Effects of GABAB receptor agonists and antagonists on the bulbar respiratory network in cat

Baclofen destabilises breathing during sleep in healthy humans: A randomised, controlled, double-blind crossover trial

AIMS

Periodic breathing is frequent in patients with severe heart failure. Apart from being an indicator of severity, periodic breathing has its own deleterious consequences (sleep-related oxygen desaturations, sleep fragmentation), which justifies attempts to correct it irrespective of the underlying disease. Animal models and human data suggest that baclofen can reconfigure respiratory central pattern generators. We hypothesised that baclofen, a GABA_B agonist, may thus be able to correct periodic breathing in humans.

METHODS

Healthy volunteers were exposed to hypoxia during sleep. Participants who developed periodic breathing (n = 14 [53 screened]) were randomly assigned to double-blind oral baclofen (progressively increased to 60 mg/d) or placebo. The primary outcome was the coefficient of variation (CoVar) of respiratory cycle total time considered as an indicator of breathing irregularity. Secondary outcomes included the CoVar of tidal volume, apnoea-hypopnoea index, sleep fragmentation index and ventilatory complexity (noise limit).

RESULTS

The analysis was conducted in 9 subjects after exclusion of incomplete datasets. CoVar of respiratory cycle total time significantly increased with baclofen during non-rapid eye movement sleep (median with placebo 56.00% [37.63-78.95]; baclofen 85.42% [68.37-86.40], P = .020; significant difference during the N1-N2 phases of sleep but not during the N3 phase). CoVar of tidal volume significantly increased during N1-N2 sleep. The apnoea-hypopnoea index, sleep fragmentation index and ventilatory complexity were not significantly different between placebo and baclofen.

CONCLUSION

Baclofen did not stabilise breathing in our model. On the contrary, it increased respiratory variability. Baclofen should probably not be used in patients with or at risk of periodic breathing.

Baclofen-Induced Neuro-Respiratory Toxicity in the Rat: Contribution of Tolerance and Characterization of Withdrawal Syndrome

Baclofen, a γ-amino-butyric acid type-B receptor agonist with exponentially increased use at high-dose to facilitate abstinence in chronic alcoholics, is responsible for increasing poisonings. Tolerance and withdrawal syndromes have been reported during prolonged treatment but their contribution to the variability of baclofen-induced neurotoxicity in overdose is unknown. We studied baclofen-induced effects on rat sedation, temperature, and ventilation and modeled baclofen pharmacokinetics and effect/concentration relationships aiming to investigate the consequences of repeated baclofen pretreatment and to characterize withdrawal syndrome. Baclofen-induced dose-dependent sedation (p <0.01), hypothermia (p <.001) and respiratory depression (p <.01) were altered in repeatedly baclofen-pretreated rats (p <.05). Repeatedly baclofen-pretreated rats did not exhibit respiratory depression following baclofen overdose due to limitations on baclofen-induced increase in inspiratory (p <.01) and expiratory times (p <.01). Only slight hypoxemia without respiratory acidosis was observed. Baclofen discontinuation resulted in hyperlocomotion and non-anxiogenic withdrawal symptoms. Regarding pharmacokinetics, repeated baclofen pretreatment increased the peak concentration (p <.05) and absorption constant rate (p <.05) and reduced the distribution volume (p <.0001) and elimination half-life (p <.05). Analysis of the effect/concentration relationships indicated that plasma baclofen concentration decreases more rapidly than all studied neuro-respiratory effects, in tolerant and non-tolerant rats. Taken together, our findings supported the role of brain distribution in baclofen-induced neurotoxicity expression and its probable involvement in tolerance-related attenuation in addition to physiological adaptations of ventilation. In conclusion, repeated pretreatment attenuates baclofen-attributed neurotoxicity in overdose and results in post-discontinuation withdrawal syndrome. Our findings suggest both pharmacodynamic and pharmacokinetic mechanisms whose relative contributions to the variability of baclofen-induced neurotoxicity in overdose remain to be established.

Safety Challenges of Using High Dose Baclofen for Alcohol Use Disorder: A Focused Review

Since the early 2000s, the gamma-aminobutyric acid type B (GABA-B) receptor agonist baclofen has been extensively used for treating alcohol use disorder (AUD). In some countries, like France, Australia, or Germany, baclofen has been used at patient-tailored dose regimens, which can reach 300 mgpd or even more in some patients. The GABA-B-related pharmacology of baclofen expose patients to a specific profile of neuropsychiatric adverse drug reactions (ADRs), primarily some frequent sedative symptoms whose risk of occurrence and severity are both related to the absolute baclofen dosing and the kinetics of dose variations. Other frequent neuropsychiatric ADRs can occur, i.e., tinnitus, insomnia, or dizziness. More rarely, other serious ADRs have been reported, like seizures, manic symptoms, or sleep apnea. However, real-life AUD patients are also exposed to other sedative drugs, like alcohol of course, but also benzodiazepines, other drugs of abuse, or other sedative medications. Consequently, the occurrence of neuropsychiatric safety issues in these patients is essentially the result of a complex multifactorial exposure, in which baclofen causality is rarely obvious by itself. As a result, the decision of initiating baclofen, as well as the daily dose management should be patient-tailored, according the medical history but also the immediate clinical situation of the patient. The overall safety profile of baclofen, as well as the clinical context in which baclofen is used, have many similarities with the use of opiate substitution medications for opiate use disorder. This empirical statement has many implications on how baclofen should be managed and dosing should be adjusted. Moreover, this constant patient-tailored adjustment can be difficult to adapt in the design of clinical trials, which may explain inconsistent findings in baclofen-related literature on AUD.

