Gaspard Montandon

PreBötzinger Complex Neurokinin-1 Receptor-Expressing Neurons Mediate Opioid-Induced Respiratory Depression

The analgesic properties of the opium poppy Papever somniferum were first mentioned by Hippocrates around 400 BC, and opioid analgesics remain the mainstay of pain management today. These drugs can cause the serious side-effect of respiratory depression that can be lethal with overdose, however the critical brain sites and neurochemical identity of the neurons mediating this depression are unknown. By locally manipulating neurotransmission in the adult rat, we identify the critical site of the medulla, the preBötzinger complex, that mediates opioid-induced respiratory depression in vivo. Here we show that opioids at the preBötzinger complex cause respiratory depression or fatal apnea, with anesthesia and deep-sleep being particularly vulnerable states for opioid-induced respiratory depression. Importantly, we establish that the preBötzinger complex is fully responsible for respiratory rate suppression following systemic administration of opioid analgesics. The site in the medulla most sensitive to opioids corresponds to a region expressing neurokinin-1 receptors, and we show in rhythmically active brainstem section in vitro that neurokinin-1 receptor-expressing preBötzinger complex neurons are selectively inhibited by opioids. In summary, neurokinin-1 receptor-expressing preBötzinger complex neurons constitute the critical site mediating opioid-induced respiratory rate depression, and the key therapeutic target for its prevention or reversal.

Long-Term Consequences of Neonatal Caffeine on Ventilation, Occurrence of Apneas, and Hypercapnic Chemoreflex in Male and Female Rats

Caffeine is an adenosine receptor antagonist commonly used as a respiratory stimulant to treat neonatal apneas of premature newborn. Neonatal caffeine treatment (NCT) has long-term effects on adenosine receptor expression and distribution; however, the potential effects of NCT on respiratory control development are unknown. To address this issue, rat pups received orally each day from postnatal d 3–12, 15 mg/kg of caffeine (NCT), water (vehicle), or were undisturbed during early life (control). Measurements of resting ventilation, apnea index, and ventilatory response to moderate hypercapnia (Fico2 = 0.05) were made using whole-body plethysmography at postnatal d 20 (juvenile) and adulthood. At d 20, resting respiratory variables were not affected by the treatments. Juvenile NCT male rats showed a 22% higher minute ventilation response to hypercapnia than vehicle rats. However, oral gavage alone increased the frequency component of the response by 11%. In adult males, caffeine increased the resting respiratory frequency by 15%. In these animals, the tidal volume response to hypercapnia was increased by 15%, whereas the frequency response was decreased by 20%. In juvenile and adult females, no differences were observed between treatments. In juvenile rats of both sexes, gavage increased the apnea index by at least 200%. These results show that NCT and gavage influence respiratory control during early life and that these effects persist until adulthood. The underlying mechanisms are unclear, but may be related to persistent changes in adenosinergic neurotransmission because neonatal caffeine administration increases A1 adenosine receptor density in adult rats.

Neonatal caffeine induces sex-specific developmental plasticity of the hypoxic respiratory chemoreflex in adult rats

Caffeine is widely used to treat apneas of prematurity during the neonatal period; however, the potential consequences of administering a neonatal caffeine treatment (NCT) during a critical period for respiratory control development are unknown. The present study therefore determined whether NCT in rats alters the hypoxic respiratory chemoreflex measured at adulthood. Newborn rats received either caffeine (15 mg/kg) or water (control) each day from postnatal day 3 to 12. The ventilatory response to a hypoxic challenge (inspired O(2) fraction = 0.12) was first evaluated in awake adult female and male rats using whole body plethysmography. Results showed that NCT increased the initial phase of the breathing frequency response to hypoxia in males only. This result was confirmed in anesthetized and artificially ventilated adult male rats where NCT also increased the phrenic burst frequency response to hypoxia. RT-PCR assessment of mRNA encoding for adenosine A(1A) and A(2A) receptors, dopamine D(2) receptors, and tyrosine hydroxylase in the rat carotid bodies showed that NCT enhanced mRNA expression levels of adenosine A(2A), dopamine D(2) receptors, and tyrosine hydroxylase of males but not females. Subsequent experiments on awake male rats showed that injection of the adenosine A(2A) receptor antagonist ZM2413855 (1 mg/kg ip) before ventilatory measurements abolished, in NCT rats, the enhanced respiratory frequency response observed during the early phase of hypoxia. We propose that NCT elicits a sex-specific increase in the hypoxic respiratory chemoreflex, which is related, at least partially, to an enhancement in adenosine A(2A) receptors in the rat carotid body.

Adenosinergic modulation of respiratory activity: developmental plasticity induced by perinatal caffeine administration.

