Subnuclear organization of the lateral tegmental field of the rat. II: Catecholamine neurons and ventral respiratory group

Systematic Morphometry of Catecholamine Nuclei in the Brainstem

Catecholamine nuclei within the brainstem reticular formation (RF) play a pivotal role in a variety of brain functions. However, a systematic characterization of these nuclei in the very same experimental conditions is missing so far. Tyrosine hydroxylase (TH) immune-positive cells of the brainstem correspond to dopamine (DA)-, norepinephrine (NE)-, and epinephrine (E)-containing cells. Here, we report a systematic count of TH-positive neurons in the RF of the mouse brainstem by using stereological morphometry. All these nuclei were analyzed for anatomical localization, rostro-caudal extension, volume, neuron number, neuron density, and mean neuronal area for each nucleus. The present data apart from inherent informative value wish to represent a reference for neuronal mapping in those studies investigating the functional anatomy of the brainstem RF. These include: the sleep-wake cycle, movement control, muscle tone modulation, mood control, novelty orienting stimuli, attention, archaic responses to internal and external stressful stimuli, anxiety, breathing, blood pressure, and innumerable activities modulated by the archaic iso-dendritic hard core of the brainstem RF. Most TH-immune-positive cells fill the lateral part of the RF, which indeed possesses a high catecholamine content. A few nuclei are medial, although conventional nosography considers all these nuclei as part of the lateral column of the RF. Despite the key role of these nuclei in psychiatric and neurological disorders, only a few of them aspired a great attention in biomedical investigation, while most of them remain largely obscure although intense research is currently in progress. A simultaneous description of all these nuclei is not simply key to comprehend the variety of brainstem catecholamine reticular neurons, but probably represents an intrinsically key base for understanding brain physiology and physiopathology.

Differences in respiratory changes and Fos expression in the ventrolateral medulla of rats exposed to hypoxia, hypercapnia, and hypercapnic hypoxia

Respiratory responses to hypoxia and/or hypercapnia, and their relationship to neural activity in the ventrolateral medulla (VLM), which includes the respiratory center, have not yet been elucidated in detail. We herein examined respiratory responses during exposure of 10% O2 (hypoxia), 10% CO2 (hypercapnia), and 10% O2-10% CO2 (hypercapnic hypoxia) using plethysmography. In addition to recording respiration, Fos expressions were examined in the VLM of the rat exposed to each gas to analyze neural activity. Respiratory frequency was increased in rats exposed to hypoxia, and Fos-positive neurons were observed in the caudal VLM (cVLM) and medial VLM (mVLM). Tidal volume was increased in rats exposed to hypercapnia, and Fos-positive neurons were observed in the rostral VLM (rVLM) includes the retrotrapezoid nucleus (RTN) and mVLM. Tidal volume was enhanced in rats exposed to hypercapnic hypoxia, similar to that in hypercapnia-exposed rats, and Fos-positive neurons were observed in the entire region of the VLM. In the mVLM and cVLM, double immunofluorescence showed Fos-immunoreactive nerve cells were also immunoreactive to dopamine β-hydroxylase, the marker for A1/C1 catecholaminergic neuron. These results suggested that hypoxia and hypercapnia modulated rhythmogenic microcircuits in the mVLM via A1/C1 neurons and the RTN, respectively.

Prostaglandin E2 differentially modulates the central control of eupnoea, sighs and gasping in mice

Prostaglandin E2 (PGE2) augments distinct inspiratory motor patterns, generated within the preBötzinger complex (preBötC), in a dose-dependent way. The frequency of sighs and gasping are stimulated at low concentrations, while the frequency of eupnoea increases only at high concentrations. We used in vivo microinjections into the preBötC and in vitro isolated brainstem slice preparations to investigate the dose-dependent effects of PGE2 on the preBötC activity. Synaptic measurements in whole cell voltage clamp recordings of inspiratory neurons revealed no changes in inhibitory or excitatory synaptic transmission in response to PGE2 exposure. In current clamp recordings obtained from inspiratory neurons of the preBötC, we found an increase in the frequency and amplitude of bursting activity in neurons with intrinsic bursting properties after exposure to PGE2. Riluzole, a blocker of the persistent sodium current, abolished the effect of PGE2 on sigh activity, while flufenamic acid, a blocker of the calcium-activated non-selective cation conductance, abolished the effect on eupnoeic activity caused by PGE2. Prostaglandins are important regulators of autonomic functions in the mammalian organism. Here we demonstrate in vivo that prostaglandin E2 (PGE2) can differentially increase the frequency of eupnoea (normal breathing) and sighs (augmented breaths) when injected into the preBötzinger complex (preBötC), a medullary area that is critical for breathing. Low concentrations of PGE2 (100-300 nm) increased the sigh frequency, while higher concentrations (1-2 μm) were required to increase the eupnoeic frequency. The concentration-dependent effects were similarly observed in the isolated preBötC. This in vitro preparation also revealed that riluzole, a blocker of the persistent sodium current (INap), abolished the modulatory effect on sighs, while flufenamic acid, an antagonist for the calcium-activated non-selective cation conductance (ICAN ) abolished the effect of PGE2 on fictive eupnoea at higher concentrations. At the cellular level PGE2 significantly increased the amplitude and frequency of intrinsic bursting in inspiratory neurons. By contrast PGE2 affected neither excitatory nor inhibitory synaptic transmission. We conclude that PGE2 differentially modulates sigh, gasping and eupnoeic activity by differentially increasing INap and ICAN currents in preBötC neurons.

Catecholamine inputs to expiratory laryngeal motoneurons in rats

Many respiration-related interneurons and motoneurons receive a catecholaminergic input, but the extent and distribution of this input to recurrent laryngeal motoneurons that innervate intrinsic muscles of the larynx are not clear. In the present study, we examined the catecholaminergic input to expiratory laryngeal motoneurons in the caudal nucleus ambiguus by combining intracellular labeling of single identified motoneurons, with immunohistochemistry to reveal tyrosine hydroxylase immunoreactive (catecholaminergic) terminal varicosities. Close appositions were found between the two structures, with 18 ± 5 close appositions per motoneuron (n = 7). Close appositions were more frequently observed on distal rather than proximal dendrites. Axosomatic appositions were not seen. In order to determine the source of this input, microinjections of cholera toxin B subunit (1%, 20 nl) were made into the caudal nucleus ambiguus. Retrogradely labeled neurons, located in the ipsilateral nucleus tractus solitarius and the area postrema, were tyrosine hydroxylase-positive. Our results not only demonstrate details of the extent and distribution of potential catecholamine inputs to the expiratory laryngeal motoneuron, but further indicate that the inputs, at least in part, originate from the dorsomedial medulla, providing a potential anatomical basis for previously reported catecholaminergic effects on the laryngeal adductor reflex.

