[Use of controlled respiratory rhythm to enhance stress resistance in pupils]

To enhance stress resistance in pupils is now a very urgent problem. The influence of a method affecting the central nervous system as acquisition of the averaged value of intrinsic respiratory rhythm has been studied. There has been a positive result of using the procedure to increase working capacity during performance by pupils and positive changes in electroencephalographic readings, by optimizing the autonomic provision for their activity.

5-HT3 receptor-dependent modulation of respiratory burst frequency, regularity, and episodicity in isolated adult turtle brainstems

To determine the role of central serotonin 5-HT(3) receptors in respiratory motor control, respiratory motor bursts were recorded from hypoglossal (XII) nerve rootlets on isolated adult turtle brainstems during bath-application of 5-HT(3) receptor agonists and antagonists. mCPBG and PBG (5-HT(3) receptor agonists) acutely increased XII burst frequency and regularity, and decreased bursts/episode. Tropisetron and MDL72222 (5-HT(3) antagonists) increased bursts/episode, suggesting endogenous 5-HT(3) receptor activation modulates burst timing in vitro. Tropisetron blocked all mCPBG effects, and the PBG-induced reduction in bursts/episode. Tropisetron application following mCPBG application did not reverse the long-lasting (2h) mCPBG-induced decrease in bursts/episode. We conclude that endogenous 5-HT(3) receptor activation regulates respiratory frequency, regularity, and episodicity in turtles and may induce a form of respiratory plasticity with the long-lasting changes in respiratory regularity.

Task2 potassium channels set central respiratory CO2 and O2 sensitivity

Task2 K(+) channel expression in the central nervous system is surprisingly restricted to a few brainstem nuclei, including the retrotrapezoid (RTN) region. All Task2-positive RTN neurons were lost in mice bearing a Phox2b mutation that causes the human congenital central hypoventilation syndrome. In plethysmography, Task2(-/-) mice showed disturbed chemosensory function with hypersensitivity to low CO(2) concentrations, leading to hyperventilation. Task2 probably is needed to stabilize the membrane potential of chemoreceptive cells. In addition, Task2(-/-) mice lost the long-term hypoxia-induced respiratory decrease whereas the acute carotid-body-mediated increase was maintained. The lack of anoxia-induced respiratory depression in the isolated brainstem-spinal cord preparation suggested a central origin of the phenotype. Task2 activation by reactive oxygen species generated during hypoxia could silence RTN neurons, thus contributing to respiratory depression. These data identify Task2 as a determinant of central O(2) chemoreception and demonstrate that this phenomenon is due to the activity of a small number of neurons located at the ventral medullary surface.

Activation of the retrotrapezoid nucleus by posterior hypothalamic stimulation

The retrotrapezoid nucleus (RTN) contains chemically defined neurons (ccRTN neurons) that provide a pH-regulated excitatory drive to the central respiratory pattern generator. Here we test whether ccRTN neurons respond to stimulation of the perifornical hypothalamus (PeF), a region that regulates breathing during sleep, stress and exercise. PeF stimulation with gabazine increased blood pressure, phrenic nerve discharge (PND) and the firing rate of ccRTN neurons in isoflurane-anaesthetized rats. Gabazine produced an approximately parallel upward shift of the steady-state relationship between ccRTN neuron firing rate and end-tidal CO(2), and a similar shift of the relationship between PND and end-tidal CO(2). The central respiratory modulation of ccRTN neurons persisted after gabazine without a change in pattern. Morphine administration typically abolished PND and reduced the discharge rate of most ccRTN neurons (by 25% on average). After morphine administration, PeF stimulation activated the ccRTN neurons normally but PND activation and the central respiratory modulation of the ccRTN neurons were severely attenuated. In the same rat preparation, most (58%) ccRTN neurons expressed c-Fos after exposure to hypercapnic hyperoxia (6-7% end-tidal CO(2); 3.5 h; no hypothalamic stimulation) and 62% expressed c-Fos under hypocapnia (approximately 3% end-tidal CO(2)) after PeF stimulation. Under baseline conditions (approximately 3% end-tidal CO(2), hyperoxia, no PeF stimulation) few (11%) ccRTN neurons expressed c-Fos. In summary, most ccRTN neurons are excited by posterior hypothalamic stimulation while retaining their normal response to CNS acidification. ccRTN neurons probably contribute both to the chemical drive of breathing and to the feed-forward control of breathing associated with emotions and or locomotion.

Retrotrapezoid nucleus, respiratory chemosensitivity and breathing automaticity

Breathing automaticity and CO(2) regulation are inseparable neural processes. The retrotrapezoid nucleus (RTN), a group of glutamatergic neurons that express the transcription factor Phox2b, may be a crucial nodal point through which breathing automaticity is regulated to maintain CO(2) constant. This review updates the analysis presented in prior publications. Additional evidence that RTN neurons have central respiratory chemoreceptor properties is presented, but this is only one of many factors that determine their activity. The RTN is also regulated by powerful inputs from the carotid bodies and, at least in the adult, by many other synaptic inputs. We also analyze how RTN neurons may control the activity of the downstream central respiratory pattern generator. Specifically, we review the evidence which suggests that RTN neurons (a) innervate the entire ventral respiratory column and (b) control both inspiration and expiration. Finally, we argue that the RTN neurons are the adult form of the parafacial respiratory group in neonate rats.

Multiple pontomedullary mechanisms of respiratory rhythmogenesis

Mammalian central pattern generators producing rhythmic movements exhibit robust but flexible behavior. However, brainstem network architectures that enable these features are not well understood. Using precise sequential transections through the pons to medulla, it was observed that there was compartmentalization of distinct rhythmogenic mechanisms in the ponto-medullary respiratory network, which has rostro-caudal organization. The eupneic 3-phase respiratory pattern was transformed to a 2-phase and then to a 1-phase pattern as the network was physically reduced. The pons, the retrotrapezoid nucleus and glycine mediated inhibition are all essential for expression of the 3-phase rhythm. The 2-phase rhythm depends on inhibitory interactions (reciprocal) between Bötzinger and pre-Bötzinger complexes, whereas the 1-phase-pattern is generated within the pre-Bötzinger complex and is reliant on the persistent sodium current. In conditions of forced expiration, the RTN region was found to be essential for the expression of abdominal late expiratory activity. However, it is unknown whether the RTN generates or simply relays this activity. Entrained with the central respiratory network is the sympathetic nervous system, which exhibits patterns of discharge coupled with the respiratory cycle (in terms of both gain and phase of coupling) and dysfunctions in this coupling appear to underpin pathological conditions. In conclusion, the respiratory network has rhythmogenic capabilities at multiple levels of network organization, allowing expression of motor patterns specific for various physiological and pathophysiological respiratory behaviors.

