Neuroanatomy of central cardiovascular control. Nucleus tractus solitarii: afferent and efferent neuronal connections in relation to the baroreceptor reflex arc

The medial prefrontal cortex and the cardiac baroreflex activity: physiological and pathological implications

The cardiac baroreflex is an autonomic neural mechanism involved in the modulation of the cardiovascular system. It influences the heart rate and peripheral vascular resistance to preserve arterial blood pressure within a narrow variation range. This mechanism is mainly controlled by medullary nuclei located in the brain stem. However, supramedullary areas, such as the ventral portion of medial prefrontal cortex (vMPFC), are also involved. Particularly, the glutamatergic NMDA/NO pathway in the vMPFC can facilitate baroreflex bradycardic and tachycardic responses. In addition, cannabinoid receptors in this same area can reduce or increase those cardiac responses, possibly through alteration in glutamate release. This vMPFC network has been associated to cardiovascular responses during stressful situations. Recent results showed an involvement of glutamatergic, nitrergic, and endocannabinoid systems in the blood pressure and heart rate increases in animals after aversive conditioning. Consequently, baroreflex could be modified by the vMPFC neurotransmission during stressful situations, allowing necessary cardiovascular adjustments. Remarkably, some mental, neurological and neurodegenerative disorders can involve damage in the vMPFC, such as posttraumatic stress disorder, major depressive disorder, Alzheimer's disease, and neuropathic pain. These pathologies are also associated with alterations in glutamate/NO release and endocannabinoid functions along with baroreflex impairment. Thus, the vMPFC seems to play a crucial role on the baroreflex control, either during pathological or physiological stress-related responses. The study of baroreflex mechanism under such pathological view may be helpful to establish causality mechanisms for the autonomic and cardiovascular imbalance found in those conditions. It can explain in the future the reasons of the high cardiovascular risk some neurological and neurodegenerative disease patients undergo. Additionally, the present work offers insights on the possible contributions of vMPFC dysfunction on baroreflex alterations, which, in turn, may raise questions in what extent other brain areas may play a role in autonomic deregulation under such pathological situations.

Reduced responses to glutamate receptor agonists follow loss of astrocytes and astroglial glutamate markers in the nucleus tractus solitarii

Saporin (SAP) or SAP conjugates injected into the nucleus tractus solitarii (NTS) of rats kill astrocytes. When injected in its unconjugated form, SAP produces no demonstrable loss of or damage to local neurons. However bilateral injections of SAP significantly attenuate responses to activation of baroreceptor reflexes that are mediated by transmission of signals through glutamate receptors in the NTS We tested the hypothesis that SAP would reduce cardiovascular responses to activation of NTS glutamate receptors despite its recognized ability to spare local neurons while killing local astrocytes. In animals treated with SAP and SAP conjugates or, as a control, with the toxin 6-hydroxydopamine (6-OHDA), we sought to determine if dose-related changes of arterial pressure (AP) or heart rate (HR) in response to injection into NTS of N-methyl-d-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were attenuated. Also we quantified changes in immunoreactivity (IR) for EAAT2, EAAC1, and VGluT2 in NTS after SAP and SAP conjugates. Our earlier studies showed that IR for NMDA and AMPA receptors was not changed after injection of SAP We found that EAAT2 and EAAC1, both found in astrocytes, were reduced by SAP or its conjugates but not by 6-OHDA In contrast, VGluT2-IR was increased by SAP or conjugates but not by 6-OHDA AP and HR responses to NMDA and AMPA were attenuated after SAP and SAP conjugate injection but not after 6-OHDA Results of this study are consistent with others that have shown interactions between astroglia and neurons in synaptic transmission mediated by glutamate receptor activation in the NTS.

Effects of acetylcholine and cholinergic antagonists on the activity of nucleus of the solitary tract neurons

Previously we have demonstrated that microinjection of acetylcholine (ACh) into the intermediate nucleus of the solitary tract (iNTS) induced sympatho-inhibition combined with a decrease in the phrenic nerve activity (PNA), whereas in the commissural NTS (cNTS), ACh did not change sympathetic nerve activity (SNA), but increased the PNA. In view of these demonstrated distinctive effects of ACh in different subnuclei of the NTS the current studies were undertaken to examine, using patch clamp techniques, the specific effects of ACh on the excitability of individual neurons in the NTS, as well as the neuropharmacology of these actions. Coronal slices of the brainstem containing either cNTS or iNTS subnuclei were used, and whole cell patch clamp recordings obtained from individual neurons in these two subnuclei. In cNTS, 58% of recorded neurons (n=12) demonstrated rapid reversible depolarizations in response to ACh (10mM), effects which were inhibited by the nicotinic antagonist mecamylamine (10μM), but unaffected by the muscarinic antagonist atropine (10μM). Similarly, bath application of ACh depolarized 76% of iNTS neurons (n=17), although in this case both atropine and mecamylamine reduced the ACh-induced depolarization. These data demonstrate that ACh depolarizes cNTS neurons through actions on nicotinic receptors, while depolarizing effects in iNTS are apparently mediated by both receptors.

Key role of 5-HT3 receptors in the nucleus tractus solitarii in cardiovagal stress reactivity

Serotonin plays a modulatory role in central control of the autonomic nervous system (ANS). The nucleus tractus solitarii (NTS) in the medulla is an area of viscerosomatic integration innervated by both central and peripheral serotonergic fibers. Influences from different origins therefore trigger the release of serotonin into the NTS and exert multiple influences on the ANS. This major influence on the ANS is also mediated by activation of several receptors in the NTS. In particular, the NTS is the central zone with the highest density of serotonin₃ (5-HT₃) receptors. In this review, we present evidence that 5-HT₃ receptors in the NTS play a key role in one of the crucial homeostatic responses to acute and chronic stress: inhibitory modulation of the parasympathetic component of the ANS. The possible functional interactions of 5-HT₃ receptors with GABA_A and NK₁ receptors in the NTS are also discussed.

Effects of stressor controllability on diurnal physiological rhythms

Disruptions in circadian and diurnal rhythms are associated with stress-related psychiatric disorders and stressor exposure can disrupt these rhythms. The controllability of the stressor can modulate various behavioral and neurochemical responses to stress. Uncontrollable, but not controllable, stress produces behaviors in rats that resemble symptoms of anxiety and depression. Whether acute stress-induced disruptions in physiological rhythms are sensitive to controllability of the stressor, however, remains unknown. To examine the role of controllability in diurnal rhythm disruption, adult male Sprague-Dawley rats were implanted with Data Sciences International (DSI) biotelemetry devices. Real-time measurements were obtained before, during and after exposure to a controllable or yoked uncontrollable stressor. Controllable and uncontrollable stress equally disrupted diurnal rhythms of locomotor activity and body temperature but not heart rate. The diurnal heart rate the day following stressor exposure was flattened to a greater extent and was significantly higher in rats with control over stress suggesting a relationship between stressor controllability and the heart rate response. Our results are consistent with the conclusion that acute stress-induced disruptions in diurnal physiological rhythms likely contribute little to the behavioral and affective consequences of stress that are sensitive to stressor controllability.

Inhibitory mechanism of the nucleus of the solitary tract involved in the control of cardiovascular, dipsogenic, hormonal, and renal responses to hyperosmolality

The nucleus of the solitary tract (NTS) is the primary site of visceral afferents to the central nervous system. In the present study, we investigated the effects of lesions in the commissural portion of the NTS (commNTS) on the activity of vasopressinergic neurons in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, plasma vasopressin, arterial pressure, water intake, and sodium excretion in rats with plasma hyperosmolality produced by intragastric 2 M NaCl (2 ml/rat). Male Holtzman rats with 15-20 days of sham or electrolytic lesion (1 mA; 10 s) of the commNTS were used. CommNTS lesions enhanced a 2 M NaCl intragastrically induced increase in the number of vasopressinergic neurons expressing c-Fos in the PVN (28 ± 1, vs. sham: 22 ± 2 c-Fos/AVP cells) and SON (26 ± 4, vs. sham: 11 ± 1 c-Fos/AVP cells), plasma vasopressin levels (21 ± 8, vs. sham: 6.6 ± 1.3 pg/ml), pressor responses (25 ± 7 mmHg, vs. sham: 7 ± 2 mmHg), water intake (17.5 ± 0.8, vs. sham: 11.2 ± 1.8 ml/2 h), and natriuresis (4.9 ± 0.8, vs. sham: 1.4 ± 0.3 meq/1 h). The pretreatment with vasopressin antagonist abolished the pressor response to intragastric 2 M NaCl in commNTS-lesioned rats (8 ± 2.4 mmHg at 10 min), suggesting that this response is dependent on vasopressin secretion. The results suggest that inhibitory mechanisms dependent on commNTS act to limit or counterbalance behavioral, hormonal, cardiovascular, and renal responses to an acute increase in plasma osmolality.

