Central chemosensitivity: a respiratory drive

Translational approach to studying panic disorder in rats: hits and misses

Panic disorder (PD) patients are specifically sensitive to 5–7% carbon dioxide. Another startling feature of clinical panic is the counterintuitive lack of increments in ‘stress hormones’. PD is also more frequent in women and highly comorbid with childhood separation anxiety (CSA). On the other hand, increasing evidence suggests that panic is mediated at dorsal periaqueductal grey matter (DPAG). In line with prior studies showing that DPAG-evoked panic-like behaviours are attenuated by clinically-effective treatments with panicolytics, we show here that (i) the DPAG harbors a hypoxia-sensitive alarm system, which is activated by hypoxia and potentiated by hypercapnia, (ii) the DPAG suffocation alarm system is inhibited by clinically-effective treatments with panicolytics, (iii) DPAG stimulations do not increase stress hormones in the absence of physical exertion, (iv) DPAG-evoked panic-like behaviours are facilitated in neonatally-isolated adult rats, a model of CSA, and (v) DPAG-evoked responses are enhanced in the late diestrus of female rats. Data are consistent with the DPAG mediation of both respiratory and non-respiratory types of panic attacks.

Cardiorespiratory and neural consequences of rats brought past their aerobic dive limit

The mammalian diving response is a dramatic autonomic adjustment to underwater submersion affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is known to be modulated by the parasympathetic nervous system, arterial blood pressure is modulated via the sympathetic system, and still other circuits modulate the respiratory changes. In the present study, we investigate the submergence of rats brought past their aerobic dive limit, defined as the diving duration beyond which blood lactate concentration increases above resting levels. Hemodynamic measurements were made during underwater submergence with biotelemetric transmitters, and blood was drawn from cannulas previously implanted in the rats' carotid arteries. Such prolonged submersion induces radical changes in blood chemistry; mean arterial PCO(2) rose to 62.4 Torr, while mean arterial PO(2) and pH reached nadirs of 21.8 Torr and 7.18, respectively. Despite these radical changes in blood chemistry, the rats neither attempted to gasp nor breathe while underwater. Immunohistochemistry for Fos protein done on their brains revealed numerous Fos-positive profiles. Especially noteworthy were the large number of immunopositive profiles in loci where presumptive chemoreceptors are found. Despite the activation of these presumptive chemoreceptors, the rats did not attempt to breathe. Injections of biotinylated dextran amine were made into ventral parts of the medullary dorsal horn, where central fibers of the anterior ethmoidal nerve terminate. Labeled fibers coursed caudal, ventral, and medial from the injection to neurons on the ventral surface of the medulla, where numerous Fos-labeled profiles were seen in the rats brought past their aerobic dive limit. We propose that this projection inhibits the homeostatic chemoreceptor reflex, despite the gross activation of chemoreceptors.

Opioid mu-receptors in medullary raphe region affect the hypoxic ventilation in anesthetized rats

Opioids can attenuate the peripheral chemoreceptor-mediated hypoxic ventilatory response (HVR) by acting on central mu-type opioid receptors. Since the medullary raphe region (MRR) expresses abundant mu-receptors and participates in modulating HVR, we tested the role of mu-receptors within the caudal, medial, and rostral MRR (cMRR, mMRR, and rMRR) in modulating the HVR. We recorded cardiorespiratory activities and their responses to isocapnic hypoxia in anesthetized rats before and after local microinjection of DAMGO into the MRR, and intravenous administration of DAMGO (100 microg/kg) alone or coupled with a previous local injection of CTAP. Microinjecting DAMGO into the cMRR or mMRR but not the rMRR significantly attenuated the HVR. However, systemic DAMGO-induced HVR attenuation was not significantly affected by pretreating the cMRR and mMRR with CTAP. Our data suggest that cMRR and mMRR mu-receptors are capable of depressing the HVR, while their contribution to the attenuated HVR by systemic DAMGO is limited.

Primary sensory neurons in the central nervous system

Published data and our own results relating to exteroceptor and a variety of interceptor neurons in the brain and spinal cord, such as intraspinal Hesse ocelli and light-sensitive epiphyseal and ependymal neurons, are presented. Light-sensitive ganglion neurons in invertebrates are also described, along with intrinsic spinal cord bipolar sensory neurons within the spinal cord, primary chemo-and thermosensitive neurons, and sensory unipolar neurons associated with the three fine "central nerves" of Motavkin, which perforate the sheath of the spinal cord and ending with bush-like receptors close to vessels or near the ependyma of the central canal. Data on all known intracortical interoceptors in vertebrates are generalized into a single scheme. It is hypothesized that the brains of animals and humans have an intrinsic sensory innervation comparable with the innervation of other organs and containing local primary sensory neurons and their asynaptic dendrites, which can be divided into two groups: interceptor and exteroceptor.

