Control of neurosecretory vasopressin cells by noradrenergic projections of the caudal ventrolateral medulla

Catecholamines modulate differentially nonapeptide precursor mRNA expression in the preoptic area and ovary of the catfish Heteropneustes fossilis: An in vitro study

In the catfish Heteropneustes fossilis, three nonapeptide hormone genes were identified in the brain preoptic area (POA) and ovary: a pro-vasotocin (pro-vt) and two isotocin gene paralogs viz., a novel pro-ita and conventional pro-itb. In the present study, the regulatory role of catecholamines [CA: dopamine (DA), noradrenaline (NA), adrenaline (AD)] on the expression of these genes were investigated in vitro. DA (1, 10, and 100 ng/mL) inhibited significantly the mRNA expression in both the POA and ovary. NA upregulated the POA mRNA expression in a biphasic manner, the lower concentrations (1 ng and 10 ng) scaled up and the higher concentration (100 ng) scaled down the expression of pro-vt and pro-itb, while only the 1 ng NA scaled up the pro-ita expression. In the ovary, NA upregulated the mRNA expressions at all concentrations; the pro-vt expression was stimulated only at 10 and 100 ng. AD stimulated pro-vt and pro-ita expression in the POA at all concentrations but the pro-itb expression was inhibited at 1 and 10 ng, and stimulated at 100 ng concentrations. In the ovary, AD elicited varied effects; no significant change in pro-vt, a stimulation of pro-ita, and an inhibition of pro-itb at 1 ng, and stimulation of pro-itb at the 10 and 100 ng. The incubation of the POA and ovary with α-methylparatyrosine (MPT, 250 µg/mL, a tyrosine hydroxylase inhibitor) for 8 h downregulated the mRNA expression in the POA but unaltered the expression in the ovary. Pre-incubation with MPT for 4 h, followed by co-incubation with DA, NA or AD for 4 h elicited varied effects. In the POA, the co-incubations with the CAs rescued the inhibition due to MPT. The MPT + DA and MPT + AD treatments reduced the magnitude of the inhibition of pro-vt and pro-itb by MPT. But the pro-ita expression was modestly stimulated in the MPT + AD group. On the other hand, the MPT + NA treatment rescued the MPT effect and elicited 10-folds increase in the expression levels. In the ovary, the changes were: an inhibition in the MPT + DA group, no significant alteration in the MPT + NA group, and a mild stimulation in the MPT + AD group. The results suggest that CAs modulate brain and ovarian nonapeptide gene expression differentially, which is important in the neuroendocrine/endocrine integration of reproduction in the catfish.

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

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

An Overview of Similarities and Differences in Metabolic Actions and Effects of Central Nervous System Between Glucagon-Like Peptide-1 Receptor Agonists (GLP-1RAs) and Sodium Glucose Co-Transporter-2 Inhibitors (SGLT-2is)

GLP-1 receptor agonists (GLP-1RAs) and SGLT-2 inhibitors (SGLT-2is) are novel antidiabetic medications associated with considerable cardiovascular benefits therapying treatment of diabetic patients. GLP-1 exhibits atherosclerosis resistance, whereas SGLT-2i acts to ameliorate the neuroendocrine state in the patients with chronic heart failure. Despite their distinct modes of action, both factors share pathways by regulating the central nervous system (CNS). While numerous preclinical and clinical studies have demonstrated that GLP-1 can access various nuclei associated with energy homeostasis and hedonic eating in the CNS via blood-brain barrier (BBB), research on the activity of SGLT-2is remains limited. In our previous studies, we demonstrated that both GLP-1 receptor agonists (GLP-1RAs) liraglutide and exenatide, as well as an SGLT-2i, dapagliflozin, could activate various nuclei and pathways in the CNS of Sprague Dawley (SD) rats and C57BL/6 mice, respectively. Moreover, our results revealed similarities and differences in neural pathways, which possibly regulated different metabolic effects of GLP-1RA and SGLT-2i via sympathetic and parasympathetic systems in the CNS, such as feeding, blood glucose regulation and cardiovascular activities (arterial blood pressure and heart rate control). In the present article, we extensively discuss recent preclinical studies on the effects of GLP-1RAs and SGLT-2is on the CNS actions, with the aim of providing a theoretical explanation on their mechanism of action in improvement of the macro-cardiovascular risk and reducing incidence of diabetic complications. Overall, these findings are expected to guide future drug design approaches.

