The effects of opiates on the respiratory activity of thoracic motoneurones in the anaesthetized and decerebrate rabbit

Brain Activation by H1 Antihistamines Challenges Conventional View of Their Mechanism of Action in Motion Sickness: A Behavioral, c-Fos and Physiological Study in Suncus murinus (House Musk Shrew)

Motion sickness occurs under a variety of circumstances and is common in the general population. It is usually associated with changes in gastric motility, and hypothermia, which are argued to be surrogate markers for nausea; there are also reports that respiratory function is affected. As laboratory rodents are incapable of vomiting, Suncus murinus was used to model motion sickness and to investigate changes in gastric myoelectric activity (GMA) and temperature homeostasis using radiotelemetry, whilst also simultaneously investigating changes in respiratory function using whole body plethysmography. The anti-emetic potential of the highly selective histamine H₁ receptor antagonists, mepyramine (brain penetrant), and cetirizine (non-brain penetrant), along with the muscarinic receptor antagonist, scopolamine, were investigated in the present study. On isolated ileal segments from Suncus murinus, both mepyramine and cetirizine non-competitively antagonized the contractile action of histamine with pK _b values of 7.5 and 8.4, respectively; scopolamine competitively antagonized the contractile action of acetylcholine with pA₂ of 9.5. In responding animals, motion (1 Hz, 4 cm horizontal displacement, 10 min) increased the percentage of the power of bradygastria, and decreased the percentage power of normogastria whilst also causing hypothermia. Animals also exhibited an increase in respiratory rate and a reduction in tidal volume. Mepyramine (50 mg/kg, i.p.) and scopolamine (10 mg/kg, i.p.), but not cetirizine (10 mg/kg, i.p.), significantly antagonized motion-induced emesis but did not reverse the motion-induced disruptions of GMA, or hypothermia, or effects on respiration. Burst analysis of plethysmographic-derived waveforms showed mepyramine also had increased the inter-retch+vomit frequency, and emetic episode duration. Immunohistochemistry demonstrated that motion alone did not induce c-fos expression in the brain. Paradoxically, mepyramine increased c-fos in brain areas regulating emesis control, and caused hypothermia; it also appeared to cause sedation and reduced the dominant frequency of slow waves. In conclusion, motion-induced emesis was associated with a disruption of GMA, respiration, and hypothermia. Mepyramine was a more efficacious anti-emetic than cetirizine, suggesting an important role of centrally-located H₁ receptors. The ability of mepyramine to elevate c-fos provides a new perspective on how H₁ receptors are involved in mechanisms of emesis control.

Functional connectivity in raphé-pontomedullary circuits supports active suppression of breathing during hypocapnic apnea

Hyperventilation is a common feature of disordered breathing. Apnea ensues if CO2 drive is sufficiently reduced. We tested the hypothesis that medullary raphé, ventral respiratory column (VRC), and pontine neurons have functional connectivity and persistent or evoked activities appropriate for roles in the suppression of drive and rhythm during hyperventilation and apnea. Phrenic nerve activity, arterial blood pressure, end-tidal CO2, and other parameters were monitored in 10 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated cats. Multielectrode arrays recorded spiking activity of 649 neurons. Loss and return of rhythmic activity during passive hyperventilation to apnea were identified with the S-transform. Diverse fluctuating activity patterns were recorded in the raphé-pontomedullary respiratory network during the transition to hypocapnic apnea. The firing rates of 160 neurons increased during apnea; the rates of 241 others decreased or stopped. VRC inspiratory neurons were usually the last to cease firing or lose rhythmic activity during the transition to apnea. Mayer wave-related oscillations (0.04-0.1 Hz) in firing rate were also disrupted during apnea. Four-hundred neurons (62%) were elements of pairs with at least one hyperventilation-responsive neuron and a correlational signature of interaction identified by cross-correlation or gravitational clustering. Our results support a model with distinct groups of chemoresponsive raphé neurons contributing to hypocapnic apnea through parallel processes that incorporate disfacilitation and active inhibition of inspiratory motor drive by expiratory neurons. During apnea, carotid chemoreceptors can evoke rhythm reemergence and an inspiratory shift in the balance of reciprocal inhibition via suppression of ongoing tonic expiratory neuron activity.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Inhibition of protein kinase A activity depresses phrenic drive and glycinergic signalling, but not rhythmogenesis in anaesthetized rat

