Dopaminergic modulation on respiratory rhythm in rat brainstem-spinal cord preparation

Amphetamines modulate fentanyl-depressed respiration in a bidirectional manner

BACKGROUND

The opioid epidemic remains one of the most pressing public health crises facing the United States. Fentanyl and related synthetic opioid agonists have largely driven the rising rates of associated overdose deaths, in part, because of their surreptitious use as substitutes for other opioids and as adulterants in psychostimulants. Deaths involving opioids typically result from lethal respiratory depression, and it is currently unknown how co-use of psychostimulants with opioids affects respiratory toxicity. Considering psychostimulant overdoses have increased over 3-fold since 2013, and half of those co-involved opioids, this is a cardinal question.

METHODS

Naloxone, d-amphetamine (AMPH), and (±)-methamphetamine (METH) were evaluated for their effects on basal and fentanyl-depressed respiration. Minute volume (MVb) was measured in awake, freely moving mice via whole-body plethysmography to quantify fentanyl-induced respiratory depression and its modulation by dose ranges of each test drug.

RESULTS

Naloxone immediately reversed respiratory depression induced by fentanyl only at the highest dose tested (10 mg/kg). Both AMPH and METH exhibited bidirectional effects on MVb under basal conditions, producing significant (p ≤ 0.05) depressions then elevations of respiration as dose increased. Under depressed conditions the bidirectional effects of AMPH and METH on respiration were exaggerated, exacerbating and then reversing fentanyl-induced depression as dose increased.

CONCLUSIONS

These results indicate that co-use of amphetamines with fentanyl may worsen respiratory depression, but conversely, monoaminergic components of the amphetamines may possibly be exploited to mitigate fentanyl overdose.

Effects of Prenatal Cannabinoids Exposure upon Placenta and Development of Respiratory Neural Circuits

Cannabis use has risen dangerously during pregnancy in the face of incipient therapeutic use and a growing perception of safety. The main psychoactive compound of the Cannabis sativa plant is the phytocannabinoid delta-9-tetrahydrocannabinol (A-9 THC), and its status as a teratogen is controversial. THC and its endogenous analogues, anandamide (AEA) and 2-AG, exert their actions through specific receptors (eCBr) that activate intracellular signaling pathways. CB1r and CB2r, also called classic cannabinoid receptors, together with their endogenous ligands and the enzymes that synthesize and degrade them, constitute the endocannabinoid system. This system is distributed ubiquitously in various central and peripheral tissues. Although the endocannabinoid system's most studied role is controlling the release of neurotransmitters in the central nervous system, the study of long-term exposure to cannabinoids on fetal development is not well known and is vital for understanding environmental or pathological embryo-fetal or postnatal conditions. Prenatal exposure to cannabinoids in animal models has induced changes in placental and embryo-fetal organs. Particularly, cannabinoids could influence both neural and nonneural tissues and induce embryo-fetal pathological conditions in critical processes such as neural respiratory control. This review aims at the acute and chronic effects of prenatal exposure to cannabinoids on placental function and the embryo-fetal neurodevelopment of the respiratory pattern. The information provided here will serve as a theoretical framework to critically evaluate the teratogen effects of the consumption of cannabis during pregnancy.

Manipulation of gut microbiota blunts the ventilatory response to hypercapnia in adult rats

BACKGROUND

It is increasingly evident that perturbations to the diversity and composition of the gut microbiota have significant consequences for the regulation of integrative physiological systems. There is growing interest in the potential contribution of microbiota-gut-brain signalling to cardiorespiratory control in health and disease.

METHODS

In adult male rats, we sought to determine the cardiorespiratory effects of manipulation of the gut microbiota following a 4-week administration of a cocktail of antibiotics. We subsequently explored the effects of administration of faecal microbiota from pooled control (vehicle) rat faeces, given by gavage to vehicle- and antibiotic-treated rats.

