Diencephalic and mesencephalic influences on ponto-medullary respiratory control in normoxic and hypoxic conditions: an in vitro study on central nervous system preparations from newborn rat

Acid Base Disorders in Cirrhosis

Metabolic and respiratory acid-base disorders are common in individuals with liver disease and cirrhosis. The most common disorder is respiratory alkalosis, which may be related to dyspnea or respiratory stimulation. Primary metabolic disorders are less common. Although the liver plays a role in metabolism of amino acids and generation of acid from dietary sources, it does not play a role in the regulation of pH. Instead, metabolic disorders may arise from alterations in normal metabolism or from medications, particularly diuretics and osmotic laxatives, used in the treatment of these complex patients. Understanding the mechanistic underpinnings of these disorders can aid in the management of individuals with liver disease in the hospital and in outpatient settings.

Passive limb training modulates respiratory rhythmic bursts

Exercise modifies respiratory functions mainly through the afferent feedback provided by exercising limbs and the descending input from suprapontine areas, two contributions that are still underestimated in vitro. To better characterize the role of limb afferents in modulating respiration during physical activity, we designed a novel experimental in vitro platform. The whole central nervous system was isolated from neonatal rodents and kept with hindlimbs attached to an ad-hoc robot (Bipedal Induced Kinetic Exercise, BIKE) driving passive pedaling at calibrated speeds. This setting allowed extracellular recordings of a stable spontaneous respiratory rhythm for more than 4 h, from all cervical ventral roots. BIKE reversibly reduced the duration of single respiratory bursts even at lower pedaling speeds (2 Hz), though only an intense exercise (3.5 Hz) modulated the frequency of breathing. Moreover, brief sessions (5 min) of BIKE at 3.5 Hz augmented the respiratory rate of preparations with slow bursting in control (slower breathers) but did not change the speed of faster breathers. When spontaneous breathing was accelerated by high concentrations of potassium, BIKE reduced bursting frequency. Regardless of the baseline respiratory rhythm, BIKE at 3.5 Hz always decreased duration of single bursts. Surgical ablation of suprapontine structures completely prevented modulation of breathing after intense training. Albeit the variability in baseline breathing rates, intense passive cyclic movement tuned fictive respiration toward a common frequency range and shortened all respiratory events through the involvement of suprapontine areas. These observations contribute to better define how the respiratory system integrates sensory input from moving limbs during development, opening new rehabilitation perspectives.

Suprapontine Structures Modulate Brainstem and Spinal Networks

Several spinal motor output and essential rhythmic behaviors are controlled by supraspinal structures, although their contribution to neuronal networks for respiration and locomotion at birth still requires better characterization. As preparations of isolated brainstem and spinal networks only focus on local circuitry, we introduced the in vitro central nervous system (CNS) from neonatal rodents to simultaneously record a stable respiratory rhythm from both cervical and lumbar ventral roots (VRs).Electrical pulses supplied to multiple sites of brainstem evoked distinct VR responses with staggered onset in the rostro-caudal direction. Stimulation of ventrolateral medulla (VLM) resulted in higher events from homolateral VRs. Stimulating a lumbar dorsal root (DR) elicited responses even from cervical VRs, albeit small and delayed, confirming functional ascending pathways. Oximetric assessments detected optimal oxygen levels on brainstem and cortical surfaces, and histological analysis of internal brain structures indicated preserved neuron viability without astrogliosis. Serial ablations showed precollicular decerebration reducing respiratory burst duration and frequency and diminishing the area of lumbar DR and VR potentials elicited by DR stimulation, while pontobulbar transection increased the frequency and duration of respiratory bursts. Keeping legs attached allows for expressing a respiratory rhythm during hindlimb stimulation. Trains of pulses evoked episodes of fictive locomotion (FL) when delivered to VLM or to a DR, the latter with a slightly better FL than in isolated cords.In summary, suprapontine centers regulate spontaneous respiratory rhythms, as well as electrically evoked reflexes and spinal network activity. The current approach contributes to clarifying modulatory brain influences on the brainstem and spinal microcircuits during development. Novel preparation of the entire isolated CNS from newborn rats unveils suprapontine modulation on brainstem and spinal networks. Preparation views (A) with and without legs attached (B). Successful fictive respiration occurs with fast dissection from P0-P2 rats (C). Decerebration speeds up respiratory rhythm (D) and reduces spinal reflexes derived from both ventral and dorsal lumbar roots (E).

