Differential baroreflex control of sympathetic drive by angiotensin II in the nucleus tractus solitarii

Microinjection of angiotensin II into the nucleus tractus solitarii attenuates the baroreceptor reflex-mediated bradycardia by inhibiting both vagal and cardiac sympathetic components. However, it is not known whether the baroreflex modulation of other sympathetic outputs (i.e., noncardiac) also are inhibited by exogenous angiotensin II (ANG II) in nucleus tractus solitarii (NTS). In this study, we determined whether there was a difference in the baroreflex sensitivity of sympathetic outflows at the thoracic and lumbar levels of the sympathetic chain following exogenous delivery of ANG II into the NTS. Experiments were performed in two types of in situ arterially perfused decerebrate rat preparations. Sympathetic nerve activity was recorded from the inferior cardiac nerve, the midthoracic sympathetic chain, or the lower thoracic-lumbar sympathetic chain. Increases in perfusion pressure produced a reflex bradycardia and sympathoinhibition. Microinjection of ANG II (500 fmol) into the NTS attenuated the reflex bradycardia (57% attenuation, P < 0.01) and sympathoinhibition of both the inferior cardiac nerve (26% attenuation, P < 0.05) and midthoracic sympathetic chain (37% attenuation, P < 0.05) but not the lower thoracic-lumbar chain (P = 0.56). We conclude that ANG II in the nucleus tractus solitarii selectively inhibits baroreflex responses in specific sympathetic outflows, possibly dependent on the target organ innervated.     

Protein kinase A-dependent Neuronal Nitric Oxide Synthase Activation Mediates the Enhancement of Baroreflex Response by Adrenomedullin in the Nucleus Tractus Solitarii of Rats

Background  

Adrenomedullin (ADM) exerts its biological functions through the receptor-mediated enzymatic mechanisms that involve protein kinase A (PKA), or neuronal nitric oxide synthase (nNOS). We previously demonstrated that the receptor-mediated cAMP/PKA pathway involves in ADM-enhanced baroreceptor reflex (BRR) response. It remains unclear whether ADM may enhance BRR response via activation of nNOS-dependent mechanism in the nucleus tractus solitarii (NTS).     

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.     

Eating-Induced Dopamine Release from Mesolimbic Neurons Is Mediated by NMDA Receptors in the Ventral Tegmental Area: A Dual-Probe Microdialysis Study

Abstract: This study was aimed at identifying the neuronal pathways that mediate the eating-induced increase in the release of dopamine in the nucleus accumbens of the rat brain. For that purpose, a microdialysis probe was implanted in the ventral tegmental area and a second probe was placed in the ipsilateral nucleus accumbens. Receptor-specific compounds acting on GABA_A (40 µ M muscimol; 50 µ M bicuculline), GABA_B (50 µ M baclofen), acetylcholine (50 µ M carbachol), NMDA [30 µ M (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP)], and non-NMDA [300 µ M 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)] receptors were infused into the ventral tegmental area by retrograde dialysis, whereas extracellular dopamine was recorded in the ipsilateral nucleus accumbens. Intrategmental infusion of muscimol or baclofen decreased extracellular dopamine in the ipsilateral nucleus accumbens; CPP and CNQX were without effect, and bicuculline and carbachol increased dopamine release. During infusion of the various compounds, food-deprived rats were allowed to eat for 10 min. The infusions of muscimol, bicuculline, baclofen, carbachol, and CNQX did not prevent the eating-induced increase in extracellular dopamine in the nucleus accumbens. However, during intrategmental infusion of CPP, the eating-induced increase in extracellular dopamine in the nucleus accumbens was suppressed. These results indicate that a glutamatergic projection to the ventral tegmental area mediates, via an NMDA receptor, the eating-induced increase in dopamine release from mesolimbic dopamine neurons.     

Muscarinic Receptor-Mediated Increase in Extracellular Inositol 1,4,5-Trisphosphate Levels in the Rat Hippocampus: An In Vivo Microdialysis Study

Abstract: The extracellular concentration of inositol 1,4,5-trisphosphate (IP₃) has been monitored in the ventral hippocampus of the anesthetized rat by using a microdialysis technique coupled to a radioreceptor assay. Three hours after the implantation of the cannula, basal extracellular concentration of IP₃ (corrected for a 9% recovery) was 71 n M (0.39 pmol/60-µl fraction) and remained stable for at least 5 h. Local infusion of carbachol for 60 min caused a significant concentration-related increase in extracellular IP₃ levels (0, 24, and 57% at 1, 50, and 100 µ M , respectively). Acetylcholine (100 µ M ) and muscarine (100 µ M ) increased IP₃ outflow by 40 and 42%, respectively. The effect of carbachol was fully prevented by coinfusion of 10 µ M pirenzepine and reduced by 1 µ M tetrodotoxin indicating that the carbachol response is mediated by neuronal muscarinic receptors. These data demonstrate the feasibility of using microdialysis and a radioreceptor assay to measure IP₃ in the extracellular space. This approach could prove useful for the study of the in vivo operation of muscarinic and, by extension, a number of receptors coupled to phosphoinositide turnover.     

