An intracellular study of respiratory neurons in the rostral ventrolateral medulla of the rat and their relationship to catecholamine-containing neurons

Detection of amino acid and peptide transmitters in physiologically identified brainstem cardiorespiratory neurons

Most of the CNS neurons that regulate circulation and respiration reside in regions of the brain characterized by extreme cellular heterogeneity (nucleus of the solitary tract, reticular formation, parabrachial nuclei, periaqueductal gray matter, hypothalamus, etc.). The chemical neuroanatomy of these regions is correspondingly complex and teasing out specific circuits in their midst remains a problem that is usually very difficult if not impossible to solve by conventional tract-tracing methods, Fos methodology or electrophysiology in slices. In addition, identifying the type of amino acid or peptide transmitter used by electrophysiologically recorded neurons has been until recently an especially difficult task either for lack of a specific marker or because such markers (many peptides for example) are exported to synaptic terminals and thus undetectable in neuronal cell bodies. In this review, we describe a general purpose method that solves many of these problems. The approach combines juxtacellular labeling in vivo with the histological identification of mRNAs that provide definitive neurochemical phenotypic identification (e.g. vesicular glutamate transporter 1 or 2, glutamic acid decarboxylase). The results obtained with this method are discussed in the general context of amino acid transmission in brainstem cardiorespiratory pathways. The presence of markers of amino acid transmission in specific aminergic pre-sympathetic neurons is especially emphasized as is the extensive co-localization of markers of GABAergic and glycinergic transmission in the brainstem reticular formation.

Mu opioid receptors in rat ventral medulla: effects of endomorphin-1 on phrenic nerve activity

Anatomical and in vitro studies suggest that mu opioid receptors (MOR) on pre-Bötzinger complex neurons are responsible for opioid induced respiratory depression (Grey et al., Science 286 (1999) 1566). However, mu opioid agonists injected in vivo, in other regions of the ventral respiratory group (VRG), produce respiratory depression, suggesting that opioids are widely distributed in the VRG. We therefore re-examined the distribution of the MOR in the ventral medulla and found MOR-immunoreactive neurons and terminals in all subdivisions of the VRG. Furthermore, we determined, in rats, the effects of a MOR agonist (endomorphin-1, 10 mM, 60 nl, unilateral), microinjected into different subdivisions of the VRG, on phrenic nerve activity. Endomorphin-1 produced changes in phrenic nerve frequency and amplitude, throughout the VRG. Unexpectedly, endomorphin-1 microinjected into the Bötzinger and pre-Bötzinger complexes consistently increased phrenic nerve frequency. These results support the widespread distribution of MOR in the VRG and also indicate that endomorphin-1, a postulated endogenous ligand, may differentially regulate respiration.

Presynaptic Δ opioid receptors differentially modulate rhythm and pattern generation in the ventral respiratory group of the rat

The specific role of the Delta opioid receptor (DOR), in opioid-induced respiratory depression in the ventral respiratory group (VRG) is largely unknown. Here, we sought to determine (1) the relationship between DOR-immunoreactive (ir) boutons, bulbospinal and functionally identified respiratory neurons in the VRG and (2) the effects of microinjection of the selective DOR agonist, D-Pen 2,5-enkephalin (DPDPE), into different subdivisions of the VRG, on phrenic nerve discharge and mean arterial pressure. Following injections of retrograde tracer into the spinal cord or intracellular labelling of respiratory neurons, in Sprague-Dawley rats, brainstem sections were processed for retrograde or intracellular labelling and DOR-ir. Bulbospinal neurons were apposed by DOR-ir boutons regardless of whether they projected to single (cervical or thoracic ventral horn) or multiple (cervical and thoracic ventral horn) targets in the spinal cord. In the VRG, a total of 24 +/- 5% (67 +/- 13/223 +/- 49) of neurons projecting to the cervical ventral horn, and 37 +/- 3% (96 +/- 22/255 +/- 37) of neurons projecting to the thoracic ventral horn, received close appositions from DOR-ir boutons. Furthermore, DOR-ir boutons closely apposed six of seven intracellularly labelled neurons, whilst the remaining neuron itself possessed boutons that were DOR-ir. DPDPE was microinjected (10 mM, 60 nl, unilateral) into regions of respiratory field activity in the VRG of anaesthetised, vagotomised rats, and the effects on phrenic nerve discharge and mean arterial pressure were recorded. DPDPE depressed phrenic nerve amplitude, with little effect on phrenic nerve frequency in the Bötzinger complex, pre-Bötzinger complex and rVRG, the greatest effects occurring in the Bötzinger complex. The results indicate that the DOR is located on afferent inputs to respiratory neurons in the VRG. Activation of the DOR in the VRG is likely to inhibit the release of neurotransmitters from afferent inputs that modulate the pattern of activity of VRG neurons.

Cannabinoid receptor activation in the rostral ventrolateral medulla oblongata evokes cardiorespiratory effects in anaesthetised rats