Naked mole rats exhibit metabolic but not ventilatory plasticity following chronic sustained hypoxia

Naked mole rats are among the most hypoxia-tolerant mammals identified and live in chronic hypoxia throughout their lives. The physiological mechanisms underlying this tolerance, however, are poorly understood. Most vertebrates hyperventilate in acute hypoxia and exhibit an enhanced hyperventilation following acclimatization to chronic sustained hypoxia (CSH). Conversely, naked mole rats do not hyperventilate in acute hypoxia and their response to CSH has not been examined. In this study, we explored mechanisms of plasticity in the control of the hypoxic ventilatory response (HVR) and hypoxic metabolic response (HMR) of freely behaving naked mole rats following 8-10 days of chronic sustained normoxia (CSN) or CSH. Specifically, we investigated the role of the major inhibitory neurotransmitter γ-amino butyric acid (GABA) in mediating these responses. Our study yielded three important findings. First, naked mole rats did not exhibit ventilatory plasticity following CSH, which is unique among adult animals studied to date. Second, GABA receptor (GABAR) antagonism altered breathing patterns in CSN and CSH animals and modulated the acute HVR in CSN animals. Third, naked mole rats exhibited GABAR-dependent metabolic plasticity following long-term hypoxia, such that the basal metabolic rate was approximately 25% higher in normoxic CSH animals than CSN animals, and GABAR antagonists modulated this increase.

Respiratory Rhythm Generation: Plasticity of a Neuronal Network

The exchange of gases between the external environment and the organism is controlled by a neural network of medullary neurons that produces rhythmic activity that ultimately leads to periodic contractions of thoracic, abdominal, and diaphragm muscles. This occurs in three neural phases: inspiration, postinspiration, and expiration. The present article deals with the mechanisms underlying respiratory rhythm generation and the processes of dynamic adjustment of respiratory activity by neuromodulation as it occurs during normoxia and hypoxia. The respiratory rhythm originates from the “pre-Bötzinger complex,” which is a morphologically defined region within the lower brainstem. There is a primary oscillating network consisting of reciprocally connected early-inspiratory and postinspiratory neurons, whereas various other subgroups of respiratory neurons shape the activity pattern. Rhythm generation and pattern formation result from neuronal interactions within the network, that is, from cooperative adjustments of intrinsic membrane properties and synaptic processes in the respiratory neurons. There is evidence that in neonatal mammals, as well as under certain pathological situations in adult mammals, the respiratory rhythm derives from early-inspiratory burster neurons that drive inspiratory output neurons. The respiratory network is influenced by a variety of neuromodulators. Stimulation of appropriate receptors mostly activates signal pathways that converge on cAMP-dependent protein kinase and protein kinase C. Both pathways exert modulatory effects on voltage- and ligand-controlled ion channels. Many neuromodulators are continuously released within the respiratory region or accumulated under pathological conditions such as hypoxia. The functional significance of such ongoing neuromodulation is seen in variations of network excitability. In this review, the authors concentrate on the modulators serotonin, adenosine, and opioids.

Early abnormalities of post-sigh breathing in a mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disease accompanied by complex, disabling symptoms, including breathing symptoms. Because Rett syndrome is caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2), Mecp2-deficient mice have been generated as experimental model. Males of Mecp2-deficient mice (Mecp2(-/y)) breathe normally at birth but show abnormal respiratory responses to hypoxia and hypercapnia from postnatal day 25 (P25). After P30, Mecp2(-/y) mice develop breathing symptoms reminiscent of Rett syndrome, aggravating until premature death at around P60. Using plethysmography, we analyzed the sighs and the post-sigh breathing pattern of unrestrained wild type male mice (WT) and Mecp2(-/y) mice from P15 to P60. Sighs are spontaneous large inspirations known to prevent lung atelectasis and to improve alveolar oxygenation. However, Mecp2(-/y) mice show early abnormalities of post-sigh breathing, with long-lasting post-sigh apnoeas, reduced tidal volume when eupnoea resumes and lack of post-sigh bradypnoea which develop from P15, aggravate with age and possibly contribute to breathing symptoms to come.