Involvement of brain stem noradrenergic neurons in the development of hypertension in spontaneously hypertensive rats

This study attempted to investigate the possible involvement of the brain stem noradrenergic system in the development of hypertension in spontaneously hypertensive rats. Steady-state norepinephrine, dopamine, serotonin and 5-hydroxyindoleacetic acid concentrations and norepinephrine turnover were determined in the individual brain stem nuclei using high performance liquid chromatography with electrochemical detection. Decreased norepinephrine contents in the nucleus tractus solitarii in spontaneously hypertensive rats compared with Wistar-Kyoto rats at the age of 4, 8, and 16 weeks were demonstrated. In later stages (8 and 16 weeks), increased norepinephrine levels were observed in the nucleus reticularis gigantocellularis, the A1 and A5 areas. Norepinephrine turnover was not different between spontaneously hypertensive rats and Wistar-Kyoto rats in the nucleus tractus solitarii at the age of 4 and 16 weeks and increased in the nucleus reticularis gigantocellularis of spontaneously hypertensive rats at 16 weeks. Our results indicate that altered norepinephrine metabolism in the specific brain stem nuclei, especially the consistently decreased norepinephrine in the nucleus tractus solitarii of spontaneously hypertensive rats, contribute to the development of genetic hypertension.     

Somatodendritic α2-Adrenoceptors in the Locus Coeruleus Are Involved in the In Vivo Modulation of Cortical Noradrenaline Release by the Antidepressant Desipramine

Abstract: The effect of the antidepressant and selective noradrenaline reuptake blocker desipramine (DMI) on noradrenergic transmission was evaluated in vivo by dual-probe microdialysis. DMI (1, 3, and 10 mg/kg, i.p.) dose-dependently increased extracellular levels of noradrenaline (NA) in the locus coeruleus (LC) area. In the cingulate cortex (Cg), DMI (3 and 10 mg/kg, i.p.) also increased NA dialysate, but at the lowest dose (1 mg/kg, i.p.) it decreased NA levels. When the α₂-adrenoceptor antagonist RX821002 (1 µ M ) was perfused in the LC, DMI (1 mg/kg, i.p.) no longer decreased but rather increased NA dialysate in the Cg. In electrophysiological experiments, DMI (1 mg/kg, i.p.) inhibited the firing activity of LC neurons by a mechanism reversed by RX821002. Local DMI (0.01–100 µ M ) into the LC increased concentration-dependently NA levels in the LC and simultaneously decreased NA levels in the Cg. This decrease was abolished by local RX821002 administration into the LC. The results demonstrate in vivo that DMI inhibits NA reuptake at somatodendritic and nerve terminal levels of noradrenergic cells. The increased NA dialysate in the LC inhibits noradrenergic activity, which in part counteracts the effects of DMI on the Cg. The modulation of cortical NA release by activity of DMI at the somatodendritic level is mediated through α₂-adrenoceptors located in the LC.     

Tonic enhancement of the sensitivity of baroreceptor reflex response by endogenous substance P in the rat.

Modulation of baroreceptor reflex (BRR) by endogenous substance P (SP) in the brain was investigated in rats anesthetized with pentobarbital sodium. Intracerebroventricular administration of the undecapeptide (15 or 30 nmol) and its antagonist, (D-Pro2, D-Trp7,9)-SP (30 or 60 nmol) or SP antiserum (1:20), respectively, promoted a significant increase and decrease in the sensitivity of BRR response. Prolonging the endogenous activity of SP with the aminopeptidase blocker, bestatin (200 nmol) or with the endopeptidase-24.11 inhibitor, phosphoramidon (200 nmol) significantly augmented the same reflex. Combining the undecapeptide with either peptidase blocker, moreover, promoted additional potentiation of the BRR response. On the other hand, simultaneous administration of bestatin and (D-Pro2, D-Trp7,9)-SP produced a reduction of the augmented effect of bestatin on the sensitivity of BRR response. Bilateral microinjection of SP (600 pmol) or an antiserum against SP (1:20) into the nucleus tractus solitarius (NTS) elicited respectively an enhancement of and reduction in the BRR response. These data suggest that neurons that contain SP may participate in central cardiovascular control by tonically enhancing the sensitivity of the BRR response, possibly via an action on the NTS.     