[Mechanisms of the respiratory activity of leptin at the level of the solitary tract nucleus]

In acute experiments on anaesthetised rats we investigated the effects of 10(-4) M leptin microinjected into the solitary tract nucleus on inspiratory-terminating Breuer-Hering reflex and ventilatory responses to hypercapnea. We found that the local administration of leptin into the area inhibited the inspiratory-terminating Breuer-Hering reflex. In contrast, ventilatory responses to hypercapnea were accentuated suggesting a modulatory effect of leptin on central chemoreceptors. The discovered physiological mechanisms are likely to play a key role for stimulating respiratory effects of leptin at the level of the solitary tract nucleus.

Central cholinergic regulation of respiration: nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in brainstem and spinal cord regions involved in the control of breathing. These receptors mediate central cholinergic regulation of respiration and effects of the exogenous ligand nicotine on respiratory pattern. Activation of alpha4* nAChRs in the preBötzinger Complex (preBötC), an essential site for normal respiratory rhythm generation in mammals, modulates excitatory glutamatergic neurotransmission and depolarizes preBötC inspiratory neurons, leading to increases in respiratory frequency. nAChRs are also present in motor nuclei innervating respiratory muscles. Activation of post- and/or extra-synaptic alpha4* nAChRs on hypoglossal (XII) motoneurons depolarizes these neurons, potentiating tonic and respiratory-related rhythmic activity. As perinatal nicotine exposure may contribute to the pathogenesis of sudden infant death syndrome (SIDS), we discuss the effects of perinatal nicotine exposure on development of the cholinergic and other neurotransmitter systems involved in control of breathing. Advances in understanding of the mechanisms underlying central cholinergic/nicotinic modulation of respiration provide a pharmacological basis for exploiting nAChRs as therapeutic targets for neurological disorders related to neural control of breathing such as sleep apnea and SIDS.

Local antagonism of intertrigeminal region metabotropic glutamate receptors exacerbates apneic responses to intravenous serotonin

Injections of a broad spectrum glutamate receptor antagonist into the pontine intertrigeminal region (ITR) exacerbate vagal reflex apnea produced by intravenous serotonin infusion. This effect is not reproduced by ITR injections with either NMDA or AMPA receptor antagonists. Here, we tested the hypothesis that ITR injection with a metabotropic glutamate antagonist would alter respiratory responses to serotonin (5-HT) intravenous infusions. In anesthetized adult male rats (N=20; Sprague-Dawley) AIDA (1-aminoindan-1,5-dicarboxylic acid), a specific antagonist of the type 1 metabotropic glutamate receptor (mGlu1R), was microinjected unilaterally into the ITR to block 5-HT evoked apnea. Respiratory pattern changes evoked by ITR-glutamate injection and by intravenous serotonin (5-HT) infusion (0.5 microl, 0.05 M; or 2.5x10(-8) mol) were characterized according to apnea expression and duration, as well as coefficients of variation for breath duration (CVTT) and amplitude (CVVT) before and after ITR AIDA injection. Unilateral AIDA blockade of the ITR significantly increased the duration of apnea evoked by 5-HT infusion (p<0.03 for each dose tested) during the 30s following infusion in a dose-dependent fashion, with the two highest doses resulting in intermittent apneas for at least 10 min following a bolus 5-HT infusion. Similar prolonged increases in CVTT and CVVT with respect to control were associated with ITR AIDA injections. These findings suggest that brief perturbations of vagal afferent pathways can produce ongoing respiratory dysrhythmia, including spontaneous apnea, and that glutamatergic neurotransmission within ITR may be important for damping such disturbances. The present observations also suggest that such respiratory damping may be mediated by mGlu1 receptors. These findings extend our understanding of the role of the intertrigeminal region in modulating respiratory reflexes.

[Identification and classification of respiratory pacemaker neurons in the medial region of nucleus retrofacialis in neonatal rats.]

The aim of the present study is to supply direct experimental proof that the medial region of nucleus retrofacialis (mNRF) is the site generating basic rhythm of respiration. Medullary slices of neonatal Sprague-Dawley rats, including hypoglossal nerve root and mNRF, were made according to Suzue's method. Simultaneous recordings of the respiratory rhythmic discharge activity (RRDA) in hypoglossal nerve root with suction electrode and the respiratory neuronal discharge in the mNRF with whole cell patch clamp were performed on the brainstem slice in vitro. Not only the electrophysiological characteristics of pacemaker and non-pacemaker neurons, but the cadmium-sensitivity of pacemaker neurons was observed. The nature of the discharge in the respiratory pacemaker neurons in mNRF was spontaneous, rhythmical and voltage-dependent burst behavior, generating an ectopic burst in response to a depolarizing or hyperpolarizing current. There were no significant differences in capacitance of membrane (C(m)), input resistance of membrane (R(m)) and leak inward current (I(leak)) between the pacemaker neurons and the non-pacemaker ones. Moreover, most pacemaker neurons in the mNRF were Cd(2+)-insensitive. In conclusion, these results support the idea that the mNRF is the site generating basic respiratory rhythm.

Respiratory response to systemic inhibition of the Na+/H+ exchanger type 3 in intact rats

The Na+/H+ exchangers (NHEs) are a family of antiporters involved in the maintenance of neural steady-state intracellular pH. The NHE3 seems to be the predominant subtype in central chemosensitive cells. We aimed to analyze the effect of a selective NHE3 inhibition on the respiratory pattern in spontaneously breathing rats with intact vagi. Rats were intravenously infused for 10 min with the selective NHE3 inhibitor AVE1599 (Aventis Pharma Deustchland, 0.5 and 2 mg/kg) or with phosphate buffer. Whole-body plethysmography was used to monitor breathing pattern before, during, and up to 30 min after the drug infusion. Immunohistochemistry for the c-Fos protein was performed in the animal brains and c-Fos-positive cells were counted along the brainstem. Selective NHE3 inhibition induced a significant increase in the respiratory frequency and in the number of c-Fos immunopositive cells in the lateral parabrachial nucleus, the pre-Bötzinger complex and a rostral extension of the retrotrapezoid nucleus/parapyramidal region (p<0.05, ANOVA). We conclude that systemic administration of AVE1599 increases respiratory frequency and activates ponto-medullary areas implicated in the central control of breathing and chemoreception.

