Water deprivation induces regional expression of c-fos protein in the brain of inbred polydipsic mice

AVP-eGFP was significantly upregulated by hypovolemia in the parvocellular division of the paraventricular nucleus in the transgenic rats

Arginine vasopressin (AVP) is produced in the paraventricular (PVN) and supraoptic nuclei (SON). Peripheral AVP, which is secreted from the posterior pituitary, is produced in the magnocellular division of the PVN (mPVN) and SON. In addition, AVP is produced in the parvocellular division of the PVN (pPVN), where corticotrophin-releasing factor (CRF) is synthesized. These peptides synergistically modulate the hypothalamic-pituitary-adrenal (HPA) axis. Previous studies have revealed that the HPA axis was activated by hypovolemia. However, the detailed dynamics of AVP in the pPVN under hypovolemic state has not been elucidated. Here, we evaluated the effects of hypovolemia and hyperosmolality on the hypothalamus, using AVP-enhanced green fluorescent protein (eGFP) transgenic rats. Polyethylene glycol (PEG) or 3% hypertonic saline (HTN) was intraperitoneally administered to develop hypovolemia or hyperosmolality. AVP-eGFP intensity was robustly upregulated at 3 and 6 h after intraperitoneal administration of PEG or HTN in the mPVN. While in the pPVN, eGFP intensity was significantly increased at 6 h after intraperitoneal administration of PEG with significant induction of Fos-immunoreactive (-ir) neurons. Consistently, eGFP mRNA, AVP hnRNA, and CRF mRNA in the pPVN and plasma AVP and corticosterone were significantly increased at 6 h after intraperitoneal administration of PEG. The results suggest that AVP and CRF syntheses in the pPVN were activated by hypovolemia, resulting in the activation of the HPA axis.

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.

Induction of Fos expression in the rat brain after intragastric administration of dried bonito dashi

Objectives: Dried bonito dashi, a traditional Japanese fish broth made from dried bonito tuna, enhances food palatability due to its specific umami flavor characteristics. However, the pattern of brain activation following dashi ingestion has not been previously investigated.Methods: We mapped activation sites of the rat brain after intragastric loads of dried bonito dashi by measuring neuronal levels of the Fos protein, a functional marker of neuronal activation.Results: Compared to intragastric saline, intragastric dashi administration produced enhanced Fos expression in four forebrain regions: the medial preoptic area, subfornical organ, habenular nucleus, and central nucleus of the amygdala. Interestingly, the medial preoptic area was found to be the only feeding-related hypothalamic area responsive to dashi administration. Moreover, dashi had no effect in the nucleus accumbens and ventral tegmental area, two connected sites known to be activated by highly palatable sugars and fats. In the hindbrain, dashi administration produced enhanced Fos expression in both visceral sensory (caudal nucleus of the solitary tract, dorsal part of the lateral parabrachial nucleus, and area postrema) and autonomic (rostral ventrolateral medulla, and caudal ventrolateral medulla) sites.Discussion: The results demonstrate the activation of discrete forebrain and hindbrain regions following intragastric loads of dried bonito dashi. Our data suggest that the gut-brain axis is the principal mediator of the postingestive effects associated with the ingestion of dashi.

Effects of acute kidney dysfunction on hypothalamic arginine vasopressin synthesis in transgenic rats

Acute loss of kidney function is a critical internal stressor. Arginine vasopressin (AVP) present in the parvocellular division of the paraventricular nucleus (PVN) plays a key role in the regulation of stress responses. However, hypothalamic AVP dynamics during acute kidney dysfunction remain unclear. In this study, we investigated the effects of bilateral nephrectomy on AVP, using a transgenic rat line that expressed the AVP-enhanced green fluorescent protein (eGFP). The eGFP fluorescent intensities in the PVN were dramatically increased after bilateral nephrectomy. The mRNA levels of eGFP, AVP, and corticotrophin-releasing hormone in the PVN were dramatically increased after bilateral nephrectomy. Bilateral nephrectomy also increased the levels of Fos-like immunoreactive cells in brainstem neurons. These results indicate that bilateral nephrectomy upregulates the AVP-eGFP synthesis. Further studies are needed to identify the neural and/or humoral factors that activate AVP synthesis and regulate neuronal circuits during acute kidney dysfunction.

Role of Trpv1 and Trpv4 in surgical incision-induced tissue swelling and Fos-like immunoreactivity in the central nervous system of mice

Pain management remains a major concern regarding the treatment of postoperative patients. Transient receptor potential (TRP) channels are considered to be new therapeutic targets for pain control. We investigated whether the genes Trpv1 and Trpv4 are involved in hind paw swelling caused after surgical incision in mice or in incision-induced Fos-like immunoreactivity (Fos-LI) levels in the central nervous system. Mice were divided into four groups: wild-type (WT) control, WT incision, Trpv1 knockout (Trpv1^-/-) incision, and Trpv4 knockout (Trpv4^-/-) incision. Mice were anesthetized, and only those in the incision, and not control, groups received a surgical incision to their right plantar hind paw. Changes in paw diameter and in Fos-LI levels in the dorsal horn of the spinal cord, paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus, and central amygdala were evaluated 2 h after the incision. There was no significant difference in the paw diameter among groups. In contrast, in laminae I-II of the dorsal horn of the spinal cord and PVN, Fos-LI was significantly higher in all incision groups than in the WT control group. A significant increase in Fos-positive cells was also observed in the dorsal horn laminae III-IV in Trpv1^-/- and Trpv4^-/- incision groups compared with the WT incision group. Our results indicate that surgical incision activates the PVN and that Trpv1 and Trpv4 might be involved in neuronal activity in the dorsal horn laminae III-IV after surgical incision.