Hypoxia recruits a respiratory-related excitatory pathway to brainstem premotor cardiac vagal neurons in animals exposed to prenatal nicotine

The most ubiquitous form of arrhythmia is respiratory sinus arrhythmia in which the heart beat slows during expiration and heart rate increases during inspiration. Whereas respiratory sinus arrhythmia benefits pulmonary gas exchange respiratory dysfunction presents a major challenge to the cardiorespiratory system. Hypoxia evokes a pronounced bradycardia mediated by increases in parasympathetic cardiac activity. It has been hypothesized that the fatal events in sudden infant death syndrome (SIDS) are exaggerated cardiorespiratory responses to hypoxia. This study tests whether premotor cardiac vagal neurons receive rhythmic respiratory-related excitatory synaptic inputs during normoxia and hypoxia, and if animals exposed to nicotine in the prenatal period have exaggerated responses to hypoxia. Premotor cardiac vagal neurons in the nucleus ambiguus were identified in rats by the presence of a fluorescent tracer in medullary slices that generate rhythmic inspiratory-related motor discharge. Respiratory activity was recorded from the hypoglossal nerve and excitatory synaptic events in cardiac vagal neurons were isolated using patch clamp techniques. Adult female rats were implanted with osmotic minipumps that delivered nicotine at a level approximately equivalent to those that occur in moderate to heavy smokers. During normal eupneic respiration, as well as during hypoxia, premotor cardiac vagal neurons from control animals did not receive any rhythmic respiratory-related excitatory inputs. However in animals exposed to nicotine throughout the prenatal period respiratory bursts during hypoxia dramatically increased the frequency of excitatory synaptic events in cardiac vagal neurons. In summary, in animals exposed to nicotine throughout the prenatal period, but not in unexposed animals, respiratory bursts that occur during hypoxia dramatically increase the frequency of excitatory synaptic events in cardiac vagal neurons. This study establishes a likely neurochemical mechanism for the heart rate responses to hypoxia and a link between prenatal nicotine exposure and exaggerated bradycardia responses during hypoxia that may contribute to sudden infant death syndrome.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

Effects of opioid dependence and tobacco use on ventilatory response to progressive hypercapnia

Respiratory depression is a serious medical risk of opioid use. Most opioid abusers also smoke cigarettes, perhaps further compromising breathing. Differences in ventilatory response to nonhypoxic hypercapnia were studied in healthy volunteers with limited substance use (LU), tobacco smokers (SM), and opioid-dependent, methadone-maintained smokers (OD). The last two groups had similar current cigarette use and all groups were similar in gender and body mass index. Because previous data suggest that SM are sensitive to hypoxia but not hypercapnia, it was predicted that only the OD group would exhibit decreased carbon dioxide (CO(2)) sensitivity. All subjects rebreathed CO(2) during three identical sessions (four trials per session). Fractional end-tidal (Fet) CO(2) levels during repeated 4-min exposures to progressive hypercapnia (6% to 10%) were similar across groups. Ventilatory response (breathing rate, tidal volume and minute volume) linearly increased with FetCO(2) concentration and did not differ significantly across sessions. Relative to the LU and SM groups (which did not significantly differ), the CO(2)-minute volume and CO(2)-breathing rate functions were significantly shifted rightward (decrease in intercept but not slope) for OD subjects. These data are consistent with the hypothesis that chronic opioid exposure and/or short-term methadone maintenance (but not tobacco or nicotine use) produces a specific decrease in CO(2) sensitivity, primarily through an inhibitory effect on respiratory frequency.

Vasomotor centre of medulla - morpho-functional and neurochemical organization

The analysis of ventrolateral medulla morpho-functional and neurochemical organization is the aim of this survey. The date on the system of activation and inhibition of the spinal cord vasomotor neurons is represented. In addition, we discuss the role of catecholamines, substance P, glutamate, gamma-aminobutiric acid as neuromediators in the regulation of circulation.

Highly H+-sensitive neurons in the caudal ventrolateral medulla of the rat

The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid-base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral-most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n = 51), were not responsive to direct stimulation with H+. Sixty-six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus-response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n = 12) showed an average value of 34.17 +/- 7.44 spikes s-1 (100 nC)-1 (mean +/- S.D.) for maximal slope and an EC50 of 126.76 +/- 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre-stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

Cardiovascular responses and neurotransmission in the ventrolateral medulla during skeletal muscle contraction following transient middle cerebral artery occlusion and reperfusion