FINDINGS

Antibiotic intervention depressed the ventilatory response to hypercapnic stress in conscious animals, owing to a reduction in the respiratory frequency response to carbon dioxide. Baseline frequency, respiratory timing variability, and the expression of apnoeas and sighs were normal. Microbiota-depleted rats had decreased systolic blood pressure. Faecal microbiota transfer to vehicle- and antibiotic-treated animals also disrupted the gut microbiota composition, associated with depressed ventilatory responsiveness to hypercapnia. Chronic antibiotic intervention or faecal microbiota transfer both caused significant disruptions to brainstem monoamine neurochemistry, with increased homovanillic acid:dopamine ratio indicative of increased dopamine turnover, which correlated with the abundance of several bacteria of six different phyla.

INTERPRETATION

Chronic antibiotic administration and faecal microbiota transfer disrupt gut microbiota, brainstem monoamine concentrations and the ventilatory response to hypercapnia. We suggest that aberrant microbiota-gut-brain axis signalling has a modulatory influence on respiratory behaviour during hypercapnic stress. FUND: Department of Physiology and APC Microbiome Ireland, University College Cork, Ireland.

Persistent Expression of Serotonin Receptor 5b Alters Breathing Behavior in Male MeCP2 Knockout Mice

Mutations in the transcription factor methyl-CpG-binding protein 2 (MeCP2) cause the neurodevelopmental disorder Rett syndrome (RTT). Besides many other neurological problems, RTT patients show irregular breathing with recurrent apneas or breath-holdings. MeCP2-deficient mice, which recapitulate this breathing phenotype, show a dysregulated, persistent expression of G-protein-coupled serotonin receptor 5-ht_5b (Htr5b) in the brainstem. To investigate whether the persistence of 5-ht_5b expression is contributing to the respiratory phenotype, we crossbred MeCP2-deficient mice with 5-ht_5b-deficient mice to generate double knockout mice (Mecp2^−/y;Htr5b^−/−). To compare respiration between wild type (WT), Mecp2^−/y and Mecp2^−/y;Htr5b^−/− mice, we used unrestrained whole-body plethysmography. While the breathing of MeCP2-deficient male mice (Mecp2^−/y) at postnatal day 40 is characterized by a slow breathing rate and the occurrence of prolonged respiratory pauses, we found that in MeCP2-deficient mice, which also lacked the 5-ht_5b receptor, the breathing rate and the number of pauses were indistinguishable from WT mice. To test for a potential mechanism, we also analyzed if the known coupling of 5-ht_5b receptors to G_i proteins is altering second messenger signaling. Tissue cAMP levels in the medulla of Mecp2^−/y mice were decreased as compared to WT mice. In contrast, cAMP levels in Mecp2^−/y;Htr5b^−/− mice were indistinguishable from WT mice. Taken together, our data points towards a role of 5-ht_5b receptors within the complex breathing phenotype of MeCP2-deficient mice.

Serotonin in the solitary tract nucleus shortens the laryngeal chemoreflex in anaesthetized neonatal rats

What is the central question of this study? Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who died of SIDS. Therefore, we tested the hypothesis that serotonin in the nucleus of the solitary tract (NTS) would shorten reflex apnoea. What is the main finding and its importance? Serotonin microinjected into the NTS shortened the apnoea and respiratory inhibition associated with the laryngeal chemoreflex. Moreover, this effect was achieved through a 5-HT3 receptor. This is a new insight that is likely to be relevant to the pathogenesis of SIDS. The laryngeal chemoreflex (LCR), an airway-protective reflex that causes apnoea and bradycardia, has long been suspected as an initiating event in the sudden infant death syndrome. Serotonin (5-HT) and 5-HT receptors may be deficient in the brainstems of babies who die of sudden infant death syndrome, and 5-HT seems to be important in terminating apnoeas directly or in causing arousals or as part of the process of autoresuscitation. We hypothesized that 5-HT in the brainstem would limit the duration of the LCR. We studied anaesthetized rat pups between 7 and 21 days of age and made microinjections into the cisterna magna or into the nucleus of the solitary tract (NTS). Focal, bilateral microinjections of 5-HT into the caudal NTS significantly shortened the LCR. The 5-HT1a receptor antagonist, WAY 100635, did not affect the LCR consistently, nor did a 5-HT2 receptor antagonist, ketanserin, alter the duration of the LCR. The 5-HT3 specific agonist, 1-(3-chlorophenyl)-biguanide, microinjected bilaterally into the caudal NTS significantly shortened the LCR. Thus, endogenous 5-HT released within the NTS may curtail the respiratory depression that is part of the LCR, and serotonergic shortening of the LCR may be attributed to activation of 5-HT3 receptors within the NTS. 5-HT3 receptors are expressed presynaptically on C fibre afferents of the superior laryngeal nerve, and serotonergic shortening of the LCR may be mediated presynaptically by enhanced activation of inhibitory interneurons within the NTS.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Opposing effects on the phrenic motor pathway attributed to dopamine-D1 and -D3/D2 receptor activation