Caffeine is an adenosine receptor antagonist that is commonly used in the clinic as a respiratory stimulant to treat apnea of prematurity. A recent clinical study showed that newborns treated with caffeine present no neuro-developmental disabilities at 2 years of age in comparison to placebo-treated children [Schmidt, B., Roberts, R.S., Davis, P., Doyle, L.W., Barrington, K.J., Ohlsson, A., Solimano, A., Tin, W., 2007. Long-term effects of caffeine therapy for apnea of prematurity. N. Engl. J. Med. 357, 1893-1902]. Although neonatal caffeine administration in this population is associated with clear short- and long-term health improvements, the consequences of this treatment on basic homeostatic functions such as respiratory regulation are unknown. This article reviews evidence indicating that neonatal caffeine treatment modifies respiratory control development and that these changes persist until adulthood. The mechanisms contributing to this form of developmental plasticity are unknown but current data indicate that caffeine treatment, especially during the perinatal period, alters adenosinergic neuromodulation of the respiratory control system. While human data show that neonatal caffeine treatment is relatively safe for some aspects of neural development, the results obtained in animal studies raise important questions pertaining to the potential long-term effects of this treatment on the respiratory control system.

G-protein–gated Inwardly Rectifying Potassium Channels Modulate Respiratory Depression by Opioids

Background:Drugs acting on &mgr;-opioid receptors (MORs) are widely used as analgesics but present side effects including life-threatening respiratory depression. MORs are G-protein–coupled receptors inhibiting neuronal activity through calcium channels, adenylyl cyclase, and/or G-protein–gated inwardly rectifying potassium (GIRK) channels. The pathways underlying MOR-dependent inhibition of rhythmic breathing are unknown. Methods:By using a combination of genetic, pharmacological, and physiological tools in rodents in vivo, the authors aimed to identify the role of GIRK channels in MOR-mediated inhibition of respiratory circuits. Results:GIRK channels were expressed in the ventrolateral medulla, a neuronal population regulating rhythmic breathing, and GIRK channel activation with flupirtine reduced respiratory rate in rats (percentage of baseline rate in mean ± SD: 79.4 ± 7.4%, n = 7), wild-type mice (82.6 ± 3.8%, n = 3), but not in mice lacking the GIRK2 subunit, an integral subunit of neuronal GIRK channels (GIRK2−/−, 101.0 ± 1.9%, n = 3). Application of the MOR agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) to the ventrolateral medulla depressed respiratory rate, an effect partially reversed by the GIRK channel blocker Tertiapin-Q (baseline: 42.1 ± 7.4 breath/min, DAMGO: 26.1 ± 13.4 breath/min, Tertiapin-Q + DAMGO: 33.9 ± 9.8 breath/min, n = 4). Importantly, DAMGO applied to the ventrolateral medulla failed to reduce rhythmic breathing in GIRK2−/− mice (percentage of baseline rate: 103.2 ± 12.1%, n = 4), whereas it considerably reduced rate in wild-type mice (62.5 ± 17.7% of baseline, n = 4). Respiratory rate depression by systemic injection of the opioid analgesic fentanyl was markedly reduced in GIRK2−/− (percentage of baseline: 12.8 ± 15.8%, n = 5) compared with wild-type mice (72.9 ± 27.3%). Conclusions:Overall, these results identify that GIRK channels contribute to respiratory inhibition by MOR, an essential step toward understanding respiratory depression by opioids.

CrossTalk proposal: The preBötzinger complex is essential for the respiratory depression following systemic administration of opioid analgesics

Drugs acting on μ-opioid receptors (MOR) are widely used as analgesics but present serious side-effects such as addiction and respiratory depression. The latter is critical considering its potential lethality and the current absence of treatments to prevent it. The development of therapies to reduce respiratory depression is limited because the critical neural sites and mechanisms of action of opioids in causing respiratory depression are unclear. Here we discuss evidence highlighting the importance of the preBotzinger complex (preBotC), a critical site in the medulla for respiratory rhythm generation, in mediating respiratory rate depression by MOR drugs. This is of significance given that the isolated preBotC in vitro is widely utilized to develop and test new therapies to prevent respiratory depression (Manzke et al. 2003; Ren et al. 2006).

Caffeine in the neonatal period induces long‐lasting changes in sleep and breathing in adult rats

Caffeine is commonly used clinically to treat apnoeas and unstable breathing associated with premature birth. Caffeine antagonizes adenosine receptors and acts as an efficient respiratory stimulant in neonates. Owing to its persistent effects on adenosine receptor expression in the brain, neonatal caffeine administration also has significant effects on maturation of the respiratory control system. However, since adenosine receptors are critically involved in sleep regulation, and sleep also modulates breathing, we tested the hypothesis that neonatal caffeine treatment disrupts regulation of sleep and breathing in the adult rat. Neonatal caffeine treatment (15 mg kg−1 day−1) was administered from postnatal days 3–12. At adulthood (8–10 weeks old), sleep and breathing were measured with a telemetry system and whole‐body plethysmography respectively. In adult rats treated with caffeine during the neonatal period, sleep time was reduced, sleep onset latency was increased, and non‐rapid eye movement (non‐REM) sleep was fragmented compared to controls. Ventilation at rest was higher in caffeine‐treated adult rats compared to controls across sleep/wake states. Hypercapnic ventilatory responses were significantly reduced in caffeine‐treated rats compared to control rats across sleep/wake states. Additional experiments in adult anaesthetized rats showed that at similar levels of arterial blood gases, phrenic nerve activity was enhanced in caffeine‐treated rats. This study demonstrates that administration of caffeine in the neonatal period alters respiratory control system activity in awake and sleeping rats, as well as in the anaesthetized rats, and also has persistent disrupting effects on sleep that are apparent in adult rats.