Serotonin and the ventilatory effects of etonogestrel, a gonane progestin, in a murine model of congenital central hypoventilation syndrome

INTRODUCTION

Congenital Central Hypoventilation Syndrome, a rare disease caused by PHOX2B mutation, is associated with absent or blunted CO₂/H⁺ chemosensitivity due to the dysfunction of PHOX2B neurons of the retrotrapezoid nucleus. No pharmacological treatment is available. Clinical observations have reported non-systematic CO₂/H⁺ chemosensitivity recovery under desogestrel.

METHODS

Here, we used a preclinical model of Congenital Central Hypoventilation Syndrome, the retrotrapezoid nucleus conditional Phox2b mutant mouse, to investigate whether etonogestrel, the active metabolite of desogestrel, led to a restoration of chemosensitivity by acting on serotonin neurons known to be sensitive to etonogestrel, or retrotrapezoid nucleus PHOX2B residual cells that persist despite the mutation. The influence of etonogestrel on respiratory variables under hypercapnia was investigated using whole-body plethysmographic recording. The effect of etonogestrel, alone or combined with serotonin drugs, on the respiratory rhythm of medullary-spinal cord preparations from Phox2b mutants and wildtype mice was analyzed under metabolic acidosis. c-FOS, serotonin and PHOX2B were immunodetected. Serotonin metabolic pathways were characterized in the medulla oblongata by ultra-high-performance liquid chromatography.

RESULTS

We observed etonogestrel restored chemosensitivity in Phox2b mutants in a non-systematic way. Histological differences between Phox2b mutants with restored chemosensitivity and Phox2b mutant without restored chemosensitivity indicated greater activation of serotonin neurons of the raphe obscurus nucleus but no effect on retrotrapezoid nucleus PHOX2B residual cells. Finally, the increase in serotonergic signaling by the fluoxetine application modulated the respiratory effect of etonogestrel differently between Phox2b mutant mice and their WT littermates or WT OF1 mice, a result which parallels with differences in the functional state of serotonergic metabolic pathways between these different mice.

DISCUSSION

Our work thus highlights that serotonin systems were critically important for the occurrence of an etonogestrel-restoration, an element to consider in potential therapeutic intervention in Congenital Central Hypoventilation Syndrome patients.

Orexin Neurons Contribute to Central Modulation of Respiratory Drive by Progestins on ex vivo Newborn Rodent Preparations

Dysfunction of central respiratory CO₂/H⁺ chemosensitivity is a pivotal factor that elicits deep hypoventilation in patients suffering from central hypoventilation syndromes. No pharmacological treatment is currently available. The progestin desogestrel has been suggested to allow recovery of respiratory response to CO₂/H⁺ in patients suffering from central hypoventilation, but except the fact that supramedullary regions may be involved, mechanisms are still unknown. Here, we tested in neonates whether orexin systems contribute to desogestrel’s central effects on respiratory function. Using isolated ex vivo central nervous system preparations from newborn rats, we show orexin and almorexant, an antagonist of orexin receptors, supressed strengthening of the increase in respiratory frequency induced by prolonged metabolic acidosis under exposure to etonogestrel, the active metabolite of desogestrel. In parallel, almorexant suppressed the increase and enhanced increase in c-fos expression in respiratory-related brainstem structures induced by etonogestrel. These results suggest orexin signalisation is a key component of acidosis reinforcement of respiratory drive by etonogestrel in neonates. Although stage of development used is different as that for progestin clinical observations, presents results provide clues about conditions under which desogestrel or etonogestrel may enhance ventilation in patients suffering from central hypoventilation syndromes.

The role of the hypothalamus in modulation of respiration

The hypothalamus is a higher center of the autonomic nervous system and maintains essential body homeostasis including respiration. The paraventricular nucleus, perifornical area, dorsomedial hypothalamus, and lateral and posterior hypothalamus are the primary nuclei of the hypothalamus critically involved in respiratory control. These hypothalamic nuclei are interconnected with respiratory nuclei located in the midbrain, pons, medulla and spinal cord. We provide an extensive review of the role of the above hypothalamic nuclei in the maintenance of basal ventilation, and modulation of respiration in hypoxic and hypercapnic conditions, during dynamic exercise, in awake and sleep states, and under stress. Dysfunction of the hypothalamus causes abnormal breathing and hypoventilation. However, the cellular and molecular mechanisms how the hypothalamus integrates and modulates autonomic and respiratory functions remain to be elucidated.