Macrophage migration inhibitory factor in the nucleus of solitary tract decreases blood pressure in SHRs

AIMS

The macrophage migration inhibitory factor (MIF) is an intracellular inhibitor of the central nervous system actions of angiotensin II on blood pressure. Considering that angiotensin II actions at the nucleus of the solitary tract are important for the maintenance of hypertension in spontaneously hypertensive rats (SHRs), we tested if increased MIF expression in the nucleus of the solitary tract of SHR alters the baseline high blood pressure in these rats.

METHODS AND RESULTS

Eight-week-old SHRs or normotensive rats were microinjected with the vector AAV2-CBA-MIF into the nucleus of the solitary tract, resulting in MIF expression predominantly in neurons. Rats also underwent recordings of the mean arterial blood pressure (MAP) and heart rate (via telemetry devices implanted in the abdominal aorta), cardiac- and baroreflex function. Injections of AAV2-CBA-MIF into the nucleus of the solitary tract of SHRs produced significant decreases in the MAP, ranging from 10 to 20 mmHg, compared with age-matched SHRs that had received identical microinjections of the control vector AAV2-CBA-eGFP. This lowered MAP in SHRs was maintained through the end of the experiment at 31 days, and was associated with an improvement in baroreflex function to values observed in normotensive rats. In contrast to SHRs, similar increased MIF expression in the nucleus of the solitary tract of normotensive rats produced no changes in baseline MAP and baroreflex function.

CONCLUSION

These results indicate that an increased expression of MIF within the nucleus of the solitary tract neurons of SHRs lowers blood pressure and restores baroreflex function.

Views of translational research from a somewhat translational scientist

This review arose from a talk entitled "Identifying Targets" and given by the author at EB2011 at the invitation of the American Federation for Medical Research (AFMR). The presentation was part of the American Federation for Medical Research workshop entitled "Keys for Translation: Science and Strategy" and focused on identifying clinically relevant targets as a result of observations made during basic scientific studies. The review emphasizes that targets do not have to be the aim that drives basic discovery, but communication between the basic scientist and clinical investigators may aid recognition of such targets and their translation to clinical applications. Using one line of investigator-initiated research from his own laboratory as an example, the author emphasizes that basic discovery must be hypothesis driven and allowed to follow its logical sequence. Finding treatments, while always an aim of biomedical research, may arise as a result of basic studies that were not originally aimed at a target of translational research.

Angiotensinergic signaling in the brain mediates metabolic effects of deoxycorticosterone (DOCA)-salt in C57 mice

Low-renin hypertension accounts for ≈ 25% of essential hypertensive patients. It is modeled in animals by chronic delivery of deoxycorticosterone acetate and excess dietary sodium (the DOCA-salt model). Previous studies have demonstrated that DOCA-salt hypertension is mediated through activation of the brain renin-angiotensin system. Here, we demonstrate robust metabolic phenotypes of DOCA-salt treatment. Male C57BL/6J mice (6 to 8 weeks old) received a subcutaneous pellet of DOCA (50 mg for 21 days) and were offered a 0.15 mol/L NaCl drink solution in addition to regular chow and tap water. Treatment resulted in mild hypertension, a blunting of weight gain, gross polydipsia, polyuria, and sodium intake, alterations in urinary sodium and potassium turnover, and serum sodium retention. Most strikingly, DOCA-salt mice exhibited no difference in food intake but did exhibited a large elevation in basal metabolic rate. Normalization of blood pressure by hydralazine (500 mg/L in drink solutions) attenuated the hydromineral phenotypes and renal renin suppression effects of DOCA-salt but had no effect on the elevated metabolic rate. In contrast, intracerebroventricular infusion of the angiotensin II type 1 receptor antagonist losartan (5 μg/h) attenuated the elevation in metabolic rate with DOCA-salt treatment. Together, these data illustrate the necessity of angiotensinergic signaling within the brain, independent of blood pressure alterations, in the metabolic consequences of DOCA-salt treatment.

Connexin 40 mediates the tubuloglomerular feedback contribution to renal blood flow autoregulation

Connexins are important in vascular development and function. Connexin 40 (Cx40), which plays a predominant role in the formation of gap junctions in the vasculature, participates in the autoregulation of renal blood flow (RBF), but the underlying mechanisms are unknown. Here, Cx40-deficient mice (Cx40-ko) had impaired steady-state autoregulation to a sudden step increase in renal perfusion pressure. Analysis of the mechanisms underlying this derangement suggested that a marked reduction in tubuloglomerular feedback (TGF) in Cx40-ko mice was responsible. In transgenic mice with Cx40 replaced by Cx45, steady-state autoregulation and TGF were weaker than those in wild-type mice but stronger than those in Cx40-ko mice. N omega-Nitro-L-arginine-methyl-ester (L-NAME) augmented the myogenic response similarly in all genotypes, leaving autoregulation impaired in transgenic animals. The responses of renovascular resistance and arterial pressure to norepinephrine and acetylcholine were similar in all groups before or after L-NAME inhibition. Systemic and renal vasoconstrictor responses to L-NAME were also similar in all genotypes. We conclude that Cx40 contributes to RBF autoregulation by transducing TGF-mediated signals to the afferent arteriole, a function that is independent of nitric oxide (NO). However, Cx40 is not required for the modulation of the renal myogenic response by NO, norepinephrine-induced renal vasoconstriction, and acetylcholine- or NO-induced vasodilation.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Emetic stimulation inhibits the swallowing reflex in decerebrate rats

The effects of emetic stimulation on the swallowing reflex were investigated in decerebrated rats. Hypoxia, gastric distension and LiCl administration were used as emetic stimulations. The swallowing reflex was elicited by electrical stimulation of the superior laryngeal nerve (SLN, 20 Hz, 3-5 V, 0.3 ms duration) for 20 s. To examine the effect of hypoxia, nitrogen gas was inhaled under artificial ventilation. There were significantly fewer swallows during a decrease in PO(2) than under air ventilation (p<0.05). The number of swallows during 3-ml stomach distension was significantly lower than that before distension (p<0.05). Intravenous administration of LiCl (100 mg/kg) also significantly reduced the number of swallows (p<0.05). The combination of SLN stimulation and emetic stimuli occasionally produced burst activity of abdominal muscles, which might be associated with the gag reflex. Both the gag and swallowing reflexes are well known to be mediated by the nucleus of the solitary tract. The physiological roles of the gag reflex and the swallowing reflex are considered to be reciprocal. Taken together, these results suggest that emetic stimulation inhibits the swallowing pattern generator via the nucleus of the solitary tract, which in turn facilitates the gag reflex.

Activation of nucleus tractus solitarius 5-HT2A but not other 5-HT2 receptor subtypes inhibits the sympathetic activity in rats

Our first aim was to elucidate the mechanisms underlying the hypotensive response elicited by 5-HT(2) receptor activation in the nucleus tractus solitarius (NTS). In pentobarbitone-anaesthetized rats, intra-NTS administration of 2,5-dimethoxy-4-iodoamphetamine (DOI), a wide spectrum 5-HT(2) receptor agonist, but not an antagonist of selective 5-HT(2B) and 5-HT(2C) receptors, produced a decrease in blood pressure and heart rate. The maximal cardiovascular changes obtained by DOI (0.5 pmol) could be almost completely abolished by prior intra-NTS microinjection (10 pmol) of MDL-100907, a selective 5-HT(2A) receptor antagonist, but not by 5-HT(2B) or 5-HT(2C) receptor antagonists. In addition, using extracellular recordings we found that the large majority of identified cardiovascular rostroventrolateral medulla (RVLM) neurons were almost totally inhibited by NTS 5-HT(2A) receptor stimulation. We then investigated whether intra-NTS administration of a subthreshold dose (0.05 pmol) of DOI, known to facilitate the cardiovagal component of the baroreflex, could also modulate the sympathoinhibitory component of this reflex. These experiments showed that neither the decrease in the activity of the cardiovascular RVLM neurons and lumbar sympathetic nerve activities produced by aortic occlusion (gain of the baroreflex), nor the hypotensive response elicited by aortic nerve stimulation, were potentiated by the microinjection of DOI under such conditions. These data show that activation of 5-HT(2A), but not 5-HT(2B) or 5-HT(2C), receptors, located on NTS neurons, elicits depressor and bradycardic responses, and that this 5-HT(2A)-mediated hypotension is produced via the inhibition of RVLM cardiovascular neurons. In addition, NTS 5-HT(2A) receptor activation facilitates the cardiac but not the sympathetic baroreflex response.