GABAA and GABAB receptors differentially modulate volume and frequency in ventilatory compensation in obese Zucker rats

The aim of this study was to investigate whether GABAA and/or GABAB receptor-mediated mechanisms contribute to the impaired ventilatory response and reduced maximal aerobic exercise capacity in obese Zucker rats. Ten lean and 10 obese Zucker rats were studied at 12 wk of age. Minute ventilation (V̇e), tidal volume (Vt), and breathing frequency (f) during room air breathing and in response to 10 min of hypercapnia (8% CO2) and 30 min of hypoxia (10% O2) were measured by the barometric method, and peak oxygen consumption (V̇o2 peak) was measured by an enclosed metabolic treadmill following the randomized blinded subcutaneous administration of equal volumes of DMSO (vehicle), bicuculline (selective GABAA receptor antagonist, 1 mg/kg), and phaclofen (selective GABAB receptor antagonist, 1 mg/kg). Administration of bicuculline and phaclofen to lean animals had no effect on V̇e and V̇o2 peak. Similarly, phaclofen failed to alter V̇e and V̇o2 peak in obese rats, although it did significantly increase f after 5–20 min of hypoxia. In contrast, bicuculline increased V̇e and Vt relative to DMSO during room air breathing and after 10–30 min of hypoxic exposure in obese rats, but it did not increase V̇e at 5 min of hypoxemia. Bicuculline increased V̇o2 peak relative to DMSO in obese Zucker rats. We conclude that endogenous GABA acting on GABAA receptors can modulate V̇e and V̇o2 peak in obese but not in lean Zucker rats, whereas endogenous GABA acting on GABAB receptors modulates f during hypoxia (5–20 min) in obese rats in a very different manner from that when acting on GABAA receptors.

GABA-mediated neurotransmission in the ventrolateral NTS plays a role in respiratory regulation in the rat

Our purpose was to determine whether endogenously released GABA in the ventrolateral nucleus of the solitary tract (vlNTS) of the rat influences respiration. Experiments were carried out in anesthetized, vagotomized and spontaneously breathing rats, and diaphragm electromyogram activity was measured while drugs affecting GABAergic neurotransmission were microinjected into the vlNTS and medial NTS (mNTS). Bilateral microinjection of nipecotic acid, 5 or 25 nmol, into the vlNTS (but not in the mNTS) produced dose-dependent increases in inspiratory duration (Ti) frequently culminating in apneustic breathing. Neither unilateral microinjection of bicuculline nor CGP-35348 (GABA(B) receptor antagonist) reversed this response; however, a combination of both GABA receptor antagonists effectively reversed apneustic breathing. Bilateral microinjection of either muscimol or baclofen into the vlNTS mimicked the effect of nipecotic acid. Microinjection of the bicuculline produced apnea, whereas microinjection of CGP-35348 produced a decrease in Ti and an increase in expiratory duration. Immunohistochemical analysis of the vlNTS region revealed GABA(A) receptors densely localized to processes, whereas GABA(B) immunoreactivity was localized to cell bodies. Our data indicate that GABA activity in the vlNTS is important for respiratory function.

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

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

Postnatal expression of neurotransmitters, receptors, and cytochrome oxidase in the rat pre-Bötzinger complex

The pre-Bötzinger complex (PBC) is postulated as the center of respiratory rhythmogenesis. Previously, we found a reduction or plateau of cytochrome oxidase (CO) activity in the PBC and other respiratory nuclei at postnatal days 3-4, despite a general increase of CO with age, suggesting a period of synaptic readjustment. The present study examined the expression of CO and a number of neurochemicals in the PBC at closer time intervals. At postnatal days 3-4 and, more prominently, at postnatal day 12, expression of CO, glutamate, and N-methyl-D-aspartate receptor subunit 1 was reduced, whereas expression of GABA, GABA(B) receptor, glycine receptor, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor subunit 2 was increased. These findings are consistent with our hypothesis that decreased CO activity is associated with an increase in inhibitory drive (mediated by GABA and glycine, their receptors, and possibly blockage of Ca(2+) entry by glutamate receptor subunit 2) and a decrease in excitatory drive (mediated by glutamate and its receptors). Our findings point to two critical periods during postnatal development of the rat when their respiratory system may be more vulnerable to respiratory insults.

Distribution and colocalization of neurotransmitters and receptors in the pre-Bötzinger complex of rats

The pre-Bötzinger complex (PBC), thought to be the center of respiratory rhythm generation, is a cell column ventrolateral to the nucleus ambiguus. The present study analyzed its cellular and neurochemical composition in adult rats. PBC neurons were mainly oval, fusiform, or multipolar in shape and small to medium in size. Neurokinin-1 receptor, a marker of the PBC, was present in the plasma membrane of mostly medium and small neurons and their associated processes and boutons. Among neurons immunoreactive for different neurotransmitter or receptor candidates, various numbers were colocalized with neurokinin-1 receptor. The highest ratio was with nitric oxide synthase (52.72%), and the lowest was with glycine receptors (31.93%). Glutamic acid decarboxylase- and glycine transporter 2-immunoreactive boutons, as well as GABA(A) receptor-immunoreactive plasma membrane processes and boutons, were also identified in the PBC. PBC neurons exhibited different levels of cytochrome oxidase activity, indicating their various energy demands. Our results suggest that synaptic interactions within the PBC of adult rats involve a variety of neurotransmitter and receptor types and that nitric oxide may play an important role in addition to glutamate, GABA, glycine, and neurokinin.