The cAMP-protein kinase A (PKA) pathway plays a critical role in regulating neuronal activity. Yet, how PKA signalling shapes the population activity of neurons that regulate respiratory rhythm and motor patterns in vivo is poorly defined. We determined the respiratory effects of focally inhibiting endogenous PKA activity in defined classes of respiratory neurons in the ventrolateral medulla and spinal cord by microinjection of the membrane-permeable PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) in urethane-anaesthetized adult Sprague Dawley rats. Phrenic nerve activity, end-tidal CO2 and arterial pressure were recorded. Rp-cAMPS in the preBötzinger complex (preBötC) caused powerful, dose-dependent depression of phrenic burst amplitude and inspiratory period. Rp-cAMPS powerfully depressed burst amplitude in the phrenic premotor nucleus, but had no effect at the phrenic motor nucleus, suggesting a lack of persistent PKA activity here. Surprisingly, inhibition of PKA activity in the preBötC increased phrenic burst frequency, whereas in the Bötzinger complex phrenic frequency decreased. Pretreating the preBötC with strychnine, but not bicuculline, blocked the Rp-cAMPS-evoked increase in frequency, but not the depression of phrenic burst amplitude. We conclude that endogenous PKA activity in excitatory inspiratory preBötzinger neurons and phrenic premotor neurons, but not motor neurons, regulates network inspiratory drive currents that underpin the intensity of phrenic nerve discharge. We show that inhibition of PKA activity reduces tonic glycinergic transmission that normally restrains the frequency of rhythmic respiratory activity. Finally, we suggest that the maintenance of the respiratory rhythm in vivo is not dependent on endogenous cAMP-PKA signalling.

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.

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 muscle activity and respiratory obstruction after abdominal surgery

BACKGROUND

Respiratory movements in patients after abdominal surgery are frequently abnormal, with associated disturbances in the pattern of inspiratory pressure generation. The reasons for these abnormalities are not clear and have been attributed to impaired action of the diaphragm. However, an alternative is that partial airway obstruction could trigger reflex activation of the inspiratory ribcage muscles, which would cause a similar pattern of inspiratory pressure change. Direct measurement of electrical activity can indicate if reflex activation of inspiratory muscles occurs when partial airway obstruction is present.

METHODS

In an open study, we implanted electrodes to measure the EMG of scalene, intercostal and external oblique abdominal muscles in patients after lower abdominal surgery. Analgesia was with morphine i.v. by patient control. We used nasal cannulae to measure nasal airflow and compared EMG activity when airway obstruction was present with activity when breathing was not obstructed.

RESULTS

The pattern of activity of the different muscles was distinct. Intercostal activity reached a maximum during inspiration, before the scalene muscles, whereas scalene activity increased in phase with increasing lung volume. Abdominal muscle activity commenced when expiratory flow had ceased and continued until the next inspiration. In all three muscle groups, partial airway obstruction did not alter muscle activity.

CONCLUSIONS

Partial airway obstruction does not activate inspiratory ribcage muscles, in patients receiving morphine for postoperative analgesia after lower abdominal surgery. Changes in respiratory pressures and abnormalities of chest wall movement described in previous studies cannot be attributed to reflex responses and probably result from increased airway resistance and abdominal muscle action.

Distinct rhythm generators for inspiration and expiration in the juvenile rat

Inspiration and active expiration are commonly viewed as antagonistic phases of a unitary oscillator that generates respiratory rhythm. This view conflicts with observations we report here in juvenile rats, where by administration of fentanyl, a selective mu-opiate agonist, and induction of lung reflexes, we separately manipulated the frequency of inspirations and expirations. Moreover, completely transecting the brainstem at the caudal end of the facial nucleus abolished active expirations, while rhythmic inspirations continued. We hypothesize that inspiration and expiration are generated by coupled, anatomically separate rhythm generators, one generating active expiration located close to the facial nucleus in the region of the retrotrapezoid nucleus/parafacial respiratory group, the other generating inspiration located more caudally in the preBötzinger Complex.

D1-dopamine receptor agonists prevent and reverse opiate depression of breathing but not antinociception in the cat

Opioids depress respiration and decrease chest wall compliance. A previous study in this laboratory showed that dopamine-D(1) receptor (D(1)R) agonists restored phrenic nerve activity after arrest by fentanyl in immobilized, mechanically ventilated cats. The reinstated phrenic nerve rhythm was slower than control, so it was not known whether D(1)R agonists can restore spontaneous breathing to levels that provide favorable alveolar gas exchange and blood oxygenation. It was also not known whether the agonists counteract opioid analgesia. In the present study, anesthetized, spontaneously breathing cats were given intravenous doses of fentanyl (18.0 +/- 3.4 microg/kg) that severely depressed depth and rate of respiration, lowered arterial hemoglobin oxygenation (HbO(2)), elevated end-tidal carbon dioxide (ETCO(2)), and abolished the nociceptive hind limb crossed-extensor reflex. Fentanyl (30 microg/kg) also evoked tonic discharges of caudal medullary expiratory neurons in paralyzed mechanically ventilated cats, which might explain decreased chest compliance. The selective D(1)R agonists 6-chloro APB (3 mg/kg) or dihydrexidine (DHD, 1 mg/kg) increased depth and rate of spontaneous breathing after opioid depression and returned HbO(2) and ETCO(2) to control levels. Opioid arrest of the nociceptive reflex remained intact. Pretreatment with DHD prevented significant depression of spontaneous breathing by fentanyl (17.5 +/- 4.3 microg/kg). Tonic firing evoked by fentanyl in expiratory neurons was converted to rhythmic respiratory discharges by DHD (1 mg/kg). The results suggest that D(1)R agonists might be therapeutically useful for the treatment of opioid disturbances of breathing without impeding analgesia.