Divergent projections of catecholaminergic neurons in the nucleus of the solitary tract to limbic forebrain and medullary autonomic brain regions

The nucleus of the solitary tract (NTS) is a critical structure involved in coordinating autonomic and visceral activities. Previous independent studies have demonstrated efferent projections from the NTS to the nucleus paragigantocellularis (PGi) and the central nucleus of the amygdala (CNA) in rat brain. To further characterize the neural circuitry originating from the NTS with postsynaptic targets in the amygdala and medullary autonomic targets, distinct green or red fluorescent latex microspheres were injected into the PGi and the CNA, respectively, of the same rat. Thirty-micron thick tissue sections through the lower brainstem and forebrain were collected. Every fourth section through the NTS region was processed for immunocytochemical detection of tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. Retrogradely labeled neurons from the PGi or CNA were distributed throughout the rostro-caudal segments of the NTS. However, the majority of neurons containing both retrograde tracers were distributed within the caudal third of the NTS. Cell counts revealed that approximately 27% of neurons projecting to the CNA in the NTS sent collateralized projections to the PGi while approximately 16% of neurons projecting to the PGi sent collateralized projections to the CNA. Interestingly, more than half of the PGi and CNA-projecting neurons in the NTS expressed TH immunoreactivity. These data indicate that catecholaminergic neurons in the NTS are poised to simultaneously coordinate activities in limbic and medullary autonomic brain regions.

Cardiac baroreflex facilitation evoked by hypothalamus and prefrontal cortex stimulation: role of the nucleus tractus solitarius 5-HT2Areceptors

We previously showed that serotonin (5-HT2) receptor activation in the nucleus of the tractus solitarius (NTS) produced hypotension, bradycardia, and facilitation of the baroreflex bradycardia. Activation of the preoptic area (POA) of the hypothalamus, which is involved in shock-evoked passive behaviors, induces similar modifications. In addition, previous studies showed that blockade of the infralimbic (IL) part of the medial prefrontal cortex, which sends projections to POA, produced an inhibitory influence on the baroreflex cardiac response. Thus, to assess the possible implication of NTS 5-HT2receptors in passive cardiovascular responses, we analyzed in anesthetized rats the effects of NTS inhibition and NTS 5-HT2receptor blockade on the cardiovascular modifications induced by chemical (0.3 M d,l-homocysteic acid) and electrical (50 Hz, 150–200 μA) stimulation of IL or POA. Intra-NTS microinjections of muscimol, a GABAAreceptor agonist, prevented the decreases in blood pressure and heart rate normally evoked by IL or POA activation. In addition, we found that intra-NTS microinjection of R(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol, a specific 5-HT2Areceptor antagonist, did not affect the decreases in cardiovascular baseline parameters induced by IL or POA stimulation but prevented the facilitation of the aortic baroreflex bradycardia normally observed during IL (+65 and +60%) or POA (+70 and +69%) electrical and chemical stimulation, respectively. These results show that NTS 5-HT2Areceptors play a key role in the enhancement of the cardiac response of the baroreflex but not in the changes in basal heart rate and blood pressure induced by IL or POA stimulation.

Disinhibition of the cardiac limb of the arterial baroreflex in rat: a role for metabotropic glutamate receptors in the nucleus tractus solitarii

The nucleus tractus solitarii (NTS) is the first site of integration for primary baroreceptor afferents, which release glutamate to excite second-order neurones through ionotropic receptors. In vitro studies indicate that glutamate may also activate metabotropic receptors (mGluRs) to modulate the excitability of NTS neurones at pre- and postsynaptic loci. We examined the functional role of metabotropic glutamate receptors (mGluRs) in modulating the baroreceptor reflex in the rat NTS. Using the working heart-brainstem preparation, the baroreflex was activated using brief pressor stimuli and the consequent cardiac (heart rate change) and non-cardiac sympathetic (T8-10 chain) baroreflex gains were obtained. Microinjections of glutamate antagonists were made bilaterally into the NTS at the site of termination of baroreceptor afferents. NTS microinjection of kynurenate (ionotropic antagonist) inhibited both the cardiac and sympathetic baroreflex gains (16 +/- 5% and 59 +/- 11% of control, respectively). The non-selective mGluR antagonist MCPG produced a dose-dependent inhibition of the cardiac gain (30 +/- 3% of control) but not the sympathetic gain. Selective inhibitions of the cardiac gain were also seen with LY341495 and EGLU suggesting the response was mediated by group II mGluRs. This effect on cardiac gain involves attenuation of the parasympathetic baroreflex as it persists in the presence of atenolol. Prior NTS microinjection of bicuculline (GABA(A) antagonist) prevented the mGluR-mediated attenuation of the cardiac gain. These results are consistent with the reported presynaptic inhibition of GABAergic transmission by group II mGluRs in the NTS and constitute a plausible mechanism allowing selective feed-forward disinhibition to increase the gain of the cardiac limb of the baroreflex without changing the sympathoinhibitory component.

Urethane inhibits the GABAergic neurotransmission in the nucleus of the solitary tract of rat brain stem slices

Because urethane is a widely used anesthetic in animal experimentation, in the present study, we evaluated its effects on neurons of the nucleus of the solitary tract (NTS) in brain stem slices from young rats (25-30 days old). Using the whole cell configuration of the patch-clamp technique, spontaneous postsynaptic currents (sPSCs) and evoked excitatory postsynaptic currents (eEPSCs) were recorded. Urethane (20 mM) decreased by approximately 60% the frequency of GABAergic sPSCs (1.0 +/- 0.2 vs. 0.4 +/- 0.1 Hz) but did not change the frequency, amplitude, or half-width of glutamatergic events or TTX-resistant inhibitory sPSCs [miniature inhibitory postsynaptic currents (IPSCs)]. Miniature IPSCs were measured in the presence of urethane plus 1 mM diazepam (1 mM), and no changes were seen in their amplitude. This suggests that the GABA concentration in the NTS synapses is set at saturating level. We also evaluated the effect of urethane on eEPSCs, and no significant change was observed in the amplitude of N-methyl-d-aspartate [NMDA; 44.2 +/- 11.5 vs. 37.6 +/- 10.6 pA (holding potential = 40 mV)] and non-NMDA currents [204.4 +/- 35.5 vs. 196.6 +/- 31.2 pA (holding potential = -70 mV)]. Current-clamp experiments showed that urethane did not alter the action potential characteristics and passive membrane properties. These data suggest that urethane has an inhibitory effect on GABAergic neurons in the NTS but does not change the spontaneous or evoked excitatory responses.