Central GABAergic mechanisms are involved in apnea induced by SLN stimulation in piglets

Stimulation of the superior laryngeal nerve (SLN) results in apnea in animals of different species, the mechanism of which is not known. We studied the effect of the GABA(A) receptor blocker bicuculline, given intravenously and intracisternally, on apnea induced by SLN stimulation. Eighteen 5- to 10-day-old piglets were studied: bicuculline was administered intravenously to nine animals and intracisternally to nine animals. The animals were anesthetized and then decerebrated, vagotomized, ventilated, and paralyzed. The phrenic nerve responses to four levels of electrical SLN stimulation were measured before and after bicuculline. SLN stimulation caused a significant decrease in phrenic nerve amplitude, phrenic nerve frequency, minute phrenic activity, and inspiratory time (P < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea during stimulation that often persisted beyond cessation of the stimulus. Bicuculline, administered intravenously or intracisternally, decreased the SLN stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (P < 0.05). Bicuculline also reduced SLN-induced apnea and duration of poststimulation apnea (P < 0.05). We conclude that centrally mediated GABAergic pathways are involved in laryngeal stimulation-induced apnea.

Baclofen eliminates cluster lung breathing of the tadpole brainstem, in vitro

Intermittent lung ventilation is a respiratory pattern wherein breaths occur in clusters. Intermittent lung ventilation is common in amphibians and can occur in mammals. Isolated brainstems from postmetamorphic tadpoles exhibiting episodic lung ventilatory bursts were superfused with baclofen, a potent gamma-aminobutyric acid (GABA)(B) receptor agonist. At moderate concentrations (0.125 to 0.5 microM), the number of lung bursts per episode decreased but their overall frequency was unchanged. At 0.5 microM, only 1.22+/-0.24 lung bursts occurred per episode, indicating virtually no clustering. Only at higher concentrations was overall breathing frequency decreased. Therefore, at moderate concentrations of baclofen continuous ventilation replaced episodic ventilation, suggesting that a GABA(B) receptor-dependent pathway may regulate the clustering of lung breaths.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

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

Changes in respiratory timing induced by hypercapnia in maturing rats

Premature infants respond to hypercapnia by an attenuated ventilatory response that is characterized by a decrease in respiratory frequency. We hypothesized that this impaired hypercapnic ventilatory response is of central origin and is mediated via gamma-aminobutyric acid-ergic (GABAergic) pathways. We therefore studied two groups of maturing Sprague-Dawley rats: unrestrained rats in a whole body plethysmograph at four postnatal ages (5, 16-17, 22-23, and 41-42 days); and ventilated, decerebrate, vagotomized, paralyzed rats in which phrenic nerve responses to hypercapnia were measured at 4-6 and 37-39 days of age. In the unrestrained group, the increase in minute ventilation induced by hypercapnia was significantly lower at 5 days vs. beyond 16 days. Although there was an increase in tidal volume at all ages, frequency decreased significantly from baseline at 5 days, whereas it increased significantly at 16-17, 22-23, and 41-42 days. The decrease in frequency at 5 days of age was mainly due to a significant prolongation in expiratory duration (TE). In the ventilated group, hypercapnia also caused prolongation in TE at 4-6 days but not at 37-39 days of age. Intravenous administration of bicuculline (GABA(A)-receptor blocker) abolished the prolongation of TE in response to hypercapnia in the newborn rats. We conclude that newborn rat pups exhibit a characteristic ventilatory response to CO(2) expressed as a centrally mediated prolongation of TE that appears to be mediated by GABAergic mechanisms.

Postnatal development of GABAB receptor-mediated modulation of voltage-activated Ca2+ currents in mouse brain-stem neurons

GABAB receptors modulate respiratory rhythm generation in adult mammals. However, little is currently known of their functional significance during postnatal development. In the present investigation, the effects of GABAB receptor activation on voltage-activated Ca2+ currents were examined in rhythmically active neurons of the pre-Bötzinger complex (PBC). Both low- (LVA) and high-voltage-activated (HVA) Ca2+ currents were present from the first postnatal day (P1). The density of LVA Ca2+ currents increased during the first week, whilst the density of HVA Ca2+ currents increased after the first week. In the second postnatal week, the HVA Ca2+ currents were composed of L- (47 +/- 10%) and N-type (21 +/- 8%) currents plus a 'residual' current, whilst there were no N-type currents detectable in the first few days. The GABAB receptor agonist baclofen (30 microM) increased LVA Ca2+ currents (30 +/- 11%) at P1-P3, but it decreased the currents (35 +/- 11%) at P7-P15 without changing its time course. At all ages, baclofen (30 microM) decreased the HVA Ca2+ currents by approximately 54%. Threshold of baclofen effects on both LVA and HVA Ca2+ currents was 5 microM at P1-P3 and lower than 1 microM at P7-P15. The effect of baclofen was abolished in the presence of the GABAB receptor antagonist CGP 55845A (50 nM). We conclude that both LVA and HVA Ca2+ currents increased postnatally. The GABAB receptor-mediated modulation of these currents undergo marked developmental changes during the first two postnatal weeks, which may contribute essentially to modulation of respiratory rhythm generation.

