classification of Molecular, Network, and Endocrine Features of Hypothalamic Neurons

Peripheral endocrine output relies on either direct or feed-forward multi-order command from the hypothalamus. Efficient coding of endocrine responses is made possible by the many neuronal cell types that coexist in intercalated hypothalamic nuclei and communicate through extensive synaptic connectivity. Although general anatomical and neurochemical features of hypothalamic neurons were described during the past decades, they have yet to be reconciled with recently discovered molecular classifiers and neurogenetic function determination. By interrogating magnocellular as well as parvocellular dopamine, GABA, glutamate, and phenotypically mixed neurons, we integrate available information at the molecular, cellular, network, and endocrine output levels to propose a framework for the comprehensive classification of hypothalamic neurons. Simultaneously, we single out putative neuronal subclasses for which future research can fill in existing gaps of knowledge to rationalize cellular diversity through function-determinant molecular marks in the hypothalamus.

Mapping neuronal inputs to Kiss1 neurons in the arcuate nucleus of the mouse

The normal function of the mammalian reproductive axis is strongly influenced by physiological, metabolic and environmental factors. Kisspeptin neuropeptides, encoded by the Kiss1 gene, are potent regulators of the mammalian reproductive axis by stimulating gonadodropin releasing hormone secretion from the hypothalamus. To understand how the reproductive axis is modulated by higher order neuronal inputs we have mapped the afferent circuits into arcuate (ARC) Kiss1 neurons. We used a transgenic mouse that expresses the CRE recombinase in Kiss1 neurons for conditional viral tracing with genetically modified viruses. CRE-mediated activation of these viruses in Kiss1 neurons allows the virus to move transynaptically to label neurons with primary or secondary afferent inputs into the Kiss1 neurons. Several regions of the brain showed synaptic connectivity to arcuate Kiss1 neurons including proopiomelanocortin neurons in the ARC itself, kisspeptin neurons in the anteroventral periventricular nucleus, vasopressin neurons in the supraoptic and suprachiasmatic nuclei, thyrotropin releasing neurons in the paraventricular nucleus and unidentified neurons in other regions including the subfornical organ, amygdala, interpeduncular nucleus, ventral premammilary nucleus, basal nucleus of stria terminalis and the visual, somatosensory and piriform regions of the cortex. These data provide an insight into how the activity of Kiss1 neurons may be regulated by metabolic signals and provide a detailed neuroanatomical map for future functional studies.

The oxytocin system of mice and men-Similarities and discrepancies of oxytocinergic modulation in rodents and primates

Nonapeptides and their respective receptors have been conserved throughout evolution and display astonishing similarities among the animal kingdom. They can be found in worms, birds, fish, amphibians, reptiles and mammals, including rodents, non-human primates and humans. In particular, the neuropeptide oxytocin (OT) has attracted the attention of scientists due to its profound effects on social behavior. However, although both the neuropeptide and its receptor are identical in rodents and primates, the effects of OT vary greatly in the two species. Here, we provide a brief overview about OT's role in the evolution of mammals and provide reasons for the manifold effects of OT within the brain with a particular focus on the discrepancy of OT's effects in rodents and primates. In addition, we suggest new approaches towards improvement of translatability of scientific studies and highlight the most recent advances in animal models for autism spectrum disorder, a disease, in which the normal function of the OT system seems to be impaired.

High salt loading increases brain derived neurotrophic factor in supraoptic vasopressin neurones

High salt loading (SL) is associated with inappropriate arginine vasopressin (AVP) release and increased mean arterial pressure. Previous work has shown that chronic high salt intake impairs baroreceptor inhibition of rat AVP neurones through brain-derived neurotrophic factor (BDNF) dependent activation of tyrosine receptor kinase B (TrkB) and down-regulation of K+/Cl- co-transporter KCC2. This mechanism diminishes the GABA_A inhibition of AVP neurones in the supraoptic nucleus (SON) by increasing intracellular chloride. However, the source of BDNF leading to this ionic plasticity is unknown. In the present study, we used adeno-associated viral vectors with short hairpin RNA against BDNF to test whether SON is the source of BDNF contributing to increased AVP release and elevated mean arterial pressure in high salt loaded rats. Virally mediated BDNF knockdown (shBDNF) in the SON of salt loaded rats significantly blocked the increases in BDNF mRNA and AVP heterogeneous RNA expression. The observed increase in the activation of TrkB receptor during salt loading is consistent with previous studies. Western blot analysis of SON punches revealed that tyrosine phosphorylation of TrkB (pTrkBY515) was significantly decreased in salt shBDNF rats compared to the salt scrambled (SCR) rats. Injections of shBDNF in the SON also significantly prevented the increase in plasma AVP concentration associated with salt loading. However, the salt loading induced increase in mean arterial pressure was not decreased with BDNF knockdown in the SON. Average daily fluid intake and urine output were significantly elevated in both salt SCR and salt shBDNF rats compared to the euhydrated controls. Daily average urine sodium concentration was significantly higher in shBDNF injected salt rats than the other groups. These findings indicate that BDNF produced in the SON contributes to the increased AVP secretion during high salt loading but not with respect to the subsequent increase in mean arterial pressure.

Central and peripheral roles of vasopressin in the circadian defense of body hydration

Vasopressin is a neuropeptide synthesized by specific subsets of neurons within the eye and brain. Studies in rats and mice have shown that vasopressin produced by magnocellular neurosecretory cells (MNCs) that project to the neurohypophysis is released into the blood circulation where it serves as an antidiuretic hormone to promote water reabsorption from the kidney. Moreover vasopressin is a neurotransmitter and neuromodulator that contributes to time-keeping within the master circadian clock (i.e. the suprachiasmatic nucleus, SCN) and is also used as an output signal by SCN neurons to direct centrally mediated circadian rhythms. In this chapter, we review recent cellular and network level studies in rodents that have provided insight into how circadian rhythms in vasopressin mediate changes in water intake behavior and renal water conservation that protect the body against dehydration during sleep.

Diversity of oxytocin neurons: beyond magno- and parvocellular cell types?