We hypothesized that static skeletal muscle contraction-induced systemic cardiovascular responses, and central glutamate/GABA release in rostral (RVLM) and caudal ventrolateral medulla (CVLM), would be modulated by cerebral ischemia. In sham-operated rats, a 2-min tibial nerve stimulation induced static contraction of the triceps surae, evoked pressor responses, increased glutamate in both the RVLM and CVLM, decreased GABA in the CVLM, and increased GABA in the RVLM. In rats with a temporary 90-min left middle cerebral artery occlusion (MCAO) followed by 24 h reperfusion, pressor responses during muscle contractions were attenuated, as were glutamate within the left RVLM and left CVLM. Glutamate within the right RVLM and right CVLM were unaltered and similar to those in sham rats. In contrast, GABA increases during muscle contractions were enhanced in the left RVLM and CVLM but changes within the right CVLM and RVLM were similar to those in sham rats. These results indicate that unilateral ischemia increases ipsilateral GABA/glutamate ratios during muscle contraction in the RVLM. In contrast, opposite changes in ipsilateral glutamate and GABA release within the RVLM and CVLM were observed following a 90-min right-sided MCAO followed by 24 h reperfusion. However, cardiovascular responses during muscle contraction were depressed following such an ischemic brain injury. These data suggest that transient ischemic brain injury attenuates cardiovascular responses to static exercise via modulating neurotransmission within the ventrolateral medulla.

CO(2)-induced c-Fos expression in brainstem preprotachykinin mRNA containing neurons

Tachykinin peptides are found in brainstem regions involved in central chemoreception and they may play a modulatory role in ventilatory response to hypercapnia. We determined whether tachykinin peptide containing neurons are activated by CO(2) by combining in situ hybridization and immunohistochemistry (IHH). Experiments were performed in 21-day-old rats exposed to 12% CO(2) for 1 h. c-Fos expression was identified by IHH on free floating sections (40 microm) that were mounted and then hybridized with anti-sense 35S labeled ribonucleotide probe of the rat preprotachykinin A (PPT-A) gene. Sections were analyzed for expression of the PPT-A gene, c-Fos protein and colocalization of PPT-A gene with c-Fos protein. Within the chemosensory region of the nucleus tractus solitarius (nTS), 19% of c-Fos positive cells expressed PPT-A mRNA after hypercapnic loading. In medullary raphe nuclei, 64% of c-Fos positive cells expressed the PPT-A gene after exposure to CO(2), while 21% of c-Fos labeled neurons in parapyramidal nuclei also expressed PPT-A mRNA. These results indicate that a subpopulation of CO(2) activated neurons within the nTS and in the parapyramidal and midline regions of the ventral aspect of the medulla oblongata express the PPT-A gene, suggesting that these are substance P- or neurokinin A-containing neurons. Furthermore, these peptides may play a role in modulation of respiratory and cardiovascular responses to changes in CO(2)/H(+) content of the extracellular fluid.

Sleep and Breathing in Recreational Climbers at an Altitude of 4200 and 6400 Meters: Observational Study of Sleep and Patterning of Respiration During Sleep in a Group of Recreational Climbers

Background: The increasing popularity of mountain climbing will result in greater numbers of the general population being at risk for the disturbances known to occur with altitude exposure. Methods: Observations of sleep and breathing were made in 6 healthy travellers (5 males and 1 female, 38 to 62 years of age) before, during, and after a recreational climb. We modified a portable seven channel polygraph to record sleep state, oxygen saturation, respiratory movements, body position, and oronasal airflow 4 weeks prior to the expedition at home (500m), at base camp (4200m) and in 3 climbers at 6400m. All had a repeat study at 500m altitude 4 weeks after the expedition. Results: For the group, the total number of obstructive apneas and hypopneas (OA/H) at night increased from 36 at home to 68 at base camp over a one night recording. Separately counted central apneas and hypopneas (CA/CS) increased from 6.7 to 45. In one climber, who had a history of recurrent snoring and observed apneas at home, the number of apneas increased from 201 at 4200m to 322 at 6400m, whereas in 2 climbers measured at 6400m, all apneas decreased. The total sleep time (TST) increased in all 6 climbers by 10% at base camp in comparison to home records. In the 3 climbers attaining an altitude of 6400m, the REM (rapid eye movement) sleep declined by 10% compared to the record at 4200m. Conclusion: Respiratory disturbances at low altitude are amplified by exposure to high and extreme altitude. In those without symptoms of sleep apnea, significant physiologic alterations will occur at high altitude but at extreme altitude regular ventilation is re-established.

Simultaneous glutamate and gamma-aminobutyric acid release within ventrolateral medulla during skeletal muscle contraction in intact and barodenervated rats

The purpose of this study was to determine if baroreflex modulates cardiovascular responses and neurotransmitter release within rostral (RVLM) and caudal (CVLM) ventrolateral medulla during static contraction of skeletal muscle using anesthetized rats. We evoked cardiovascular responses by a static muscle contraction and measured simultaneous release of glutamate and gamma-aminobutyric acid (GABA) in both the RVLM and CVLM using microdialysis probes, two inserted bilaterally into the RVLM and two into the CVLM. In intact anesthetized rats, a muscle contraction increased release of glutamate concomitantly in both the RVLM and CVLM along with significant increases in heart rate and arterial blood pressure. In contrast, concentrations of GABA increased within the RVLM, but decreased significantly within the CVLM during the pressor response. These changes were due to contraction-evoked activation of muscle afferents since tibial nerve stimulation following muscle paralysis failed to evoke glutamate, GABA, or any cardiovascular changes. On the other hand, static muscle contractions in baroreceptor denervated rats augmented the increases in heart rate and blood pressure. Furthermore, muscle contraction significantly enhanced the release of glutamate in the RVLM but attenuated its release in the CVLM. In addition, concentrations of GABA within the RVLM were attenuated following a muscle contraction in denervated rats without any changes in GABA within the CVLM. These results demonstrate that the baroreceptors influence cardiovascular responses to static muscle contraction associated with dynamic changes in glutamate and GABA release within the RVLM and CVLM.