Current Perspectives for the use of Gonane Progesteronergic Drugs in the Treatment of Central Hypoventilation Syndromes

BACKGROUND

Central alveolar hypoventilation syndromes (CHS) encompass neurorespiratory diseases resulting from congenital or acquired neurological disorders. Hypercapnia, acidosis, and hypoxemia resulting from CHS negatively affect physiological functions and can be lifethreatening. To date, the absence of pharmacological treatment implies that the patients must receive assisted ventilation throughout their lives.

OBJECTIVE

To highlight the relevance of determining conditions in which using gonane synthetic progestins could be of potential clinical interest for the treatment of CHS.

METHODS

The mechanisms by which gonanes modulate the respiratory drive were put into the context of those established for natural progesterone and other synthetic progestins.

RESULTS

The clinical benefits of synthetic progestins to treat respiratory diseases are mixed with either positive outcomes or no improvement. A benefit for CHS patients has only recently been proposed. We incidentally observed restoration of CO2 chemosensitivity, the functional deficit of this disease, in two adult CHS women by desogestrel, a gonane progestin, used for contraception. This effect was not observed by another group, studying a single patient. These contradictory findings are probably due to the complex nature of the action of desogestrel on breathing and led us to carry out mechanistic studies in rodents. Our results show that desogestrel influences the respiratory command by modulating the GABAA and NMDA signaling in the respiratory network, medullary serotoninergic systems, and supramedullary areas.

CONCLUSION

Gonanes show promise for improving ventilation of CHS patients, although the conditions of their use need to be better understood.

The c-FOS Protein Immunohistological Detection: A Useful Tool As a Marker of Central Pathways Involved in Specific Physiological Responses In Vivo and Ex Vivo

Many studies seek to identify and map the brain regions involved in specific physiological regulations. The proto-oncogene c-fos, an immediate early gene, is expressed in neurons in response to various stimuli. The protein product can be readily detected with immunohistochemical techniques leading to the use of c-FOS detection to map groups of neurons that display changes in their activity. In this article, we focused on the identification of brainstem neuronal populations involved in the ventilatory adaptation to hypoxia or hypercapnia. Two approaches were described to identify involved neuronal populations in vivo in animals and ex vivo in deafferented brainstem preparations. In vivo, animals were exposed to hypercapnic or hypoxic gas mixtures. Ex vivo, deafferented preparations were superfused with hypoxic or hypercapnic artificial cerebrospinal fluid. In both cases, either control in vivo animals or ex vivo preparations were maintained under normoxic and normocapnic conditions. The comparison of these two approaches allows the determination of the origin of the neuronal activation i.e., peripheral and/or central. In vivo and ex vivo, brainstems were collected, fixed, and sliced into sections. Once sections were prepared, immunohistochemical detection of the c-FOS protein was made in order to identify the brainstem groups of cells activated by hypoxic or hypercapnic stimulations. Labeled cells were counted in brainstem respiratory structures. In comparison to the control condition, hypoxia or hypercapnia increased the number of c-FOS labeled cells in several specific brainstem sites that are thus constitutive of the neuronal pathways involved in the adaptation of the central respiratory drive.

Thalamic mediation of hypoxic respiratory depression in lambs

Immaturity of respiratory controllers in preterm infants dispose to recurrent apnea and oxygen deprivation. Accompanying reductions in brain oxygen tensions evoke respiratory depression, potentially exacerbating hypoxemia. Central respiratory depression during moderate hypoxia is revealed in the ventilatory decline following initial augmentation. This study determined whether the thalamic parafascicular nuclear (Pf) complex involved in adult nociception and sensorimotor regulation (Bentivoglio M, Balerecia G, Kruger L. Prog Brain Res 87: 53-80, 1991) also becomes a postnatal controller of hypoxic ventilatory decline. Respiratory responses to moderate isocapnic hypoxia were studied in conscious lambs. Hypoxic ventilatory decline was compared with peak augmentation. Pf and/or adjacent thalamic structures were destroyed by the neuron-specific toxin ibotenic acid (IB). IB lesions involving the thalamic Pf abolished hypoxic ventilatory decline. Lesions of adjacent thalamic nuclei that spared Pf and control injections of vehicle failed to blunt hypoxic respiratory depression. Our findings reveal that the thalamic Pf region is a critical controller of hypoxic ventilatory depression and thus a key target for exploring molecular concomitants of forebrain pathways regulating hypoxic ventilatory depression in early development.