The hypothalamic neuropeptide oxytocin (OT), which is evolutionarily conserved among different species throughout the animal kingdom, is a key modulator of a variety of socio-emotional behaviors such as fear, trust and empathy. OT cells in the mammalian hypothalamus have been traditionally divided into two distinct types - magnocellular (magnOT) and parvocellular (parvOT) or preautonomic neurons. This distinction is based on OT cell sizes and shapes, projections, electrophysiological activity and functions. Indeed, while neuroendocrine magnOT neurons are known to primarily project their axons to the posterior pituitary and to a number of forebrain regions, non-neuroendocrine parvOT neurons have been seen as the main source of OT innervation of the brainstem and spinal cord to control autonomic functions and pain perception. However, very recent findings demonstrated distinct genetic profiles in OT neurons, allowing discrimination of at least four types of cells expressing OT. Furthermore, unexpected axonal projections of parvOT neurons to the forebrain and magnOT neurons to the midbrain have been newly reported. In this review, we focus on the detailed analysis of methods of distinction between OT cell types, in- and output sites, morphology as well as on the direct connectivity between OT neurons and its physiological significance. At the end, we propose a hypothesis that the central OT system is composed of more than just two OT cell types, which should be further verified by the application of available genetic and anatomical techniques. This article is protected by copyright. All rights reserved.

Phosphatidylinositol 4,5-bisphosphate (PIP2 ) modulates afterhyperpolarizations in oxytocin neurons of the supraoptic nucleus

KEY POINTS

Afterhyperpolarizations (AHPs) generated by repetitive action potentials in supraoptic magnocellular neurons regulate repetitive firing and spike frequency adaptation but relatively little is known about PIP₂ 's control of these AHPs. We examined how changes in PIP₂ levels affected AHPs, somatic [Ca²⁺ ]_i , and whole cell Ca²⁺ currents. Manipulations of PIP₂ levels affected both medium and slow AHP currents in oxytocin (OT) neurons of the supraoptic nucleus. Manipulations of PIP₂ levels did not modulate AHPs by influencing Ca²⁺ release from IP₃ -triggered Ca²⁺ stores, suggesting more direct modulation of channels by PIP₂ . PIP₂ depletion reduced spike-evoked Ca²⁺ entry and voltage-gated Ca²⁺ currents. PIP₂ appears to influence AHPs in OT neurons by reducing Ca²⁺ influx during spiking.

ABSTRACT

Oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurons of the supraoptic nucleus (SON) display calcium-dependent afterhyperpolarizations (AHPs) following a train of action potentials that are critical to shaping the firing patterns of these cells. Previous work demonstrated that the lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ) enabled the slow AHP component (sAHP) in cortical pyramidal neurons. We investigated whether this phenomenon occurred in OT and VP neurons of the SON. Using whole cell recordings in coronal hypothalamic slices from adult female rats, we demonstrated that inhibition of PIP₂ synthesis with wortmannin robustly blocked both the medium and slow AHP currents (I_mAHP and I_sAHP ) of OT, but not VP neurons with high affinity. We further tested this by introducing a water-soluble PIP₂ analogue (diC₈ -PIP₂ ) into neurons, which in OT neurons not only prevented wortmannin's inhibitory effect, but slowed rundown of the I_mAHP and I_sAHP . Inhibition of phospholipase C (PLC) with U73122 did not inhibit either I_mAHP or I_sAHP in OT neurons, consistent with wortmannin's effects not being due to reducing diacylglycerol (DAG) or IP₃ availability, i.e. PIP₂ modulation of AHPs is not likely to involve downstream Ca²⁺ release from inositol 1,4,5-trisphosphate (IP₃ )-triggered Ca²⁺ -store release, or channel modulation via DAG and protein kinase C (PKC). We found that wortmannin reduced [Ca²⁺ ]_i increase induced by spike trains in OT neurons, but had no effect on AHPs evoked by uncaging intracellular Ca²⁺ . Finally, wortmannin selectively reduced whole cell Ca²⁺ currents in OT neurons while leaving VP neurons unaffected. The results indicate that PIP₂ modulates both the I_mAHP and I_sAHP in OT neurons, most likely by controlling Ca²⁺ entry through voltage-gated Ca²⁺ channels opened during spike trains.

Activation of Supraoptic Oxytocin Neurons by Secretin Facilitates Social Recognition

BACKGROUND

Social recognition underlies social behavior in animals, and patients with psychiatric disorders associated with social deficits show abnormalities in social recognition. Oxytocin is implicated in social behavior and has received attention as an effective treatment for sociobehavioral deficits. Secretin receptor-deficient mice show deficits in social behavior. The relationship between oxytocin and secretin concerning social behavior remains to be determined.

METHODS

Expression of c-Fos in oxytocin neurons and release of oxytocin from their dendrites after secretin application were investigated. Social recognition was examined after intracerebroventricular or local injection of secretin, oxytocin, or an oxytocin receptor antagonist in rats, oxytocin receptor-deficient mice, and secretin receptor-deficient mice. Electron and light microscopic immunohistochemical analysis was also performed to determine whether oxytocin neurons extend their dendrites into the medial amygdala.

RESULTS

Supraoptic oxytocin neurons expressed the secretin receptor. Secretin activated supraoptic oxytocin neurons and facilitated oxytocin release from dendrites. Secretin increased acquisition of social recognition in an oxytocin receptor-dependent manner. Local application of secretin into the supraoptic nucleus facilitated social recognition, and this facilitation was blocked by an oxytocin receptor antagonist injected into, but not outside of, the medial amygdala. In the medial amygdala, dendrite-like thick oxytocin processes were found to extend from the supraoptic nucleus. Furthermore, oxytocin treatment restored deficits of social recognition in secretin receptor-deficient mice.

CONCLUSIONS

The results of our study demonstrate that secretin-induced dendritic oxytocin release from supraoptic neurons enhances social recognition. The newly defined secretin-oxytocin system may lead to a possible treatment for social deficits.