Chemosensitivity of serotonergic neurons in the rostral ventral medulla

The medullary raphé contains two subtypes of chemosensitive neuron: one that is stimulated by acidosis and another that is inhibited. Both types of neuron are putative chemoreceptors, proposed to act in opposite ways to modulate respiratory output and other pH sensitive brain functions. In this review, we will discuss the cellular properties of these chemosensitive raphé neurons when studied in vitro using brain slices and primary dissociated cell culture. Quantification of chemosensitivity of raphé neurons indicates that they are highly sensitive to small changes in extracellular pH (pH(o)) between 7.2 and 7.6. Stimulation by acidosis occurs only in the specific phenotypic subset of neurons within the raphé that are serotonergic. These serotonergic neurons also have other properties consistent with a specialized role in chemoreception. Homologous serotonergic neurons are present within the ventrolateral medulla (VLM), and may have contributed to localization of respiratory chemoreception to that region. Chemosensitivity of raphé neurons increases in the postnatal period in rats, in parallel with development of respiratory chemoreception in vivo. An abnormality of serotonergic neurons of the ventral medulla has been identified in victims of sudden infant death syndrome (SIDS). The cellular properties of serotonergic raphé neurons suggest that they play a role in the CNS response to hypercapnia, and that they may contribute to interactions between the sleep/wake cycle and respiratory control.

Relationship between cerebro-spinal fluid pH and pulmonary ventilation of the South American lungfish, Lepidosiren paradoxa (Fitz.)

The respiratory control in land vertebrates (Tetrapoda) is mainly linked to regulation of acid-base status, which involves peripheral and central chemoreceptors. The lungfish (Dipnoi) might constitute the sister group of all land vertebrates (Tetrapoda) and possess a combination of real lungs and reduced gills. In this context, we evaluated the possible presence of central respiratory chemoreceptors in the South American Lungfish, Lepidosiren paradoxa. Pulmonary ventilation and respiratory frequency increased significantly with reductions of CSF pH by means of mock CSF solutions. This suggests that Lepidosiren possess central acid-base receptors.

CNS innervation of posterior cricoarytenoid muscles: a transneuronal labeling study

The CNS cell groups that project to neurons, which innervate the posterior cricoarytenoid muscles (PCA), were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the PCA of C8 spinal rats and after 5 days survival, brain tissue sections were processed for immunohistochemical detection of PRV. Retrogradely labeled motor neurons innervating the PCA were seen in the nucleus ambiguus and in the area ventral to it. Neurons innervating the PCA motoneurons were found throughout the ventral aspect of the medulla oblongata, in the nucleus tractus solitarius, and in the pons. Labeling was present in the midbrain periaquaductal gray, in the lateral and paraventricular hypothalamic nuclei, in the amygdaloid complex, in the hippocampus, and within the piriform cortex. In summary, the motor neurons that control PCA activity are innervated predominantly by a network of neurons that lie along the neuraxis, in the regions known to be involved in regulation of respiratory output and autonomic functions.

Acidosis-Stimulated Neurons of the Medullary Raphe Are Serotonergic

Neurons of the medullary raphe project widely to respiratory and autonomic nuclei and contain co-localized serotonin, thyrotropin-releasing hormone (TRH), and substance P, three neurotransmitters known to stimulate ventilation. Some medullary raphe neurons are highly sensitive to pH and CO2 and have been proposed to be central chemoreceptors. Here it was determined whether these chemosensitive neurons are serotonergic. Cells were microdissected from the rat medullary raphe and maintained in primary cell culture for 13–70 days. Immunoreactivity for serotonin, substance P, and TRH was present in these cultures. All acidosis-stimulated neurons ( n = 22) were immunoreactive for tryptophan hydroxylase (TpOH-IR), the rate-limiting enzyme for serotonin biosynthesis, whereas all acidosis-inhibited neurons ( n= 16) were TpOH-immunonegative. The majority of TpOH-IR medullary raphe neurons (73%) were stimulated by acidosis. The electrophysiological properties of TpOH-IR neurons in culture were similar to those previously reported for serotonergic neurons in vivo and in brain slices. These properties included wide action potentials (4.55 ± 0.5 ms) with a low variability of the interspike interval, a postspike afterhyperpolarization (AHP) that reversed 25 mV more positive than the Nernst potential for K+, prominent A current, spike frequency adaptation and a prolonged AHP after a depolarizing pulse. Thus the intrinsic cellular properties of serotonergic neurons were preserved in cell culture, indicating that the results obtained using this in vitro approach are relevant to serotonergic neurons in vivo. These results demonstrate that acidosis-stimulated neurons of the medullary raphe contain serotonin. We propose that serotonergic neurons initiate a homeostatic response to changes in blood CO2 that includes increased ventilation and modulation of autonomic function.