Cervical prephrenic interneurons in the normal and lesioned spinal cord of the adult rat

Although monosynaptic bulbospinal projections to phrenic motoneurons have been extensively described, little is known about the organization of phrenic premotor neurons in the adult rat spinal cord. Because interneurons may play an important role in normal breathing and recovery following spinal cord injury, the present study has used anterograde and transneuronal retrograde tracing to study their distribution and synaptic relations. Exclusive unilateral, first-order labeling of the phrenic motoneuron pool with pseudorabies virus demonstrated a substantial number of second-order, bilaterally distributed cervical interneurons predominantly in the dorsal horn and around the central canal. Combined transneuronal and anterograde tracing revealed ventral respiratory column projections to prephrenic interneurons, suggesting that some propriospinal relays exist between medullary neurons and the phrenic nucleus. Dual-labeling studies with pseudorabies virus recombinants also showed prephrenic interneurons integrated with either contralateral phrenic or intercostal motoneuron pools. The stability of interneuronal pseudorabies virus labeling patterns following lateral cervical hemisection was then addressed. Except for fewer infected contralateral interneurons at the level of the central canal, the number and distribution of phrenic-associated interneurons was not significantly altered 2 weeks posthemisection (i.e., the point at which the earliest postinjury recovery of phrenic activity has been reported). These results demonstrate a heterogeneous population of phrenic-related interneurons. Their connectivity and relative stability after cervical hemisection raise speculation for potentially diverse roles in modulating phrenic function normally and postinjury.

[Glycine is involved in the modulation of respiratory rhythmical discharge activity in neonatal rat medullary brain slices]

OBJECTIVE

To determine the role of glycine (Gly) in the generation and modulation of basic respiratory rhythm.

METHODS

Neonatal (0-3 days) SD rats of either sex were used in this study. The medulla oblongata brain slice containing the medial region of the nucleus retrofacialis (mNRF) and the hypoglossal nerve rootlets was prepared, and the surgical procedure was performed in the modified Kreb's solution (MKS) with continuous carbogen (95% O(2) and 5% CO(2)) within 3 min. The rhythmical respiratory discharge activity (RRDA) of the hypoglossal nerve rootlets was recorded using suction electrode. Eighteen medulla oblongata slice preparations were divided into 3 groups and treated for 20 min with Gly receptor specific agonist Gly (10 micromol/L), Gly receptor antagonist strychnine (STR, 1 micromol/L), or Gly+STR after a 20 min Gly application. The changes in RRDA of the hypoglossal nerve rootlets were observed.

RESULTS

Gly significantly decreased the inspiratory time and integral amplitude (IA), but the changes of respiratory cycle (RC) and expiratory time (TE) were not statistically significant. STR induced a decrease in expiratory time and respiratory cycle without significantly affecting the inspiratory time or integral amplitud. The effect of Gly on the respiratory rhythm was partially reversed by additional application of STR.

CONCLUSION

Gly may play an important role in the modulation of RRDA in the medulla oblongata slice of neonatal rats.

[Role of 5-HT(2A) receptor in increase in respiratory-related rhythmic discharge activity by nikethamide in neonatal rat transverse medullary slices]

To investigate the effects of nikethamide on the generation and modulation of rhythmic respiration of neonatal rats and the role of 5-HT(2A) receptor in this course, experiments were performed on the transverse medullary slices of neonatal rats (both sexes, 1-3 d) in vitro. The slices containing the medial region of the nucleus retrofacialis (mNRF) with the hypoglossal nerve rootlets were prepared in which the respiratory-related rhythmic discharge activity (RRDA) was recorded from the hypoglossal nerve rootlets by suction electrode. The possible role of nikethamide on RRDA was investigated by administration of an agonist of 5-HT(2A) receptor, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), and an antagonist of 5-HT(2A) receptor, ketanserine, dissolved in modified Krebos solution (MKS). Thirty slices were randomly divided into five groups: Group 1: the slices were perfused with different concentrations of nikethamide (0.5, 1, 3, 5, 7, 10 μg/mL), and the most effective concentration was selected; Group 2: the slices were perfused with DOI (40 μmol/L); Group 3: the slices were perfused with ketanserine (40 μmol/L); Group 4: the slices were perfused with ketanserine + DOI; Group 5: the slices were perfused with nikethamide, then perfused with nikethamide + ketanserine after washout of nikethamide. Nikethamide increased RRDA in transverse medullary slices at 0.5-7 μg/mL, and 5 μg/mL was the most effective concentration. DOI increased RRDA with prolonged inspiratory time (TI), increased integral amplitude (IA), and shortened respiratory cycle (RC). Ketanserine decreased RRDA with shortened TI, decreased IA and prolonged RC. Ketanserine + DOI had no significant effects on RRDA. The effects of nikethamide on RC and IA were totally and partially reversed by additional application of ketanserine, but the effect of nikethamide on TI was not influenced by ketanserine. It is proposed that nikethamide increases RRDA partly via 5-HT(2A) receptors.

[GABA A receptor participates in respiratory enhancement induced by nikethamide in neonatal rats]

OBJECTIVE

To investigate the role of GABA A receptor in nikethamide-induced respiratory enhancement in the medullary slices of neonatal rats.

METHODS

Ex vivo medullary slices of neonatal rats (1 to 3 days old) containing the medial region of the nucleus retrofacialis with the hypoglossal nerve rootlets were prepared and perfused with modified Kreb's solution to record respiration-related rhythmic discharge activity (RRDA) from the hypoglossal nerve rootlets using suction electrodes. Thirty RRDA-positive slices were randomized into 5 equal groups and perfused with nikethamide (at concentrations of 0.5, 1, 3, 5, 7, and 10 microg/ml with the optimal nikethamide concentration determined), GABA (at 10, 20, 40, and 60 micromol/ to determine the optimal concentration), 10 micromol/ bicuculline, 10 micromol/ bicuculline plus 40 micromol/L GABA, and 5 microg/ml nikethamide followed by 5 microg/ml nikethamide plus 10 micromol/ bicuculline after wash out, respectively.