1. The nature of the cardiorespiratory effects mediated by cannabinoids in the hindbrain is poorly understood. In the present study we investigated whether cannabinoid receptor activation in the rostral ventrolateral medulla oblongata (RVLM) affects cardiovascular and/or respiratory function. 2. Initially, we looked for evidence of CB1 receptor gene expression in rostral and caudal sections of the rat ventrolateral medulla (VLM) using reverse transcription-polymerase chain reaction. Second, the potent cannabinoid receptor agonists WIN55,212-2 (0.05, 0.5 or 5 pmol per 50 nl) and HU-210 (0.5 pmol per 50 nl) or the CB1 receptor antagonist/inverse agonist AM281 (1 pmol per 100 nl) were microinjected into the RVLM of urethane-anaesthetised, immobilised and mechanically ventilated male Sprague-Dawley rats (n=22). Changes in splanchnic nerve activity (sSNA), phrenic nerve activity (PNA), mean arterial pressure (MAP) and heart rate (HR) in response to cannabinoid administration were recorded. 3. The CB1 receptor gene was expressed throughout the VLM. Unilateral microinjection of WIN55,212-2 into the RVLM evoked short-latency, dose-dependent increases in sSNA (0.5 pmol; 175+/-8%, n=5) and MAP (0.5 pmol; 26+/-3%, n=8) and abolished PNA (0.5 pmol; duration of apnoea: 5.4+/-0.4 s, n=8), with little change in HR (P<0.005). HU-210, structurally related to Delta9-tetrahydrocannabinol (THC), evoked similar effects when microinjected into the RVLM (n=4). Surprisingly, prior microinjection of AM281 produced agonist-like effects, as well as significantly attenuated the response to subsequent injection of WIN55,212-2 (0.5 pmol, n=4). 4. The present study reveals CB1 receptor gene expression in the rat VLM and demonstrates sympathoexcitation, hypertension and respiratory inhibition in response to RVLM-administered cannabinoids. These findings suggest a novel link between CB1 receptors in this region of the hindbrain and the central cardiorespiratory effects of cannabinoids. The extent to which these central effects contribute to the cardiovascular and respiratory outcomes of cannabis use remains to be investigated.

Substance P inputs to laryngeal motoneurons in the rat

Substance P terminals have previously been demonstrated around retrogradely labelled posterior cricoarytenoid (PCA) motoneurons, but little is known regarding substance P inputs to other functionally identified laryngeal motoneurons. In the present study, we determined the number and distribution of close appositions between substance P immunoreactive boutons and three types of laryngeal motoneuron by using a combination of intracellular recording, dye-filling and immunocytochemistry in the rat. Cricothyroid (CT) motoneurons received 15+/-5 substance P appositions/neuron (mean+/-S.D., n = 6), PCA motoneurons received 13+/-5 (n = 6), and laryngeal constrictor (LCS) motoneurons received 11+/-4 (n = 5). In contrast to our previous finding of a preferential serotonin innervation of CT motoneurons, we found no significant difference between the substance P inputs to CT, PCA and LCS motoneurons. Our results indicate a modest role for substance P in control of laryngeal motoneuronal function.

Serotonin inputs to inspiratory laryngeal motoneurons in the rat

Serotonergic neurons are distributed widely throughout the central nervous system and exert a tonic influence on a range of activities in relation to the sleep-wake cycle. Previous morphologic and functional studies have indicated a role for serotonin in control of laryngeal motoneurons. In the present study, we used a combination of intracellular recording, dye-filling, and immunocytochemistry in rats to demonstrate close appositions between serotonin immunoreactive boutons and posterior cricoarytenoid (PCA) and cricothyroid (CT) motoneurons, both of which are located in the nucleus ambiguus and exhibit phasic inspiratory activity. PCA motoneurons received 29 +/- 5 close appositions/neuron (mean +/- SD, n = 6), with the close appositions distributed more frequently on the distal dendrites, less frequently on the proximal dendrites, and sparsely on the axons and somata. CT motoneurons received 56 +/- 15 (n = 6), with close appositions found on both the somata and dendrites, especially proximal dendrites. Close appositions on the axons were only seen on one CT motoneuron. These results demonstrate a significant serotonin input to inspiratory laryngeal motoneurons, which is more prominent on CT compared with PCA motoneurons, and may reflect the different functional role of the muscles that they innervate during the sleep-wake cycle.

Cytoarchitecture of central chemoreceptors in the mammalian ventral medulla

We reviewed the previous reports on the fine anatomy of the mammalian ventral medulla with special attention to the cytoarchitecture of the superficial chemosensitive regions to summarize what is known, what is not yet known, and what should be studied in the future. We also reviewed studies on anatomical relationship between neurons and vessels, and morphological studies on dendrites of respiratory or chemosensitive neurons. When we compared the morphological reports on the ventral and dorsal putative chemosensitive regions, similarities were found as follows. Chemosensitive cells were often found not only near the ventral surface but near the dorsal surface of the brainstem. Dendritic projection towards the surface was a common characteristic in the ventral and dorsal chemosensitive neurons. Morphological abnormality in the brainstem of sudden infant death syndrome victims was also summarized. On the basis of the previous reports we discussed the perspective on the future study on central chemoreception. Among various unanswered questions in central chemosensitivity studies, physiological significance of surface cells and surface extending dendrites is the most important topic, and must be thoroughly investigated.

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.

Firing patterns of pre-Bötzinger and Bötzinger neurons during hypocapnia in the adult rat

Controversy exists about how a coordinated respiratory rhythm is generated in the brainstem. Some authors suggest that neurons in the pre-Bötzinger complex are key to initiation of all types of breathing. While, on the other hand, it has been reported that some pre-Bötzinger neurons fail to maintain a rhythmic discharge in phase with phrenic nerve discharge during mechanical hyperventilation. Extracellular recordings were made from respiratory units in the pre-Bötzinger and Bötzinger complexes of 13 anaesthetised, paralysed and vagotomised rats. Central respiratory activity was monitored from the C5 phrenic nerve. During mechanical hyperventilation, several changes were observed in the phrenic neurogram. Firstly, the frequency and amplitude of integrated phrenic nerve discharge were reduced and reversibly stopped. Secondly, the patterned discharges changed from an augmenting to a variety of non-augmenting patterns in 53 of 60 cases. In some cases (n=9) we observed that the pattern appeared to have two components, an early short duration discharge followed by a longer duration discharge. Respiratory units also started to show different firing patterns during mechanical hyperventilation. In general, they were divided into those units that fired tonically (n=28) and units that became silent (n=32), before phrenic nerve discharge ceased coincidently with complete apnoea. Of particular interest were those expiratory-inspiratory units in the pre-Bötzinger complex (n=8) that narrowed their firing period towards late expiration and early inspiration during mechanical hyperventilation. Given their firing features, it is possible that these expiratory-inspiratory units may participate in generation of the early inspiratory component of phrenic nerve discharge.

