Effects of opiates and methionine-enkephalin on pontine and bulbar respiratory neurones of the cat

A Subregion of the Parabrachial Nucleus Partially Mediates Respiratory Rate Depression from Intravenous Remifentanil in Young and Adult Rabbits

BACKGROUND

The efficacy of opioid administration to reduce postoperative pain is limited by respiratory depression. We investigated whether clinically relevant opioid concentrations altered the respiratory pattern in the parabrachial nucleus, a pontine region contributing to respiratory pattern generation, and compared these effects with a medullary respiratory site, the pre-Bötzinger complex.

METHODS

Studies were performed in 40 young and 55 adult artificially ventilated, decerebrate rabbits. We identified an area in the parabrachial nucleus where α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjections elicited tachypnea. Two protocols were performed in separate sets of animals. First, bilateral microinjections of the μ-opioid receptor agonist [D-Ala, N-MePhe, Gly-ol]-enkephalin (100 μM) into the "tachypneic area" determined the effect of maximal μ-opioid receptor activation. Second, respiratory rate was decreased with continuous IV infusions of remifentanil. The opioid antagonist naloxone (1 mM) was then microinjected bilaterally into the "tachypneic area" of the parabrachial nucleus to determine whether the respiratory rate depression could be locally reversed.

RESULTS

Average respiratory rate was 27 ± 10 breaths/min. First, [D-Ala, N-MePhe, Gly-ol]-enkephalin injections decreased respiratory rate by 62 ± 20% in young and 45 ± 26% in adult rabbits (both P < 0.001). Second, during IV remifentanil infusion, bilateral naloxone injections into the "tachypneic area" of the parabrachial nucleus reversed respiratory rate depression from 55 ± 9% to 20 ± 14% in young and from 46 ± 20% to 18 ± 27% in adult rabbits (both P < 0.001). The effects of bilateral [D-Ala, N-MePhe, Gly-ol]-enkephalin injection and IV remifentanil on respiratory phase duration in the "tachypneic area" of the parabrachial nucleus was significantly different from the pre-Bötzinger complex.

CONCLUSIONS

The "tachypneic area" of the parabrachial nucleus is highly sensitive to μ-opioid receptor activation and mediates part of the respiratory rate depression by clinically relevant administration of opioids.

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.

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.

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Opioid medication and sleep-disordered breathing

There has been a growing recognition of chronic pain that may be experienced by patients. There has been a movement toward treating these patients aggressively with pharmacologic and nonpharmacologic modalities. Opioids have been a significant component of the treatment of acute pain, with their increasing use in cases of chronic pain, albeit with some controversy. In addition to analgesia, opioids have many accompanying adverse effects, particularly with regard to stability of breathing during sleep. This article reviews the existing literature on the effects of opioids on sleep, particularly sleep-disordered breathing.

Excitatory and inhibitory effects of opioid agonists on respiratory motor output produced by isolated brainstems from adult turtles (Trachemys)

To determine how central opioid receptor activation alters turtle breathing, respiratory-related hypoglossal (XII) motor bursts were recorded from isolated adult turtle brainstems during 60 min bath applications of agonists for delta- (DOR), kappa- (KOR), or nociceptin/orphanin (NOR) receptors. DADLE (DOR agonist) abolished XII burst frequency at 0.3-0.5 microM. DPDPE (DOR agonist) increased frequency by 40-44% at 0.01-0.1 microM and decreased frequency by 88+/-8% at 1.0 microM. U-50488 and U-59693 (KOR agonists) decreased frequency by 65-68% at 100 and 50 microM, respectively. Orphanin (NOR agonist) decreased frequency by 31-51% at 1.0-2.0 microM during the first 30 min period. Orphanin (0.5 and 2.0 microM) increased bursts/episode. Although morphine (10 microM) abolished frequency in nearly all brainstems, subsequent co-application of phenylephrine (alpha(1)-adrenergic agonist, 20-100 microM) with morphine restored activity to 16-78% of baseline frequency. Thus, DOR, KOR, and NOR activation regulates frequency and NOR activation regulates episodicity, while alpha(1)-adrenergic receptor activation reverses opioid-induced respiratory depression in turtles.