A novel central pathway links arterial baroreceptors and pontine parasympathetic neurons in cerebrovascular control

1. We tested the hypothesis that arterial baroreceptor reflexes modulate cerebrovascular tone through a pathway that connects the cardiovascular nucleus tractus solitarii with parasympathetic preganglionic neurons in the pons. 2. Anesthetized rats were used in all studies. Laser flowmetry was used to measure cerebral blood flow. We assessed cerebrovascular responses to increases in arterial blood pressure in animals with lesions of baroreceptor nerves, the nucleus tractus solitarii itself, the pontine preganglionic parasympathetic neurons, or the parasympathetic ganglionic nerves to the cerebral vessels. Similar assessments were made in animals after blockade of synthesis of nitric oxide, which is released by the parasympathetic nerves from the pterygopalatine ganglia. Finally the effects on cerebral blood flow of glutamate stimulation of pontine preganglionic parasympathetic neurons were evaluated. 3. We found that lesions at any one of the sites in the putative pathway or interruption of nitric oxide synthesis led to prolongation of autoregulation as mean arterial pressure was increased to levels as high as 200 mmHg. Conversely, stimulation of pontine parasympathetic preganglionic neurons led to cerebral vasodilatation. The second series of studies utilized classic anatomical tracing methods to determine at the light and electron microscopic level whether neurons in the cardiovascular nucleus tractus solitarii, the site of termination of baroreceptor afferents, projected to the pontine preganglionic neurons. Fibers were traced with anterograde tracer from the nucleus tractus solitarii to the pons and with retrograde tracer from the pons to the nucleus tractus solitarii. Using double labeling techniques we further studied synapses made between labeled projections from the nucleus tractus solitarii and preganglionic neurons that were themselves labeled with retrograde tracer placed into the pterygopalatine ganglion. 4. These anatomical studies showed that the nucleus tractus solitarii directly projects to pontine preganglionic neurons and makes asymmetric, seemingly excitatory, synapses with those neurons. These studies provide strong evidence that arterial baroreceptors may modulate cerebral blood flow through direct connections with pontine parasympathetic neurons. Further study is needed to clarify the role this pathway plays in integrative physiology.

Angiotensin potentiates excitatory sensory synaptic transmission to medial solitary tract nucleus neurons

Femtomole doses of angiotensin (ANG) II microinjected into nucleus tractus solitarii (nTS) decrease blood pressure and heart rate, mimicking activation of the baroreflex, whereas higher doses depress this reflex. ANG II might generate cardioinhibitory responses by augmenting cardiovascular afferent synaptic transmission onto nTS neurons. Intracellular recordings were obtained from 99 dorsal medial nTS region neurons in rat medulla horizontal slices to investigate whether ANG II modulated short-latency excitatory postsynaptic potentials (EPSPs) evoked by solitary tract (TS) stimulation. ANG II (200 fmol) increased TS-evoked EPSP amplitudes 20-200% with minimal membrane depolarization in 12 neurons excited by ANG II and glutamate, but not substance P (group A). Blockade of non-N-methyl-d-aspartate receptors eliminated TS-evoked EPSPs and responses to ANG II. ANG II did not alter TS-evoked EPSPs in 14 other neurons depolarized substantially by ANG II and substance P (group B). ANG II appeared to selectively augment presynaptic sensory transmission in one class of nTS neurons but had only postsynaptic effects on another group of cells. Thus ANG II is likely to modulate cardiovascular function by more than one nTS neuronal pathway.

Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [125I] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy

We have examined the binding distribution of a selective AT(2) receptor ligand [125I] CGP42112 in the brain of adult Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). AT(2) receptor localization was also examined in the rat brainstem following unilateral nodose ganglionectomy. Specific [125I] CGP42112 binding was observed in discrete brain regions from both rat strains, including the nucleus of the solitary tract (NTS), and did not differ between WKY and SHR. [125I] CGP42112 binding in the NTS revealed an AT(2) receptor component that was displaceable by PD 123319 and Ang II (50-58%), as well as a non-angiotensin II receptor component (42-49%). Following unilateral nodose ganglionectomy, [125I] CGP42112 binding density on the denervated side of the NTS was increased approximately two-fold in both WKY and SHR. This increased [125I] CGP42112 binding density in the ipsilateral NTS was comprised of a greater non-angiotensin II component than that observed in the sham groups, since only approximately 30% was displaced by PD123319 and angiotensin II. Furthermore, [125I] CGP42112 also revealed high binding density on the denervated side in the dorsal motor nucleus and the nucleus ambiguus in both WKY and SHR. AT(2) receptor immunoreactivity was also visualised in the NTS of sham operated rats, but was not observed in the dorsal motor nucleus or the nucleus ambiguus, nor was it up-regulated following nodose ganglionectomy. These results demonstrate, for the first time, an AT(2) receptor binding site in the NTS, as well as a non-angiotensin II [125I] CGP42112 binding site. These studies also demonstrate that nodose ganglionectomy represents a useful model in which to study a non-angiotensin II [125I] CGP42112 binding site that is up-regulated following degeneration of afferent vagal nerves.

Antagonism of glutamatergic metabotropic receptors in the NTS of awake rats does not affect the gain of the baroreflex

There is evidence suggesting that metabotropic receptors may play a role in the neurotransmission of the baroreflex in the nucleus tractus solitarius (NTS) of rats. In a recent study from our laboratory, we verified that microinjection of a metabotropic receptor agonist, trans-1-amino-1,3-cyclopentanediocarboxylic acid, into the NTS of awake and anesthetized rats produced baroreflex-like responses (hypotension and bradycardia). In the present study, we evaluated the possible role of L-glutamate metabotropic receptors of the NTS in the neuromodulation of the parasympathetic component of baroreflex activation in awake rats. Bilateral microinjection (50 nl) of a metabotropic receptor antagonist (alpha-methyl-4-carboxyphenylglycine, MCPG, 100 mM) into the rostral commissural NTS produced no change in the gain of the baroreflex bradycardia. In addition, microinjection of MCPG into the NTS produced no changes in baseline mean arterial pressure (MAP) or heart rate (HR), indicating that metabotropic receptors play no tonic role in the neurotransmission of the baroreflex. The dose of MCPG used to block the metabotropic receptors was effective in reducing the bradycardic and hypotensive responses to microinjection (50 nl) of trans-1-amino-1,3-cyclopentanediocarboxylic acid (5 mM) into the NTS. The data show that metabotropic glutamate receptors play no major role in the neuromodulation of the parasympathetic component of the baroreflex at the NTS level.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Renal injury caused by intrarenal injection of phenol increases afferent and efferent renal sympathetic nerve activity

Intrarenal injection of phenol in rats causes a persistent elevation in blood pressure (BP) and in norepinephrine (NE) secretion from the posterior hypothalamus (PH), and downregulation of neuronal nitric oxide synthase (nNOS) and interleukin-1beta (IL-1beta) in the PH. These studies suggest that afferent impulses from the kidney to the brain may be responsible for hypertension associated with renal injury. Downregulation of nNOS and IL-1beta, two modulators of sympathetic nervous system (SNS) activity may mediate this activation. In this study we measured the effects of intrarenal phenol injection on peripheral SNS activity by direct renal nerve recording, plasma NE, nNOS, and IL-1beta abundance in the brain. We also determined whether renal denervation or administration of clonidine prevented these effects of phenol. Acutely, the phenol injection increased both afferent and efferent renal sympathetic nerve activity, decreased urinary sodium excretion, and increased plasma NE. Three weeks after the phenol injection, BP and plasma NE remained elevated. Renal denervation and pretreatment with clonidine prevented the increase in BP and plasma NE caused by phenol. Chronic renal injury caused by phenol was associated with decreased abundance of IL-1beta and nNOS in the PH. These studies have shown that a renal injury caused by phenol injection increases BP and central as well as peripheral SNS activity, which persist long after the injury. Renal denervation and antiadrenergic drugs abolish the effects of phenol on BP and plasma NE. Because NO and IL-1beta modulate SNS activity, the stimulatory action of phenol on the SNS could be mediated by downregulation of nNOS and IL-1beta in the brain.

Downregulation of neuronal nitric oxide synthase and interleukin-1beta mediates angiotensin II-dependent stimulation of sympathetic nerve activity

There is substantial evidence that angiotensin II (Ang II) enhances sympathetic nervous system (SNS) activity. We recently observed that nitric oxide and interleukin-1beta (IL-1beta) exert a tonic inhibitory action on central SNS activity. Moreover, in 2 rat models of neurogenic hypertension, one caused by intrarenal injection of phenol and the other by 5/6 nephrectomy, we observed that losartan, an Ang II type 1 receptor blocker, inhibits SNS activity and increases the abundance of IL-1beta and the neuronal isoform of nitric oxide synthase (nNOS) in the posterior hypothalamic nuclei (PH), paraventricular nuclei (PVN), and locus ceruleus (LC). This raises the possibility that the stimulatory effects of Ang II on central SNS activity may be mediated by inhibition of nNOS and IL-1beta. To test this hypothesis, we studied the effect of an intracerebroventricular (ICV) infusion of Ang II on blood pressure (BP), norepinephrine (NE) secretion from the PH, renal SNS activity (RSNA), and abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC of normal Sprague-Dawley rats. Finally, we measured the concentration of nitrite/nitrate in the dialysate collected from the PH after Ang II or vehicle. ICV infusion of Ang II (100 ng/kg body wt dissolved in 10 microL of artificial cerebrospinal fluid) raised BP, RSNA, and NE secretion from the PH compared with control rats. Ang II reduced the abundance of IL-1beta and nNOS mRNA in the PH, PVN, and LC. Pretreatment with losartan (10 microg/kg body wt dissolved in 10 microL of aCSF) given ICV 20 minutes before Ang II abolished the effects of Ang II on BP, RSNA, and NE secretion from the PH and IL-1beta and nNOS mRNA. Ang II also decreased the secretion of NO from the PH. In conclusion, these studies suggest that Ang II inhibits the expression of IL-1beta and nNOS in the brain. Because locally produced NO exerts a tonic inhibitory action on SNS activity, the decrease in NO expression caused by Ang II results in greater SNS activity.