Oxytocin Signaling in Pain: Cellular, Circuit, System, and Behavioral Levels

Originally confined to the initiation of parturition and milk ejection after birth, the hypothalamic nonapeptide oxytocin (OT) is now recognized as a critical determinant of social behavior and emotional processing. It accounts for the modulation of sensory processing and pain perception as OT displays a potent analgesic effect mediated by OT receptors (OTRs) expressed in the peripheral and central nervous systems. In our chapter, we will first systemically analyze known efferent and afferent OT neuron projections, which form the anatomical basis for OT modulation of somatosensory and pain processing. Next, we will focus on the synergy of distinct types of OT neurons (e.g., magno- and parvocellular OT neurons) which efficiently control acute inflammatory pain perception. Finally, we will describe how OT signaling mechanisms in the spinal cord control nociception, as well as how OT is able to modulate emotional pain processing within the central amygdala. In the conclusions at the end of the chapter, we will formulate perspectives in the study of OT effects on pain anticipation and pain memory, as well as propose some reasons for the application of exogenous OT for the treatment of certain types of pain in human patients.

Bidirectional Anticipation of Future Osmotic Challenges by Vasopressin Neurons

Ingestion of water and food are major hypo- and hyperosmotic challenges. To protect the body from osmotic stress, posterior pituitary-projecting, vasopressin-secreting neurons (VP_pp neurons) counter osmotic perturbations by altering their release of vasopressin, which controls renal water excretion. Vasopressin levels begin to fall within minutes of water consumption, even prior to changes in blood osmolality. To ascertain the precise temporal dynamics by which water or food ingestion affect VP_pp neuron activity, we directly recorded the spiking and calcium activity of genetically defined VP_pp neurons. In states of elevated osmolality, water availability rapidly decreased VP_pp neuron activity within seconds, beginning prior to water ingestion, upon presentation of water-predicting cues. In contrast, food availability following food restriction rapidly increased VP_pp neuron activity within seconds, but only following feeding onset. These rapid and distinct changes in activity during drinking and feeding suggest diverse neural mechanisms underlying anticipatory regulation of VP_pp neurons.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Sodium-calcium exchanger and R-type Ca(2+) channels mediate spontaneous [Ca(2+)]i oscillations in magnocellular neurones of the rat supraoptic nucleus

Isolated supraoptic neurones generate spontaneous [Ca(2+)]i oscillations in isolated conditions. Here we report in depth analysis of the contribution of plasmalemmal ion channels (Ca(2+), Na(+)), Na(+)/Ca(2+) exchanger (NCX), intracellular Ca(2+) release channels (InsP3Rs and RyRs), Ca(2+) storage organelles, plasma membrane Ca(2+) pump and intracellular signal transduction cascades into spontaneous Ca(2+) activity. While removal of extracellular Ca(2+) or incubation with non-specific voltage-gated Ca(2+) channel (VGCC) blocker Cd(2+) suppressed the oscillations, neither Ni(2+) nor TTA-P2, the T-type VGCC blockers, had an effect. Inhibitors of VGCC nicardipine, ω-conotoxin GVIA, ω-conotoxin MVIIC, ω-agatoxin IVA (for L-, N-, P and P/Q-type channels, respectively) did not affect [Ca(2+)]i oscillations. In contrast, a specific R-type VGCC blocker SNX-482 attenuated [Ca(2+)]i oscillations. Incubation with TTX had no effect, whereas removal of the extracellular Na(+) or application of an inhibitor of the reverse operation mode of Na(+)/Ca(2+) exchanger KB-R7943 blocked the oscillations. The mitochondrial uncoupler CCCP irreversibly blocked spontaneous [Ca(2+)]i activity. Exposure of neurones to Ca(2+) mobilisers (thapsigargin, cyclopiazonic acid, caffeine and ryanodine); 4-aminopyridine (A-type K(+) current blocker); phospholipase C and adenylyl cyclase pathways blockers U-73122, Rp-cAMP, SQ-22536 and H-89 had no effect. Oscillations were blocked by GABA, but not by glutamate, apamin or dynorphin. In conclusion, spontaneous oscillations in magnocellular neurones are mediated by a concerted action of R-type Ca(2+) channels and the NCX fluctuating between forward and reverse modes.

Oxytocin Neurones: Intrinsic Mechanisms Governing the Regularity of Spiking Activity

Oxytocin neurones of the rat supraoptic nucleus are osmoresponsive and, with all other things being equal, they fire at a mean rate that is proportional to the plasma sodium concentration. However, individual spike times are governed by highly stochastic events, namely the random occurrences of excitatory synaptic inputs, the probability of which is increased by increasing extracellular osmotic pressure. Accordingly, interspike intervals (ISIs) are very irregular. In the present study, we show, by statistical analyses of firing patterns in oxytocin neurones, that the mean firing rate as measured in bins of a few seconds is more regular than expected from the variability of ISIs. This is consistent with an intrinsic activity-dependent negative-feedback mechanism. To test this, we compared observed neuronal firing patterns with firing patterns generated by a leaky integrate-and-fire model neurone, modified to exhibit activity-dependent mechanisms known to be present in oxytocin neurones. The presence of a prolonged afterhyperpolarisation (AHP) was critical for the ability to mimic the observed regularisation of mean firing rate, although we also had to add a depolarising afterpotential (DAP; sometimes called an afterdepolarisation) to the model to match the observed ISI distributions. We tested this model by comparing its behaviour with the behaviour of oxytocin neurones exposed to apamin, a blocker of the medium AHP. Good fits indicate that the medium AHP actively contributes to the firing patterns of oxytocin neurones during non-bursting activity, and that oxytocin neurones generally express a DAP, even though this is usually masked by superposition of a larger AHP.

Age-Related (Aged vs. Adult) Comparison of the Effect of Two Mild Stressors on the Nerve Growth Factor (NGF) in the Rat Hypothalamic Supraoptic Nucleus (SON) - Immunohistochemical Study

The ontogenetic period of life and stress can have different effects on the nerve growth factor (NGF) in the hypothalamus. The aim of our study was to investigate the influence of two mild stressors, acute and chronic exposure to forced swim (FS) or high-light open field (HL-OF), on neurons containing NGF. Immunofluorescence staining was used to reveal the density of NGF-immunoreactive (ir) cells in the hypothalamic supraoptic nucleus (SON) in adult (postnatal day 90; P90) and aged (P720) rats. The P90 and P720 rats that were subjected to acute and chronic FS showed no differences in the density of NGF-ir neurons in the SON compared with nonstressed rats. However, a significant increase in NGF-ir cells was noted after acute but not after chronic HL-OF only in P90 rats. What is more, there were no age-related (P90 vs. P720) changes in the density of NGF-ir neurons in non-stressed and FS- or HL-OF-stressed rats. Our results indicate that acute HL-OF was the only factor inducing changes in the density of NGF-ir neurons in the SON of adult rats. This could be related to the neuroprotective role of NGF-ir cells in response to acute HL-OF. The absence of age-dependent changes in the density of NGF-ir neurons may indicate that the ageing processes in SON do not generate changes in the NGF immunoreactivity of its neurons.