Previous in vivo studies revealed that dopamine-D1-agonists elevate excitability of ventral respiratory column (VRC) neurons and increase discharge activity in the phrenic motor output through actions in the brainstem. In this in vivo study performed on pentobarbital-anesthetized cats, we show that D1-agonists (SKF-38393, dihydrexidine) given intravenously enhanced discharge activity in VRC inspiratory neurons and the phrenic nerve in two stages; discharge intensity first increased to a peak and then discharge duration increased. Cross-correlation analysis of VRC inspiratory neuron and phrenic nerve discharges showed that both stages increased strength of coupling between medullary inspiratory neurons and the phrenic motoneuron output. Intracellular recording and microiontophoresis experiments indicated that D1-agonists produced their stimulatory effects indirectly through actions on synaptic inputs to VRC inspiratory neurons. Because other laboratories have provided evidence that dopamine acting on other types of receptors depresses respiratory neuron excitability we tested the effects of piribedil, an agonist that activates receptors of the generally depressant D3/D2-dopamine receptor family, on phrenic nerve activity. Piribedil depressed phrenic nerve inspiratory discharge intensity, prolonged discharge duration, slowed burst frequency and slowed rate of action potential augmentation. The effects of piribedil were partially counteracted by intravenous injection of dihydrexidine. We propose that under normal, steady state conditions, D1-receptor-mediated excitatory modulation of phrenic motor output overrides D3/D2-receptor mediated inhibition.

Two types of independent bursting mechanisms in inspiratory neurons: an integrative model

The network of coupled neurons in the pre-Bötzinger complex (pBC) of the medulla generates a bursting rhythm, which underlies the inspiratory phase of respiration. In some of these neurons, bursting persists even when synaptic coupling in the network is blocked and respiratory rhythmic discharge stops. Bursting in inspiratory neurons has been extensively studied, and two classes of bursting neurons have been identified, with bursting mechanism depends on either persistent sodium current or changes in intracellular Ca(2+), respectively. Motivated by experimental evidence from these intrinsically bursting neurons, we present a two-compartment mathematical model of an isolated pBC neuron with two independent bursting mechanisms. Bursting in the somatic compartment is modeled via inactivation of a persistent sodium current, whereas bursting in the dendritic compartment relies on Ca(2+) oscillations, which are determined by the neuromodulatory tone. The model explains a number of conflicting experimental results and is able to generate a robust bursting rhythm, over a large range of parameters, with a frequency adjusted by neuromodulators.

Adrenaline modulates on the respiratory network development

Adrenaline regulates respiratory network, however, adrenergic contribution to the developing respiratory center has not well studied. Adrenaline application on embryonic day 17 medulla-spinal cord block preparations abolished non-respiratory activity and enhanced respiratory frequency. Phentolamine application on neonatal brainstem-spinal cord preparations that produced stable neonatal respiration resulted in respiratory destabilization. In E19 rat, adrenaline switched from enhancement to depression of the respiratory rhythm. Adrenaline modulated GABAergic synaptic transmission to respiratory neurons in late developmental stage. These results suggest that the involvement of central adrenergic modulation on the respiratory network maturation.

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.

