Different patterns of respiratory and cardiovascular responses elicited by chemical stimulation of dorsal medulla in the rat

Deficiency of CPEB2-Confined Choline Acetyltransferase Expression in the Dorsal Motor Nucleus of Vagus Causes Hyperactivated Parasympathetic Signaling-Associated Bronchoconstriction

Cytoplasmic polyadenylation element binding protein 2 (CPEB2) is an RNA-binding protein and translational regulator. To understand the physiological function of CPEB2, we generated CPEB2 knock-out (KO) mice and found that most died within 3 d after birth. CPEB2 is highly expressed in the brainstem, which controls vital functions, such as breathing. Whole-body plethysmography revealed that KO neonates had aberrant respiration with frequent apnea. Nevertheless, the morphology and function of the respiratory rhythm generator and diaphragm neuromuscular junctions appeared normal. We found that upregulated translation of choline acetyltransferase in the CPEB2 KO dorsal motor nucleus of vagus resulted in hyperactivation of parasympathetic signaling-induced bronchoconstriction, as evidenced by increased pulmonary acetylcholine and phosphorylated myosin light chain 2 in bronchial smooth muscles. Specific deletion of CPEB2 in cholinergic neurons sufficiently caused increased apnea in neonatal pups and airway hyper-reactivity in adult mice. Moreover, inhalation of an anticholinergic bronchodilator reduced apnea episodes in global and cholinergic CPEB2-KO mice. Together, the elevated airway constriction induced by cholinergic transmission in KO neonates may account for the respiratory defect and mortality.

SIGNIFICANCE STATEMENT

This study first generated and characterized cpeb2 gene-deficient mice. CPEB2-knock-out (KO) mice are born alive but most die within 3 d after birth showing no overt defects in anatomy. We found that the KO neonates showed severe apnea and altered respiratory pattern. Such respiratory defects could be recapitulated in mice with pan-neuron-specific or cholinergic neuron-specific ablation of the cpeb2 gene. Further investigation revealed that cholinergic transmission in the KO dorsal motor nucleus of vagus was overactivated because KO mice lack CPEB2-suppressed translation of the rate-limiting enzyme in the production of acetylcholine (i.e., choline acetyltransferase). Consequently, increased parasympathetic signaling leads to hyperactivated bronchoconstriction and abnormal respiration in the KO neonates.

Cannabinoid type 1 and type 2 receptor antagonists prevent attenuation of serotonin-induced reflex apneas by dronabinol in Sprague-Dawley rats

The prevalence of obstructive sleep apnea (OSA) in Americans is 9% and increasing. Increased afferent vagal activation may predispose to OSA by reducing upper airway muscle activation/patency and disrupting respiratory rhythmogenesis. Vagal afferent neurons are inhibited by cannabinoid type 1 (CB₁) or cannabinoid type 2 (CB₂) receptors in animal models of vagally-mediated behaviors. Injections of dronabinol, a non-selective CB₁/CB₂ receptor agonist, into the nodose ganglia reduced serotonin (5-HT)-induced reflex apneas. It is unknown what role CB₁ and/or CB₂ receptors play in reflex apnea. Here, to determine the independent and combined effects of activating CB₁ and/or CB₂ receptors on dronabinol’s attenuating effect, rats were pre-treated with CB₁ (AM251) and/or CB₂ (AM630) receptor antagonists. Adult male Sprague-Dawley rats were anesthetized, instrumented with bilateral electrodes to monitor genioglossus electromyogram (EMGgg) and a piezoelectric strain gauge to monitor respiratory pattern. Following intraperitoneal treatment with AM251 and/or AM630, or with vehicle, serotonin was intravenously infused into a femoral vein to induce reflex apnea. After baseline recordings, the nodose ganglia were exposed and 5-HT-induced reflex apneas were again recorded to confirm that the nerves remained functionally intact. Dronabinol was injected into each nodose ganglion and 5-HT infusion was repeated. Prior to dronabinol injection, there were no significant differences in 5-HT-induced reflex apneas or phasic and tonic EMGgg before or after surgery in the CB₁, CB₂, combined CB₁/CB₂ antagonist, and vehicle groups. In the vehicle group, dronabinol injections reduced 5-HT-induced reflex apnea duration. In contrast, dronabinol injections into nodose ganglia of the CB₁, CB₂, and combined CB₁/CB₂ groups did not attenuate 5-HT-induced reflex apnea duration. However, the CB₁ and CB₂ antagonists had no effect on dronabinol’s ability to increase phasic EMGgg. These findings underscore the therapeutic potential of dronabinol in the treatment of OSA and implicate participation of both cannabinoid receptors in dronabinol’s apnea suppression effect.