Opioid receptors on bulbospinal respiratory neurons are not activated during neuronal depression by clinically relevant opioid concentrations

Opioids depress the activity of brain stem respiratory-related neurons, but it is not resolved whether the mechanism at clinical concentrations consists of direct neuronal effects or network effects. We performed extracellular recordings of discharge activity of single respiratory neurons in the caudal ventral respiratory group of decerebrate dogs, which were premotor neurons with a likelihood of 90%. We used multibarrel glass microelectrodes, which allowed concomitant highly localized picoejection of opioid receptor agonists or antagonists onto the neuron. Picoejection of the mu receptor agonist [d-Ala(2), N-Me-phe(4), gly-ol(5)]-enkephalin (DAMGO, 1 mM) decreased the peak discharge frequency (mean +/- SD) of expiratory neurons to 68 +/- 22% (n = 12), the delta(1) agonist d-Pen(2,5)-enkephalin (DPDPE, 1 mM) to 95 +/- 11% (n = 15), and delta(2) receptor agonist [d-Ala(2)] deltorphin-II to 86 +/- 17% (1 mM, n = 15). The corresponding values for inspiratory neurons were: 64 +/- 12% (n = 11), 48 +/- 30% (n = 12), and 75 +/- 15% (n = 11), respectively. Naloxone fully reversed these effects. Picoejection of morphine (0.01-1 mM) depressed most neurons in a concentration dependent fashion to maximally 63% (n = 27). Picoejection of remifentanil (240-480 nM) did not cause any significant depression of inspiratory (n = 11) or expiratory neurons (n = 9). 4. Intravenous remifentanil (0.2-0.6 microg.kg(-1).min(-1)) decreased neuronal peak discharge frequency to 60 +/- 12% (inspiratory, n = 7) and 58 +/- 11% (expiratory, n = 11). However, local picoejection of naloxone did not reverse the neuronal depression. Our data suggest that mu, delta(1), and delta(2) receptors are present on canine respiratory premotor neurons. Clinical concentrations of morphine and remifentanil caused no local depression. This lack of effect and the inability of local naloxone to reverse the neuronal depression by intravenous remifentanil suggest that clinical concentrations of opioids produce their depressive effects on mechanisms upstream from respiratory bulbospinal premotor neurons.

Inhibitory and excitatory effects of micro-, delta-, and kappa-opioid receptor activation on breathing in awake turtles, Trachemys scripta

For ectothermic vertebrates, such as reptiles, the effects of opioid receptor subtype activation on breathing are poorly understood. On the basis of previous studies on mammals and lampreys, we hypothesized that mu- and delta-opioid receptor (MOR and DOR, respectively) activation would cause respiratory depression, whereas kappa-opioid receptor (KOR) activation would have no effect. To address this question, we measured respiration in awake, freely swimming adult red-eared slider turtles (Trachemys scripta) before and after injection with agonists for specific opioid receptors. Injection of the MOR agonist [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin acetate salt (DAMGO, 1.5 or 6.5 mg/kg) decreased ventilation (Ve) by 72 +/- 9% and 95 +/- 3%, respectively, 4.0 h after injection as a result of decreased breathing frequency and no change in tidal volume (Vt). DOR agonists, such as [d-Pen(2,5)]-enkephalin hydrate (DPDPE, 5.0 mg/kg) and [d-Ala(2),d-Leu(5)]-enkephalin acetate salt (DADLE, 6.3 mg/kg), decreased Ve by 44 +/- 10% and 89 +/- 4%, respectively, 4.0 h after injection as a result of decreased breathing frequency and no change in Vt. DADLE also increased breath duration by a maximum of 25 +/- 9% at 6.0 h after injection. The KOR agonist U-50488 (6.2 mg/kg) increased Vt by a maximum of 52 +/- 30% at 5.0 h after injection, with variable nonsignificant changes in Ve and breathing frequency. Naloxone injections (0.25-0.5 mg/kg) 1.0 h before opioid agonist injections blocked all DAMGO-dependent effects, DPDPE-dependent frequency depression, and DADLE-dependent breath duration augmentation for 2.0 h after agonist injections. These results show that MOR and DOR activation causes respiratory depression as a result of decreased breathing frequency, whereas Vt is increased after KOR activation.