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.

Comparison of the induction of c-fos-eGFP and Fos protein in the rat spinal cord and hypothalamus resulting from subcutaneous capsaicin or formalin injection

We evaluated whether a c-fos-enhanced green fluorescent protein (eGFP) transgenic rat line, which expresses the c-fos and eGFP fusion gene, can be useful for the study of nociceptive pathways and processing. Capsaicin solution (15%) or formalin (5%) was subcutaneously injected bilaterally into the hind paws (100μL per each paw) of adult male c-fos-eGFP transgenic or wild-type rats. Control rats were injected with ethanol or physiological saline respectively. Transgenic and wild-type rats were perfused at 1.5, 3 and 6h post injection, with some transgenic rats being perfused 24h post injection. A comparison of eGFP in transgenic rats and Fos-like immunoreactivity (LI) in wild-type rats was made in the dorsal spinal cord, paraventricular nucleus (PVN) and supraoptic nucleus (SON). Oxytocin-LI (OXT-LI) was carried out to examine the activation of OXT neurons in the PVN and SON. Following capsaicin or formalin treatment, eGFP was maximally expressed at 6h in the spinal cord and 3h in the PVN and SON, whereas, Fos-LI was maximally expressed at 1.5h in all the regions we analyzed. Induction of eGFP in the OXT neurons was observed after capsaicin or formalin treatment, while Fos-LI in the OXT neurons was observed only after formalin treatment. These results demonstrate that the peak induction of c-fos-eGFP following exposure to acute nociceptive stimuli was delayed by around 1.5-4.5h, but more sensitive than endogenous Fos, suggesting that the c-fos-eGFP rat line can be useful for the study of nociceptive pathways and processing.

Possible involvement of TRPV1 and TRPV4 in nociceptive stimulation- induced nocifensive behavior and neuroendocrine response in mice

Members of the transient receptor potential (TRP) family of ion channels play important roles in inflammation and pain. Here, we showed that both TRPV1 and TRPV4 might contribute to biphasic nocifensive behavior and neuroendocrine response following a formalin test. We subcutaneously injected saline, formalin, or the TRPV4 agonist, 4α-phorbol 12,13-didecanoate (4α-PDD) into one hindpaw of wild-type (WT), TRPV1-deficient (Trpv1(-/-)), and TRPV4-deficient (Trpv4(-/-)) mice to investigate nocifensive behaviors (phase I [0-10 min] and phase II [10-60 min]) and Fos expression in the dorsal horn of the spinal cord and other brain regions related to pain, in the paraventricular nucleus (PVN), paraventricular nucleus of the thalamus, the medial habenular nucleus, the medial nucleus of the amygdala and capsular part of the central amygdala. Subcutaneous (s.c.) injection of formalin caused less nocifensive behavior in Trpv1(-/-) and Trpv4(-/-) mice than in WT mice during phase I. In phase II, however, formalin induced less nocifensive behavior only in the Trpv1(-/-) mice, but not in the Trpv4(-/-) mice, relative to WT mice. The number of Fos-like immunoreactive (LI) neurons in laminae I-II of the dorsal horn increased in all types of mice 90 min after s.c. injection of formalin; however, there was no difference in the other regions between saline- and formalin-treated mice. Furthermore, s.c. injection of 4α-PDD did not induce nociceptive behavior nor influence the number of Fos-LI neurons in the all above mentioned regions in any of the mice. These results suggest that TRPV4-mediated nociceptive information from the peripheral tissue excluding the spinal pathway might be involved the formalin behavioral response during phase I. Only TRPV1 might regulate the formalin behavioral response in peripheral neuron.

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

We have previously shown that an acute osmotic stimulation induces the expression of a c-fos and monomeric red fluorescent protein 1 (mRFP1) fusion transgene in osmosensitive rat brain areas, including the supraoptic (SON) and paraventricular nuclei (PVN). However, the effects of chronic stimuli, such as dehydration, have not been investigated. In the present study, the expression patterns of the c-fos-mRFP1 fusion gene in the forebrain and the brainstem of male and female transgenic rats were studied in seven experimental groups: ad lib. water (euhydration), water deprivation for 12, 24 or 48 h (dehydration) and water deprivation for 46 h + ad lib. water for 2, 6 or 12 h (rehydration). The number of cells that express nuclear mRFP1 fluorescence was quantified in the hypothalamus, the circumventricular organs and the brainstem. Compared to the euhydrated state, the number of transgene expressing cells significantly increased in all forebrain areas and in the rostral ventrolateral medulla after dehydration and 2 h of rehydration. In the nucleus of the solitary tract and area postrema, the number of mRFP1 fluorescent cells was markedly increased after 2 h of rehydration. Although the number of mRFP1 fluorescent cells in the organum vasculosum laminae terminalis, median preoptic nucleus and subfornical organ remained significantly increased after 6 h of rehydration, reaching control levels after 12 h of rehydration, the number of mRFP1 fluorescent cells in the SON and the PVN reached control levels after 6 h of rehydration. There were no significant differences between male and female rats. These results show that the expression of the c-fos-mRFP1 fusion gene changes in the forebrain and the brainstem not only after acute osmotic stimulation, but also after chronic osmotic stimulation. Interestingly, these studies reveal the differential activation of different neuronal groups over the time course of dehydration and rehydration.