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

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

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.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

Serum-free culture of rat post-natal and fetal brainstem neurons

Serum-free medium is essential for cell culture studies in which complete control of the environment is required. Primary culture of post-natal brainstem neurons in defined medium has not been described in the literature, and successful culture of primary brainstem neurons is typically restricted to embryonic ages E14-E18. This study describes a method for culture of fetal and post-natal brainstem neurons using a serum-free culture medium. The culture system is based on Neurobasal medium supplemented with antioxidant-rich B27. Media and supplements are commercially available products from Life Technologies. Neuron survival was optimized by replacing glutamine with GlutaMaxI, by matching osmolality with neuronal age, and by using Hibernate medium to increase neuron survival during tissue dissociation. Fetal E14, E16, E20, and post-natal P3 and P6 cultures were examined after 4, 7, and 9 days in culture. Neuron and glial cells present in the cultures were identified using immunocytochemistry with antibodies raised against microtubule-associated protein 2 (MAP2) and glial fibrillary acidic protein (GFAP), respectively. Fetal E14 cultures had more bipolar neurons than multipolar neurons compared with developmentally older P6 cultures. Early fetal cultures had a higher percentage of neurons than late fetal and early post-natal cultures. Neuron survival was similar between 4 and 9 days in culture for all age groups tested. This is the first reliable, defined culture medium that supports brainstem neurons from late fetal and early post-natal stages of the rat for up to 6 days post-partum.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

Nonvagal tachypnea following alpha2-adrenoceptor stimulation in awake goats

To assess the influence of vagal afferent feedback in the development of respiratory instabilities induced by alpha2-adrenoceptor (alpha2-AR) stimulation in the goat, we examined the ventilatory effects of clonidine, an alpha2-AR agonist, in awake tracheostomized goats before and after bilateral mid-cervical vagotomy. Prior to vagal section, systemic administration of clonidine (0.5-3.0 microg kg(-1)) induced a highly dysrhythmic pattern of breathing in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns including prolonged and variable expiratory time (TE) intervals. Periods of apnea were commonly observed. Bilateral vagotomy resulted in a slower deeper breathing pattern and abolished the tachypnea evoked by intravenous administration of phenylbiguanide (PBG; 20-50 microg kg(-1)), a selective serotonin type 3 (5-HT3) receptor agonist. However, respiratory disturbances associated with alpha2-AR stimulation (including tachypnea) persisted after vagal section and were qualitatively and quantitatively similar to pre-vagotomy data demonstrating that vagal afferent feedback is not necessary for the development of respiratory disturbances induced by clonidine. The results suggest that respiratory dysrhythmias caused by alpha2-AR agonists in the goat are mediated by alpha2-ARs in the CNS.

Differential effects of clonidine on upper airway abductor and adductor muscle activity in awake goats

The purpose of this study was to determine the extent to which alpha(2)-adrenoceptor (alpha(2)-AR) pathways affect the central motor output to upper airway muscles that regulate airflow. Electromyogram (EMG) measurements were made from posterior cricoarytenoid (PCA), cricothyroid (CT), thyroarytenoid (TA), and middle (MPC) and inferior (IPC) pharyngeal constrictor muscles in awake standing goats. Systemic administration of the alpha(2)-AR agonist clonidine induced a highly dysrhythmic pattern of ventilation in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns, including prolonged and variable expiratory time intervals. Periods of apnea were commonly observed. Dysrhythmic ventilatory patterns induced by clonidine were associated with differential recruitment of upper airway muscles. alpha(2)-AR stimulation preferentially decreased the activity of the PCA, CT, and IPC muscles while increasing TA and MPC EMG activities. Clonidine-induced apneas were associated with continuous tonic activation of laryngeal (TA) and pharyngeal (MPC) adductors, leading to airway closure and arterial oxygen desaturation. Tonic activation of the TA and MPC muscles was interrupted only during the first inspiratory efforts after central apnea. Laryngeal abductor, diaphragm, and transversus abdominis EMG activities were completely silenced during apneic events. Ventilatory and EMG effects were reversed by selective alpha(2)-AR blockade with SKF-86466. The results demonstrate that alpha(2)-AR pathways are important modulators of central respiratory motor outputs to the upper airway muscles.

Substance P-immunoreactive boutons closely appose inspiratory protruder hypoglossal motoneurons in the cat

In anesthetized cats, we recorded intracellularly from 26 hypoglossal motoneurons which were antidromically activated following electrical stimulation of either the medial or lateral branches of the hypoglossal nerve. Twenty-one of these neurons were protruder motoneurons 6 of which had inspiratory activity. Three of the protruder motoneurons with inspiratory activity were filled with Neurobiotin and found to be closely apposed to substance P-like immunoreactive nerve terminals.

Evidence for glycinergic respiratory neurons: Bötzinger neurons express mRNA for glycinergic transporter 2

Bötzinger (BOTZ) neurons in the rostral ventrolateral medulla fire during the late expiratory phase of the respiratory cycle. These cells inhibit phrenic motor neurons and several types of respiratory neurons in the medulla oblongata. BOTZ cells produce a fast, chloride-mediated inhibition of their target neurons, but the neurotransmitter used by these cells has not been determined. In the present study, we examine whether gamma-aminobutyric acid (GABA) or glycine could be the inhibitory neurotransmitter of BOTZ cells. In chloralose-anesthetized rats, we individually filled 20 physiologically characterized BOTZ neurons with biotinamide by using a juxtacellular labeling method. Medullary sections containing the labeled BOTZ neurons were processed for in situ hybridization by using digoxigenin-labeled riboprobes for glutamic acid decarboxylase isoform 67 (GAD67), a marker for GABAergic neurons, or for glycine transporter 2 (GLYT2), a marker for glycinergic neurons. All BOTZ cells examined contained GLYT2 mRNA (n = 10), whereas none had detectable levels of GAD67 mRNA (n = 10). For a positive control, 12 GABAergic neurons in the substantia nigra pars reticulata also were recorded and filled with biotinamide in vivo. Most of these cells, as expected, had detectable levels of GAD67 mRNA (11 out of 12). These results demonstrate that the juxtacellular labeling method can be combined with in situ hybridization to identify physiologically characterized cells with probable GABAergic or glycinergic phenotypes. Furthermore, these data suggest that BOTZ neurons use the neurotransmitter glycine and not GABA to provide widespread inhibition of respiratory-related neurons.