Opioid-induced depression in the lamprey respiratory network

The role of opioid receptors in modulating respiratory activity was investigated in in vitro brainstem preparations of adult lampreys by bath application of agonists and antagonists. The vagal motor output was used to monitor respiratory activity. Neuronal recordings were also performed to characterize the rostrolateral trigeminal region that has been suggested to be critical for respiratory rhythmogenesis. Microinjections of the micro-opioid receptor agonist [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) were also made into this region and at different locations within the brainstem. Bath application of DAMGO (0.5-2 microM) caused marked decreases in respiratory frequency up to complete apnea. Bath application of the delta-opioid receptor agonist [d-Pen(2,5)]-enkephalin (DPDPE) at 10-40 microM induced less pronounced depressant respiratory effects, while no changes in respiratory activity were induced by the kappa-opioid receptor agonist trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50488) at 10-40 microM. Bath application of the opioid receptor antagonists naloxone and naltrindole did not affect baseline respiratory activity, but prevented agonist-induced effects. DAMGO microinjections (1 mM; 0.5-1 nl) at sites rostrolateral to the trigeminal motor nucleus, where respiration-related neuronal activity was recorded, abolished the respiratory rhythm. The results show that opioids may have an important role in the lamprey respiratory network and that micro-opioid receptor activation is the most effective in causing respiratory depression. They also indicate that endogenous opioids are not required for the generation of baseline respiratory activity. Apneic responses induced by DAMGO microinjections support the hypothesis that a specific opioid-sensitive region rostrolateral to the trigeminal motor nucleus, that we have termed the paratrigeminal respiratory group (pTRG), likely has a pivotal role in respiratory rhythmogenesis. Since the lamprey diverged from the main vertebrate line around 450 million years ago, our results also imply that the inhibitory role of opioids on respiration is present at an early stage of vertebrate evolution.

Ontogeny of central rhythm generation in chicks and rodents

Recent studies help in understanding how the basic organization of brainstem neuronal circuits along the anterior-posterior (AP) axis is set by the Hox-dependent segmentation of the neural tube in vertebrate embryos. Neonatal respiratory abnormalities in Krox20(-/-), Hoxa1(-/-) and kreisler mutant mice indicate the vital role of a para-facial (Krox20-dependent, rhombomere 4-derived) respiratory group, that is distinct from the more caudal rhythm generator called Pre-Bötzinger complex. Embryological studies in the chick suggest homology and conservation of this Krox20-dependent induction of parafacial rhythms in birds and mammals. Calcium imaging in embryo indicate that rhythm generators may derive from different cell lineages within rhombomeres. In mice, the Pre-Bötzinger complex is found to be distinct from oscillators producing the earliest neuronal activity, a primordial low-frequency rhythm. In contrast, in chicks, maturation of the parafacial generator is tightly linked to the evolution of this primordial rhythm. It seems therefore that ontogeny of brainstem rhythm generation involves conserved processes specifying distinct AP domains in the neural tube, followed by diverse, lineage-specific regulations allowing the emergence of organized rhythm generators at a given AP level.

Neural tube patterning by Krox20 and emergence of a respiratory control

Recent data begin to bridge the gap between developmental events controlling hindbrain neural tube regional patterning and the emergence of breathing behaviour in the fetus and its vital adaptive function after birth. In vertebrates, Hox paralogs and Hox-regulating genes orchestrate, in a conserved manner, the transient formation of developmental compartments in the hindbrain, the rhombomeres, in which rhythmic neuronal networks of the brainstem develop. Genetic inactivation of some of these genes in mice leads to pathological breathing at birth pointing to the vital importance of rhombomere 3 and 4 derived territories for maintenance of the breathing frequency. In chick embryo at E7, we investigated neuronal activities generated in neural tube islands deriving from combinations of rhombomeres isolated at embryonic day E1.5. Using a gain of function approach, we reveal a role of the transcription factor Krox20, specifying rhombomeres 3 and 5, in inducing a rhythm generator at the parafacial level of the hindbrain. The developmental genes selecting and regionally coordinating the fate of CNS progenitors may hold further clues to conserved aspects of neuronal network formation and function. However, the most immediate concern is to take advantage of early generated rhythmic activities in the hindbrain to pursue their downstream cellular and molecular targets, for it seems likely that it will be here that rhythmogenic properties will eventually take on a vital role at birth.

Parabrachial-lateral pontine neurons link nociception and breathing

We investigated the role of the parabrachial complex in cutaneous nociceptor-induced respiratory stimulation in chloralose-urethane anesthetized, vagotomized rats. Noxious stimulation (mustard oil, MO) applied topically to a forelimb or hindlimb enhanced the peak amplitude of the integrated phrenic nerve discharge and, with forelimb application, increased phrenic nerve burst frequency. Bilateral inactivation of neural activity in the parabrachial complex with injection of the GABA agonist muscimol (3nl) markedly attenuated the response to MO application. Injection of the retrograde tracer FluoroGold within the medullary ventral respiratory column labeled neurons in dorsolateral pontine regions known to receive nociceptive inputs (i.e., Kolliker-Fuse, lateral crescent, and superior lateral subnuclei of the parabrachial complex). Extracellular recordings of 65 dorsolateral parabrachial neurons revealed about 15% responded to a noxious cutaneous pinch with either an increase or a decrease in discharge and approximately 40% of these exhibited a phasic respiratory-related component to their discharge. In conclusion, parabrachial pontine neurons contribute to cutaneous nociceptor-induced increases in breathing.

