Use of c-fos functional mapping to identify the central baroreceptor reflex pathway: advantages and limitations

Brainstem sources of input to the central mesencephalic reticular formation in the macaque

Physiological studies indicate that the central mesencephalic reticular formation (cMRF) plays a role in gaze changes, including control of disjunctive saccades. Neuroanatomical studies have demonstrated strong interconnections with the superior colliculus, along with projections to extraocular motor nuclei, the preganglionic nucleus of Edinger-Westphal, the paramedian pontine reticular formation, nucleus raphe interpositus, medullary reticular formation and cervical spinal cord, as might be expected for a structure that is intimately involved in gaze control. However, the sources of input to this midbrain structure have not been described in detail. In the present study, the brainstem cells of origin supplying the cMRF were labeled by retrograde transport of tracer (wheat germ agglutinin conjugated horseradish peroxidase) in macaque monkeys. Within the diencephalon, labeled neurons were noted in the ventromedial nucleus of the hypothalamus, pregeniculate nucleus and habenula. In the midbrain, labeled cells were found in the substantia nigra pars reticulata, medial pretectal nucleus, superior colliculus, tectal longitudinal column, periaqueductal gray, supraoculomotor area, and contralateral cMRF. In the pons they were located in the paralemniscal zone, parabrachial nucleus, locus coeruleus, nucleus prepositus hypoglossi and the paramedian pontine reticular formation. Finally, in the medulla they were observed in the medullary reticular formation. The fact that this list of input sources is very similar to those of the superior colliculus supports the view that the cMRF represents an important gaze control center.

Both central sympathoexcitation and peripheral angiotensin II-dependent vasoconstriction contribute to hypertension development in immature heterozygous Ren-2 transgenic rats

Recently, we demonstrated that chronic blockade of the renin-angiotensin system (RAS) lowered the blood pressure (BP) of adult Ren-2 transgenic rats (TGR) mainly through the attenuation of central sympathoexcitation. However, the participation of central and peripheral mechanisms in the development of high BP in immature TGR remains unclear. In the present study, 6-week-old heterozygous TGR males were chronically treated with intracerebroventricular (ICV) or intraperitoneal (IP) infusions of the AT₁ receptor inhibitor losartan (1 or 2 mg/kg/day) for 4 weeks. The influence of these treatments on sympathetic- and angiotensin II-dependent BP components (BP response to pentolinium or captopril, respectively) as well as on BP response to exogenous angiotensin II were determined to evaluate the participation of central and peripheral RAS in hypertension development. Chronic IP losartan administration (1 or 2 mg/kg/day) lowered the BP of immature TGR by reducing both sympathetic and angiotensin II-dependent BP components. The central action of IP-administered losartan was indicated by a reduced BP response to acute ICV angiotensin II injection. Chronic ICV administration of a lower losartan dose (1 mg/kg/day) reduced only the sympathetic BP component, whereas a higher ICV administered dose (2 mg/kg/day) was required to influence the angiotensin II-dependent BP component. Accordingly, chronic ICV losartan administration of 2 mg/kg/day (but not 1 mg/kg/day) attenuated the BP response to acute intravenous angiotensin II application. In conclusion, central sympathoexcitation seems to play an important role in hypertension development in immature TGR. Central sympathoexcitation is highly susceptible to inhibition by low doses of RAS-blocking agents, whereas higher doses also affect peripheral angiotensin II-dependent vasoconstriction.

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.

The neural pathway for lacrimal gland tear secretion in New Zealand White rabbits

OBJECTIVE

We investigated the neural pathway for tear secretion from the lacrimal gland of New Zealand White rabbits.

METHODS

Nine healthy adult New Zealand White rabbits were randomly divided into three experimental groups, namely, an irritant-stimulated group, a non-stimulated group, and a saline-stimulated group. Sanitized dry cotton swabs with menthol were used to wipe both of the rabbits' eyelids in the irritant-stimulated group, and the non-stimulated group and saline- stimulated group were compared as controls. The animals in the three groups were killed 2h later and the expressions of c-Fos in the frontal cortex, hippocampus, hypothalamus, pons, and medulla oblongata of the rabbits were detected using immunofluorescence labeling. According to the distribution of c-Fos protein expression, 12 healthy adult New Zealand rabbits were similarly divided into three groups for retrograde tract tracing via pseudorabies virus (PRV) injection into the lacrimal gland. Immunofluorescence labeling was used to analyze PRV-infected neurons in the brains of rabbits after survival for 30h, 38h, and 46h.

RESULTS

The most c-Fos-positive immunolabeled cells were observed in the menthol-stimulated group, whereas fewer c-Fos-positive immunolabeled cells were observed in the saline-stimulated group.The non-treated group showed the least c-Fos-positive immunolabeled cells. At 30h after PRV injection, PRV-positive neurons were found only in the superior salivary nucleus of the pons (SSN). At 38h, PRV-infected neurons were observed in the lateral nucleus of the superior olive (LSO) and the medial nucleus of the superior olive (MSO). At 46h, PRV-infected neurons were found in the nucleus of the trapezoid body (Tz) and the hypothalamic paraventricular nucleus (PVN), and their distributions were dense in the LSO and MSO.

CONCLUSIONS

Menthol-induced c-Fos protein expression and PRV-mediated tract tracing suggest that in New Zealand White rabbits, the neural pathway that regulates tear secretion from the lacrimal gland proceeds from the PVN to the superior olivary complex of the pons to the SSN and finally to the lacrimal gland.

CNS sites activated by renal pelvic epithelial sodium channels (ENaCs) in response to hypertonic saline in awake rats

In some patients, renal nerve denervation has been reported to be an effective treatment for essential hypertension. Considerable evidence suggests that afferent renal nerves (ARN) and sodium balance play important roles in the development and maintenance of high blood pressure. ARN are sensitive to sodium concentrations in the renal pelvis. To better understand the role of ARN, we infused isotonic or hypertonic NaCl (308 or 500mOsm) into the left renal pelvis of conscious rats for two 2hours while recording arterial pressure and heart rate. Subsequently, brain tissue was analyzed for immunohistochemical detection of the protein Fos, a marker for neuronal activation. Fos-immunoreactive neurons were identified in numerous sites in the forebrain and brainstem. These areas included the nucleus tractus solitarius (NTS), the lateral parabrachial nucleus, the paraventricular nucleus of the hypothalamus (PVH) and the supraoptic nucleus (SON). The most effective stimulus was 500mOsm NaCl. Activation of these sites was attenuated or prevented by administration of benzamil (1μM) or amiloride (10μM) into the renal pelvis concomitantly with hypertonic saline. In anesthetized rats, infusion of hypertonic saline but not isotonic saline into the renal pelvis elevated ARN activity and this increase was attenuated by simultaneous infusion of benzamil or amiloride. We propose that renal pelvic epithelial sodium channels (ENaCs) play a role in activation of ARN and, via central visceral afferent circuits, this system modulates fluid volume and peripheral blood pressure. These pathways may contribute to the development of hypertension.

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.

Molecular neuroimaging of post-injury plasticity

Nerve injury induces long-term changes in neuronal activity in the primary somatosensory cortex (S1), which has often been implicated as the origin of sensory dysfunction. However, the cellular mechanisms underlying this phenomenon remain unclear. C-fos is an immediate early gene, which has been shown to play an instrumental role in plasticity. By developing a new platform to image real-time changes in gene expression in vivo, we investigated whether injury modulates the levels of c-fos in layer V of S1, since previous studies have suggested that these neurons are particularly susceptible to injury. The yellow fluorescent protein, ZsYellow1, under the regulation of the c-fos promoter, was expressed throughout the rat brain. A fiber-based confocal microscope that enabled deep brain imaging was utilized, and local field potentials were collected simultaneously. In the weeks following limb denervation in adult rats (n=10), sensory stimulation of the intact limb induced significant increases in c-fos gene expression in cells located in S1, both contralateral (affected, 27.6±3 cells) and ipsilateral (8.6±3 cells) to the injury, compared to controls (n=10, 13.4±3 and 1.0±1, respectively, p value<0.05). Thus, we demonstrated that injury activates cellular mechanisms that are involved in reshaping neuronal connections, and this may translate to neurorehabilitative potential.