Stimulation of 5-HT2 receptors in the nucleus tractus solitarius enhances NMDA receptor-mediated reflex-evoked bradycardiac responses in the rat

The modulation by 5-HT2 receptors in the nucleus tractus solitarius of the reflex bradycardia evoked by stimulation of peripheral baroreceptors and cardiopulmonary chemoreceptors, and their possible functional interactions with local NMDA receptors, were investigated in pentobarbital- and urethane-anaesthetized rats, respectively. Microinjection of the 5-HT2 receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (0.1-0.5 pmol), into the nucleus tractus solitarius elicited a dose-dependent hypotension and bradycardia. Bilateral microinjections at the same site of a subthreshold dose of 2,5-dimethoxy-4-iodoamphetamine (0.05 pmol) significantly enhanced the aforementioned reflex-evoked bradycardiac responses. In contrast, local bilateral microinjections of the NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (500 and 1000 pmol), reduced, in a dose-dependent manner, both reflex-evoked responses. The facilitatory effect of 2,5-dimethoxy-4-iodoamphetamine upon these reflex-evoked bradycardiac responses was prevented by prior local microinjection of low doses of either the selective 5-HT2 receptor antagonist, ketanserin (10 pmol), or 2-amino-5-phosphonopentanoic acid (100 pmol), which, on their own, did not affect the reflex-associated bradycardia. These data suggest that 5-HT2 receptors within the nucleus tractus solitarius participate in a facilitatory modulation of the reflex control of heart rate, probably through functional interactions with local NMDA receptors.

Distribution of alpha 2-adrenergic receptor binding in the developing human brain stem

Rapid and dramatic changes occur in cardiorespiratory function during early human life. Catecholamines within select brain stem nuclei are implicated in the control of autonomic and respiratory function, including in the nucleus of the solitary tract and the dorsal motor nucleus of X. Animal and adult human studies have shown high binding to alpha 2-adrenergic receptors in these regions. To determine the developmental profile of brainstem alpha 2-adrenergic binding across early human life, we studied brain stems from five fetuses at midgestation, three newborns (37-38 postconceptional weeks), and six infants (44-61 postconceptional weeks). We used quantitative tissue receptor autoradiography with [3H]para-aminoclonidine as the radioligand and phentolamine as the displacer. In the fetal group, binding was high (63-93 fmol/mg tissue) in the nucleus of the solitary tract, dorsal motor nucleus of X, locus coeruleus, and reticular formation; it was low (< 32 fmol/mg tissue) in the principal inferior olive and basis pontis. Binding decreased in all regions with age: in infancy, the highest binding was in the intermediate range (32-62 fmol/mg tissue) and was localized to the nucleus of the solitary tract and dorsal motor nucleus of X. The most substantial decrease in binding (75%-85%) between the fetal and infant periods occurred in the pontine and medullary reticular formation and hypoglossal nucleus. Binding remained low in the principal inferior olive and basis pontis. The decreases in binding with age remained significant after quench correction. These data suggest that rapid and dramatic changes occur in early human life in the brain stem catecholaminergic system in regions related to cardiorespiratory control.

Interleukin-1beta and neurogenic control of blood pressure in normal rats and rats with chronic renal failure

Increased sympathetic nervous system (SNS) activity plays a role in the genesis of hypertension in rats with chronic renal failure (CRF). The rise in central SNS activity is mitigated by increased local expression of neuronal nitric oxide synthase (NOS) mRNA and NO(2)/NO(3) production. Because interleukin (IL)-1beta may activate nitric oxide in the brain, we have tested the hypothesis that IL-1beta may modulate the activity of the SNS via regulation of the local expression of neuronal NOS (nNOS) in the brain of CRF and control rats. To this end, we first found that administration of IL-1beta in the lateral ventricle of control and CRF rats decreased blood pressure and norepinephrine (NE) secretion from the posterior hypothalamus (PH) and increased NOS mRNA expression. Second, we observed that an acute or chronic injection of an IL-1beta-specific antibody in the lateral ventricle raised blood pressure and NE secretion from the PH and decreased NOS mRNA abundance in the PH of control and CRF rats. Finally, we measured the IL-1beta mRNA abundance in the PH, locus coeruleus, and paraventricular nuclei of CRF and control rats by RT-PCR and found it to be greater in CRF rats than in control rats. In conclusion, these studies have shown that IL-1beta modulates the activity of the SNS in the central nervous system and that this modulation is mediated by increased local expression of nNOS mRNA.

Autonomic ganglionic neurones in rabbits with differing resistance to emotional stress

The aim of the present study was to evaluate the reactions of autonomic neurones of the nodose ganglion of the vagus nerve, and the stellate and superior cervical ganglia in rabbits under emotional stress, and to correlate these reactions with the individual variations in responses to the stressor. Emotional stress was induced in immobilized adult male Chinchilla rabbits by electrical stimulation of the ventromedial hypothalamus and skin. During the experiment (3 hours) arterial blood pressure (BP) was recorded. Metabolic activities of the stellate and superior cervical sympathetic ganglia and nodose ganglion were measured as contents of biogenic amines and their synthesizing and degrading enzymes, neuronal size and dry mass and total RNA; these corresponded to the changes in BP. One group of rabbits showed small fluctuations of BP throughout the experiment and were defined as resistant to stress, whereas in the other group (predisposed to stress) BP progressively decreased. In the former, there was a smaller increase of sympathetic and nodose ganglia metabolic activity than in the latter, in which changes included reduced neuronal dry mass, increased RNA content compared with controls, and reduced tyrosine hydroxylase activity and increased norepinephrine content compare with controls and stress- resistant rabbits. The predisposed rabbits showed earlier and greater increases in circulating norepinephrine concentrations than the resistant rabbits, indicating sustained sympathetic activation. The data indicate that the ganglia of the sympathetic nervous system are part of a major mechanism of BP regulation under acute experimental emotional/painful stress. The nodose ganglion participates in the maintenance of stable cardiovascular function in extreme conditions.

Activation of GABA receptors in the NTS of awake rats reduces the gain of baroreflex bradycardia

In the present study we evaluated the effects of bilateral microinjection of muscimol (a GABA(A) receptor agonist) and baclofen (a GABA(B) receptor agonist) into the lateral commissural nucleus tractus solitarii (NTS) of awake rats on the gain of the baroreflex (BG) activated by a short duration (10-15 s) infusion of phenylephrine (Phe, 2.5 microg/0.05 ml, i.v.). Microinjection of muscimol (50 pmol/50 nl, n=8) into the NTS produced a significant increase in baseline mean arterial pressure ((MAP) 122+/-6 vs. 101+/-2 mmHg), no changes in baseline heart rate (HR) and a reduction in BG (-1.59+/-0. 1 vs. -0.69+/-0.1 beats/mmHg). Microinjection of baclofen (6.25 pmol/50 nl, n=6) into the NTS also produced a significant increase in baseline MAP (138+/-5 vs. 103+/-2 mmHg), no changes in baseline HR and a reduction in BG (-1.54+/-0.3 vs. -0.53+/-0.2 beats/mmHg). Considering that the reduction in BG could be secondary to the increase in MAP in response to microinjection of muscimol (n=6) or baclofen (n=7) into the NTS, in these two groups of rats we brought the MAP back to baseline by infusion of sodium nitroprusside (NP, 3.0 microg/0.05 ml, i.v.). Under these conditions, we verified that the BG remained significantly reduced after muscimol (-1.49+/-0.2 vs. -0.35+/-0.2 beats/mmHg) and after baclofen (-1.72+/-0.2 vs. -0.33+/-0.2 beats/mmHg) when compared to control. Reflex tachycardia was observed during the normalization of MAP by NP infusion and, in order to prevent the autonomic imbalance from affecting BG, we used another group of rats treated with atenolol (5 mg/kg, i.v.), a beta1 receptor antagonist. In rats previously treated with atenolol and submitted to NP infusion, we verified that BG remained reduced after microinjection of muscimol or baclofen into the NTS. The data show that activation of GABA(A) and GABA(B) receptors, independently of the changes in the baseline MAP or HR, inhibited the neurons of the NTS involved in the parasympathetic component of the baroreflex.