Colocalized neurotransmitters in the hindbrain cooperate in adaptation to chronic hypernatremia

Chronic hypernatremia activates the central osmoregulatory mechanisms and inhibits the function of the hypothalamic-pituitary-adrenal (HPA) axis. Noradrenaline (NE) release into the periventricular anteroventral third ventricle region (AV3V), the supraoptic (SON) and hypothalamic paraventricular nuclei (PVN) from efferents of the caudal ventrolateral (cVLM) and dorsomedial (cDMM) medulla has been shown to be essential for the hypernatremia-evoked responses and for the HPA response to acute restraint. Notably, the medullary NE cell groups highly coexpress prolactin-releasing peptide (PrRP) and nesfatin-1/NUCB2 (nesfatin), therefore, we assumed they contributed to the reactions to chronic hypernatremia. To investigate this, we compared two models: homozygous Brattleboro rats with hereditary diabetes insipidus (DI) and Wistar rats subjected to chronic high salt solution (HS) intake. HS rats had higher plasma osmolality than DI rats. PrRP and nesfatin mRNA levels were higher in both models, in both medullary regions compared to controls. Elevated basal tyrosine hydroxylase (TH) expression and impaired restraint-induced TH, PrRP and nesfatin expression elevations in the cVLM were, however, detected only in HS, but not in DI rats. Simultaneously, only HS rats exhibited classical signs of chronic stress and severely blunted hormonal reactions to acute restraint. Data suggest that HPA axis responsiveness to restraint depends on the type of hypernatremia, and on NE capacity in the cVLM. Additionally, NE and PrRP signalization primarily of medullary origin is increased in the SON, PVN and AV3V in HS rats. This suggests a cooperative action in the adaptation responses and designates the AV3V as a new site for PrRP's action in hypernatremia.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Acute systemic hypoxia activates hypothalamic paraventricular nucleus-projecting catecholaminergic neurons in the caudal ventrolateral medulla

Hypoxia activates catecholamine neurons in the caudal ventrolateral medulla (CVLM). The hypothalamic paraventricular nucleus (PVN) modulates arterial chemoreflex responses and receives catecholaminergic projections from the CVLM, but it is not known whether the CVLM-PVN projection is activated by chemoreflex stimulation. We hypothesized that acute hypoxia (AH) activates PVN-projecting catecholaminergic neurons in the CVLM. Fluoro-Gold (2%, 60-90 nl) was microinjected into the PVN of rats to retrogradely label CVLM neurons. After recovery, conscious rats underwent 3 h of normoxia (21% O2, n = 4) or AH (12, 10, or 8% O2; n = 5 each group). We used Fos immunoreactivity as an index of CVLM neuronal activation and tyrosine hydroxylase (TH) immunoreactivity to identify catecholaminergic neurons. Positively labeled neurons were counted in six caudal-rostral sections containing CVLM. Hypoxia progressively increased the number of Fos-immunoreactive CVLM cells (21%, 19 ± 6; 12%, 49 ± 2; 10%, 117 ± 8; 8%, 179 ± 7; P < 0.001). Catecholaminergic cells colabeled with Fos immunoreactivity in the CVLM were observed following 12% O2, and further increases in hypoxia severity caused markedly more activation. PVN-projecting CVLM cells were activated following more severe hypoxia (10% and 8% O2). A large proportion (89 ± 3%) of all activated PVN-projecting CVLM neurons were catecholaminergic, regardless of hypoxia intensity. Data suggest that catecholaminergic, PVN-projecting CVLM neurons are particularly hypoxia-sensitive, and these neurons may be important in the cardiorespiratory and/or neuroendocrine responses elicited by the chemoreflex.

C1 neurons: the body's EMTs

The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception, and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brain stem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/inflammation, hemorrhage, and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic, and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension, and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses, and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity associated with diseases such as hypertension.

Distribution and localisation of Gα proteins in the rostral ventrolateral medulla of normotensive and hypertensive rats: focus on catecholaminergic neurons

About 860 G-protein-coupled receptors (GPCRs) mediate their actions via heterotrimeric G-proteins. Their activation releases Gα from Gβλ subunits. The type of Gα subunit dictates the major signalling proteins involved: adenylyl cyclase, PLC and rhoGEF. The rostral ventrolateral medulla (RVLM), containing the rostral C1 (rC1) cell group, sets and maintains the tonic and reflex control of blood pressure and a plethora of inputs converge onto these neurons. We determined the relative abundance of 10 Gα subunit mRNAs, representing the four major families, within the RVLM, using quantitative RT-PCR. In situ hybridisation (ISH) combined with immunohistochemistry (IHC) was used to quantify and compare this expression in rC1 with that in the A1 and A5 cell groups. The relative abundance of Gα subunit mRNAs and a comparison of gene expression levels were quantitatively determined in normotensive and hypertensive rat strains. All 10 Gα mRNAs were detected in the RVLM of Sprague-Dawley (SD) rats with relative abundance such that Gαs>Gαi2>Gαo>Gαq>GαL>Gα11>Gαi3>Gαi1>Gα12>Gα13. The high abundance of Gα mRNAs signalling via adenylyl cyclase indicates the importance of associated GPCRs. Within the rC1 and A1 groups similar differential Gα mRNA expression profiles were seen with Gαs being found in all rC1 cells, Gα11 absent and Gαi3 rarely expressed. Thus functionally distinct subgroups exist within the rC1 and A1 cell groups as differing distributions of Gα subunits must reflect the array of GPCRs that influence their activity. In contrast, all A5 cells expressed all Gα mRNAs suggesting a functionally homogeneous group. When the 10 Gα mRNAs of the RVLM in spontaneously hypertensive rats (SHR) were compared quantitatively to Wistar-Kyoto (WKY), only Gαs and Gα12 were significantly elevated. However when the expression in normotensive SD and WKY was compared with SHR no significant differences were evident. These findings demonstrate a range of GPCR signalling capabilities in brainstem neurons important for homeostasis and suggest a prominent role for signalling via adenylyl cyclase.

