A c-fos-monomeric red fluorescent protein 1 fusion transgene is differentially expressed in rat forebrain and brainstem after chronic dehydration and rehydration

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.

Oestrogen-dependent hypothalamic oxytocin expression with changes in feeding and body weight in female rats

Oxytocin (OXT) is produced in the hypothalamic nuclei and secreted into systemic circulation from the posterior pituitary gland. In the central nervous system, OXT regulates behaviours including maternal and feeding behaviours. Our aim is to evaluate whether oestrogen regulates hypothalamic OXT dynamics. Herein, we provide the first evidence that OXT dynamics in the hypothalamus vary with sex and that oestrogen may modulate dynamic changes in OXT levels, using OXT-mRFP1 transgenic rats. The fluorescence intensity of OXT-mRFP1 and expression of the OXT and mRFP1 genes in the hypothalamic nuclei is highest during the oestrus stage in female rats and decreased significantly in ovariectomised rats. Oestrogen replacement caused significant increases in fluorescence intensity and gene expression in a dose-related manner. This is also demonstrated in the rats' feeding behaviour and hypothalamic Fos neurons using cholecystokinin-8 and immunohistochemistry. Hypothalamic OXT expression is oestrogen-dependent and can be enhanced centrally by the administration of oestrogen.

Transgenic approaches to opening up new fields of vasopressin and oxytocin research

Transgenic approaches have been applied to generate transgenic rats that express exogenous genes in arginine vasopressin (AVP)- and oxytocin (OXT)-producing magnocellular neurosecretory cells (MNCs) of the hypothalamic-neurohypophyseal system (HNS). First, the fusion gene that expresses AVP-enhanced green fluorescent protein (eGFP) and OXT-monomeric red fluorescent protein 1 (mRFP1) was used to visualize AVP- and OXT-producing MNCs and their axon terminals in the HNS under fluorescence microscopy. Second, the fusion gene that expresses c-fos-eGFP and c-fos-mRFP1 was used to identify activated neurons physiologically in the central nervous system, including MNCs, circumventricular organs and spinal cord. In addition, AVP-eGFP x c-fos-mRFP1 and OXT-mRFP1 × c-fos-eGFP double transgenic rats were generated to identify activated AVP- and OXT-producing MNCs using appropriate physiological stimuli. Third, the fusion gene that expresses AVP-chanelrhodopsin 2 (ChR2)-eGFP and AVP-hM3Dq-mCherry was used to activate AVP- and OXT-producing MNCs by optogenetic and chemogenetic approaches. In each step, these transgenic approaches in rats have provided new insights on the physiological roles of AVP and OXT not only in the HNS, but also in the whole body. In this review, we summarize the transgenic rats that we generated, as well as related physiological findings.

Oxytocin-monomeric red fluorescent protein 1 synthesis in the hypothalamus under osmotic challenge and acute hypovolemia in a transgenic rat line

We generated a transgenic rat line that expresses oxytocin (OXT)-monomeric red fluorescent protein 1 (mRFP1) fusion gene to visualize the dynamics of OXT. In this transgenic rat line, hypothalamic OXT can be assessed under diverse physiological and pathophysiological conditions by semiquantitative fluorometry of mRFP1 fluorescence intensity as a surrogate marker for endogenous OXT. Using this transgenic rat line, we identified the changes in hypothalamic OXT synthesis under various physiological conditions. However, few reports have directly examined hypothalamic OXT synthesis under hyperosmolality or hypovolemia. In this study, hypothalamic OXT synthesis was investigated using the transgenic rat line after acute osmotic challenge and acute hypovolemia induced by intraperitoneal (i.p.) administration of 3% hypertonic saline (HTN) and polyethylene glycol (PEG), respectively. The mRFP1 fluorescence intensity in the paraventricular (PVN) and supraoptic nuclei (SON) was significantly increased after i.p. administration of HTN and PEG, along with robust Fos-like immunoreactivity (co-expression). Fos expression showed neuronal activation in the brain regions that are associated with the hypothalamus and/or are involved in maintaining water and electrolyte homeostasis in HTN- and PEG-treated rats. OXT and mRFP1 gene expressions were dramatically increased after HTN and PEG administration. The plasma OXT level was extremely increased after HTN and PEG administration. Acute osmotic challenge and acute hypovolemia induced upregulation of hypothalamic OXT in the PVN and SON. These results suggest that not only endogenous arginine vasopressin (AVP) but also endogenous OXT has a key role in maintaining body fluid homeostasis to cope with hyperosmolality and hypovolemia.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

Effect of oestrogen-dependent vasopressin on HPA axis in the median eminence of female rats