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.

Neurocardiac dysregulation and neurogenic arrhythmias in a transgenic mouse model of Huntington's disease

Huntington's disease (HD) is a heritable neurodegenerative disorder, with heart disease implicated as one major cause of death. While the responsible mechanism remains unknown, autonomic nervous system (ANS) dysfunction may play a role. We studied the cardiac phenotype in R6/1 transgenic mice at early (3 months old) and advanced (7 months old) stages of HD. While exhibiting a modest reduction in cardiomyocyte diameter, R6/1 mice had preserved baseline cardiac function. Conscious ECG telemetry revealed the absence of 24-h variation of heart rate (HR), and higher HR levels than wild-type littermates in young but not older R6/1 mice. Older R6/1 mice had increased plasma level of noradrenaline (NA), which was associated with reduced cardiac NA content. R6/1 mice also had unstable R-R intervals that were reversed following atropine treatment, suggesting parasympathetic nervous activation, and developed brady- and tachyarrhythmias, including paroxysmal atrial fibrillation and sudden death. c-Fos immunohistochemistry revealed greater numbers of active neurons in ANS-regulatory regions of R6/1 brains. Collectively, R6/1 mice exhibit profound ANS-cardiac dysfunction involving both sympathetic and parasympathetic limbs, that may be related to altered central autonomic pathways and lead to cardiac arrhythmias and sudden death.

Comparison of ΔFosB immunoreactivity induced by vagal nerve stimulation with that caused by pharmacologically diverse antidepressants

Vagal nerve stimulation (VNS) has been approved for treatment of refractory depression. However, there have been few, if any, studies directly comparing the effects produced by VNS in animals with those caused by antidepressants, particularly using clinically relevant stimulation parameters in nonanesthetized animals. In this study, ΔFosB immunohistochemistry was used to evaluate different brain regions activated by long-term administration of VNS. Effects of VNS were compared with those caused by sertraline or desipramine (DMI). Double-labeling of ΔFosB and serotonin was used to determine whether serotonergic neurons in the dorsal raphe nucleus (DRN) were activated by long-term VNS. VNS significantly increased ΔFosB staining in the nucleus tractus solitarius (NTS), parabrachial nucleus (PBN), locus ceruleus (LC), and DRN, as well as in many cortical and limbic areas of brain including those involved in mood and cognition. Most, but not all, of these effects were seen also upon long-term treatments of rats with sertraline or DMI. Some areas where VNS increased ΔFosB (e.g., the NTS, PBN, LC, and peripeduncular nucleus) were not affected significantly by either drug. Sertraline was similar to VNS in causing an increase in the DRN whereas DMI did not. Double-labeling of the DRN with ΔFosB and an antibody for serotonin revealed that only a small percentage of ΔFosB staining in the DRN colocalized with serotonergic neurons. The effects of VNS were somewhat more widespread than those caused by the antidepressants. The increases in ΔFosB produced by VNS were either equivalent to and/or more robust than those seen with antidepressants.

Gene expression profiling in rodent models for schizophrenia

The complex neurodevelopmental disorder schizophrenia is thought to be induced by an interaction between predisposing genes and environmental stressors. In order to get a better insight into the aetiology of this complex disorder, animal models have been developed. In this review, we summarize mRNA expression profiling studies on neurodevelopmental, pharmacological and genetic animal models for schizophrenia. We discuss parallels and contradictions among these studies, and propose strategies for future research.

Exposure to a high-fat diet alters leptin sensitivity and elevates renal sympathetic nerve activity and arterial pressure in rabbits

The activation of the sympathetic nervous system through the central actions of the adipokine leptin has been suggested as a major mechanism by which obesity contributes to the development of hypertension. However, direct evidence for elevated sympathetic activity in obesity has been limited to muscle. The present study examined the renal sympathetic nerve activity and cardiovascular effects of a high-fat diet (HFD), as well as the changes in the sensitivity to intracerebroventricular leptin. New Zealand white rabbits fed a 13.5% HFD for 4 weeks showed modest weight gain but a 2- to 3-fold greater accumulation of visceral fat compared with control rabbits. Mean arterial pressure, heart rate, and plasma norepinephrine concentration increased by 8%, 26%, and 87%, respectively (P<0.05), after 3 weeks of HFD. Renal sympathetic nerve activity was 48% higher (P<0.05) in HFD compared with control diet rabbits and was correlated to plasma leptin (r=0.87; P<0.01). Intracerebroventricular leptin administration (5 to 100 microg) increased mean arterial pressure similarly in both groups, but renal sympathetic nerve activity increased more in HFD-fed rabbits. By contrast, intracerebroventricular leptin produced less neurons expressing c-Fos in HFD compared with control rabbits in regions important for appetite and sympathetic actions of leptin (arcuate: -54%, paraventricular: -69%, and dorsomedial hypothalamus: -65%). These results suggest that visceral fat accumulation through consumption of a HFD leads to marked sympathetic activation, which is related to increased responsiveness to central sympathoexcitatory effects of leptin. The paradoxical reduction in hypothalamic neuronal activation by leptin suggests a marked "selective leptin resistance" in these animals.

Delta 9-tetrahydrocannabinol suppresses vomiting behavior and Fos expression in both acute and delayed phases of cisplatin-induced emesis in the least shrew

Cisplatin chemotherapy frequently causes severe vomiting in two temporally separated clusters of bouts dubbed the acute and delayed phases. Cannabinoids can inhibit the acute phase, albeit through a poorly understood mechanism. We examined the substrates of cannabinoid-mediated inhibition of both the emetic phases via immunolabeling for serotonin, Substance P, cannabinoid receptors 1 and 2 (CB(1), CB(2)), and the neuronal activation marker Fos in the least shrew (Cryptotis parva). Shrews were injected with cisplatin (10mg/kg i.p.), and one of vehicle, Delta(9)-THC, or both Delta(9)-THC and the CB(1) receptor antagonist SR141716A (2mg/kg i.p.), and monitored for vomiting. Delta(9)-THC-pretreatment caused concurrent decreases in the number of shrews expressing vomiting and Fos-immunoreactivity (Fos-IR), effects which were blocked by SR141716A-pretreatment. Acute phase vomiting induced Fos-IR in the solitary tract nucleus (NTS), dorsal motor nucleus of the vagus (DMNX), and area postrema (AP), whereas in the delayed phase Fos-IR was not induced in the AP at all, and was induced at lower levels in the other nuclei when compared to the acute phase. CB(1) receptor-IR in the NTS was dense, punctate labeling indicative of presynaptic elements, which surrounded Fos-expressing NTS neurons. CB(2) receptor-IR was not found in neuronal elements, but in vascular-appearing structures. All areas correlated with serotonin- and Substance P-IR. These results support published acute phase data in other species, and are the first describing Fos-IR following delayed phase emesis. The data suggest overlapping but separate mechanisms are invoked for each phase, which are sensitive to antiemetic effects of Delta(9)-THC mediated by CB(1) receptors.