Losartan reduces sympathetic nerve outflow from the brain of rats with chronic renal failure

Sympathetic nervous system (SNS) activity, measured by norepinephrine (NE) turnover rate, was greater in the posterior hypothalamic (PH) nuclei, the paraventricular nuclei (PVN), and the locus coeruleus (LC) of 5/6 nephrectomised (CRF) rats than of control rats. NE secretion from the PH was also greater in CRF than in control rats. These findings demonstrate that SNS activity plays an important role in the genesis of hypertension associated with CRF. The increase in central SNS activity was mitigated by increased local expression of nitric oxide synthase (NOS)-mRNA and nitric oxide (NOx) production. Because angiotensin II may stimulate the central SNS, we tested the hypothesis that losartan, a specific angiotensin II AT(1)-receptor antagonist, may lower blood pressure (BP), at least in part, by central noradrenergic inhibition. To this end, we studied two groups of CRF rats. One group received losartan (10 mg/kg body weight) in drinking water between the 3rd and 4th week after nephrectomy, the second group received drinking water without losartan. SNS activity was measured by NE secretion from the PH using the microdialysis technique. NOS-mRNA gene expression was also measured by RT-PCR in the PH, PVN, and LC of CRF and control rats. Losartan reduced systolic BP from 184+/-3.7 to 152+/-3.1 mmHg and NE secretion from the PH from 340+/-9.7 to 247+/-4.8 pg/ml. CRF rats treated with losartan manifested a significant (p<0.01) increase in the expression of nNOS-mRNA in the PH (from 84+/-1.2 to 99+/-2.6), the PVN (from 44+/-1.5 to 63+/-2.1), and the LC (from 59+/-6.7 to 76+/-2.1). CRF rats also manifested a significant increase (p<0.01) in the expression of IL-1beta the PH (from 41.6+/-2.8 to 54.3+/-1.4), PVN (from 44+/-1.9 to 54+/-1.5), and LC (from 35.5+/-1.6 to 53.5+/-1.9). In conclusion, these studies suggest that the antihypertensive action of losartan in CRF rats may be mediated, at least in part, by inhibition of central SNS outflow. The studies also suggest that the inhibitory action of losartan on the SNS may be mediated by activation of IL-1beta, which, in turn, stimulates nNOS, an important modulator of central SNS activity.

Expression of 15 glutamate receptor subunits and various splice variants in tissue slices and single neurons of brainstem nuclei and potential functional implications

Brainstem nuclei serve a diverse array of functions in many of which ionotropic glutamate receptors are known to be involved. However, little detailed information is available on the expression of different glutamate receptor subunits in specific nuclei. We used RT-PCR in mice to analyze the glutamate receptor subunit composition of the pre-Bötzinger complex, the hypoglossal nucleus, the nucleus of the solitary tract, and the inferior olive. Analyzing 15 receptor subunits and five variants, we found all four alpha-amino-3-hydroxy-5-methyl-4-propionic acid (AMPA) and six NMDA receptor (NR) subunits as well as three of five kainate (KA) receptors (GluR5, GluR6, and KA1) to be expressed in all nuclei. However, some distinct differences were observed: The inferior olive preferentially expresses flop variants of AMPA receptors, GluR7 is more abundant in the pre-Bötzinger complex than in the other nuclei, and NR2C is most prominent in the nucleus of the solitary tract. In single hypoglossal motoneurons and interneurons of the pre-Bötzinger complex investigation of GluR2 editing revealed strong expression of the GluR2-R editing variant, suggesting low Ca2+ permeability of AMPA receptors. Thus, Ca2+-permeable AMPA receptors are unlikely to be the cause for the reported selective vulnerability of hypoglossal motoneurons during excitotoxic events.

Localization and action of adenosine A2a receptors in regions of the brainstem important in cardiovascular control

In vitro autoradiography and central microinjections of a P1 adenosine A2a receptor antagonist have been employed to investigate a possible role for centrally located adenosine A2a receptors in modulation of the baroreceptor reflex. In vitro autoradiography using [125I]4-(2-[7-amino-2-[2-furyl][3,2,4]triazolol[2,3-a][1,3,5]tr iazin-5-yl-amino]ethyl)phenol ([125I]ZM241385), the high-affinity adenosine A2a receptor antagonist, revealed a heterogeneous distribution of adenosine A2a binding sites within the lower brainstem of the rat. Image analysis showed high levels of binding in rostral regions of both the nucleus tractus solitarius and the ventrolateral medulla. Intermediate levels of binding were observed in the commissural nucleus tractus solitarius and the dorsal vagal motor nucleus, with low levels of binding in caudal regions of the nucleus tractus solitarius and the ventrolateral medulla, and the hypoglossal nucleus. Unilateral microinjections of unlabelled ZM241385 into the nucleus tractus solitarius had no effect on baseline levels of arterial pressure, heart rate and phrenic nerve activity recorded in anaesthetized, artificially ventilated rats. However, microinjections of ZM241385 reduced the bradycardia evoked by stimulation of the ipsilateral aortic nerve. In contrast, ZM241385 had no effect on the depressor response or the reduction in phrenic nerve activity evoked by aortic nerve stimulation. Our results indicate that adenosine A2a binding sites are located in a number of brainstem regions involved in autonomic function, consistent with the idea that adenosine acts as a neuromodulator of a variety of cardiorespiratory reflexes. Specifically, the data support the hypothesis that adenosine A2a receptors located within the nucleus tractus solitarius are activated during baroreceptor stimulation and have an important modulatory role in the pattern of cardiovascular changes associated with this reflex.

Interconnections between the neuroendocrine hypothalamus and the central autonomic system. Geoffrey Harris Memorial Lecture, Kitakyushu, Japan, October 1998

Tract-tracing techniques in combination with immunohistochemistry and in situ hybridization were used in intact and operated rats (hypothalamic lesions, transections of neuronal pathways) to localize and characterize neuronal connections between the hypothalamus and autonomic centers. Viscerosensory and somatosensory signals which relay in the spinal cord and the medulla oblongata reach the hypothalamus through various catecholaminergic and noncatecholaminergic neuronal pathways. Vice versa, the hypothalamus influences autonomic activities through humoral and neurohumoral pathways. Descending hypothalamic efferents carry feedback signals to viscerosensory and brainstem catecholaminergic neurons and regulatory inputs to parasympathetic (dorsal vagal nucleus) and sympathetic (thoracolumbar intermediolateral cell column) preganglionic neurons. These fibers arise mainly from neurons of the paraventricular, arcuate, perifornical, and dorsomedial nuclei and the lateral hypothalamus. The major neuroanatomical observations are the following: (1) pathways between the hypothalamus and autonomic centers are bidirectional: the ascending and descending fibers may use the same avenues; (2) the descending axons are mainly peptidergic (CRF, vasopressin, oxytocin, somatostatin, enkephalin, POMC, and cANP), while the ascending fibers are both peptidergic (enkephalin, NPY, neurotensin, dynorphins) and catecholaminergic; (3) descending hypothalamic axons terminate directly on the sensory, preganglionic, and catecholaminergic neurons in the medulla and the spinal cord; (4) hypothalamic projections to the autonomic centers are always bilateral; (5) while medullary autonomic and catecholaminergic fibers innervate hypothalamic neurons directly, spinohypothalamic axons are relayed on neurons in the lateral hypothalamus.