Involvement of cholinergic mechanisms in the central control of respiration in the cane toad, Bufo marinus

Chemical substrates, central sites and central mechanisms underlying the regulation of breathing in lower vertebrates have not been well characterized. The present study was undertaken to determine the effect of pH changes and cholinergic agents on the central control of respiration in the cane toad, Bufo marinus. Adult toads were anesthetized, catheterized and unidirectionally ventilated before exposing the brainstem. An airtight buccal cannula was also inserted through the tympanum to record buccal pressure. The animal was decerebrated, anesthetic removed and the responses to pH changes of solutions bathing the ventral surface of the medulla (VSM) were tested by superfusing the VSM with mock cerebrospinal fluid (mCSF) of pH 7.8-normal, 7.2-acidic and 8.4-basic. The acidic solution increased respiratory activity, the basic solution decreased activity and the normal solution had no effect. In addition, cholinergeric agents (acetylcholine-ACh, physostigmine-Phy, nicotine-Nic, and atropine-Atr) dissolved in mCSF were applied bilaterally onto the VSM using filter paper pledgets. ACh, Phy and Nic increased episodic breathing frequency by 14.3+/-9.7, 9.4+/-5.4 and 29.1+/-11.8 %, respectively, whereas, Atr caused a decrease (-26.6+/-16.6%). These agents had no effect on blood pressure. It is therefore, concluded that the VSM is pH sensitive and a cholinergic mechanism is involved in the central modulation of respiration in Bufo.

Neurotransmitters in central respiratory control

A diverse group of processes are involved in central control of ventilation. Both fast acting neurotransmitters and slower acting neuromodulators are involved in the central respiratory drive. This review deals with fast acting neurotransmitters that are essential centrally in the ventilatory response to H(+)/CO(2) and to acute hypoxia. Data are reviewed to show that the central response to H(+)/CO(2) is primarily at sites in the medulla, the most prominent being the ventral medullary surface (VMS), and that acetylcholine is the key neurotransmitter in this process. Genetic abnormalities in the cholinergic system lead to states of hypoventilation in man and that knock out mice for genes responsible for neural crest development have none or diminished CO(2) ventilatory response. In the acute ventilatory response to hypoxia the afferent impulses from the carotid body reach the nucleus tractus solitarius (NTS) releasing glutamate which stimulates ventilation. Glutamate release also occurs in the VMS. Hypoxia is also associated with release of GABA in the mid-brain and a biphasic change in concentration of another inhibitory amino acid, taurine. Collectively changes in these amino acids can account for the ventilatory output in response to acute hypoxia. Future studies should provide more data on molecular and genetic basis of central respiratory drive and the role of neurotransmitter in this essential function.

Differential cardiorespiratory control elicited by activation of ventral medullary sites in mice

We studied the respiratory and blood pressure responses to chemical stimulation of two regions of the ventral brainstem in mice: the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). Stimulation of the RVLM by microinjections of the excitatory amino acid L-glutamate induced increases in diaphragm activity and breathing frequency, elevation of blood pressure (BP), and a slight increase in heart rate (HR). However, activation of the CVLM induced a decrease in breathing frequency, mainly due to prolongation of expiratory time (TE), and hypotension associated with a slight slowing of HR. Because adrenergic mechanisms are known to participate in the control of respiratory timing, we examined the role of alpha(2)-adrenergic receptors in the RVLM region in mediating these inhibitory effects. The findings demonstrated that blockade of the alpha(2)-adrenergic receptors within the RVLM by prior microinjection of SKF-86466 (an alpha(2)-adrenergic receptor blocker) significantly reduced changes in TE induced by CVLM stimulation but had little effect on BP responses. These results indicate that, in mice, activation of the RVLM increases respiratory drive associated with an elevation of BP, but stimulation of CVLM induces prolongation of TE via an alpha(2)-adrenergic signal transduction pathway.