Enkephalinergic inhibition of raphe pallidus inputs to rat hypoglossal motoneurones in vitro

Hypoglossal motoneurones play a major role in maintaining the patency of the upper airways and in determining airways resistance. These neurones receive inputs from many different regions of the neuroaxis including the caudal raphe nuclei. Whilst we have previously shown that glutamate is utilised in projections from one of these caudal raphe nuclei, the raphe pallidus, to hypoglossal motoneurones, these raphe pallidus-hypoglossal projections also contain multiple co-localised neuropeptides, including a population that are immunopositive for enkephalin. The role of enkephalin in the control of hypoglossal motoneurones is unknown. Therefore the aim of these studies was to determine whether enkephalins modulate caudal raphe glutamatergic inputs to hypoglossal motoneurones. Whole cell recordings were made from rat hypoglossal motoneurones in vitro, with glutamate-mediated excitatory postsynaptic currents (EPSCs) evoked in these neurones following electrical stimulation within the raphe pallidus. Superfusion of enkephalin significantly decreased the amplitude of these raphe pallidus evoked EPSCs (56.1+/-29% of control, P<0.001), an action that was mirrored by the tau-opioid receptor agonist, [D-Ala, N-Me-Phe, Gly-ol]-enkephalin acetate (DAMGO;53.8+/-26%, P<0.01), but not by the delta-opioid receptor agonist, [D-Pen]-enkephalin (DPDPE). Enkephalin also increased the amplitude ratio (1.57+/-0.36 vs. 1.14+/-0.27, P<0.01) of pairs of evoked EPSCs (paired pulse ratio), decreased the frequency (P<0.0001) but not the amplitude of miniature EPSCs, whilst having no effect on the inward current evoked by glutamate applied directly to the postsynaptic cell (97.8+/-2.2% of control, P=n.s.). Likewise, DAMGO also increased the paired pulse ratio (1.62+/-0.35 vs. 1.31+/-0.14, P<0.05) and decreased the frequency of miniature EPSCs (P<0.0001). Together, these data suggest that enkephalin acts at tau-opioid receptors located on the presynaptic terminals of raphe pallidus inputs to hypoglossal motoneurones to significantly decrease glutamate release from these projections.

Developmental gene control of brainstem function: views from the embryo

The respiratory rhythm is generated within the hindbrain reticular formation, rostrally in the vicinity of the facial nucleus and caudally within the vagal/glossopharyngeal domain. This is probably one of the best models to understand how genes have been selected and conserved to control adaptive behaviour in vertebrates. The para-facial region is well understood with respect to the transcription factors that underlie antero-posterior specification of neural progenitors in the embryo. Hox paralogs and Hox-regulating genes kreisler and Krox-20 govern transient formation of developmental compartments, the rhombomeres, in which rhythmic neuronal networks develop. Hox are master genes selecting and coordinating the developmental fate of reticular and motor neurons thereby specifying patterns of motor activities operating throughout life. Neuronal function and development are also tightly linked in the vagal/glossopharyngeal domain. At this level, bdnf acts as a neurotrophin of peripheral chemoafferent neural populations and as a neuromodulator of the central rhythmogenic respiratory circuits. A general view is now emerging on the role of developmental transcription and trophic factors allowing the coordinated integration of different neuronal types to produce, and eventually refine, respiratory rhythmic pattern in a use-dependent manner.

Mu opioid receptors in rat ventral medulla: effects of endomorphin-1 on phrenic nerve activity

Anatomical and in vitro studies suggest that mu opioid receptors (MOR) on pre-Bötzinger complex neurons are responsible for opioid induced respiratory depression (Grey et al., Science 286 (1999) 1566). However, mu opioid agonists injected in vivo, in other regions of the ventral respiratory group (VRG), produce respiratory depression, suggesting that opioids are widely distributed in the VRG. We therefore re-examined the distribution of the MOR in the ventral medulla and found MOR-immunoreactive neurons and terminals in all subdivisions of the VRG. Furthermore, we determined, in rats, the effects of a MOR agonist (endomorphin-1, 10 mM, 60 nl, unilateral), microinjected into different subdivisions of the VRG, on phrenic nerve activity. Endomorphin-1 produced changes in phrenic nerve frequency and amplitude, throughout the VRG. Unexpectedly, endomorphin-1 microinjected into the Bötzinger and pre-Bötzinger complexes consistently increased phrenic nerve frequency. These results support the widespread distribution of MOR in the VRG and also indicate that endomorphin-1, a postulated endogenous ligand, may differentially regulate respiration.