Activation of GABAA but not GABAB receptors in the NTSblocked bradycardia of chemoreflex in awake rats

In the present study we analyzed effects of bilateral microinjections of muscimol (a GABAA agonist) and baclofen (a GABAB agonist) into the nucleus tractus solitarius (NTS) on bradycardic and pressor responses to chemoreflex activation (potassium cyanide, 40 micrograms/rat iv) in awake rats. Bilateral microinjections of muscimol (25 and 50 pmol/50 nl) into the NTS increased baseline mean arterial pressure (MAP): 119 +/- 8 vs. 107 +/- 2 mmHg (n = 6) and 121 +/- 8 vs. 103 +/- 3 mmHg (n = 6), respectively. Muscimol at 25 pmol/50 nl reduced the bradycardic response to chemoreflex activation 5 min after microinjection; with 50 pmol/50 nl the bradycardic response to chemoreflex activation was reduced 5, 15, 30, and 60 min after microinjection. Neither muscimol dose produced an effect on the pressor response of the chemoreflex. Effects of muscimol (50 pmol/50 nl) on basal MAP and on the bradycardic response of the chemoreflex were prevented by prior microinjection of bicuculline (a GABAA antagonist, 40 pmol/50 nl) into the NTS. Bilateral microinjections of baclofen (12.5 and 25 pmol/50 nl) into the NTS produced an increase in baseline MAP [137 +/- 9 vs. 108 +/- 4 (n = 7) and 145 +/- 5 vs. 105 +/- 2 mmHg (n = 7), respectively], no changes in basal heart rate, and no effects on the bradycardic response; 25 pmol/50 nl only attenuated the pressor response to chemoreflex activation. The data show that activation of GABAA receptors in the NTS produces a significant reduction in the bradycardic response, whereas activation of GABAB receptors produces a significant reduction in the pressor response of the chemoreflex. We conclude that 1) GABAA but not GABAB plays an inhibitory role in neurons of the lateral commissural NTS involved in the parasympathetic component of the chemoreflex and 2) attenuation of the pressor response of the chemoreflex by activation of GABAB receptors may be due to inhibition of sympathoexcitatory neurons in the NTS or may be secondary to the large increase in baseline MAP produced by baclofen.

Distribution of pre-pro-glucagon and glucagon-like peptide-1 receptor messenger RNAs in the rat central nervous system

Glucagon-like peptide-1 (GLP-1) is derived from the peptide precursor pre-pro-glucagon (PPG) by enzymatic cleavage and acts via its receptor, glucagon-like peptide-1 receptor (GLP-1R). By using riboprobes complementary to PPG and GLP-1R, we described the distribution of PPG and GLP-1R messenger RNAs (mRNAs) in the central nervous system of the rat. PPG mRNA-expressing perikarya were restricted to the nucleus of the solitary tact or to the dorsal and ventral medulla and olfactory bulb. GLP-1R mRNA was detected in numerous brain regions, including the mitral cell layer of the olfactory bulb; temporal cortex; caudal hippocampus; lateral septum; amygdala; nucleus accumbens; ventral pallium; nucleus basalis Meynert; bed nucleus of the stria terminalis; preoptic area; paraventricular, supraoptic, arcuate, and dorsomedial nuclei of the hypothalamus; lateral habenula; zona incerta; substantia innominata; posterior thalamic nuclei; ventral tegmental area; dorsal tegmental, posterodorsal tegmental, and interpeduncular nuclei; substantia nigra, central gray; raphe nuclei; parabrachial nuclei; locus ceruleus, nucleus of the solitary tract; area postrema; dorsal nucleus of the vagus; lateral reticular nucleus; and spinal cord. These studies, in addition to describing the sites of GLP-1 and GLP-1R synthesis, suggest that the efferent connections from the nucleus of the solitary tract are more widespread than previously reported. Although the current role of GLP-1 in regulating neuronal physiology is not known, these studies provide detailed information about the sites of GLP-1 synthesis and potential sites of action, an important first step in evaluating the function of GLP-1 in the brain. The widespread distribution of GLP-1R mRNA-containing cells strongly suggests that GLP-1 not only functions as a satiety factor but also acts as a neurotransmitter or neuromodulator in anatomically and functionally distinct areas of the central nervous system.

Catechol activation in the vasomotor center upon emergence from anesthesia: specificity

The rostral ventrolateral medulla (RVLM) controls the vascular system. It may contribute to postoperative hypertension observed upon emergence from anesthesia. This structure contains adrenergic cardiovascular neurons. Therefore, one question was addressed: does a change in RVLM catechol activity occur upon emergence from anesthesia? Halothane-anesthetized, paralyzed rats had their ventilatory, circulatory, and acid-base stability controlled. All pressure points and incisions were infiltrated with local anesthetic. With in vivo electrochemistry, a catechol signal was recorded in the RVLM in the following circumstances: (1) under stable halothane anesthesia for 120 minutes (halothane group), (2) during 120 minutes after halothane discontinuation (saline-emergence group), (3) during 60 minutes after halothane discontinuation followed by 60 minutes after halothane readministration (readministration group), (4) emergence in rats treated with atenolol and nitroprusside to hold blood pressure as close as possible to baseline, (5) emergence after morphine 1 mg.kg(-1) i.v., (6) emergence after decerebration, and (7) emergence upon recording in the mid-brain dopaminergic A10 area. Stable halothane anesthesia (n = 6) led to no change in mean arterial pressure (MAP), heart rate (HR), and catechol signal (CAOC). During emergence from anesthesia (n = 6), MAP, HR, and catechol signal increased and did not return to baseline. By contrast, a return of MAP, HR, and catechol signal to baseline was observed upon readministration of halothane (n = 6). Whereas blood pressure and heart rate were maintained as closely as possible to baseline, a large catechol activation (n = 5) was observed upon emergence from anesthesia. A catechol activation from a lowered baseline was observed upon emergence following morphine administration (n = 5). A minor circulatory activation without RVLM catechol activation was observed upon emergence following decerebration (n = 5). Recordings in the A10 area revealed no increase in the catechol signal following emergence (n = 5). Adrenergic RVLM neurons appear to be responsive upon emergence from anesthesia, possibly being activated by suprapontine afferents impinging on the RVLM.

Catecholamines in rabbit nodose ganglion following exposure to an acute emotional stressor

The aim of the present investigation was to examine catecholamine content and the activities of catecholamine synthesizing and degrading enzymes in the nodose ganglia of rabbits with different patterns of arterial blood pressure during exposure to an acute emotional stressor. The stress protocol involved exposure of immobilized adult male rabbits to electrical stimulation of the ventromedial hypothalamus and the skin for 3 hours. Stress-resistant rabbits that had small fluctuations in arterial pressure during exposure to the stressor had significant reductions in levels ofnorepinephrine (NE) in the nodose ganglion during the 3 hours of stress exposure. In contrast, stress-sensitive rabbits that had progressive decreases in arterial pressure exhibited significant elevations in nodose ganglion content of NE, dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) throughout the period of stress. Tyrosine hydroxylase (TH) activity was changed during the course of the experiment while monoamine oxidase (MAO) activity was unaffected by stress exposure. The changes in nodose ganglion catecholamine content of stress-sensitive and stress-resistant rabbits suggest that the nodose ganglion plays an important role in the maintenance of cardiovascular homeostasis during exposure of animals to an acute emotional stressor.

Direct evidence for nitric oxide synthase in vagal afferents to the nucleus tractus solitarii

The anatomical relationship between vagal afferents and brain nitric oxide synthase containing terminals in the nucleus tractus solitarii was studied by means of anterograde tracing combined with immunocytochemistry and immuno-electron microscopy. Biotinylated dextran amine was injected into the nodose ganglion with a glass micropipette. Four to eight days following the injection, regions of the nucleus tractus solitarii containing biotinylated dextran amine-labelled vagal afferents and those containing nitric oxide synthase-immunopositive terminals were congruent. Many neurons exhibiting nitric oxide synthase immunoreactivity were found within the biotinylated dextran amine-containing terminal field. However dense labeling of terminals with biotinylated dextran amine precluded determination if the terminals were nitric oxide synthase-immunoreactive. Therefore, we combined degeneration of vagal afferents after removal of one nodose ganglion with nitric oxide synthase immuno-electron microscopy. Axon terminals that possessed characteristic vesicle clusters and were partially or completely engulfed by glial processes were identified as degenerating vagal afferents. Degenerating axon terminals comprised 38% of the total axon terminals in the nucleus tractus solitarii in a sample of sections; and of the degenerating axon terminals, 67% were nitric oxide synthase-immunoreactive. Nitric oxide synthase immunoreactivity was present in 41% of the non-degenerating axon terminals. Prominent staining of dendrites for nitric oxide synthase immunoreactivity indicated that much of the nitric oxide synthase in the nucleus tractus solitarii is not derived from peripheral afferents. Of the total number of dendritic profiles sampled, half were nitric oxide synthase-immunoreactive. Our data support the hypothesis that nitric oxide or nitric oxide donors may be present in primary vagal afferents that terminate in the nucleus tractus solitarii. While this study confirms that vagal afferents contain brain nitric oxide synthase, it demonstrates for the first time that the majority of nitric oxide synthase immunoreactivity in the nucleus tractus solitarii is found in intrinsic structures in the nucleus. In addition, our data show that second or higher order neurons in the nucleus tractus solitarii may be nitroxidergic and receive both nitroxidergic and non-nitroxidergic vagal input.