GABA receptors in the phrenic nucleus of the rat

The phrenic nucleus was identified by microinjections of N-methyl-D-aspartic acid in urethan-anesthetized adult male Wistar rats. Microinjections of GABAA and GABAB receptor agonists (muscimol and baclofen, respectively) at the same site decreased the phrenic nerve burst amplitude. Microinjections of GABAA and GABAB receptor antagonists (bicuculline and 2-hydroxysaclofen, respectively) blocked as well as reversed the effects of their respective agonists. These results were confirmed by recording extracellular action potentials from single phrenic neurons. Micropressure applications of muscimol and baclofen decreased the activity of single neurons in the phrenic nucleus; this effect was blocked as well as reversed by micropressure applications of bicuculline and 2-hydroxysaclofen, respectively. These results demonstrated the presence of GABA receptors on the neurons in the phrenic nucleus and suggested that their activation results in the decrease of the phrenic nerve burst amplitude. The importance of these results in the identification of neural circuits mediating inhibition of phrenic neurons is discussed.

Synaptic inhibition in the isolated respiratory network of neonatal rats

Gramicidin-perforated patch-clamp recording revealed phasic Cl(-)-mediated hyperpolarizations in respiratory neurons of the brainstem-spinal cord preparation from newborn rats. The in vitro respiratory rhythm persisted after block of gamma-aminobutyric acid (GABA), i.e. GABAA, receptor-mediated inhibitory postsynaptic potentials (IPSPs) with bicuculline and/or glycinergic IPSPs with strychnine. In one class of expiratory neurons, bicuculline unmasked inspiration-related excitatory postsynaptic potentials (EPSPs), leading to spike discharge. Bicuculline also blocked hyperpolarizations and respiratory arrest due to bath-applied muscimol, whereas strychnine antagonized similar responses to glycine. The reversal potential of respiration-related IPSPs and responses to GABA, muscimol or glycine was not affected by CO2/HCO3(-)-free solutions, but shifted from about -65 mV to values more positive than -20 mV upon dialysis of the cells with 144 instead of 4 mM Cl-. Impairment of GABA uptake with nipecotic acid or glycine uptake with sarcosine evoked a bicuculline- or strychnine-sensitive decrease of respiratory frequency which could lead to respiratory arrest. Also, the GABAB receptor agonist baclofen led to reversible suppression of respiratory rhythm. This in vitro apnoea was accompanied by a K+ channel-mediated hyperpolarization (reversal potential -88 mV) of tonic cells, whereas membrane potential of neighbouring respiratory neurons remained almost unaffected. Both baclofen-induced hyperpolarization and respiratory depression were antagonised by 2-OH-saclofen, which did not affect respiration-related IPSPs per se. The results show that synaptic inhibition is not essential for rhythmogenesis in the isolated neonatal respiratory network, although (endogenous) GABA and glycine have a strong modulatory action. Hyperpolarizing IPSPs mediated by GABAA and glycine receptors provide a characteristic pattern of membrane potential oscillations in respiratory neurons, whereas GABAB receptors rather appear to be a feature of non-respiratory neurons, possibly providing excitatory drive to the network.

Driving respiration: the respiratory central pattern generator

1. The central pattern generator (CPG) for respiration is located in the brainstem and produces rhythmic synaptic drive for motoneurons controlling respiratory muscles. Based on respiratory nerve discharge, the respiratory cycle can be divided into three phases: inspiration, postinspiration and stage 2 expiration. 2. Six basic types of respiratory neuron participate in respiratory rhythmogenesis. Their firing and membrane potential patterns are locked to different phases of the respiratory cycle. 3. In adult mammals, respiratory neurons are subject to excitatory and inhibitory synaptic inputs and show extensive synaptic interconnections that are mainly inhibitory. There are differences in the relative importance of excitatory and inhibitory synaptic drives and the neurotransmitters involved in respiratory rhythmogenesis in neonates compared with adults. 4. Respiratory neurons possess a number of intrinsic membrane currents that may be involved in central pattern generation, including low- and high-voltage-activated calcium, potassium, calcium-dependent potassium, sodium and mixed cationic currents. More quantitative information is needed about the distribution and characteristics of these ionic currents if we are to understand rhythmogenesis. 5. The two main theories for the origin of respiratory rhythm are those of pacemaker neuron-driven and synaptic network-driven CPG. Evidence derived from in vivo and in vitro experiments exists to support both of these theories. There may be a significant switch in the underlying mechanism driving the respiratory CPG during postnatal development.