The nucleus of the solitary tract and the coordination of respiratory and sympathetic activities

It is well known that breathing introduces rhythmical oscillations in the heart rate and arterial pressure levels. Sympathetic oscillations coupled to the respiratory activity have been suggested as an important homeostatic mechanism optimizing tissue perfusion and blood gas uptake/delivery. This respiratory-sympathetic coupling is strengthened in conditions of blood gas challenges (hypoxia and hypercapnia) as a result of the synchronized activation of brainstem respiratory and sympathetic neurons, culminating with the emergence of entrained cardiovascular and respiratory reflex responses. Studies have proposed that the ventrolateral region of the medulla oblongata is a major site of synaptic interaction between respiratory and sympathetic neurons. However, other brainstem regions also play a relevant role in the patterning of respiratory and sympathetic motor outputs. Recent findings suggest that the neurons of the nucleus of the solitary tract (NTS), in the dorsal medulla, are essential for the processing and coordination of respiratory and sympathetic responses to hypoxia. The NTS is the first synaptic station of the cardiorespiratory afferent inputs, including peripheral chemoreceptors, baroreceptors and pulmonary stretch receptors. The synaptic profile of the NTS neurons receiving the excitatory drive from afferent inputs is complex and involves distinct neurotransmitters, including glutamate, ATP and acetylcholine. In the present review we discuss the role of the NTS circuitry in coordinating sympathetic and respiratory reflex responses. We also analyze the neuroplasticity of NTS neurons and their contribution for the development of cardiorespiratory dysfunctions, as observed in neurogenic hypertension, obstructive sleep apnea and metabolic disorders.

Ventilatory effects of substance P-saporin lesions in the nucleus tractus solitarii of chronically hypoxic rats

During ventilatory acclimatization to hypoxia (VAH), time-dependent increases in ventilation lower Pco(2) levels, and this persists on return to normoxia. We hypothesized that plasticity in the caudal nucleus tractus solitarii (NTS) contributes to VAH, as the NTS receives the first synapse from the carotid body chemoreceptor afferents and also contains CO(2)-sensitive neurons. We lesioned cells in the caudal NTS containing the neurokinin-1 receptor by microinjecting the neurotoxin saporin conjugated to substance P and measured ventilatory responses in awake, unrestrained rats 18 days later. Lesions did not affect hypoxic or hypercapnic ventilatory responses in normoxic control rats, in contrast to published reports for similar lesions in other central chemosensitive areas. Also, lesions did not affect the hypercapnic ventilatory response in chronically hypoxic rats (inspired Po(2) = 90 Torr for 7 days). These results suggest functional differences between central chemoreceptor sites. However, lesions significantly increased ventilation in normoxia or acute hypoxia in chronically hypoxic rats. Hence, chronic hypoxia increases an inhibitory effect of neurokinin-1 receptor neurons in the NTS on ventilatory drive, indicating that these neurons contribute to plasticity during chronic hypoxia, although such plasticity does not explain VAH.

Hypoxia-excited neurons in NTS send axonal projections to Kölliker-Fuse/parabrachial complex in dorsolateral pons

Hypoxic respiratory and cardiovascular responses in mammals are mediated by peripheral chemoreceptor afferents which are relayed centrally via the solitary tract nucleus (NTS) in dorsomedial medulla to other cardiorespiratory-related brainstem regions such as ventrolateral medulla (VLM). Here, we test the hypothesis that peripheral chemoafferents could also be relayed directly to the Kölliker-Fuse/parabrachial complex in dorsolateral pons, an area traditionally thought to subserve pneumotaxic and cardiovascular regulation. Experiments were performed on adult Sprague-Dawley rats. Brainstem neurons with axons projecting to the dorsolateral pons were retrogradely labeled by microinjection with choleras toxin subunit B (CTB). Neurons involved in peripheral chemoreflex were identified by hypoxia-induced c-Fos expression. We found that double-labeled neurons (i.e. immunopositive to both CTB and c-Fos) were localized mostly in the commissural and medial subnuclei of NTS and to a lesser extent in the ventrolateral NTS subnucleus, VLM and ventrolateral pontine A5 region. Extracellular recordings from the commissural and medial NTS subnuclei revealed that some hypoxia-excited NTS neurons could be antidromically activated by electrical stimulations at the dorsolateral pons. These findings demonstrate that hypoxia-activated afferent inputs are relayed to the Kölliker-Fuse/parabrachial complex directly via the commissural and medial NTS and indirectly via the ventrolateral NTS subnucleus, VLM and A5 region. These pontine-projecting peripheral chemoafferent inputs may play an important role in the modulation of cardiorespiratory regulation by dorsolateral pons.