RESULTS

Nikethamide increased RRDA at the concentrations of 0.5-7 microg/ml, and 5 microg/ml nikethamide showed the most distinct effect on the inspiratory time (TI), integral amplitude (IA), and respiratory cycle (RC). GABA at 40 micromol/ showed the most effective inhibition of RRDA in terms of TI, IA, and RC. Bicuculline at 10 micromol/ could increase the IA, TI and RC, but the combination of 10 micromol/ bicuculline and 40 micromol/ GABA had no significant effects on RRDA. Compared with nikethamide used alone, nikethamide plus bicuculline significantly increased TI and IA without affecting RC.

CONCLUSION

Nikethamide can enhance RRDA of the hypoglossal nerve rootlets in the medullary slices of neonatal rats, and the effect can be partially mediated by the GABA A receptor.

Effects of microinjections of glutamate and glutamate receptor antagonists into A5 zone on generation of respiratory rhythm in ponto-bulbospinal preparations from newborn rats in vitro

Activation of glutamate receptors in A5 neurons enhanced their tonic inhibitory influence on the respiratory rhythm generator in isolated newborn rat ponto-bulbospinal preparations. The stimulating effect of glutamate on A5 neurons is determined by its effect mainly on non-NMDA receptors and less so on NMDA receptors.

Respiratory Alterations Due to Chronic Long-Term Intermittent Hypobaric Hypoxia in Rabbits: Importance of Peripheral Chemoreceptors

BACKGROUND

None of the studies carried out so far investigated the effect of denervation of peripheral chemoreceptors on basal ventilation and respiratory responses to acute hypoxia in subjects exposed to chronic long-term intermittent hypobaric hypoxia (CLTIHH). We aimed to research (i) the effect of CLTIHH (430 mmHg, 5 h/day, 5 days/week, 5 weeks) on basal ventilation and respiratory responses to hypoxia and (ii) the effects of CLTIHH on central respiratory mechanisms after peripheral chemodenervation.

METHODS

Sixteen adult albino rabbits were divided into two groups: CLTIHH (n = 8) and control (n = 8). The tidal volume (V(T)) and respiratory frequency (f/min) were initially recorded in both groups and respiratory minute volume (V(E)) was calculated. PaO(2), PaCO(2), and pHa values were determined.

RESULTS

The initial values of f/min and V(E) in CLTIHH group were significantly higher than that of control group. After exposure to hypoxic gas mixture (8% O(2)-92% N(2)), the elevations in f/min, V(T), and V(E) in CLTIHH group were significantly higher than those of control group. After denervation of peripheral chemoreceptors, the decrease in V(E) in CLTIHH group was found to be significantly less than that of control group. When the animals in control group were allowed to breathe hypoxic gas mixture, f/min, V(T,) and V(E) decreased significantly and hypoxic depression was obtained. In contrast, hypoxic depression did not occur in the CLTIHH group.

CONCLUSIONS

Our results suggested that CLTIHH increases the basal ventilation and hypoxic respiratory responses and that enhanced ventilatory responses were due not only to the augmentation of peripheral chemoreceptor activity but also to the augmentation of central respiratory activity.

Transgenic expression of fluorescent proteins in respiratory neurons

We screened transgenic mouse lines with Thy1.2 promoter-induced expression of fluorescent proteins (FPs) for targeting of respiratory neuronal populations in the medulla oblongata. Respiratory neurons were found to be tagged by FPs within the ventral respiratory column (VRC), the pre-Bötzinger complex (preBötC) and the rostral ventral respiratory group (rVRG) interneurons. A subset of neurons in the preBötC, labeled with the enhanced yellow fluorescent protein (EYFP), showed inspiratory activity during whole cell recordings from rhythmic slice preparations. Additionally, a subpopulation of EYFP-labeled preBötC neurons expressed both NK1- and mu-opioid receptors. Furthermore, the spinal trigeminal nucleus, the lateral reticular nucleus (LRT) and the hypoglossal nucleus demonstrated intense EYFP expression whereas other regions of the medulla were devoid of neuronal EYFP labeling (e.g. the nucleus ambiguous). In conclusion, Thy1.2-FP transgenic mice will facilitate the functional analysis of respiratory-related neurons in the medulla and improve the three dimensional analysis of cells contributing to this important neuronal circuit.

Similarities of the neuronal circuit for the induction of fictive vomiting between ferrets and dogs

Previous studies suggested that the following neuronal circuit participates in the induction of vomiting by afferent vagal stimulation in decerebrated paralyzed dogs: (1) afferent fibers of the vagus nerve, (2) neurons of the solitary nucleus (NTS), (3) neurons of the prodromal sign center near the semicompact part of the nucleus ambiguus (scAMB), (4) neurons of the central pattern generator in the reticular area adjacent to the compact part of nucleus ambiguus (cAMB), (5) respiratory premotor neurons in the caudal medulla, (6) motor neurons of the diaphragm and abdominal muscles. However, the commonality of this neuronal circuit in different species has not yet been clarified. Thus, this study was conducted to clarify this point. This study clarified for the first time that fictive vomiting in decerebrated paralyzed ferrets could be induced by vagal stimulation, and could be identified by centrifugal activity patterns of the phrenic and abdominal muscle nerves. The distributions of c-Fos immunoreactive neurons in the NTS, scAMB and cAMB areas in ferrets that exhibited fictive vomiting were denser than those in ferrets that did not. Application of the nonNMDA receptor antagonist into the 4th ventricle produced the reversible suppression of fictive vomiting. The NK1 receptor immunoreactive puncta were found in the reticular area adjacent to the scAMB. Microinjections of NK1 receptor antagonist into the reticular areas on both sides abolished fictive vomiting. All these results in the ferrets are identical with results previously obtained in dogs and cats. Therefore, this suggests that the above neuronal circuit commonly participates in the induction of emesis in these animal species.