A RNA-Seq Analysis of the Rat Supraoptic Nucleus Transcriptome: Effects of Salt Loading on Gene Expression

Magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS) are highly specialized to release large amounts of arginine vasopressin (Avp) or oxytocin (Oxt) into the blood stream and play critical roles in the regulation of body fluid homeostasis. The MCNs are osmosensory neurons and are excited by exposure to hypertonic solutions and inhibited by hypotonic solutions. The MCNs respond to systemic hypertonic and hypotonic stimulation with large changes in the expression of their Avp and Oxt genes, and microarray studies have shown that these osmotic perturbations also cause large changes in global gene expression in the HNS. In this paper, we examine gene expression in the rat supraoptic nucleus (SON) under normosmotic and chronic salt-loading SL) conditions by the first time using "new-generation", RNA sequencing (RNA-Seq) methods. We reliably detect 9,709 genes as present in the SON by RNA-Seq, and 552 of these genes were changed in expression as a result of chronic SL. These genes reflect diverse functions, and 42 of these are involved in either transcriptional or translational processes. In addition, we compare the SON transcriptomes resolved by RNA-Seq methods with the SON transcriptomes determined by Affymetrix microarray methods in rats under the same osmotic conditions, and find that there are 6,466 genes present in the SON that are represented in both data sets, although 1,040 of the expressed genes were found only in the microarray data, and 2,762 of the expressed genes are selectively found in the RNA-Seq data and not the microarray data. These data provide the research community a comprehensive view of the transcriptome in the SON under normosmotic conditions and the changes in specific gene expression evoked by salt loading.

In vitro differentiation between oxytocin- and vasopressin-secreting magnocellular neurons requires more than one experimental criterion

The phenotypic differentiation between oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurosecretory cells (MNCs) from the supraoptic nucleus is relevant to understanding how several physiological and pharmacological challenges affect their electrical activity. Although the firing patterns of OT and VP neurons, both in vivo and in vitro, may appear different from each other, much is assumed about their characteristics. These assumptions make it practically impossible to obtain a confident phenotypic differentiation based exclusively on the firing patterns. The presence of a sustained outward rectifying potassium current (SOR) and/or an inward rectifying hyperpolarization-activated current (IR), which are presumably present in OT neurons and absent in VP neurons, has been used to distinguish between the two types of MNCs in the past. In this study, we aimed to analyze the accuracy of the phenotypic discrimination of MNCs based on the presence of rectifying currents using comparisons with the molecular phenotype of the cells, as determined by single-cell RT-qPCR and immunohistochemistry. Our results demonstrated that the phenotypes classified according to the electrophysiological protocol in brain slices do not match their molecular counterparts because vasopressinergic and intermediate neurons also exhibit both outward and inward rectifying currents. In addition, we also show that MNCs can change the relative proportion of each cell phenotype when the system is challenged by chronic hypertonicity (70% water restriction for 7 days). We conclude that for in vitro preparations, the combination of mRNA detection and immunohistochemistry seems to be preferable when trying to characterize a single MNC phenotype.

Characteristics of GABAergic and cholinergic neurons in perinuclear zone of mouse supraoptic nucleus

The perinuclear zone (PNZ) of the supraoptic nucleus (SON) contains some GABAergic and cholinergic neurons thought to innervate the SON proper. In mice expressing enhanced green fluorescent protein (eGFP) in association with glutamate decarboxylase (GAD)65 we found an abundance of GAD65-eGFP neurons in the PNZ, whereas in mice expressing GAD67-eGFP, there were few labeled PNZ neurons. In mice expressing choline acetyltransferase (ChAT)-eGFP, large, brightly fluorescent and small, dimly fluorescent ChAT-eGFP neurons were present in the PNZ. The small ChAT-eGFP and GAD65-eGFP neurons exhibited a low-threshold depolarizing potential consistent with a low-threshold spike, with little transient outward rectification. Large ChAT-eGFP neurons exhibited strong transient outward rectification and a large hyperpolarizing spike afterpotential, very similar to that of magnocellular vasopressin and oxytocin neurons. Thus the large soma and transient outward rectification of large ChAT-eGFP neurons suggest that these neurons would be difficult to distinguish from magnocellular SON neurons in dissociated preparations by these criteria. Large, but not small, ChAT-eGFP neurons were immunostained with ChAT antibody (AB144p). Reconstructed neurons revealed a few processes encroaching near and passing through the SON from all types but no clear evidence of a terminal axon arbor. Large ChAT-eGFP neurons were usually oriented vertically and had four or five dendrites with multiple branches and an axon with many collaterals and local arborizations. Small ChAT-eGFP neurons had a more restricted dendritic tree compared with parvocellular GAD65 neurons, the latter of which had long thin processes oriented mediolaterally. Thus many of the characteristics found previously in unidentified, small PNZ neurons are also found in identified GABAergic neurons and in a population of smaller ChAT-eGFP neurons.

ANG II receptor subtype 1a gene knockdown in the subfornical organ prevents increased drinking behavior in bile duct-ligated rats

Bile duct ligation (BDL) causes congestive liver failure that initiates hemodynamic changes, resulting in dilutional hyponatremia due to increased water intake and vasopressin release. This project tested the hypothesis that angiotensin signaling at the subfornical organ (SFO) augments drinking behavior in BDL rats. A genetically modified adeno-associated virus containing short hairpin RNA (shRNA) for ANG II receptor subtype 1a (AT1aR) gene was microinjected into the SFO of rats to knock down expression. Two weeks later, BDL or sham surgery was performed. Rats were housed in metabolic chambers for measurement of fluid and food intake and urine output. The rats were euthanized 28 days after BDL surgery for analysis. A group of rats was perfused for immunohistochemistry, and a second group was used for laser-capture microdissection for analysis of SFO AT1aR gene expression. BDL rats showed increased water intake that was attenuated in rats that received SFO microinjection of AT1aR shRNA. Among BDL rats treated with scrambled (control) and AT1aR shRNA, we observed an increased number of vasopressin-positive cells in the supraoptic nucleus that colocalized with ΔFosB staining, suggesting increased vasopressin release in both groups. These results indicate that angiotensin signaling through the SFO contributes to increased water intake, but not dilutional hyponatremia, during congestive liver failure.