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.

The pre-Bötzinger complex and phase-spanning neurons in the adult rat

To characterise respiratory neurons in the pre-Bötzinger complex of adult rats, extracellular recordings were made from 302 respiratory neurons in the ventral respiratory group of sodium pentobarbitone anaesthetised adult rats. Neurons were located 0 to 1.6 mm caudal to the facial nucleus, and ventral to the nucleus ambiguus. The pre-Bötzinger complex comprised expiratory neurons (22%, 22/100), inspiratory neurons (37%, 37/100) and phase-spanning neurons (41%, 41/100). In contrast, 80% (125/157) of Bötzinger neurons were expiratory, and 80% (36/45) of rostral ventral respiratory group neurons were inspiratory. Rostrocaudally, the pre-Bötzinger complex extended about 400 microns, starting at the caudal pole of the nucleus ambiguus compact formation. The pre-Bötzinger complex was also characterised by a predominance of propriobulbar neurons (81%, 13/16). Furthermore, 68% (33/48) of expiratory-inspiratory neurons found were located within the pre-Bötzinger complex. The variety of neuronal subtypes in the pre-Bötzinger complex, including many firing during the expiratory-inspiratory transition is consistent with the hypothesis that this nucleus plays a key role in respiratory rhythm generation in the adult rat.

Lability of the pacemaker activity in the rat rostro-ventrolateral medulla: effects of noradrenaline

Cardiovascular neurons in the Rostro-Ventrolateral Medulla (RVLM) have been shown to display a regular action potential discharge activity in vitro which has been proposed to result either from pacemaker conductances or from the activity of an oscillating network. Using intracellularly recordings in vitro from regularly-discharging RVLM neurons, we observed in young adult rats (> 19 days) that the regular discharge activity in RVLM is labile, as many neurons were or spontaneously became quiescent during the recording. A regular discharge could be induced or restored in quiescent neurons by superfusing an external concentration of K+ ions ([K+]ext) of 6 mM. In order to elucidate how neurotransmitters might influence this discharge activity, we studied the effects of a catecholamine, noradrenaline (present in this brain region). Noradrenaline depolarized or increased the spiking frequency of regularly-discharging neurons. This excitatory effect was sensitive to prazosin and propranolol. In the presence of these two blockers, noradrenaline induced a hyperpolarization sensitive to idazoxan and mimicked by alpha 2-adrenergic agonists. These effects persisted in the presence of tetrodotoxin. In spontaneously active neurons, prazosin plus propranolol abolished the discharge activity. At hyperpolarized potentials, the adrenoceptor blockers reduced the baseline synaptic/oscillatory activity. Our results demonstrate that the regular discharge activity of RVLM neurons is labile and depends on external ionic conditions, such as [K+]ext. This discharge activity can be modulated by catecholamines, acting at the 3 subtypes of adrenoceptors which co-exist on the same neuron. We propose that an endogenous release of catecholamines may condition the discharge activity of RVLM neurons.

Ventral medullary chemoreceptor inputs to neurons within the nucleus ambiguus

1. The neural mechanisms by which neurons within the nucleus ambiguus respond to chemoreceptor stimuli applied to the ventral medullary surface (VMS) were investigated by determining the effect of low pH on the membrane potential and synaptic activity of these neurons in vitro. 2. Small reductions in pH evoked an indirect membrane depolarization and/or an increase in excitatory synaptic activity in the majority of neurons. A direct membrane hyperpolarization was observed in the remaining neurons. 3. Acetylcholine evoked a direct nicotinic receptor-mediated membrane depolarization in these neurons. In addition, 37% of neurons received muscarinic synaptic input that originated from neurons located near the VMS. 4. Low-pH artificial cerebrospinal fluid (aCSF) potentiated the cholinergic component of the excitatory post-synaptic potential (EPSP) evoked from near the VMS. Both this EPSP and the spontaneous EPSP evoked by low-pH aCSF could be blocked by atropine. 5. It is concluded that at least two different mechanisms exist to transmit chemoreceptive information from the VMS to neurons within the nucleus ambiguus. In the majority of neurons, an indirect excitatory response is observed that is due, in part, to activation of a polysynaptic cholinergic pathway originating near the VMS. In the remaining neurons, low pH evokes a hyperpolarization that is due to a direct action of the dendrites of the neurons themselves that project near to the VMS.

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.

