Serotonin 1A-receptor activation suppresses respiratory apneusis in the cat

Hypoxia evokes a sequence of raphe-pontomedullary network operations for inspiratory drive amplification and gasping

Hypoxia can trigger a sequence of breathing-related behaviors, from tachypnea to apneusis to apnea and gasping, an autoresuscitative behavior that, via large tidal volumes and altered intrathoracic pressure, can enhance coronary perfusion, carotid blood flow, and sympathetic activity, and thereby coordinate cardiac and respiratory functions. We tested the hypothesis that hypoxia-evoked gasps are amplified through a disinhibitory microcircuit within the inspiratory neuron chain and a distributed efference copy mechanism that generates coordinated gasp-like discharges concurrently in other circuits of the raphe-pontomedullary respiratory network. Data were obtained from 6 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated adult cats. Arterial blood pressure, phrenic nerve activity, end-tidal CO2, and other parameters were monitored. Hypoxia was produced by ventilation with a gas mixture of 5% O2 in nitrogen (N2). Neuron spike trains were recorded at multiple pontomedullary sites simultaneously and evaluated for firing rate modulations and short-time scale correlations indicative of functional connectivity. Experimental perturbations evoked reconfiguration of raphe-pontomedullary circuits during tachypnea, apneusis and augmented bursts, apnea, and gasping. The functional connectivity, altered firing rates, efference copy of gasp drive, and coordinated step increments in blood pressure reported here support a distributed brain stem network model for amplification and broadcasting of inspiratory drive during autoresuscitative gasping that begins with a reduction in inhibition by expiratory neurons and an initial loss of inspiratory drive during hypoxic apnea.

Countermeasures for Preventing and Treating Opioid Overdose

The only medication available currently to prevent and treat opioid overdose (naloxone) was approved by the US Food and Drug Administration (FDA) nearly 50 years ago. Because of its pharmacokinetic and pharmacodynamic properties, naloxone has limited utility under some conditions and would not be effective to counteract mass casualties involving large-scale deployment of weaponized synthetic opioids. To address shortcomings of current medical countermeasures for opioid toxicity, a trans-agency scientific meeting was convened by the US National Institute of Allergy and Infectious Diseases/National Institutes of Health (NIAID/NIH) on August 6 and 7, 2019, to explore emerging alternative approaches for treating opioid overdose in the event of weaponization of synthetic opioids. The meeting was initiated by the Chemical Countermeasures Research Program (CCRP), was organized by NIAID, and was a collaboration with the National Institute on Drug Abuse/NIH (NIDA/NIH), the FDA, the Defense Threat Reduction Agency (DTRA), and the Biomedical Advanced Research and Development Authority (BARDA). This paper provides an overview of several presentations at that meeting that discussed emerging new approaches for treating opioid overdose, including the following: (1) intranasal nalmefene, a competitive, reversible opioid receptor antagonist with a longer duration of action than naloxone; (2) methocinnamox, a novel opioid receptor antagonist; (3) covalent naloxone nanoparticles; (4) serotonin (5-HT)_1A receptor agonists; (5) fentanyl-binding cyclodextrin scaffolds; (6) detoxifying biomimetic "nanosponge" decoy receptors; and (7) antibody-based strategies. These approaches could also be applied to treat opioid use disorder.

Serotonin and substance P: Synergy or competition in the control of breathing

Numerous neurotransmitters identified in the central nervous system play role in ventilatory control. This mini-review focuses on the respiratory effects of two neurotransmitters: serotonin (5-HT) and substance P (SP). We discuss their co-localization in medullary raphe nuclei, expression of proper receptors within the specific regions of respiratory related structures and contribution to respiratory rhythmogenesis.

Dose-effect study of the serotonin agonist R-8-OH-DPAT on opioid-induced respiratory depression in blesbok (Damaliscus pygargus philipsi) and impala (Aepyceros melampus)

OBJECTIVE

To determine whether the R-enantiomer of 8-hydroxy-2-(di-n-propylamino) tetralin (R-8-OH-DPAT) alleviates respiratory depression in antelope species immobilized with etorphine. The experiment also aimed to establish the most clinically effective dose of this serotonin 5- HT_1A receptor agonist.

ANIMALS

A group of six female blesbok and six female impala.

STUDY DESIGN

Each animal was subjected to four immobilization treatments in a prospective four-way crossover design-control treatment consisting of only etorphine at 0.09 mg kg^-1 and three treatments consisting of etorphine at 0.09 mg kg^-1 combined with 0.005, 0.02 and 0.07 mg kg^-1 of R-8-OH-DPAT, respectively. Induction, quality of immobilization and recovery were monitored in each treatment. Physiological variables including heart rate, respiratory rate, arterial blood pressure and blood gases were measured for 35 minutes during immobilization. A linear mixed model was used to assess the effects of treatments over the recumbency period.

RESULTS

R-8-OH-DPAT did not influence induction, immobilization or recovery scores. Respiratory rate in blesbok was increased in the medium- and high-dosage R-8-OH-DPAT treatment group. However, this increased respiratory rate did not translate into improvements of arterial partial pressure of oxygen (PaO₂) values in the blesbok. The medium and higher dosages of R-8-OH-DPAT in impala led to an improved PaO₂ as well as to decreased opioid-induced tachycardia during the first 10 minutes of immobilization.

CONCLUSIONS AND CLINICAL RELEVANCE

Previous reports indicated that the racemic mixture of 8-OH-DPAT injected intravenously had a positive effect on blood-gas values in etorphine-treated hypoxemic goats. In this experiment, similar effects could be seen in impala at the higher dosage rates of R-8-OH-DPAT. However, failure to achieve an improvement of blood-gas values in blesbok was an unexpected result. It could be speculated that the dosage, species-specific differences of serotonin receptors or the use of the R-enantiomer of 8-OH-DPAT might play a role.

Pharmacologically evoked apnoeas. Receptors and nervous pathways involved

This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT_1A serotonin receptors in overcoming opioid-induced respiratory inhibition.

Effect of Serotonin 1A Agonists and Selective Serotonin Reuptake Inhibitors on Behavioral and Nighttime Respiratory Symptoms in Rett Syndrome

BACKGROUND

Rett syndrome is characterized by psychomotor regression during early childhood, autistic-like behaviors, and aberrant breathing patterns. Dysfunction of the serotonergic system has been postulated to play a role in the pathophysiology of these symptoms.