Abdominal muscle action during expiration can impair pressure controlled ventilation

Pressure controlled ventilation, and pressure support for spontaneous breathing are often used in intensive care because coordination of the ventilator with patient efforts can improve comfort and possibly reduce sedation. However we report a series of 10 patients whose efforts did not synchronise with pressure controlled ventilation. This was incorrectly diagnosed as inadequate sedation, and treated with increased sedation or muscle paralysis. Better recognition of this condition showed that slow respiratory rates and increased abdominal muscle action during expiration can affect pressure-controlled ventilation and pressure assisted breathing. If the condition is not recognised, treatment for poor synchronisation may delay weaning or be inappropriate.

Mu-opioid receptor agonist effects on medullary respiratory neurons in the cat: evidence for involvement in certain types of ventilatory disturbances

Mu-opioid receptor agonists depress tidal volume, decrease chest wall compliance, and increase upper airway resistance. In this study, potential neuronal sites and mechanisms responsible for the disturbances were investigated, dose-response relationships were established, and it was determined whether general anesthesia plays a role. Effects of micro-opioid agonists on membrane properties and discharges of respiratory bulbospinal, vagal, and propriobulbar neurons and phrenic nerve activity were measured in pentobarbital-anesthetized and unanesthetized decerebrate cats. In all types of respiratory neurons tested, threshold intravenous doses of the micro-opioid agonist fentanyl slowed discharge frequency and prolonged duration without altering peak discharge intensity. Larger doses postsynaptically depressed discharges of inspiratory bulbospinal and inspiratory propriobulbar neurons that might account for depression of tidal volume. Iontophoresis of the micro-opioid agonist DAMGO also depressed the intensity of inspiratory bulbospinal neuron discharges. Fentanyl given intravenously prolonged discharges leading to tonic firing of bulbospinal expiratory neurons in association with reduced hyperpolarizing synaptic drive potentials, perhaps explaining decreased inspiratory phase chest wall compliance. Lowest effective doses of fentanyl had similar effects on vagal postinspiratory (laryngeal adductor) motoneurons, whereas in vagal laryngeal abductor and pharyngeal constrictor motoneurons, depression of depolarizing synaptic drive potentials led to sparse, very-low-frequency discharges. Such effects on three types of vagal motoneurons might explain tonic vocal fold closure and pharyngeal obstruction of airflow. Measurements of membrane potential and input resistance suggest the effects on bulbospinal Aug-E neurons and vagal motoneurons are mediated presynaptically. Opioid effects on the respiratory neurons were similar in anesthetized and decerebrate preparations.

Biphasic effects of morphine on bulbar respiratory neuronal activities in decerebrate cats

To understand neuronal mechanisms underlying respiratory depression induced by morphine, membrane potential, input resistance and burst discharge in different types of respiratory neurons were recorded in decerebrate and vagotomized cats. Intravenous morphine (0.3-3.0 mg/kg) dose-dependently decreased the respiratory discharge in the phrenic and iliohypogastric nerves. The drug changed the respiratory frequency in a biphasic fashion, a transient increase (early phase) followed by a long-lasting decrease (late phase). During the early phase, the membrane was hyperpolarized throughout the respiratory cycle and the burst discharge was decreased in all types of respiratory neurons. During the late phase, the active phase depolarization and the inactive phase hyperpolarization were decreased, resulting in a decline of membrane potential fluctuations. Input resistance was decreased during the early phase and increased during the late phase. Iontophoresed (50-100 nA) morphine produced hyperpolarization of the membrane and a decrease in input resistance in respiratory neurons. This hyperpolarization remained unaltered after iontophoresed tetrodotoxin depressed the synaptic transmission. These effects of morphine were completely blocked by naloxone and beta-funaltrexamine, but not by naltrindole. The present results suggest that morphine depresses the respiratory neuronal activity through two different mechanisms, both of which are mediated by mu receptors.