Endogenous calcitonin gene-related peptide modulates tachycardiac but not bradycardiac baroreflex in rats

Effects of intracisternally administered human calcitonin gene-related peptide (8-37) [hCGRP-(8-37)], an antagonist, and hCGRP, an agonist of the CGRP receptor in the rat central nervous system, on baroreflex sensitivity (BRS) were studied in conscious male rats. Each rat sequentially received intracisternally injected 0.9% saline and then hCGRP-(8-37) at doses of 1, 2.5, and 5 nmol in a volume of 10 microliters at an interval of 15 min. Five minutes after each injection, sodium nitroprusside (SNP, 10 micrograms/kg) or phenylephrine hydrochloride (PE, 2 micrograms/kg) was intravenously administered to induce reflex tachycardia or bradycardia, respectively. Intracisternally administered hCGRP-(8-37) increased BRS of the reflex tachycardia induced by SNP in a dose-related manner but did not change the BRS after PE. Intracisternally injected hCGRP significantly decreased the BRS after SNP. The lowering effect of hCGRP on BRS after SNP was inhibited by hCGRP-(8-37) injected before hCGRP. These results suggest that endogenous CGRP in the lower brain stem is selectively involved in the tachycardiac but not the bradycardiac baroreflex and modulates the baroreflex in an inhibitory rather than facilitatory fashion.

Dorsal medullary 5-HT3 receptors and sympathetic premotor neurones in the rat

1. Our aim was to determine whether the cardiovascular neurones in the rostro-ventrolateral medulla (CV-RVLM neurones) were involved in the sympathoexcitation induced by stimulation of 5-HT3 receptors in the region of the nucleus tractus solitarii (NTS). Experiments were performed in pentobarbitone-anaesthetized rats, artificially ventilated and paralysed with pancuronium bromide. 2. Using extracellular recordings, different types of RVLM neurones were characterized: cardiovascular (CV), ventilation-related and baroreflex-insensitive (unidentified) neurones. The CV-RVLM cells were further subdivided into three populations according to their axonal conduction velocities: A (1.2 +/- 0.1 m s-1), B (2.5 +/- 0.2 m s-1) and C (6.8 +/- 1.1 m s-1). 3. Only the CV-RVLM neurones of the A and B categories were partially inhibited (-30 %) by a hypotensive dose (2.5 microg kg-1 i.v.) of clonidine. 4. Microinjections into the region of the commissural NTS of 1-(m-chlorophenyl)-biguanide (CPBG, 2 nmol), a selective 5-HT3 receptor agonist, elicited an increase in both lumbar sympathetic nerve discharge (SND) and arterial pressure. In addition, this treatment produced a marked excitation of CV-RVLM neurones of the A and B categories, without affecting those of the C type, as well as ventilation-related and unidentified RVLM cells. 5. The activity of the CV neurones in the caudo-ventrolateral part of the medulla oblongata (CV-CVLM) was not modified by 5-HT3 receptor stimulation in the NTS. 6. Prior intra-NTS microinjections of ondansetron (300 pmol, a selective 5-HT3 receptor antagonist) into the region of the commissural NTS prevented the excitation of A and B CV-RVLM neurones induced by CPBG. 7. Intracarotid administration of saline saturated with CO2 (chemoreceptor activation) elicited both an increase in the SND and an excitation of the clonidine-insensitive CV-RVLM neurones of the C type, without affecting A and B neurones. 8. In conclusion, the sympathoexcitation elicited following 5-HT3 receptor stimulation in the region of the commissural NTS of pentobarbitone-anaesthetized rats seems to result from the excitation of two different pools of clonidine-sensitive CV-RVLM neurones. These neurones are apparently not involved in the sympathetic component of the chemoreceptor reflex.

Microinjection of a serotonin3 receptor agonist into the NTS of unanesthetized rats inhibits the bradycardia evoked by activation of the baro- and chemoreflexes

In the present study we investigated the effects of microinjection into the commissural nucleus tractus solitarius (NTS) of unanesthetized rats of 2-methylserotonin (2-methyl-5-HT), a 5-HT3 receptor agonist, on the cardiac component of the baro- and chemoreflexes. The study was performed in conscious freely moving rats in order to avoid the possible effects of anesthetics on the cardiovascular responses to microinjection of neuroactive substances into the NTS. The baroreflex (phenylephrine, 0.5-2.0 micrograms/kg, i.v.) and the chemoreflex (potassium cyanide, 40 micrograms/rat, i.v) were activated in different groups of rats before and after bilateral microinjection of 2-methyl-5-HT into the NTS. Microinjections of 2-methyl-5-HT (5 nmol/50 nl) into the NTS produced a significant increase in basal mean arterial pressure (101 +/- 3 versus 125 +/- 8 mmHg), no changes in basal HR and a significant reduction in the reflex bradycardia triggered by baroreflex activation at 3 (-28 +/- 7 bpm), 10 (-35 +/- 4 bpm) and 20 min (-34 +/- 5 bpm) in comparison with the control value (-68 +/- 9 bpm). A similar reduction in the bradycardic response to chemoreflex activation was observed at 3 (-94 +/- 35 bpm), 10 (-98 +/- 38 bpm) and 20 min (-110 +/- 29 bpm) after 2-methyl-5-HT in comparison with the control value (-178 +/- 19 bpm). The effect of 2-methyl-5-HT on the basal mean arterial pressure and on the bradycardia evoked by stimulation of the baro- and chemoreflexes was blocked by pretreatment with granisetron bilaterally microinjected (500 pmol/50 nl) into the NTS. The data show that the stimulation of 5-HT3 receptors in the NTS of unanesthetized rats elicits a significant increase in basal mean arterial pressure and decreases the bradycardic response to baro- or chemoreflex activation.

Nitric oxide (NO) modulates the neurogenic control of blood pressure in rats with chronic renal failure (CRF)

Increased sympathetic nervous system (SNS) activity plays a role in the genesis of hypertension in rats with chronic renal failure (CRF). Because nitric oxide (NO) modulates the activity of the SNS, a deficit of NO synthesis could be responsible for the increased SNS activity in these animals. In the present study, we evaluated the effects of L-arginine and L-NAME on blood pressure and SNS activity-in Sprague Dawley 5/6 nephrectomized or sham-operated rats. SNS activity was determined by measuring norepinephrine turnover rate in several brain nuclei involved in the regulation of blood pressure. In the same brain nuclei, we measured NO content and nitric oxide synthase (NOS) gene expression by semiquantitative measurements of NOS mRNA reverse transcription polymerase chain reaction. In CRF rats, norepinephrine turnover rate was increased in the posterior hypothalamic nuclei, locus coeruleus, paraventricular nuclei, and the rostral ventral medulla, whereas NOS mRNA gene expression and NO2/NO3 content were increased in all brain nuclei tested. L-NAME increased blood pressure and NE turnover rate in several brain nuclei of both control and 5/6 nephrectomized rats. In CRF rats, a significant relationship was present between the percent increment in NOS mRNA gene expression related to the renal failure, and the percent increase in norepinephrine turnover rate caused by L-NAME. This suggests that endogenous NO may partially inhibit the activity of the SNS in brain nuclei involved in the neurogenic regulation of blood pressure, and this inhibition is enhanced in CRF rats. In summary, the increase in SNS activity in the posterior hypothalamic nuclei and in the locus coeruleus of CRF rats is partially mitigated by increased local expression of NOS m-RNA.