State-Dependent Contribution of the Hyperpolarization-Activated Na+/K+ and Persistent Na+ Currents to Respiratory Rhythmogenesis In Vivo

How rhythms are generated by neuronal networks is fundamental to understand rhythmic behaviors such as respiration, locomotion, and mastication. Respiratory rhythm is generated by the preBötzinger complex (preBötC), an anatomically and functionally discrete population of brainstem neurons, central and necessary for respiratory rhythm. In specific in vitro conditions, preBötC neurons depend on voltage-dependent inward currents to generate respiratory rhythm. In the mature and intact organism, where preBötC neurons are deeply embedded in the respiratory network, the contribution of ionic currents to respiratory rhythm is unclear. We propose that a set of ionic currents plays a key role in generating respiratory rhythm in the mature organism in vivo. By microperfusing ionic current blockers into the preBötC of adult rats, we identify the hyperpolarization-activated cation current as a critical component of the mechanism promoting respiratory rhythm, and that this current, in combination with the persistent sodium current, is essential to respiratory rhythm in vivo. Importantly, both currents contribute to rhythmic activity in states of anesthesia, quiet wakefulness, and sleep, but not when the organism is engaged in active behaviors. These data show that a set of ionic currents at the preBötC imparts the network with rhythmicity in reduced states of arousal, although the network can override their contribution to adjust its activity for nonrhythmic behaviors in active wakefulness.

Activation of the Hypoglossal to Tongue Musculature Motor Pathway by Remote Control

Reduced tongue muscle tone precipitates obstructive sleep apnea (OSA), and activation of the tongue musculature can lessen OSA. The hypoglossal motor nucleus (HMN) innervates the tongue muscles but there is no pharmacological agent currently able to selectively manipulate a channel (e.g., Kir2.4) that is highly restricted in its expression to cranial motor pools such as the HMN. To model the effect of manipulating such a restricted target, we introduced a “designer” receptor into the HMN and selectively modulated it with a “designer” drug. We used cre-dependent viral vectors (AAV8-hSyn-DIO-hM3Dq-mCherry) to transduce hypoglossal motoneurons of ChAT-Cre+ mice with hM3Dq (activating) receptors. We measured sleep and breathing in three conditions: (i) sham, (ii) after systemic administration of clozapine-N-oxide (CNO; 1 mg/kg) or (iii) vehicle. CNO activates hM3Dq receptors but is otherwise biologically inert. Systemic administration of CNO caused significant and sustained increases in tongue muscle activity in non-REM (261 ± 33% for 10 hrs) and REM sleep (217 ± 21% for 8 hrs), both P < 0.01 versus controls. Responses were specific and selective for the tongue with no effects on diaphragm or postural muscle activities, or sleep-wake states. These results support targeting a selective and restricted “druggable” target at the HMN (e.g., Kir2.4) to activate tongue motor activity during sleep.

Distinct Cortical Signatures Associated with Sedation and Respiratory Rate Depression by Morphine in a Pediatric Population.

Background:Opioid analgesia is an essential component of perioperative care, but effective analgesia can be limited by excessive sedation and respiratory depression. The cortical signatures associated with sedation by opioids and the relationship between changes in cortical activity and respiratory function are not well understood. The objectives of this study were to identify the electroencephalogram signatures of sedation and respiratory changes induced by morphine in a pediatric population after elective surgery. Methods:After otologic surgery, patients (14.8 ± 2.8 yr, n = 10) stayed overnight for pain relief with morphine (3 to 10 mg), hydration, and clinical observation. Electroencephalogram activity and polysomnography were performed before and after morphine, and electroencephalogram spectral properties and cardiorespiratory activities were analyzed. Results:Compared to wakefulness and non–rapid eye movement sleep, morphine reduced high-frequency &bgr;1 (13.5 to 20 Hz) and &bgr;2 (20 to 30Hz) electroencephalogram powers (n = 10) and decreased coherence between frontal and occipital &bgr;2 electroencephalogram activities (n = 9), therefore indicating that morphine induced a deep sedative state. Morphine also reduced respiratory rate by 8.3% (n = 10). Interestingly, there was a significant correlation between the reduction in &bgr;1 electroencephalogram activity and the depression in respiratory rate induced by morphine (R = 0.715, n = 10). With significant reduction in &bgr;1 power, respiratory rate was decreased by more than 25%, suggesting that reduction in cortical arousal is associated with the severity of respiratory rate depression. Conclusions:Analgesic doses of morphine are associated with reduction in respiratory rate when accompanied by reduction in &bgr;1 electroencephalogram power, indicating a powerful effect of cortical arousal state per se in respiratory rate depression by morphine.