Fos expression in non-catecholaminergic neurons in medullary and pontine nuclei after volume depletion induced by polyethylene glycol

Early oxytocin inhibition of salt intake after furosemide treatment in rats?

Body fluid homeostasis requires a complex suite of physiological and behavioral processes. Understanding of the role of the central nervous system (CNS) in integrating these processes has been advanced by research employing immunohistochemical techniques to assess responses to a variety of body fluid challenges. Such techniques have revealed sex/estrogen differences in CNS activation in response to hypotension and hypernatremia. In contrast, it has been difficult to conclusively identify specific CNS areas and neurotransmitter systems that are activated by hyponatremia using these techniques. In part, this difficulty is due to the temporal disconnect between the physiological effects of treatments commonly used to deplete body sodium and the behavioral response to such depletion. In some methods, sodium ingestion is delayed in association with increased oxytocin (OT), suggesting an inhibitory role for OT in sodium intake. Urinary sodium loss increases within an hour after treatment with furosemide, a natriuretic-diuretic, but sodium intake is delayed for 18-24h. Accordingly, we hypothesized that acute furosemide-induced sodium loss activates centrally-projecting OT neurons which provide an initial inhibition of sodium intake, and tested this hypothesis in ovariectomized Sprague-Dawley rats with or without estrogen using immunohistochemical methods. Neuronal activation in the hypothalamic paraventricular nuclei (PVN) after administration of furosemide corresponded to the timing of the physiological effects. The activation was not different in estrogen-treated rats, nor did estrogen alter the initial suppression of sodium intake. However, virtually no fos immunoreactive (fos-IR) neurons in the parvocellular PVN were also immunolabeled for OT. Thus, acute sodium loss after furosemide produces neural activation and an early inhibition of sodium intake that does not appear to involve activation of centrally-projecting OT neurons and is not influenced by estrogen.

A role for the lateral parabrachial nucleus in cardiovascular function and fluid homeostasis

The lateral parabrachial nucleus (LPBN) is located in an anatomical position that enables it to perform a critical role in relaying signals related to the regulation of fluid and electrolyte intake and cardiovascular function from the brainstem to the forebrain. Early neuroanatomical studies have described the topographic organization of blood pressure sensitive neurons and functional studies have demonstrated a major role for the LPBN in regulating cardiovascular function, including blood pressure, in response to hemorrhages, and hypovolemia. In addition, inactivation of the LPBN induces overdrinking of water in response to a range of dipsogenic treatments primarily, but not exclusively, those associated with endogenous centrally acting angiotensin II. Moreover, treatments that typically cause water intake stimulate salt intake under some circumstances particularly when serotonin receptors in the LPBN are blocked. This review explores the expanding body of evidence that underlies the complex neural network within the LPBN influencing salt appetite, thirst and the regulation of blood pressure. Importantly understanding the interactions among neurons in the LPBN that affect fluid balance and cardiovascular control may be critical to unraveling the mechanisms responsible for hypertension.

Regional differences in serotonin content in the nucleus of the solitary tract of male rats after hypovolemia produced by polyethylene glycol

Serotonin (5-HT) has been implicated in centrally mediated compensatory responses to volume loss in rats. Accordingly, we hypothesized that slowly developing, non-hypotensive hypovolemia increases serotonin in the hindbrain nucleus of the solitary tract (NTS). We produced volume loss in adult male rats by administering hyperoncotic polyethylene glycol (PEG) and then assessed 5-HT levels in the NTS using measurements of tissue 5-HT content or 5-HT immunohistochemistry. The results show selective increases of 5-HT in the caudal NTS after PEG treatment, but no change in the primary 5-HT metabolite, 5-HIAA. Moreover, the intensity of 5-HT immunolabeled fibers in the caudal NTS was increased after PEG treatment. These findings suggest that, after PEG-induced hypovolemia, 5-HT accumulates in neural elements in the caudal NTS. We propose that this accumulation is attributable to an initial release of 5-HT that then acts at presynaptic autoreceptors to inhibit subsequent 5-HT release.

Estradiol selectively reduces central neural activation induced by hypertonic NaCl infusion in ovariectomized rats

We recently reported that the latency to begin drinking water during slow, intravenous infusion of a concentrated NaCl solution was shorter in estradiol-treated ovariectomized rats compared to oil vehicle-treated rats, despite comparably elevated plasma osmolality. To test the hypothesis that the decreased latency to begin drinking is attributable to enhanced detection of increased plasma osmolality by osmoreceptors located in the CNS, the present study used immunocytochemical methods to label fos, a marker of neural activation. Increased plasma osmolality did not activate the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), or the nucleus of the solitary tract (NTS) in either oil vehicle-treated rats or estradiol-treated rats. In contrast, hyperosmolality increased fos labeling in the area postrema (AP), the paraventricular nucleus of the hypothalamus (PVN) and the rostral ventrolateral medulla (RVLM) in both groups; however, the increase was blunted in estradiol-treated rats. These results suggest that estradiol has selective effects on the sensitivity of a population of osmo-/Na(+)-receptors located in the AP, which, in turn, alters activity in other central areas associated with responses to increased osmolality. In conjunction with previous reports that hyperosmolality increases blood pressure and that elevated blood pressure inhibits drinking, the current findings of reduced activation in AP, PVN, and RVLM-areas involved in sympathetic nerve activity-raise the possibility that estradiol blunts HS-induced blood pressure changes. Thus, estradiol may eliminate or reduce the initial inhibition of water intake that occurs during increased osmolality, and facilitate a more rapid behavioral response, as we observed in our recent study.