Expression of the c-fos-monomeric red fluorescent protein 1 fusion gene in the spinal cord and the hypothalamic paraventricular nucleus in transgenic rats after nociceptive stimulation

We generated transgenic rats expressing the c-fos and monomeric red fluorescent protein 1 (mRFP1) fusion gene in the central nervous system after adequate stimulation. In the present study, the time-course of the induction patterns of mRFP1 fluorescence in the spinal cord and the paraventricular nucleus (PVN) was compared with that of Fos-like immunoreactivity (LI) within 24 h after subcutaneous (s.c.) injection of 0.9% saline and 5% formalin in both hind paws. Control rats were not treated. In the control and saline/formalin injected rats, scattered mRFP1 fluorescence in the spinal cord and the PVN was observed at 0 min, though there was little Fos-LI in the same region. The mRFP1 fluorescence in the spinal cord and the PVN was increased at 3h after formalin. On the other hand, the changes of Fos-LI in the spinal cord and the PVN were relatively shorter than those of the mRFP1 fluorescence after formalin. These results suggest that the c-fos-mRFP1 fusion gene expression is slightly upregulated in normal conditions and nociceptive stimulation-induced induction of the fusion gene may be maintained longer than the endogenous c-fos gene expression in the spinal cord and the PVN. Next, nocifensive behavior and mRFP1 fluorescence and Fos-LI in the spinal cord and the PVN after s.c. injection of formalin, 4α-phorbol 12,13-didecanoate (4α-PDD) and saline were compared. Although the 4α-PDD injected rats seldom displayed nocifensive behaviors like s.c. saline injection, 4α-PDD injection caused mRFP1 fluorescence and Fos-LI significantly in the spinal cord and the PVN unlike s.c. saline injection.

Induction of Fos expression in the rat forebrain after intragastric administration of monosodium L-glutamate, glucose and NaCl

l-glutamate, an umami taste substance, is a key molecule coupled to a food intake signaling pathway. Furthermore, recent studies have unveiled new roles for dietary glutamate on gut-brain axis communication via activation of gut glutamate receptors and subsequent vagus nerve. In the present study, we mapped activation sites of the rat forebrain after intragastric load of 60 mM monosodium l-glutamate (MSG) by measurement of Fos protein, a functional marker of neuronal activation. The same concentration of d-glucose (sweet) and NaCl (salty) was used as controls. MSG administration exclusively produced enhanced Fos expression in four hypothalamic regions (the medial preoptic area, lateral hypothalamic area, dorsomedial nucleus, and arcuate nucleus). On the other hand, glucose administration exclusively enhanced Fos induction in the nucleus accumbens. Both MSG and glucose enhanced Fos induction in three brain regions (the habenular nucleus, paraventricular nucleus, and central nucleus of the amygdala). However, MSG induced Fos inductions were more potent than those of glucose in the habenular nucleus and paraventricular nucleus. Importantly, the present study identified for the first time two brain areas (the paraventricular and arcuate hypothalamic nuclei) that are more potently activated by intragastric MSG loads compared with glucose and NaCl. Overall, our results suggest significant activation of a neural network comprising the habenular nucleus, amygdala, and the hypothalamic subnuclei following intragastric load with glutamate.

Centrally administered relaxin-3 induces Fos expression in the osmosensitive areas in rat brain and facilitates water intake

The expression of the relaxin-3 gene, detected as a new member of the insulin superfamily using human genomic databases, is abundantly present in the brain and testis. Intracerebroventricularly (icv) administered relaxin-3 stimulates food intake. Icv administered relaxin (identical to relaxin-2 in humans) affects the secretion of vasopressin and drinking behavior. Relaxin-3 partly binds relaxin family peptide receptor 1, which is a specific receptor to relaxin. Thus, we hypothesized that relaxin-3 would have physiological effects in the body fluid balance. However, the effects of relaxin-3 in the body fluid balance remain unknown. In the present study, we revealed that icv administered relaxin-3 induced dense Fos-like immunoreactivity (Fos-LI) in the rat hypothalamus and circumventricular organs including the organum vasculosum of the lamina terminalis, the median preoptic nucleus, supraoptic nucleus (SON), the subfornical organ (SFO) and the paraventricular nucleus (PVN), that are related to the central regulation of body fluid balance. Icv administered relaxin-3 (54, 180 and 540 pmol/rat) also induced a significant increase in c-fos gene expression in a dose-dependent manner in the SON, SFO and PVN. Further, icv administered relaxin-3 (180 pmol/rat) significantly increased water intake, and the effect was as strong as that of relaxin-2 (180 pmol/rat). These results suggest that icv administered relaxin-3 activates osmosensitive areas in the brain and plays an important role in the regulation of body fluid balance.