Glutamatergic Antagonism in the NTS Decreases Post-Inspiratory Drive and Changes Phrenic and Sympathetic Coupling During Chemoreflex Activation

For a better understanding of the processing at the nucleus tractus solitarius (NTS) level of the autonomic and respiratory responses to peripheral chemoreceptor activation, herein we evaluated the role of glutamatergic neurotransmission in the intermediate (iNTS) and caudal NTS (cNTS) on baseline respiratory parameters and on chemoreflex-evoked responses using the in situ working heart-brain stem preparation (WHBP). The activities of phrenic (PND), cervical vagus (cVNA), and thoracic sympathetic (tSNA) nerves were recorded before and after bilateral microinjections of kynurenic acid (Kyn, 5 nmol/20 nl) into iNTS, cNTS, or both simultaneously. In WHBP, baseline sympathetic discharge markedly correlated with phrenic bursts (inspiration). However, most of sympathoexcitation elicited by chemoreflex activation occurred during expiration. Kyn microinjected into iNTS or into cNTS decreased the postinspiratory component of cVNA and increased the duration and frequency of PND. Kyn into iNTS produced no changes in sympathoexcitatory and tachypneic responses to peripheral chemoreflex activation, whereas into cNTS, a reduction of the sympathoexcitation, but not of the tachypnea, was observed. The pattern of phrenic and sympathetic coupling during the chemoreflex activation was an inspiratory-related rather than an expiratory-related sympathoexcitation. Kyn simultaneously into iNTS and cNTS produced a greater decrease in postinspiratory component of cVNA and increase in frequency and duration of PND and abolished the respiratory and autonomic responses to chemoreflex activation. The data show that glutamatergic neurotransmission in the iNTS and cNTS plays a tonic role on the baseline respiratory rhythm, contributes to the postinspiratory activity, and is essential to expiratory-related sympathoexcitation observed during chemoreflex activation.

Identification of different types of respiratory neurones in the dorsal brainstem nucleus tractus solitarius of the rat

In Nembutal anaesthetised, spontaneously breathing rats, stereotaxic mapping of the nucleus tractus solitarius (NTS) for respiratory neuronal activity was undertaken. Eight different types of respiratory cells were found between 0.25 and 1.5 mm lateral to midline, extending 0.5 mm caudal to 1.5 mm rostral to obex, and 0.4-1.5 mm below the dorsal surface. A study of the respiratory motor (diaphragm EMG) and neuronal responses to excitatory amino acid (EAA) stimulation of the NTS areas was undertaken. Electrical stimulation of the vagus nerve was employed to study the NTS cellular responses to activation of pulmonary afferents. The effects of chemical activation of the midbrain periaqueductal grey (PAG) on NTS respiratory neuronal activity were investigated. EAA microinjections into the ventrolateral NTS rostral to the obex resulted in an increase in respiratory motor frequency along with increases to inspiratory cell discharge, whilst microinjections into the medial NTS caudal to the obex caused respiratory depression. EAA stimulation of calamus scriptorius produced apnea. NTS inspiratory neurones were inhibited following stimulation of ipsilateral vagus nerve, suggesting their involvement in the Hering-Breuer reflex pathway. PAG stimulation caused excitation of the NTS inspiratory cells indicating the presence of an excitatory respiratory pathway between the two nuclei. Following beta-adrenergic antagonist pre-treatment of ventrolateral NTS, EAA microinjections into PAG did not evoke a cardiorespiratory effect. Based on the various findings the role of NTS in organising respiration in the rat is discussed.

Autonomic and respiratory responses to microinjection of L-glutamate into the commissural subnucleus of the NTS in the working heart-brainstem preparation of the rat

Changes in heart rate (HR), thoracic sympathetic nerve activity (tSNA) and frequency of phrenic nerve discharge (PND) in response to microinjection of L-glutamate before and after local microinjection of ionotropic or metabotropic glutamate receptors antagonists into the commissural subnucleus of the NTS (comNTS) were investigated. The experiments were performed in an in situ unanesthetized decerebrated working heart-brainstem preparation (WHBP), and the main findings were as follows: (a) microinjection of increasing concentrations of L-glutamate (5, 25, 50, 250 and 500 mM) into the comNTS produced bradycardia, increase in tSNA and reduction in the frequency of the PND in a concentration-dependent manner; (b) both bradycardia and increase in tSNA were almost abolished by kynurenic acid (KYN, 250 mM, a nonselective ionotropic glutamate receptor antagonist); (c) the reduction in the frequency of the PND was reversed to an increase in the frequency of the PND after KYN and this increase was blocked by the sequential microinjection of MCPG (100 mM, a nonselective metabotropic glutamate receptor antagonist); and (d) microinjection of increasing concentrations of trans-ACPD (0.5, 1.0, 2.5, 5.0 and 10 mM, a metabotropic glutamate receptor agonist), elicited bradycardia and increase in the frequency of the PND in a concentration-dependent manner, which were blocked by MCPG. Taken together, these data indicate that l-glutamate and its ionotropic receptors are involved in the sympathoexcitatory, bradycardic and reduction in the frequency of the PND responses whereas/although its metabotropic receptors are involved in the bradycardic and mainly in the increase in the frequency of the PND to microinjection of L-glutamate into the comNTS in the WHBP.