PATIENT DESCRIPTION

We present an 11-year-old girl with Rett syndrome who exhibited marked respiratory symptoms, including frequent apneic events during sleep. She had been treated for these respiratory symptoms using noninvasive positive pressure ventilation since age six years. Treatment with serotonin 1A receptor agonist was initiated at age eight years, whereas treatment using a selective serotonin reuptake inhibitor began at age nine years. Noninvasive positive pressure ventilation therapy was effective in reducing symptoms of sleep apnea, and administration of serotonergic agents resulted in amelioration of sleep apneic events even in the absence of noninvasive positive pressure ventilation. In addition, improvements in hand stereotypy and social skills were observed after initiation of serotonin-based therapy.

DISCUSSION

The respiratory difficulties our patient experienced during non-rapid eye movement (REM) sleep are characteristic of post-sigh central apnea. Exaggerated activity of expiratory neurons during such apneic events has been observed in mouse models of Rett syndrome. We suggest that prescribed serotonergic agents might serve to inhibit such activity, attenuating the imbalance between inspiratory and expiratory neurons. These agents might also be useful in the treatment of autistic-like behaviors caused by impaired serotonergic transmission in the brain.

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.

A latent serotonin-1A receptor-gated spinal afferent pathway inhibiting breathing

Spinal afferents such as nociceptive afferents and group III-IV muscle afferents are known to exert an acute excitatory effect on breathing when activated. Here, we report the surprising existence of latent spinal afferents which exerted tonic inhibitory influence on breathing subliminally in anesthetized rats, an effect which was reversed upon activation of serotonin-1A receptors (5-HT_1ARs) in lumbar spinal cord, lesion of pontine lateral parabrachial nucleus or suppression of the adjacent Kölliker-Fuse nucleus with NMDA receptor blockade. Small-interfering RNA knockdown of 5-HT_1ARs in lumbar spinal cord unequivocally localized the site of 5-HT_1AR-mediated gating of these respiratory-inhibiting interoceptive afferents to relay neurons in the spinal superficial dorsal horn at the lumbar level and not cervical spinal or supraspinal levels. Our results reveal a novel somatosensory/viscerosensory mechanism which exerts tonic inhibitory influence on homeostatic regulation of breathing independent from the classical chemoreflex excitatory pathways, and suggest a hitherto unrecognized therapeutic target in spinal dorsal horn for 5-HT_1AR-based treatment of a variety of respiratory abnormalities.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

Effects of cholinesterase inhibitors and serotonin-1A receptor agonists on morphine-induced ventilatory depression and antinociception in rats

Ventilatory depression is a serious side-effect of opioid analgesics. Naloxone, an antagonist of opioid receptors, eliminates not only ventilatory depression but also analgesic effect of opioids. Pharmacological dissociation of adverse reactions from the main action is important clinically and basically. Cholinergic and serotonergic mechanisms are suggested to counteract the opioid-induced ventilatory disturbances, but their influence on analgesia is still controversial. The present study evaluated the effects of cholinesterase inhibitors and serotonin-1A (5-HT1A) receptor agonists on morphine (1.0mg/kg, i.v.)-induced ventilatory depression and analgesia in rats. In anesthetized animals, spontaneous ventilation and hind leg withdrawal reflexes against nociceptive thermal stimuli were measured simultaneously. Physostigmine (0.1 and 0.2mg/kg, i.v.) and donepezil (0.5 and 1.0mg/kg, i.v.) relieved the morphine-induced ventilatory depression and enhanced its antinociception. On the other hand, (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.03 and 0.1mg/kg, i.v.) and buspirone (0.1 and 0.3mg/kg, i.v.) did not influence antinociception of morphine while they restored the decreased ventilation. In unanesthetized animals, hypercapnic ventilatory response was measured by using whole-body plethysmography. Physostigmine (0.3mg/kg, i.p.), donepezil (1.0mg/kg, i.p.), 8-OH-DPAT (0.3mg/kg, i.p.) and buspirone (3.0mg/kg, i.p.) all recovered the morphine (10mg/kg, i.p.)-induced depression of hypercapnic ventilatory response. The present study suggests that activation of cholinergic or serotonergic (5-HT1A) mechanisms may be a useful therapeutic approach for morphine-induced ventilatory depression without loss of its analgesic action.

[Serotonin receptor 1A-modulated dephosphorylation of glycine receptor α3: a new molecular mechanism of breathing control for compensation of opioid-induced respiratory depression without loss of analgesia]

To control the breathing rhythm the medullary respiratory network generates periodic salvo activities for inspiration, post-inspiration and expiration. These are under permanent modulatory control by serotonergic neurons of the raphe which governs the degree of phosphorylation of the inhibitory glycine receptor α3. The specific activation of serotonin receptor type 1A (5-HTR(1A)), which is strongly expressed in the respiratory neurons, functions via inhibition of adenylate cyclase and the resulting reduction of the intracellular cAMP level and a gradual dephosphorylation of the glycine receptor type α3 (GlyRα3). This 5-HTR(1A)-GlyRα3 signal pathway is independent of the µ-opioidergic transduction pathway and via a synaptic inhibition caused by an increase in GlyRα3 stimulates a disinhibition of some target neurons not only from excitatory but also from inhibitory neurons. Our physiological investigations show that this 5-HTR(1A)-GlyRα3 modulation allows treatment of respiratory depression due to opioids without affecting the desired analgesic effects of opioids. The molecular mechanism presented here opens new pharmacological possibilities to treat opioid-induced respiratory depression and respiratory disorders due to disturbed inhibitory synaptic transmission, such as hyperekplexia.

Expression and function of serotonin 2A and 2B receptors in the mammalian respiratory network

Neurons of the respiratory network in the lower brainstem express a variety of serotonin receptors (5-HTRs) that act primarily through adenylyl cyclase. However, there is one receptor family including 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors that are directed towards protein kinase C (PKC). In contrast to 5-HT_2ARs, expression and function of 5-HT_2BRs within the respiratory network are still unclear. 5-HT_2BR utilizes a Gq-mediated signaling cascade involving calcium and leading to activation of phospholipase C and IP3/DAG pathways. Based on previous studies, this signal pathway appears to mediate excitatory actions on respiration. In the present study, we analyzed receptor expression in pontine and medullary regions of the respiratory network both at the transcriptional and translational level using quantitative RT-PCR and self-made as well as commercially available antibodies, respectively. In addition we measured effects of selective agonists and antagonists for 5-HT_2ARs and 5-HT_2BRs given intra-arterially on phrenic nerve discharges in juvenile rats using the perfused brainstem preparation. The drugs caused significant changes in discharge activity. Co-administration of both agonists revealed a dominance of the 5-HT_2BR. Given the nature of the signaling pathways, we investigated whether intracellular calcium may explain effects observed in the respiratory network. Taken together, the results of this study suggest a significant role of both receptors in respiratory network modulation.

