8-OH-DPAT abolishes the pulmonary C-fiber-mediated apneic response to fentanyl largely via acting on 5HT1A receptors in the nucleus tractus solitarius

Impact of Serotonergic 5HT1A and 5HT2A Receptor Activation on the Respiratory Response to Hypercapnia in a Rat Model of Parkinson's Disease

In Parkinson's disease (PD), along with typical motor dysfunction, abnormal breathing is present; the cause of which is not well understood. The study aimed to analyze the effects of stimulation of the serotonergic system with 5-HT1A and 5-HT2A agonists in a model of PD induced by injection of 6-hydroxydopamine (6-OHDA). To model PD, bilateral injection of 6-OHDA into both striata was performed in male Wistar rats. Respiratory disturbances in response to 7% hypercapnia (CO2 in O2) in the plethysmographic chamber before and after stimulation of the serotonergic system and the incidence of apnea were studied in awake rats 5 weeks after 6-OHDA or vehicle injection. Administration of 6-OHDA reduced the concentration of serotonin (5-HT), dopamine (DA) and norepinephrine (NA) in the striatum and the level of 5-HT in the brainstem of treated rats, which have been associated with decreased basal ventilation, impaired respiratory response to 7% CO2 and increased incidence of apnea compared to Sham-operated rats. Intraperitoneal (i.p.) injection of the 5-HT1AR agonist 8-OH-DPAT and 5-HT2AR agonist NBOH-2C-CN increased breathing during normocapnia and hypercapnia in both groups of rats. However, it restored reactivity to hypercapnia in 6-OHDA group to the level present in Sham rats. Another 5-HT2AR agonist TCB-2 was only effective in increasing normocapnic ventilation in 6-OHDA rats. Both the serotonergic agonists 8-OH-DPAT and NBOH-2C-CN had stronger stimulatory effects on respiration in PD rats, compensating for deficits in basal ventilation and hypercapnic respiration. We conclude that serotonergic stimulation may have a positive effect on respiratory impairments that occur in PD.

Apneic response to fentanyl in adult rats: Role of laryngeal afferents

Intravenous (systemic) bolus injection of fentanyl (FNT) reportedly induces an immediate vagal-mediated apnea; however, the precise origin of vagal afferents responsible for this apnea remains unknown. We tested whether intralaryngeal (local) application of FNT would also trigger an apnea and whether the apneic response to both local and systemic administration of FNT was laryngeal afferent-mediated. Cardiorespiratory responses to FNT were recorded in anesthetized male adult rats with and without bilateral sectioning of the superior laryngeal nerve (SLNx) or peri-SLN capsaicin treatment (SLNcap) to block local C-fiber signal conduction. Opioid mu-receptor (MOR)-immunoreactivity was detected in laryngeal C- and myelinated neurons. We found that local and systemic administration of FNT elicited an immediate apnea. SLNx, rather than SLNcap, abolished the apneic response to local FNT application though MORs were abundantly expressed in both laryngeal C- and myelinated neurons. Importantly, SLNx failed to affect the apneic response to systemic FNT administration. These results lead to the conclusion that laryngeal afferents' MORs are responsible for the apneic response to local, but not systemic, administration of FNT.

L-cysteine ethylester reverses the adverse effects of morphine on breathing and arterial blood-gas chemistry while minimally affecting antinociception in unanesthetized rats

L-cysteine ethylester (L-CYSee) is a membrane-permeable analogue of L-cysteine with a variety of pharmacological effects. The purpose of this study was to determine the effects of L-CYSee on morphine-induced changes in ventilation, arterial-blood gas (ABG) chemistry, Alveolar-arterial (A-a) gradient (i.e., a measure of the index of alveolar gas-exchange), antinociception and sedation in male Sprague Dawley rats. An injection of morphine (10 mg/kg, IV) produced adverse effects on breathing, including sustained decreases in minute ventilation. L-CYSee (500 μmol/kg, IV) given 15 min later immediately reversed the actions of morphine. Another injection of L-CYSee (500 μmol/kg, IV) after 15 min elicited more pronounced excitatory ventilatory responses. L-CYSee (250 or 500 μmol/kg, IV) elicited a rapid and prolonged reversal of the actions of morphine (10 mg/kg, IV) on ABG chemistry (pH, pCO2, pO2, sO2) and A-a gradient. L-serine ethylester (an oxygen atom replaces the sulfur; 500 μmol/kg, IV), was ineffective in all studies. L-CYSee (500 μmol/kg, IV) did not alter morphine (10 mg/kg, IV)-induced sedation, but slightly reduced the overall duration of morphine (5 or 10 mg/kg, IV)-induced analgesia. In summary, L-CYSee rapidly overcame the effects of morphine on breathing and alveolar gas-exchange, while not affecting morphine sedation or early-stage analgesia. The mechanisms by which L-CYSee modulates morphine depression of breathing are unknown, but appear to require thiol-dependent processes.