The role of hypothalamic ingestive behavior controllers in generating dehydration anorexia: a Fos mapping study

Giving rats 2.5% saline to drink for 3-5 days simply and reliably generates anorexia. Despite having the neurochemical and hormonal markers of negative energy balance, dehydrated anorexic rats show a marked suppression of spontaneous food intake, as well as the feeding that is usually stimulated by overnight starvation or a 2-deoxy-d-glucose (2DG) challenge. These observations are consistent with a dehydration-dependent inhibition of the core circuitry that controls feeding. We hypothesize that this inhibition is directed at those neurons in the paraventricular nucleus and lateral hypothalamic area that constitute the hypothalamic "behavior controller" for feeding rather than their afferent inputs from the arcuate nucleus or hindbrain that convey critical feeding-related sensory information. To test this hypothesis, we mapped and quantified the Fos-immunoreactive response to 2DG in control and dehydrated rats drinking 2.5% saline. Our rationale was that regions showing an attenuated Fos response to 2DG in dehydrated animals would be strong candidates as the targets of dehydration-induced suppression of 2DG feeding. We found that the Fos response to combined dehydration and 2DG was attenuated only in the lateral hypothalamic area, with dehydration alone increasing Fos in the lateral part of the paraventricular nucleus. In the arcuate nucleus and those regions of the hindbrain that provide afferent inputs critical for the feeding response to 2DG, the Fos response to 2DG was unaffected by dehydration. Therefore, dehydration appears to target the lateral hypothalamic area and possibly the lateral part of the paraventricular nucleus to suppress the feeding response to 2DG.

Centrally and peripherally administered ghrelin potently inhibits water intake in rats

Ghrelin is known as a potent orexigenic hormone through its action on the brain. In this study, we examined the effects of intracerebroventricular (icv) and iv injection of ghrelin on water intake, food intake, and urine volume in rats deprived of water for 24 h. Water intake that occurred after water deprivation was significantly inhibited by icv injection of ghrelin (0.1, 1, and 10 nmol/rat) in a dose-related manner, although food intake was stimulated by the hormone. The antidipsogenic effect was as potent as the orexigenic effect. Similarly, water intake was inhibited, whereas food intake was stimulated dose dependently after iv injection of ghrelin (0.1, 1, and 10 nmol/kg). The inhibition of drinking was comparable with, or even more potent than, atrial natriuretic peptide (ANP), an established antidipsogenic hormone, when administered icv, although the antidipsogenic effect lasted longer. ANP had no effect on food intake. Urine volume decreased dose relatedly after icv injection of ghrelin but not by ANP. Intravenous injection of ghrelin had no effect on urine volume. Because drinking usually occurs with feeding, food was withdrawn to remove the prandial drinking. Then the antidipsogenic effect of ghrelin became more potent than that of ANP and continued longer than when food was available. Expression of Fos was increased in the area postrema and the nucleus of the tractus solitarius by using immunohistochemistry after icv and iv injection of ghrelin. The present study convincingly showed that ghrelin is a potent antidisogenic peptide in rats.

Differential effects of water deprivation and rehydration on Fos and FosB/DeltaFosB staining in the rat brainstem

This study examined the effects of dehydration and rehydration with water on Fos and FosB staining in the brainstem of rats. Male rats were water deprived for 48 h (Dehyd, n=7) or 46 h followed by 2 h access to water (Rehyd, n=7). Controls had ad libitum access to water (Con, n=9). Brainstems were stained for Fos and FosB/DeltaFosB using commercially available antibodies. In the nucleus of the solitary tract (NTS), the number of Fos stained neurons was significantly increased by dehydration and increased further following rehydration (Con 5+/-1; Dehyd 22+/-1; Rehyd 48+/-5). The average number of Fos-positive cells in the parabrachial nucleus (PBN) was significantly increased only by rehydration (Con 12+/-2; Dehyd 6+/-2; Rehyd 51+/-4). The area postrema (AP) showed significant increases in Fos staining after dehydration and rehydration (Fos: Con 4+/-1; Dehyd 28+/-3; Rehyd 24+/-3). In the rostral ventrolateral medulla (RVL), Fos staining significantly increased after dehydration and this effect was reduced by rehydration (Con 3+/-1; Dehyd 21+/-2; Rehyd 12+/-1). In contrast, Fos staining in the caudal ventrolateral medulla (CVL) was not significantly influenced following either dehydration or rehydration with water (Con 4+/-1; Dehyd 4+/-1; Rehyd 5+/-1). FosB/DeltaFosB staining in the NTS, AP, and RVL was comparably increased by dehydration and rehydration. In the PBN and CVL, FosB/DeltaFosB staining was not affected by the treatments. Dehydration and rehydration have regionally specific effects on Fos and FosB/DeltaFosB staining in the brainstem.