Altered responses of MeCP2-deficient mouse brain stem to severe hypoxia

Rett syndrome (RTT) patients suffer from respiratory arrhythmias with frequent apneas causing intermittent hypoxia. In a RTT mouse model (methyl-CpG-binding protein 2-deficient mice; Mecp2−/ y) we recently discovered an enhanced hippocampal susceptibility to hypoxia and hypoxia-induced spreading depression (HSD). In the present study we investigated whether this also applies to infant Mecp2−/ y brain stem, which could become life-threatening due to failure of cardiorespiratory control. HSD most reliably occurred in the nucleus of the solitary tract (NTS) and the spinal trigeminal nucleus (Sp5). HSD susceptibility of the Mecp2−/ y NTS and Sp5 was increased on 8 mM K+-mediated conditioning. 5-HT1A receptor stimulation with 8-hydroxy-2-(di-propylamino)tetralin (8-OH-DPAT) postponed HSD by up to 40%, mediating genotype-independent protection. The deleterious impact of HSD on in vitro respiration became obvious in rhythmically active slices, where HSD propagation into the pre-Bötzinger complex (pre-BötC) immediately arrested the respiratory rhythm. Compared with wild-type, the Mecp2−/ y pre-BötC was invaded less frequently by HSD, but if so, HSD occurred earlier. On reoxygenation, in vitro rhythms reappeared with increased frequency, which was less pronounced in Mecp2−/ y slices. 8-OH-DPAT increased respiratory frequency but failed to postpone HSD in the pre-BötC. Repetitive hypoxia facilitated posthypoxic recovery only if HSD occurred. In 57% of Mecp2−/ y slices, however, HSD spared the pre-BötC. Although this occasionally promoted residual hypoxic respiratory activity (“gasping”), it also prolonged the posthypoxic recovery, and thus the absence of central inspiratory drive, which in vivo would lengthen respiratory arrest. In view of the breathing disorders in RTTs, the increased hypoxia susceptibility of MeCP2-deficient brain stem potentially contributes to life-threatening disturbances of cardiorespiratory control.

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

The potency of different serotonergic agonists in counteracting opioid evoked cardiorespiratory disturbances

Serotonin receptor (5-HTR) agonists that target 5-HT(4(a))R and 5-HT(1A)R can reverse mu-opioid receptor (mu-OR)-evoked respiratory depression. Here, we have tested whether such rescuing by serotonin agonists also applies to the cardiovascular system. In working heart-brainstem preparations in situ, we have recorded phrenic nerve activity, thoracic sympathetic chain activity (SCA), vascular resistance and heart rate (HR) and in conscious rats, diaphragmatic electromyogram, arterial blood pressure (BP) and HR via radio-telemetry. In addition, the distribution of 5-HT(4(a))R and 5-HT(1A)R in ponto-medullary cardiorespiratory networks was identified using histochemistry. Systemic administration of the mu-OR agonist fentanyl in situ decreased HR, vascular resistance, SCA and phrenic nerve activity. Subsequent application of the 5-HT(1A)R agonist 8-OH-DPAT further enhanced bradycardia, but partially compensated the decrease in vascular resistance, sympathetic activity and restored breathing. By contrast, the 5-HT(4(a))R agonist RS67333 further decreased vascular resistance, HR and sympathetic activity, but partially rescued breathing. In conscious rats, administration of remifentanyl caused severe respiratory depression, a decrease in mean BP accompanied by pronounced bradyarrhythmia. 8-OH-DPAT restored breathing and prevented the bradyarrhythmia; however, BP and HR remained below baseline. In contrast, RS67333 further suppressed cardiovascular functions in vivo and only partially recovered breathing in some cases. The better recovery of mu-OR cardiorespiratory disturbance by 5-HT(1A)R than 5-HT(4(a))R is supported by the finding that 5-HT(1A)R was more densely expressed in key brainstem nuclei for cardiorespiratory control compared with 5-HT(4(a))R. We conclude that during treatment of severe pain, 5-HT(1A)R agonists may provide a useful tool to counteract opioid-mediated cardiorespiratory disturbances.

Raphé neurons stimulate respiratory circuit activity by multiple mechanisms via endogenously released serotonin and substance P

Brainstem serotonin (5-HT) neurons modulate activity of many neural circuits in the mammalian brain, but in many cases endogenous mechanisms have not been resolved. Here, we analyzed actions of raphé 5-HT neurons on respiratory network activity including at the level of the pre-Bötzinger complex (pre-BötC) in neonatal rat medullary slices in vitro, and in the more intact nervous system of juvenile rats in arterially perfused brainstem-spinal cord preparations in situ. At basal levels of activity, excitation of the respiratory network via simultaneous release of 5-HT and substance P (SP), acting at 5-HT(2A/2C), 5-HT(4), and/or neurokinin-1 receptors, was required to maintain inspiratory motor output in both the neonatal and juvenile systems. The midline raphé obscurus contained spontaneously active 5-HT neurons, some of which projected to the pre-BötC and hypoglossal motoneurons, colocalized 5-HT and SP, and received reciprocal excitatory connections from the pre-BötC. Experimentally augmenting raphé obscurus activity increased motor output by simultaneously exciting pre-BötC and motor neurons. Biophysical analyses in vitro demonstrated that 5-HT and SP modulated background cation conductances in pre-BötC and motor neurons, including a nonselective cation leak current that contributed to the resting potential, which explains the neuronal depolarization that augmented motor output. Furthermore, we found that 5-HT, but not SP, can transform the electrophysiological phenotype of some pre-BötC neurons to intrinsic bursters, providing 5-HT with an additional role in promoting rhythm generation. We conclude that raphé 5-HT neurons excite key circuit components required for generation of respiratory motor output.

Neuromodulation and the orchestration of the respiratory rhythm

The respiratory system is continuously modulated by numerous aminergic and peptidergic substances that act at all levels of integration: from the sensory level to the level of central networks and motor nuclei. The same neuronal networks receive inputs from multiple modulators released locally as well as from distal nuclei. All parameters of respiratory control are controlled by multiple neuromodulators. By partly converging onto similar G-proteins and second messenger systems, acetylcholine, norepinephrine, histamine, serotonin (5-HT), dopamine, ATP, substance P, cholecystokinin (CCK) can increase frequency, regularity and amplitude of respiratory activity. Yet, the same modulator can also exert differential effects on respiratory activity by acting on different receptors partly in the same neurons. In the pre-Bötzinger complex (pre-BötC) modulators can differentially modulate frequency and amplitude in different types of pacemaker neurons. Similarly motoneurons located in different motor nuclei receive differential amplitude modulation from different modulators. Thus, modulators are capable of orchestrating and modulating different parameters of respiratory activity by differentially targeting different cellular targets. A disturbance in modulatory control may lead to Sudden Infant Death Syndrome (SIDS) and erratic breathing.