Dynamic transcriptomic response to acute hypertension in the nucleus tractus solitarius

Baroreceptor afferents project to the cardiovascular region of the nucleus tractus solitarius (cvNTS), and their cvNTS target neurons may play a role in governing the sensitivity and operating range of the arterial baroreceptor reflex (baroreflexes). Recent studies have shown differential gene and protein expression in the cvNTS in response to changed arterial pressure. However, the extent of these responses is unknown. Therefore, we collected differential global gene expression data in a time series following acute hypertension in awake, freely moving rats. To acquire statistically significant results and place them in functional context, we overcame several quality control requirements and developed novel analytical approaches. The physiologically new findings from the study are that acute hypertension causes very extensive, time-varying gene regulatory changes, many involving neuronal function-specific genes and systems of genes. We use standard genomic analysis methods to manage the large data sets and to develop results such as heat maps to examine patterns and clusters in the gene regulation. We used the Gene Ontology categories to provide functional context. To place our findings in the context of the relevant literature, we developed two graphical representations of the networks implicated, linking receptors and channels to signaling pathways. The results point to the multivariate complexity of the response and implicate a group of receptors as candidates for mediating nucleus tractus solitarius baroreflex function in hypertension by identifying concurrent upregulation of receptor genes. We were able to make transcription factor binding predictions and record dysregulation of heart rate correlated with the transcriptional response.

Fos-like immunoreactivity in Siberian hamster brain during initiation of torpor-like hypothermia induced by 2DG

Systemic 2-deoxy-d-glucose (2DG) produces pronounced torpor-like hypothermia (not< approximately 15 degrees C) in the Siberian hamster. Siberian hamsters are heterothermic, naturally undergoing photoperiod-dependent torpor during winter-like photoperiods. Fos was used to identify neural structures activated during the initiation of torpor-like hypothermia induced by 2DG treatment. The Fos-like immunoreactivity (Fos-li) in the area postrema and nucleus of the solitary tract that predominantly characterizes other 2DG-induced responses was absent during 2DG-induced torpor in the present experiment. Fos-li was seen in a number of forebrain and hindbrain sites during entry into hypothermia, but the densest Fos-li was found in the parvocellular portion of the paraventricular nucleus. Fos-li in the medial nucleus of the amygdala and the dorsal lateral septum also distinguished 2DG-induced torpor from other 2DG-induced behaviors. The possible involvement of neuropeptide Y pathways during 2DG-induced expression of reversible hypothermia is discussed.

Fos-Related Antigen Immunoreactivity After Acute and Chronic Angiotensin II–Induced Hypertension in the Rabbit Brain

Several brain regions are proposed as contributing to chronic sympatho-excitatory effects of elevated circulating angiotensin II. However, earlier c-Fos studies have been limited to acute angiotensin II exposure. This study aims to determine brain regions responding with chronic elevated angiotensin II. Rabbits were administered angiotensin II (50 ng/kg per minute) or saline for 3 hours, 3 days, or 14 days. Basal mean arterial pressure was 71±2 mm Hg and increased 23±2 mm Hg, 32±4 mm Hg, and 22±2 mm Hg for 3 hours, 3 days, and 14 days, respectively, with angiotensin II infusion. Neuronal activation was detected using Fos-related antigens, which recognizes all of the known members of the Fos family. Neurons located in the amygdala and area postrema were activated transiently after acute infusion of angiotensin II but were not responsive by days 3 or 14. Neurons located in the nucleus of the solitary tract, caudal ventrolateral medulla, and lateral parabrachial nucleus were activated for ≤3 days after infusion of angiotensin II but were not responsive by day 14, which is consistent with their role in response to baroreceptor pathways that reset with sustained hypertension. The vascular organ of the lamina terminalis and subfornical organ showed sustained but diminishing activation over the 14-day period. However, the downstream hypothalamic nuclei that receive inputs from these nuclei, the paraventricular, supraoptic, and arcuate nuclei, showed marked sustained activation. These findings suggest that there is desensitization of circumventricular organs but sensitization of neurons in hypothalamic regions to long-term angiotensin II infusion.

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.

Hypotension and short-term anaesthesia induce ERK1/2 phosphorylation in autonomic nuclei of the brainstem

The aims of this study were: first, to investigate the effects of anaesthesia on phosphorylated extracellular signal-regulated kinase (p-ERK)1/2-immunoreactivity (-ir) in the brainstem; second, to choose the best anaesthetic for p-ERK1/2 studies; and third, to determine the effect of short-term hypotension on p-ERK1/2-ir in the brainstem. Rats were anaesthetized with halothane, sodium pentobarbital or 100% CO2 narcosis, or were cervically dislocated and within 5 min perfused and the brains processed immunohistochemically for pERK1/2-ir. p-ERK1/2-ir was primarily observed in regions associated with cardiovascular and/or respiratory control. Several regions consistently showed dense p-ERK1/2 labelling, including a restricted region of the ventrolateral medulla (VLM). In contrast, other regions showed differential labelling depending on the mode of death. Cervical dislocation showed the least VLM labelling, limited to a discrete area approximately 0.6-1.4 mm caudal to the facial nucleus. Anaesthetics induced labelling throughout the VLM, with halothane inducing the most. Many p-ERK1/2-ir VLM neurons were catecholaminergic following halothane or sodium pentobarbital anaesthesia, but no double labelling was seen following cervical dislocation. Of the anaesthetics, sodium pentobarbital induced the least labelling and was used subsequently. Intravenous hydralazine was used to induce a 20-min period of hypotension, whereas arterial pressure did not change in vehicle-treated animals. Hydralazine evoked more pERK-ir neurons in specific regions, including the VLM, nucleus tractus solitarius (NTS), parabrachial nuclei, Kolliker-Fuse nucleus and locus coeruleus. Approximately twice as many p-ERK1/2-positive neurons were seen in the intermediate NTS and rostral VLM following hydralazine compared with the vehicle. In conclusion, p-ERK1/2-ir identifies neurons in central autonomic regions, and their number and distribution are markedly affected by anaesthetics, and are increased in some regions by short-term hypotension.

The effects of osmotic stimulation and water availability on c-Fos and FosB staining in the supraoptic and paraventricular nuclei of the hypothalamus

We studied the effects of osmotic stimulation on the expression of FosB and c-Fos in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). Adult male rats were divided into two groups that were injected with lidocaine (0.1-0.2 ml sc) followed by either 0.9% or 6% NaCl (1 ml/100 g bw sc). After the NaCl injections, the rats were anesthetized and perfused 2, 6, or 8 h after the injections. Their brains were prepared for immunocytochemistry and stained with FosB and c-Fos antibodies. The number of c-Fos-positive cells was significantly increased only at 2 h in the SON and PVN. In contrast, the number of FosB-positive cells was significantly increased at 6, and 8 h in both the SON and PVN. In a second experiment, the effect of water availability on FosB staining 8 h after injections of 6% NaCl was tested in 3 groups of rats: water ad libitum, rats that had no access to water, and rats that were given water 2 h prior to perfusion. FosB staining was significantly reduced in both the SON and the PVN of rats that had ad libitum water compared to the two water-restricted groups. In the third experiment, rats were injected with either 0.9% NaCl or 6% NaCl and were either given ad libitum access to water or water restricted for 6 h after the injections and perfused 24 h after the saline injections. FosB staining was not increased when water was available ad libitum. FosB staining was significantly increased at 24 h in the rats injected with 6% NaCl when water was restricted. Thus, FosB may continue to influence protein expression in the SON and PVN for at least 24 h following acute osmotic stimulation.

Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit

In the present study we used the expression of the c-Fos-like protein as a "functional marker" to map populations of brainstem neurons involved in the generation of mastication. Experiments were conducted on urethane-anesthetized and paralyzed rabbits. In five animals (experimental group), rhythmical bouts of fictive masticatory-like motoneuron activity (cumulative duration 60-130 min) were induced by electrical stimulation of the left cortical "masticatory area" and recorded from the right digastric motoneuron pool. A control group of five animals (non-masticatory) were treated in the same way as the experimental animals with regard to surgical procedures, anesthesia, paralysis, and survival time. To detect the c-Fos-like protein, the animals were perfused, and the brainstems were cryosectioned and processed immunocytochemically. In the experimental group, the number of c-Fos-like immunoreactive neurons increased significantly in several brainstem areas. In rostral and lateral areas, increments occurred bilaterally in the borderzones surrounding the trigeminal motor nucleus (Regio h); the rostrodorsomedial half of the trigeminal main sensory nucleus; subnucleus oralis-gamma of the spinal trigeminal tract; nuclei reticularis parvocellularis pars alpha and nucleus reticularis pontis caudalis (RPc) pars alpha. Further caudally-enhanced labeling occurred bilaterally in nucleus reticularis parvocellularis and nucleus reticularis gigantocellularis (Rgc) including its pars-alpha. Our results provide a detailed anatomical record of neuronal populations that are correlated with the generation of the masticatory motor behavior.