Differential regulation of TRPC4 in the vasopressin magnocellular system by water deprivation and hepatic cirrhosis in the rat

Transient receptor potential canonical subtype 4 (TRPC4) is expressed in the magnocellular paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. In this study, the regulation of TRPC4 expression was investigated in water deprivation and hepatic cirrhosis. We used laser capture microdissection technique for precise dissection of pure AVP cell population in the PVN and SON followed by quantitative real-time RT-PCR, and immunodetection techniques by Western blot analysis and immunofluorescence. Bile duct ligation elevated TRPC4 transcripts in the SON but not PVN with correlated changes in the protein expression in these regions, as well as increased colocalization with AVP in the SON, with no changes in the PVN. Water deprivation resulted in increased TRPC4 mRNA expression in the PVN, while it decreased channel expression levels in the SON. In both of these regions, protein expression measured from tissue punches were unaltered following water deprivation, with no changes in the number of TRPC4-positive cells. Thus, TRPC4 expression is differentially regulated in physiological and pathophysiological models of vasopressin release.

Whole transcriptome organisation in the dehydrated supraoptic nucleus

The supraoptic nucleus (SON) is part of the central osmotic circuitry that synthesises the hormone vasopressin (Avp) and transports it to terminals in the posterior lobe of the pituitary. Following osmotic stress such as dehydration, this tissue undergoes morphological, electrical and transcriptional changes to facilitate the appropriate regulation and release of Avp into the circulation where it conserves water at the level of the kidney. Here, the organisation of the whole transcriptome following dehydration is modelled to fit Zipf's law, a natural power law that holds true for all natural languages, that states if the frequency of word usage is plotted against its rank, then the log linear regression of this is -1. We have applied this model to our previously published euhydrated and dehydrated SON data to observe this trend and how it changes following dehydration. In accordance with other studies, our whole transcriptome data fit well with this model in the euhydrated SON microarrays, but interestingly, fit better in the dehydrated arrays. This trend was observed in a subset of differentially regulated genes and also following network reconstruction using a third-party database that mines public data. We make use of language as a metaphor that helps us philosophise about the role of the whole transcriptome in providing a suitable environment for the delivery of Avp following a survival threat like dehydration.

Prenatal exposure to dietary fat induces changes in the transcriptional factors, TEF and YAP, which may stimulate differentiation of peptide neurons in rat hypothalamus

Gestational exposure to a high-fat diet (HFD) stimulates the differentiation of orexigenic peptide-expressing neurons in the hypothalamus of offspring. To examine possible mechanisms that mediate this phenomenon, this study investigated the transcriptional factor, transcription enhancer factor-1 (TEF), and co-activator, Yes-associated protein (YAP), which when inactivated stimulate neuronal differentiation. In rat embryos and postnatal offspring prenatally exposed to a HFD compared to chow, changes in hypothalamic TEF and YAP and their relationship to the orexigenic peptide, enkephalin (ENK), were measured. The HFD offspring at postnatal day 15 (P15) exhibited in the hypothalamic paraventricular nucleus a significant reduction in YAP mRNA and protein, and increased levels of inactive and total TEF protein, with no change in mRNA. Similarly, HFD-exposed embryos at embryonic day 19 (E19) showed in whole hypothalamus significantly decreased levels of YAP mRNA and protein and TEF mRNA, and increased levels of inactive TEF protein, suggesting that HFD inactivates TEF and YAP. This was accompanied by increased density and fluorescence intensity of ENK neurons. A close relationship between TEF and ENK was suggested by the finding that TEF co-localizes with this peptide in hypothalamic neurons and HFD reduced the density of TEF/ENK co-labeled neurons, even while the number and fluorescence intensity of single-labeled TEF neurons were increased. Increased YAP inactivity by HFD was further evidenced by a decrease in number and fluorescence intensity of YAP-containing neurons, although the density of YAP/ENK co-labeled neurons was unaltered. Genetic knockdown of TEF or YAP stimulated ENK expression in hypothalamic neurons, supporting a close relationship between these transcription factors and neuropeptide. These findings suggest that prenatal HFD exposure inactivates both hypothalamic TEF and YAP, by either decreasing their levels or increasing their inactive form, and that this contributes to the stimulatory effect of HFD on ENK expression and possibly the differentiation of ENK-expressing neurons.

Multivesicular release underlies short term synaptic potentiation independent of release probability change in the supraoptic nucleus

Magnocellular neurons of the supraoptic nucleus receive glutamatergic excitatory inputs that regulate the firing activity and hormone release from these neurons. A strong, brief activation of these excitatory inputs induces a lingering barrage of tetrodotoxin-resistant miniature EPSCs (mEPSCs) that lasts for tens of minutes. This is known to accompany an immediate increase in large amplitude mEPSCs. However, it remains unknown how long this amplitude increase can last and whether it is simply a byproduct of greater release probability. Using in vitro patch clamp recording on acute rat brain slices, we found that a brief, high frequency stimulation (HFS) of afferents induced a potentiation of mEPSC amplitude lasting up to 20 min. This amplitude potentiation did not correlate with changes in mEPSC frequency, suggesting that it does not reflect changes in presynaptic release probability. Nonetheless, neither postsynaptic calcium chelator nor the NMDA receptor antagonist blocked the potentiation. Together with the known calcium dependency of HFS-induced potentiation of mEPSCs, our results imply that mEPSC amplitude increase requires presynaptic calcium. Further analysis showed multimodal distribution of mEPSC amplitude, suggesting that large mEPSCs were due to multivesicular glutamate release, even at late post-HFS when the frequency is no longer elevated. In conclusion, high frequency activation of excitatory synapses induces lasting multivesicular release in the SON, which is independent of changes in release probability. This represents a novel form of synaptic plasticity that may contribute to prolonged excitatory tone necessary for generation of burst firing of magnocellular neurons.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Analysis of transcription factor mRNAs in identified oxytocin and vasopressin magnocellular neurons isolated by laser capture microdissection