When norepinephrine becomes a driver of breathing irregularities: how intermittent hypoxia fundamentally alters the modulatory response of the respiratory network

Neuronal networks are endogenously modulated by aminergic and peptidergic substances. These modulatory processes are critical for maintaining normal activity and adapting networks to changes in metabolic, behavioral, and environmental conditions. However, disturbances in neuromodulation have also been associated with pathologies. Using whole animals (in vivo) and functional brainstem slices (in vitro) from mice, we demonstrate that exposure to acute intermittent hypoxia (AIH) leads to fundamental changes in the neuromodulatory response of the respiratory network located within the preBötzinger complex (preBötC), an area critical for breathing. Norepinephrine, which normally regularizes respiratory activity, renders respiratory activity irregular after AIH. Respiratory irregularities are caused both in vitro and in vivo by AIH, which increases synaptic inhibition within the preBötC when norepinephrine is endogenously or exogenously increased. These irregularities are prevented by blocking synaptic inhibition before AIH. However, regular breathing cannot be reestablished if synaptic inhibition is blocked after AIH. We conclude that subtle changes in synaptic transmission can have dramatic consequences at the network level as endogenously released neuromodulators that are normally adaptive become the drivers of irregularity. Moreover, irregularities in the preBötC result in irregularities in the motor output in vivo and in incomplete transmission of inspiratory activity to the hypoglossus motor nucleus. Our finding has basic science implications for understanding network functions in general, and it may be clinically relevant for understanding pathological disturbances associated with hypoxic episodes such as those associated with myocardial infarcts, obstructive sleep apneas, apneas of prematurity, Rett syndrome, and sudden infant death syndrome.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

Projections of preBötzinger complex neurons in adult rats

The preBötzinger Complex (preBötC) contains neural microcircuitry essential for normal respiratory rhythm generation in rodents. A subpopulation of preBötC neurons expresses somatostatin, a neuropeptide with a modulatory action on breathing. Acute silencing of a subpopulation of preBötC neurons transfected by a virus driving protein expression under the somatostatin promoter results in persistent apnea in awake adult rats. Given the profound effect of silencing these neurons, their projections are of interest. We used an adeno-associated virus to overexpress enhanced green fluorescent protein driven by the somatostatin promoter in preBötC neurons to label their axons and terminal fields. These neurons send brainstem projections to: 1) contralateral preBötC; 2) ipsi- and contralateral Bötzinger Complex; 3) ventral respiratory column caudal to preBötC; 4) parafacial respiratory group/retrotrapezoid nucleus; 5) parahypoglossal nucleus/nucleus of the solitary tract; 6) parabrachial/Kölliker-Fuse nuclei; and 7) periaqueductal gray. We did not find major projections to either cerebellum or spinal cord. We conclude that there are widespread projections from preBötC somatostatin-expressing neurons specifically targeted to brainstem regions implicated in control of breathing, and provide a network basis for the profound effects and the essential role of the preBötC in breathing.

Apnea produces neuronal degeneration in the pons and medulla of guinea pigs

Obstructive sleep apnea and other sleep-related breathing disorders result in recurrent periods of oxygen deprivation (hypoxia), hypercapnia and an increase in the cellular production of reactive oxygen species (oxidative stress-related injury). Individuals with these disorders suffer from a variety of cellular abnormalities that result in cardiopulmonary dysfunctions, disturbances in sleep and other pathologies. In the present experiment, using an animal model of sleep apnea, we determined that the degeneration of neurons and glia, due to apoptosis, occurs in specific regions of the pons and medulla. Adult guinea pigs, which were divided into control (normoxic) and experimental (hypoxic) groups, were anesthetized with alpha-chloralose and immobilized with Flaxedil. Apnea (hypoxia) was induced by ventilatory arrest in order to desaturate the oxyhemoglobin to 75% SpO(2). A sequence of apnea, followed by ventilation with recovery to >95% SpO(2), was repeated for a period of 3h. At the end of the period of recurrent apnea, the animals were perfused and brain sections were immunostained with a mouse monoclonal antibody raised against single-stranded DNA (ssDNA). Apoptotic neurons and glia, which were not found in the control group of animals, were present in brainstem regions in hypoxic group of animals; these regions involved in the control of respiration (e.g., the parafacial respiratory group and the ventral respiratory group), cardiovascular functions (e.g., the nucleus ambiguus, the nucleus tractus solitarius and the dorsal motor nucleus of the vagus) as well as REM sleep (the nucleus pontis oralis) and wakefulness (e.g., the dorsal raphe and locus ceruleus). We suggest apoptotic neurons and glia in critical areas of the pons and medulla results in many of the comorbidities experienced by patients with sleep-disordered breathing pathologies.

Alpha2-adrenoceptors coordinate swallowing and respiration

Because the discoordination between swallowing and respiration may cause severe respiratory disorders such as aspiration pneumonia, understanding the neuronal mechanisms underlying such coordination is important. Recently, it was reported that medullary noradrenergic neurons are involved in evoking esophageal-gastric relaxation reflex, leading to a hypothesis that such neurons are also involved in swallowing-respiration coordination. We tested this hypothesis using an in vitro brain-stem preparation obtained from neonatal rats. A temporal inhibition of respiratory rhythm was consistently observed when swallowing activity was induced by electrical stimulations to the supralaryngeal nerve. We found that a broad adrenergic receptor agonist, norepinephrine, markedly blocked the swallowing-induced temporal inhibition of respiration. Further studies revealed that swallowing-induced respiratory inhibition is blocked by an alpha2-adrenergic receptor agonist and enhanced by an alpha2-adrenergic receptor antagonist, indicating an important role of alpha2-adrenergic receptors in regulation of the coordination between swallowing and respiration in vitro.

Diabetes induces neural degeneration in nucleus ambiguus (NA) and attenuates heart rate control in OVE26 mice

Baroreflex sensitivity is impaired by diabetes mellitus. Previously, we found that diabetes induces a deficit of central mediation of baroreflex-mediated bradycardia. In this study, we assessed whether diabetes induces degeneration of the nucleus ambiguus (NA) and reduces heart rate (HR) responses to l-Glutamate (L-Glu) microinjection into the NA. FVB control and OVE26 diabetic mice (5-6 months) were anesthetized. Different doses of L-Glu (0.1-5 mM/l, 20 nl) were delivered into the left NA using a multi-channel injector. In other animals, the left vagus was electrically stimulated at 1-40 Hz (1 ms, 0.5 mA, 20 s). HR and mean arterial blood pressure (MAP) responses to L-Glu microinjections into the NA and to the electrical stimulation of the vagus were measured. The NA region was defined by tracer TMR-D injection into the ipsilateral nodose ganglion to retrogradely label vagal motoneurons in the NA. Brainstem slices at -600, -300, 0, +300, and +600 mum relative to the obex were processed using Nissl staining and the number of NA motoneurons was counted. Compared with FVB control, we found in OVE26 mice that: 1) HR responses to L-Glu injection into the NA at doses of 0.2-0.4 (mM/l, 20 nl) were attenuated (p<0.05), but MAP responses were unchanged (p>0.05). 2) HR responses to vagal stimulation were increased (p<0.05). 3) The total number of NA (left and right) motoneurons was reduced (p<0.05). Taken together, we concluded that diabetes reduces NA control of HR and induces degeneration of NA motoneurons. Degeneration of NA cardiac motoneurons may contribute to impairment of reflex-bradycardia in OVE26 diabetic mice.

Maturational changes in pontine and medullary alpha-adrenoceptor influences on respiratory rhythm generation in neonatal rats

We examined developmental changes in alpha-adrenoceptor influences and descending pontine inputs on the medullary respiratory network in the neonatal rat in vitro brainstem-spinal cord preparation. Using a split bath preparation to isolate the pons from the medulla, antagonists for alpha1 and alpha2 adrenoreceptors were applied to only the medulla at postnatal days 0, 2 and 4, before and after transection of the pons. Blocking alpha1 and alpha2 receptors in the medulla in the absence of a pons reduced burst frequency at all ages with a more pronounced effect in younger animals. At all ages the presence of a pons diminished the effect of blocking alpha2 receptors in the medulla and eliminated the effect of blocking alpha1 receptors. These results indicate that there is a tonic release of catecholamines within the medulla that is under influence from the pons. Additionally, transection experiments indicated that during development, the net influence of the pons changed from one of excitation to one of inhibition.