Origins of arousal: roles for medullary reticular neurons

The existence of a primitive CNS function involved in the activation of all vertebrate behaviors, generalized arousal (GA), has been proposed. Here, we provide an overview of the neuroanatomical, neurophysiological and molecular properties of reticular neurons within the nucleus gigantocellularis (NGC) of the mammalian medulla, and propose that the properties of these neurons equip them to contribute powerfully to GA. We also explore the hypothesis that these neurons may have evolved from the Mauthner cell in the medulla of teleost fish, although NGC neurons have a wider range of action far beyond the specific escape network served by Mauthner cells. Understanding the neuronal circuits that control and regulate GA is central to understanding how motivated behaviors such as hunger, thirst and sexual behaviors arise.

Multimodal sensory responses of nucleus reticularis gigantocellularis and the responses' relation to cortical and motor activation

The connectivity of large neurons of the nucleus reticularis gigantocellularis (NRGc) in the medullary reticular formation potentially allows both for the integration of stimuli, in several modalities, that would demand immediate action, and for coordinated activation of cortical and motoric activity. We have simultaneously recorded cortical local field potentials, neck muscle electromyograph (EMG), and the neural activity of medullary NRGc neurons in unrestrained, unanesthetized rats to determine whether the activity of the NRGc is consistent with the modulation of general arousal. We observed excitatory responses of individual NRGc neurons to all modalities tested: tactile, visual, auditory, vestibular, and olfactory. Excitation was directly linked to increases in neck muscle EMG amplitude and corresponded with increases in the power of fast oscillations (30 to 80 Hz) of cortical activity and decreases in the power of slow oscillations (2 to 8 Hz). Because these reticular formation neurons can respond to broad ranges of stimuli with increased firing rates associated with the initiation of behavioral responses, we infer that they are part of an elementary "first responder" CNS arousal mechanism.

Oestrogen affects the cardiovascular and central responses to isoproterenol of female rats

This study examined the influence of oestrogen on cardiovascular responses to hypotension produced by administration of isoproterenol (Isop) and on neural activation in hindbrain nuclei mediating these responses. We first measured mean arterial pressure (MAP) and heart rate (HR) after administration of isoproterenol, a beta-adrenergic agonist that increases circulating levels of AngII, in ovariectomized (OVX) rats treated with oestradiol benzoate (EB). We then evaluated EB effects on Isop-induced Fos immunoreactivity (Fos-IR) in the hindbrain baroreflex circuit. To control for weight loss associated with oestrogen replacement in OVX rats, we food restricted a separate group of OVX rats and evaluated Isop-induced changes in MAP, HR and Fos-IR. The depressor response to Isop was significantly attenuated by EB, which also produced a disproportionate increase in HR. These effects were not secondary to loss of body weight after EB treatment, because cardiovascular responses to Isop in food restricted rats were similar to those in OVX rats treated with the oil vehicle. Isop significantly increased Fos-IR in the nucleus of the solitary tract (NTS), area postrema (AP), rostral ventrolateral medulla (RVLM), and lateral parabrachial nucleus (lPBN); however, EB significantly attenuated the increase in the AP and in the lPBN. Again, these effects were not secondary to body weight loss, because food restricted rats had the same pattern of Fos-IR as did rats treated with the oil vehicle. These results suggest that EB modifies cardiovascular responses to Isop, possibly by decreasing activation of the AP and lPBN.

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.

Influence of dehydration on the expression of neuropeptide Y Y1 receptors in hypothalamic magnocellular neurons

Regulation of vasopressin (VP) and oxytocin (OT) secretion involves integration of neural signals from hypothalamic osmoreceptors, ascending catecholaminergic and peptidergic cell groups in the brain stem, and local and autoregulatory afferents. Neuropeptide Y (NPY) is one factor that stimulates the release of VP and OT from the supraoptic (SON) and paraventricular nuclei of the hypothalamus via activation of Y1 receptors (Y1R). The current studies were designed to assess the regulation and distribution of NPY Y1R expression in the SON of male rats that were either given 2% NaCl drinking water (24-72 h) or water deprived (48 h). Subjecting male rats to these conditions resulted in significant increases in both the number of cells expressing Y1R immunoreactivity (ir) and the amount of Y1R protein per cell within the SON. Y1R immunoreactivity was increased in the magnocellular but not medial parvocellular paraventricular nuclei, and Y1R mRNA levels were increased in the SON of salt-loaded rats. Subpopulations of both VP and OT cells in the hypothalamus express Y1R immunoreactivity and a greater percentage of VP-ir cells express Y1R after salt loading. To control for potential effects of dehydration-induced anorexia, a group of euhydrate animals was pair fed with animals consuming 2% NaCl. No detectable change in Y1R expression was observed in the SON of pair-fed animals, even though body weights were significantly lower than controls. These data demonstrate that NPY Y1R gene and protein expression are increased in the SON of salt-loaded and water-deprived animals and provide a mechanism whereby NPY can support VP/OT release during prolonged challenges to fluid homeostasis.