The oxytocin (Oxt) and vasopressin (Avp) magnocellular neurons (MCNs) in the hypothalamus are the only neuronal phenotypes that are present in the supraoptic nucleus (SON), and are characterized by their robust and selective expression of either the Oxt or Avp genes. In this paper, we take advantage of the differential expression of these neuropeptide genes to identify and isolate these two individual phenotypes from the rat SON by laser capture microdissection (LCM), and to analyze the differential expression of several of their transcription factor mRNAs by qRT-PCR. We identify these neuronal phenotypes by stereotaxically injecting recombinant Adeno-Associated Viral (rAAV) vectors which contain cell-type specific Oxt or Avp promoters that drive expression of EGFP selectively in either the Oxt or Avp MCNs into the SON. The fluorescent MCNs are then dissected by LCM using a novel Cap Road Map protocol described in this paper, and the purified MCNs are extracted for their RNAs. qRT-PCR of these RNAs show that some transcription factors (RORA and c-jun) are differentially expressed in the Oxt and Avp MCNs.

Variation in sodium current amplitude between vasopressin and oxytocin hypothalamic supraoptic neurons

Biophysical characteristics of tetrodotoxin-sensitive sodium (Na(+)) currents were studied in vasopressin (VP) and oxytocin (OT) supraoptic neurons acutely isolated from rat hypothalamus. Na(+) current density (pA/pF) was significantly greater in VP neurons than in OT neurons. No significant difference between VP and OT neurons was detected regarding the voltage dependence of activation and steady-state inactivation, or rate of recovery from inactivation of Na(+) currents. In both VP and OT neurons, the macroscopic inactivation of the Na(+) currents was best fitted with a double-exponential expression suggesting two rates of inactivation. Also in both types, the time course of recovery from inactivation proceeded with fast and slow time constants averaging around 8 and 350 ms, respectively, suggesting the presence of multiple pathways of recovery from inactivation. The slower time constant of recovery of inactivation may be involved in the decrease in action potential (AP) amplitude that occurs after the first spike during burst firing in both neuronal types. The larger amplitude of Na(+) currents in VP vs. OT neurons may explain the previous observations that VP neurons exhibit a lower AP threshold and greater AP amplitude than OT neurons, and may serve to differently tune the firing properties and responses to neuromodulators of the respective neuronal types.

Cell-type specific expression of the vasopressin gene analyzed by AAV mediated gene delivery of promoter deletion constructs into the rat SON in vivo

The magnocellular neurons (MCNs) in the supraoptic nucleus (SON) of the hypothalamus selectively express either oxytocin (Oxt) or vasopressin (Avp) neuropeptide genes. In this paper we examine the cis-regulatory domains in the Avp gene promoter that are responsible for its cell-type specific expression. AAV vectors that contain various Avp gene promoter deletion constructs using EGFP as the reporter were stereotaxically injected into the rat SON. Two weeks following the injection immunohistochemical assays of EGFP expression from these constructs were done to determine whether the expressed EGFP reporter co-localizes with either the Oxt- or Avp-immunoreactivity in the MCNs. The results identify three major enhancer domains located at −2.0 to −1.5 kbp, −1.5 to −950 bp, and −950 to −543 bp in the Avp gene promoter that regulate the expression in Avp MCNs. The results also show that cell–type specific expression in Avp MCNs is maintained in constructs containing at least 288 bp of the promoter region upstream of the transcription start site, but this specificity is lost at 116 bp and below. Based on these data, we hypothesize that the −288 bp to −116 bp domain contains an Avp MCN specific activator and a possible repressor that inhibits expression in Oxt-MCNs, thereby leading to the cell-type specific expression of the Avp gene only in the Avp-MCNs.

ΔFosB in the supraoptic nucleus contributes to hyponatremia in rats with cirrhosis

Bile duct ligation (BDL), a model of hepatic cirrhosis, is associated with dilutional hyponatremia and inappropriate vasopressin release. ΔFosB staining was significantly increased in vasopressin and oxytocin magnocellular neurosecretory cells in the supraoptic nucleus (SON) of BDL rats. We tested the role of SON ΔFosB in fluid retention following BDL by injecting the SON (n = 10) with 400 nl of an adeno-associated virus (AAV) vector expressing ΔJunD (a dominant negative construct for ΔFosB) plus green fluorescent protein (GFP) (AAV-GFP-ΔJunD). Controls were either noninjected or injected with an AAV vector expressing only GFP. Three weeks after BDL or sham ligation surgery, rats were individually housed in metabolism cages for 1 wk. Average daily water intake was significantly elevated in all BDL rats compared with sham ligated controls. Average daily urine output was significantly greater in AAV-GFP-ΔJunD-treated BDL rats compared with all other groups. Daily average urine sodium concentration was significantly lower in AAV-GFP-ΔJunD-treated BDL rats than the other groups, although average daily sodium excretion was not different among the groups. SON expression of ΔJunD produced a diuresis in BDL rats that may be related to decreased circulating levels of vasopressin or oxytocin. These findings support the view that ΔFosB expression in SON magnocellular secretory cells contribute to dilutional hyponatremia in BDL rats.

Cell-type specific oxytocin gene expression from AAV delivered promoter deletion constructs into the rat supraoptic nucleus in vivo

The magnocellular neurons (MCNs) in the hypothalamus selectively express either oxytocin (OXT) or vasopressin (AVP) neuropeptide genes, a property that defines their phenotypes. Here we examine the molecular basis of this selectivity in the OXT MCNs by stereotaxic microinjections of adeno-associated virus (AAV) vectors that contain various OXT gene promoter deletion constructs using EGFP as the reporter into the rat supraoptic nucleus (SON). Two weeks following injection of the AAVs, immunohistochemical assays of EGFP expression from these constructs were done to determine whether the EGFP reporter co-localizes with either the OXT- or AVP-immunoreactivity in the MCNs. The results show that the key elements in the OT gene promoter that regulate the cell-type specific expression the SON are located -216 to -100 bp upstream of the transcription start site. We hypothesize that within this 116 bp domain a repressor exists that inhibits expression specifically in AVP MCNs, thereby leading to the cell-type specific expression of the OXT gene only in the OXT MCNs.