Defective interaction between dual oscillators for respiratory rhythm generation in Na+,K+-ATPase {alpha}2 subunit-deficient mice

The current concept regarding the respiratory centre in mammals is that it is composed of two distinct rhythm-generating neuronal networks in the ventrolateral medulla. These two rhythm generators can be active independently but are normally coupled in newborn and juvenile rats. Detailed characteristics of each generator and the neuronal mechanisms of coupling during development remain to be elucidated. Here, we report a knockout mouse (Na(+),K(+)-ATPase alpha2 subunit gene (Atp1a2) knockout) that may be defective in functional coupling between the two respiration-related rhythm generators. We investigated respiration-related neuron activity in an en bloc brainstem-spinal cord preparation isolated from embryonic day 18.5 Atp1a2(-/)(-) mouse fetuses. In the presence of adrenaline, two different types of rhythm generators were identified. One produced inspiratory burst activity that correlated with C4 inspiratory activity and was thought to be the inspiratory rhythm generator on the basis of its location and sensitivity to a mu-opiate receptor agonist, [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO). The other was presumed to be the preinspiratory rhythm generator because it was insensitive to DAMGO and correlated with facial nerve activity. Coupling between these rhythm generators did not function in the normal manner in Atp1a2(-/)(-) mice, as shown by disruption of the linkage between the preinspiratory burst and the inspiratory burst. Coupling was partially restored by repeated activation of the neurons within the networks, suggesting the involvement of an activity-dependent process in the prenatal development of this coupling.

Development of vagal afferent projections circumflex to the obex in the embryonic chick brainstem visualized with voltage-sensitive dye recording

Using voltage-sensitive dye recording, we surveyed neural responses related to the vagus nerve in the embryonic chick brainstem. In our previous studies, we identified four vagus nerve-related response areas in the brainstem. On the stimulated side, they included (1) the nucleus of the tractus solitarius (NTS: the primary sensory nucleus) and (2) the dorsal motor nucleus of the vagus nerve (DMNV), whereas on the contralateral side, they corresponded to (3) the parabrachial nucleus (PBN: the second/higher-ordered nucleus) and (4) the medullary non-NTS region. In the present study, in addition to these areas, we identified another response area circumflex to the obex. The intensity of the optical signal in the response area was much smaller than that in the NTS/DMNV, and the spatio-temporal pattern could be discerned after signal averaging. The conduction rate to the response area was slower than that to the other four areas. Ontogenetically, the response area was distributed on the stimulated side at the 6-day embryonic stage, and it spread into the contralateral side in 7- and 8-day embryonic stages. These distribution patterns were consistent with projection patterns of vagal afferent fibers stained with a fluorescent tracer, suggesting that the response area included a primary sensory nucleus. In comparison with the functional development of the other four response areas, we traced the functional organization of vagus nerve-related nuclei in the embryonic brainstem.

Respiratory rhythms generated in the lamprey rhombencephalon

Brainstem networks generating the respiratory rhythm in lampreys are still not fully characterized. In this study, we described the patterns of respiratory activities and we identified the general location of underlying neural networks. In a semi-intact preparation including the brain and gills, rhythmic discharges were recorded bilaterally with surface electrodes placed over the vagal motoneurons. The main respiratory output driving rhythmic gill movements consisted of short bursts (40.9+/-15.6 ms) of discharge occurring at a frequency of 1.0+/-0.3 Hz. This fast pattern was interrupted by long bursts (506.3+/-174.6 ms) recurring with an average period of 37.4+/-24.9 s. After isolating the brainstem by cutting all cranial nerves, the frequency of the short respiratory bursts did not change significantly, but the slow pattern was less frequent. Local injections of a glutamate agonist (AMPA) and antagonists (6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or D,L-amino-5-phosphonopentanoic acid (AP5)) were made over different brainstem regions to influence respiratory output. The results were similar in the semi-intact and isolated-brainstem preparations. Unilateral injection of AP5 or CNQX over a rostral rhombencephalic region, lateral to the rostral pole of the trigeminal motor nucleus, decreased the frequency of the fast respiratory rhythm bilaterally or stopped it altogether. Injection of AMPA at the same site increased the rate of the fast respiratory rhythm and decreased the frequency of the slow pattern. The activity recorded in this area was synchronous with that recorded over the vagal motoneurons. After a complete transverse lesion of the brainstem caudal to the trigeminal motor nucleus, the fast rhythm was confined to the rostral area, while only the slow activity persisted in the vagal motoneurons. Our results support the hypothesis that normal breathing depends on the activity of neurons located in the rostral rhombencephalon in lampreys, whereas the caudal rhombencephalon generates the slow pattern.

Multifunctional laryngeal premotor neurons: their activities during breathing, coughing, sneezing, and swallowing

To examine whether motor commands of two or more distinct laryngeal motor patterns converge onto a common premotor network, we conducted dual recordings from the laryngeal adductor motoneuron and its premotor neuron within the brainstem respiratory circuitry during fictive breathing, coughing, sneezing, and swallowing in decerebrate paralyzed cats. Expiratory neurons with an augmenting firing pattern (EAUG), whose action potentials evoked monosynaptic IPSPs in the adductor motoneurons, sharply fired during the expulsive phases of fictive coughing and sneezing, during which the adductor motoneurons transiently repolarized. In contrast, these premotor neurons were silent during the swallow-related hyperpolarization in adductor motoneurons. These results show that one class of medullary respiratory neuron, EAUG, is multifunctional and shared among the central pattern generators (CPGs) for breathing, coughing, and sneezing. In addition, although the CPGs underlying these three behaviors and the swallowing CPG do overlap, EAUG neurons are not part of the swallowing CPG and, in contrast to the other three behaviors, are not a source of inhibitory input to adductor motoneurons during swallowing.