Serotoninergic receptor 1A in the sudden infant death syndrome brainstem medulla and associations with clinical risk factors

The immunoreactivity of the serotoninergic receptor subtype 1A (5HT(1A)R) was quantitatively analyzed in the human infant brainstem medulla (caudal and rostral levels). We hypothesized that immunoreactivity of 5HT(1A)R would be reduced in infants diagnosed with sudden infant death syndrome (SIDS). In particular that those infants with known clinical risk factors (including cigarette smoke exposure, bed sharing and sleep position) would have greater changes than those without clinical risks. Comparing SIDS (n = 67) to infants who died suddenly with another diagnosis (non-SIDS, n = 25), we found decreased 5HT(1A)R immunoreactivity in the majority of the nuclei studied at the rostral medulla level including dorsal motor nucleus of the vagus (DMNV), nucleus of the solitary tract, vestibular, and inferior olivary nucleus (ION). There was a significant relationship with all risk factors for 5HT(1A)R, especially for DMNV, suggesting that 5HT(1A)Rs are highly vulnerable to various insults within the SIDS DMNV. This study not only provides further evidence of abnormalities within the brainstem serotoninergic system of SIDS infants, but also shows that these changes may be associated with exposure to clinical risk factors.

Apneusis responding to buspirone in multiple sclerosis

Apneusis is a disturbance of respiratory rhythm characterized by severely prolonged inspiratory effort, and is caused by bilateral lesions in the dorsal pons. In humans it is most commonly caused by pontine infarction and has rarely been reported in multiple sclerosis (MS). Here we report on a patient with MS who developed episodic apneusis which responded to treatment with buspirone, a serotonin type 1A receptor agonist.

Effects of buspirone on posthypoxic ventilatory behavior in the C57BL/6J and A/J mouse strains

Buspirone, a partial agonist of the serotonergic 5-HT1A receptor, improves breathing irregularities in humans with Rett syndrome or brain stem injury. The purpose of this study was to examine whether buspirone alters posthypoxic ventilatory behavior in C57BL/6J (B6) and A/J mouse strains. Measurements of ventilatory behavior were collected from unanesthetized adult male mice (n=6 for each strain) using the plethysmographic method. Mice were given intraperitoneal injections of vehicle or several doses of buspirone and exposed to 2 min of hypoxia (10% O2) followed by rapid reoxygenation (100% O2). Twenty minutes later, mice were tested for hypercapnic response (8% CO(2)-92% O2). On a separate day, mice were injected with the 5-HT1A receptor antagonist 4-iodo-N-{2-[4-(methoxyphenyl)-1-piperazinyl] ethyl}-N-2-pyridinylbenzamide (p-MPPI) before the injection of buspirone, and measurements were repeated. In separate studies, arterial blood-gas analysis was performed for each strain (n=12 in B6 and 10 in A/J) with buspirone or vehicle. In both strains, buspirone stimulated ventilation at rest. In the B6 mice, the hypoxic response was unchanged, but the response to hypercapnia was reduced with buspirone (5 mg/kg; P<0.05). With reoxygenation, vehicle-treated B6 exhibited periodic breathing and greater variation in ventilation compared with A/J (P<0.01). In B6 animals, >or=3 mg/kg of buspirone reduced variation and prevented the occurrence of posthypoxic periodic breathing. Both effects were reversed by p-MPPI. Treatment effect of buspirone was not explained by a difference in resting arterial blood gases. We conclude that buspirone improves posthypoxic ventilatory irregularities in the B6 mouse through its agonist effects on the 5-HT1A receptor.

Time- and dose-related effects of three 5-HT receptor ligands on the genioglossus activity in anesthetized and conscious rats

Clinical trials in obstructive sleep apnea syndrome patients reported moderate effects of serotoninergic drugs on oropharyngeal apneas, although numerous specific 5-HT ligands highly modulate the genioglossus muscle (GG) activity in experiments performed in anesthetized animals. The purpose of this study was to investigate time- and dose-related effects of central and systemic injections of 8-OHDPAT (5-HT1A agonist), SB224289 (5-HT1B antagonist), and DOI (5-HT2A/2C agonist) on the GG activity in anesthetized and conscious rats. Electromyographic recordings of the GG activity (GGemg) were analyzed after central and systemic injections of each drug in ketamine-xylazine anesthetized rats. Electroencephalograms (EEG), as well as neck and GG muscle activities (Nemg and GGemg), were recorded in 15 additional rats to analyze changes in sleep-wake states before and after systemic injection of the drugs. Central injections of 8-OHDPAT and DOI in anesthetized rats induced clear dose-related increases in phasic and tonic GGemg activities, respectively. The time-responses were inferior to 30 min with 8-OHDPAT and over 50 min with DOI. Moderate increases in phasic GGemg activity were also observed after central, but not peripheral injection of SB and DOI. The total sleep time measured in conscious rats significantly decreased after systemic injections of DOI and 8-OHDPAT, although no change was observed in phasic or tonic GGemg activity. The dose- and time-responses of the DOI in anesthetized rat partly explain the lack of GGemg tonic change in conscious rat. The moderate effect on the GGemg phasic activity of peripheral 5-HT1A ligand injection easily explains the lack of change in conscious rat. The serotonergic modulation of the respiratory component of the GGemg remains complex, but is highly sensitive to 5-HT1A receptors after central injection in rats under anesthesia. Forthcoming therapy in OSAS should be made of mixed profiled neurotransmitters and different routes of administration.

8-OH-DPAT suppresses spontaneous central apneas in the C57BL/6J mouse strain

Apneas are common and prognostically relevant disorders of the central control of breathing, but pharmacological interventions are dissatisfying. The respiratory phenotype of C57BL/6J mice is characterized by the occurrence of spontaneous central apneas with laryngeal closure. In the present study we investigated the impact of the 5-HT(1A) receptor agonist 8-OH-DPAT on apneas in C57BL/6J mice, because of the important role of serotonin in the regulation of breathing and previous reports showing that serotonergic drugs can affect central apneas. Whole-body plethysmography in awake, unrestrained mice revealed that intraperitoneal application of 8-OH-DPAT (10microgkg(-1)) decreased the occurrence of spontaneous apneas from 1.91+/-0.25 to 1.05+/-0.05 apneas min(-1). The efficacy of 5-HT(1A) receptor activation was further verified in the in situ working heart-brainstem preparation. Here the apneas occurred at a frequency of 1.33+/-0.19min(-1). Intra-arterial perfusion with 1-2microM 8-OH-DPAT completely abolished spontaneous apneas. These results suggest that 5-HT(1A) receptor activation may be a potential treatment option for central apneas.