Chronic intermittent hypoxia impairs heart rate responses to AMPA and NMDA and induces loss of glutamate receptor neurons in nucleus ambiguous of F344 rats

Chronic intermittent hypoxia (CIH), as occurs in sleep apnea, impairs baroreflex-mediated reductions in heart rate (HR) and enhances HR responses to electrical stimulation of vagal efferent. We tested the hypotheses that HR responses to activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the nucleus ambiguous (NA) are reduced in CIH-exposed rats and that this impairment is associated with degeneration of glutamate receptor (GluR)-immunoreactive NA neurons. Fischer 344 rats (3-4 mo) were exposed to room air (RA) or CIH for 35-50 days (n = 18/group). At the end of the exposures, AMPA (4 pmol, 20 nl) and NMDA (80 pmol, 20 nl) were microinjected into the same location of the left NA (-200 microm to +200 microm relative to caudal end of area postrema; n = 6/group), and HR and arterial blood pressure responses were measured. In addition, brain stem sections at the level of -800, -400, 0, +400, and +800 microm relative to obex were processed for AMPA and NMDA receptor immunohistochemistry. The number of NA neurons expressing AMPA receptors and NMDA receptors (NMDARs) was quantified. Compared with RA, we found that after CIH 1) HR responses to microinjection of AMPA into the left NA were reduced (RA -290 +/- 30 vs. CIH -227 +/- 15 beats/min, P < 0.05); 2) HR responses to microinjection of NMDA into the left NA were reduced (RA -302 +/- 16 vs. CIH -238 +/- 27 beats/min, P < 0.05); and 3) the number of NMDAR1, AMPA GluR1, and AMPA GluR2/3-immunoreactive cells in the NA was reduced (P < 0.05). These results suggest that degeneration of NA neurons expressing GluRs contributes to impaired baroreflex control of HR in rats exposed to CIH.

Attenuation of heart rate control and neural degeneration in nucleus ambiguus following chronic intermittent hypoxia in young adult Fischer 344 rats

Chronic intermittent hypoxia (CIH) attenuates baroreflex control of heart rate (HR). In this study, we assessed whether CIH exposure reduced nucleus ambiguus (NA) control of HR and induced neural degeneration in the NA. Fischer 344 (age: 3-4 months) rats were exposed to either room air (RA: normoxia) or intermittent hypoxia for 35-50 days. At the end of these exposures, animals were anesthetized with pentobarbital. HR responses to arterial blood pressure (AP) changes induced by phenylephrine (PE) and sodium nitroprusside (SNP) were measured. In another set of rats, HR and AP responses to L-glutamate (L-Glu) microinjections (10 mM, 20 nl) into the left NA and electrical stimulation of the left cervical vagus nerve at 1-30 Hz (0.5 mA, 1 ms) for 20 s were measured. Brainstem slices at the level of -800, -400, 0, +400, +800 microm relative to the obex were processed in additional rats using Nissl staining. The NA was identified by retrogradely labeling vagal motoneurons using the tracer tetramethylrhodamine dextran (TMR-D) which was injected into the ipsilateral nodose ganglion. We found that CIH significantly 1) reduced the baroreflex control of HR (slope RA: -1.2+/-0.2 bpm/mmHg; CIH -0.5+/-0.1 bpm/mmHg; P<0.05); 2) attenuated the HR responses to l-Glu injections into the NA [HR: -280+/-15 (RA) vs. -235+/-16 (CIH) beats/min; P<0.05]; 3) augmented the HR responses to electrical stimulation of the vagus (P<0.05); 4) induced a significant cellular loss in the NA region (P<0.05). Thus, CIH induces a cell loss in the NA region which may contribute to attenuation of baroreflex sensitivity and NA control of HR following CIH.

Possible modulation of the mouse respiratory rhythm generator by A1/C1 neurones

Although compelling evidence exist that the respiratory rhythm generator is modulated by endogenous noradrenaline released from pontine A5 and A6 neurones, we examined whether medullary catecholaminergic neurones also participated in respiratory rhythm modulation. Experiments were performed in neonatal (postnatal days 0-6, P0-P6) and young mice (P14-P18) using "en bloc" medullary preparations (pons resected) and transverse medullary slices. In "en bloc" preparations, blockade of medullary alpha2 adrenoceptors with yohimbine and activation of catecholamine biosynthesis with L-tyrosine significantly depresses and facilitates the respiratory rhythm, respectively. In slices from neonatal and young mice, blockade of medullary alpha2 adrenoceptors also depressed the respiratory rhythm. Yohimbine local applications and lesion-ablation experiments of the dorsal medulla revealed implication of A1/C1 neurones in the yohimbine depressing effect. Although the mechanisms responsible for the yohimbine-depressing effect remain to be elucidated, our in vitro results in neonatal and young mice suggest that endogenous catecholamines released from A1/C1 neurones participate in respiratory rhythm modulation via medullary alpha2 adrenoceptors.

Arterial pulse modulated activity is expressed in respiratory neural output

Although it is well-established that sympathetic activity is modulated with respiration, it is unknown whether neural control of respiration is reciprocally influenced by cardiovascular function. Even though previous studies have suggested the existence of pulse modulation in respiratory neurons, they could not exclude the possibility that such cells were involved in cardiovascular rather than respiratory motor control, owing to neuroanatomic and functional overlaps between brain stem neurons involved in respiratory and cardiovascular control. The aim of this study was to test the hypothesis that respiratory motoneurons and putative premotoneurons are modulated by arterial pulse. An existing data set composed of 72 well-characterized, respiratory-modulated brain stem motoneurons and putative premotoneurons was analyzed using delta(2), a recently described statistic that quantifies the magnitude of arterial pulse-modulated spike activity [Dick TE and Morris KF. J Physiol 556: 959-970, 2004]. Neuronal activity was recorded in the rostral and caudal ventral respiratory groups of 19 decerebrate, neuromuscular-blocked, ventilated cats. Axonal projections were identified by rectified and unrectified spike-triggered averages of recurrent laryngeal nerve activity or by antidromic activation from spinal stimulation electrodes. The firing rates of approximately 30% of these neurons were modulated in phase with both the respiratory and cardiac cycles. Furthermore, arterial pulse modulation occurred preferentially in the expiratory phase in that only expiratory neurons had high delta(2) values and only expiratory activity had significant delta(2) values after partitioning tonic activity into the inspiratory and expiratory phases. The results demonstrate that both respiratory motoneurons and putative premotoneuronal activity can be pulse modulated. We conclude that a cardiac cycle-related modulation is expressed in respiratory motor activity, complementing the long-recognized respiratory modulation of sympathetic nerve activity.

Ultrastructure of the rostral ventral respiratory group neurons in the ventrolateral medulla of the rat

The neurons in the ventrolateral medulla that project to the spinal cord are called the rostral ventral respiratory group (rVRG) because they activate spinal respiratory motor neurons. We retrogradely labeled rVRG neurons with Fluoro-Gold (FG) injections into the fourth cervical spinal cord segment to determine their distribution. The rostral half of the rVRG was located in the area ventral to the semicompact formation of the nucleus ambiguus (AmS). A cluster of the neurons moved dorsally and intermingled with the palatopharyngeal motor neurons at the caudal end of the AmS. The caudal half of the rVRG was located in the area including the loose formation of the nucleus ambiguus caudal to the AmS. We also labeled the rVRG neurons retrogradely with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to determine their ultrastructural characteristics. The neurons of the rVRG were medium to large (38.1 x 22.1 microm), oval or ellipsoid in shape, and had a dark cytoplasm containing numerous free ribosomes, rough endoplasmic reticulum (rER), mitochondria, Golgi apparatuses, lipofuscin granules and a round nucleus with an invaginated nuclear membrane. The average number of axosomatic terminals in a profile was 33.2. The number of axosomatic terminals containing round vesicles and making asymmetric synaptic contacts (Gray's type I) was almost equal to those containing pleomorphic vesicles and making symmetric synaptic contacts (Gray's type II). The axodendritic terminals were large (1.55 microm), and about 60% of them were Gray's type I. The rVRG neurons have ultrastructural characteristics, which are different from the palatopharyngeal motor neurons or the prorpiobulbar neurons.