Activation of Orexin B receptors in the pontine Kölliker-Fuse nucleus modulates pre-inspiratory hypoglossal motor activity in rat

Orexins (splice variants A and B) are hypothalamic neuropeptides that have essential functions in control of arousal and nutrition. Lack of Orexins is strongly associated with narcolepsy and sleep disordered breathing. However, the role of Orexins and particularly that of Orexin-B (OXB), in respiratory centres controlling upper-airway patency are less defined. In the present study we performed microinjections of OXB into the pontine Kölliker-Fuse nucleus (KF) of the dorsolateral pons, since this nucleus is particularly involved in the pre-motor control of upper airway muscles. The OXB mediated effects on heart, phrenic (PNA) and hypoglossal (XII-A) nerve activities were analysed in an in situ perfused brainstem preparation. Injection of OXB into the KF evoked significant augmentation of the respiratory frequency. Importantly, OXB provoked particularly prolonged pre-inspiratory discharge of the XII nerve, while no cardiovascular response was observed after KF microinjections. In summary, OXB in the KF exerts an excitatory effect on XII pre-motoneurones. Since pre-inspiratory activity of the XII is important for the decrease in upper airway resistance during inspiration, we conclude that OXB release in the KF has strong implications in the state-dependent control of upper airway patency under physiological and pathophysiological conditions.

Acute role of the brain-derived neurotrophic factor (BDNF) on the respiratory neural network activity in mice in vitro

In humans, several pathologies are associated with disturbances of the respiratory control, some of them including alteration in the brain-derived neurotrophic factor (BDNF) signalling pathway. BDNF has long been known as a neurotrophic factor involved in survival, differentiation and maintenance of neuronal populations in the peripheral and central nervous system. More recently BDNF has also been discovered to be a potent neuromodulator with acute effects on neuronal excitability and synaptic plasticity. Animals deleted for the gene encoding BDNF exhibit respiratory alteration suggesting an important but yet undefined role of the neurotrophin in respiratory rhythmogenesis either by a trophic and/or an acute action. The possibility that BDNF might exert an acute regulatory role on the rhythmic activity of the respiratory generator of the pre-Bötzinger complex has been recently examined in newborn mice in vitro. Results obtained, reviewed in the present paper, will help getting insights in respiratory rhythm regulatory mechanisms that involve BDNF signalling.

The pre-Bötzinger oscillator in the mouse embryo

Studies of the sites and mechanisms involved in mammalian respiratory rhythm generation point to two clusters of rhythmic neurons forming a coupled oscillator network within the brainstem. The location of these oscillators, the pre-Bötzinger complex (preBötC) at vagal level, and the para-facial respiratory group at facial level, probably result from regional patterning schemes specifying neural types in the hindbrain during embryogenesis. Here, we report evidence that the preBötC oscillator (i) is first active at embryonic stages, (ii) originates in the post-otic hindbrain neural tube and (iii) requires the glutamate vesicular transporter 2 for rhythm generation.

Are pacemaker properties required for respiratory rhythm generation in adult turtle brain stems in vitro?

The role of pacemaker properties in vertebrate respiratory rhythm generation is not well understood. To address this question from a comparative perspective, brain stems from adult turtles were isolated in vitro, and respiratory motor bursts were recorded on hypoglossal (XII) nerve rootlets. The goal was to test whether burst frequency could be altered by conditions known to alter respiratory pacemaker neuron activity in mammals (e.g., increased bath KCl or blockade of specific inward currents). While bathed in artificial cerebrospinal fluid (aCSF), respiratory burst frequency was not correlated with changes in bath KCl (0.5-10.0 mM). Riluzole (50 microM; persistent Na(+) channel blocker) increased burst frequency by 31 +/- 5% (P < 0.05) and decreased burst amplitude by 42 +/- 4% (P < 0.05). In contrast, flufenamic acid (FFA, 20-500 microM; Ca(2+)-activated cation channel blocker) reduced and abolished burst frequency in a dose- and time-dependent manner (P < 0.05). During synaptic inhibition blockade with bicuculline (50 microM; GABA(A) channel blocker) and strychnine (50 muM; glycine receptor blocker), rhythmic motor activity persisted, and burst frequency was directly correlated with extracellular KCl (0.5-10.0 mM; P = 0.005). During synaptic inhibition blockade, riluzole (50 microM) did not alter burst frequency, whereas FFA (100 microM) abolished burst frequency (P < 0.05). These data are most consistent with the hypothesis that turtle respiratory rhythm generation requires Ca(2+)-activated cation channels but not pacemaker neurons, which thereby favors the group-pacemaker model. During synaptic inhibition blockade, however, the rhythm generator appears to be transformed into a pacemaker-driven network that requires Ca(2+)-activated cation channels.

Influence of acute inspiratory loading upon diaphragm motor-evoked potentials in healthy humans

Acute prior activity of the inspiratory muscles can enhance inspiratory muscle strength and reduce effort perception during subsequent inspiratory efforts. However, the mechanisms subserving these changes are poorly understood. Responses to magnetic stimulation in 10 subjects were studied after an acute bout of nonfatiguing inspiratory muscle loading (IML), corresponding to 40% of subjects’ initial maximal inspiratory pressure (MIP), and after an acute bout of nonloaded, forced inspiration (NLF). Motor-evoked potentials elicited by cortical stimulation (MEPc) and by phrenic nerve stimulation (MEPp) were recorded transcutaneously from the diaphragm before, immediately after, and 15 min after two sets of 30 inspiratory efforts, at rest and during an MIP effort. After IML, MIP increased to 113 ± 3% (SE) of baseline and diaphragm MEPp (during MIP) significantly increased (129 ± 10% of baseline). Diaphragmatic MEPc (during MIP), expressed as a percentage of maximal MEPp, decreased after IML (from 29 ± 9% to 20 ± 6%; P = 0.017) and after NLF (from 43 ± 5% to 31 ± 5%; P = 0.032). Observations from the biceps brachi demonstrated that changes after IML and NLF were specific to the inspiratory muscle, since no significant changes were observed in biceps force generation or in MEPp or MEPc amplitudes. These data indicate that after IML increased global inspiratory strength is accompanied by increased peripheral excitability and by a dampening of corticospinal excitability of the diaphragm.

[Regulation of respiration: visceral and behavioural components]

The review segregates two aspects of respiration regulation: autonomous respiration regulation as a visceral function ensuring metabolic needs of a body by maintaining stability of own respiratory environment, on the one hand, and behavioural regulation of respiration under control of the volitional sphere, on the other hand. The authors focus on respiratory rythmogenesis, the problem that has not yet been resolved, and on the mechanism of precise correlation of lung ventillation with the metabolic level, in case of muscular exercise, in particular. The authors discuss interaction of visceral and behavioural mechanisms of respiratory regulation. The substance of the phenomenon of respiratory embarrassment is considered in this connection as a visceral signal addressed to the behavioural sphere. Reasonableness of introduction of a new breathing system in a healthy person is doubted. The article justifies the pracice of bioregulation of the respiratory function.