Intraventricular injections of tachykinin NK3 receptor agonist reduce the gain of the baroreflex in unrestrained rats

The tachykinin neuropeptides acting at NK3 receptors affect mean arterial pressure (MAP) through both neuroendocrine and neural mechanisms. NK3 receptors are found in brainstem nuclei that mediate the baroreflex, but the effects of NK3 receptor stimulation on baroreflex function are unknown. The present study tests the effects of intraventricular injections of senktide, a selective NK3 receptor agonist, on the sensitivity of the baroreflex in three stains of rats: Charles River Laboratory, Long-Evans, and Brattleboro rats, which lack the ability to synthesize vasopressin. Rats with lateral ventricle cannulas were administered injections of isotonic saline, 100 ng, or 200 ng senktide, and 5 min later arterial baroreceptor-heart rate (HR) function was examined by constructing full-range blood pressure-HR curves using alternating doses (5-20 microg kg min) of phenylephrine and nitroprusside to raise and decrease blood pressure approximately 50 mm Hg over a period of 1 min, respectively. Intraventricular injections of 200 ng senktide had no significant effect on baseline MAP, but significantly decreased the gain of the baroreflex in all three rat strains whereas the 100 ng dose had no effect on the baroreflex. These results show that NK3 receptor stimulation modulates the baroreflex that is independent of any action of vasopressin.

Effects of water deprivation and rehydration on c-Fos and FosB staining in the rat supraoptic nucleus and lamina terminalis region

We studied cFos and FosB staining in the supraoptic nucleus (SON) the organum vasculosum of the lamina terminalis (OVLT) and the median preoptic nucleus (MnPO) in adult male rats after water deprivation (24 h, n = 11; 48 h, n = 12) and water deprivation with rehydration (22 h + water, n = 11; 46 h + water, n = 10). Control rats ( n = 15) had water available ad libitum. Separate sets of serial sections from each brain were processed for immunocytochemistry using primary antibodies against either c-Fos or FosB protein. Plasma osmolality, vasopressin, hematocrit, and plasma proteins were measured in separate groups ( n = 6–7). The number of c-Fos-positive cells in the SON was significantly increased after 24 and 48 h of water deprivation. In contrast, rehydrated groups were not different from control. Water deprivation significantly increased c-Fos staining in both the OVLT and the MnPO, but c-Fos staining was not altered by rehydration. FosB staining in the SON was significantly increased only by 48-h water deprivation, and this effect was significantly decreased by rehydration. In the MnPO and OVLT, FosB staining was significantly increased by water deprivation, and, like c-Fos staining, these increases were not affected by rehydration. Water deprivation significantly increased osmolality and hematocrit, as well as plasma protein and vasopressin concentrations. Plasma measurements from rehydrated rats were not different from control. We conclude that water deprivation and rehydration differentially affect c-Fos and FosB staining in a region-dependent manner.

Enhanced activity of central adrenergic neurons in two-kidney, one clip hypertension in Sprague–Dawley rats

The present study was aimed at investigating whether two-kidney, one clip (2K1C) hypertension is associated with an enhanced central adrenergic activity. Rats were made 2K1C hypertensive, and the expression of Fos-like immunoreactivity (FLI) and phenylethanalamine N-methyltransferase (PNMT)-immunoreactivity was determined in the brain areas related to the cardiovascular regulation. In 2K1C hypertension, the basal Fos-immunoreactivity was significantly increased in rostral ventrolateral medulla (RVLM), paraventricular nucleus (PVN), and supraoptic nucleus (SON). PNMT-immunoreactivities were noted in RVLM, but not in PVN or SON. Intracerebroventricular administration of angiotensin II (AII) markedly increased Fos-immunoreactivities, the degree of which was greater in hypertension. Furthermore, AII increased the ratio of PNMT-positive/Fos-positive neurons in RVLM in hypertension. It is suggested that the responsiveness to AII of the central adrenergic system is enhanced in 2K1C hypertension.

Barosensitive neurons in the rat tractus solitarius and paratrigeminal nucleus: a new model for medullary, cardiovascular reflex regulation

The nucleus of the solitary tract (NTS), a termination site for primary afferent fibers from baroreceptors and other peripheral cardiovascular receptors, contains blood pressure-sensitive neurons, some of which have rhythmic activity locked to the cardiac cycle, making them key components of the central pathway for cardiovascular regulation. The paratrigeminal nucleus (Pa5), a small collection of medullary neurons in the dorsal lateral spinal trigeminal tract, like the NTS, receives primary somatosensory inputs of glossopharyngeal, vagal, and other nerves. Recent studies show that the Pa5 has efferent connections to the rostroventrolateral reticular nucleus (RVL), NTS, and ambiguus nucleus, suggesting that its structure may play a role in the baroreceptor reflex modulation. In the present study, simultaneous recording from multiple single neurons in freely behaving rats challenged with i.v. phenylephrine administration, showed that 83% of NTS units and 72% of Pa5 units were baroreceptor sensitive. Whereas most of the baroreceptor-sensitive NTS and Pa5 neurons (86 and 61%, respectively) increased firing rate during the ascending phase of the pressor response, about 16% of Pa5 and NTS baroreceptor-sensitive neurons had a decreased firing rate. On one hand, the decrease in firing rate occurred during the ascending phase of the pressor response, indicating sensitivity to rapid changes in arterial pressure. On the other hand, the increases in neuron activity in the Pa5 or NTS occurred during the entire pressor response to phenylephrine. Cross-correlational analysis showed that 71% of Pa5 and 93% of NTS baroreceptor-activated neurons possessed phasic discharge patterns locked to the cardiac cycle. These findings suggest that the Pa5, like the NTS, acts as a terminal for primary afferents in the medullary-baroreflex or cardiorespiratory-reflex pathways.Key words: cardiovascular reflexes, baroreflex response, arterial blood pressure, multiple single unit recording.

Detection of amino acid and peptide transmitters in physiologically identified brainstem cardiorespiratory neurons

Most of the CNS neurons that regulate circulation and respiration reside in regions of the brain characterized by extreme cellular heterogeneity (nucleus of the solitary tract, reticular formation, parabrachial nuclei, periaqueductal gray matter, hypothalamus, etc.). The chemical neuroanatomy of these regions is correspondingly complex and teasing out specific circuits in their midst remains a problem that is usually very difficult if not impossible to solve by conventional tract-tracing methods, Fos methodology or electrophysiology in slices. In addition, identifying the type of amino acid or peptide transmitter used by electrophysiologically recorded neurons has been until recently an especially difficult task either for lack of a specific marker or because such markers (many peptides for example) are exported to synaptic terminals and thus undetectable in neuronal cell bodies. In this review, we describe a general purpose method that solves many of these problems. The approach combines juxtacellular labeling in vivo with the histological identification of mRNAs that provide definitive neurochemical phenotypic identification (e.g. vesicular glutamate transporter 1 or 2, glutamic acid decarboxylase). The results obtained with this method are discussed in the general context of amino acid transmission in brainstem cardiorespiratory pathways. The presence of markers of amino acid transmission in specific aminergic pre-sympathetic neurons is especially emphasized as is the extensive co-localization of markers of GABAergic and glycinergic transmission in the brainstem reticular formation.