Microinjections of glycine into the pre-Bötzinger complex inhibit phrenic nerve activity in the rat

Microinjections of L-glutamate were used to identify the pre-Bötzinger complex in urethane-anesthetized, immobilized, bilaterally vagotomized, artificially ventilated, adult male Wistar rats. Unilateral microinjections (20-30 nl) of L-glutamate into the pre-Bötzinger complex on either side elicited a bilateral continuous phrenic nerve discharge superimposed on which was an increase in burst-frequency. Neurokinin-1 receptor immunoreactivity in the semi-compact region of the nucleus ambiguus and the area immediately ventral to it indicated that the site of microinjections was in the general region of pre-Bötzinger complex. Unilateral microinjections of glycine into the pre-Bötzinger complex caused an inhibition of phrenic nerve activity bilaterally in a concentration-dependent manner. At lower concentrations (1 and 3 mM) phrenic nerve burst-frequency as well as burst-amplitude were decreased. At higher concentrations (6 mM), complete bilateral cessation of phrenic nerve activity was observed. The effects of glycine were prevented by a prior microinjection of strychnine (0.5 mM) into the pre-Bötzinger complex. The specificity of strychnine as an antagonist for glycine receptors was established by its lack effect on GABA(A) receptors; muscimol was used as a GABA(A) receptor agonist. Unilateral microinjections of muscimol (0.01 and 0.1 mM) into previously identified pre-Bötzinger complex also caused a bilateral decrease in phrenic nerve burst-frequency and burst-amplitude. At higher concentrations (0.3 and 1 mM) muscimol microinjections into the pre-Bötzinger elicited a complete bilateral cessation of phrenic nerve activity. The effects of muscimol were not altered by prior microinjections of strychnine (0.5 mM) at the same site. These results demonstrate pharmacologically the presence of glycine receptors in the pre-Bötzinger complex. The role of these receptors in the regulation of respiration remains to be elucidated.

Respiratory rhythm generation: converging concepts from in vitro and in vivo approaches?

The timing and activation pattern of breathing movements are determined by the respiratory network. This network is amenable to a variety of in vivo and in vitro approaches, which offers a unique opportunity to investigate multiple organizational levels. It is only recently, however, that concepts obtained under in vivo and in vitro conditions are being integrated into a coherent model of breathing behavior. For example, the pre-Bötzinger complex as an essential site for rhythm generation was first identified in vitro, but has since been verified in vivo. Conversely, timing signals provided by other central and peripheral neuronal areas have so far been investigated in vivo, but it is now possible to address these issues with more complex in vitro preparations. Several key issues remain unresolved. For example, to what extent is the respiratory pattern controlled independently of the underlying rhythm? Answers to this and other questions require a dissection of mechanisms that is only possible through a complementary combination of experimental approaches.

Differential ontogeny of GABA(B)-receptor-mediated pre- and postsynaptic modulation of GABA and glycine transmission in respiratory rhythm-generating network in mouse

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABA(A)-receptor- and glycine-receptor-mediated inhibition is not present, thus the question arises as to whether GABA(B)-receptor-mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0-P15), we analysed the role of GABA(B)-mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25-2 mM) reduced the respiratory frequency. This reduction was prevented by the selective GABA(B) receptor antagonist CGP55845A (CGP) alone at P0-P3, but by bicuculline as well as CGP at P7-P15. Blockade of GABA(B) receptors by CGP increased the respiratory frequency at P0-P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABA(B)-receptor-mediated pre- and postsynaptic modulation was examined by analysing the effect of GABA(B) receptors on spontaneous and miniature IPSCs. In younger animals (P0-P3), the GABA(B) receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7-P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABA(B)-receptor-mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABA(B)-receptor-mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.

MASH-1/RET pathway involvement in development of brain stem control of respiratory frequency in newborn mice

Respiratory abnormalities have been described in MASH-1 (mammalian achaete-scute homologous gene) and c-RET ("rearranged during transfection") mutant newborn mice. However, the neural mechanisms underlying these abnormalities have not been studied. We tested the hypothesis that the MASH-1 mutation may impair c-RET expression in brain stem neurons involved in the control of breathing. To do this, we analyzed brain stem c-RET expression and respiratory phenotype in MASH-1 +/+ wild-type, MASH-1 +/- heterozygous, and MASH-1 -/- knock-out newborn mice during the first 2 h of life. In MASH-1 -/- newborns, c-RET gene expression was absent in the noradrenergic nuclei (A2, A5, A6, A7) that contribute to modulate respiratory frequency and in scattered cells of the rostral ventrolateral medulla. The c-RET transcript levels measured by quantitative RT-PCR were lower in MASH-1 -/- and MASH-1 +/- than in MASH-1 +/+ brain stems (P = 0.001 and P = 0.003, respectively). Breath durations were shorter in MASH-1 -/- and MASH-1 +/- than in MASH-1 +/+ mice (P = 0.022) and were weakly correlated with c-RET transcript levels (P = 0.032). Taken together, these results provide evidence that MASH-1 is upstream of c-RET in noradrenergic brain stem neurons important for respiratory rhythm modulation.

A brainstem area mediating cerebrovascular and EEG responses to hypoxic excitation of rostral ventrolateral medulla in rat

We sought to identify the medullary relay area mediating the elevations of regional cerebral blood flow (rCBF) and synchronization of the electroencephalogram (EEG) in the rat cerebral cortex elicited by hypoxic excitation of reticulospinal sympathoexcitatory neurons of the rostral ventrolateral medulla (RVLM ). In anaesthetized spinalized rats electrical stimulation of RVLM elevated rCBF (laser-Doppler flowmetry) by 31 +/- 6 %, reduced cerebrovascular resistance (CVR) by 26 +/- 8 %, and synchronized the EEG, increasing the power of the 5-6 Hz band by 98 +/- 25 %. Stimulation of a contiguous caudal region, the medullary cerebral vasodilator area (MCVA), had comparable effects which, like responses of RVLM, were replicated by microinjection of L-glutamate (5 nmol, 20 nl). Microinjection of NaCN (300 pmol in 20 nl) elevated rCBF (17 +/- 5 %) and synchronized the EEG from RVLM, but not MCVA, while nicotine (1.2 nmol in 40 nl) increased rCBF by 13 +/- 5 % and synchronized the EEG from MCVA. In intact rats nicotine lowered arterial pressure only from MCVA (101 +/- 3 to 52 +/- 9 mmHg). Bilateral electrolytic lesions of MCVA significantly reduced, by over 59 %, elevations in rCBF and, by 78 %, changes in EEG evoked from RVLM. Bilateral electrolytic lesions of RVLM did not affect responses from MCVA. Anterograde tracing with BDA demonstrated that RVLM and MCVA are interconnected. The MCVA is a nicotine-sensitive region of the medulla that relays signals elicited by excitation of oxygen-sensitive reticulospinal neurons in RVLM to reflexively elevate rCBF and slow the EEG as part of the oxygen-conserving (diving) reflex initiated in these neurons by hypoxia or ischaemia.