The respiratory drive to thoracic motoneurones in the cat and its relation to the connections from expiratory bulbospinal neurones

The descending control of respiratory-related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike-triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5-T9 of the spinal cord of anaesthetized cats. Spike-triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 microV. The overall strength of the connection supports previous measurements made by cross-correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike-triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

[Role of 5-HT2A-receptors coupled with superoxide anion in the medial area of nucleus retrofacialis]

OBJECTIVE

To explore the effect of 5-HT2A-receptors coupled with superoxide anion (O2-) on respiratory regulation signal transductionin passageway in the medial area of nucleus retrofacialis (mNRF).

METHODS

mNRF island was prepared from medullary slices of neonatal SD rats according to Johnson's method and transferred separately into 24-well culture plates with reagents according to protocol, followed by incubation for 60 min at 37 degrees C in a humidified incubator with 5% CO2. Absorbance of 100 microl supernatant was measured by spectrophotometry at 550 nm and the effect of 5-HT and 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI, agonist of 5-HT2A-receptors) on O2- generation in the mNRF was observed, along with the inhibition of this effect by ketanserin (antagonist of 5-HT2A-receptors) and alpha-lipoic acid (alpha-LA, a antioxidant).

RESULTS

5-HT concentration-response curve demonstrated that absorbance peak occurred at 1 micromol/L without further increment with higher concentration. DOI concentration-response curve showed the absorbance peak at 20 micromol/L without further increment. 5-HT and DOI significantly increased the absorbance with comparable effects. Ketanserin and alpha-LA significantly decreased the absorbance generated by 5-HT and DOI.

CONCLUSION

Activation of 5-HT2A receptors results in obvious O2- production in mNRF, suggesting that 5-HT2A receptors regulate respiratory function in association with O2-.

Effects of riluzole and flufenamic acid on eupnea and gasping of neonatal mice in vivo

The pre-Bötzinger complex (PBC), part of the ventral respiratory group that is responsible for inspiratory rhythm generation, contains at least two types of pacemaker neurons. In vitro studies have shown that bursting properties of one type of pacemaker relies on a riluzole-sensitive persistent sodium current, whereas bursting of a second type is sensitive to flufenamic acid (FFA), a calcium-dependent nonspecific cationic current blocker. In vitro, under control conditions, the PBC generates fictive eupneic activity that depends on both riluzole-sensitive and FFA-sensitive pacemaker neurons. During hypoxia the PBC generates fictive gasping activity and only riluzole-sensitive pacemaker neurons appear to be necessary for this rhythm. We carried out pharmacological experiments to test the role of respiratory pacemaker neurons in vivo by performing plethysmographic recordings on neonate mice. As reported in vitro, eupnea activity in vivo is abolished only if both FFA and riluzole are coadministered intracisternally, but not when either of them is administered independently. On the other hand riluzole, but not FFA, drastically reduced gasping generation and compromised the ability of mice to autoresucitate. Neither substance P nor forskolin was able to reestablish respiratory activity after riluzole and FFA coapplication. Our results confirm in vitro reports and suggest that eupnea generation in neonates requires a complex neuronal network that includes riluzole- and FFA-sensitive elements and that gasping activity depends mostly on a riluzole-sensitive mechanism.

[Development of respiratory function in perinatal ontogenesis]

In development of respiratory function in rats, mice, and other representatives of placental animals there exists the general plan of formation of rhythm: from single contraction of respiratory musculature to formation of bursts and complexes alternating periodically with pauses and apnea intervals and subsequent rhythm stabilization. These peculiarities are closely connected with the states of sleep and consciousness. A concept is put forward about a certain sequence of functional maturation and ways of regulation of activity of the respiratory rhythm central pacemaker. At the first stage the autogenic rhythmical activity is determined by pacemaker properties of a part of neurons of the medulla rostral ventrolateral part. It is not ruled out that the first respiratory discharges in spinal cord ventral roots might have been a manifestation of the nervous network rhythmogenic properties. The direct sensitivity of central neurons to chemical composition if the medium and to some neutomodulators serves as the first regulatory mechanism. Somewhat later, inhibitory control is established from supramedullary structures, with an increase of role of peripheral receptors in regulation of respiration.

Pre-/post-otic rhombomeric interactions control the emergence of a fetal-like respiratory rhythm in the mouse embryo

How regional patterning of the neural tube in vertebrate embryos may influence the emergence and the function of neural networks remains elusive. We have begun to address this issue in the embryonic mouse hindbrain by studying rhythmogenic properties of different neural tube segments. We have isolated pre- and post-otic hindbrain segments and spinal segments of the mouse neural tube, when they form at embryonic day (E) 9, and grafted them into the same positions in stage-matched chick hosts. Three days after grafting, in vitro recordings of the activity in the cranial nerves exiting the grafts indicate that a high frequency (HF) rhythm (order: 10 bursts/min) is generated in post-otic segments while more anterior pre-otic and more posterior spinal territories generate a low frequency (LF) rhythm (order: 1 burst/min). Comparison with homo-specific grafting of corresponding chick segments points to conservation in mouse and chick of the link between the patterning of activities and the axial origin of the hindbrain segment. This HF rhythm is reminiscent of the respiratory rhythm known to appear at E15 in mice. We also report on pre-/post-otic interactions. The pre-otic rhombomere 5 prevents the emergence of the HF rhythm at E12. Although the nature of the interaction with r5 remains obscure, we propose that ontogeny of fetal-like respiratory circuits relies on: (i) a selective developmental program enforcing HF rhythm generation, already set at E9 in post-otic segments, and (ii) trans-segmental interactions with pre-otic territories that may control the time when this rhythm appears.

Cluster breathing associated with bihemispheric infarction and sparing of the brainstem

OBJECTIVE

To report cluster breathing pattern associated with a nonbrainstem lesion.

DESIGN

Case report.

SETTING

Neurointensive care unit, St Mary's Hospital, Rochester, Minn.