FosB expression in the central nervous system following isotonic volume expansion in unanesthetized rats

We examined c-Fos and FosB staining in the central nervous system 8 and 24 h following acute volume expansion in unanesthetized rats. Male rats were instrumented with a femoral artery catheter for measurement of blood pressure and heart rate (HR), a jugular venous catheter for measurement of central venous pressure (CVP), and a femoral vein catheter for i.v. infusion. After 48 h, rats were volume expanded with isotonic saline (10% of body weight for 10 min i.v.) or given a control infusion (0.01 ml/min for 10 min i.v.). After a period of 8 or 24 h, the rats were deeply anesthetized and perfused transcardially with 4% paraformaldehyde. Separate sets of serial sections of the hypothalamus were processed for either FosB (Santa Cruz) or c-Fos (Oncogene AB-5) immunocytochemistry. The volume expansion protocol significantly increased central venous pressure but did not affect blood pressure or heart rate. Volume expansion produced a significant increase in FosB-positive cells in the paraventricular nucleus (PVN) of the hypothalamus, the supraoptic nucleus (SON), the perinuclear zone (PNZ) of the supraoptic nucleus, the nucleus of the solitary tract (NST), and the caudal ventrolateral medulla (CVL) in both the 8- and 24-h groups. In the area postrema (AP), the number of FosB-positive cells was significantly increased only at 8 h post-infusion. However, c-Fos was not significantly increased above control levels at either time point. The results demonstrate that FosB activation is maintained for at least 24 h following an acute increase in central venous pressure.

Functional organization of brain pathways subserving the baroreceptor reflex: studies in conscious animals using immediate early gene expression

1. This paper reviews studies carried out in our laboratory in which we have used the c-fos functional mapping method, in combination with other methods, to determine the functional organization of central baroreceptor pathways as they operate in the conscious rabbit. 2. First, we showed that periods of induced hypertension or hypotension each result in a specific and reproducible pattern of activation of neurons in the brainstem and forebrain. In particular, hypotension (but not hypertension) results in the activation of catecholamine neurons in the medulla and pons and vasopressin-synthesizing neurons in the hypothalamus. 3. The activation of medullary cell groups in response to induced hypertension or hypotension in the conscious rabbit is almost entirely dependent on inputs from arterial baroreceptors, while the activation of hypothalamic vasopressin-synthesising neurons in response to hypotension is largely dependent on baroreceptors, although an increase in circulating angiotensin also appears to contribute. 4. Discrete groups of neurons in the rostral ventrolateral medulla (RVLM) and A5 area in the pons are the major groups of spinally projecting neurons activated by baroreceptor unloading. In contrast, spinally projecting neurons in the paraventricular nucleus in the hypothalamus appear to be largely unaffected by baroreceptor signals. 5. Direct afferent inputs to RVLM neurons in response to increases or decreases in arterial pressure originate primarily from other medullary nuclei, particularly neurons located in the caudal and intermediate levels of the ventrolateral medulla (CVLM and IVLM), as well as in the nucleus tractus solitarius (NTS). 6. There is also a direct projection from barosensory neurons in the NTS to the CVLM/IVLM region, which is activated by baroreceptor inputs. 7. Collectively, the results of our studies in conscious animals indicate that baroreceptor signals reach all levels of the brain. With regard to the baroreceptor reflex control of sympathetic activity, our studies are consistent with previous studies in anesthetized animals, but in addition reveal other previously unrecognized pathways that also contribute to this reflex regulation.

Functional organisation of central cardiovascular pathways: studies using c-fos gene expression

Until about 10 years ago, knowledge of the functional organisation of the central pathways that subserve cardiovascular responses to homeostatic challenges and other stressors was based almost entirely on studies in anaesthetised animals. More recently, however, many studies have used the method of the expression of immediate early genes, particularly the c-fos gene, to identify populations of central neurons that are activated by such challenges in conscious animals. In this review we first consider the advantages and limitations of this method. Then, we discuss how the application of the method of immediate early gene expression, when used alone or in combination with other methods, has contributed to our understanding of the central mechanisms that regulate the autonomic and neuroendocrine response to various cardiovascular challenges (e.g., hypotension, hypoxia, hypovolemia, and other stressors) as they operate in the conscious state. In general, the results of studies of central cardiovascular pathways using immediate early gene expression are consistent with previous studies in anaesthetised animals, but in addition have revealed other previously unrecognised pathways that also contribute to cardiovascular regulation. Finally, we briefly consider recent evidence indicating that immediate early gene expression can modify the functional properties of central cardiovascular neurons, and the possible significance of this in producing long-term changes in the regulation of the cardiovascular system both in normal and pathological conditions.

Sustained activation of the central baroreceptor pathway in obesity hypertension

The major goal of this study was to determine whether there is increased activation of medullary neurons that participate in the central baroreceptor reflex pathway in dogs with obesity-induced hypertension, a model of hypertension that is associated with increased sympathetic activity. We used Fos-like (Fos-Li) protein immunohistochemical methods to determine activation of neurons in the nucleus tractus solitarius (NTS), caudal ventrolateral medulla (CVLM), and rostral ventrolateral medulla (RVLM). Dogs were fed either a regular diet or an identical diet with the addition of 0.5 to 0.9 kg of cooked beef fat. After approximately 6 weeks of the high fat diet, body weight (36.3+/-0.4 vs 21.5+/-0.5 kg), mean arterial pressure (105+/-4 vs 91+/-3 mm Hg), and heart rate (97+/-4 vs 70+/-3 bpm) were significantly greater in obese than in control dogs, respectively. There was little Fos-Li immunoreactivity in medullary neurons of control dogs but marked reactivity in obese dogs. Specifically, the number of Fos-Li-positive cells in the NTS and CVLM was 3 to 5 times greater in obese than in control dogs. Furthermore, despite sustained activation of these baroreceptor-sensitive neurons, there was a significantly greater number of Fos-Li positive cells in the RVLM of dogs fed the high fat diet. As baroreceptor suppression of sympathoexcitatory cells in the RVLM is mediated by activation of neurons in the NTS and CVLM, these results support recent findings indicating that baroreflex suppression of sympathetic activity is a long-term compensatory response in hypertension. However, sympathoexcitatory inputs onto RVLM neurons would appear to predominate over the inhibitory effects of the baroreflex in obesity hypertension.

Fos, RVLM-projecting neurons, and spinally projecting neurons in the PVN following hypertonic saline infusion

Hypertonic saline (HTS; 1.7 M) infused intravenously into conscious rats increases the production of Fos, a marker of cell activation, in the hypothalamic paraventricular nucleus (PVN). The parvocellular PVN contains subpopulations of neurons. However, which subpopulations are activated by HTS is unknown. We determined whether PVN neurons that innervate the rostral ventrolateral medulla (RVLM) or the spinal cord (important autonomic sites) expressed Fos following HTS. Experiments were performed 24-96 h after chronic implantation of an intravenous cannula. HTS significantly increased the number of Fos-positive cells. In the parvocellular PVN, the maximum number of Fos-positive cells occurred rostral of the anterior-posterior level at which the number of neurons that projected to the medulla or spinal cord peaked. Compared with controls, HTS did not significantly increase the number of double-labeled neurons. These findings demonstrate that an elevation in plasma osmolality activates PVN neurons but not the subgroups of PVN neurons with projections to the RVLM or to the spinal cord.