O2-sensing after carotid chemodenervation: hypoxic ventilatory responsiveness and upregulation of tyrosine hydroxylase mRNA in brainstem catecholaminergic cells

Ventilatory responses to acute and long-term hypoxia are classically triggered by carotid chemoreceptors. The chemosensory inputs are carried within the carotid sinus nerve to the nucleus tractus solitarius and the brainstem respiratory centres. To investigate whether hypoxia acts directly on brainstem neurons or secondarily via carotid body inputs, we tested the ventilatory responses to acute and long-term hypoxia in rats with bilaterally transected carotid sinus nerves and in sham-operated rats. Because brainstem catecholaminergic neurons are part of the chemoreflex pathway, the ventilatory response to hypoxia was studied in association with the expression of tyrosine hydroxylase (TH). TH mRNA levels were assessed in the brainstem by in situ hybridization and hypoxic ventilatory responses were measured in vivo by plethysmography. After long-term hypoxia, TH mRNA levels in the nucleus tractus solitarius and ventrolateral medulla increased similarly in chemodenervated and sham-operated rats. Ventilatory acclimatization to hypoxia developed in chemodenervated rats, but to a lesser extent than in sham-operated rats. Ventilatory response to acute hypoxia, which was initially low in chemodenervated rats, was fully restored within 21 days in long-term hypoxic rats, as well as in normoxic animals which do not overexpress TH. Therefore, activation of brainstem catecholaminergic neurons and ventilatory adjustments to hypoxia occurred independently of carotid chemosensory inputs. O2-sensing mechanisms unmasked by carotid chemodenervation triggered two ventilatory adjustments: (i) a partial acclimatization to long-term hypoxia associated with TH upregulation; (ii) a complete restoration of acute hypoxic responsivity independent of TH upregulation.

Prenatal hypoxia impairs the postnatal development of neural and functional chemoafferent pathway in rat

1. To define the effects of prenatal hypoxia on the postnatal development of the chemoafferent pathway, ventilation and metabolism, pregnant rats were exposed to normobaric hypoxia (10 % oxygen) from embryonic day 5 to embryonic day 20. Offspring were studied at 1, 3 and 9 weeks of age in three separate protocols. 2. Prenatal hypoxia decreased the dopamine content in the carotid bodies at all ages, and decreased the utilisation rate of noradrenaline in the caudal part of the A2 (A2c), A1 and A5 noradrenergic brainstem cell groups at 3 weeks after birth. At 9 weeks of age, the level of dopamine in the carotid bodies was still reduced but the utilisation rate of noradrenaline was enhanced in A1. 3. Rats from dams subjected to hypoxia during pregnancy hyperventilated until 3 weeks after birth. In these rats, the biphasic hypoxic ventilatory response was absent at 1 week and the increase in minute ventilation was amplified at 3 weeks. 4. Prenatal hypoxia disturbed the metabolism of offspring until 3 weeks after birth. A weak or absent hypometabolism in response to hypoxia was observed in these rats in contrast to control animals. 5. Prenatal hypoxia impairs the postnatal development of the chemoafferent pathway, as well as the ventilatory and metabolic responses to hypoxia. These alterations were mostly evident until 3 weeks after birth.

Modulation of respiratory frequency by peptidergic input to rhythmogenic neurons in the preBötzinger complex

Neurokinin-1 receptor (NK1R) and mu-opioid receptor (muOR) agonists affected respiratory rhythm when injected directly into the preBötzinger Complex (preBötC), the hypothesized site for respiratory rhythmogenesis in mammals. These effects were mediated by actions on preBötC rhythmogenic neurons. The distribution of NK1R+ neurons anatomically defined the preBötC. Type 1 neurons in the preBötC, which have rhythmogenic properties, expressed both NK1Rs and muORs, whereas type 2 neurons expressed only NK1Rs. These findings suggest that the preBötC is a definable anatomic structure with unique physiological function and that a subpopulation of neurons expressing both NK1Rs and muORs generate respiratory rhythm and modulate respiratory frequency.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Phrenic nerve responses to chemical stimulation of the subregions of ventral medullary respiratory neuronal group in the rat

Phrenic nerve (PN) responses to unilateral microinjections of L-glutamate (L-Glu, 5 mM) or N-methyl-D-aspartic acid (NMDA, 1 mM) into different subregions of ventral respiratory neuronal group (VRG) were studied in urethane-anesthetized, immobilized, and artificially ventilated, adult male Wistar rats. A 50-nl volume of microinjection was used in all the subregions of VRG except in Pre-Bötzinger complex (Pre-BötC) where a 20-nl volume was used. Unilateral microinjections of L-Glu or NMDA into the Bötzinger complex (BötC) and caudal VRG (cVRG), caused a transient cessation of phrenic nerve (PN) activity. Expiratory neurons, abundant in BötC and cVRG, were excited by stimulation of cardiopulmonary receptors while their responses to carotid chemoreceptor stimulation were variable. Microinjections of L-Glu or NMDA into the Pre-BötC caused an increase in the PN background discharge (this response was unique to Pre-BötC) superimposed on which was an increase in the PN burst frequency. Microinjections of L-Glu or NMDA into the rostral VRG (rVRG) caused an increase in the frequency and amplitude of PN bursts. Inspiratory neurons, abundant in Pre-BötC and rVRG, were excited and inhibited by activation of carotid chemoreceptors and cardiopulmonary receptors, respectively. The coordinates for the location of different subregions of VRG were as follows (reference points are listed in parentheses). BötC: 1.6-2.6 mm rostral (calamus scriptorius), 1.7-2.7 mm lateral (midline), and 2.3-2.8 mm deep (dorsal surface of medulla); Pre-BötC: 1.4-1.6 mm rostral, 1. 8-2.5 mm lateral, and 2.3-2.8 mm deep; rVRG: 0.4-1.4 mm rostral, 1. 6-2.5 mm lateral, and 2.3-2.8 mm deep; and cVRG: 0.5 mm caudal to 0. 5 mm rostral, 1.0-2.2 mm lateral, and 2.1-2.6 mm deep. A detailed map of the subregions of VRG, functionally identified by L-Glu and NMDA-microinjections, has been presented. These data are likely to prove useful in future studies on respiratory reflex mechanisms.

Catecholaminergic modulation of respiratory rhythm in an in vitro turtle brain stem preparation

An in vitro brain stem preparation from adult turtles was used to determine effects of dopamine (DA) and norepinephrine (NE) on the pattern of respiratory motor output recorded from hypoglossal nerve roots (XII). Bath-applied DA (10-200 microM) increased the frequency of respiratory bursts (peaks) from 0.9 +/- 0.2 to 2.4 +/- 0.3 (SE) peaks/min, resulting in a 99 +/- 9% increase in neural minute activity. R[+]-SCH-23390 (10 microM, D1 antagonist) and eticlopride (20 microM, D2 antagonist) attenuated the DA-mediated increase in peak frequency by 52 and 59%, respectively. On the other hand, the DA-receptor agonists apomorphine (D1, D2), quinelorane (D2), and SKF-38393 (D1) had no effect on peak frequency. Prazosin, an alpha1-adrenergic antagonist (250 nM) abolished the DA-mediated frequency increase. Although NE (10-200 microM) and phenylephrine (10-200 microM, alpha1-adrenergic agonist) increased peak frequency from 0.5 +/- 0.1 to 1.2 +/- 0.3 peaks/min and from 0.6 +/- 0.1 to 1. 0 +/- 0.2 peaks/min, respectively, these effects were not as large as that with DA alone. The data suggest that both dopaminergic and adrenergic receptor activation in the brain stem increase respiratory frequency in turtles, but the DA receptor-mediated increase is dependent on coactivation of alpha1-adrenergic receptors.