Respiratory and autonomic responses to microinjection of NMDA and AMPA into the commissural subnucleus of the NTS of awake rats

The changes in mean arterial pressure (MAP) and respiratory frequency (RF) in response to microinjection of NMDA or AMPA into the commissural subnucleus of the NTS (comNTS) at the calamus scriptorius level of awake rats were evaluated. Under tribromoethanol anesthesia, the rats received guide-cannulae in direction of the NTS and a catheter was inserted into the femoral artery for measurement of arterial pressure. Changes in RF were evaluated with the rats inside a plethysmographic chamber. Randomly microinjections of 5 doses of NMDA (0.001, 0.01, 0.1, 1.0 and 2 nmol/50 nL; n = 10) or AMPA (1, 5, 10, 25 and 50 pmol/50 nL; n = 8) into the comNTS were performed at 15 min intervals and produced a dose-dependent increase in MAP [NMDA (3 +/- 2, 4 +/- 3, 25 +/- 4, 41 +/- 4 and 51 +/- 4 mm Hg) and AMPA (0 +/- 1, 14 +/- 4, 17 +/- 3, 27 +/- 5 and 34 +/- 3 mm Hg)]. Microinjection of NMDA (1 nmol/50 nL; n = 7) or AMPA (50 pmol/50 nL; n = 4) into the comNTS produced a long lasting apnea. The pressor responses to microinjection of NMDA or AMPA into the comNTS were blocked by prazosin, a alpha(1)-adrenoceptor antagonist, indicating that the increase in arterial pressure in both cases was sympathetically mediated. The data show that microinjection of NMDA and AMPA into the comNTS produced pressor response and apnea, indicating that both ionotropic l-glutamate receptors may play a role in the neurotransmission/neuromodulation of the autonomic and respiratory components of the cardiovascular reflexes at this level.

Angiotensin II evokes hypotension and renal sympathoinhibition from a highly restricted region in the nucleus tractus solitarii

Microinjections of low doses (in the femtomolar or low picomolar range) of angiotensin II (Ang II) into the nucleus tractus solitarii (NTS) evoke depressor responses. In this study we have mapped in the rat the precise location of the subregion within the NTS at which Ang II evokes significant sympathoinhibitory and depressor responses. Microinjections of 1 pmol of Ang II evoked large decreases (>or=20% of baseline) in renal sympathetic nerve activity (RSNA), from a highly restricted region in the medial NTS, at or very close to the level 0.2 mm caudal to the obex. Microinjections of the same dose of Ang II into the commissural or lateral NTS at the same rostrocaudal level, or into the medial and lateral NTS at the level of the obex evoked significantly smaller sympathoinhibitory responses, while microinjections into more rostral or caudal levels of the NTS evoked significant sympathoinhibitory responses even less frequently. In most cases (71%), the sympathoinhibitory responses were accompanied by depressor responses, the magnitudes of which were also greater within the medial NTS at the level 0.2 mm caudal to obex, as compared to the surrounding subregions. The results demonstrate that the cardiovascular effects of Ang II in the NTS are highly site-specific. Taken together with previous studies, the results also indicate that the neurons in the NTS that mediate the Ang II-evoked sympathoinhibition are a discrete subgroup of the population of sympathoinhibitory neurons within the nucleus.

A randomized placebo-controlled trial of an NMDA receptor antagonist in sleep-disordered breathing

Hypoxemia is a powerful stimulus of glutamate release in the central nervous system (CNS) and a hallmark phenomenon in sleep disordered breathing (SDB). Glutamate effects that include neuronal damage and apoptosis following hypoxemia and apnea following microinjections in animal models are in part mediated via postjunctional N-methyl-D-aspartate (NMDA) receptors. This was a double blind, randomized, placebo-controlled single dose cross-over study of the NMDA receptor antagonist AR-R15896AR in 15 male patients with moderate to severe SDB. Seven patients received 120 mg and eight patients received 350 mg AR-R15896AR or corresponding placebo (given by 2 h infusion) starting half an hour before estimated sleep onset. AR-R15896AR concentrations were in line with the predicting kinetic model. A standard polysomnographic montage was applied. Repeated plasma samples were obtained in nine patients for analysis of plasma glutamate. Glutamate concentration in plasma did not change overnight and was unrelated to severity of SDB. Overall AHI (apnea-hypopnea index; primary efficacy variable) or investigated oxygen saturation variables were not significantly changed after AR-R15896AR at either dosage level. Side effects were mostly confined to the higher dose level and included vivid dreams, nightmares as well as in two cases mild hallucinations. The previously postulated role of glutamate in SDB could not be confirmed after AR-R15896AR induced NMDA-receptor blockade.