An inhibitory role of beta-endorphin in central cardiovascular regulation

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitarii (NTS) of urethane anaesthetised rats were investigated. Unilateral injection resulted in a dose related fall in mean arterial pressure and heart rate. No change in respiratory frequency was prevented and the bradycardia reduced by pretreatment with locally applied naloxone (10 ng). This dose of the opiate antagonist had no effect on mean arterial pressure or heart rate when administered alone. Antiserum to beta-endorphin (1:50 dilution) caused a rise in pressure and a tendency towards tachycardia on injection into the NTS, while it completely blocked the depressor response and bradycardia induced by beta-endorphin. These results are consistent with the view that a beta-endorphin-like peptide has a depressor role in the central nervous system. The hypotension may result from an effect within the central connections of the baroreceptor reflex arc, probably at the level of the NTS.     

Paraventricular noradrenergic systems participate in angiotensin II-induced drinking

The present study was designed to examine the role of noradrenergic systems in the hypothalamic paraventricular nucleus (PVN) in the drinking response induced by microinjection of angiotensin II (ANG II) into the subfornical organ (SFO) in the awake rat. Intracerebral microdialysis techniques were utilized to quantify the extracellular concentration of noradrenaline (NA) in the region of the PVN. Injections of ANG II (10⁻⁶ M, 0.2 μl) into the SFO significantly increased NA release in the PVN area. The increase in the NA concentration caused by the ANG II injection was significantly attenuated by water ingestion. In urethane-anesthetized rats, injections of ANG II into the SFO elicited an elevation in mean arterial pressure (MAP). On the other hand, intravenous injections of the -agonist metaraminol (5 μg) slightly decreased the release of NA in the PVN area that accompanied an elevation in MAP. These results show that the noradrenergic system in the PVN area may be involved in the dipsogenic response induced by ANG II acting at the SFO.     

Neurochemical Evidence that Postsynaptic Nucleus Accumbens D3 Receptor Stimulation Enhances Cocaine Reinforcement

Abstract: The mechanism by which two D₃ receptor-preferring agonists, 7-hydroxydipropylaminotetralin (7-OH-DPAT) and quinelorane, modulate cocaine reinforcement was examined by monitoring nucleus accumbens dopamine levels with in vivo microdialysis while rats intravenously self-administered the following four different drug solutions consecutively: (1) cocaine; (2) a combination of cocaine plus a low dose of either agonist; (3) either agonist alone; and finally, (4) a physiological saline solution. Both 7-OH-DPAT (4 µg/infusion) and quinelorane (0.25 µg/infusion) decreased cocaine (0.25 mg/infusion) intake in a manner indicating an enhancement of cocaine reinforcement and simultaneously decreased the cocaine-induced elevations in nucleus accumbens dopamine levels by >50%. Subsequent self-administration of either 7-OH-DPAT (4 µg/infusion) or quinelorane (0.25 µg/infusion) alone resulted in significant, but stable, increases in drug intake, with a concurrent decrease in nucleus accumbens dopamine levels to ∼50% below nondrug baseline levels. These findings indicate that postsynaptic D₃ receptor stimulation in the nucleus accumbens enhances the reinforcing properties of cocaine. In a second experiment, local application of 7-OH-DPAT via reverse dialysis (30 and 100 n M perfusate concentrations) dose-dependently decreased nucleus accumbens dopamine efflux to 76 ± 3.9 and 61 ± 6.3% of baseline, respectively, whereas there was no effect of this agonist on dopamine efflux in the ipsilateral striatum of these same animals. Coperfusion with the D₃ receptor-preferring antagonist nafadotride dose-dependently blocked the effect of 7-OH-DPAT on nucleus accumbens dopamine efflux. These results suggest that, at low concentrations, 7-OH-DPAT selectively activates D₃ receptors in vivo.     