Organization and transmitter specificity of medullary neurons activated by sustained hypertension: implications for understanding baroreceptor reflex circuitry

In situ expression of c-fos observed in response to phenylephrine (PE)-induced hypertension provided a basis for characterizing the organization and neurotransmitter specificity of neurons at nodal points of medullary baroreflex circuitry. Sustained hypertension induced by a moderate dose of PE provoked patterns of c-fos mRNA and protein expression that conformed in the nucleus of the solitary tract (NTS) to the termination patterns of primary baroreceptor afferents and in the caudal ventrolateral medulla (CVLM) to a physiologically defined depressor region. A majority of barosensitive CVLM neurons concurrently displayed markers for the GABAergic phenotype; few were glycinergic. Phenylephrine-sensitive GABAergic neurons that were retrogradely labeled after tracer deposits in pressor sites of the rostral ventrolateral medulla (RVLM) occupied a zone extending approximately 1.4 mm rostrally from the level of the calamus scriptorius, intermingled partly with catecholaminergic neurons of the A1 and C1 cell groups. By contrast, barosensitive neurons of the NTS were found to be phenotypically complex, with very few projecting directly to the RVLM. Extensive colocalization of PE-induced Fos-IR and markers for the nitric oxide phenotype were seen in a circumscribed, rostral, portion of the baroreceptor afferent zone of the NTS, whereas only a small proportion of PE-sensitive neurons in the NTS were found to be GABAergic. PE treatment parameters have been identified that provide a basis for defining and characterizing populations of neurons at the first station in the central processing of primary baroreceptor input and at a key inhibitory relay in the CVLM.

Distribution of N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptor subunits on respiratory motor and premotor neurons in the rat

Glutamate is required for the transmission of inspiratory drive in respiratory premotor and motor neurons. The glutamate receptors (GluRs) responsible for this essential function have yet to be anatomically characterized. We mapped the GluR subtypes expressed by respiratory premotor and motor neurons by using combined immunohistochemistry and retrograde labeling in adult rats. Phrenic motoneurons and bulbospinal ventral respiratory group (VRG) neurons were retrogradely labeled and immunolabeled with subunit-specific antibodies against the N-methyl-D-aspartate (NMDA) receptor subtype (NMDAR1) and the non-NMDA receptor subtypes, alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA; GluR1, GluR2/3, GluR4) and kainate (GluR5-7). Phrenic motoneurons and bulbospinal VRG neurons showed positive immunolabeling for all five GluR subunits. These results support the hypothesis that NMDA and non-NMDA receptor subtypes underlie the excitation of bulbospinal VRG neurons and phrenic motoneurons. Furthermore, immunolabeling for each receptor subtype demonstrated a unique distribution along the neuronal membrane. Immunoreactivity for AMPA receptor subunits was distributed throughout somata and proximal dendrites, NMDAR1 subunit immunolabeling was localized to somata, and GluR5-7 subunit immunolabeling was confined largely to dendrites. The differential distribution of AMPA, kainate, and NMDA receptors on the somal and dendritic surface of respiratory neurons suggests that the location of glutamatergic synapses along the neuronal surface is an important determinant of glutamate-mediated postsynaptic currents. Consequently, different patterns of glutamatergic excitation of respiratory neurons could be achieved by selective activation of different profiles of GluR subtypes on different portions of the neuronal membrane.

Expression of c-fos in the rat brainstem after exposure to hypoxia and to normoxic and hyperoxic hypercapnia

In this study, Fos immunohistochemistry was used to map brainstem neuronal pathways activated during hypercapnia and hypoxia. Conscious rats were exposed to six different gas mixtures: (a) air; (b) 8% CO2 in air; (c) 10% CO2 in air; (d) 15% CO2 in air; (e) 15% CO2 + 60% O2, balance N2; (f) 9% O2, balance N2. Double-staining was performed to show the presence of tyrosine hydroxylase. Hypercapnia, in a dose-dependent way caused Fos expression in the following areas: caudal nucleus tractus solitarius (NTS), with few labeled A2 noradrenergic neurons; noradrenergic A1 cells and noncatecholaminergic neurons in the caudal ventrolateral medulla; raphe magnus and gigantocellular nucleus pars alpha (GiA); many noncatecholaminergic (and relatively few C1) neurons in the lateral paragigantocellular nucleus (PGCl), and in the retrotrapezoid nucleus (RTN); locus coeruleus (LC), external lateral parabrachial and Kölliker-Fuse nuclei, and A5 noradrenergic neurons at pontine level; and in caudal mesencephalon, the ventrolateral column of the periaqueductal gray (vlPAG). In most of these nuclei, hypoxia also induced Fos expression, albeit generally less than after hypercapnia. However, hypoxia did not cause labeling in RTN, juxtafacial PGCl, GiA, LC, or vlPAG. After normoxic hypercapnia, more labeled cells were present in NTS and PGCl than after hyperoxic hypercapnia. Part of the observed labeling could be caused by stress- or cardiovascular-related sequelae of hypoxia and hypercapnia. Possible implications for the neural control of breathing are also discussed, particularly with regard to the finding that several nuclei, not belonging to the classical brainstem respiratory centres, contained labeled cells.

Cardiorespiratory effects of L-glutamate microinjected into the rat ventral medulla

We investigated the cardiorespiratory effects elicited by microinjections of L-glutamate (L-glu, 25 nmol, 200 nl) at various sites in the ventral medulla (VMS) of urethane-anesthetized rats. The results demonstrated that regions responsive to the drug are located along a column in the VMS extending from the VI cranial nerve to the first cervical nerve in the caudal medulla. Within this column three breathing patterns were elicited from four distinct areas. In the most rostral and caudal portion of this hypothetical column, the breathing patterns observed in response to L-glu were similar and characterized by increases in minute ventilation, tidal volume, inspiratory drive, respiratory frequency, mean arterial blood pressure (MAP) and heart rate (HR). In the regions located between the areas described above two different breathing patterns were obtained without significant changes in MAP or HR. These patterns were characterized by decreases and increases in the respiratory indices analyzed, with the exception of respiratory frequency, which decreased in both regions. These results suggest that within the VMS discrete areas may act as functional units modulating cardiorespiratory responses while in others these functions are spatially segregated.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.

Monosynaptic projections from the nucleus tractus solitarii to C1 adrenergic neurons in the rostral ventrolateral medulla: comparison with input from the caudal ventrolateral medulla

The rostral ventrolateral medulla (RVL) contains reticulospinal adrenergic (C1) neurons that are thought to be sympathoexcitatory and that form the medullary efferent limb of the baroreceptor reflex pathway. The RVL receives direct projections from two important autonomic regions, the caudal ventrolateral medulla (CVL) and the nucleus tractus solitarii with immunocytochemical identification of C1 adrenergic neurons in the RVL to compare the morphology of afferent input from these two autonomic regions into the RVL. NTS (n = 203) and CVL (n = 380) efferent terminals had similar morphology and vesicular content, but CVL efferent terminals were slightly larger than NTS efferent terminals. Overall, efferent terminals from either region were equally likely to contact adrenergic neurons in the RVL (21% for NTS, 25% for CVL). Although efferents from both regions formed both symmetric and asymmetric synapses, NTS efferent terminals were statistically more likely to form asymmetric synapses than CVL efferent terminals. CVL efferent terminals were more likely to contact adrenergic somata than were NTS efferents, which usually contacted dendrites. These findings 1) support the hypothesis that a portion of NTS efferents to the RVL may be involved in sympathoexcitatory, e.g., chemoreceptor, reflexes (via asymmetric synapses), whereas those from the CVL mediate sympathoinhibition (via symmetric synapses); and 2) provide an anatomical substrate for differential postsynaptic modulation of C1 neurons by projections from the NTS and CVL. With their more frequent somatic localization, CVL inhibitory inputs may be more influential than excitatory NTS inputs in determining the discharge of RVL neurons.