Hemodynamic and respiratory responses to microinjection of ATP into the intermediate and caudal NTS of awake rats

The nucleus tractus solitarii (NTS) is the site of integration of the peripheral chemoreceptor afferents in the brainstem. Previous studies from our laboratory have shown that microinjection of ATP into the intermediate NTS produced increases in arterial pressure and bradycardia. In the present study, we evaluated the hemodynamic and respiratory responses to microinjection of ATP into the intermediate and caudal commissural NTS. In the same group of rats the responses were compared with cardiorespiratory responses to chemoreflex activation (KCN, i.v.). The data show that microinjection of ATP into the intermediate NTS produced pressor and bradycardic responses similar to those observed in response to chemoreflex activation but apnoea instead of tachypnoea. Microinjection of ATP into caudal commissural NTS produced increase in arterial pressure and tachypnoea similar to the chemoreflex but a minor bradycardia. The data show that microinjection of ATP into different sub-regions of the NTS produces a diverse pattern of hemodynamic and respiratory responses and suggest the involvement of this purine in the neurotransmission of the cardiovascular reflex in the NTS.

Carotid sinus nerve section abolishes NMDA evoked respiratory effects in anaesthetised rats

Respiratory effects of NMDA injection into the right atrium were investigated in 11 urethane-chloralose anaesthetised and spontaneously breathing rats. The animals were initially vagotomised and six of them were subdued to the subsequent carotid sinus nerve section, and the other five were treated by NMDA antagonist. Bolus injection of NMDA (27 micromol/kg) induced the depression of ventilation in all rats, due to the decrease in tidal volume from a baseline of 2.98 +/- 0.4 to 2.63 +/- 0.3 ml (P < 0.01), and slowing down of the respiratory rate from a baseline of 56 +/- 2.6 to 27 +/- 2.0 breaths min(-1) (P < 0.0001). Section of the carotid sinus nerves (CSNs) precluded the respiratory depression. Prolongation of the expiratory time was reduced by this neurotomy from 5.07 +/- 2.6 to 1.04 +/- 0.03 (P < 0.05). In five rats the blockade of NMDA receptors with the selective antagonist (AP-7) was likewise efficient in eliminating the post-NMDA respiratory response. NMDA increased mean arterial blood pressure and this rise occurred beyond the afferentation from the carotid bodies and the blockade of NMDA receptors. Results of this study indicate that inhibition of the respiratory drive evoked by NMDA administered via the peripheral circulation requires intact carotid bodies and activation of NMDA receptors.

Excitatory convergence of periaqueductal gray and somatic afferents in the solitary tract nucleus: role for neurokinin 1 receptors

Our previous studies (Boscan P, Kasparov S, and Paton JF. Eur J Neurosci 16: 907–920, 2002) showed that activation of somatic afferents attenuated the baroreceptor reflex via neurokinin type 1 (NK1) and GABAA receptors within the nucleus of the solitary tract (NTS). The periaqueductal gray matter (PAG) can also depress baroreceptor reflex function and project to the NTS. In the present study, we have tested the possibility that the dorsolateral (dl)-PAG projects to the NTS neurons that also respond to somatic afferent input. In an in situ, arterially perfused, unanesthetized decerebrate rat preparation, somatic afferents (brachial plexus), cervical spinal cord, and dl-PAG were stimulated electrically, whereas NTS neurons were recorded extracellularly. From 45 NTS neurons excited by either brachial plexus or dl-PAG stimulation, 41 received convergence excitatory inputs from both afferents. Onset latency and evoked peak discharge frequency from brachial plexus afferents were 39.4 ± 4.7 ms and 10.7 ± 1.1 Hz, whereas this was 43.9 ± 6.4 ms and 7.9 ± 1 Hz, respectively, following dl-PAG stimulation. As revealed by using a paired pulse stimulation protocol, monosynaptic connections were found in 9 of 36 neurons tested from both spinal cord and dl-PAG. We tested NK1-receptor sensitivity in 38 neurons that received convergent inputs from brachial plexus/PAG. Fifteen neurons were sensitive to selective antagonism of NK1 receptors. CP-99994, the NK1 antagonist, failed to alter ongoing firing activity but reduced the evoked peak discharge frequency following stimulation of both brachial plexus (from 12.3 ± 1.8 to 7.2 ± 1.3 Hz; P < 0.01) and PAG (from 7.8 ± 1.5 to 4.5 ± 1 Hz; P < 0.01). We conclude that 1) somatic brachial and PAG afferents can converge onto single NTS neurons; 2) this convergence occurs via either direct or indirect pathways; and 3) NK1 receptors are activated by some of these inputs.