Dichotomous cellular properties of mouse orexin/hypocretin neurons

Hypothalamic hypocretin/orexin (Hcrt/Orx) neurons recently emerged as critical regulators of sleep–wake cycles, reward seeking and body energy balance. However, at the level of cellular and network properties, it remains unclear whether Hcrt/Orx neurons are one homogeneous population, or whether there are several distinct types of Hcrt/Orx cells. Here, we collated diverse structural and functional information about individual Hcrt/Orx neurons in mouse brain slices, by combining patch-clamp analysis of spike firing, membrane currents and synaptic inputs with confocal imaging of cell shape and subsequent 3-dimensional Sholl analysis of dendritic architecture. Statistical cluster analysis of intrinsic firing properties revealed that Hcrt/Orx neurons fall into two distinct types. These two cell types also differ in the complexity of their dendritic arbour, the strength of AMPA and GABAA receptor-mediated synaptic drive that they receive, and the density of low-threshold, 4-aminopyridine-sensitive, transient K+ current. Our results provide quantitative evidence that, at the cellular level, the mouse Hcrt/Orx system is composed of two classes of neurons with different firing properties, morphologies and synaptic input organization.

Performance, properties and plasticity of identified oxytocin and vasopressin neurones in vitro

The neurohypophysial hormones oxytocin (OT) and vasopressin (VP) originate from hypothalamic neurosecretory cells in the paraventricular and supraoptic (SON) nuclei. The firing rate and pattern of action potentials arising from these neurones determine the timing and quantity of peripheral hormone release. We have used immunochemical identification of biocytin-filled SON neurones in hypothalamic slices in vitro to uncover differences between OT and VP neurones in membrane and synaptic properties, firing patterns, and plasticity during pregnancy and lactation. In this review, we summarise some recent findings from this approach: (i) VP neuronal excitability is influenced by slow (sDAP) and fast (fDAP) depolarising afterpotentials that underlie phasic bursting activity. The fDAP may relate to a transient receptor potential (TRP) channel, type melastatin (TRPM4 and/or TRPM5), both of which are immunochemically localised more to VP neurones, and especially, to their dendrites. Both TRPM4 and TRPM5 mRNAs are found in the SON, but single cell reverse transcriptase-polymerisation suggests that TRPM4 might be the more prominent channel. Phasic bursting in VP neurones is little influenced by spontaneous synaptic activity in slices, being shaped largely by intrinsic currents. (ii) The firing pattern of OT neurones ranges from irregular to continuous, with the coefficient of variation determined by randomly distributed, spontaneous GABAergic, inhibitory synaptic currents (sIPSCs). These sIPSCs are four- to five-fold more frequent in OT versus VP neurones, and much more frequent than spontaneous excitatory synaptic currents. (iii) Both cell types express Ca(2+)-dependent afterhyperpolarisations (AHPs), including an apamin-sensitive, medium duration AHP and a slower, apamin-insensitive AHP (sAHP). In OT neurones, both AHPs are enhanced during pregnancy and lactation. During pregnancy, the plasticity of the sAHP is blocked by antagonism of central OT receptors. AHP enhancement is mimicked by exposing slices from day 19 pregnant rats to OT and oestradiol, suggesting that central OT and sex steroids programme this plasticity during pregnancy by direct hypothalamic actions. In conclusion, the differences in VP and OT neuronal function are underlain by differences in both membrane and synaptic properties, and differentially modulated by reproductive state.

The adaptive brain: Glenn Hatton and the supraoptic nucleus

In December 2009, Glenn Hatton died, and neuroendocrinology lost a pioneer who had done much to forge our present understanding of the hypothalamus and whose productivity had not faded with the passing years. Glenn, an expert in both functional morphology and electrophysiology, was driven by a will to understand the significance of his observations in the context of the living, behaving organism. He also had the wit to generate bold and challenging hypotheses, the wherewithal to expose them to critical and elegant experimental testing, and a way with words that gave his papers and lectures clarity and eloquence. The hypothalamo-neurohypophysial system offered a host of opportunities for understanding how physiological functions are fulfilled by the electrical activity of neurones, how neuronal behaviour changes with changing physiological states, and how morphological changes contribute to the physiological response. In the vision that Glenn developed over 35 years, the neuroendocrine brain is as dynamic in structure as it is adaptable in function. Its adaptability is reflected not only by mere synaptic plasticity, but also by changes in neuronal morphology and in the morphology of the glial cells. Astrocytes, in Glenn's view, were intimate partners of the neurones, partners with an essential role in adaptation to changing physiological demands.

Interleukin-6 activates arginine vasopressin neurons in the supraoptic nucleus during immune challenge in rats

The increase of plasma arginin-vasopressin (AVP) release, which translates hypothalamic AVP neuron activation in response to immune challenge, appears to occur independently of plasma osmolality or blood pressure changes. Many studies have shown that major inflammatory mediators produced in response to peripheral inflammation, such as prostaglandin (PG)-E(2) and interleukin (IL)-1beta, excite AVP neurons. However, in vivo electrical activation of AVP neurons was still not assessed in relation to plasma AVP release, osmolality, or blood pressure or to the expression and role of inflammatory molecules like PG-E(2), IL-1beta, IL-6, and tumor necrosis factor-alpha (TNFalpha). This study aims at elucidating those factors that underlie the activation of AVP neurons in response to immune stimulation mimicked by an intraperitoneal injection of lipopolysaccharide (LPS) in male Wistar rats. LPS treatment concomittanlty decreased diuresis and increased plasma AVP as well as AVP neuron activity in vivo, and these effects occurred as early as 30 min. Activation was sustained for more than 6 h. Plasma osmolality did not change, whereas blood pressure only transiently increased during the first hour post-LPS. PG-E(2), IL-1beta, and TNFalpha mRNA expression were raised 3 h after LPS, whereas IL-6 mRNA level increased 30 min post-LPS. In vivo electrophysiological recordings showed that brain IL-6 injection increased AVP neuron activity similarly to peripheral LPS treatment. In contrast, brain injection of anti-IL-6 antibodies prevented the LPS induced-activation of AVP neurons. Taken together, these results suggest that the early activation of AVP neurons in response to LPS injection is induced by brain IL-6.