Identification of C1 presympathetic neurons in rat rostral ventrolateral medulla by juxtacellular labeling in vivo

The rostral ventrolateral medulla (RVLM) contains barosensitive, bulbospinal neurons that provide the main supraspinal excitatory input to sympathetic vasomotor preganglionic neurons. However, the phenotype of the critical RVLM cells has not been conclusively determined. The goal of the current study was to identify the proportion of electrophysiologically defined, putative, presympathetic RVLM neurons that are C1 cells. We used a juxtacellular labeling technique to individually fill spontaneously active, barosensitive, bulbospinal RVLM neurons with biotinamide following electrophysiological characterization in chloralose-anesthetized rats. To determine whether these neurons could be classified as C1 cells, the biotinamide-labeled cells were processed for detection of tyrosine hydroxylase. The majority of barosensitive bulbospinal RVLM neurons were tyrosine hydroxylase immunoreactive (TH-ir; 28 of 39). All of the barosensitive bulbospinal RVLM neurons with axonal conduction velocities in the C fiber range (<1 m/second) were TH-ir (n = 16), whereas faster conducting cells (1 to 7 m/second) were either lightly TH-ir (n = 12) or not detectably TH-ir (n = 11). Adjacent respiratory-related RVLM units labeled with biotinamide were not detectably TH-ir (n = 10). To verify that TH-ir cells were indeed adrenergic, a subset of barosensitive bulbospinal cells labeled with biotinamide were examined for phenylethanolamine N-methyltransferase immunoreactivity (PNMT-ir). Three slowly conducting cells had detectable PNMT-ir, and two fast-conducting cells had no detectable PNMT-ir. These results indicate that the majority of bulbospinal RVLM neurons with putative sympathoexcitatory function are C1 cells.

Phosphate-activated glutaminase immunoreactivity in brainstem respiratory neurons

The aim of this study was to determine if immunoreactivity for phosphate activated glutaminase (PAG), an enzyme involved in the biosynthesis of glutamate and a putative marker for neurons that use glutamate as a neurotransmitter, is present within respiratory neurons in the ventrolateral medulla oblongata. Intracellular recordings were obtained from neurons in the ventrolateral medulla of adult anaesthetised Sprague-Dawley rats. Neurons with a respiratory-related modulation of their membrane potential were filled with Neurobiotin (Vector, CA). After histochemical processing, sections of brainstem were examined by fluorescence and light microscopy. Some PAG immunoreactivity was found in all of the four types of respiratory neurons examined. PAG immunoreactivity was graded as strong or weak. (1) Of six inspiratory neurons in the rostral ventral respiratory group five were strongly PAG immunoreactive and one was weakly PAG immunoreactive. (2) Of six expiratory neurons in the caudal ventral respiratory group five were strongly PAG immunoreactive while one was weak. (3) Seven motoneurons in the nucleus ambiguous were all strongly PAG immunoreactive. (4) Five neurons in the Bötzinger area were examined. Four were weakly PAG immunoreactive while one contained strong PAG immunoreactivity. These data demonstrate a heterogeneity of PAG immunoreactivity amongst brainstem respiratory neurons.

The morphology and connections of neurons in the gasping centre of adult rats

Neuronal activities in the intermediate reticular nucleus and adjacent lateral tegmental field are critical for the neurogenesis of the ventilatory pattern of gasping. We report herein the anatomical features of these neurons, their axonal projections and the location of neurons providing afferent inputs. These neuroanatomical evaluations were performed by iontophoretic injection of the tracer Neurobiotin into the region of the intermediate reticular nucleus of the rat. At the site of injection, neurons having soma of 30-50 microns were filled. Labelled axons and terminals were observed in ipsilateral regions which contain neurons having established functions in the control of ventilatory activity. These regions include the nucleus ambiguous and motor nuclei of the hypoglossal and facial nerves. In addition, axonal projections extended to the contralateral region of the intermediate reticular nucleus. From this contralateral region, retrograde tracing revealed projections to the site of injection. Similarly, many ipsilateral regions which received axonal terminals from the region of the intermediate reticular nucleus had reciprocal projections to this region. These anatomical results support the physiological observation that the neurogenesis of gasping involves a synchronized activation of diverse components of the brainstem ventilatory control system.

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.

Optical imaging of respiratory burst activity in newborn rat medullary block preparations

We report on the optical imaging of excitation propagation induced by electrical stimulation of the nucleus tractus solitarius (NTS) area and subsequent inspiratory burst activity in the ventrolateral medulla (VLM) of a medullary block preparation. A medullary block preparation with a thickness of 1.0-1.4 mm was made from brainstems isolated from 0- to 4-day-old rats and stained with a fluorescent voltage-sensitive dye, RH795. Neuronal responses in the VLM evoked by electrical stimulation were recorded as a fluorescence change using an optical recording apparatus with a 128 x 128 photodiode array and a maximum time resolution of 0.6 ms. Motoneuronal activity was simultaneously recorded at the contralateral hypoglossal nerve roots. Neuronal excitation evoked by stimulation of the NTS area propagated to the VLM through the intermediate reticular zone (IRt). In contrast, caudal VLM stimulation induced excitation which propagated to the rostral VLM without any detectable excitation propagation in the IRt toward the NTS area from the VLM. NTS stimulation also induced an inspiratory burst activity in the hypoglossal nerve root activity with a 150-200 ms delay. Fluorescence changes corresponding to the inspiratory burst activity were observed in the VLM which coincided with the area in which the localization of many respiratory neurons had been demonstrated in previous studies using whole-brainstem preparations. The present results show the feasibility of using optical recordings for the analysis of respiratory neuron activity as well as for analysis of the transmission pathway of afferent and/or efferent information in the medulla.

Thyrotropin-releasing hormone immunoreactive boutons form close appositions with medullary expiratory neurons in the rat

The aim of the present study was to assess the size of the input from TRH immunoreactive varicosities to medullary respiratory neurons in the Bötzinger complex and caudal ventral respiratory group. Neurobiotin was intracellularly injected into seven neurons in the Bötzinger complex, between 0.4 and 0.9 mm caudal to the facial nucleus. Five of the seven Bötzinger neurons had extensive local axonal projections, with bouton-like varicosities clustered predominantly between their somata and the nucleus ambiguus. Seven neurons in the caudal ventral respiratory group, located between 1.6 and 2.4 mm caudal to the facial nucleus, were also labelled. All but one caudal respiratory neurons had no, or very few, medullary collaterals. TRH immunoreactive fibres were seen in many medullary nuclei, including the ventral reticular formation. Bötzinger neurons were closely apposed by an average of 29 +/- 8 TRH immunoreactive boutons/neuron (mean +/- S.D., n = 7). In contrast, caudal ventral respiratory group neurons were apposed by only 5 +/- 3 TRH immunoreactive boutons/neuron (n = 7). Bötzinger neurons form many intramedullary and bulbospinal inhibitory connections with premotoneurons and motoneurons that are important in the timing, amplitude and shape, of respiratory activity. Our findings suggest a role for endogenous TRH-containing neurons in modulating the activity of inhibitory Bötzinger neurons and neurons in the caudal ventral respiratory group. The significance of the apparent difference in size of this input remains to be determined.