The median eminence (ME) anatomically consists of external (eME) and internal (iME) layers. The hypothalamic neurosecretory cells terminate their axons in the eME and secrete their neurohormones regulating anterior pituitary hormone secretion involved in stress responses into the portal vein located in the eME. Magnocellular neurosecretory cells (MNCs) which produce arginine vasopressin (AVP) and oxytocin in the paraventricular (PVN) and supraoptic nuclei (SON) terminate their axons in the posterior pituitary gland (PP) through the iME. Here, we provide the first evidence that oestrogen modulates the dynamic changes in AVP levels in the eME axon terminals in female rats, using AVP-eGFP and AVP-DREADDs transgenic rats. Strong AVP-eGFP fluorescence in the eME was observed at all oestrus cycle stages in adult female rats but not in male transgenic rats. AVP-eGFP fluorescence in the eME was depleted after bilateral ovariectomy but re-appeared with high-dose 17β-oestradiol. AVP-eGFP fluorescence in the MNCs and PP did not change significantly in most treatments. Peripheral clozapine-N-oxide administration induced AVP-DREADDs neurone activation, causing a significant increase in plasma corticosterone levels in the transgenic rats. These results suggest that stress-induced activation of the hypothalamic-pituitary-adrenal axis may be caused by oestrogen-dependent upregulation of AVP in the eME of female rats.

Advanced genetic and viral methods for labelling and manipulation of oxytocin and vasopressin neurones in rats

Rats have been widely used as one of the most common laboratory animals for biological research, because their physiology, pathology, and behavioral characteristics are highly similar to humans. Recent developments in rat genetic modification techniques have now led to further their utility for a broad range of research questions, including the ability to specifically label individual neurones, and even manipulate neuronal function in rats. We have succeeded in generating several transgenic rat lines that enable visualization of specific neurones due to their expression of fluorescently-tagged oxytocin, vasopressin, and c-fos protein. Furthermore, we have been able to generate novel transgenic rat lines in which we can activate vasopressin neurones using optogenetic and chemogenetic techniques. In this review, we will summarize the techniques of genetic modification for labeling and manipulating the specific neurones. Successful examples of generating transgenic rat lines in our lab and usefulness of these rats will also be introduced. These transgenic rat lines enable the interrogation of neuronal function and physiology in a way that was not possible in the past, providing novel insights into neuronal mechanisms both in vivo and ex vivo.

Upregulation of hypothalamic arginine vasopressin by peripherally administered furosemide in transgenic rats expressing arginine vasopressin-enhanced green fluorescent protein

Furosemide, which is used worldwide as a diuretic agent, inhibits sodium reabsorption in the Henle's loop, resulting in diuresis and natriuresis. Arginine vasopressin (AVP) is synthesized in the supraoptic nucleus (SON), paraventricular nucleus (PVN), and suprachiasmatic nucleus (SCN) of the hypothalamus. The synthesis AVP in the magnocellular neurons of SON and PVN physiologically regulated by plasma osmolality and blood volume and contributed water homeostasis by increasing water reabsorption in the collecting duct. Central AVP dynamics after peripheral administration of furosemide remain unclear. Here, we studied the effects of intraperitoneal (i.p.) administration of furosemide (20 mg/kg) on hypothalamic AVP by using transgenic rats expressing AVP-enhanced green fluorescent protein (eGFP) under the AVP promoter. The i.p. administration of furosemide did not affect plasma osmolality in the present study; however, eGFP in the SON and magnocellular divisions of the PVN (mPVN) were significantly increased after furosemide administration compared to the control. Immunohistochemical analysis revealed Fos-like immunoreactivity (IR) in eGFP-positive neurons in the SON and mPVN 90 min after i.p. administration of furosemide, and AVP heteronuclear (hn) RNA and eGFP mRNA levels were significantly increased. These furosemide-induced changes were not observed in the suprachiasmatic AVP neurons. Furthermore, furosemide induced a remarkable increase in Fos-IR in the organum vasculosum laminae terminals (OVLT), median preoptic nucleus (MnPO), subfornical organ (SFO), locus coeruleus (LC), nucleus of the solitary tract (NTS), and rostral ventrolateral medulla (RVLM) after i.p. administration of furosemide. In conclusion, we were able to visualize and quantitatively evaluate AVP-eGFP synthesis and neuronal activations after peripheral administration of furosemide, using the AVP-eGFP transgenic rats. The results of this study may provide new insights into the elucidation of physiological mechanisms underlying body fluid homeostasis induced by furosemide. This article is protected by copyright. All rights reserved.