Serotonin(3) receptor stimulation in the nucleus tractus solitarii activates non-catecholaminergic neurons in the rat ventrolateral medulla

The present study was performed to determine whether or not the increased arterial pressure triggered by 5-HT(3) receptor stimulation in the nucleus tractus solitarii and underlain by a sympathoexcitation is associated with the activation of ventromedullary cells known to be involved in vascular regulation, i.e. the C1 and A1 catecholaminergic cells. For this purpose, double immunohistochemical labeling for tyrosine hydroxylase and c-fos protein was performed all along the ventrolateral medulla after microinjection of 1-(m-chlorophenyl)-biguanide, a selective and potent 5-HT(3) receptor agonist, into the nucleus tractus solitarii of alpha-chloralose/urethane-anaesthetized rats. This treatment produced a significant elevation of arterial pressure ( approximately +35 mm Hg). Concomitantly, a significant increase in the number of c-fos expressing neurons was observed in the rostral ventrolateral medulla (+63%), in particular in its most anterior part (+78%), and in the medullary region surrounding the caudal part of the facial nucleus (+91%). Retrograde labeling with gold-horseradish peroxidase complex showed that at least some of these activated c-fos expressing cells project to the spinal cord. However, the number of double-stained neurons, i.e. c-fos and tyrosine hydroxylase positive neurons, did not increase at any level of the ventrolateral medulla. In contrast, under the same alpha-chloralose/urethane anesthesia, systemic infusion of sodium nitroprusside appeared to produce a hypotension and a marked increase in the density of such double c-fos and tyrosine hydroxylase expressing cells in the rostral ventrolateral medulla and the caudal medullary region surrounding the caudal part of the facial nucleus. These data indicate that medullary catecholaminergic C1 and A1 neurons are not involved in the pressor effect elicited by 5-HT(3) receptor stimulation in the nucleus tractus solitarii. However, this 5-HT(3) receptor-mediated effect is clearly associated with the excitation of (non-catecholaminergic) neurons within the pressor region of the ventral medulla.

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

Fos immunocytochemistry was combined with immunolabeling for dopamine-beta-hydroxylase (DBH) to examine neuronal activation in the medulla and pons after administration of polyethelene glycol (PEG), which produces volume depletion without altering arterial blood pressure. Increased Fos immunoreactivity was observed in the area postrema, nucleus of the solitary tract, rostral ventrolateral medulla, lateral parabrachial nucleus, and the dorsomedial pons at the level of the locus coeruleus. Fos immunolabeling in the caudal nucleus of the solitary tract, rostral ventrolateral medulla, and the dorsomedial pons occurred primarily in neurons that did not contain DBH. Thus, PEG activates non-catecholaminergic neurons in medullary and pontine areas associated with cardiovascular and body fluid regulation.

Differences between SHR and WKY following the airpuff startle stimulus in the number of Fos expressing, RVLM projecting neurons

The neurocircuitry responsible for excessive stress-induced cardiovascular responses in genetic hypertensive rats remains elusive. Prior studies detailed a differential cardiovascular response profile to airpuff startle stimuli between Spontaneously Hypertensive (SHR) and Wistar Kyoto (WKY) rats. We recently identified strain differential Fos expression in the rostroventrolateral medulla (RVLM) and several RVLM projecting sites following airpuff startle. The current study sought to define RVLM projecting neurons that also express Fos following placement in the test chamber and administration of the airpuff startle stimulus. Unilateral iontophoretic micro-injections of fluorogold were made into the RVLM of 9-10 week old SHR and WKY rats. Two to three weeks later, animals were subjected to a series of 60 airpuff startle stimuli. Brains were double labeled for Fos and fluorogold. Single fluorogold and single Fos cells, and double labeled cells were found in the nucleus tractus solitarius (NTS), caudal ventral lateral medulla (CVLM), Kölliker fuse (KF), ventral lateral, lateral, and dorsal central gray, lateral hypothalamus (LH), and paraventricular nucleus of the hypothalamus (PVN). These data are consistent with the notion that the RVLM receives differential excitatory and/or inhibitory input from higher brain centers, perhaps contributing to differential Fos expression in the RVLM, differential autonomic responding, or both.

Intrapericardial procaine affects volume expansion-induced fos immunoreactivity in unanesthetized rats

Acute volume expansion is associated with a specific pattern of Fos expression and the goal of the present study was to evaluate the contribution of cardiac receptors to this response. Adult male rats were instrumented with pericardial catheters introduced at the level of the thymus. Rats were also catheterized for measuring blood pressure, heart rate, central venous pressure, and intravenous infusion. Each rat received a 200-microl intrapericardial (i.p.c) injection of 2% procaine or 0.9% NaCl. Rats were then volume expanded with isotonic saline (10% body weight in 10 min) or given a control infusion (0.01 ml/min for 10 min). Ninety minutes after the start of the infusion, the rats were anesthetized and perfused transcardially. Their brains were sectioned and processed for Fos, dopamine-beta-hydroxylase, and oxytocin immunocytochemistry. Volume expansion plus i.p.c. saline produced a significant increase in Fos expression in the nucleus of the solitary tract, the ventrolateral medulla, the area postrema, the locus coeruleus, the paraventricular nucleus of the hypothalamus, the perinuclear zone of the supraoptic nucleus, and oxytocin neurons in the supraoptic nucleus. The i.p.c. procaine significantly blocked Fos expression produced by the volume expansion in the all of the regions examined except for the area postrema and the SON oxytocin neurons.

Sustained activation of the central baroreceptor pathway in angiotensin hypertension

Recent studies indicate that renal sympathetic nerve activity is chronically suppressed in angiotensin (Ang II) hypertension and that baroreflexes play a critical role in mediating this response. To support these findings, we determined whether the hypertension associated with chronic infusion of Ang II at 4.8 pmol/kg per minute (5ng/kg per minute) produces sustained activation of medullary neurons that participate in the central baroreceptor reflex pathway. We used Fos-like (Fos-Li) protein immunohistochemical methods to determine activation of neurons in the nucleus tractus solitarius (NTS), caudal ventrolateral medulla (CVLM), and rostral ventrolateral medulla (RVLM). Results were compared in three groups of chronically instrumented dogs subjected to infusion of: 1) saline (control); 2) Ang II-2 hours (acute); and 3) Ang II-5 days (chronic). Mean arterial pressure increased 22 +/- 3 and 35 +/- 3 mm Hg during acute and chronic Ang II infusion, respectively. There was little Fos-Li immunoreactivity in medullary neurons in control dogs. In contrast, during acute Ang II infusion there was a 2- to 3-fold increase in Fos-Li staining in the NTS and CVLM, but no increase in staining in RVLM neurons. As baroreceptor suppression of sympathoexcitatory cells in the RVLM is mediated by activation of neurons in the NTS and CVLM, these results were expected. More importantly, this same pattern of central neuronal activation was observed during chronic Ang II hypertension. Therefore, these results support recent findings indicating that baroreflex suppression of renal sympathetic nerve activity is a long-term compensatory response in Ang II hypertension.

Elucidation of neuronal circuitry: protocol(s) combining intracellular labeling, neuroanatomical tracing and immunocytochemical methodologies

We describe a protocol combining either intracellular biotinamide staining or anterograde biotinylated dextran amine (BDA) tracing with retrograde horseradish peroxidase (HRP) labeling and immunocytochemistry in order to map physiologically identified neuronal pathways. Presynaptic neurons including their boutons are labeled by either intracellular injection of biotinamide or extracellular injection of BDA while postsynaptic neurons are labeled with HRP via retrograde transport. Tissues are first processed to detect HRP using a tetramethylbenzidine and sodium-tungstate method. Biotinamide or BDA staining is then visualized using an ABC-diaminobenzidine-Ni method and finally the tissue is immunocytochemically stained using choline acetyltransferase (ChAT) or parvalbumin antibodies and a peroxidase-anti-peroxidase method. After processing, biotinamide, BDA, HRP and immunocytochemical staining can readily be distinguished by differences in the size, color and texture of their reaction products. We have utilized this methodology to explore synaptic relationships between trigeminal primary afferent neurons and brainstem projection and motoneurons at both the light and electron microscopic levels. This multiple labeling methodology could be readily adapted to characterize the physiological, morphological and neurochemical properties of other neuronal pathways.