Sex differences in the effects of MDMA (ecstasy) on plasma copeptin in healthy subjects

BACKGROUND

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) misuse is associated with hyponatremia particularly in women. Hyponatremia is possibly due to inappropriate secretion of plasma arginine vasopressin (AVP).

OBJECTIVE

To assess whether MDMA increases plasma AVP and copeptin in healthy male and female subjects and whether effects depend on MDMA-induced release of serotonin and norepinephrine. Copeptin, the C-terminal part of the AVP precursor preprovasopressin, is cosecreted with AVP and can be determined more reliably.

METHODS

We used a randomized placebo-controlled crossover design. Plasma and urine osmolalities as well as AVP and copeptin levels were measured in 16 healthy subjects (eight female, eight male) at baseline and after MDMA (125 mg) administration. In addition, we tested whether effects of MDMA on AVP and copeptin secretion can be prevented by pretreatment with the serotonin and norepinephrine transporter inhibitor duloxetine (120 mg), which blocks MDMA-induced transporter-mediated release of serotonin and norepinephrine.

RESULTS

MDMA significantly elevated plasma copeptin levels at 60 min and at 120 min compared with placebo in women but not in men. The copeptin response to MDMA in women was prevented by duloxetine. MDMA also nonsignificantly increased plasma AVP levels in women, and the effect was prevented by duloxetine. Although subjects drank more water after MDMA compared with placebo administration, MDMA tended to increase urine sodium levels and urine osmolality compared with placebo, indicating increased renal water retention.

CONCLUSION

MDMA increased plasma copeptin, a marker for AVP secretion, in women but not in men. This sex difference in MDMA-induced AVP secretion may explain why hyponatremia is typically reported in female ecstasy users. The copeptin response to MDMA is likely mediated via MDMA-induced release of serotonin and/or norepinephrine because it was prevented by duloxetine, which blocks the interaction of MDMA with the serotonergic and noradrenergic system.

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.

Hypotensive hypovolemia and hypoglycemia activate different hindbrain catecholamine neurons with projections to the hypothalamus

To better understand the involvement of hindbrain catecholamine neurons in hypovolemia-induced secretion of AVP, we injected antidopamine beta-hydroxylase saporin (DSAP) or unconjugated saporin (SAP) control solution into the hypothalamic paraventricular nucleus (PVH) of anesthetized rats to retrogradely lesion catecholamine neurons innervating magnocellular areas of the hypothalamus. Subsequently, hypotensive hypovolemia was induced by remote blood withdrawal (4.5 ml, 1 ml/min) using an intra-atrial catheter. Blood was sampled at 2, 5, 20, and 50 min after onset of blood withdrawal. The AVP response was severely impaired by DSAP. Peak responses at 50 min were 51 pg/ml in SAP control and 17 pg/ml in DSAP-lesioned rats, indicating the importance of catecholamine neurons for this response. We also measured AVP responses to osmotic challenge induced by administration of hypertonic saline (1 M, 15 ml/kg, sc) and to insulin-induced hypoglycemia. Osmotic challenge increased AVP levels, but the response was not impaired by DSAP, indicating that AVP neurons were not damaged by the DSAP injection. Insulin-induced hypoglycemia did not increase AVP levels in either DSAP- or SAP-treated rats. However, the same dose of insulin increased food intake and corticosterone secretion in SAP controls, and these responses were profoundly impaired by DSAP. Thus catecholamine neurons are required for both the AVP response to hypotensive hypovolemia and for feeding and corticosterone responses to hypoglycemia. Lack of an AVP response to insulin-induced hypoglycemia in intact rats therefore indicates that responses to hypovolemia and hypoglycemia are mediated by different catecholamine neurons under distinct sensory controls.

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.

Prolactin-releasing peptide is a potent mediator of stress responses in the brain through the hypothalamic paraventricular nucleus