Peripheral 5-HT1Areceptors are not essential for increased ventilation evoked by systemic 8-OH-DPAT challenge in anaesthetized rats

The respiratory effects resulting from stimulation of 5-HT(1A) receptors were studied in spontaneously breathing rats that were: (i) neurally intact and subsequently bilaterally vagotomized; (ii) subjected to bilateral midcervical vagotomy followed by supranodosal vagotomy; (iii) midcervically vagotomized and treated by carotid sinus/body denervation; or (iv) subjected to infra- and supranodosal vagotomy followed by pharmacological blockade of 5-HT(1A) receptors. An intravenous bolus of the 5-HT(1A) receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT, 10 microg kg(-1)) evoked increases in both breathing rate and tidal volume. After section of the midcervical and supranodosal vagi, 8-OH-DPAT challenge still increased the respiratory rate and tidal volume. Carotid sinus/body denervation did not reduce the augmentation of the tidal volume, but prevented the increase in breathing rate. Blockade of 5-HT(1A) receptors with intravenous doses of 1-(2-metoxyphenyl)-4-[4-(2-phthalimido) butyl] piperazine (NAN 190; 20 microg kg(-1)) abolished all respiratory effects of 8-OH-DPAT challenge. In all the neural states, 8-OH-DPAT evoked a significant fall in mean arterial blood pressure. Pretreatment with NAN 190 reduced baseline values of mean arterial pressure and prevented 8-OH-DPAT-induced hypotension. These results indicate that: (i) 8-OH-DPAT-evoked activation of 5-HT(1A) receptors increases breathing rate and tidal volume, which persists after section of the lung vagi and the nodose ganglia, but only the increase in breathing rate was abolished by carotid sinus/body denervation; and (ii) 8-OH-DPAT hyperventilatory and hypotensive responses result from the excitation of presumed 5-HT(1A) carotid receptors and the central 5-HT(1A)-expressing neurones.

The Partial 5-Hydroxytryptamine1A Receptor Agonist Buspirone does not Antagonize Morphine-induced Respiratory Depression in Humans

Based on experiments in rats, serotonin receptor 5-hydroxytryptamine (5-HT)(1A) agonists have been proposed as a potential therapeutic strategy for the selective treatment of opioid-induced respiratory depression. We investigated the clinical applicability of this principle in healthy volunteers. Twelve subjects received 0.43 mg/kg morphine (30 mg for 70 kg body weight) administered intravenously (i.v.) over approximately 2 h. At the start of the morphine infusion, they received in a randomized, double-blind cross-over design 60 mg p.o. buspirone or placebo. Respiratory depression (hypercapnic challenge) and pain (electrical stimuli: 5 Hz sinus 0-20 mA; chemical stimuli: 200 ms gaseous CO(2) pulses applied to the nasal mucosa) were assessed at baseline, at the end of the morphine infusion, and a third time after antagonizing the opioid effects by i.v. administration of 2 mg naloxone. The linear relationship between the minute ventilation and the CO(2) concentration in the inspired air of 1.07+/-0.27 l/mm Hg CO(2) at baseline conditions became shallower (0.45+/-0.23 l/mm Hg CO(2)) after morphine administration (P<0.001), indicating respiratory depression, which was significantly reversed by naloxone (0.95+/-0.43 l/mm Hg CO(2); P=0.001). Co-administration of buspirone had no effect on morphine-induced respiratory depression (slope 0.45+/-0.23 l/mm Hg CO(2) under morphine plus placebo versus 0.38+/-0.25 l/mm Hg CO(2) under morphine plus buspirone; P=0.7). Significant morphine-induced analgesia was observed in both pain models and was reversed by naloxone but unaffected by buspirone. Buspirone significantly increased the nausea induced by morphine (P=0.011). Oral co-administration of a high dose of the clinically available 5-HT(1A) agonist buspirone cannot be advised as a remedy for opioid-induced respiratory depression. This is indicated by its lack of anti-respiratory depressive effects and by the buspirone-associated increase of morphine-induced nausea.

Opiate slowing of feline respiratory rhythm and effects on putative medullary phase-regulating neurons

Opiates have effects on respiratory neurons that depress tidal volume and air exchange, reduce chest wall compliance, and slow rhythm. The most dose-sensitive opioid effect is slowing of the respiratory rhythm through mechanisms that have not been thoroughly investigated. An in vivo dose-response analysis was performed on medullary respiratory neurons of adult cats to investigate two untested hypotheses related to mechanisms of opioid-mediated rhythm slowing: 1) Opiates suppress intrinsic conductances that limit discharge duration in medullary inspiratory and expiratory neurons, and 2) opiates delay the onset and lengthen the duration of discharges postsynaptically in phase-regulating postinspiratory and late-inspiratory neurons. In anesthetized and unanesthetized decerebrate cats, a threshold dose (3 μg/kg) of the μ-opioid receptor agonist fentanyl slowed respiratory rhythm by prolonging discharges of inspiratory and expiratory bulbospinal neurons. Additional doses (2–4 μg/kg) of fentanyl also lengthened the interburst silent periods in each type of neuron and delayed the rate of membrane depolarization to firing threshold without altering synaptic drive potential amplitude, input resistance, peak action potential frequency, action potential shape, or afterhyperpolarization. Fentanyl also prolonged discharges of postinspiratory and late-inspiratory neurons in doses that slowed the rhythm of inspiratory and expiratory neurons without altering peak membrane depolarization and hyperpolarization, input resistance, or action potential properties. The temporal changes evoked in the tested neurons can explain the slowing of network respiratory rhythm, but the lack of significant, direct opioid-mediated membrane effects suggests that actions emanating from other types of upstream bulbar respiratory neurons account for rhythm slowing.