Effects of selective sinoaortic denervations on phenylephrine-induced activational responses in the nucleus of the solitary tract

Intravenous administration of phenylephrine provokes a pattern of cellular activation in the nucleus of the solitary tract that resembles the central distributions of primary baroreceptor afferents supplied by the carotid sinus and aortic depressor nerves. Transganglionic transport and denervation methods were used in an experimental setting to test the dependence of phenylephrine-induced Fos immunoreactivity on the integrity of buffer nerve afferents, and to identify the subregions of the nucleus of the solitary tract supplied by each. Cholera toxin B-horseradish peroxidase injections into either or both nerves revealed terminal labeling concentrated in, but not restricted to, the dorsal commissural part of the nucleus of the solitary tract at the level of the apex of calamus scriptorius, and extending into the dorsal subnucleus at the level of the area postrema. Preferential ramifications of carotid sinus and aortic depressor nerve afferents at the levels of the commissural part of the nucleus and the area postrema, respectively, were reflected in the extent to which labeled fibers comingled with neurons exhibiting phenylephrine-induced Fos in dual labeling experiments. Complete sinoaortic denervation reduced by 90% the number of neurons exhibiting drug-induced Fos expression. Selective carotid and aortic sinus denervations effected partial reductions manifest preferentially in the caudal and rostral foci of the distribution, respectively. Reduced activational responses at the level of the area postrema of aortic sinus-denervated rats were accompanied by a reduction in cellular nicotinamide adenine dinucleotide phosphate-diaphorase activity in this region. Animals killed 30 days after complete sinoaortic denervation displayed no evidence of recovery of phenylephrine-induced Fos, while the strength and distribution of the response in rats that received selective carotid sinus denervation were indistinguishable from those seen in controls. These findings (i) support the dependence of phenylephrine-induced Fos expression on the integrity of carotid sinus and aortic depressor nerve afferents, (ii) provide anatomical and functional evidence that the two buffer nerves distribute differentially within the nucleus of the solitary tract, and (iii) implicate central reorganization as a likely basis for functional recovery of baroreflex mechanisms following partial sinoaortic denervation.

Double immunocytochemistry for the detection of Fos protein in retrogradely identified neurons using cholera toxin B subunit

The focus of this paper was to describe a method combining the neuroanatomical technique of retrograde transport of cholera toxin B subunit (CTB) with the technique of Fos functional labeling. This method allowed us to evaluate whether neurons identified by retrograde tracing were activated following chemical stimulation of another brain area. We have used this method at the light microscopic level to determine whether the stimulation of the rostral ventrolateral medulla activated retrogradely labeled adrenal sympathetic preganglionic neurons in the spinal cord. CTB-containing neurons, Fos immunoreactive neurons and double labeled neurons were observed in spinal autonomic areas. These results suggest that the rostral ventrolateral medulla exerts a descending activation upon identified adrenal preganglionic neurons. The method described in this protocol can be applied for other brain areas in order to establish if a given structure can activate an identified population of neurons linked with a particular target of central or peripheral nervous system.

Neuronal expression of Fos protein in the hypothalamus of rats with heart failure

We sought to identify the areas that have altered neuronal activity within the hypothalamus of rats with heart failure (HF) by mapping neuronal staining of c-Fos protein (Fos) 6-8 weeks following coronary artery ligation (HF group; n=17) or sham surgery (sham-operated control group, n=15). Fos-like immunoreactivity was observed in the paraventricular nucleus (PVN), supraoptic nucleus (SON), median preoptic nucleus (MnPO), anterior hypothalamus (AH) and posterior hypothalamus (PH) using a standard ABC immunocytochemical protocol. The rats in the HF group displayed infarcts averaging 34+/-2% of the outer circumference and 41+/-1% of the inner circumference of the left ventricular wall. Sham-operated control rats had no observable damage to the myocardium. Rats with chronic heart failure (n=5) but no manipulation (no surgery) had a similar number of Fos-staining cells in PVN SON, MnPO, AH and PH compared to sham-operated rats. Acute surgery for isolation of vagus nerves and anesthesia for 90 min increased the number of Fos positive cells in PVN, SON and MnPO of both sham-operated rats and rats with HF. Furthermore, rats with heart failure (n=5) had significantly higher number of Fos-staining cells in PVN (four times), SON (4.5 times) and MnPO (1.5 times) compared to sham-operated rats after acute surgery for isolation of the vagus. The number of Fos-staining cells remained unaltered in AH and PH in both groups of rats. However, in a third series of experiments vagotomy reduced the number of Fos-staining cells in the PVN, SON or MnPO of rats with HF (n=5) to those observed in sham-operated vagotomized rats. This study shows that: (1) there is augmented neuronal activity as indicated by increased number of Fos staining neurons in the PVN, SON and MnPO due to acute surgical stress in rats with HF, and (2) vagal afferents are responsible for the increased neuronal activity in PVN, SON and MnPO of rats with HF during acute surgical stress. These data support the conclusion that vasopressin producing neurons and autonomic areas within the hypothalamus influenced by vagal afferents are activated during HF and are sensitive to 'acute surgical stress' and may contribute to the elevated levels of vasopressin and sympatho-excitation commonly observed in heart failure.

Area postrema and sympathetic nervous system effects of vasopressin and angiotensin II

1. Precise control over the cardiovascular system requires the integration of both neural and humoral signals related to blood volume and blood pressure. Humoral signals interact with neural systems, modulating their control over the efferent mechanisms that ultimately determine the level of pressure and volume. 2. Peptide hormones such as angiotensin (Ang)II and arginine vasopressin (AVP) act through circumventricular organs (CVO) to influence cardiovascular regulation. 3. The area postrema (AP), a CVO in the brainstem, mediates at least some of the central actions of these peptides. Vasopressin appears to act in the AP to cause sympathoinhibition and a shift in baroreflex control of the sympathetic nervous system (SNS) to lower pressures. These effects of AVP and the AP appear to be mediated by alpha2-adrenoceptor and glutamatergic mechanisms in the nucleus tractus solitarius. 4. In contrast to AVP AngII has effects in the AP to blunt baroreflex control of heart rate and cause sympathoexcitation. The effects of chronic AngII to increase activity of the SNS may be due to AP-dependent activation of neurons in the rostral ventrolateral medulla.

Immunocytochemical detection of fos protein combined with anterograde tract-tracing using biotinylated dextran

The present report deals with an axonal tract-tracing procedure in rat enabling visualization of anterogradely transported biotinylated dextran amine (BDA) combined with immnunocytochemical detection of Fos protein following electrical stimulation of the brain. This method allows us to evaluate whether a given structure, receiving both injection of BDA and electrical stimulation, elicits neuronal activation in another part of the brain via direct or indirect projections. We have used the method at the light microscopic level to determine the connectivity of the sensorimotor cortex in the rat. In various parts of the forebrain and brainstem, BDA-labeled fibers originating from the cortex were observed in close apposition to Fos-like immunoreactive cells (FLI) activated by stimulation. This result suggests a direct (probably monosynaptic) projection. On the contrary, FLI neurons were observed in areas devoid of direct afferents, indicating a cascade of activations. The method described in this protocol is applicable for functional anatomy purposes elsewhere within the central nervous system. It constitutes a preliminary step in identifying the validity of a pathway before examination of the reality of the monosynaptic relationship at the electron microscopic level.

Airpuff startle stress elicited fos expression in brain cardiovascular areas of young SHR and WKY rats

Prior studies comparing Fos expression in adult Wistar Kyoto (WKY) and Spontaneously Hypertensive rats (SHR) identified more Fos-positive neurons in a subset of brain regions following two stressors: placement in a startle chamber and presentation of an airpuff startle stimulus. The present study assessed Fos expression in five week old SHR and WKY rats in those same brain areas. Like adults, young SHR expressed more Fos-positive neurons than WKY in response to the startle chamber alone. Unlike adults, in the SHR only the locus coeruleus showed a increases in Fos expression following addition of the airpuff. Otherwise, startle chamber and airpuff startle treatments induced roughly equivalent Fos expression in the SHR, possibly reflecting a ceiling effect. Young WKY exhibited predominant airpuff-induced elevations. The present results demonstrate that certain brain regions are strain-differentially activated by stressors prior to overt hypertension and that differential Fos expression is an early developmental feature of these strains.