Chemosensitive medullary neurones in the brainstem--spinal cord preparation of the neonatal rat

1. Using the isolated medulla and spinal cord of the neonatal rat, the response to CO2-induced changes in superfusate pH was examined in whole cell and perforated patch recordings from ventral medullary neurones which were identified by injection of Lucifer Yellow. The respiratory response to changing the CO2 concentration (from 2 to 8%) consisted of an increase in phrenic burst frequency, which could be accompanied by an increase, decrease or no change in burst amplitude. 2. Five classes of neurone - inspiratory, post-inspiratory, expiratory, respiration-modulated and ionic - were distinguished on the basis of their membrane potential and discharge patterns. Almost all (112 of 123) responded rapidly to 8% CO2 with a sustained change in membrane potential. Depolarizing responses (3-18 mV) occurred in inspiratory, respiration-modulated and 45% of tonic neurones. Hyperpolarizing responses (2-19 mV) occurred in expiratory and post-inspiratory neurones. The remaining tonic neurones were inhibited or showed no response. 3. In representatives of each class of neurone, membrane potential responses to 8% CO2 were retained when tested in the presence of tetrodotoxin (n = 7), low (0.2 mM) Ca(2+)-high (5 mM) Mg2+ (n = 23) or Cd2+ (0.2 mM) (n = 3)-containing superfusate, implying that they are mediated by intrinsic membrane or cellular mechanisms. 4. Neurones were distributed between 1200 microns rostral and 400 microns caudal to obex, and their cell bodies were located between 50 and 700 microns below the ventral surface (n = 104). Almost all responsive neurones (n = 78) showed dendritic projections to within 50 microns of the surface. 6. These experiments indicate that significant numbers of ventral medullary neurones, including respiratory neurones, are intrinsically chemosensitive. The consistency with which these neurones show surface dendritic projections suggests that this sensitivity may arise in part at this level.

Recruitment order and dendritic morphology of rat phrenic motoneurons

The detailed morphology of rat phrenic motoneurons (PMs) was studied in 40 electrophysiologically identified cells with intracellular injection of Neurobiotin. In 15 cells, the dendritic trees were fully analyzed by using path-distance analysis, and total surface area and volume were estimated. Based on their relative onset times (ROT; i.e., the time of firing onset relative to the onset of whole phrenic activity), PMs were classified into three types; early recruited (type E; ROT < 10%), late recruited (type L; ROT > 12.5%), and quiescent (type Q; not recruited under normal conditions). Dendrites constituted 93.3% of the surface area of cells and 38.9% of the cell volumes. The number of primary dendrites (nPD) averaged 10.1, and the mean number of terminations was 38.8. The combined diameters of primary dendrites of PMs correlated well with the total dendritic surface area and the number of dendritic terminations. Comparisons among cell types revealed that type Q cells had greater dendritic surface areas and volumes than type E or type L cells. With path-distance analysis, this difference was found to be due to differences between the cell types in the numbers of their dendrites, their combined dendritic lengths, and the number of their branches. The differences between these data and those available for cat motoneurons are discussed. The input resistance of PMs correlated with their total surface area but did not correlate with their somal surface area, indicating that, in rat, PM input resistance is a function of the entire neuronal membrane rather than of the somal surface alone.

Characterizations and comparisons of eupnoea and gasping in neonatal rats

1. Our purpose was to characterize the ventilatory patterns of eupnoea and gasping in the neonatal rat. This study was precipitated by reports, using in vitro brainstem spinal cord preparations, that only a single pattern is present in neonatal rats. 2. In anaesthetized or decerebrate rat pups aged less than 13 days, eupnoea was characterized by a sudden onset of inspiratory activity and then a more gradual rise to peak levels. Following vagotomy, frequency fell and peak phrenic activity and tidal volume increased. The rate of rise of inspiratory activity also rose, but peak levels were still achieved during the latter half of inspiration. Vagal efferent activity exhibited bursts during both inspiration and the early expiration. This basic eupnoeic rhythm was not altered after sectioning of the carotid sinus nerves. 3. Upon exposure to hypoxia or anoxia, phrenic activity, tidal volume and frequency initially increased and then declined. In many animals, ventilatory activity then ceased, but later returned with a gasping pattern. 4. Gasping was characterized by a sudden onset of phrenic activity, which reached a peak intensity during the early portion of inspiration. The expiratory burst of vagal activity was eliminated. 5. Reductions of body temperature from 37 to 27 degrees C resulted in prolongations of inspiration and expiration and decreases of phrenic amplitude; phasic phrenic activity completely disappeared in some animals. Upon exposure to anoxia, gasping was observed, even in animals in which phrenic activity had disappeared in hyperoxia. 6. We conclude that, from the day of birth, rats can exhibit eupnoea and gasping patterns which are very similar to those of adult animals. 7. The rhythmic neural activities of the in vitro brainstem-spinal cord preparation, reported by others, differ markedly from eupnoea but are identical with gasping. We therefore conclude that this preparation is not suitable for investigation of the mechanisms that generate eupnoeic breathing.