Brainstem amino acid neurotransmitters and hypoxic ventilatory response

The ventilatory response to acute hypoxia in mammalian species is biphasic, an initial hyperventilatory response is followed by a reduction in ventilation within 2-3 min below the peak level (roll-off). Brain amino acid neurotransmitters also change during hypoxia. This study explores the role of neurotransmitters in anesthetized adult Sprague Dawley rats mechanically ventilated during 20 min of 10% O2 breathing. Phrenic nerve activity was recorded, and microdialysate concentrations of selected amino acids were determined at 3- to 5-min intervals in respiratory chemosensitive areas of the ventrolateral medulla (VMS) 1.25-2.00 mm below the surface. Phrenic nerve output was biphasic during hypoxia, concurrent with a rapid glutamate and gradual GABA increase. Taurine first decreased, then increased. In both intact and chemodenervated animals, time-dependent change in phrenic nerve activity during hypoxia was associated with corresponding changes in glutamate, GABA, and taurine concentrations, suggesting that cumulative effects of changes in the concentration of these three amino acids could account for response of the phrenic nerve to hypoxia.

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.

CO2-induced c-fos expression in medullary neurons during early development

In this study, we characterized the responses of brainstem neurons to hypercapnic loading at 5, 15, and 40 postnatal days, using c-fos gene encoded protein (Fos), as a marker of neuronal activity. At any of these studied ages exposure to 10% CO2 for 1 h produced a significant increase in the number of activated neurons within the ventral and the dorsal aspects of the brainstem. In the ventrolateral aspect of the medulla oblongata, Fos positive cells were observed within the ventrolateral medulla, extending from the pontomedullary border to the decussation of the pyramids. In the most rostral regions, within the retrotrapezoid field, the number of Fos positive cells was lower than in caudal ventral medullary regions at the levels of the area postrema and the caudal to it. No age related differences were observed in the number of neurons exhibiting CO2-induced Fos expression. Fos positive cells were additionally observed in the lateral paragigantocellular and gigantocellular reticular nuclei, in the medullary midline complex, in the raphe pallidus and in the raphe obscurus. The number of activated cells in the midline neurons was higher at 5 than at 40 days of age. In the dorsal aspect of the medulla oblongata Fos positive neurons were observed mainly within the caudal nucleus tractus solitarius (nTS). Postnatal age had no effect on the distribution and number of nTS cells activated by hypercapnic loading. These findings indicate that neurons activated by increases in CO2/H+ concentrations appear to be well developed from the first days of postnatal life in maturing rat pups.

CO2-induced prolongation of expiratory time during early development

In these studies, we determined the contribution of central mechanisms and the role of GABA(A)-receptor signal transduction pathways in mediating hypercapnia-induced slowing of breathing frequency. Experiments were performed in decerebrate, vagotomized, paralyzed and mechanically ventilated piglets of 3-5 days and 2-3 weeks of age (n=19). Repeated exposure to progressive hyperoxic hypercapnia induced a reproducible increase in phrenic nerve activity, accompanied by a CO2 concentration-dependent increase in expiratory duration. No differences were observed in piglets with intact or cut carotid sinus nerves. Intravenous administration of bicuculline (2 mg/kg: n=7), a gamma-aminobutyric acid (GABA(A)) receptor antagonist, significantly reduced the CO2-induced prolongation of TE. These data demonstrate for the first time that in early postnatal life, hypercapnia induced increase in phrenic activity is associated with centrally mediated prolongation of expiratory duration. Furthermore. the results suggest that brainstem GABAergic mechanisms play an important role in CO2-induced prolongation of expiratory time during early development.

Identification of a critical motif responsible for gating of Kir2.3 channel by intracellular protons

Protons are involved in gating Kir2.3. To identify the molecular motif in the Kir2.3 channel protein that is responsible for this process, experiments were performed using wild-type and mutated Kir2. 3 and Kir2.1. CO2 and low pHi strongly inhibited wild-type Kir2.3 but not Kir2.1 in whole cell voltage clamp and excised inside-out patches. This CO2/pH sensitivity was completely eliminated in a mutant Kir2.3 in which the N terminus was substituted with that in Kir2.1, whereas a similar replacement of its C terminus had no effect. Site-specific mutations of all titratable residues in the N terminus, however, did not change the CO2/pH sensitivity. Using several chimeras generated systematically in the N terminus, a 10-residue motif near the M1 region was identified in which only three amino acids are different between Kir2.3 and Kir2.1. Mutations of these residues, especially Thr53, dramatically reduced the pH sensitivity of Kir2.3. Introducing these residues or even a single threonine to the corresponding positions of Kir2.1 made the mutant channel pH-sensitive. Thus, a critical motif responsible for gating Kir2.3 by protons was identified in the N terminus, which contained about 10 residues centered by Thr53.

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 involvement of rostral ventromedullary neuronal structures in regulating the mechanism of formation of the respiratory rhythm in rats

The role of neuronal structures in the rostral parts of the ventral surface of the medulla oblongata of the rat in regulating the central inspiratory activity of the respiratory center was analyzed. It is suggested that neuronal structures of the subretrofascial area, located close to the ventral surface of the medulla oblongata have direct associations with the mechanisms generating and regulating the respiratory rhythm. These have excitatory effects on neurons of the respiratory center which generate inspiratory activity.