Differential effects of water and saline intake on water deprivation-induced c-Fos staining in the rat

We studied c-Fos staining in adult male rats after 48 h of water deprivation and after 46 h of water deprivation with 2 h of access to water or physiological saline. Controls were allowed ad libitum access to water and physiological saline. For immunocytochemistry, anesthetized rats were perfused with a commercially available antibody for c-Fos. Dehydration significantly increased plasma vasopressin (AVP), osmolality, plasma renin activity (PRA), hematocrit, and sodium concentration and decreased urinary volume. Fos staining was significantly increased in the median preoptic nucleus, organum vasculosum of the lamina terminalis, supraoptic nucleus (SON), and magnocellular and parvocellular paraventricular nucleus (PVN), as well as the area postrema, nucleus of the solitary tract (NTS), and rostral ventrolateral medulla (RVL). Rehydration with water significantly decreased AVP levels and Fos staining in the SON, PVN, and RVL and significantly increased Fos expression in the perinuclear zone of the SON, NTS, and parabrachial nucleus. Rehydration with water was associated with decreased urinary sodium concentration and hypotonicity, and hematocrit and PRA were comparable to levels seen after dehydration. After rehydration with saline, plasma osmolality, hematocrit, and PRA were not different from control, but plasma AVP and urinary sodium concentration were increased. In the SON, Fos staining was significantly increased, with a great percentage of the Fos cells also stained for oxytocin compared with water deprivation. Changes in Fos staining were also observed in the NTS, RVL, parabrachial nucleus, and PVN. Rehydration with water or saline produces differential effects on plasma AVP, Fos staining, and sodium concentration.

The subfornical organ, a specialized sodium channel, and the sensing of sodium levels in the brain

Dehydration causes an increase in the sodium (Na) concentration and osmolarity of body fluid. For Na homeostasis of the body, controls of Na and water intake and excretion are of prime importance. However, though the circumventricular organs (CVOs) are suggested to be involved in body-fluid homeostasis, the system for sensing the Na level within the brain that is responsible for the control of Na- and water-intake behavior has long been an enigma. The authors found that the Na(x) channel is preferentially expressed in the CVOs in the brain and that Na(x) knockout mice ingest saline in excess under dehydrated conditions. Subsequently, the authors demonstrated that Na(x) is an Na-level-sensitive Na channel. When Na(x) cDNA was introduced into the brain of the knockout mice with an adenoviral expression vector, only animals that received a transduction of the Na(x) gene into the subfornical organ (SFO) among the CVOs recovered salt-avoiding behavior under dehydrated conditions. Here, the authors advocate that the SFO is the center of the control of salt-intake behavior in the brain, where the Na-level-sensitive Na(x) channel is involved in sensing the physiological increase in the level of Na in body fluids.

Effects of cholecystokinin (CCK)-8 on hypothalamic oxytocin-secreting neurons in rats lacking CCK-A receptor

Peripheral administration of cholecystokinin (CCK)-8 selectively activates oxytocin (OXT)-secreting neurons in the supraoptic (SON) and the paraventricular nuclei (PVN) with the elevation of plasma OXT level in rats. We examined the effects of intravenous (iv) administration of CCK-8 on the neuronal activity of hypothalamic OXT-secreting neurons and plasma OXT level in Otsuka Long-Evans Tokushima Fatty (OLETF) rats that have a congenital defect in the expression of the CCK-A receptor gene. In situ hybridization histochemistry (ISH) for c-fos mRNA revealed that the expression of the c-fos gene was not induced in the SON, the PVN, the nucleus of the tractus solitarius (NTS) and the area postrema (AP) 30 min after iv administration of CCK-8 (20 and 40 microg/kg) in OLETF rats. In Long-Evans Tokushima Otsuka (LETO) rats (controls), c-fos mRNA was detected abundantly in those nuclei 30 min after iv administration of CCK-8 (20 microg/kg). Immunohistochemistry for c-fos protein (Fos) showed that the distributions of Fos-like immunoreactivity (LI) were identical to the results obtained from ISH. Dual immunostaining for OXT and Fos revealed that Fos-LI was mainly observed in OXT-secreting neurons in the SON and the PVN of LETO rats 90 min after iv administration of CCK-8 (20 microg/kg). Radioimmunoassay for OXT and arginine vasopressin (AVP) showed that iv administration of CCK-8 did not cause significant change in the plasma OXT and AVP levels in OLETF rats, while iv administration of CCK-8 caused a significant elevation of plasma OXT level without changing the plasma AVP level in LETO rats. These results suggest that peripheral administration of CCK-8 may selectively activate the hypothalamic OXT-secreting neurons and brainstem neurons through CCK-A receptor in rats.