Passive smoke effects on cough and airways in young guinea pigs: role of brainstem substance P

Children raised with extended exposure to environmental tobacco smoke (ETS) experience increased cough and wheeze. This study was designed to determine whether extended ETS exposure enhances citric acid-induced cough and bronchoconstriction in young guinea pigs via a neurokinin-1 (NK-1) receptor mechanism at the first central synapse of lung afferent neurons, the nucleus tractus solitarius. Guinea pigs were exposed to ETS from 1 to 6 weeks of age. At 5 weeks of age, guide cannulae were implanted bilaterally in the medial nucleus tractus solitarius at a site that produced apnea in response to the glutamate agonist D,L-homocysteic acid. At 6 weeks of age, either vehicle or a NK-1 receptor antagonist, SR 140333, was injected into the nucleus tractus solitarius of the conscious guinea pigs who were then exposed to citric acid aerosol. ETS exposure significantly enhanced citric acid-induced cough by 56% and maximal Penh (a measure of airway obstruction) by 43%, effects that were attenuated by the NK-1 receptor antagonist in the nucleus tractus solitarius. We conclude that in young guinea pigs extended exposure to ETS increases citric acid-induced cough and bronchoconstriction in part by an NK-1 receptor mechanism in the nucleus tractus solitarius.

Cardiovascular responses to chemical stimulation of the lateral tegmental field and adjacent medullary reticular formation in the rat

Relatively few studies have been done to characterize cardiovascular responses to the chemical stimulation of sites located in the medullary lateral tegmental field (LTF) and most of them have been carried out in anesthetized animals. Our experiments were carried out in decerebrated, artificially ventilated, adult male Wistar rats. In the LTF, two types of cardiovascular responses were elicited. One type consisted of pressor responses accompanied by bradycardia. Such responses were elicited from a region 0.4 mm caudal to 0.8 mm rostral to the calamus scriptorius (CS); maximum responses were elicited from a site 0.6 mm rostral to the CS, 1.2 mm lateral to the midline and 1.2 mm deep from the dorsal medullary surface. Another type consisted of pressor responses without any change in heart rate; such responses were elicited from a region 1-1.6 mm rostral to the CS. Nucleus ambiguus (nAmb) and dorsal motor nucleus of the vagus (nDMX) and the reticular formation surrounding these areas were the main sites from which bradycardia (accompanied by either no or small changes in BP) was elicited. In the nAmb, maximum bradycardia was elicited from a site 0.6 mm rostral to the CS, 1.8 mm lateral to the midline and 2.4 mm deep from the dorsal medullary surface. In the nDMX, most prominent bradycardic responses were elicited at 0-0.6 mm rostral to the CS, and 0.6 mm lateral to the midline and 1 mm deep from the dorsal medullary surface. Cardiovascular effects elicited from sites in other well-known areas, such as the rostral ventrolateral medullary pressor area (RVLM) and caudal ventrolateral medullary depressor area (CVLM), and the nucleus tractus solitarius (nTS) were also included for comparison of different responses. These results are expected to prove useful in studies in which the microinjection technique is used to characterize cardiovascular responses.

Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate is a major neurotransmitter of chemoreceptor and baroreceptor afferent pathways in mammals and therefore plays a central role in the development of cardiorespiratory reflexes. In fish, the gills are the major sites of these receptors, and, consequently, the terminal field (sensory area) of their afferents (glossopharyngus and vagus) in the medulla must be an important site for the integration of chemoreceptor and baroreceptor signals. This investigation explored whether fish have glutamatergic mechanisms in the vagal sensory area (Xs) that could be involved in the generation of cardiorespiratory reflexes. The locations of the vagal sensory and motor (Xm) areas in the medulla were established by the orthograde and retrograde axonal transport of the neural tract tracer Fast Blue following its injection into the ganglion nodosum. Glutamate was then microinjected into identified sites within the Xs in an attempt to mimic chemoreceptor- and baroreceptor-induced reflexes commonly observed in fish. By necessity, the brain injections were performed on anaesthetised animals that were fixed by 'eye bars' in a recirculating water system. Blood pressure and heart rate were measured using an arterial cannula positioned in the afferent branchial artery of the 3rd gill arch, and ventilation was measured by impedance probes sutured onto the operculum. Unilateral injection of glutamate (40-100 nl, 10 mmol l(-1)) into the Xs caused marked cardiorespiratory changes. Injection (0.1-0.3 mm deep) in different rostrocaudal, medial-lateral positions induced a bradycardia, either increased or decreased blood pressure, ventilation frequency and amplitude and, sometimes, an initial apnea. Often these responses occurred simultaneously in various different combinations but, occasionally, they appeared singly, suggesting specific projections into the Xs for each cardiorespiratory variable and local determination of the modality of the response. Response patterns related to chemoreceptor reflex activation were predominantly located rostral of obex, whereas patterns related to baroreceptor reflex activation were more caudal, around obex. The glutamate-induced bradycardia was N-methyl-D-aspartate (NMDA) receptor dependent and atropine sensitive. Taken together, our data provide evidence that glutamate is a putative player in the central integration of chemoreceptor and baroreceptor information in fish.