Effects of oxygen deprivation on parapyramidal neurons of the ventrolateral medulla in the rat

We characterized the electrophysiological properties and responses of neurons located in the parapyramidal region of the ventral aspect of the rat medulla oblongata (parapyramidal neurons, PP neurons) to oxygen deprivation, in order to understand the mechanisms involved in hypoxia induced respiratory depression. The responses of PP neurons to oxygen deprivation were compared to those of the functionally dissimilar neurons of the dentate gyrus (DG). Neurons from the PP region were found to fire spontaneously with a frequency of 3-3.5 spikes/sec in both adults and neonates and responded to an anoxic insult with a complete loss of spontaneous firing. Discrete metabolite analysis showed a small (about 17%) decrease in tissue adenosine triphosphate (ATP) levels of the PP neurons during an anoxic insult and the decrease was significantly smaller than in the DG cell region (28%). In contrast to the DG neurons, the PP neurons recovered from an anoxic insult lasting more than 30 min, indicating a greater survival capacity of the PP neurons during oxygen deprivation. The PP neurons were also capable of withstanding successive anoxic insults better than the DG cells as demonstrated by their complete recovery following reoxygenation. It is suggested that the PP neurons may depress their electrical activity as an energy conservation mechanism, and thereby survive anoxic insults longer than the dentate neurons, whereas the loss of cellular activity in the DG neurons may be a result of energy depletion.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

Respiratory effects of kynurenic acid microinjected into the ventromedullary surface of the rat

Several studies demonstrate that, within the ventral medullary surface (VMS), excitatory amino acids are necessary components of the neural circuits involved in the tonic and reflex control of respiration and circulation. In the present study we investigated the cardiorespiratory effects of unilateral microinjections of the broad spectrum glutamate antagonist kynurenic acid (2 nmol/200 nl) along the VMS of urethane-anesthetized rats. Within the VMS only one region was responsive to this drug. This area includes most of the intermediate respiratory area, partially overlapping the rostral ventrolateral medulla (IA/RVL). When microinjected into the IA/RVL, kynurenic acid produced a respiratory depression, without changes in mean arterial pressure or heart rate. The respiratory depression observed was characterized by a decrease in ventilation, tidal volume and mean inspiratory flow and an increase in respiratory frequency. Therefore, the observed respiratory depression was entirely due to a reduction in the inspiratory drive. Microinjections of vehicle (200 nl of saline) into this area produced no significant changes in breathing pattern, blood pressure or heart rate. Respiratory depression in response to the blockade of glutamatergic receptors inside the rostral VMS suggests that neurons at this site have an endogenous glutamatergic input controlling the respiratory cycle duration and the inspiratory drive transmission.

Nitric oxide inhalation therapy for newborn infants

Binding of NO to heavy metal-containing proteins probably accounts for many of its physiologic actions. NO inhalation is a promising new treatment for various disorders of neonates. The therapy is most likely to benefit premature neonates who are hypoxemic despite breathing pure oxygen and those who suffer from impaired carbon dioxide elimination. Newborn infants who have congenital heart disease may benefit from inhaled NO therapy if their disease involves some form of pulmonary venous hypertension or if they have recently undergone surgery involving cardiopulmonary bypass grafting. The use of NO in infants with PPHN might obviate the need for ECMO or other invasive treatment methods. Neonates with CDH seem likely to benefit marginally from NO therapy. Minimizing the toxicities of NO inhalation therapy requires that the physicians understand the nuances of infant care. The therapeutic value of increasing carbon dioxide elimination with NO inhalation warrants further investigation.

Catechol activation in rat rostral ventrolateral medulla after systemic isocapnic metabolic acidosis

The catechol signal recorded using in vivo voltammetry within the rat rostral ventrolateral medulla (RVLM) can be interpreted as a catechol-specific index of the integrated activity of RVLM adrenergic barosensitive bulbospinal and nonbulbospinal neurons. To test the hypothesis that systemic acidosis leads to the activation of RVLM adrenergic neurons, the RVLM catechol signal was observed in rats after mild systemic acidosis (pH 7.20-7.25 for 30 min) induced by 1 M HCl under halothane anesthesia, controlled mechanical ventilation, and continuous infusion of Ringer lactate. Particular attention was paid to ensure that changes in mean arterial pressure (MAP) were <15 mmHg during HCl challenge. Saline administration was not associated with any significant change in all considered variables (n = 5). Mild isocapnic systemic acidosis was associated with an increase in catechol signal (n = 5), irrespective of carotid sinus nerve section (n = 5). In keeping with the aim of the study, there were minor (<15 mmHg) but significant changes in MAP among saline, intact, and deafferented groups. Changes in heart rate were not significant. In conclusion, a catechol activation is observed in the RVLM when arterial pressure is maintained during isocapnic systemic metabolic acidosis. This catechol activation appears primarily centrally mediated. Therefore, adrenergic RVLM neurons may relay inputs from the central respiratory generator to the sympathetic system and/or act as chemosensors for H+ next to the surface of the ventrolateral medulla.