Centrally administered adrenomedullin 2 activates hypothalamic oxytocin-secreting neurons, causing elevated plasma oxytocin level in rats

We examined the effects of intracerebroventricular (i.c.v.) administration of adrenomedullin 2 (AM2) on plasma oxytocin (OXT) and arginine vasopressin (AVP) levels in conscious rats. Plasma OXT levels were markedly increased 5 min after i.c.v. administration of AM2 (1 nmol/rat) compared with vehicle and remained elevated in samples taken at 10, 15, 30, and 60 min. By contrast, plasma AVP levels were not significantly elevated in samples taken between 5 and 180 min after i.c.v. administration of AM2 except at the 30-min time point. Fos-like immunoreactivity (Fos-LI) was observed in various brain areas, including the paraventricular (PVN) and the supraoptic nuclei (SON) after i.c.v. administration of AM2 (2 nmol/rat) in conscious rats (measured at 90 min post-AM2 infusion). Dual immunostaining for OXT/Fos and AVP/Fos showed that OXT-LI neurons predominantly exhibited nuclear Fos-LI compared with AVP-LI neurons in the PVN and the SON. In situ hybridization histochemistry showed that i.c.v. administration of AM2 (0.2, 1, and 2 nmol/rat) caused marked induction of the expression of the c-fos gene in the PVN and the SON. This induction was significantly reduced by pretreatment with both the calcitonin gene-related peptide (CGRP) antagonist CGRP-(8-37) (3 nmol/rat) and the AM receptor antagonist AM-(22-52) (27 nmol/rat). These results suggest that centrally administered AM2 mainly activates OXT-secreting neurons in the PVN and the SON, at least in part through the CGRP and/or AM receptors with marked elevation of plasma OXT levels in conscious rats.

Role of angiotensin in body fluid homeostasis of mice: fluid intake, plasma hormones, and brain Fos

CD1 mice injected peripherally with either ANG I or ANG II failed to drink substantial amounts of water or NaCl, yet showed strong Fos immunoreactivity (ir) in subfornical organ (SFO). Mice injected with furosemide showed modest stimulation of NaCl intake either 3 or 24 h later, were hypovolemic, and showed elevated plasma renin activity (PRA). The pattern of Fos-ir in the brain after furosemide was similar to that seen after peripheral injection of ANG II. Mice became hypovolemic after subcutaneous injection of polyethylene glycol (PEG), showed large increases in PRA, aldosterone, and water intake, but did not show sodium appetite. PEG-treated mice had strong activation of SFO as well as other brain regions previously shown to be related to ANG-associated drinking in rats. ANG II appears to have a modified role in the behavioral response to fluid loss in mice compared with rats.

Water deprivation-induced sodium appetite: humoral and cardiovascular mediators and immediate early genes

Adult rats deprived of water for 24-30 h were allowed to rehydrate by ingesting only water for 1-2 h. Rats were then given access to both water and 1.8% NaCl. This procedure induced a sodium appetite defined by the operational criteria of a significant increase in 1.8% NaCl intake (3.8 +/- 0.8 ml/2 h; n = 6). Expression of Fos (as assessed by immunohistochemistry) was increased in the organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MnPO), subfornical organ (SFO), and supraoptic nucleus (SON) after water deprivation. After rehydration with water but before consumption of 1.8% NaCl, Fos expression in the SON disappeared and was partially reduced in the OVLT and MnPO. However, Fos expression did not change in the SFO. Water deprivation also 1) increased plasma renin activity (PRA), osmolality, and plasma Na+; 2) decreased blood volume; and 3) reduced total body Na+; but 4) did not alter arterial blood pressure. Rehydration with water alone caused only plasma osmolality and plasma Na+ concentration to revert to euhydrated levels. The changes in Fos expression and PRA are consistent with a proposed role for ANG II in the control of the sodium appetite produced by water deprivation followed by rehydration with only water.

Action sites of adrenomedullin in the rat brain: functional mapping by Fos expression

The effects of intracerebroventricular (icv) administration of adrenomedullin (AM) and proadrenomedullin NH2-terminal 20 peptide (PAMP) on the expression of Fos in the central nervous system (CNS) were examined in conscious rats, using immunohistochemistry. Fos-like immunoreactivity (LI) was detected in various brain areas of the rats, including the supraoptic nucleus, the paraventricular nucleus, the locus coeruleus, the area postrema and the nucleus of the tractus solitarius 90 min after icv administration of AM. Few cells with Fos-LI were found in the CNS 90 min after icv administration of saline. Fos-LI was also detected in the various hypothalamic areas after icv administration of PAMP. These results suggest that centrally administered AM and PAMP may cause physiological responses through the activation of a neural network in the hypothalamus and the brainstem.

Nav2/NaG channel is involved in control of salt-intake behavior in the CNS

Na(v)2/NaG is a putative sodium channel, whose physiological role has long been an enigma. We generated Na(v)2 gene-deficient mice by inserting the lacZ gene. Analysis of the targeted mice allowed us to identify Na(v)2-producing cells by examining the lacZ expression. Besides in the lung, heart, dorsal root ganglia, and Schwann cells in the peripheral nervous system, Na(v)2 was expressed in neurons and ependymal cells in restricted areas of the CNS, particularly in the circumventricular organs, which are involved in body-fluid homeostasis. Under water-depleted conditions, c-fos expression was markedly elevated in neurons in the subfornical organ and organum vasculosum laminae terminalis compared with wild-type animals, suggesting a hyperactive state in the Na(v)2-null mice. Moreover, the null mutants showed abnormal intakes of hypertonic saline under both water- and salt-depleted conditions. These findings suggest that the Na(v)2 channel plays an important role in the central sensing of body-fluid sodium level and regulation of salt intake behavior.