The effects of i.c.v. administration of prolactin-releasing peptide on neurons in the paraventricular nucleus of rats and plasma corticosterone levels were examined by measuring changes in Fos-like immunoreactivity, c-fos mRNA using in situ hybridization histochemistry, and plasma corticosterone using a specific radioimmunoassay. Approximately 80% of corticotropin-releasing hormone immunoreactive cells exhibited Fos-like immunoreactivity in the parvocellular division of the paraventricular nucleus 90 min after i.c.v. administration of prolactin-releasing peptide. The greatest induction of the c-fos mRNA expression in the paraventricular nucleus was observed 30 min after administration of prolactin-releasing peptide, and occurred in a dose-related manner. Plasma corticosterone levels were also significantly increased 30 min after administration of prolactin-releasing peptide. Next, the effects of restraint stress, nociceptive stimulus and acute inflammatory stress on the expression of the prolactin-releasing peptide mRNA in the dorsomedial hypothalamic nucleus, nucleus of the solitary tract and ventrolateral medulla were examined using in situ hybridization histochemistry for prolactin-releasing peptide mRNA. Restraint stress and acute inflammatory stress upregulated the prolactin-releasing peptide mRNA expression in the nucleus of the solitary tract and ventrolateral medulla. Nociceptive stimulus upregulated the prolactin-releasing peptide mRNA expression in the ventrolateral medulla. Finally, we observed that pretreatment (i.c.v. administration) with an anti-prolactin-releasing peptide antibody significantly attenuated nociceptive stimulus-induced c-fos mRNA expression in the paraventricular nucleus. These results suggest that prolactin-releasing peptide is a potent and important mediator of the stress response in the brain through the hypothalamic paraventricular nucleus.

Preprotachykinin A mRNA is colocalized with tyrosine hydroxylase-immunoreactivity in bulbospinal neurons

Previous studies have generated controversy about the extent of co-localization between substance P- and catecholamine-containing neurons that project to the spinal cord. In earlier studies, estimates using immunofluorescence after colchicine have ranged from almost all, to almost none. We sought to resolve this issue by combining in situ hybridization and immunofluorescence. Catecholamine (A1 to A7, C1 to C3; tyrosine hydroxylase immunoreactive) neurons in the rat brainstem were examined to determine their content of mRNA for the preprotachykinin-A gene. In the A1 to A7 and the C1 to C3 cell groups, preprotachykinin-A mRNA was found only in substantial amounts in the C1-C3 cell groups. On average 20.9+/-0.9% (234/1120, n=3) of rostral C1 neurons contained preprotachykinin-A mRNA. Co-localization was also observed in C2 and C3 neurons to a similar extent. Retrograde tract-tracing with cholera toxin B subunit was used to identify bulbospinal neurons and 17.9+/-2.7% (96/529 cells) of the bulbospinal tyrosine hydroxylase-containing neurons of the rostral C1 cell group were found to contain preprotachykinin-A mRNA. In addition a new population of non-catecholaminergic bulbospinal preprotachykinin-A neurons is described in an area corresponding to the recently described caudal pressor area. To confirm that the preprotachykinin-A mRNA observed in cells in the medulla was converted to protein, dual immunofluorescence for fiber labeling at the confocal level was carried out. This confirmed colocalization of substance P and tyrosine hydroxylase in the intermediolateral cell column, but nowhere else, in a small number of cases. The results provide evidence for a much larger population of substance P/neurokinin A containing neurons in the brainstem than was previously suspected. Furthermore, many of these neurons are catecholaminergic and spinally projecting. The specific sympathetic outflow that these neurons influence remains to be determined.

Haemorrhage-evoked decompensation and recompensation mediated by distinct projections from rostral and caudal midline medulla in the rat

The haemodynamic response to blood loss consists of three phases: (i) an initial compensatory phase during which resting arterial pressure is maintained; (ii) a decompensatory phase characterized by a sudden, life-threatening hypotension and bradycardia; and (iii) if blood loss ceases, a recompensatory phase during which arterial pressure returns to normal. Previous research indicates that topographically distinct, rostral and caudal parts of the caudal midline medulla (CMM) contain neurons that differentially regulate the timing and magnitude of each of the three phases. Specifically, decompensation depends critically on the integrity of the rostral CMM; whereas compensation and recompensation depend upon the integrity of the caudal CMM. This study aimed to determine, using retrograde and anterograde tracing techniques, if the rostral and caudal CMM gave rise to different sets of projections to the major cardiovascular region of the ventrolateral medulla (VLM) and spinal cord. It was found that rostral and caudal CMM each have projections of varying density to the region containing bulbospinal (presympathetic) motor neurons in the rostral VLM and preganglionic sympathetic motor neurons in the intermediolateral cell column of the spinal cord. Via these projections vasomotor tone and hence arterial pressure can be regulated. More strikingly: (i) consistent with a role in mediating bradycardia during decompensation, the rostral CMM projects uniquely to VLM regions containing vagal cardiac motor neurons; and (ii) consistent with its role in mediating recompensation, the caudal CMM projects uniquely onto tyrosine hydroxylase-containing, caudal VLM (A1) neurons whose activity mediates vasopressin release, on which recompensation depends.

Effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in peripheral and central areas related to cardiovascular regulation

Results from previous studies have demonstrated that ethanol influences central neural mechanisms involved in the control of blood pressure. We studied the effects of ethanol consumption on vasopressin and neuropeptide Y immunoreactivity and mRNA expression in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, as well as in the petrosal and nodose ganglia of rats. The ethanol-fed rats received liquid diet ad libitum containing 37.5% ethanol-derived calories (6.7% volume/volume), and the pair-fed rats received the same volume of diet containing isocaloric amounts of maltose-dextrin substituted for ethanol for 3 or 28 days. Arterial blood pressure was evaluated in a separate group of rats, which was unchanged by 3 days, but elevated by 21% after 28 days of ethanol consumption. Vasopressin immunoreactivity and mRNA signal were not detected in the ganglia, nor were they changed in the nucleus tractus solitarius and paraventricular hypothalamic nucleus, by 3 days of ethanol consumption. However, after 28 days of ethanol liquid diet consumption, vasopressin-positive terminals were decreased in the nucleus tractus solitarius and vasopressin immunoreactivity cell bodies and mRNA signal were decreased in the paraventricular hypothalamic nucleus. Neuropeptide Y-immunoreactive terminals were increased in the nucleus tractus solitarius only after 28 days of ethanol liquid diet consumption, but they were decreased in the paraventricular hypothalamic nucleus in rats treated with ethanol for 3 or 28 days. We concluded that the levels of both vasopressin and neuropeptide Y neurotransmitters are changed by long-term ethanol consumption in the neuronal pathways related to control of blood pressure.

The effect of acute and chronic restraint on the central expression of prepro-neuropeptide Y mRNA in normotensive and hypertensive rats

Neuropeptide Y (NPY), one of the most abundant neuropeptides found in the central nervous system (CNS), has been implicated in the regulation of many autonomic functions, including cardiovascular control and the central stress response. The present study represents a detailed investigation of the effects of acute and chronic restraint stress on the expression of the mRNA encoding the NPY precursor, prepro-NPY, in the CNS of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) using in situ hybridization histochemistry. Basal (unstressed) levels of prepro-NPY mRNA expression were found to be significantly increased in the hypothalamic arcuate nucleus of SHR compared to WKY rats, with similar levels of prepro-NPY mRNA expression found in the remaining central nuclei. Following exposure to both acute and chronic restraint, significant changes in prepro-NPY mRNA expression were found in a variety of central regions in both strains, including the arcuate nucleus and hippocampus (both strains), medial amygdala and cortex (WKY only), and dentate gyrus, nucleus of the solitary tract and ventrolateral medulla (SHR only). A comparison of the temporal response to restraint revealed that significant differences between strains existed in regions such as the arcuate nucleus, hippocampus and dentate gyrus, providing further evidence that hypertensive rats apparently have an impaired neural stress response. The present study demonstrates that exposure to restraint results in significant changes in prepro-NPY mRNA expression in specific nuclei of both WKY and SHR that are components of not only the central circuitry regulating the stress response, but also the neural network modulating autonomic function.

Prolactin-releasing peptide as a novel stress mediator in the central nervous system

A1/A2 noradrenergic neurons in the medulla oblongata are well known to mediate stress signals in the central nervous system. Stress activates A1/A2 noradrenergic neurons, and then noradrenaline (NA) stimulates ACTH secretion through hypothalamic CRH. On the other hand, PRL-releasing peptide (PrRP) was recently isolated and was found to be produced by some A1/A2 neurons and the dorsomedial hypothalamic nucleus. We previously demonstrated that PrRP neurons make synapse-like contact with hypothalamic CRH neurons. In fact, we demonstrated that the central administration of PrRP stimulates CRH-mediated ACTH secretion. Furthermore, it has been reported that PrRP neurons in A1/A2 cell groups are colocalized with tyrosine hydroxylase (TH), which is known as the marker enzyme of catecholaminergic neurons. These data strongly suggest that PrRP is related to stress-responsive signal transduction, and PrRP and NA cooperatively modulate the hypothalamo-pituitary-adrenal axis. We therefore examined the effect of water immersion-restraint stress on c-Fos protein accumulation in PrRP- and TH-immunoreactive neurons. The synergistic effects of PrRP and NA on plasma ACTH elevation were also examined. The results clearly showed that c-Fos protein accumulation dramatically increased in the nuclei of A1/A2 and dorsomedial hypothalamic nucleus PrRP neurons. In addition, it was revealed that c-Fos protein was specifically expressed in the PrRP/TH double positive cells in the A1/A2 cell groups. We also demonstrated that the central administration of PrRP and NA in combination at subactive (noneffective) doses clearly induced plasma ACTH elevation. Here we report that PrRP is a novel and important mediator of the hypothalamo-pituitary-adrenal axis for the stress response.

Sensitivity to glucocorticoid-mediated fast-feedback regulation of the hypothalamic-pituitary-adrenal axis is dependent upon stressor specific neurocircuitry

Fos-protein immunoreactivity (Fos-IR) was used to identify neurocircuits potentially participating in the regulation of hypothalamic-pituitary-adrenal (HPA) axis sensitivity to glucocorticoid-mediated fast-feedback in rats exposed to the physical stressor, hemorrhage, or the psychological stressor, airpuff startle. Marked regional brain differences in the Fos-IR expression were observed in response to these stressors. Specifically, after hemorrhage, nuclear Fos-IR increased in the nucleus of the solitary tract and other brainstem regions known to regulate hemodynamic processes including the supraoptic nucleus, and the magnocellular division of hypothalamic paraventricular nucleus (PVN). In contrast, after airpuff startle Fos-IR increased in the dorsomedial and lateral hypothalamus as well as in the lateral septum. Thus, activation of brainstem neurocircuits predominated after hemorrhage whereas activation of forebrain neurocircuits predominated after airpuff startle. In other regions, the magnitude of stressor-induced Fos-IR expression varied in a region-specific manner. When stressor exposure was preceded by administration of corticosterone to achieve levels within the physiological range after stressors, HPA axis responses were suppressed in response to the airpuff startle but not to either a small or moderate hemorrhage.