The in vivo respiratory phenotype of the adenosine A1 receptor knockout mouse

The nucleoside adenosine has been implicated in the regulation of respiration, especially during hypoxia in the newborn. In this study the role of adenosine A1 receptors for the control of respiration was investigated in vivo. To this end, respiration of unrestrained adult and neonatal adenosine A1 receptor knockout mice (A1R(-/-)) was measured in a plethysmographic device. Under control conditions (21% O2) and mild hypoxia (12-15% O2) no difference of respiratory parameters was observed between adult wildtype (A1R(+/+)) and A1R(-/-) mice. Under more severe hypoxia (6-10% O2) A1R(+/+) mice showed, after a transient increase of respiration, a decrease of respiration frequency (fR) and tidal volume (VT) leading to a decrease of minute volume (MV). This depression of respiration during severe hypoxia was absent in A1R(-/-) mice which displayed a stimulated respiration as indicated by the enhancement of MV by some 50-60%. During hypercapnia-hyperoxia (3-10% CO2/97-90 % O2), no obvious differences in respiration of A1R(-/-) and A1R(+/+) was observed. In neonatal mice, the respiratory response to hypoxia was surprisingly similar in both genotypes. However, neonatal A1R(-/-) mice appeared to have more frequently periods of apnea during hypoxia and in the post-hypoxic control period. In conclusion, these data indicate that the adenosine A1 receptor is an important molecular component mediating hypoxic depression in adult mice and it appears to stabilize respiration of neonatal mice.

Electrophysiology on Isolated Brainstem-spinal Cord Preparations from Newborn Rodents Allows Neural Respiratory Network Output Recording

While it is well known that the central respiratory drive is located in the brainstem, several aspects of its basic function, development, and response to stimuli remain to be fully understood. To overcome the difficulty of accessing the brainstem in the whole animal, isolation of the brainstem and part of the spinal cord is performed. This preparation is maintained in artificial cerebro-spinal fluid where gases, concentrations, and temperature are controlled and monitored. The output signal from the respiratory network is recorded by a suction electrode placed on the fourth ventral root. In this manner, stimuli can be directly applied onto the brainstem, and the effect can be recorded directly. The signal recorded is linked to the inspiratory signal sent to the diaphragm via the phrenic nerve, and can be described as bursts (around 8 bursts per minute). Analysis of these bursts (frequency, amplitude, length, and area under the curve) allows precise characterization of the stimulus effect on the respiratory network. The main limitation of this method is the viability of the preparation beyond the early post-natal stages. Thus, this method greatly focuses on the study of the whole network without the peripheral inputs in the newborn rat.

The progestin etonogestrel enhances the respiratory response to metabolic acidosis in newborn rats. Evidence for a mechanism involving supramedullary structures

Central congenital hypoventilation syndrome is a neuro-respiratory disease characterized by the dysfunction of the CO2/H(+) chemosensitive neurons of the retrotrapezoid nucleus/parafacial respiratory group. A recovery of CO2/H(+) chemosensitivity has been observed in some central congenital hypoventilation syndrome patients coincidental with contraceptive treatment by a potent progestin, desogestrel (Straus et al., 2010). The mechanisms of this progestin effect remain unknown, although structures of medulla oblongata, midbrain or diencephalon are known to be targets for progesterone. In the present study, on ex vivo preparations of central nervous system of newborn rats, we show that acute exposure to etonogestrel (active metabolite of desogestrel) enhanced the increased respiratory frequency induced by metabolic acidosis via a mechanism involving supramedullary structures located in pontine, mesencephalic or diencephalic regions.

The kreisler mutation leads to the loss of intrinsically hypoxia-activated spots in the region of the retrotrapezoid nucleus/parafacial respiratory group