GABAergic inhibition of hiccup-like reflex induced by electrical stimulation in medulla of cats

We hypothesize that the hiccup reflex is actively inhibited through GABA(B) receptor within central connections of the hiccup reflex arc. Because the hiccup-like reflex can be elicited by electrical stimulation to a limited area within the medullary reticular formation, the hiccup-evoking site (HES), electrical stimulation (50-100 microA, three train pulses at 20 Hz) was delivered to HES by means of a metal electrode containing 1.0 mM baclofen, in anesthetized spontaneously breathing cats. The evoked response was characterized by a brief powerful increase in diaphragmatic activity and a temporal suppression of the posterior cricoarytenoid muscle, laryngeal dilator, which corresponded to the fixed motor pattern of hiccup reflex. The hiccup-like response was rapidly suppressed after microinjection of baclofen (0.1-0.5 nmol) into HES, indicating that HES has GABA(B) receptors. In the other experiments, to histologically examine the inputs to the hiccup reflex arc, unconjugated cholera toxin subunit B (UCTB) was injected into HES. Following injections of UCTB, retrogradely labelled cells were found distributed in various areas of the lower brainstem. Among these areas, the nucleus raphe magnus (RM) is reported to have GABA-containing cells. It is thus hypothesized that RM is most likely to be the source of the GABAergic inhibitory inputs to the hiccup reflex arc.

Effects of baclofen on the Hering-Breuer inspiratory-inhibitory and deflation reflexes in rats

The objective of this study was to evaluate effects of baclofen, a gamma-aminobutyric acid type B (GABAB) receptor agonist, injected into the nucleus of the solitary tract, on the Hering-Breuer inspiratory-inhibitory (TI-inhibitory) and deflation reflexes in urethan-anesthetized adult Wistar rats (n = 7). The TI-inhibitory reflex was estimated from changes in peak amplitude of the integrated diaphragmatic electromyogram and inspiratory time (TI) provoked by airway occlusion at end expiration. The deflation reflex was evaluated from changes in TI and expiration (TE) of the first two breaths (TI-1, TE-1 and TI-2, TE-2) immediately after a decrease in tracheal pressure (Ptr). Under control conditions, airway occlusion at end-TE prolonged TI (66 +/- 5%; mean +/- SE) and the following TE (54 +/- 11%). Decreases in Ptr, from -2 to -5 cmH2O, evoked an increase in TI and shortening of TE of both breaths. Both effects were Ptr dependent, and TI-1 and TE-1 differed from TI-2 and TE-2, suggesting a rapid adaptation to the stimulus. At Ptr of -5 cmH2O, TI-1 and TI-2 increased by 30 +/- 2 and 43 +/- 6%, respectively, and TE-1 and TE-2 decreased by 53 +/- 4 and 33 +/- 7%, respectively. During unloaded breathing, 60 pmol baclofen prolonged TI by 120 +/- 11% and left TE unaffected. Baclofen abolished vagally mediated changes in TE. On the other hand, the TI increases caused by either airway occlusion (24 +/- 8%) or Ptr of -5 cmH2O (TI-1; 16 +/- 5%) were still significant, but TI-1 and TI-2 were not different. A GABAB receptor antagonist, CGP-35348 (2.8 nmol), reversed these effects of baclofen. These results imply that stimulation of GABAB receptors attenuates but does abolish vagally mediated control of TI. The difference in effects of baclofen on the central and vagal control of TI and TE suggests different distribution of GABAB receptors in neuronal networks controlling each of these respiratory phases.

Intracellular signal pathways controlling respiratory neurons

Medullary respiratory neurons are influenced by a variety of neuromodulators, but there is a lack of information about the specific intracellular signal pathways involved. In this report we describe the modulatory effects of the cyclic adenosine-triphosphate (cAMP)-dependent protein kinase and of protein kinase C pathways on voltage- and ligand-controlled ionic conductances and demonstrate their functional significance in regulating the excitability of medullary respiratory neurons of the vivo cat. Evidence is presented that PKA and PKC pathways are persistently activated. PKA regulates current flow through persistently activated and GABAB receptor-controlled potassium channels as well as GABAA receptor-controlled chloride channels. PKC also depresses persistent potassium currents but it potentiates excitatory and inhibitory synaptic currents. The clinical significance of these intracellular signal pathways is demonstrated in a case of a child suffering from apneustic breathing, who was successfully treated with a 5HT-1A receptor agonist.

cAMP modulates an S-type K+ channel coupled to GABAB receptors in mammalian respiratory neurons

Premotor respiratory neurons from neonatal rats express a Ba(2+)-insensitive outward rectifying K+ channel (KOR) which is activated by gamma-aminobutyric acid acting at its beta receptor. The biophysical properties of KOR are similar to those described for the S-channel (KS) which underlies simple forms of non-associative learning in the marine mollusc Aplysia. We show here that KOR, like the S-channel, is inhibited by cAMP. In addition, we demonstrate that this inhibition is due to a change in closed time kinetics. Our data suggests that the ionic and biochemical substrates underlying synaptic plasticity in Aplysia have been phylogenetically conserved in mammalian motor circuits such as that controlling rhythmic breathing movements.