NO formation in nucleus tractus solitarii attenuates pressor response evoked by skeletal muscle afferents

We have previously shown that static muscle contraction induces the expression of c-Fos protein in neurons of the nucleus tractus solitarii (NTS) and that some of these cells were codistributed with neuronal NADPH-diaphorase [nitric oxide (NO) synthase]-positive fibers. In the present study, we sought to determine the role of NO in the NTS in mediating the cardiovascular responses elicited by skeletal muscle afferent fibers. Static contraction of the triceps surae muscle was induced by electrical stimulation of the L7 and S1 ventral roots in anesthetized cats. Muscle contraction during microdialysis of artificial extracellular fluid increased mean arterial pressure (MAP) and heart rate (HR) 51 +/- 9 mmHg and 18 +/- 3 beats/min, respectively. Microdialysis of L-arginine (10 mM) into the NTS to locally increase NO formation attenuated the increases in MAP (30 +/- 7 mmHg, P < 0.05) and HR (14 +/- 2 beats/min, P > 0.05) during contraction. Microdialysis of D-arginine (10 mM) did not alter the cardiovascular responses evoked by muscle contraction. Microdialysis of N(G)-nitro-L-arginine methyl ester (2 mM) during contraction attenuated the effects of L-arginine on the reflex cardiovascular responses. These findings demonstrate that an increase in NO formation in the NTS attenuates the pressor response to static muscle contraction, indicating that the NO system plays a role in mediating the cardiovascular responses to static muscle contraction in the NTS.     

Impact of Corticotropin-Releasing Hormone on Extracellular Norepinephrine in Prefrontal Cortex After Chronic Cold Stress

Abstract: We have previously demonstrated that exposing rats to cold (5°C) for 3–4 weeks potentiates the increase in extracellular norepinephrine (NE) in the medial prefrontal cortex produced by acute tail shock. In the present study, we used microdialysis to determine the duration of cold exposure required to produce this sensitization and explored the mechanism of the phenomenon. Tail shock elicited a twofold greater increase in extracellular NE in the medial prefrontal cortex of rats exposed to cold for 2 weeks than in naive control rats or in rats exposed to cold for 1 week and tested either immediately or after a 2-week delay. Local infusion of 10 µ M d -amphetamine or 30 m M K⁺ increased extracellular NE in the medial prefrontal cortex (∼350 and 190%, respectively) comparably in control rats and rats exposed to cold for 3 weeks. In contrast, intraventricular administration of 3.0 µg of corticotropin-releasing hormone increased extracellular NE in the medial prefrontal cortex by 65% in rats exposed to cold for 2 weeks, but only 35% in control rats. These results indicate that an enhanced responsiveness of noradrenergic neurons to acute tail shock (1) requires ∼2 weeks of cold exposure to develop and (2) may be mediated by a change at the level of the noradrenergic cell bodies rather than the nerve terminals.     

Glutamatergic mechanisms in the nucleus tractus solitarius in blood pressure control

The nucleus tractus solitarius (NTS), the site of termination of visceral afferents of the ninth and tenth cranial nerves, mediates and integrates the reflex cardiovascular and noncardiovascular responses to stimulation of cardiopulmonary and other visceral afferents. On injection into the NTS, the amino acid L-glutamate (L-Glu) and its excitatory analogs produce dose-dependent hypotension and bradycardia, a baroreceptor reflex-like response. The L-Glu antagonist glutamate diethyl ester blocks the response both to L-Glu and to baroreceptor reflex activation. Electrical stimulation of vagal c-fibers selectively releases 3H into a push-pull cannula after preloading of the NTS with L-[3H]Glu or D-[3H]aspartate. The NTS contains a high-affinity uptake system for inactivation of L-Glu. Like L-Glu, acetylcholine and serotonin, which are also found in the NTS, both elicit a baroreceptor reflex-like response when microinjected into the NTS. However, cholinergic and serotonergic antagonists do not block the baroreceptor reflex. A glutamatergic neuron (or neurons) projecting into NTS appears to be an integral part of the baroreceptor reflex arc.     

Modulation of the cough reflex by antitussive agents within the caudal aspect of the nucleus tractus solitarii in the rabbit