Activation of endogenous arginine vasopressin neurons inhibit food intake: by using a novel transgenic rat line with DREADDs system

Various studies contributed to discover novel mechanisms of central arginine vasopressin (AVP) system responsible for the behaviour albeit endogenous vasopressin activation. We established a novel transgenic rat line which expresses both human muscarinic acetylcholine receptors (hM3Dq), of which ligand is clozapine-N-oxide (CNO), and mCherry fluorescence specifically in AVP neurons. The mCherry neurons that indicate the expression of the hM3Dq gene were observed in the suprachiasmatic (SCN), supraoptic (SON), and paraventricular nuclei (PVN). hM3Dq-mCherry fluorescence was localized mainly in the membrane of the neurons. The mCherry neurons were co-localized with AVP-like immunoreactive (LI) neurons, but not with oxytocin-LI neurons. The induction of Fos, which is the indicator for neuronal activity, was observed in approximately 90% of the AVP-LI neurons in the SON and PVN 90 min after intraperitoneal (i.p.) administration of CNO. Plasma AVP was significantly increased and food intake, water intake, and urine volume were significantly attenuated after i.p. administration of CNO. Although the detailed mechanism has unveiled, we demonstrated, for the first time, that activation of endogenous AVP neurons decreased food intake. This novel transgenic rat line may provide a revolutionary insight into the neuronal mechanism regarding central AVP system responsible for various kind of behaviours.

Area-specific analysis of the distribution of hypothalamic neurons projecting to the rat ventral tegmental area, with special reference to the GABAergic and glutamatergic efferents

The ventral tegmental area (VTA) is a main regulator of reward and integrates a wide scale of hormonal and neuronal information. Feeding-, energy expenditure-, stress, adaptation- and reproduction-related hypothalamic signals are processed in the VTA and influence the reward processes. However, the neuroanatomical origin and chemical phenotype of neurons mediating these signals to the VTA have not been fully characterized. In this study we have systematically mapped hypothalamic neurons that project to the VTA using the retrograde tracer Choleratoxin B subunit (CTB) and analyzed their putative gamma-aminobutyric acid (GABA) and/or glutamate character with in situ hybridization in male rats. 23.93 ± 3.91% of hypothalamic neurons projecting to the VTA was found in preoptic and 76.27 ± 4.88% in anterior, tuberal and mammillary hypothalamic regions. Nearly half of the retrogradely-labeled neurons in the preoptic, and more than one third in the anterior, tuberal and mammillary hypothalamus appeared in medially located regions. The analyses of vesicular glutamate transporter 2 (VGLUT2) and glutamate decarboxylase 65 (GAD65) mRNA expression revealed both amino acid markers in different subsets of retrogradely-labeled hypothalamic neurons, typically with the predominance of the glutamatergic marker VGLUT2. About one tenth of CTB-IR neurons were GAD65-positive even in hypothalamic nuclei expressing primarily VGLUT2. Some regions were populated mostly by GAD65 mRNA-containing retrogradely-labeled neurons. These included the perifornical part of the lateral hypothalamus where 58.63 ± 19.04% of CTB-IR neurons were GABAergic. These results indicate that both the medial and lateral nuclear compartments of the hypothalamus provide substantial input to the VTA. Furthermore, colocalization studies revealed that these projections not only use glutamate but also GABA for neurotransmission. These GABAergic afferents may underlie important inhibitory mechanism to fine-tune the reward value of specific signals in the VTA.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

Effects of a subconvulsive dose of kainic acid on the gene expressions of the arginine vasopressin, oxytocin and neuronal nitric oxide synthase in the rat hypothalamus

Arginine vasopressin (AVP) synthesis in the hypothalamo-neurohypophysial system (HNS) is up-regulated by kainic acid (KA)-induced seizure in rats. However, it remains unknown whether a subconvulsive dose of KA affects the HNS. Here we examined the effects of subcutaneous (s.c.) administration of a low dose of KA (4 mg/kg) on the gene expressions of the AVP, oxytocin (OXT) and neuronal nitric oxide synthase (nNOS) in the supraoptic (SON) and paraventricular nuclei (PVN) of the rat hypothalamus, using in situ hybridization histochemistry. The expression of the AVP gene in the SON and PVN was judged to be up-regulated in KA-treated rats in comparison with saline-treated rats as controls. Next, the expression of the OXT gene was significantly increased in the SON at 6-24h and in the PVN at 6 and 12h after s.c. administration of KA. Finally, the expression of the nNOS gene was significantly increased in the SON and PVN at 3 and 6h after s.c. administration of KA. These results suggest that up-regulation of the gene expressions of the AVP, OXT and nNOS in the rat hypothalamus may be differentially affected by peripheral administration of a subconvulsive dose of KA.