GABA(B) receptor blockade reduces resistive loading-induced hypoventilation in anesthetized rabbits

We hypothesized that hypoventilation induced by resistive loaded breathing may result in part from the inhibition of central respiratory-related structures by gamma-aminobutyric acid (GABA), through the stimulation of GABA(B) receptors. In that case, ventilatory depression should be minimized by GABA(B) receptor blockade. To test this assumption, the ventilatory effects of a GABA(B) receptor antagonist (CGP 35348) were evaluated in two groups of urethane anesthetized rabbits, breathing either through an inspiratory resistive load (IRL group) or not (Control group). CGP 35348 did not modify baseline ventilation in the Control group. On the other hand, it partially reversed IRL-induced hypoventilation through a higher respiratory rate and central inspiratory drive. These data suggest that, unlike GABA(A) receptors, GABA(B) receptors would not play a part in eupneic breathing, but that they could participate in the hypoventilation resulting from an acute increase in the work of breathing.

Modeling neural mechanisms for genesis of respiratory rhythm and pattern. I. Models of respiratory neurons

The general objectives of our research, presented in this series of papers, were to develop a computational model of the brain stem respiratory neural network and to explore possible neural mechanisms that provide the genesis of respiratory oscillations and the specific firing patterns of respiratory neurons. The present paper describes models of single respiratory neurons that have been used as the elements in our network models of the central respiratory pattern generator presented in subsequent papers. The models of respiratory neurons were developed in the Hodgkin-Huxley style employing both physiological and biophysical data obtained from brain stem neurons in mammals. Two single respiratory neuron models were developed to match the two distinct firing behaviors of respiratory neurons described in vivo: neuron type I shows an adapting firing pattern in response to synaptic excitation, and neuron type II shows a ramp firing pattern during membrane depolarization after a period of synaptic inhibition. We found that a frequency ramp firing pattern can result from intrinsic membrane properties, specifically from the combined influence of calcium-dependent K(AHP)(Ca), low-threshold Ca(T) and K(A) channels. The neuron models with these ionic channels (type II) demonstrated ramp firing patterns similar to those recorded from respiratory neurons in vivo. Our simulations show that K(AHP)(Ca) channels in combination with high-threshold Ca(L) channels produce spike frequency adaptation during synaptic excitation. However, in combination with low-threshold Ca(T) channels, they cause a frequency ramp firing response after release from inhibition. This promotes a testable hypothesis that the main difference between the respiratory neurons that adapt (for example, early inspiratory, postinspiratory, and decrementing expiratory) and those that show ramp firing patterns (for example, ramp inspiratory and augmenting expiratory) consists of a ratio between the two types of calcium channels: Ca(L) channels predominate in the former and Ca(T) channels in the latter respiratory neuron types. We have analyzed the dependence of adapting and ramp firing patterns on maximal conductances of different ionic channels and values of synaptic drive. The effect of adjusting specific membrane conductances and synaptic interactions revealed plausible neuronal mechanisms that may underlie modulatory effects on respiratory neuron firing patterns and network performances. The results of computer simulation provide useful insight into functional significance of specific intrinsic membrane properties and their interactions with phasic synaptic inputs for a better understanding of respiratory neuron firing behavior.

Protein kinase C pathways modulate respiratory pattern generation in the cat

1. The significance of protein kinase C (PKC) in respiratory pattern generation was investigated in forty-three expiratory neurones of anaesthetized cats. 2. Intracellular injection of R-2,6-diamino-N-([1-(oxotridecyl)-2-piperidinyl]-methyl)-hexana mide dihydrochloride reversibly hyperpolarized twenty-six neurones. Respiratory drive potentials decreased to 92% of control, and action potential discharges were reduced. Neuronal input resistance (Rin) decreased during inspiration and increased during expiration. 3. Voltage clamp revealed that blockade of PKC induced an increase of inhibitory drive currents and a decrease of excitatory drive currents in sixteen neurones. The amplitude of respiratory drive currents was decreased to 91% of control. The slope of synaptic inward currents during postinspiration was reduced. 4. After blockade of K+ conductances by TEA, additional blockade of PKC caused a hyperpolarization during postinspiration and expiration, but depolarization during inspiration in fourteen neurones. The respiratory drive currents were reduced to 61% of control. Respiratory drive potentials decreased to 72% of control, leading to reduced spontaneous discharge. Rin was increased throughout the respiratory cycle. 5. Stimulus-evoked postsynaptic currents and potentials decreased after blockade of PKC with and without TEA. 6. The results indicate that PKC is endogenously active in expiratory neurones, modulating their excitability in three different ways: (a) it downregulates persistent K+ currents, (b) it upregulates Cl(-)-mediated inhibitory postsynaptic currents (IPSCs), and (c) it upregulates excitatory postsynaptic currents (EPSCs).