PATIENT

A patient with subarachnoid hemorrhage developed severe, diffuse, distal bilateral middle cerebral artery vasospasm with resultant cortical laminar necrosis and transient cluster breathing. Intervention Magnetic resonance imaging revealed bihemispheric lesions but no brainstem lesion.

CONCLUSION

Cluster breathing may occur with nonbrainstem lesions.

The airway-related parasympathetic motoneurones in the ventrolateral medulla of newborn rats were dissociated anatomically and in functional control

The respiratory-related synaptic control of the airway-related preganglionic parasympathetic motoneurones (APPMs) has not been investigated, and whether differently targeted APPMs receive differential respiratory-related synaptic modulation is unknown. In this study, putative APPMs in the ventrolateral medulla of newborn rats were retrogradely traced with fluorescent tracer and were examined using the patch-clamp method in brainstem slices with respiratory rhythm. The results indicate that tracer application directly to the recurrent laryngeal nerve only labelled the putative APPMs within the compact portion of nucleus ambiguus (cNA), while tracer injection into the trachea wall labelled the putative APPMs both in cNA and in the area ventral/ventrolateral to cNA (vNA). The putative APPMs within cNA received mainly inhibitory inputs, which in some (9 of 20) neurones showed an inspiratory-related attenuation and in others (7 of 20) showed an inspiratory-related augmentation. At least some putative APPMs within cNA, of which the inhibitory synaptic inputs showed inspiratory-related changes, might be related to the control of laryngeal muscles. The putative APPMs in vNA receive both excitatory and inhibitory inputs, and central inspiratory activity excited some (11 of 19) neurones via augmentation of their excitatory inputs and inhibited others (8 of 19) via augmentation of their inhibitory inputs. At least some putative APPMs in vNA might be trachea-related motoneurones. These results provide evidence that APPMs controlling different segments of the airway might be dissociated in the ventrolateral medulla both anatomically and in functional control.

Ontogeny of central rhythm generation in chicks and rodents

Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior-posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20(-/-), Hoxa1(-/-) and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.

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

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

[Effects of pentobarbital sodium on rhythmical respiration of neonatal rat medullary preparations]

OBJECTIVE

To study the effects of pentobarbital sodium in generation and modulation of rhythmical respiration in neonatal rats.

METHODS

The effects of pentobarbital sodium were examined on hypoglossal nerve (XII) rootlets and inspiratory neurons in the medullary preparations including the medial region of the nucleus retrofacialis, pre-Bötzinger complex and the dorsal respiratory group of neonatal rats aged 0-3 days. The electrical activity of XII nerve rootlets and inspiratory neurons were recorded. Different doses of pentobarbital sodium (20, 40, 60, 80 micromol/L) were added into modified Krebs solution to observe changes in the discharge activity of XII nerve and inspiratory neurons. Bicuculline was used to further investigate the mechanisms that pentobarbital sodium suppresses respiration.

RESULTS

The discharge activity inhibition of XII nerve was increased as pentobarbital sodium doses increased from 20 to 60 micromol/L, but no significant difference was observed between the doses of 60 and 80 micromol/L. Bicuculline can partly restore the rhythmical respiration discharge activity.

CONCLUSION

Pentobarbital sodium can suppress respiration partly via GABAA receptors.

Association of nicotinic acetylcholine receptors with central respiratory control in isolated brainstem-spinal cord preparation of neonatal rats

Nicotine exposure is a risk factor in several breathing disorders Nicotinic acetylcholine receptors (nAChRs) exist in the ventrolateral medulla, an important site for respiratory control. We examined the effects of nicotinic acetylcholine neurotransmission on central respiratory control by addition of a nAChR agonist or one of various antagonists into superfusion medium in the isolated brainstem-spinal cord from neonatal rats. Ventral C4 neuronal activity was monitored as central respiratory output, and activities of respiratory neurons in the ventrolateral medulla were recorded in whole-cell configuration. RJR-2403 (0.1-10 mM), alpha4beta2 nAChR agonist induced dose-dependent increases in respiratory frequency. Non-selective nAChR antagonist mecamylamine (0.1-100 mM), alpha4beta2 antagonist dihydro-beta-erythroidine (0.1-100 mM), alpha7 antagonist methyllycaconitine (0.1-100 mM), and a-bungarotoxin (0.01-10 mM) all induced dose-dependent reductions in C4 respiratory rate. We next examined effects of 20 mM dihydro-beta-erythroidine and 20mM methyllycaconitine on respiratory neurons. Dihydro-beta-erythroidine induces hyperpolarization and decreases intraburst firing frequency of inspiratory and preinspiratory neurons. In contrast, methyllycaconitine has no effect on the membrane potential of inspiratory neurons, but does decrease their intraburst firing frequency while inducing hyperpolarization and decreasing intraburst firing frequency in preinspiratory neurons. These findings indicate that alpha4beta2 nAChR is involved in both inspiratory and preinspiratory neurons, whereas alpha7 nAChR functions only in preinspiratory neurons to modulate C4 respiratory rate.

Purinergic P2 receptors modulate excitability but do not mediate pH sensitivity of RTN respiratory chemoreceptors

The cellular mechanism(s) by which the brain senses changes in pH to regulate breathing (i.e., central chemoreception) have remained incompletely understood, in large part because the central respiratory chemoreceptors have themselves eluded detection. Here, we recorded from a newly identified population of central chemoreceptors located in the retrotrapezoid nucleus (RTN) on the ventral surface of the brainstem to test a recently proposed role for purinergic P2 receptor signaling in central respiratory chemoreception (Gourine et al., 2005). Using loose-patch current-clamp recordings in brainstem slices from rat pups (postnatal day 7-12), we indeed show purinergic modulation of pH-sensitive RTN neurons: activation of P2X receptors indirectly inhibited RTN firing by increasing inhibitory input, whereas P2Y receptor stimulation caused direct excitation of RTN chemoreceptors. However, after blocking P2 receptors with the broad-spectrum antagonists PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate) or RB2 (reactive blue 2), the pH sensitivity of RTN neurons remained intact. Therefore, we conclude that purinergic signaling can modulate RTN neuron activity but does not mediate the pH sensing intrinsic to these central respiratory chemoreceptors.