Rhythmic phrenic, intercostal and sympathetic activity in relation to limb and trunk motor activity in spinal cats

During L-DOPA-induced fictive spinal locomotion rhythmic activities in nerves to internal intercostal and external oblique abdominal muscles and in phrenic and sympathetic nerves were observed which were always coordinated with locomotor activity in forelimb and hindlimb muscle nerves. A periodicity with longer lasting tonic phases could be induced by cutaneous nerve stimulation or asphyxia. This activity was observed in limb motor nerves as well as in respiratory motor and sympathetic nerves. A slow independent activity of the phrenic and intercostal nerves or the sympathetic nerves, which could be related to a normal respiratory rhythm or independent sympathetic rhythms was not observed. The findings indicate that during fictive spinal locomotion the activity of spinal rhythm generators for locomotion also projects onto respiratory and sympathetic spinal neurones.

Opioid-resistant respiratory pathway from the preinspiratory neurones to abdominal muscles: in vivo and in vitro study in the newborn rat

We report that after spontaneous breathing movements are stopped by administration of opioids (opioid-induced apnoea) in neonatal rats, abdominal muscles continue to contract at a rate similar to that observed during periods of ventilation. Correspondingly, in vitro bath application of a mu opioid receptor agonist suppresses the activity of the fourth cervical root (C4) supplying the diaphragm, but not the rhythmic activity of the first lumbar root (L1) innervating the abdominal muscles. This indicates the existence of opioid-resistant rhythmogenic neurones and a neuronal pathway transmitting their activity to the abdominal motoneurones. We have investigated this pathway by using a brainstem-spinal cord preparation of the neonatal rat. We identified bulbospinal neurones with a firing pattern identical to that of the L1 root. These neurones were located caudal to the obex in the vicinity of the nucleus retroambiguus. Resting potentials ranged from -49 to -40 mV (mean +/- S.D. -44.0 +/- 4.3 mV). The mean input resistance was 315.5 +/- 54.8 MOmega. The mean antidromic latency from the L1 level was 42.8 +/- 4.4 ms. Axons crossed the midline at the level of the cell body. The activity pattern of the bulbospinal neurones and the L1 root consisted of two bursts per respiratory cycle with a silent period during inspiration. This pattern is characteristic of preinspiratory neurones. We found that 11 % of the preinspiratory neurones projected to the area where the bulbospinal neurones were located. These preinspiratory neurones were found in the rostral ventrolateral medulla close (200-350 microm) to the ventral surface at the level of the rostral half of the nucleus retrofacialis. Our data suggest the operation of a disynaptic pathway from the preinspiratory neurones to the L1 motoneurones in the in vitro preparation. We propose that the same pathway is responsible for rhythmic activation of the abdominal muscles during opioid-induced apnoea in the newborn rat.

Influence of opioids and naloxone on rhythmic motor activity in spinal cats

The effects of L-DOPA, naloxone, and the opioids (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAGO) and D-Ser2-Leu-enkephalin-Thr6 (DSLET) on spinal motor rhythm generation were compared in anemically decapitated high spinal cats. After premedication with nialamide, DOPA caused the well-known, slow rhythmic motor activity with a locomotor pattern. The cycle duration of the evoked rhythm was usually between 3.9 and 5.0 s. The opioids DAGO and DSLET, injected intravenously (1.2-2 mg/kg) or suffused over the lumbar spinal cord (10(-3)-10(-4) M in Ringer's solution), severely depressed the DOPA-induced rhythmic activity, sometimes completely abolishing efferent motor activity. Naloxone (0.5-1 mg/kg i.v.) exerted different rhythm-facilitating effects, depending on the experimental condition. In the acute phase after spinalization, without paralysis and without nialamide and DOPA, naloxone induced rhythmic movements with a main frequency of 1.2-2 Hz. In the same preparation with paralysis, naloxone induced a rhythmic motor activity with a distinctly higher frequency (main range 4.3-5.8 Hz). After premedication with nialamide and DOPA, naloxone facilitated or, if a rhythm was absent, induced the slow-frequency DOPA type of rhythm. Given after i.v. or topical opioid application, naloxone antagonized the rhythm-depressing action of the opioid and caused an additional facilitation of rhythmic activity. Dopa and naloxone facilitated the long-latency, segmental reflex pathways from flexor reflex afferents (FRA), while the opioids depressed them. The short-latency FRA pathways were depressed by DOPA and opioids but were facilitated by naloxone. The influence of the different drugs on spinal motor rhythm generation is discussed in relation to their influence on short- and long-latency segmental pathways from FRA. If the rhythm generation induced by DOPA is based on the release of the long-latency FRA pathways, as has been proposed before, the rhythm-depressing action of opioids may be due to the suppression of these pathways, and the particular rhythm-generating function of naloxone may be related to its facilitation of short- and long-latency FRA pathways.