What is eupnea

Respiratory neuronal networks in vertebrates appear to be able to generate a variety of rhythmic patterns in vivo, leading to the biological diversity of eupneic patterns as well as to life-threatening dyspneic patterns. Eupnea is best viewed as the collection of respiratory strategies preventing potential dyspneas, the major (and perhaps the only) criterion for a definition being that eupnea allows survival. Specific criteria can then be derived from the physiological identification of neurobiological mechanisms underlying identified dyspneic patterns, by exaggerating (pro-dyspneic mechanisms) or suppressing them (anti-dyspneic mechanisms). Because eupnea is vital, and one of the major targets of evolutionary pressure, identification of dyspnea-related neuronal systems seems to be important to understand the normal biological organization of the respiratory neuronal system.

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.

Distribution of mu receptors in the ventral respiratory group neurons; immunohistochemical and pharmacological studies in decerebrate cats

Immunoreactivity for mu receptors was investigated in 21 bulbar respiratory neurons, individually identified by intracellular recording and labeling with neurobiotin. In 14 of these neurons, effects of iontophoresed morphine were examined. Morphine hyperpolarized the membrane and decreased spike discharges in 4/6 augmenting inspiratory (aug-I), 4/5 postinspiratory (post-I) and 3/3 augmenting expiratory (aug-E) neurons. It had no effect on two aug-I and one post-I neurons. Strong immunoreactivity for mu receptor was detected in the soma and dendrites of 5/8 aug-I, 5/7 post-I and 6/6 aug-E neurons. In the remaining three aug-I and two post-I neurons that included cells unresponsive to morphine, weak immunoreactivity was detected only in the dendrites. These results demonstrated wide, but uneven, distribution of mu receptors in bulbar respiratory neurons and suggest their contribution to respiratory depression by opioids.

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.

5-HT4(a) receptors avert opioid-induced breathing depression without loss of analgesia

Opiates are widely used analgesics in anesthesiology, but they have serious adverse effects such as depression of breathing. This is caused by direct inhibition of rhythm-generating respiratory neurons in the Pre-Boetzinger complex (PBC) of the brainstem. We report that serotonin 4(a) [5-HT4(a)] receptors are strongly expressed in respiratory PBC neurons and that their selective activation protects spontaneous respiratory activity. Treatment of rats with a 5-HT4 receptor-specific agonist overcame fentanyl-induced respiratory depression and reestablished stable respiratory rhythm without loss of fentanyl's analgesic effect. These findings imply the prospect of a fine-tuned recovery from opioid-induced respiratory depression, through adjustment of intracellular adenosine 3',5'-monophosphate levels through the convergent signaling pathways in neurons.

Respiratory function in adult mice lacking the mu-opioid receptor: role of delta-receptors

Mice lacking the mu-opioid receptor (MOR) provide a unique model to determine whether opioid receptors are functionally interactive. Recent results have shown that respiratory depression produced by delta-opioid receptor agonists is suppressed in mice lacking the mu-opioid receptor. Here we investigated the involvement of mu- and delta-opioid receptors in the control of ventilation and mu/delta receptor interactions in brainstem rhythm-generating structures. Unrestrained MOR-/- and wild-type mice showed similar ventilatory patterns at rest and similar chemosensory responses to hyperoxia (100% O2), hypoxia (10% O2) or hypercapnia (5%CO2-95%O2). Blockade of delta-opioid receptors with naltrindole affected neither the ventilatory patterns nor the ventilatory responses to hypoxia in MOR-/- and wild-type mice. In-vitro, respiratory neurons were recorded in the pre-Bötzinger complex of thick brainstem slices of MOR-/- and wild-type young adult mice. Respiratory frequency was not significantly different between these two groups. The delta2 receptor agonist deltorphin II (0.1-1.0 microM) decreased respiratory frequency in both groups whereas doses of the delta1 receptor agonist enkephalin[D-Pen2,5] (0.1-1.0 microM) which were ineffective in wild-type mice significantly decreased respiratory frequency in MOR-/- mice. We conclude that deletion of the mu-opioid receptor gene has no significant effect on ensuing respiratory rhythm generation, ventilatory pattern, or chemosensory control. In MOR-/- mice, the loss of respiratory-depressant effects of delta2-opioid receptor agonists previously observed in vivo does not result from a blunted response of delta receptors in brainstem rhythm-generating structures. These structures show an unaltered response to delta2-receptor agonists and an augmented response to delta1-receptor agonists.