IN CONCLUSION

(1) fast-feedback mediated inhibition of HPA axis activity is critically dependent upon stressor modality; (2) this apparent selectivity is reflected by differences in the nature of the neurocircuitry mediating these stressors. It is suggested that determination of the central actions of glucocorticoids in mediating fast-feedback regulation of the HPA axis requires evaluation of the interactions between activated glucocorticoid receptors and intracellular signaling cascades evoked by convergent neuronal input.

Activation of brain neurons following central hypervolaemia and hypovolaemia: contribution of baroreceptor and non-baroreceptor inputs

In the present study we have used the detection of Fos, the protein product of c-fos, to determine the distribution of neurons in the medulla and hypothalamus that are activated by changes in central blood volume. Experiments were conducted in both barointact and barodenervated conscious rabbits, to determine the contribution of arterial baroreceptors to the pattern of Fos expression evoked by changes in central blood volume, induced either by intravenous infusion of an isotonic modified gelatin solution, or by partial occlusion of the vena cava. These procedures resulted in a significant increase and decrease, respectively, in right atrial pressure over a 60 min period. In control experiments, barointact and barodenervated rabbits were subjected to the identical procedures except that no changes in central blood volume were induced. In comparison with the control observations, central hypervolaemia produced a significant increase in the number of Fos-immunoreactive neurons in the nucleus tractus solitarius, area postrema, the caudal, intermediate and rostral parts of the ventrolateral medulla, supraoptic nucleus, paraventricular nucleus, arcuate nucleus, suprachiasmatic nucleus and median preoptic nucleus. The overall pattern of Fos expression induced by central hypervolaemia did not differ significantly between barointact and barodenervated animals. Similarly, the overall pattern of Fos expression induced by central hypovolaemia did not differ significantly between barointact and barodenervated animals, but did differ significantly from that produced by hypervolaemia. In particular, central hypovolaemia produced a significant increase in Fos expression in the same regions as above, but also in the subfornical organ and organum vasculosum lamina terminalis. In addition, compared with central hypervolaemia, hypovolaemia produced a significantly greater degree of Fos expression in the rostral ventrolateral medulla and supraoptic nucleus. Furthermore, double-labelling for tyrosine hydroxylase immunoreactivity demonstrated that neurons in the ventrolateral medulla that expressed Fos following hypovolaemia were predominantly catecholamine cells, whereas following hypervolaemia they were predominantly non-catecholamine cells. Finally, double-labelling for vasopressin immunoreactivity demonstrated that the number of Fos/vasopressin immunoreactive cells in the supraoptic nucleus was approximately 10 times greater following hypovolaemia compared with hypervolaemia, but there were very few such double-labelled neurons in the paraventricular nucleus in response to either stimulus. The results demonstrate that central hypervolaemia and hypovolaemia each induces reproducible and specific patterns of Fos expression in the medulla and hypothalamus. The degree and pattern of Fos expression was unaffected by arterial baroreceptor denervation, indicating that it is primarily a consequence of inputs from cardiac receptors, together with an increase in the level of circulating hormones such as atrial natriuretic peptide, angiotensin II or vasopressin. Furthermore, the pattern of Fos expression produced by central hypervolaemia and hypovolaemia is distinctly different from that evoked by hypertension and hypotension, respectively [Li and Dampney (1994) Neuroscience 61, 613-634], particularly in hypothalamic regions. These findings therefore indicate that the central pathways activated by changes in blood volume are, at least in part, separate from those activated by changes in arterial pressure.

Renal responses to increases in renal sympathetic nerve activity induced by brainstem stimulation in rabbits

We have stimulated the rostral ventrolateral medulla of the central nervous system to increase renal sympathetic nerve activity (RSNA), and measured the effect on renal blood flow, glomerular filtration rate, and urinary excretion. Increases in RSNA were produced by infusion of 0.02 M glutamate at a rate of 30-50 nl/min into the subretrofacial nucleus for 40 min, in 10 urethane anaesthetized rabbits. Changes in RSNA were quantified as the mean nerve activity per 1 s period and as the frequency and amplitude of individual discharges (reflecting the number of activated nerve fibres). Glutamate infusion increased RSNA 59 +/- 11% over control levels. This was predominantly due to a 65 +/- 15% increase in the frequency of discharges (3.0 +/- 0.35 to 4.6 +/- 0.4 Hz), rather than the amplitude of the discharges (+9 +/- 3% over control). The effects of these changes on the kidney were made against data collected in the last 20 min of the infusion and the 40 min pre-and post-stimulation periods, when arterial pressure and heart rate were unchanged from control levels. Renal blood flow fell significantly from 31.3 +/- 4.5 to 17.7 +/- 5.1 ml/min (47% decrease) and filtration fraction significantly increased from 12.7 +/- 1.1 to 15.7 +/- 2.1% (24% increase) during glutamate infusion. Each of these variables returned to their pre-stimulus levels after ceasing the central stimulation. Fluid, sodium and potassium excretion were not changed by this stimulus. In conclusion, the results in this study suggest that a selective increase in sympathetic nerve activity to the kidney without change in renal perfusion pressure can cause constriction of the renal vasculature without alteration in sodium and water excretion.