NMDA as well as non-NMDA receptors mediate the neurotransmission of inspiratory drive to phrenic motoneurons in the adult rat

The neurotransmission of bulbospinal respiratory drive is believed to involve primarily non-NMDA receptors located in the phrenic motonucleus (PMN). This conclusion is based on studies carried out mainly on in vitro brainstem-spinal cord preparations of the neonatal rat. The present study was undertaken to investigate the transmitter/receptor mechanisms in the PMN which are involved in the neurotransmission of inspiratory drive, using an in vivo adult rat model. Microinjections of glutamate, NMDA and AMPA into the PMN elicited an increase in the phrenic nerve (PN) background discharge. These injections did not alter significantly the frequency of spontaneously occurring PN bursts confirming that mechanisms responsible for respiratory rhythm reside in the supraspinal structures. Microinjections of an NMDA receptor blocker (AP-7), in concentrations that did not alter the responses to a non-NMDA receptor agonist (AMPA), reduced the PN amplitude significantly. Similarly, microinjections of a potent non-NMDA receptor blocker (NBQX), in concentrations that did not alter responses to NMDA, reduced the PN amplitude significantly. Sequential microinjections, within an interval of 5 min, of AP-7 and NBQX into the PMN, resulted in a dramatic reduction in the spontaneous PN bursts. The reduction of PN amplitude started immediately after the microinjection of AP-7 and NBQX, either alone or in combination, and reached a maximum within 5-10 min. These results indicate that, unlike in the neonatal rat, both NMDA and non-NMDA receptors located in the PMN play a significant role in the neurotransmission of the inspiratory drive in the adult rat.

Extensive projections from the midbrain periaqueductal gray to the caudal ventrolateral medulla: a retrograde and anterograde tracing study in the rat

We investigated the innervation of the caudal ventrolateral medulla by the midbrain periaqueductal gray in the rat using retrograde and anterograde tract-tracing. Iontophoretic injection of Fluoro-Gold or cholera toxin B subunit into the caudal ventrolateral medulla resulted in retrogradely labeled neurons in discrete regions of the periaqueductal gray. These labeled cells were observed throughout the rostrocaudal extent of the periaqueductal gray and were distributed (as percentage of total labeled cells) in its lateral (53-67%), ventrolateral (14-28%), ventromedial (7-16%) and dorsomedial aspects (7-10%). About 70-72% of labeled cells were found in the caudal half of the periaqueductal gray and 28-30% in the rostral half. In the ventromedial periaqueductal gray, more labeled cells were seen in the contralateral side (5-13%) than the ipsilateral side (2-3%), whereas for other periaqueductal gray areas labeling was preferentially ipsilateral. Phaseolus vulgaris leucoagglutinin anterograde tracing was used to confirm the retrograde labeling results. Following iontophoretic injection into the periaqueductal gray, labeled fibers and terminals were observed throughout the rostrocaudal extent of the caudal ventrolateral medulla. Injections in the lateral and/or ventrolateral aspect of the periaqueductal gray yielded more anterograde labeling in the ipsilateral than the contralateral caudal ventrolateral medulla, while injections in the ventromedial aspect of the periaqueductal gray produced labeling preferentially in the contralateral caudal ventrolateral medulla. The present study indicates that specific regions of the periaqueductal gray project to the caudal ventrolateral medulla and may regulate cardiovascular and respiratory functions through these connections.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

The efferent projections of the periaqueductal gray in the rat: a Phaseolus vulgaris-leucoagglutinin study. II. Descending projections

The descending projections of the periaqueductal gray (PAG) have been studied in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The tracer was injected into the dorsolateral or ventrolateral subdivisions of the PAG at rostral or caudal sites. It was found that the patterns of the descending projections of the rostral and caudal parts of the dorsolateral PAG were the same and that the patterns of the descending projections of the rostral and caudal parts of the ventrolateral PAG were the same. However, the patterns of projections of the dorsolateral and ventrolateral PAG subregions were substantially different. These results suggest that the dorsolateral and ventrolateral parts of the PAG are organized into longitudinal columns that extend throughout the length of the PAG. The axons of PAG neurons descended through the pons and medulla via two routes. A small fiber bundle was present in the periaqueductal gray and in the periventricular area. This bundle distributed fibers and terminals locally within the periaqueductal gray and in the locus coeruleus and Barrington's nucleus. A larger bundle had a diffuse arrangement in the pontine reticular formation, however, and it had a more restricted distribution in the medulla, where it occupied a position dorsolateral to the pyramid. This bundle supplied structures in the pontine and medullary tegmentum. The dorsolateral column preferentially supplied the locus coeruleus, subcoeruleus, the gigantocellular nucleus pars alpha, the rostral part of the paragigantocellular nucleus, and the region of the A5 noradrenergic cell group. The ventrolateral column preferentially supplied the nucleus raphe magnus, the caudal part of the lateral paragigantocellular nucleus, and the rostroventrolateral reticular nucleus.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

A1 catecholamine cell group: fine structure and synaptic input from the nucleus of the solitary tract

Preembedding immunoperoxidase staining methods were used to characterize tyrosine hydroxylase-immunoreactive (TH-ir) elements in the caudal ventrolateral medulla, and to determine the extent to which neurons of the A1 cell group are directly innervated by projections of the nucleus of the solitary tract (NTS). TH-ir neurons in the A1 region were medium-sized and multipolar. They possessed rounded nuclei with infrequent invaginations, well-developed Golgi apparati, high cytoplasmic densities of mitochondria, and a low to moderate tendency for rough endoplasmic reticulum (RER) to align in parallel stacks. A1 cell bodies were commonly juxtaposed to TH-positive and TH-negative neurons, myelinated profiles, glia and/or vascular elements, but close membrane appositions were only seen with glial elements. Synaptic input to A1 neurons was predominantly asymmetric, provided virtually exclusively by non-TH-ir terminals, and directed principally to dendritic shafts; A1 somata are relatively sparsely innervated. In a second experiment, silver-intensified immunogold localization of TH-ir was combined with immunoperoxidase labeling for anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L), following tracer injections in the caudal aspect of the medial division of the NTS. These experiments revealed a small proportion of PHA-L-labeled axon terminals that made asymmetric contacts with dendritic shafts of TH-ir neurons. These results suggest that the fine structure and synaptic input of A1 neurons are somewhat distinct from that of rostrally situated C1 catecholamine cells. In addition, while they document a direct NTS-A1 projection that may participate in the interoceptive control of vasopressin secretion, the bulk of ventrolaterally directed projections from the caudomedial NTS contact noncatecholaminergic elements in the A1 region, some of which may correspond to so-called depressor neurons implicated in the baroreflex control of sympathetic outflow and vasopressin secretion.