Acute hypoxia elicits a biphasic respiratory response characterized in the newborn by a transient hyperventilation followed by a severe decrease in respiratory drive known as hypoxic respiratory depression. Medullary O(2) chemosensitivity is known to contribute to respiratory depression induced by hypoxia, although precise involvement of cell populations remains to be determined. Having a thorough knowledge of these populations is of relevance because perturbations in the respiratory response to hypoxia may participate in respiratory diseases in newborns. We aimed to analyze the hypoxic response of ponto-medullary cell populations of kreisler mutant mice. These mice have defects in a gene expressed in two rhombomeres encompassing a part of the medulla oblongata implicated in hypoxic respiratory depression. Central responses to hypoxia were analyzed in newborn mice by measuring respiratory rhythm in ex vivo caudal pons-medullary-spinal cord preparations and c-fos expression in wild-type and kreisler mutants. The homozygous kreisler mutation, which eliminates most of rhombomere 5 and mis-specifies rhombomere 6, abolished (1) an early decrease in respiratory frequency within 10 min of hypoxia and (2) an intrinsic hypoxic activation, which is characterized by an increase in c-fos expression in the region of the ventral medullary surface encompassing the retrotrapezoid nucleus/parafacial respiratory group expressing Phox2b. This increase in c-fos expression persisted in wild-type Phox2b-negative and Phox2b-positive cells after blockade of synaptic transmission and rhythmogenesis by a low [Ca(2+)](0). Another central response was retained in homozygous kreisler mutant mice; it was distinguished by (1) a delayed (10-30 min) depression of respiratory frequency and (2) a downregulation of c-fos expression in the ventrolateral reticular nucleus of the medulla, the nucleus of the solitary tract, and the area of the A5 region. Thus, two types of ponto-medullary cell groups, with distinct anatomical locations, participate in central hypoxic respiratory depression in newborns.

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.

Puberty, steroids and GABA(A) receptor plasticity

GABA(A) receptors (GABAR) mediate most inhibition in the CNS and are also a target for neuroactive steroids such as 3alpha,5[alpha]beta-THP (3alphaOH-5[alpha]beta-OH-pregnan-20-one or [allo]pregnanolone). Although these steroids robustly enhance current gated by alpha1beta2delta GABAR, we have shown that 3alpha,5[alpha]beta-THP effects at recombinant alpha4beta2delta GABAR depend on the direction of Cl(-) flux, where the steroid increases outward flux, but decreases inward flux through the receptor. This polarity-dependent inhibition of alpha4beta2delta GABAR resulted from an increase in the rate and extent of rapid desensitization of the receptor, recorded from recombinant receptors expressed in HEK-293 cells with whole cell voltage clamp techniques. This inhibitory effect of 3alpha,5[alpha]beta-THP was not observed at other receptor subtypes, suggesting it was selective for alpha4beta2delta GABAR. Furthermore, it was prevented by a selective mutation of basic residue arginine 353 in the intracellular loop of the receptor, suggesting that this might be a putative chloride modulatory site. Expression of alpha4betadelta GABAR increases markedly at extrasynaptic sites at the onset of puberty in female mice. At this time, 3alpha,5[alpha]beta-THP decreased the inhibitory tonic current, recorded with perforated patch techniques to maintain the physiological Cl(-) gradient. By decreasing this shunting inhibition, 3alpha,5[alpha]beta-THP increased the excitability of CA1 hippocampal pyramidal cells at puberty. These effects of the steroid were opposite to those observed before puberty when 3alpha,5[alpha]beta-THP reduced neuronal excitability as a pre-synaptic effect. Behaviorally, the excitatory effect of 3alpha,5[alpha]beta-THP was reflected as an increase in anxiety at the onset of puberty in female mice. Taken together, these findings suggest that the emergence of alpha4beta2delta GABAR at the onset of puberty reverses the effect of a stress steroid. These findings may be relevant for the mood swings and increased response to stressful events reported in adolescence.

Differential regulation of the central neural cardiorespiratory system by metabotropic neurotransmitters

Central neurons in the brainstem and spinal cord are essential for the maintenance of sympathetic tone, the integration of responses to the activation of reflexes and central commands, and the generation of an appropriate respiratory motor output. Here, we will discuss work that aims to understand the role that metabotropic neurotransmitter systems play in central cardiorespiratory mechanisms. It is well known that blockade of glutamatergic, gamma-aminobutyric acidergic and glycinergic pathways causes major or even complete disruption of cardiorespiratory systems, whereas antagonism of other neurotransmitter systems barely affects circulation or ventilation. Despite the lack of an 'all-or-none' role for metabotropic neurotransmitters, they are nevertheless significant in modulating the effects of central command and peripheral adaptive reflexes. Finally, we propose that a likely explanation for the plethora of neurotransmitters and their receptors on cardiorespiratory neurons is to enable differential regulation of outputs in response to reflex inputs, while at the same time maintaining a tonic level of sympathetic activity that supports those organs that significantly autoregulate their blood supply, such as the heart, brain, retina and kidney. Such an explanation of the data now available enables the generation of many new testable hypotheses.