Altered central TRPV4 expression and lipid raft association related to inappropriate vasopressin secretion in cirrhotic rats

Inappropriate vasopressin (AVP) release causes dilutional hyponatremia in many pathophysiological states such as cirrhosis. The central molecular mechanisms that mediate inappropriate AVP release are unknown. We tested the hypothesis that changes in the expression or trafficking of TRPV4 in the central nervous system may contribute to inappropriate AVP release in the bile duct ligation (BDL) model of cirrhosis in the rat. Four weeks after surgery, BDL rats demonstrated significantly increased plasma vasopressin and plasma renin activity (PRA), hypervolemia, and decreased plasma osmolality. These effects were blocked by providing BDL rats with 2% saline to drink for 15 days. TRPV4 protein expression was significantly increased in brain punches from BDL rats containing the supraoptic nucleus (SON) of the hypothalamus (100% +/- 11 to 157% +/- 4.8), and this effect was blocked in BDL rats given saline. Immunohistochemistry demonstrated a significant increase in TRPV4-positive cells and the percentage of AVP neurons that also were TRPV4-positive in the SON of BDL rats. In the hypothalamus of BDL rats, TRPV4 lipid raft association increased compared with sham (from 100% +/- 2.1 to 326.1% +/- 16). This effect was significantly attenuated in BDL rats given 2% saline to drink (174% +/- 11). In the brain stem, TRPV4 lipid raft association was reduced by BDL and inversely related to plasma AVP and PRA. We speculate that changes in TRPV4 expression and compartmentalization within lipid rafts could contribute to a feed-forward mechanism related to AVP release in cirrhosis.

Response of substances co-expressed in hypothalamic magnocellular neurons to osmotic challenges in normal and Brattleboro rats

The intention of this review is to emphasize the current knowledge about the extent and importance of the substances co-localized with magnocellular arginine vasopressin (AVP) and oxytocin (OXY) as potential candidates for the gradual clarification of their actual role in the regulation of hydromineral homeostasis. Maintenance of the body hydromineral balance depends on the coordinated action of principal biologically active compounds, AVP and OXY, synthesized in the hypothalamic supraoptic and paraventricular nuclei. However, on the regulation of water-salt balance, other substances, co-localized with the principal neuropetides, participate. These can be classified as (1) peptides co-localized with AVP or OXY with unambiguous osmotic function, including angiotensin II, apelin, corticotropin releasing hormone, and galanin and (2) peptides co-localized with AVP or OXY with an unknown role in osmotic regulation, including cholecystokinin, chromogranin/secretogranin, dynorphin, endothelin-1, enkephalin, ferritin protein, interleukin 6, kininogen, neurokinin B, neuropeptide Y, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, TAFA5 protein, thyrotropin releasing hormone, tyrosine hydroxylase, and urocortin. In this brief review, also the responses of these substances to different hyperosmotic and hypoosmotic challenges are pointed out. Based on the literature data published recently, the functional implication of the majority of co-localized substances is still better understood in non-osmotic than osmotic functional circuits. Brattleboro strain of rats that does not express functional vasopressin was also included in this review. These animals suffer from chronic hypernatremia and hyperosmolality, accompanied by sustained increase in OXY mRNA in PVN and SON and OXY levels in plasma. They represent an important model of animals with constantly sustained osmolality, which in the future, will be utilizable for revealing the physiological importance of biologically active substances co-expressed with AVP and OXY, involved in the regulation of plasma osmolality.

A new method of spike modelling and interval analysis

Here we develop a new model of spike firing, based on the leaky integrate and fire model, modified to simulate afterpotentials. We also develop new analysis techniques, applying these to recorded and model generated data in order to make a comparative analysis and develop the model as a hypothesis for the functional components of the neuron. The model is based in this first instance on hypothalamic oxytocin neurons. We demonstrate how model parameters and cell properties relate to features observed in inter-spike intervals histograms, and the limits of these in being able to detect patterning features in spike recordings. A new technique, spike train analysis, is able to detect previously unobserved patterning, showing a dependence of spike intervals on previous firing activity. This effect is reproduced in the model by adding the small amplitude but long lasting after hyper-polarising potential (AHP). A fit measure based on log likelihood is used to compare model generated data to recorded spike intervals, taking account of interval dependence on previous activity. This measure is used with the simplex multiple parameter search algorithm to develop an automated method for fitting the model to recorded data.

Small-conductance calcium-activated potassium channels control excitability and firing dynamics in gonadotropin-releasing hormone (GnRH) neurons

The cellular mechanisms determining the firing patterns of GnRH neurons are presently under intense investigation. In this study, we used GnRH-green fluorescent protein transgenic mice and perforated-patch electrophysiology to examine the role of small conductance calcium-activated potassium (SK) channels in determining the electrical excitability and burst-firing characteristics of adult GnRH neurons. After establishing an appropriate protocol for examining the afterhyperpolarization potential (AHP) currents in GnRH neurons, the highly selective SK channel blocker apamin was used to demonstrate that all GnRH neurons express functional SK channels (35.7 +/- 2.7 pA, mean decay time constant = 2167 msec, apamin IC(50) = 9.6 nm) and that this channel underlies approximately 90% of the AHP in these cells. Current-clamp experiments showed that apamin-sensitive SK channels were tonically active in the majority (74%) of GnRH neurons, with apamin (100 nm) administration resulting in a mean 6.9 +/- 0.5 mV membrane depolarization. Apamin also elevated the firing rate of GnRH neurons, including increased burst frequency and duration in spontaneously bursting cells as well as the ability of GnRH neurons to fire action potentials in response to current injection. In GnRH neurons activated by current injection, apamin significantly enhanced the amplitude of the afterdepolarization potential after a single action potential and eliminated spike frequency adaptation. Together, these studies show that apamin-sensitive SK channels play a key role in restraining GnRH neuron excitability. Through direct modulation of the AHP and indirect actions on the afterdepolarization potential, the SK channel exerts a powerful tonic influence upon the firing dynamics of GnRH neurons.