Rhythm generation in organotypic medullary cultures of newborn rats

Organotypic transverse medullary slices (obex level) from six-day-old rats, cultured for two to four weeks in chemically defined medium contained rhythmically discharging neurones which were activated by CO2 and H+. The mechanisms underlying this rhythmicity and the spread of excitation and synaptic transmission within this organotypic tissue were examined by modifying the composition of the external solution. Our findings showed that (1) Exposure to tetrodotoxin (0.2 microM) or to high magnesium (6 mM) and low calcium (0.2 mM) concentrations abolished periodic activity. (2) Neither the blockade of GABAergic potentials with bicuculline methiodide (200 microM) and/or hydroxysaclofen (200 microM) nor the blockade of glycinergic potentials with strychnine hydrochloride (100 microM) abolished rhythmicity. (3) While atropine sulphate (5 microM) was ineffective in modulating periodic discharges nicotine (100 microM) - like CO2-shortened the intervals between the periodic events; hexamethonium (50-100 microM) reduced both periodic and aperiodic activity. (4) Exposure to the NMDA antagonist 2-aminophosphonovaleric acid (50 microM) suppressed periodic events only transiently. In the presence of 2-aminophosphonovaleric acid rhythmicity recovered. However, the AMPA-antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10-50 microM), abolished periodic activity reversibly within less than 5 min. When 6-cyano-7-nitroquinoxaline-2,3-dione and nicotine were administered simultaneously periodic events persisted for up to 10 min. These findings indicate that synaptic excitatory drive is a prerequisite for the generation of rhythmic discharges of medullary neurones in this preparation. This drive may activate voltage-dependent channels or it may facilitate endogenous cellular mechanisms which initiate oscillations of intracellular calcium concentration. To test the latter possibility (5) calcium antagonists were added to the bath saline. The organic calcium antagonists verapamil and flunarizine (50-100 microM each) and the inorganic calcium antagonists cobalt (2 mM) and magnesium (6 mM) suppressed periodic activity and abolished or weakened the chemosensitivity towards CO2/acidosis. (6) Dantrolene (10 microM). an inhibitor of intracellular calcium release decreased the periodicity, while thapsigargin (2 microM) which blocks endoplasmic Ca(2+)-ATPase, transiently accelerated the occurrence of periodic events. (7) Oscillations of intracellular free calcium concentrations in Fura-2 AM-loaded cells were weakened or abolished by cobalt (2 mM). The results of (5)-(7) indicate that transmembrane calcium fluxes as well as intracellular Ca(2+)-release and -clearance mechanisms are a prerequisite for intracellular free calcium oscillations which may be important in the generation of rhythmic discharges in medullary neurones.

Thyrotropin-releasing hormone inputs are preferentially directed towards respiratory motoneurons in rat nucleus ambiguus

In the present study, we assessed the extent of the thyrotropin-releasing hormone (TRH) input to motoneurons in the ambigual, facial, and hypoglossal nuclei of the rat using a combination of intracellular recording, dye filling, and immunohistochemistry. Twelve motoneurons in the rostral nucleus ambiguus were labelled by intracellular injection in vivo of Neurobiotin (Vector). Seven out of 12 ambigual motoneurons displayed rhythmic fluctuations of their membrane potential in phase with phrenic nerve discharge, whereas the other five had no modulations of any kind. Seven facial motoneurons and seven hypoglossal motoneurons were also filled with Neurobiotin. All three motor nuclei contained TRH-immunoreactive varicosities, with the largest numbers found in the nucleus ambiguus. Close appositions were seen between TRH-immunoreactive boutons and every labelled motoneuron. Respiratory-related motoneurons in the nucleus ambiguus received the largest number of TRH appositions with 74 +/- 38 appositions/neuron (mean +/- S.D.; n = 7). In contrast, nonrespiratory ambigual motoneurons received significantly fewer TRH appositions (11 +/- 5; n = 5; P < 0.05; Mann-Whitney U test). Facial motoneurons received about the same number of TRH appositions as nonrespiratory ambigual motoneurons, with 13 +/- 4 (n = 7). Hypoglossal motoneurons received the fewest appositions from TRH-containing boutons, with 8 +/- 2 (n = 7). There were no differences in the TRH inputs to respiratory and nonrespiratory motoneurons in the facial and hypoglossal nuclei. These results demonstrate that, among motoneurons in the medulla, respiratory motoneurons in the rostral nucleus ambiguus are preferentially innervated by the TRH-immunoreactive boutons.

A comparative study of pre-sympathetic and Bötzinger neurons in the rostral ventrolateral medulla (RVLM) of the rat

The aim of this study was to investigate the degree of functional and anatomical overlap between two major neuronal subpopulations in the rostral ventrolateral medulla: pre-sympathetic (sympathoexcitatory) neurons, and expiratory neurons of the Bötzinger complex. Extracellular recordings were made with dye-filled microelectrodes in pentobarbital anesthetized, paralyzed and artificially ventilated adult Wistar rats. Tests applied included stimulation of baroreceptor afferents, activation of peripheral chemoreceptors and lung stretch receptors, changes in central respiratory drive with hyper- or hypoventilation, nociceptive stimulation, and antidromic stimulation from the T2 segment of the spinal cord or medulla oblongata at obex level. The two groups of neurons showed different patterns of spontaneous activity and generally different responses to these stimuli. The recording positions showed some overlap, but the majority of Bötzinger neurons were dorsolateral to pre-sympathetic neurons. There was a large overlap between the location of pre-sympathetic neurons and the lateral part of the C1 adrenergic group, but only a small overlap between these adrenergic neurons and Bötzinger neurons. These results indicate that the anatomically adjacent pre-sympathetic and Bötzinger expiratory neurons form two functionally distinct neuronal subpopulations.

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.