A possible role for protein kinase C in CO2/H+-induced c-fos mRNA expression in PC12 cells

Recently we have found that hypercapnia induces nuclear protein (FOS) expression in the brainstem chemosensitive neurons, including catecholamine-containing cells. In the present studies we examined the role of protein kinase C (PKC) pathway in CO2-induced c-fos expression. Because of the complexity of the CNS system, experiments were performed in pheochromocytoma cells (PC12 cells). These cells originate from neuronal crest and express catecholaminergic traits. We depleted PKC from PC12 cells by prolonged (48 h) exposure to high concentration of phorbol 12-myristate, 13-acetate (PMA, 100 nM), and then determined the expression of: (1) c-fos mRNA by Northern blot (2) PKC isoforms, tyrosine phosphorylated and unphosphorylated MAP (mitogen activated protein) kinases by Western blot. Depletion of PKC abolished the effect of CO2 on c-fos mRNA expression, inhibited MAP kinases tyrosine phosphorylation and suppressed the expression of PKC(alpha) and PKC(zeta). These results suggest that MAP kinases, PKC(alpha) and/or PKC(beta) might be involved in CO2-induced c-fos mRNA expression.

CO2-sensitive neurons in organotypic cultures of the fetal rat medulla

Medullary slices of the fetal rat at gestational day 16 were cultivated (organotypic culture) for up to 20 days and current clamp experiments were performed on outgrowing neurons. CO2-sensitivity was tested by changing the P(CO2) in the bath solution (equilibrating CO2 fraction from 0.02 to 0.09). Two groups of CO2-sensitive neurons were found; one with and the other without intrinsic CO2-chemosensitivity. Neurons with intrinsic CO2-sensitivity maintained their spontaneous activity and chemosensitivity after blockade of synaptic transmission. These neurons exhibited action potentials that were preceeded by a spontaneous interspike depolarization and followed by an afterhyperpolarization (beating neurons). Increasing P(CO2) either decreased (inhibited neurons, n = 55) or increased the spike frequency of these neurons (stimulated neurons, n = 31). The reduced activity of CO2-inhibited neurons was associated with membrane hyperpolarization and/or decreases in the slope of interspike depolarization. In contrast CO2-stimulated neurons were depolarized and the slope of their interspike depolarization was augmented during acidosis. In addition, we demonstrated a strong voltage dependence of CO2-induced effects on membrane potential and spike frequency. Neurons with non-beating activity did not show a spontaneous interspike depolarization and their spike generation and CO2-sensitivity appeared to be entirely produced through synaptic inputs. The CO2-mediated changes in electrical properties of these neurons closely resemble those of various CNS neurons, including respiratory neurons, in whole animal or neonatal brainstem-spinal cord preparations.

Role of the medullary raphe nuclei in the respiratory response to CO2

We characterized the role of neurons within the midline of the medulla oblongata on phrenic and hypoglossal nerve responses to hypercapnia during early-development. Studies were performed on decorticate or anesthetized; vagotomized and mechanically ventilated 14-20 day old piglets. Reversible withdrawal of midline neuronal activity was induced by microinjections of lidocaine (2%, 300 nl; n = 10) and lesioning was caused by microinjections of the neurotoxic agent, ibotenic acid (n = 12), at the same sites. At any given end-tidal CO2, peak phrenic and hypoglossal activities after lidocaine were significantly lower than in the control period (P < 0.01). Similarly, 1-2 h after injections of ibotenic acid, both phrenic and hypoglossal nerve responses to CO2 were significantly lower than in the control period (P < 0.01). The results indicate for the first time that the medullary midline neurons are required for full expression of ventilatory responses to hypercapnia and raise the possibility that dysfunction of these nuclei may contribute to respiratory instability during early postnatal life.

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.

Sudden Infant Death Syndrome: The Role of Central Chemosensitivity

Clinical and basic research on Sudden Infant Death Syndrome (SIDS) has focused on sleep-disordered cardiorespiratory control during a critical period of brainstem maturation. Recently, some SIDS cases have been reported to have abnormalities of the arcuate nucleus of the medulla. The human arcuate nucleus is thought to be homologous to the medullary raphe in rats and cats, a widely projecting serotonergic system that is functionally linked to both respiration and sleep. Neurons of the medullary raphe are now known to be highly sensitive to respiratory acidosis in vitro and are candidates for central chemoreceptors. The relevance of changes in the arcuate nucleus to the mechanisms of death in SIDS remains controversial. However, based on this new data, a specific hypothesis is proposed here. In combination with Immaturity of respiratory control mechanisms, dysfunction of arcuate neurons may lead to a fatal exaggeration of a normal physiologic inhibition of central chemoreception during sleep. The major elements of this working hypothesis are testable in animal experiments and clinical studies.