Respiratory abnormalities resulting from midcervical spinal cord injury and their reversal by serotonin 1A agonists in conscious rats

Respiratory dysfunction after cervical spinal cord injury (SCI) has not been examined experimentally using conscious animals, although clinical SCI most frequently occurs in midcervical segments. Here, we report a C5 hemicontusion SCI model in rats with abnormalities that emulate human post-SCI pathophysiology, including spontaneous recovery processes. Post-C5 SCI rats demonstrated deficits in minute ventilation (Ve) responses to a 7% CO2 challenge that correlated significantly with lesion severities (no injury or 12.5, 25, or 50 mm x 10 g weight drop; New York University impactor; p < 0.001) and ipsilateral motor neuron loss (p = 0.016). Importantly, C5 SCI resulted in at least 4 weeks of respiratory abnormalities that ultimately recovered afterward. Because serotonin is involved in respiration-related neuroplasticity, we investigated the impact of activating 5-HT1A receptors on post-C5 SCI respiratory dysfunction. Treatment with the 5-HT1A agonist 8-hydroxy-2-(di-n-propylmino)tetralin (8-OH DPAT) (250 microg/kg, i.p.) restored hypercapnic Ve at 2 and 4 weeks after injury (i.e., approximately 39.2% increase vs post-SCI baseline; p < or = 0.033). Improvements in hypercapnic Ve response after single administration of 8-OH DPAT were dose dependent and lasted for approximately 4 h(p < or = 0.038 and p < or = 0.024, respectively). Treatment with another 5-HT1A receptor agonist, buspirone (1.5 mg/kg, i.p.), replicated the results, whereas pretreatment with a 5-HT1A-specific antagonist, 4-iodo-N-[2-[4(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (3 mg/kg, i.p.) given 20 min before 8-OH DPAT negated the effect of 8-OH DPAT. These results imply a potential clinical use of 5-HT1A agonists for post-SCI respiratory disorders.

Role of the arterial baroreflex in 5-HT1A receptor agonist-mediated sympathoexcitation following hypotensive hemorrhage

5-HT1A-receptor agonists rapidly restore blood pressure and sympathetic activity in conscious rats subjected to hypotensive hemorrhage. 5-HT1A-receptor activation has also been shown to produce a robust increase in baroreceptor-dependent, pulse-synchronous firing of cardiac sympathetic nerves in anesthetized cats. To determine whether 5-HT1A-receptor agonists reverse hemorrhage-induced suppression of sympathetic activity through facilitation of the arterial baroreflex, the effects of the 5-HT1A-receptor agonist, 8-OH-DPAT, were assessed in male Sprague-Dawley rats subjected to sinoaortic baroreceptor denervation and subsequent hypotensive hemorrhage. 8-OH-DPAT produced rapid pressor and sympathoexcitatory responses in hemorrhaged animals that were attenuated, but not blocked, by sinoaortic denervation (SAD) (+49 +/- 4 vs. +37 +/- 4 mmHg; +165 +/- 30 vs. +92 +/- 24% baseline, P < 0.01). Spectral analysis of sympathetic activity showed that SAD abolished the 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT)-mediated increases in pulse-synchronous (13 +/- 1 vs. 5 +/- 1% total power for intact vs. SAD rats, P < 0.01) and Mayer wave-related bursting (18 +/- 3 vs. 8 +/- 1% total power, P < 0.05). However, 8-OH-DPAT continued to increase total power (+72 +/- 22 vs. -63 +/- 7% prehemorrhage total power, P < 0.05) and power at the respiratory frequency (35 +/- 2 vs. 25 +/- 4% total power) in SAD animals. These data indicate that full expression of the sympathoexcitatory effect of 8-OH-DPAT requires a functional arterial baroreflex. However, a portion of the effect is due to activation of arterial baroreflex-independent sympathetic pathways.

Serotoninergic receptors in the anteroventral preoptic region modulate the hypoxic ventilatory response

Hypothalamus is a site of integration of the hypoxic and thermal stimuli on breathing and there is evidence that serotonin (5-HT) receptors in the anteroventral preoptic region (AVPO) mediate hypoxic hypothermia. Once 5-HT is involved in the hypoxic ventilatory response (HVR), we investigated the participation of the 5-HT receptors (5-HT1, 5-HT2 and 5-HT7) in the AVPO in the HVR. To this end, pulmonary ventilation (V(E)) of rats was measured before and after intra-AVPO microinjection of methysergide (a 5-HT1 and 5-HT2 receptor antagonist), WAY-100635 (a 5-HT1A receptor antagonist) and SB-269970 (a 5-HT7 receptor antagonist), followed by 60 min of hypoxia exposure (7% O2). Intra-AVPO microinjection of vehicles or 5-HT antagonists did not change V(E) during normoxic conditions. Exposure of rats to 7% O2 evoked typical hypoxia-induced hyperpnea after vehicle microinjection, which was not affected by methysergide. WAY-100635 and SB-269970 treatment caused an increased HVR, due to a higher tidal volume. Therefore, the current data provide the evidence that 5-HT acting on 5-HT1A and 5-HT7 receptors in the AVPO exert an inhibitory modulation on the HVR.

Zacopride and 8-OH-DPAT reverse opioid-induced respiratory depression and hypoxia but not catatonic immobilization in goats

Neurophysiological studies have shown that serotonergic ligands that bind to 5-HT1A, 5-HT7, and 5-HT4 serotonin receptors in brain stem have beneficial effects on respiratory neurons during opioid-induced respiratory depression. The effect of these ligands on respiratory function and pulmonary performance has not been studied. We therefore examined the effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), an agonist of 5-HT1A and 5-HT7 receptors, and zacopride, an agonist of 5-HT4 receptors, to establish whether these ligands would reverse opioid-induced respiratory depression and hypoxia without affecting the immobilizing properties of the opioid drug etorphine. When etorphine was used to sedate and immobilize goats, it significantly decreased respiratory rate (P = 0.013), percent hemoglobin oxygen saturation (P < 0.0001), and arterial oxygen partial pressure [Pa(O2); F(10,70) = 5.67, P < 0.05] and increased arterial carbon dioxide partial pressure [F(10,70) = 3.87, P < 0.05] and alveolar-arterial oxygen partial pressure gradient [A-a gradients; F(10,70) = 8.23, P < 0.0001]. Zacopride and 8-OH-DPAT, coadministered with etorphine, both attenuated the effects of etorphine; respiration rates did not decrease, and percent hemoglobin oxygen saturation and Pa(O2) remained elevated. Zacopride decreased the hypercapnia, indicating an improvement in ventilation, whereas 8-OH-DPAT did not affect the hypercapnia and, therefore, did not improve ventilation. The main beneficial effect of 8-OH-DPAT was on the pulmonary circulation; it improved oxygen diffusion, indicated by the normal A-a gradients, presumably by improving ventilation perfusion ratios. Neither zacopride nor 8-OH-DPAT reversed etorphine-induced catatonic immobilization. We conclude that serotonergic drugs that act on 5-HT1A, 5-HT7, and 5-HT4 receptors reverse opioid-induced respiratory depression and hypoxia without reversing catatonic immobilization.