Effects of the potent analgesic enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan on respiration

We investigated whether the enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan, which have been shown to be potent analgesics, depress respiration as do opioid analgesics. Ventilation was measured in cats and rodents by the barometric method, in the awake state and during anesthesia. Tissue distribution of the inhibitors was either generalized (RB101, 40-160 mg/kg i.p.), largely restricted by the blood-brain barrier to the periphery (kelatorphan, 0.7-20 mg/kg i.v.), or restricted to the brainstem (i.c.v. injection of RB101 in the fourth ventricle). RB101 did not affect ventilation in any condition tested, and large doses of kelatorphan produced a naloxone-reversible increase in ventilation and breathing frequency. Thus endogenous opioids released during conditions of normal ventilation do not exert any depressant neuromodulatory effect on this function, even when their extracellular concentrations are increased by peptidase inhibitors. The differential effect of these inhibitors on ventilation and nociception is discussed. We conclude that kelatorphan and RB101 are devoid of respiratory-depressant effects and might be interesting pharmacological alternatives to morphine and other opioid agonists.

Is there tonic activity in the endogenous opioid systems? A c-Fos study in the rat central nervous system after intravenous injection of naloxone or naloxone-methiodide

This study examined the possibility that a tonic activity in the endogenous opioid systems (EO systems) exists in animals under normal conditions. In a first set of experiments, concurrent changes in behavioral responses and in the numbers of c-Fos-like immunoreactive (Fos-LI) neurons in 58 structures of the brain and lumbosacral spinal cord were analyzed in rats after systemic administration of the opioid antagonist naloxone (NAL; 2 mg/kg). Possible roles of the EO systems were inferred from changes in the numbers of Fos-LI neurons between normal rats that received either NAL or the same volume of saline. Free-floating sections were processed immunohistochemically for c-Fos protein using standard avidin-biotin complex methods. After NAL, the numbers of Fos-LI neurons were significantly increased in the area postrema; in the caudal, intermediate, and rostral parts of the nucleus tractus solitarii; in the rostral ventrolateral medulla; in the Kölliker-Fuse nucleus; in the supramammillary nucleus; and in the central nucleus of the amygdala. In a second set of experiments examining changes in c-Fos expression in the latter structures, similar increases were found after NAL but not after an equimolar dose of NAL-methiodide, a preferential, peripherally acting opioid receptor antagonist. Therefore, Fos-LI was likely triggered after blockade of central opioid receptors, but not peripheral opioid receptors, releasing neurons from EO system-mediated inhibition. The results of this study suggest the existence of a tonic activity of the EO systems exerted on a restricted number of brain regions in normal rats. This tonic activity of the EO systems may control part of the neural networks involved in cardiorespiratory functions and in emotional and learning processes.

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.

Exposure to prenatal carbon monoxide and postnatal hyperthermia: short and long-term effects on neurochemicals and neuroglia in the developing brain

The effects of prenatal exposure to carbon monoxide (CO), a major component of cigarette smoke, was studied alone or in combination with postnatal hyperthermia, on the structural and neurochemical development of the postnatal brain at 1 and 8 weeks. Pregnant guinea pigs (n = 11) were exposed to 200 p.p.m CO for 10 h/day from midgestation until term (68 days), whereas control mothers (n = 10) breathed room air. On postnatal day 4, neonates from the control and CO-exposed pregnancies were exposed to hyperthermia (35 degrees C) for 75 min or remained at ambient (23 degrees C) temperature. Using semiquantitative immunohistochemical techniques the following neurotransmitter alterations were found in the medulla at 1 week: a decrease in met-enkephalin-immunoreactivity (IR) following postnatal hyperthermia and an increase in 5-hydroxytryptamine-IR following a combination of CO and hyperthermia. No alterations were observed in substance P- or tyrosine-hydroxylase-IR in any paradigm. At 8 weeks of age the combination of prenatal CO exposure followed by a brief hyperthermic stress postnatally resulted in lesions throughout the brain and an increase in glial fibrillary acidic protein-IR in the medulla. Such effects on brain development could be of relevance in cardiorespiratory control in the neonate and could have implications for the etiology of Sudden Infant Death Syndrome, where smoking and hyperthermia are major risk factors.