Nucleus tractus solitarius efferent terminals synapse on neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla

The caudal ventrolateral medulla (CVL) contains neurons that are vasodepressor and are a critical component of the baroreceptor reflex pathway. While electrophysiological studies suggest that CVL neurons are intercalated in the baroreceptor pathway between the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVL), there is no direct evidence for this projection. Therefore, we identified CVL neurons that project to RVL by retrogradely labelling them with wheat germ agglutinin-apo-horseradish peroxidase conjugated to colloidal gold (WAHG) injected into the RVL. Retrogradely labelled neurons were seen in previously identified vasodepressor areas of the rostral CVL that are critical for the baroreceptor reflex. Double labelling for WAHG and tyrosine hydroxylase (TH) immunocytochemistry indicated that CVL neurons that project to the RVL (CVL --> RVL neurons) are distinct from the noradrenergic neurons of the A1 cell group. To establish the presence of a direct projection from the NTS to CVL --> RVL neurons, the retrograde tracer WAHG was pressure injected into the RVL and the anterograde tracer biocytin was iontophoresed into the NTS of anesthetized rats. After 4-6 h, anesthetized rats were perfused transcardially with 3.75% acrolein in 2% paraformaldehyde and sections through the CVL were processed for both markers. By light microscopy, numerous biocytin-labelled varicose processes overlapped neurons containing WAHG in the CVL. By electron microscopy, biocytin was found in myelinated and unmyelinated axons and in axon terminals (0.9 + 0.02 microns) that contained primarily small clear vesicles. These terminals formed predominantly asymmetric synapses on large (1.5-6.0 microns in diameter) dendrites within the CVL. Some of the post-synaptic perikarya and large dendrites contained WAHG associated with lysosomes and multivesicular bodies, indicating that they belong to neurons which project to the RVL. We conclude that CVL --> RVL neurons are (a) distinct from A1 noradrenergic cells; (b) receive direct synaptic contacts from NTS efferent terminals; (c) are potently and monosynaptically excited (asymmetric synapses) by NTS efferent terminals. These data support the hypothesis that CVL neurons are intercalated between the NTS and the RVL in the baroreceptor reflex pathway.

Good vibrations? Respiratory rhythms in the central control of blood pressure

1. Arterial blood pressure is maintained, and reflexly controlled, by the activity of neurons in the medulla and spinal cord. 2. Rhythmic, automatic, respiratory activity is generated by neurons in the ventral medulla and transmitted to premotoneurons and motoneurons in the medulla and spinal cord. 3. Sympathetic nerve activity often has a respiratory rhythmicity. 4. One site at which the interaction between respiratory and sympathetic neurons occurs is the ventrolateral medulla. 5. Different types of sympathetic neurons, such as muscle vasoconstrictor, sudomotor and pilo-erector, have different patterns of respiratory rhythmicity. 6. Inputs from medullary respiratory neurons to medullary sympathetic premotor neurons may be the mechanism that co-ordinates the activity of these two vital systems.

Morphology and dendritic domains of neurons in the lateral parabrachial nucleus of the rat

The present study provides a description of the dendritic morphology and the dendritic domains of neurons in the lateral parabrachial nucleus (PB) of the rat. The cells were intracellularly stained in vitro with Lucifer yellow. A subpopulation of these cells was characterized beforehand as neurons projecting to the amygdaloid complex by retrograde transport with rhodamine beads. With respect to their dendritic arborization, different types of "spatially" organized PB neurons were discriminated. One major cell type in the external lateral PB (PBel) is characterized by long, elongated dendritic trees that are preferentially oriented parallel to the superior cerebellar peduncle. The majority of their dendrites appears to respect subnuclear boundaries, yet their distal dendrites often exceed the limits of the PBel to encroach upon adjacent subnuclei located dorsally and ventrolaterally to the PBel. Another prominent cell type in the PBel has fairly small and locally restricted dendritic trees that are also elongated, running with their main axis from ventrolateral to dorsomedial. The dendrites of the majority of these neurons apparently stay within the confines of the PBel. A distinct group of neurons is found in the ventral portion of the PBel. The majority of their dendrites is mediolaterally oriented and not confined to the PBel subnucleus. In addition, we found a smaller number of neurons scattered within the lateral PB whose dendrites do not show a preferential orientation but travel across subnuclear boundaries into several different PB subnuclei. Our data show that the dendrites of a large proportion of neurons in the lateral PB either stay within the confines of a particular subnucleus or slightly extend across subnuclear limits. In any case, they appear to match with terminal territories of afferent axons and, thus, maintain the functional specificity of inputs by their relay through the PB. In contrast, PB neurons that extend their dendrites across subnuclear boundaries or known terminal territories are likely to receive inputs of different qualities from a variety of sources and therefore transmit a more general, integrated signal to the forebrain.

Studies of the respiratory center using isolated brainstem-spinal cord preparations

It has been ten years since a brainstem-spinal cord preparation isolated from a newborn rat was introduced for study of the mammalian respiratory center. Here, I briefly summarize first, these studies, which include the tissue condition of in vitro preparations, respiratory reflexes, pharmacology, rhythm generation, respiratory chemoreception, phrenic motoneurons, regulation from pons, and development of a respiratory center. In the latter half of this paper, I focus on the neural mechanisms of respiratory rhythm generation. A current hypothesis for the central pattern generator of respiration proposed by the author's group is that the respiratory rhythm generator, composed of pre-inspiratory neurons in the rostral ventrolateral medulla, produces the primary rhythm of respiration and triggers an inspiratory pattern generator composed of inspiratory neurons in the rostral and the caudal ventrolateral medulla.

Substance P and serotonergic inputs to sympathetic preganglionic neurons

Sympathetic preganglionic neurons are the final central links in the sympathetic pathways that control the heart and blood vessels. The neurotransmitters present in the supraspinal pathways that control the activity of sympathetic preganglionic neurons include amino acids, amines and peptides. In this paper we discuss evidence that suggests a role for serotonin and substance P in these pathways. Both of these neurotransmitters are present in bulbospinal neurons. Our results suggest that they have an important physiological role in the central regulation of blood pressure.