Neurosteroid regulation of GABA(A) receptors: Focus on the alpha4 and delta subunits

Neurosteroids, such as the progesterone metabolite 3alpha-OH-5alpha[beta]-pregnan-20-one (THP or [allo]pregnanolone), function as potent positive modulators of the GABA(A) receptor (GABAR) when acutely administered. However, fluctuations in the circulating levels of this steroid at puberty, across endogenous ovarian cycles, during pregnancy or following chronic stress produce periods of prolonged exposure and withdrawal, where changes in GABAR subunit composition may occur as compensatory responses to sustained levels of inhibition. A number of laboratories have demonstrated that both chronic administration of THP as well as its withdrawal transiently increase expression of the alpha4 subunit of the GABAR in several areas of the central nervous system (CNS) as well as in in vitro neuronal systems. Receptors containing this subunit are insensitive to benzodiazepine (BDZ) modulation and display faster deactivation kinetics, which studies suggest underlie hyperexcitability states. Similar increases in alpha4 expression are triggered by withdrawal from other GABA-modulatory compounds, such as ethanol and BDZ, suggesting a common mechanism. Other studies have reported puberty or estrous cycle-associated increases in delta-GABAR, the most sensitive target of these steroids which underlies a tonic inhibitory current. In the studies reported here, the effect of steroids on inhibition, which influence anxiety state and seizure susceptibility, depend not only on the subunit composition of the receptor but also on the direction of Cl(-) current generated by these target receptors. The effect of neurosteroids on GABAR function thus results in behavioral outcomes relevant for pubertal mood swings, premenstrual dysphoric disorder and catamenial epilepsy, which are due to fluctuations in endogenous steroids.

Hypoxia-sensing properties of the newborn rat ventral medullary surface in vitro

The ventral medullary surface (VMS) is a region known to exert a respiratory stimulant effect during hypercapnia. Several studies have suggested its involvement in the central inhibition of respiratory rhythm caused by hypoxia. We studied brainstem-spinal cord preparations isolated from newborn rats transiently superfused with a very low O(2) medium, causing reversible respiratory depression, to characterize the participation of the VMS in hypoxic respiratory adaptation. In the presence of 0.8 mM Ca(2+), very low O(2) medium induced an increase in c-fos expression throughout the VMS. The reduction of synaptic transmission and blockade of the respiratory drive by 0.2 mM Ca(2+)-1.6 mM Mg(2+) abolished c-fos expression in the medial VMS (at the lateral edge of the pyramidal tract) but not in the perifacial retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) VMS, suggesting the existence of perifacial RTN/pFRG hypoxia-sensing neurons. In the presence of Ca(2+) (0.8 mM), lesioning experiments suggested a physiological difference in perifacial RTN/pFRG VMS between the lateral VMS (beneath the ventrolateral part of the facial nucleus) and the middle VMS (beneath the ventromedial part of the facial nucleus), at least in newborn rats. The lateral VMS lesion, corresponding principally to the most rostral part of the pFRG, produced hypoxia-induced stimulation, whereas the middle VMS lesion, corresponding to the main part of the RTN, abolished hypoxic excitation. This may involve relay via the medial VMS, which is thought to be the parapyramidal group.

Caffeine in the milk prevents respiratory disorders caused by in utero caffeine exposure in rats

Consequences of postnatal caffeine exposure by the milk on ponto-medullary respiratory disturbances observed following an in utero caffeine exposure were analysed. Ponto-medullary-spinal cord preparations from newborn rats exposed to caffeine during gestation but not after the birth display an increase in respiratory frequency and an exaggeration of the hypoxic respiratory depression compared to not treated preparations. These data suggest that tachypneic and apneic episodes encountered in human newborns whose mother consumed caffeine during pregnancy are due in large part to central effect of caffeine at the ponto-medullary level. Both baseline respiratory frequency increase and emphasis of hypoxic respiratory depression are not encountered if rat dams consumed caffeine during nursing. Our hypothesis is that newborn rats exposed to caffeine during gestation but not after the birth would be in withdrawal situation whereas, when caffeine is present in drinking fluid of lactating dams, it goes down the milk and is able to prevent ponto-medullary respiratory disturbances.