Anatomical distribution of sodium-dependent [(3)H]naloxone binding sites in rat brain

The sulfhydryl alkylating reagent N-ethylmaleimide (NEM) blocks opioid receptor binding and receptor/G-protein coupling. Sodium partially restores [(3)H]naloxone binding after inhibition by NEM to reveal sodium-dependent [(3)H]naloxone sites, defined as binding in the presence of 50-100 mM NaCl after treatment of membranes or sections with 750 microM NEM. In the present study, receptor autoradiography of [(3)H]naloxone binding in control and NEM-treated tissue was used to examine the anatomical distribution of sodium-dependent [(3)H]naloxone sites in rat brain. In brain membranes, the pharmacology of sodium-dependent [(3)H]naloxone sites was consistent with that of mu opioid receptors. Relatively high IC(50) values for agonists and lack of effect of Gpp(NH)p on DAMGO displacement of [(3)H]naloxone binding in NEM-treated membranes indicated that the sodium-dependent sites were low affinity sites, presumably uncoupled from G-proteins. Autoradiograms revealed that NEM treatment dramatically reduced [(3)H]naloxone binding in all brain regions. However, [(3)H]naloxone binding was increased in specific regions in NEM-treated sections in the presence of sodium, including bed nucleus of the stria terminalis, interpeduncular nucleus, periaqueductal gray, parabrachial nucleus, locus coeruleus, and commissural nucleus tractus solitarius. Sodium-dependent [(3)H]naloxone binding sites were not found in other areas that exhibited [(3)H]naloxone binding in control tissue, including the striatum and thalamus. These studies revealed the presence of a subpopulation of [(3)H]naloxone binding sites which are sodium-dependent and have a unique regional distribution in the rat brain.

Prenatal cocaine raises mu-opioid receptor density in piglet cardiorespiratory medulla

Repeated prenatal exposure to cocaine attenuates arousal and cardiorespiratory functions in neonates. This study explored the possible role of brainstem mu- and delta-opioid systems in these effects. Medullary sections were obtained from 6 to 7 (young) and 20 to 21-day-old (older) piglets either unexposed or exposed prenatally to a 2-mg/kg intravenous cocaine hydrochloride dose, injected to the pregnant sow four times a day during the last third of gestation. Mu- and delta-opioid receptor binding was assessed by quantitative autoradiography using, respectively, 125I-DAMGO (Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol) and 125I-DPDPE (Tyr-D-Pen-Gly-pCl-Phe-D-Pen). At control, delta-, but not mu-opioid, receptor density increased with postnatal age. In contrast, cocaine-induced mu-, but not delta-opioid, receptor density increased 1) in the dorsal motor vagal (dmnX) and facial (nF) nuclei, and, at borderline significance level, in the cardiorespiratory-related gigantocellular reticular nucleus (nRG) of the young, and 2) in the spinal trigeminal nucleus and tract (nSp5), and in the cardiorespiratory-related medial solitary tract (nTSm) and lateral reticular (nRL) nuclei of both age groups. These findings support a possible participation of the mu-opioid system in the attenuation of arousal and cardiorespiration after repeated prenatal exposure to cocaine.

Mu and kappa1 opioid-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate binding in cynomolgus monkey brain

Agonist-stimulated [35S]GTPgammaS binding allows the visualization of receptor-activated G-proteins, thus revealing the anatomical localization of functional receptor activity. In the present study, agonist-stimulated [35S]GTPgammaS binding was used to demonstrate mu and kappa1 opioid-stimulated [35S]GTPgammaS binding in tissue sections and membranes from cynomolgus monkey brain using DAMGO and U50,488H, respectively. Concentrations of agonists required to produce maximal stimulation of [35S]GTPgammaS binding were determined in membranes from the frontal poles of the brain. Receptor specificity was verified in both membranes and sections by inhibiting agonist-stimulated [35S]GTPgammaS binding with the appropriate antagonist. Mu opioid-stimulated [35S]GTPgammaS binding was high in areas including the amygdala, ventral striatum, caudate, putamen, medial thalamus and hypothalamus. Dense mu-stimulated [35S]GTPgammaS binding was also found in brainstem nuclei including the interpeduncular nucleus, parabrachial nucleus and nucleus of the solitary tract. Kappa1 opioid-stimulated [35S]GTPgammaS binding was high in limbic and association cortex, ventral striatum, caudate, putamen, globus pallidus, claustrum, amygdala, hypothalamus and substantia nigra. These results demonstrate the applicability of [35S]GTPgammaS autoradiography to examine receptor-activated G-proteins in the primate brain and reveal functional mu and kappa1 opioid receptor activity that may contribute to the reported central nervous system effects of opiates.