Fos expression in brain stem nuclei of pregnant rats after hydralazine-induced hypotension

Fos and dopamine beta-hydroxylase immunoreactivity were evaluated in the brain stems of 21-day pregnant and virgin female rats injected with either hydralazine (HDZ; 10 mg/kg iv) or vehicle. HDZ produced significant hypotension in both groups, although baseline blood pressure was lower in pregnant rats (96 +/- 2.5 mmHg) than in virgin female rats (121 +/- 2.8 mmHg). There were no differences in Fos immunoreactivity in the brain stems of pregnant and virgin female rats after vehicle treatment. HDZ-induced hypotension significantly increased Fos expression in both groups; however, the magnitude of the increases differed in the caudal ventrolateral medulla (CVL), the area postrema (AP), and the rostral ventrolateral medulla (RVL). Fos expression after HDZ in pregnant rats was augmented in noncatecholaminergic neurons of the CVL but was attenuated in the AP and in noncatecholaminergic neurons in the RVL. These results are consistent with differences in the sympathetic response to hypotension between pregnant and virgin female rats and indicate that the central response to hypotension may be different in pregnant rats.

Medial prefrontal cortex depressor response: role of the solitary tract nucleus in the rat

The depressor response elicited by unilateral low intensity electrical stimulation of the rat ventral medial prefrontal cortex may be mediated by a connection with the solitary tract nucleus. We tested this hypothesis by (i) examining the influence of medial prefrontal cortex stimulation on the induction of Fos-like immunoreactivity in neurons in the medulla oblongata, and (ii) by testing the effect of inhibition of solitary tract nucleus neurons on the medial prefrontal cortex stimulation-evoked depressor response. Depressor responses (>10 mmHg) were elicited by electrical stimulation of the medial prefrontal cortex every minute for 1 h ('Stimulated' group). Control animals were treated identically but did not receive electrical stimulation ('Unstimulated' group). Neurons exhibiting Fos-like immunoreactivity were abundant at the stimulation site which included the infralimbic area, and dorsal peduncular cortex. Medullary Fos-like immunoreactivity observed in the 'Stimulated' and 'Unstimulated' groups exceeded levels observed in untreated rats and was detected in the rostral, caudal and intermediate areas of the ventrolateral medulla, and the commissural, intermediate, medial and lateral regions of the solitary tract nucleus, as well as the medial vestibular nucleus, and the dorsal motor nucleus of the vagus. The number of neurons displaying Fos-like immunoreactivity in the ipsilateral solitary tract nucleus and caudal ventrolateral medulla of the 'Stimulated' group was found to be significantly elevated compared to the contralateral side (P<0.05), and the 'Unstimulated' group bilaterally. Inhibition of solitary tract nucleus neurons using bilateral injections of the GABA(A) receptor agonist muscimol (44 pmol/25 nl) inhibited the sympathetic vasomotor baroreflex and attenuated the depressor and sympathoinhibitory response to medial prefrontal cortex stimulation by 62% and 65%, respectively. These findings suggest that the projection from the medial prefrontal cortex to the solitary tract nucleus is excitatory and support the hypothesis that the depressor response elicited by medial prefrontal cortex stimulation is mediated, in part, by a cortico-solitary projection which activates the intramedullary baroreflex pathway.

Distribution of neurons in the anterior hypothalamic nucleus activated by blood pressure changes in the rat

Electrophysiological and Fos-like protein immunocytochemical methods were used to identify the number and distribution of anterior hypothalamic neurons that are activated by changes in arterial pressure. First, in anesthetized, male Sprague-Dawley rats, arterial pressure increases and decreases led to differential activation of neurons in the anterior hypothalamic nucleus. Most of the units that responded to a rise in arterial pressure with a decrease in activity (pressor units) were located in the central part of the anterior hypothalamic nucleus, whereas units that increased firing when arterial pressure rose (the depressor units) were found throughout the nucleus. Second, in awake, male Sprague-Dawley rats, Fos-like protein immunoreactivity was mapped following sustained arterial pressure changes. Within the anterior hypothalamus, reduction in arterial pressure increased the number of Fos-labeled neurons primarily in the paraventricular nucleus and to a lesser extent in the anterior half of the anterior hypothalamic nucleus. In contrast, elevation in arterial pressure increased Fos labeling throughout the anterior hypothalamic nucleus and to a lesser extent in the paraventricular nucleus.

Differential roles of NMDA and non-NMDA receptors in synaptic responses of neurons in nucleus tractus solitarii of the rat

The relative role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in synaptic responses of neurons in caudal nucleus tractus solitarii (cNTS) was delineated by immunohistochemical and electrophysiologic experiments in rats. Double immunohistochemical staining in in vivo experiments revealed that approximately 80% of cNTS neurons that showed Fos-like immunoreactivity induced by baroreceptor activation were generally also immunoreactive to non-NMDA receptor subunits GluR1 or GluR2. On the other hand, only 20% of Fos-labeled cNTS neurons showed immunoreactivity to NMDA receptor subunits NMDAR1 or NMDAR2. Stimulation of the ipsilateral solitary tract at suprathreshold intensity in slice preparations induced Fos expression in the cNTS and evoked either a single action potential or a complex synaptic response consisting of an initial action potential followed by a secondary slow depolarization. In a majority (70%) of cNTS neurons that exhibited the complex synaptic response, both the initial and secondary components were eliminated reversibly by 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). This non-NMDA antagonist also inhibited the single action potential manifested by the other population of cNTS neurons. On the other hand, only the secondary slow depolarization was blocked by D(-)-2-amino-5-phosphonopentanoic acid (250 microM) or potentiated by NMDA (1.7 microM). Our results suggested that NMDA and non-NMDA receptors are involved differentially in the synaptic responses of cNTS neurons. Non-NMDA receptors may be distributed predominantly on a majority of the second-order cNTS neurons that may receive primary baroreceptor afferent inputs. On the other hand, NMDA receptors are located primarily on higher-order neurons, which may be connected reciprocally with the second-order cNTS neurons.

Phosphorylation of transcription factor cyclic-AMP response element binding protein mediates c-fos induction elicited by sustained hypertension in rat nucleus tractus solitarii

We investigated the role of cyclic-AMP response element binding protein signaling in the induction of the immediate-early gene c-fos by baroreceptor activation in neurons of the nucleus tractus solitarii of anesthetized rats. Activation of the arterial baroreceptors with sustained hypertension significantly increased the number of neurons in the caudal nucleus tractus solitarii that were immunoreactive to an antiserum that detects Ser133-phosphorylated cyclic-AMP response element binding protein. This implied increase in phosphorylation of cyclic-AMP response element binding protein was subsequently followed by an elevation in the expression of Fos protein in neurons of the nucleus tractus solitarii. Microinjection bilaterally into the nucleus tractus solitarii of a phosphorothioated antisense oligonucleotide directed against the initiation site of cyclic-AMP response element binding protein messenger RNA discernibly reduced the manifested immunoreactivity of phosphorylated cyclic-AMP response element binding protein in response to baroreceptor activation. This was accompanied by a decline in the transcription of c-fos messenger RNA and the expression of Fos protein, along with an appreciable potentiation of the baroreceptor reflex response. Control injections of the sense oligonucleotide or artificial cerebrospinal fluid were ineffective. These findings suggest that phosphorylation of cyclic-AMP response element binding protein is crucial to Fos expression in the nucleus tractus solitarii elicited by sustained hypertension. As such, phosphorylation of cyclic-AMP response element binding protein may be an important early nuclear event that mediates the long-term inhibitory modulation of the baroreceptor reflex response by Fos protein at the nucleus tractus solitarii.