D1/D2-dopamine receptor agonist dihydrexidine stimulates inspiratory motor output and depresses medullary expiratory neurons

It is now accepted that dopamine plays an important neuromodulatory role in the central nervous control of respiration. D1, D2, and D4 subtypes of the receptor seem to be important players, but the assignment of various respiratory tasks to specific subtypes of the dopamine receptor is a work in progress. In the present investigation, dihydrexidine (DHD), a full dopamine receptor agonist with affinity for both D1- and D2-subtypes of receptor, was tested for its effects on inspiratory neurons and motor output and on membrane potential properties of medullary bulbospinal expiratory augmenting expiratory neurons in the pentobarbital anesthetized adult cat. The effects of DHD were compared with those of the highly selective D1-dopamine receptor (D1R) agonists SKF-38393 and 6-chloro-APB. DHD increased the intensity and duration of inspiratory motor output. Phrenic nerve discharge intensity was increased and prolonged, contributing to elevated inspiratory effort and duration when spontaneous breathing was monitored with tracheal pressure measurements. Intracellular recording from rostral medullary inspiratory neurons revealed that DHD, like SKF-38393, increases and prolongs inspiratory phase membrane depolarization, resulting in a longer and more intense discharge of action potentials. Remarkably, DHD had opposite effects on Aug-E neurons. Membrane potential was hyperpolarized, and action potential discharges were suppressed or abolished. In association with reduction of discharge intensity, action potential half width was reduced and after-hyperpolarization increased. The stimulatory action of DHD on inspiratory motor output is attributed to D1R effects, while the depression of Aug-E neurons seems to be linked to D2R actions on the postsynaptic membrane.

Opioidergic and dopaminergic modulation of respiration

Opioids, dopamine and their receptors are present in many regions of the bulbar respiratory network. The physiological importance of endogenous opioids to respiratory control has not been explicitly demonstrated. Nonetheless, studies of opioidergic respiratory mechanisms are important because synthetic opiate drugs have respiratory side effects that in some situations pose health risks and limit their therapeutic usefulness. They can depress breathing depth and rate, blunt respiratory responsiveness to CO2 and hypoxia, increase upper airway resistance and reduce pulmonary compliance. The opiate respiratory disturbances are mainly due to agonist activation of mu- and delta-subtypes of receptor and involve specific types of respiratory-related neurons in the ventrolateral medulla and the dorsolateral pons. Endogenous dopaminergic modulation in the CNS and carotid bodies enhances CO2-dependent respiratory drive and depresses hypoxic drive. In the CNS, synthetic agonists with selectivity for D1-and D4-types of receptor slow respiratory rhythm, whereas D2-selective agonists modulate acute and chronic responses to hypoxia. D1-receptor agonists also act centrally to increase respiratory responsiveness to CO2, and counteract opiate blunting of CO2-dependent respiratory drive and depression of breathing. Cellular targets and intracellular mechanisms responsible for opioidergic and dopaminergic respiratory effects for the most part remain to be determined.

Spinal activation of the cAMP-PKA pathway induces respiratory motor recovery following high cervical spinal cord injury

The present study investigated the involvement of the adenosine 3'5'-cyclic monophosphate-dependent protein kinase A (cAMP-PKA) pathway in the activation of the crossed-phrenic pathways after left C2 spinal cord hemisection. Experiments were conducted on left C2 spinal cord hemisected, anesthetized, vagotomized, pancuronium paralyzed, and artificially ventilated male Sprague-Dawley rats. One week post-injury, the ipsilateral phrenic nerve exhibited no respiratory-related activity indicating a functionally complete hemisection. Intrathecal spinal cord administration of the cAMP analog, 8-Br-cAMP at the level of the phrenic nucleus resulted in an enhancement of contralateral phrenic nerve output and a restoration of respiratory-related activity in the phrenic nerve ipsilateral to the hemisection. Furthermore, pre-treatment with Rp-8-Br-cAMP, a PKA inhibitor, abolished the effects of 8-Br-cAMP. These results suggest that PKA activation is necessary for the cAMP-mediated respiratory recovery following high cervical spinal cord injury and that activation of intracellular signaling cascades may represent an important strategy for improving respiratory function after spinal cord injury.