Serotonin receptors: guardians of stable breathing

Disturbances of breathing arising from failures of the respiratory center are not uncommon. Among them, breath holding and apnea occur most frequently as consequences of pulmonary and cardiac diseases, hypoxia, head trauma, cerebral inflammatory processes, genetic defects, degenerative brain diseases, alcoholism, deep anesthesia and drug overdose. They are often life-threatening and fail to respond to existing pharmacotherapies. After extensive research, there is now a reliable basis for new strategies to treat respiratory disturbances by pharmacological manipulation of intracellular signaling pathways, particularly those involving the serotonin receptor family. Specific activation of these pathways effectively prevails respiratory disturbances and can be extended to treatment of life-threatening respiratory disorders in patients.

Maturation of respiratory reflex responses in the fetus and neonate

Respiratory control in the fetus and neonate is quite immature when compared to that of adults. This immaturity involves all facets of respiration including respiratory responses to hypoxia, hypercapnia, an exaggerated apnoeic response to laryngeal stimulation and immature responses to activation of pulmonary afferents. The net result of this immaturity of breathing responses is the vulnerability of neonates and especially preterm infants to apnoea and respiratory pauses. The mechanisms behind immature control of breathing are not fully understood, but seem to originate from a predominance of inhibitory input early in life on respiratory centres. The relative contribution of up-regulation of inhibitory pathways versus down-regulation of excitatory ones is not clear. Multiple neurotransmitters have been implicated in the regulation of breathing in mammals and some of them are discussed in this chapter.

Serotonin 1A receptor agonists reverse respiratory abnormalities in spinal cord-injured rats

Contusion spinal cord injury (SCI) at T8 produces respiratory abnormalities in conscious rats breathing room air and challenged with CO2. In seeking ways to improve respiration after SCI, we tested drugs that stimulate serotonin 1A (5-HT1A) receptors, based on our previous findings that these agents can counteract respiratory depression produced by morphine overdose. Respiratory function was measured with a head-out plethysmograph system in conscious rats. T8 SCI rats (n = 5) showed decreased tidal volume (Vt; 0.90 +/- 0.02-0.66 +/- 0.03 ml; p < 0.05) and increased respiratory rate (f;91 +/- 3.7-132 +/- 5.7 breaths/min; p < 0.05) with room air ventilation at 24 hr after injury. They also exhibited a diminished response to the respiratory stimulating effect of 7% CO2; minute ventilation increased to 250 +/- 17 ml/min before, but only to 162 +/- 15 ml/min at 24 hr after SCI (p < 0.05). Respiratory deficits during room air ventilation were also observed at 7 d after injury (n = 3). Treatment with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylmino)tetralin (8-OH-DPAT; 250 microg/kg, i.p.) at 24 hr (n = 5) or 7 d (n = 3) after injury normalized Vt, f, and the respiratory response to 7% CO2. Identical results were obtained with another 5-HT1A receptor agonist, buspirone (1.5 mg/kg, i.p.; n = 3). In contrast, intraperitoneal saline vehicle administration (n = 5) showed no beneficial effects on SCI-impaired respiration. Finally, pretreatment with a specific antagonist of 5-HT1A receptors, 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (3 mg/kg, i.p.; n = 3) given 20 min before 8-OH-DPAT, prevented 8-OH-DPAT from restoring respiration to normal. Our results demonstrate that drugs that stimulate 5-HT1A receptors counteract respiratory abnormalities in conscious rats after SCI.

Serotonin 1A receptor agonists reverse respiratory abnormalities in spinal cord-injured rats

For over a century, the behavior of the aorta and other large arteries has been described as passive elastic tubes in which no active contraction occurs in the smooth muscle wall. In response to pulsatile pressure changes, the vessels undergo a 'passive' elastic dilatation–contraction cycle, described as a “Windkessel” effect. However, Mangel and colleagues have presented evidence that is contrary to this view. They reported that in the rabbit, the aorta undergoes rhythmic 'active' (contraction) during the cardiac cycle; but these findings have been largely ignored. In the present study, we observed spontaneous contractions in synchrony with the heartbeat in another species (rat). In addition we demonstrate that aorta contractions are of neurogenic origin. Electrical stimulation of the aorta evoked contractions that occur at a rate that is in the range of the animal's heartbeat and are suppressed by tetrodotoxin and the alpha-adrenergic receptor blocker, phentolamine. Altogether, these findings indicate that aortic contractions are under neural control from the heart.

Serotonergic modulation of respiratory motoneurons and interneurons in brainstem slices of perinatal rats

Respiration-related membrane potential fluctuations were recorded in hypoglossal (XII) motoneurons and pre-Bötzinger complex (pre-BötC) interneurons in medullary slices from perinatal rats. Bath application of serotonin (5-HT) evoked a ketanserine-sensitive depolarization (approximately 11 mV) and tonic spike discharge in XII motoneurons, whereas pre-BötC neurons responded with a <6 mV depolarization and no tonic discharge. The membrane effects were accompanied by an increase in respiratory frequency by up to 260% in 64% of preparations. A frequency decrease leading to block of respiratory activity could also occur (20%) as well as an initial acceleration that turned into a frequency depression (16%). In contrast, iontophoresis of 5-HT into the pre-BötC exclusively increased respiratory frequency by 30-220%, whereas iontophoresis into the XII nucleus did not change respiratory frequency but induced tonic nerve discharge. The effects of local iontophoretic administration of 5-HT on membrane properties of XII and pre-BötC cells were very similar to those upon bath application. Bath application and iontophoresis of the 5-HT2 receptor agonist -methyl-hydroxytryptamine mimicked the effects of 5-HT. Bath application of the 5-HT1A receptor agonist 8-hydroxydipropylaminotetralin hydrobromide did not affect XII nerve bursting or pre-BötC neurons. Iontophoresis of 8-hydroxydipropylaminotetralin hydrobromide had almost no effect on respiratory frequency and induced in the interneurons either a depolarization or hyperpolarization (<5 mV) which was blocked by the 5-HT1A receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)N-2-pyridinylcyclohexane carboxamide. In conclusion, 5-HT-evoked tonic excitation of respiratory XII motoneurons is mediated by postsynaptic 5-HT2 receptors. The excitatory effects on respiratory rhythm are also primarily attributable to postsynaptic 5-HT2 receptors of pre-BötC neurons. Additional modulatory effects on the interneurons appear to be mediated by postsynaptic 5-HT1A receptors.

Hyperpolarization-activated current I(h) in nucleus of solitary tract neurons: regional difference in serotonergic modulation

The nucleus of solitary tract (NTS) contains diverse neural circuits responsible for basic vital functions. We examined the effect of serotonin (5-HT) on hyperpolarization-activated current (I(h)) in neurons acutely isolated from caudal, medial and rostral parts of the NTS. Caudal and medial NTS neurons showed a large amplitude of I(h) compared with rostral neurons. In these neurons, perfusion with 5-HT potentiated Ih amplitude in a concentration-dependent manner. The effect of 5-HT was blocked by NAN-190, a 5-HT1A receptor antagonist. Thus, 5-HT1A receptors may regulate I(h) channel activity in caudal and medial NTS neurons.