CNS pathways mediating cardiovascular regulation of vasopressin

1. The release of vasopressin from the neurohypophysial terminals of hypothalamic magnocellular neurosecretory neurons is subject to regulation by peripheral baroreceptors, cardiopulmonary volume receptors and circulating angiotensin II. Information from these sources is transmitted through different pathways to achieve different influences on the excitability of the vasopressin-secreting cells. 2. A brief increase in arterial pressure, sufficient to activate baroreceptors, is associated with a transient and selective GABAergic inhibition of these neurosecretory neurons, achieved through a multisynaptic pathway that involves ascending catecholaminergic fibres and neurons in the diagonal band of Broca. A decrease in arterial pressure activates peripheral low volume receptors and initiating neural inputs that result in an increase in the excitability of vasopressin-secreting neurons, achieved via pathways that include direct projections from caudal ventrolateral medulla A1 neurons. 3. Hypotension also releases renal renin and leads to the formation of angiotensin II; binding of this hormone to AT1 receptors on subfornical organ neurons promotes activation of a central angiotensinergic input that evokes a predominantly excitatory effect on vasopressin neurons.

Anatomical evidence for inputs to ventrolateral medullary catecholaminergic neurons from the midbrain periaqueductal gray of the rat

Previous studies have shown that the midbrain periaqueductal gray (PAG) projects to the ventrolateral medulla (VLM). Here, we studied PAG projections to the area of A1/C1 neurons in the VLM in the rat using phaseolus vulgaris leucoagglutinin (PHA-L) anterograde tracing combined with immunocytochemistry for tyrosine hydroxylase (TH) or phenylethanolamine N-methyl transferase (PNMT). Following PAG injections, PHA-L labeled fibers and terminals were intermingled among TH-immunoreactive (TH-ir) neurons in the VLM. High-power light microscopic examination revealed that some of the PHA-L labeled varicose fibers and boutons were in close contiguity with TH-ir elements. Such apparent appositions appeared more frequently on TH-ir elements in the A1 area than on TH-ir or PNMT-ir neurons in the C1 area. These results indicate that some PAG inputs to the VLM may directly innervate A1/C1 neurons.

Basal and stress-induced release of noradrenaline in hypothalamus of spontaneously hypertensive rats at different ages

Basal and stress-induced noradrenaline (NA) release was studied by intracerebral microdialysis in the hypothalamic paraventricular nucleus of spontaneously hypertensive rats (SHR) at different ages (9 weeks, 6, 18 and 24 months). NA was measured in 20-min dialysate samples by high performance liquid chromatography with electrochemical detection. Microdialysis sampling was done at baseline, during a 20-min immobilization stress and for the next 100 min. Basal NA levels decreased with age, showing a highly significant correlation. Immobilization stress raised NA similarly in the four age groups (respectively 281%, 235%, 243%, 251% of baseline at 9 weeks, 6, 18, 24 months), indicating that the response to stress is maintained at all these ages and is not affected by the development of hypertension or by aging.

Inhibition of the A1 area prevents hemorrhage-induced secretion of vasopressin in rats

In rats, secretion of vasopressin initiated by hemorrhage was completely abolished after muscimol-induced inhibition of neuronal function in the A1 region of the ventrolateral medulla oblongata. The A1 neurons are essential links in the central pathway to hypothalamic vasopressin-synthesizing neurons in this species.

c-fos expression in hypothalamic neurosecretory and brainstem catecholamine cells following noxious somatic stimuli

Noxious somatic stimuli elicit vasopressin secretion, an effect thought to result from activation of a facilitatory input from A1 catecholamine cells of the medulla oblongata. To better characterize the A1 cell response and effects on other neuroendocrine A1 projection targets, particularly within the paraventricular nucleus, we have now mapped c-fos expression in neurochemically identified catecholamine and neurosecretory cells following a noxious somatic stimulus. Unilateral hind paw pinch significantly increased c-fos expression in contralateral A1 cells whereas other brainstem catecholamine cell groups were unaffected. Expression of c-fos was also increased in the supraoptic nucleus, this effect being more pronounced for vasopressin than oxytocin neurosecretory cells and, as with A1 cells, primarily on the side contralateral to the stimulated paw. In contrast, the increase in the paraventricular nucleus was greater in oxytocin rather than in vasopressin cells. Additionally there was a significant rise in c-fos expression in medial parvocellular paraventricular nucleus cells of noxiously stimulated animals. Notably, the majority of tuberoinfundibular corticotropin-releasing factor cells are located in this medial parvocellular zone. These results are consistent with and expand on those previously reported from electrophysiological and anatomical studies. The finding of differing neurosecretory cell responses between supraoptic and paraventricular nuclei has interesting implications with regard to the afferent control of neurosecretory cell activity. For example, the substantially greater activation of supraoptic versus paraventricular nucleus vasopressin cells, despite being innervated by the same medullary noradrenergic cell group, raises the possibility of a differential input or differences in responsiveness. Furthermore, the activation of paraventricular nucleus parvocellular cells is consistent with suggestions that the A1 cell group provides an excitatory input to this population.