Oestrogen and weight loss decrease isoproterenol-induced Fos immunoreactivity and angiotensin type 1 mRNA in the subfornical organ of female rats

Studies from our laboratory and others show that oestrogen reduces angiotensin II (Ang II)-induced water intake by ovariectomized rats. Elimination of endogenous oestrogen by ovariectomy causes weight gain that can be reversed or prevented by oestrogen replacement. Changes in body weight modify cardiovascular responses to Ang II but whether such changes have similar effects on central and behavioural responses to Ang II is unknown. The goal of this study was to evaluate the contributions of oestrogen and weight loss to isoproterenol (isoprenaline; Iso)-induced Fos immunoreactivity (IR) and to angiotensin type 1 (AT1) receptor mRNA in forebrain regions implicated in the control of fluid balance. Isoproterenol significantly increased Fos IR in the hypothalamic paraventricular and supraoptic nuclei, the subfornical organ (SFO), and the organum vasculosum of the lamina terminalis, but had no effect on AT1 mRNA expression. However, both Iso-induced Fos IR and the AT1 mRNA were attenuated in the SFO of the oestrogen and weight loss groups compared with that of the control group. Consequently, we examined the effect of weight loss on Iso-induced water intake and plasma renin activity (PRA) and found that weight loss decreased water intake after Iso, but had no effect on PRA. Thus, we propose that weight loss decreases Ang II-elicited water intake in the female rat by down-regulating the expression of the AT1 receptor.

Characterisation of c-Fos expression in the central nervous system of mice following right atrial injections of the 5-HT3 receptor agonist phenylbiguanide

Cardiopulmonary receptors relay signals to the central nervous system via vagal and spinal visceral afferents. To date there are no detailed topographical studies in mice indicating the distribution of central neurones activated following stimulation of cardiopulmonary afferents. In anaesthetised mice, we injected the 5-HT(3) receptor agonist phenylbiguanide (PBG), a drug that is known to stimulate cardiopulmonary afferent C-fibres, into the right atrium of the heart and mapped c-Fos expression within specific regions of the central nervous system. Intra-atrial injection of PBG produced a reflex cardiorespiratory response including a pronounced bradycardia and a respiratory depression. Using immunohistochemical detection of the protein product of the immediate-early gene c-fos, we mapped the brain regions affected by cardiopulmonary 5-HT(3) receptor stimulation. Within the nucleus of the solitary tract (nTS) of PBG-injected mice, we detected an increased number of c-Fos-positive nuclei in the dorsolateral and gelatinous parts at the level of the area postrema (-7.48 mm bregma) but not at more rostral or caudal levels (-7.76, -7.20, -6.84 and -6.36 mm bregma) relative to vehicle-injected control mice. In addition, c-Fos expression in the crescent part of the lateral parabrachial nucleus was decreased in PBG-injected mice whereas no significant differences were detected between PBG-injected and control mice in the number of c-Fos-positive nuclei in the dorsal part of the lateral parabrachial nucleus. PBG injections had no significant effects on the number of c-Fos-positive catecholaminergic neurones within the C1/A1, C2/A2, A5, A6 and A7 cell groups. Likewise, PBG injections had no significant effects on c-Fos expression in other central regions involved in cardiorespiratory control or cardiorespiratory reflexes (selected non-catecholaminergic nuclei in the medulla and midbrain periaqueductal gray, the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala). Identification of specific regions of the nTS complex involved in relaying signals from afferent cardiopulmonary C-fibres to the central nervous system will be useful for future studies aimed at understanding neural mechanisms underlying cardiopulmonary reflexes and physiological responses to cardiopulmonary disease.

Catecholamine and oxytocin cells respond to hypovolaemia as well as hypotension

Medullary catecholamine and hypothalamic neurosecretory oxytocin cells are activated by hypotension, but previous studies have provided uncertain outcomes concerning their ability to respond to a purely hypovolaemic stimulus. In the present study, injections of PEG/water and pentolinium were used to elicit non-hypotensive, isosmotic hypovolaemia and isovolaemic, isosmotic hypotension, respectively, in conscious rats. Animals were sacrificed 2 h after treatment. Immunolabelling for Fos, tyrosine hydroxylase and oxytocin established that these two stimuli activate almost identical populations of catecholamine neurons in the ventrolateral and dorsomedial medulla, and very similar populations of oxytocin cells in the supraoptic and paraventricular nuclei of the hypothalamus.