Effect of 8-OH DPAT and ketanserin on the ventilatory acclimatization to hypoxia in awake goats

We have previously reported that broad-spectrum serotonergic blockade increased the acute hypoxic ventilatory response in awake goats. The purpose of the present study was to examine the putative serotonin (5-HT) receptor subtype(s) that may have contributed to this response. Following the administration of the selective 5-HT(1A)-receptor agonist, 8-hydroxy-(2-di-n-propylamino) tetralin (8-OH DPAT, 0.1 mg x kg(-1)i.v.), there was an increase in normoxic expired minute ventilation (V(E)) that was due to an increased breathing frequency. V(E) increased during hypoxia but the change in V(E) (Delta V(E)) associated with hypoxic exposure was not different from the Delta V(E) of saline treated goats. The combination of 8-OH DPAT and a selective 5-HT(2A/2C) receptor antagonist, ketanserin (0.1 and 1.0 mg x kg(-1)i.v., respectively), also increased normoxic V(E) but did not alter the hypoxia induced Delta V(E). Both 8-OH DPAT alone and in combination with ketanserin attenuated the change in V(E) associated with sustained hypoxia but neither was able to attenuate the increased hypoxic ventilatory response that occurs following acclimatization. The augmented acute hypoxic ventilatory response that we previously reported does not appear to be mediated via the activation of the 5-HT(1A) receptor or through the combination of 5-HT(1A) activation and 5-HT(2A/2C) blockade. The results of this study further suggest that while 5-HT may modulate hypoxic ventilation it does not appear to be necessary for the development of ventilatory acclimatization to hypoxia.

Short-term modulation of the exercise ventilatory response in goats: effects of 8-OH-DPAT and MPPI

Increased respiratory dead space increases the exercise ventilatory response, a response known as short-term modulation (STM). We hypothesized that STM results from a spinal, serotonin (5-HT)-dependent mechanism. Because 5-HT(1A) autoreceptors on caudal brain stem raphe neurons inhibit 5-HT release, we hypothesized that 5-HT(1A)-receptor agonists would inhibit, whereas 5-HT(1A)-receptor antagonists would enhance, STM. Ventilatory and arterial blood-gas measurements were made at rest and during exercise (4.0-4.5 km/h, 5% grade) in goats with the respiratory mask alone or with increased dead space (0.20-0.25 liter), before and after intravenous administration of the 5-HT(1A)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg/kg) or the antagonist 4-iodo-N-(2-[4-(methoxyphenyl)-1-piperazinyl]ethyl)-N-2-pyridinylbenz amide (MPPI; 0.08 mg/kg). 8-OH-DPAT increased the slope of the arterial PCO(2) vs. metabolic CO(2) production relationship and decreased the ventilation vs. metabolic CO(2) production relationship during exercise with increased dead space (not with the mask alone), indicating an impairment of STM. In contrast, MPPI had minimal effects on any measured variable. Although nonspecific effects of 8-OH-DPAT cannot be ruled out, impaired STM is consistent with the hypothesis that STM requires active raphe serotonergic neurons and 5-HT release.

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.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

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

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

Methysergide augments the acute, but not the sustained, hypoxic ventilatory response in goats

Ventilatory acclimatization to hypoxia (VAH) is the time-dependent increase in ventilation that occurs during sustained hypoxia. As serotonin (5-HT) has been reported to be an important modulator of respiratory output, 5-HT may also play a role in VAH. Methysergide (a broad-spectrum 5-HT antagonist), was given to awake goats (1 mg kg(-1) i.v.) 30 min prior to being exposed to 4 h of isocapnic hypoxia. Although methysergide slightly decreased arterial pH, presumably due to a non-significant increase in arterial P(CO2), it did not alter normoxic ventilation. Following methysergide, the expired minute ventilation (VE) was significantly elevated above the control (saline) response after 30 min of hypoxia, but methysergide did not otherwise alter VAH. We repeated the study in the same goats using ketanserin, a specific 5-HT2A/2C receptor antagonist (1.2 mg kg(-1) i.v.). Ketanserin had no effect on the acute hypoxic ventilatory response, or on VAH. We conclude that while 5-HT modulates the acute hypoxic ventilatory response in goats, it does not appear to act through the 5-HT2A/2C receptor subtypes.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Respiratory depression produced by intravenously administered NBQX

To determine whether blockade of non-N-methyl-D-aspartate (non-NMDA) excitatory amino acid receptors affects breathing, we administered the non-NMDA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX), to anesthetized cats while monitoring phrenic nerve discharge, blood pressure and heart rate. NBQX, 3 and 10 mg/kg, i.v., reduced phrenic amplitude 59 +/- 20% (n = 3) and 88 +/- 6% (n = 5), respectively, and decreased respiratory rate. Phrenic activity was completely silenced in 3 animals. These effects were accompanied by decreased blood pressure and heart rate. Our data indicate that NBQX, a competitive antagonist of non-NMDA receptors, is a powerful depressant of cardiorespiratory activity.

Blockade of synaptic inhibition within the pre-Bötzinger complex in the cat suppresses respiratory rhythm generation in vivo

1. The role of synaptic inhibition in respiratory rhythm generation was analysed by microinjections of GABAA and glycine receptor antagonists into the bilateral pre-Botzinger complex (PBC) of anaesthetized cats. Central respiratory activity was monitored by phrenic nerve recordings. 2. Bilateral injections of bicuculline (50 or 100 microM) irreversibly slowed respiratory frequency and induced apneustic patterns. 3. Bilateral injections of strychnine (50 or 100 microM) greatly reduced phrenic burst amplitudes leading to increased burst frequency or irreversibly blocked rhythmic phrenic discharges. After unilateral tetrodotoxin (TTX) blockade in the PBC, strychnine injection into the contralateral PBC blocked rhythmic phrenic discharges. 4. Bilateral blockade of both GABAergic and glycinergic inhibition abolished rhythmic burst discharges and only tonic phrenic activity remained. Such tonic activity was blocked only by TTX (1 microM). 5. Potentiation of synaptic inhibition by the serotonin 1A receptor agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT; 50 microM) restored rhythmic activity only when given shortly after strychnine and bicuculline applications. It was, however, ineffective after blockade of synaptic inhibition was complete. 6. The study demonstrates the significance of synaptic inhibition in the process of respiratory generation in the adult cat in vivo.

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.