GABA-mediated neurotransmission in the ventrolateral NTS plays a role in respiratory regulation in the rat

Our purpose was to determine whether endogenously released GABA in the ventrolateral nucleus of the solitary tract (vlNTS) of the rat influences respiration. Experiments were carried out in anesthetized, vagotomized and spontaneously breathing rats, and diaphragm electromyogram activity was measured while drugs affecting GABAergic neurotransmission were microinjected into the vlNTS and medial NTS (mNTS). Bilateral microinjection of nipecotic acid, 5 or 25 nmol, into the vlNTS (but not in the mNTS) produced dose-dependent increases in inspiratory duration (Ti) frequently culminating in apneustic breathing. Neither unilateral microinjection of bicuculline nor CGP-35348 (GABA(B) receptor antagonist) reversed this response; however, a combination of both GABA receptor antagonists effectively reversed apneustic breathing. Bilateral microinjection of either muscimol or baclofen into the vlNTS mimicked the effect of nipecotic acid. Microinjection of the bicuculline produced apnea, whereas microinjection of CGP-35348 produced a decrease in Ti and an increase in expiratory duration. Immunohistochemical analysis of the vlNTS region revealed GABA(A) receptors densely localized to processes, whereas GABA(B) immunoreactivity was localized to cell bodies. Our data indicate that GABA activity in the vlNTS is important for respiratory function.

Site-specific effects of apelin-13 in the rat medulla oblongata on arterial pressure and respiration

Apelin peptides are now known to be endogenous ligands at the orphan G-protein coupled receptor, APJ. Apelin and its receptor have been found in the brainstem and shown to have a role in haemodynamic homeostasis when injected intravenously. The physiological role of this peptide and its receptor centrally are yet to be elucidated. In this study, urethane anaesthetised, paralysed and ventilated male Sprague-Dawley rats (350-450 g, n=4) were used to investigate the action of apelin-13 microinjected directly into the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM) on arterial pressure and phrenic nerve activity. Apelin-13 microinjections (4 mmol/l, 50 nl) into the NTS resulted in either apnea or decreased phrenic nerve discharge amplitude by up to 30%. In the RVLM, apelin-13 caused either a 100-200% increase, or a 10-30% increase in phrenic nerve discharge amplitude depending on the exact site of injection. Increases of 10-20 mm Hg in arterial pressure were also evoked from both the NTS and the RVLM. These data suggest a role for apelin-13 in arterial pressure and respiratory control in the NTS and the RVLM.

Inhibition of neurons in commissural nucleus of solitary tract reduces sympathetic nerve activity in SHR

Neurons in the commissural nucleus of the solitary tract (commNTS) play an important role in certain cardiovascular responses dependent on sympathetic vasoconstrictor activation, including the arterial chemoreceptor reflex. Electrolytic lesions of the commNTS elicit a fall in arterial pressure (AP) in spontaneously hypertensive rats (SHR). To determine whether the latter result 1) arose from elimination of commNTS neuronal activity rather than en passant axons and 2) was accompanied by a reduction in sympathetic nerve activity, we evaluated the effect of inhibition of neurons in the commNTS on basal splanchnic sympathetic nerve activity (SNA), AP, and heart rate (HR) in SHR, Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats. In chloralose-anesthetized, paralyzed, and artificially ventilated SHR, microinjection of GABA into the commNTS markedly decreased splanchnic SNA, AP, and HR. The reductions in SNA and AP following similar microinjections in WKY and SD rats were significantly less than those in SHR. Our findings suggest that tonically active neurons in the commNTS contribute to the maintenance of SNA and the hypertension in SHR. The level of tonic discharge of these commNTS neurons in normotensive WKY and SD rats may be lower than in SHR.

Selective cardiorespiratory and catecholaminergic areas express the hypoxia-inducible factor-1alpha (HIF-1alpha) under in vivo hypoxia in rat brainstem

Under severe oxygen deprivation, all cells are able to express the transcription factor HIF-1, which activates a wide range of genes. Under tolerable hypoxia, chemosensory inputs are integrated in brainstem areas, which control cardiorespiratory responses. However, the molecular mechanisms of this functional acclimatization are unknown. We investigated when and where the inducible HIF-1alpha subunit is expressed in the rat brainstem in vivo, under physiological hypoxia. The regional localization of HIF-1alpha mRNA and protein was determined by in situ hybridization and immunocytochemistry in adult male rats exposed to moderate hypoxia (10% O2) for 1-6 h. HIF-1alpha protein was found in cell types identified by immunocytochemistry as catecholaminergic neurons. Hypoxia induced HIF-1alpha mRNA and protein in only some parts of the brainstem located dorsomedially and ventrolaterally, which are those involved in the cardiorespiratory control. No labelling was detected under normoxia. The protein was detected in glia and neurons after 1 and 6 h of hypoxia, respectively. A subset of A2C2 and A1C1 catecholaminergic neurons colocalized tyrosine hydroxylase and HIF-1alpha proteins under hypoxia, but no HIF-1alpha was detected in more rostral catecholaminergic areas. In contrast to cardiorespiratory areas, HIF-1alpha protein was already present under normoxia in glial cells of brainstem tracts but was not overexpressed under hypoxia, although HIF-1alpha mRNA was up-regulated. In conclusion, there appear to be two regulatory mechanisms for HIF-1alpha expression in the brainstem: hypoxic induction of HIF-1alpha protein in cardiorespiratory-related areas and constitutive protein expression unaffected by hypoxia in brainstem tracts.