We have previously shown that ionotropic glutamate receptors in the caudal portion of the nucleus tractus solitarii (NTS), especially in the commissural NTS, play a prominent role in the mediation of tracheobronchial cough and that substance P potentiates this reflex. This NTS region could be a site of action of some centrally acting antitussive agents and a component of a drug-sensitive gating mechanism of cough. To address these issues, we investigated changes in baseline respiratory activity and cough responses to tracheobronchial mechanical stimulation following microinjections (30-50 nl) of centrally acting antitussive drugs into the caudal NTS of pentobarbitone-anesthetized, spontaneously breathing rabbits. [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and baclofen decreased baseline respiratory frequency because of increases in the inspiratory time only at the higher concentration employed (5 mM and 1 mM, respectively). DAMGO (0.5 mM) and baclofen (0.1 mM) significantly decreased cough number, peak abdominal activity, peak tracheal pressure, and increased cough-related total cycle duration. At the higher concentrations, these agents suppressed the cough reflex. The effects of these two drugs were counteracted by specific antagonists (10 mM naloxone and 25 mM CGP-35348, respectively). The neurokinin-1 (NK1) receptor antagonist CP-99,994 (10 mM) abolished cough responses, whereas the NK2 receptor antagonist MEN 10376 (5 mM) had no effect. The results indicate that the caudal NTS is a site of action of some centrally acting drugs and a likely component of a neural system involved in cough regulation. A crucial role of substance P release in the mediation of reflex cough is also suggested.     

Role of area postrema pressor mechanisms in the regulation of arterial pressure

This article discusses the data which established that angiotensin II modulates the tonic and reflex control of cardiovascular function by actions on the nuclear regions of the dorsal medulla oblongata. Although physiological evidence for the modulatory actions of angiotensin II in structures of the lower brainstem has been gathered over the past 16 years, only the recent application of new neurobiological techniques has allowed a more definitive understanding of its role. The identification of high affinity angiotensin II binding sites within the parenchyma of the area postrema with the technique of in vitro receptor autoradiography has provided anatomical validity for a role of angiotensin II in the central nervous system. The added discovery of angiotensin II binding sites in subnuclear components of the nucleus tractus solitarii and the motor nucleus of the tenth cranial nerve provides additional information on the various mechanisms through which angiotensin II may affect the intrinsic activity of the brainstem neuronal circuits involved in the integration of baroreceptor and sensory visceromotor function.     

Characterization of Excitatory Amino Acid Modulation of Dopamine Release in the Prefrontal Cortex of Conscious Rats

Abstract: The effect of various classes of excitatory amino acid agonists on the release of dopamine in the medial prefrontal cortex (PFC) of awake rats was examined using intracerebral microdialysis. Local infusion of 20 µ M α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), through the microdialysis probe, produced a significant increase of more than twofold in extracellular levels of dopamine. Application of 100 µ M AMPA increased these levels nearly 15 fold. The AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (50 µ M ) blocked the increase in dopamine release produced by 20 µ M AMPA. Local infusion of kainate at concentrations of 5 and 20 µ M increased dopamine release by nearly 150 and 500%, respectively. Local application of CNQX (50 µ M ) before 20 µ M kainate significantly attenuated the stimulatory effect of kainate on dopamine levels. In contrast to AMPA and kainate, infusion of N -methyl- d -aspartate (NMDA) at 20 or 100 µ M did not increase dopamine release. In fact, a trend toward a decrease in dopamine release was evident after 100 µ M NMDA. The present study indicates that the in vivo release of dopamine in the PFC is facilitated by AMPA and kainate receptors. This modulation is more profound than that previously reported in the basal ganglia. The lack of an excitatory effect of NMDA is in agreement with recent reports that the NMDA receptor may inhibit indirectly dopaminergic neurotransmission in the PFC.     

Hypoxia-mediated in vivo release of dopamine in nucleus tractus solitarii of rabbits

A wide variety of neuroactive substances have been suggested to be involved in the respiratory depression observed in response to severe hypoxia. By use of the technique of microdialysis, the release of dopamine (DA) was measured in the nucleus tractus solitarii during severe hypoxic provocations (6% O2 in N2) in the adult pentobarbital-anesthetized rabbit. DA release was analyzed by high-performance liquid chromatography with electrochemical detection. Such hypoxic provocations caused pronounced phase of depression in the phrenic nerve activity and enhanced release of DA. After bilateral carotid sinus nerve denervation, acute severe hypoxia did not give rise to enhanced release of DA or to phrenic nerve depression. Mild hypoxic (9% or 12% O2 in N2) or hypercapnic (6% CO2) stimuli resulted in an increased phrenic nerve activity without any concomitant changes in DA release. Decerebration at the midcollicular level in rabbits prevented an enhanced release of DA in the nucleus tractus solitarii during severe hypoxia. The results suggest that 1) DA is involved in the central ventilatory response to severe hypoxia, 2) not only the initial excitatory but also the second depressive phase in response to severe hypoxia is mediated partially by the peripheral chemoreceptors, and 3) the depressive phase is dependent on intact connections from suprapontine structures.