Inhibitions mediated by glycine and GABAA receptors shape the discharge pattern of bulbar respiratory neurons

Experiments were performed to identify the glycinergic or GABAergic nature, and the timing of discharge, of the neurons which produce chloride-dependent inhibitions on other bulbar respiratory neurons (RNs) during their silent and active phases. RNs recorded extracellularly in pentobarbital-anesthetized or decerebrate cats, were subjected to iontophoretic applications of glutamate, of the glycine antagonist strychnine, and of the GABAA receptor antagonist bicuculline. Both antagonists induced discharge or increased discharge frequency in restricted parts of the respiratory cycle without affecting the discharge frequency in other parts of the cycle. Strychnine most often elicited activity in late-inspiration and early-expiration, but also in early inspiration and in late expiration. Bicuculline was most often effective throughout the entire discharge period of each neuron with no effect during the silent period, although it also acted selectively during late-inspiration in inspiratory neurons, an effect attributed to GABAA receptor blockade. The convergence of glycinergic afferent inputs during late inspiration and early expiration suggests that glycinergic neurons may play an important role in the inspiratory to expiratory phase transition.

Localization of chemosensitive structures in the isolated brainstem of adult guinea-pig

1. Central respiratory chemosensitivity has been intensively examined but some questions remain unsolved; namely, what is the nature of the stimulus (fixed acid and/or CO2) and where is the site of brainstem chemosensitivity (near the ventral medullary surface or structures deeper within the brainstem)? To examine these questions, we used the in vitro isolated brainstem of adult guinea-pig perfused independently through the basilar artery and the bath. 2. Respiratory motor output was recorded with a suction electrode from cranial hypoglossal (XII) roots. Changes in pH and CO2 in the Krebs perfusate were made by changing either the bicarbonate concentration or the PCO2 saturating the Krebs solution. 3. Changes in basilar artery perfusate consisting of (i) an acidifying increase in PCO2 (hypercapnic acidic Krebs solution), (ii) an increase in PCO2 with no change in pH (hypercapnic Krebs solution), or (iii) a decrease in pH with no change in PCO2 (acidic Krebs solution) evoked increases in respiratory frequency and a concomitant decrease in inspiratory burst amplitude. 4. Bath superfusion with hypercapnic acidic Krebs solution increased the inspiratory burst amplitude with no effect on respiratory burst frequency. 5. Bath superfusion with hypercapnic non-acidic Krebs solution increased the inspiratory burst amplitude and decreased the respiratory frequency, while normocapnic acidic Krebs solution increased the respiratory frequency with no change in burst amplitude. 6. These results show that respiratory responses to changes in CO2 and pH depend upon the sites of action. While a CO2 increase or a pH decrease affected the respiratory frequency in the deep brainstem structures (perfused through the basilar artery), CO2 respiratory chemosensitivity at the ventral surface could be differentiated from the hydrogen ion chemosensitivity. This suggests that different mechanisms mediated respiratory responses when deep versus superficial brainstem structures were stimulated.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

The roles of K+ conductance in expiratory pattern generation in anaesthetized cats

1. The potassium current blockers caesium and tetraethylammonium were injected intracellularly by ionophoretic current into brainstem expiratory neurones of the ventral group. Neurones were identified by their spontaneous activity and by antidromic excitation from the spinal cord at the C2-C3 level. 2. The duration of action potentials increased and the early and late after-hyperpolarizations were completely suppressed. These effects on action potentials were reversible, recovered with an exponential time course within 3 min, and could be reproduced when blockers were applied repetitively into the same neurone. They were ascribed to blockade of potassium channels in the somatic membrane region. 3. Potassium channel blockers modified postsynaptic potentials: early-inspiratory hyperpolarizations were reversibly depressed while postinspiratory and expiratory depolarizations were irreversibly enhanced. The former effect was associated with a decrease of the neuronal input conductance. The latter effect was cumulative upon repetitive ionophoretic applications of potassium blockers. 4. The results demonstrate that potassium currents exert two different roles in expiratory pattern generation. Together with chloride currents, they contribute to the phasic early-inspiratory inhibition. They seem to be calcium-dependent and GABAB receptor-controlled currents which predominate near to the cell body. 5. Potassium currents also operate throughout the postinspiratory and late-expiratory periods. They seem to include persistent potassium currents which modulate the excitatory respiratory drive provided by the respiratory rhythm generator. We assume that these currents, widely distributed over the somatodendritic membrane area, are a target for neuromodulation by transmitters and intracellular second messengers.

Microiontophoresis of baclofen on membrane potential and input resistance in bulbar respiratory neurons in the cat

Iontophoresis of baclofen produced hyperpolarization and a decrease in input resistance in 32 neurons and no effect in 24 neurons of the ventral respiratory group in cats. Iontophoresed phaclofen antagonized the effect of baclofen, but had negligible effects on periodic fluctuations in membrane potential and spike activity in these neurons. The hyperpolarizing effect of baclofen persisted after iontophoresis of tetrodotoxin, suggesting that baclofen acted directly at the postsynaptic gamma-aminobutyric acid (GABA)B receptors.