Acute hypotension and bradycardia by medulla oblongata compression in spinal surgery

A 71-year-old man was admitted to the hospital with subarachnoid hemorrhage caused by a cervical dural arteriovenous shunt. During surgery, the patient developed acute hypotension and bradycardia, probably caused by surgical compression of the medulla oblongata. During posterior fossa and upper cervical surgery, monitoring cardiovascular, respiratory, and evoked potential parameters is advocated. In the current case, only cardiovascular monitoring detected alteration of brain stem function. Anesthesiologists should be aware that surgical manipulation of the dorsal medulla might cause hemodynamic changes and expose patients to danger. Through close cardiovascular monitoring we can rapidly detect changes in vital signs, which allows prompt intervention to prevent irreversible neurologic deficits and potentially catastrophic patient outcome.

Specific subnuclei of the nucleus tractus solitarius play a role in determining the duration of inspiration in the rat

Our previous data obtained in the cat suggest that the neurons of the ventrolateral subnucleus of the tractus solitarius (vlNTS) act as an inspiratory off-switch and terminate the inspiratory phase of the respiratory cycle (Berger et al., Eur. J. Pharmacol. 277 (1995) 195-208; Gillis et al., Neurosci. Abstr. 23 (1997) 725). The purpose of the present study was to determine whether inhibition of the region of the vlNTS of the rat using drugs that hyperpolarize, disfacilitate or block both axonal conduction and action potential generation would alter the inspiratory phase of the respiratory cycle. Experiments were conducted in anesthetized, vagotomized and spontaneously breathing rats while monitoring diaphragmatic electromyogram activity. Vagus nerves were sectioned in order to rule out prolongation of inspiration evoked by microinjection of agents into the vlNTS which block excitatory drive from lung afferent inputs. Bilateral microinjection of the inhibitory amino acid gamma-aminobutyric acid (GABA) 25 nmol/45 nl produced an immediate prolongation of inspiratory duration (484+/-18 to 1291+/-84 ms) and an apneustic pattern of breathing. Other effects observed were a significant shortening of expiratory duration (778+/-36 to 432+/-38 ms), rise in blood pressure (83+/-4 to 108+/-6 mmHg) and a small but significant increase in heart rate (439+/-17 to 452+/-18 beats/min). Bilateral microinjection of the ionotropic glutamate receptor antagonist kynurenic acid (1 nmol) and the Na(+) channel blocker tetrodotoxin (10 pmol) into the region of the vlNTS consistently produced a similar prolongation of inspiratory duration and an apneustic pattern of breathing. These results support the hypothesis that neurons in the region of the vlNTS promote the transition from inspiration to expiration and function as part of the 'Inspiratory Off Switch'.

Receptor subtypes mediating depressor responses to microinjections of nicotine into medial NTS of the rat

Microinjections (50 nl) of nicotine (0.01-10 microM) into the nucleus of the solitary tract (NTS) of adult, urethan-anesthetized, artificially ventilated, male Wistar rats, elicited decreases in blood pressure and heart rate. Prior microinjections of alpha-bungarotoxin (alpha-BT) and alpha-conotoxin ImI (specific toxins for nicotinic receptors containing alpha7 subunits) elicited a 20-38% reduction in nicotine responses. Similarly, prior microinjections of hexamethonium, mecamylamine, and alpha-conotoxin AuIB (specific blockers or toxin for nicotinic receptors containing alpha3beta4 subunits) elicited a 47-79% reduction in nicotine responses. Nicotine responses were completely blocked by prior sequential microinjections of alpha-BT and mecamylamine into the NTS. Complete blockade of excitatory amino acid receptors (EAARs) in the NTS did not attenuate the responses to nicotine. It was concluded that 1) the predominant type of nicotinic receptor in the NTS contains alpha3beta4 subunits, 2) a smaller proportion contains alpha7 subunits, 3) the presynaptic nicotinic receptors in the NTS do not contribute to nicotine-induced responses, and 4) EAARs in the NTS are not involved in mediating responses to nicotine.