Reduced interleukin-1 responsiveness in immune system and central nervous system of inbred polydipsic (STR/N) mice

Inbred polydipsic mice (STR/N strain) have primary polydipsia. The previous studies found abnormalities in the central nervous system (CNS), especially in the hypothalamus and circumventricular organ. As a part of pursuing to find the cause of the polydipsia, we investigated immunological characteristics of STR/N mice, using the ICR strain of mice as control. Their thymic subset cells showed that CD4+CD8+ double positive cells were increased, CD4+ single positive cells were decreased and CD5 expression was deficient, compared to ICR mice. T cell proliferative response and interleukin (IL)-2 production caused by IL-1beta stimulation were reduced in STR/N mice than those in the ICR mice. In in vivo studies the degree of thymic atrophy and the increases in serum level of ACTH and corticosterone induced by intraperitoneal IL-1beta injection were much less in STR/N mice than those in controls. Furthermore, adipsic response also induced by IL-1beta injection was greatly reduced compared to their control mice. All these results suggest that the responsiveness to IL-1 is impaired both in the immune system and the CNS of STR/N mice.

Disorder of salivary secretion in inbred polydipsic mouse

To find mechanisms of an extreme polydipsia in an inbred strain of mice, STR/N, this study was undertaken using Institute of Cancer Research (ICR) mice as a control. During food deprivation, daily water intake of both strains decreased. The decrement in the STR/N mice was larger than that in the ICR mice. During dehydration, daily food intake of the STR/N mice was smaller than that of the ICR mice. These data indicate that prandial drinking was more severely affected for the STR/N mice. Under anesthesia, the stimulated salivary secretion by pilocarpine of the STR/N mice was significantly smaller than that of the ICR mice. The submandibular gland of the STR/N mice was lighter and harder than that of the ICR mice. After desalivation from the major three salivary glands, the ICR mice drank as much as the STR/N mice. Young STR/N mice with undeveloped polydipsia did not show different salivary secretion stimulated by pilocarpine from the young ICR mice. These findings indicate a dysfunction with age in the salivary glands of the STR/N mice, and they suggest that the decreased saliva induces thirst and triggers extraordinary drinking in the polydipsic mice.

Analysis of the vasopressin system and water regulation in genetically polydipsic mice

Polydipsic mice, STR/N, which show extreme polydipsia and polyuria, were discovered in 1958. In the STR/N, urine outputs are much higher than in control mice. The possibility of an abnormal regulation of the arginine vasopressin (AVP) system, or an abnormality in the renal susceptibility to AVP, should be considered. In this study we investigated the AVP system and water regulation in STR/N. We sequenced the AVP and the AVP V(2)-receptor genes of the STR/N by direct sequencing. No mutation was found in either of them. AVP gene expression examined by in situ hybridization and plasma sodium in 8-wk-old STR/N was significantly lower than in control mice, whereas it was significantly higher at 20 wk. Renal sensitivity to injected AVP was attenuated in 20-wk-old STR/N. The suppression of AVP synthesis due to excessive water retention in 8-wk-old STR/N suggests that polydipsia may be the primary cause in this strain. The 20-wk-old STR/N became dehydrated with the acceleration of AVP synthesis, which might have resulted from secondary desensitization to AVP.

Increase of vasopressin mRNA in the hypothalamus of inbred polydipsic mice

The genetically inbred polydipsic mice, STR/N strain, are characterized by extreme polydipsia and polyuria without arginine vasopressin (AVP) deficiency. The expression of AVP gene in the hypothalamus of polydipsic and non-polydipsic mice was examined by Northern blot analysis and in situ hybridization histochemistry. Northern blot analysis revealed that the total amount of AVP mRNA in the hypothalamus of the STR/N mice was approximately three-fold of that in the control, ICR mice. In situ hybridization histochemistry showed that the signals of AVP mRNA in the paraventricular (PVN) and supraoptic nuclei (SON) of the STR/N were stronger than those in the ICR. Although AVP gene transcripts were detected in the anteroventral parts of the PVN (avPVN) in the STR/N, there was a few AVP transcripts in the same area (avPVN) in the ICR. There were no differences in plasma osmolality and hematocrit between STR/N and ICR mice. These results suggest that upregulation of AVP mRNA in the hypothalamus of STR/N may be involved in the central mechanism responsible for the polydipsia in genetically polydipsic mice.

Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons

Unlike many neuron populations, supraoptic nucleus (SON) neurons are rich in both nitric oxide synthase (NOS) and the NO receptor-soluble guanylyl cyclase (GC), the activation of which leads to cGMP accumulation. Elevations in cGMP result in increased coupling among SON neurons. We investigated the effect of NO on dye coupling in SONs from male, proestrus virgin female, and lactating rats. In 167 slices 263 SON neurons were recorded; 210 of these neurons were injected intracellularly (one neuron per SON) with Lucifer yellow (LY). The typically minimal coupling seen in virgin females was increased nearly fourfold by the NO precursor, L-arginine, or the NO donor, sodium nitroprusside (SNP). L-Arginine-induced coupling was abolished by a NOS inhibitor. In slices from male and lactating rats who have a higher basal incidence of coupling, SNP increased coupling by approximately twofold over control (p < 0.03). SNP effects were prevented by the NO scavenger hemoglobin (20 microM) and by the selective blocker of NO-activated GC, ODQ (10 microM). These results suggest that NO released from cells within the SON can expand the coupled network of neurons and that this action occurs via cGMP-dependent processes. Because increased coupling is associated with elevated SON neuronal excitability, we also studied the effects of 8-bromo-cGMP on excitability. In both phasically and continuously firing neurons 8-bromo-cGMP (1-2 mM), but not cGMP, produced membrane depolarizations accompanied by membrane conductance increases. Conductance increases remained when depolarizations were eliminated by current-clamping the membrane potential. Thus, NO-induced cGMP increases SON neuronal coupling and excitability.

Brain mechanisms of mammalian fluid homeostasis: insights from use of immediate early gene mapping

A comprehensive review of the literature through mid-1997 is presented on the application of immediate early gene mapping to problems related to brain mechanisms of fluid homeostasis and cardiovascular regulation in mammals. First, the basic mechanisms of fluid intake and the principles and pitfalls of immediate early gene mapping are briefly introduced. Then, data from several principal paradigms are reviewed. These include fluid deprivation and intracellular dehydration, both of which are associated with thirst and water intake. The contributions of peripheral sodium receptors, and of both hindbrain and forebrain integrative mechanisms are evaluated. Extracellular dehydration, and associated aspects of both thirst and sodium appetite are then reviewed. The contributions of both structures along the lamina terminalis and the hypothalamic magnocellular neurosecretory groups figure prominently in most of these paradigms. Effects of hypotension and hypertension are discussed, including data from the endogenous generation and the exogenous application of angiotensin II. Lastly, we summarize the contribution of the early gene mapping technique and consider briefly the prospects for new advances using this method.

Differential stimulation of c-fos expression in hypothalamic nuclei of the rat brain during short-term heat acclimation and mild dehydration

Activation of central nervous structures involved in the perception and integration of thermo- and osmoregulatory signals was investigated in the Sabra rat. Male rats were either non-treated (C-E), water-deprived for 24 h (C-D), short-term acclimated to 34 degrees C for two days (STHA-E) or subjected to both stimuli (STHA-D). Immunoreactivity for c-Fos protein (Fos-IR) as marker for neuronal activation was quantified in (extra-)hypothalamic structures: organum vasculosum laminae terminalis (OVLT); subfornical organ (SFO); medial (MPA), ventromedial preoptic (VMPO) and lateral hypothalamic (LHA) areas; median preoptic (MnPO), magnocellular supraoptic (SON) and paraventricular (mPVN) nuclei; limbic lateral septal (LS) and thalamic paraventricular (PV) nuclei. Compared to C-E rats, dehydration markedly increased Fos-IR exclusively in neurons of the OVLT, SFO and MnPO known to be involved in osmoreception, in the mPVN and SON, and to a minor extent in the VMPO. The VMPO, MPA, LHA and LS-important (extra-)hypothalamic sites for the perception and integration within the thermoregulatory control circuit-exhibited intense elevation of Fos-IR upon short-term heat acclimation. Of all (extra-)hypothalamic structures involved in central osmoregulation, only the MnPO revealed heat-induced Fos-IR in numerous cells located preferentially in its rostral component. Thus, the MnPO proved to be activated during both thermal and osmotic stimulations applied separately. Subjected to the combined stress (STHA-D), most brain structures investigated showed striking Fos-IR due to thermally enhanced osmotic stimulation, with additive effects demonstrated in the MnPO. The data support differential central activation of c-fos expression due to thermal or osmotic stimulations, with the MnPO acting as putative integrative center for both autonomic control circuits.

Effect of AV3V lesions on Fos expression and cell size increases in magnocellular neurons of the rat hypothalamus during chronic dehydration

The effects of osmotic stimulation on Fos expression and cell size increase in the supraoptic nucleus were evaluated in intact, sham-operated, and AV3V-lesioned rats. Fos-positive neurons were found mainly in the AV3V regions and the hypothalamic magnocellular neurons in the forebrain in dehydrated intact rats. Intraperitoneal injection of hypertonic saline and chronic dehydration induced a significant increase in number of Fos-positive neurons in the supraoptic nucleus of intact and sham-operated rats. AV3V lesions completely abolished the expression of Fos in SON neurons of rats that were intraperitoneally injected with hypertonic saline and were chronically dehydrated. Chronic dehydration increased significantly cell size of the OXT and AVP magnocelluar neurons in intact and sham-operated rats. However, there was no increase in cell size of those in the AV3V-lesioned rats. These results demonstrate that neural input derived from AV3V regions plays a significant role in causing Fos expression and structural changes such as cell size increase in the hypothalamic magnocellular neurons with osmotic stimulation.