In hamsters the D1 receptor antagonist SCH23390 depresses ventilation during hypoxia

During exposure of animals to hypoxia, brain and blood dopamine levels increase stimulating dopaminergic receptors which influence the integrated ventilatory response to low oxygen. The purpose of the present study is to test the hypothesis that in conscious hamsters, systemic antagonism of D(1) receptors would depress their breathing in air and in response to hypoxic and hypercapnic challenges. Nine male hamsters were treated with saline or 0.25 mg/kg SCH-23390 (SCH), a D(1) receptor antagonist that crosses the blood-brain barrier. Ventilation was determined using the barometric method, and oxygen consumption and CO(2) production were evaluated utilizing the flow-through method. During exposure to air, SCH decreased frequency of breathing. During exposure to hypoxia (10% oxygen in nitrogen), relative to saline, SCH-treated hamsters decreased minute ventilation by decreasing tidal volume and oxygen consumption but not CO(2) production. During exposure to hypercapnia (5% CO(2) in 95% O(2)), frequency of breathing was decreased with SCH, but there was no significant effect on minute ventilation. Relative to saline treatment body temperature was lower in SCH-treated hamsters by 0.6 degrees C. These results demonstrate that in hamsters D(1) receptors can modulate control of ventilation in air and during hypoxia and hypercapnic exposures. Whether D(1) receptors located centrally or on carotid bodies modulate these effects is not clear from this study.

Adrenaline contributes to prenatal respiratory maturation in rat medulla–spinal cord preparation

Adrenaline is a potent respiratory regulator. However, adrenergic contribution to the developing respiratory center has not been studied extensively. Adrenaline application on embryonic day 17 medulla-spinal cord block preparations abolished non-respiratory activity and enhanced respiratory frequency. Phentolamine application on neonatal blocks that produced stable neonatal respiration resulted in respiratory destabilization. These results suggest that central adrenergic modulation is involved in fetal respiratory development and maintenance of stable respiration.

Dopamine desynchronizes the pace-making neuronal activity of rat respiratory rhythm generation

In an excised Wistar rat medulla-spinal cord block preparation we previously found that dopamine slows respiratory rhythm by activation of dopamine D(4) receptors [Fujii et al., (2004)Neurosci. Res., 50, 355-359.] In the present paper, we investigated the effect of dopamine on pre-inspiratory (Pre-I) and inspiratory (I) neurons using the combination of an optical recording technique with a voltage-sensitive dye, unit recording and patch-clamp recording. Optical imaging of the ventral surface of the block preparation disclosed different locations and activity patterns of Pre-I and I neurons. In addition to slowing the rhythm, dopamine depressed respiratory activity of Pre-I neurons collectively but not that of I neurons. The dopaminergic suppression of Pre-I neurons was mimicked by a dopamine D(4) receptor agonist, PD168077. Unit recording and patch-clamp recording demonstrated that dopamine depolarizes Pre-I neurons, disperses Pre-I firing and depresses Pre-I phase postsynaptic potentials (PSPs) of I neurons. Immunohistological investigation revealed that Pre-I neurons express dopamine D(4) receptors. We found that approximately 60% of Pre-I neurons express dopamine D(4) receptors. These results show that dopaminergic respiratory rhythm depression is due to dispersion of synchronized Pre-I driving of I neurons caused by dopamine D(4) receptor stimulation of Pre-I neurons.

Polychlorinated biphenyl (PCB) alters acid-sensitivity of cultured neurons derived from the medulla oblongata

Polychlorinated biphenyls (PCBs) are known as environmental pollutants that may cause adverse health problems. However, little is known about the effects of PCBs on acid-sensitive neurons of the medulla oblongata, which regulate respiration. Therefore, the present study was designed to examine whether PCB alters acid-sensitivity of cultured neurons derived from the rat medulla oblongata. When extracellular pH was shifted from 7.4 to 7.0, acid-sensitive neurons showed depolarization, which was measured by voltage-sensitive fluorescent dye. Exposure to PCB (Aroclor 1254) decreased the amplitude of depolarization in low pH and increased the resting membrane potential in a dose-dependent manner. Taken together, our results indicate that PCB potentially influences acid-sensitivity through alteration of the membrane potential of acid-sensitive neurons, which could affect the regulation of respiration.