Brainstem network controlling descending drive to phrenic motoneurons in rat

Contraction of the diaphragm is controlled by phrenic motoneurons that receive input from sources that are not fully established. Bulbospinal (second-order) neurons projecting to phrenic motoneurons and propriobulbar (third-order) neurons projecting to these bulbspinal neurons were investigated in rat by transsynaptic transport of the neuroinvasive pseudorabies virus. Bulbospinal neurons were located predominantly in the medullary lateral tegmental field in two functionally described regions, the ventral respiratory group and Bötzinger complex. An intervening region, the pre-Bötzinger complex, contained essentially no phrenic premotoneurons. Bulbospinal neurons were also located in ventral, interstitial, and ventrolateral subnuclei of the solitary tract, and gigantocellular, Kölliker-Fuse, parabrachial, and medullary raphe nuclei. A monosynaptic pathway to phrenic motoneurons from the nucleus of the solitary tract was confirmed; monosynaptic pathways from upper cervical spinal cord, spinal trigeminal nucleus, medical and lateral vestibular nuclei, and medial pontine tegmentum were not verified. Locations of third-order neurons were consistent with described projections to the ventral respiratory group, from contralateral ventral respiratory group, Bötzinger complex, A5 noradrenergic cell group, and the following nuclei; solitary, raphe, Kölliker-Fuse, parabrachial, retrotrapezoid, and paragigantocellular. Novel findings included a projection from locus coeruleus to respiratory premotoneurons and the lack of previously described pathways from area postrema and spinal trigeminal nucleus. These second- and third-order neurons from the output network for diphragm motor control which includes numerous behaviors (e.g., respiration, phonation, defecation). Of the premotoneurons, the rostral ventral respiratory group is the primary population controlling phrenic motoneurons.

Close appositions between tyrosine hydroxylase immunoreactive boutons and respiratory neurons in the rat ventrolateral medulla

The extent of the adrenergic input to respiratory neurons in the ventrolateral medulla oblongata of rats was assessed by using a combination of intracellular recording, dye filling, and immunohistochemistry. Twenty-two neurons that displayed a pronounced respiration-related modulation of their membrane potential, and could not be antidromically activated by electrical stimulation of the superior laryngeal, vagus, or facial nerves, were labelled by intracellular injection of biocytin. Three types of respiration-related neurons were labelled: small neurons located in the Bötzinger complex between 0.5 and 1.0 mm caudal to the facial nucleus; medium-sized neurons located in the ventral respiratory group 1.0 to 2.0 mm caudal to the facial nucleus; and large motoneurons located within the nucleus ambiguus 0.5 to 2.0 mm caudal to the facial nucleus. Small Bötzinger neurons [length = 22 +/- 5 microns, width = 13 +/- 3 microns, area = 222 +/- 79 microns2; (mean +/- SD, n = 5)] had membrane potentials of -15 to -27 mV during the recording period. Four of five of these cells had profuse axonal terminations between 50 microns caudal and 450 microns rostral to their somata, suggesting that they may form part of local networks responsible for generating respiratory activity. Medium-sized ventral respiratory group neurons (length = 26 +/- 5 microns, width = 18 +/- 4 microns, area = 377 +/- 141 microns2; n = 5) were found in the vicinity of the nucleus ambiguus dorsal to the lateral reticular nucleus. Three of five of these neurons had an axon that crossed the midline and travelled caudally. One axon had a collateral with varicosities close to its soma. The somata of motoneurons (length = 29 +/- 6 microns, width = 21 +/- 4 microns, area = 485 +/- 142 microns2; n = 12) were located within the nucleus ambiguus, and had axons that could be traced to exist points from the medulla. Tyrosine hydroxylase immunoreactive cells and their terminal fibres within the medulla were localised by immunocytochemistry. Small Bötzinger neurons received the largest number of close appositions from tyrosine hydroxylase immunoreactive boutons (13 +/- 2 appositions/neuron; n = 5). Medium-sized ventral respiratory group neurons received fewer appositions (8 +/- 4 appositions/neuron; n = 5). Most motoneurons (n = 10) received few appositions from tyrosine hydroxylase immunoreactive boutons, while two received none. The average number was 3 +/- 3 appositions/neuron (n = 12).(ABSTRACT TRUNCATED AT 400 WORDS)

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, FluoroGold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei.

Expiratory neurons of the Bötzinger Complex in the rat: a morphological study following intracellular labeling with biocytin

The term "Bötzinger Complex" (BOT) refers to a distinct group of neurons, located near the rostral portion of the nucleus ambiguus, which are known to play an important role in the control of respiratory movements. Previous studies conducted in cats have demonstrated that most of these neurons are active during expiration, exerting a monosynaptic inhibitory action on several subpopulations of inspiratory neurons in the medulla and spinal cord. The aim of this study was to examine morphological properties and possible synaptic targets of BOT neurons in the rat. Forty-one expiratory neurons were labeled intracellularly with biocytin; 12 were interneurons (BOT neurons) and 29 were motoneurons. The latter could not be antidromically activated following stimulation of the superior laryngeal or vagal nerves. BOT neurons showed extensive axonal arborisations in the ipsilateral medulla, with some projections to the contralateral side. Bouton-like axon varicosities mainly clustered in two areas: near the parent cell bodies, and in the area corresponding to the rostral part of the ventral respiratory group (VRG). In five pairs of labeled neurons, each consisting of one BOT neuron and one inspiratory neuron in the rostral VRG, no appositions were identified at the light microscopic level between axons of BOT neurons and dendrites or cell bodies of inspiratory neurons. These results demonstrate that some features of BOT expiratory neurons in the rat are similar to those previously described in cats. The differences include their more ventral location in relation to the compact formation of nucleus ambiguus (retrofacial nucleus), and the relative paucity in the rat of neurons displaying an augmenting pattern of activity and of neurons with spinally projecting axons. In addition, we were unable to find morphological evidence for contacts between labeled BOT neurons and ipsilateral inspiratory neurons near the obex level, a finding not consistent with previous electrophysiological studies in the cat in which such synaptic connections have been identified.

CNS innervation of airway-related parasympathetic preganglionic neurons: a transneuronal labeling study using pseudorabies virus

The CNS cell groups that innervate the tracheal parasympathetic preganglionic neurons were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the tracheal wall of C8 spinal rats and after 4 days survival, brain tissue sections from these animals were processed for immunohistochemical detection of PRV. Retrogradely labeled parasympathetic preganglionic neurons were seen in three sites in the medulla: the compact portion of the nucleus ambiguus, the area ventral to the nucleus ambiguus, and the rostralmost portion of the medial nucleus tractus solitarius (NTS); this labeling pattern correlated well with the retrograde cell body labeling seen following cholera toxin beta-subunit injections in the tracheal wall. PRV transneuronally labeled neurons were found throughout the CNS with the most abundant labeling concentrated in the ventral medulla oblongata. Labeled neurons were identified along the ventral medullary surface, and in nearby areas including the parapyramidal, retrotrapezoid, gigantocellular and lateral paragigantocellular reticular nuclei as well as the caudal raphe nuclei (raphe pallidus, obscurus, and magnus). Serotonin (5-HT) neurons of the caudal raphe complex (B1-B3 cell groups) and ventromedial medulla were labeled as well as a few C1 adrenergic neurons. The A5 cell group was the major noradrenergic area labeled although a small number of locus coeruleus neurons were also labeled. Several NTS regions contained labeled cells including the commissural, intermediate, medial, central, ventral, and ventrolateral subnuclei. PRV infected neurons were present in the Kölliker-Fuse and Barrington's nuclei. In the rostral mesencephalon, the precommissural nucleus of the dorsal periventricular gray matter was labeled. Labeling was present in the dorsal, lateral and paraventricular hypothalamic nuclei. In summary, the airway parasympathetic preganglionic neurons are innervated predominantly by a network of lower brainstem neurons that lie in the same regions known to be involved in respiratory and cardiovascular regulation. These findings are discussed in relationship to some of the potential CNS mechanisms that may be operative in airway disorders as well as potentially involved in certain fatal respiratory conditions such as Ondine's curse and sudden infant death syndrome (SIDS).