The central autonomic network: functional organization, dysfunction, and perspective

The central autonomic network (CAN) is an integral component of an internal regulation system through which the brain controls visceromotor, neuroendocrine, pain, and behavioral responses essential for survival. It includes the insular cortex, amygdala, hypothalamus, periaqueductal gray matter, parabrachial complex, nucleus of the tractus solitarius, and ventrolateral medulla. Inputs to the CAN are multiple, including viscerosensory inputs relayed on the nucleus of the tractus solitarius and humoral inputs relayed through the circumventricular organs. The CAN controls preganglionic sympathetic and parasympathetic, neuroendocrine, respiratory, and sphincter motoneurons. The CAN is characterized by reciprocal interconnections, parallel organization, state-dependent activity, and neurochemical complexity. The insular cortex and amygdala mediate high-order autonomic control, and their involvement in seizures or stroke may produce severe cardiac arrhythmias and other autonomic manifestations. The paraventricular and other hypothalamic nuclei contain mixed neuronal populations that control specific subsets of preganglionic sympathetic and parasympathetic neurons. Hypothalamic autonomic disorders commonly produce hypothermia or hyperthermia. Hyperthermia and autonomic hyperactivity occur in patients with head trauma, hydrocephalus, neuroleptic malignant syndrome, and fatal familial insomnia. In the medulla, the nucleus of the tractus solitarius and ventrolateral medulla contain a network of respiratory, cardiovagal, and vasomotor neurons. Medullary autonomic disorders may cause orthostatic hypotension, paroxysmal hypertension, and sleep apnea. Neurologic catastrophes, such as subarachnoid hemorrhage, may produce cardiac arrhythmias, myocardial injury, hypertension, and pulmonary edema. Multiple system atrophy affects preganglionic autonomic, respiratory, and neuroendocrine outputs. The CAN may be critically involved in panic disorders, essential hypertension, obesity, and other medical conditions.

Reflex control of magnocellular vasopressin and oxytocin secretion

Reflex control of magnocellular vasopressin and oxytocin secretion has captured the curiosity and investigative imagination of neuroendocrinologists for nearly 50 years. While it may seem obvious that brisk elevations in circulating levels of vasopressin in response to hemorrhage, or of oxytocin in response to suckling, must of necessity arise from magnocellular neurosecretory neurons in the hypothalamus, the central pathways mediating these reflexes have, until quite recently, remained elusive. In this brief review, ongoing attempts to delineate these pathways are summarized. Evidence for plasticity and local modulation of magnocellular reflexes in response to prolonged stimulation, such as chronic dehydration and lactation, is also presented.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution and origins of substance P-immunoreactive projections to the paraventricular and supraoptic nuclei: partial overlap with ascending catecholaminergic projections

Anatomical and pharmacological evidence suggests a role for substance P (SP) in the control of vasopressin secretion, but the origins of SP-immunoreactive (IR) projections to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus have not yet been identified. Combined axonal transport, immunohistochemical, and ablation approaches were used to characterize the organization of SP-IR projections to the PVH. The results may be summarized as follows: (1) SP-IR projections are broadly and prominently distributed throughout the SO and both the magnocellular and parvicellular divisions of the PVH. The distribution within the PVH is quite uniform. (2) Combined retrograde transport-immunohistochemical analyses identified multiple potential sources of SP-IR inputs to the PVH. These included a number of hypothalamic cell groups, the laterodorsal and peduculopontine tegmental nuclei, and the rostral and caudal aspects of the ventrolateral medulla. Portions of the tegmental and medullary SP-IR neurons that were retrogradely labelled following tracer deposits in the PVH also stained positively for choline acetyltransferase or tyrosine hydroxylase, respectively. (3) To evaluate the distribution and prominence of medullary SP-IR projections to the PVH and SO, staining for SP and catecholamine-synthesizing enzymes was carried out in animals that had previously received knife cuts at the level of the pontomedullary border. Pronounced, and roughly parallel decrements in staining for peptide and amines were seen in the magnocellular division of the PVH and in the SO; less marked reductions in SP-IR varicosities are in a position to influence multiple visceral regulatory cell types in the PVH and SO. Inputs to the magnocellular neurosecretory system arise in large measure from medullary neurons in which SP coexists with catecholamines. SP-IR projections to the parvicellular division of the PVH appear to originate from a number of sources.