Tritiated-naloxone binding to brainstem opioid receptors in the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is defined as the sudden death of an infant under 1 year of age that remains unexplained after a thorough case investigation, including a complete autopsy. We hypothesized that SIDS is associated with altered 3H - naloxone binding to opioid receptors in brainstem nuclei related to respiratory and autonomic control. We analyzed 3H - naloxone binding in 21 regions in SIDS and control brainstems using quantitative tissue receptor autoradiography. Three groups were analyzed: SIDS (n = 45); acute controls (n = 14); and a chronic group with oxygenation disorders (n = 15). Opioid binding was heavily concentrated in the caudal nucleus of the solitary tract, nucleus parabrachialis medialis, spinal trigeminal nucleus, inferior olive, and interpeduncular nucleus in all cases analyzed (n = 74). The arcuate nucleus on the ventral medullary surface contained negligible binding in all cases (n = 74), and therefore binding was not measurable at this site. We found no significant differences among the three groups in the age-adjusted mean 3H - naloxone binding in 21 brainstem sites analyzed. The only differences we have found to date between SIDS and acute controls are decreases in 3H - quinuclidinyl benzilate binding to muscarinic cholinergic receptors and in 3H - kainate binding to kainate receptors in the arcuate nucleus in alternate sections of this same data set. The present study suggests that there is not a defect in opioid receptor binding in cardiorespiratory nuclei in SIDS brainstems.

The enkephalinergic innervation of the genioglossus musculature in the rat: implications for the respiratory control of the tongue

This study sought to determine if the enkephalinergic (ENK) innervation of the hypoglossal nucleus (nXII) in the rat was organized differentially for the control of the genioglossus musculature whose activity is essential in maintaining the patency of the upper airway. Immunocytochemical results revealed that the genioglossus motoneuron pool, comprising the ventrolateral subcompartment of the nXII, was consistently and heavily labeled throughout its rostrocaudal dimension. Labeling was characterized by dense focal clustering throughout the neuropil, and by the appearance of numerous perisomatic-like profiles. Similarly, the ventromedial subcompartment mainly rostrally, and the dorsal compartment caudally, whose motoneurons control the caudal intrinsic protrusor and rostral retrusor muscles, respectively, were also consistently labeled. While these results demonstrate that the genioglossus musculature is targeted by ENK inputs, they also suggest that other selected musculature of the tongue is controlled by ENK. It is argued that the innervation pattern identified in the present study is consistent with a functional role for ENK in the respiratory control of the tongue.

Actions of opioids on respiratory activity via activation of brainstem mu-, delta- and kappa-receptors; an in vitro study

Opioid-induced respiratory depression is well documented. However, exact sites of action and mechanisms for opioid-induced effects on respiration have not yet been elucidated. The present study was carried out on isolated brainstem-spinal cord preparations from newborn rats in order to explore the opioid activity on brainstem mu-, delta- and kappa-receptors. The brainstem-spinal cord was isolated from 0- to 4-day-old Sprague-Dawley rats. The preparation was perfused with artificial cerebrospinal fluid (28.5 degrees C) equilibrated with 95% O2 and 5% CO2 at a pH of 7.4. Neuronal respiratory activity was recorded from the ventrolateral part of the medulla oblongata and efferent impulses from C4 or C5 ventral roots. Effects of the mu-receptor agonist DAGO, the delta-receptor agonist DPDPE and the kappa-receptor agonist U50,488 on respiratory frequency (fR), inspiratory time (Ti) and peak integrated C4 amplitude (Int[C4]) were measured. In addition, the effect of pre-treatment with the mu1 receptor antagonist naloxanazine (35 mg/kg, subcutaneous injection) was evaluated. DAGO reduced fR and Ti in a concentration-dependent manner and caused a reduction of Int(C4) at high concentrations (10 microM). The mu1 receptor antagonist naloxanazine shifted the fR concentration-response curve for DAGO to the right (P < 0.05). DPDPE had no effect on respiratory activities whereas U50,488, like DAGO, reduced fR and Int(C4) in a concentration-dependent manner. It was concluded that mu-opioid receptors, including the mu1 were involved in fR reduction whereas kappa-opioid receptors were involved in reduction of both fR and respiratory amplitude. Delta-opioid receptors do not seem to participate in respiratory modulation at this age.