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

L-cysteine methyl ester overcomes the deleterious effects of morphine on ventilatory parameters and arterial blood-gas chemistry in unanesthetized rats

We are developing a series of thiolesters that produce an immediate and sustained reversal of the deleterious effects of opioids, such as morphine and fentanyl, on ventilation without diminishing the antinociceptive effects of these opioids. We report here the effects of systemic injections of L-cysteine methyl ester (L-CYSme) on morphine-induced changes in ventilatory parameters, arterial-blood gas (ABG) chemistry (pH, pCO₂, pO₂, sO₂), Alveolar-arterial (A-a) gradient (i.e., the index of alveolar gas-exchange within the lungs), and antinociception in unanesthetized Sprague Dawley rats. The administration of morphine (10 mg/kg, IV) produced a series of deleterious effects on ventilatory parameters, including sustained decreases in tidal volume, minute ventilation, inspiratory drive and peak inspiratory flow that were accompanied by a sustained increase in end inspiratory pause. A single injection of L-CYSme (500 μmol/kg, IV) produced a rapid and long-lasting reversal of the deleterious effects of morphine on ventilatory parameters, and a second injection of L-CYSme (500 μmol/kg, IV) elicited pronounced increases in ventilatory parameters, such as minute ventilation, to values well above pre-morphine levels. L-CYSme (250 or 500 μmol/kg, IV) also produced an immediate and sustained reversal of the deleterious effects of morphine (10 mg/kg, IV) on arterial blood pH, pCO₂, pO₂, sO₂ and A-a gradient, whereas L-cysteine (500 μmol/kg, IV) itself was inactive. L-CYSme (500 μmol/kg, IV) did not appear to modulate the sedative effects of morphine as measured by righting reflex times, but did diminish the duration, however, not the magnitude of the antinociceptive actions of morphine (5 or 10 mg/kg, IV) as determined in tail-flick latency and hindpaw-withdrawal latency assays. These findings provide evidence that L-CYSme can powerfully overcome the deleterious effects of morphine on breathing and gas-exchange in Sprague Dawley rats while not affecting the sedative or early stage antinociceptive effects of the opioid. The mechanisms by which L-CYSme interferes with the OR-induced signaling pathways that mediate the deleterious effects of morphine on ventilatory performance, and by which L-CYSme diminishes the late stage antinociceptive action of morphine remain to be determined.

D-Cysteine Ethyl Ester Reverses the Deleterious Effects of Morphine on Breathing and Arterial Blood-Gas Chemistry in Freely-Moving Rats

Cell-penetrant thiol esters including the disulfides, D-cystine diethyl ester and D-cystine dimethyl ester, and the monosulfide, L-glutathione ethyl ester, prevent and/or reverse the deleterious effects of opioids, such as morphine and fentanyl, on breathing and gas exchange within the lungs of unanesthetized/unrestrained rats without diminishing the antinociceptive or sedative effects of opioids. We describe here the effects of the monosulfide thiol ester, D-cysteine ethyl ester (D-CYSee), on intravenous morphine-induced changes in ventilatory parameters, arterial blood-gas chemistry, alveolar-arterial (A-a) gradient (i.e., index of gas exchange in the lungs), and sedation and antinociception in freely-moving rats. The bolus injection of morphine (10 mg/kg, IV) elicited deleterious effects on breathing, including depression of tidal volume, minute ventilation, peak inspiratory flow, and inspiratory drive. Subsequent injections of D-CYSee (2 × 500 μmol/kg, IV, given 15 min apart) elicited an immediate and sustained reversal of these effects of morphine. Morphine (10 mg/kg, IV) also A-a gradient, which caused a mismatch in ventilation perfusion within the lungs, and elicited pronounced changes in arterial blood-gas chemistry, including pronounced decreases in arterial blood pH, pO2 and sO2, and equally pronounced increases in pCO2 (all responses indicative of decreased ventilatory drive). These deleterious effects of morphine were immediately reversed by the injection of a single dose of D-CYSee (500 μmol/kg, IV). Importantly, the sedation and antinociception elicited by morphine (10 mg/kg, IV) were minimally affected by D-CYSee (500 μmol/kg, IV). In contrast, none of the effects of morphine were affected by administration of the parent thiol, D-cysteine (1 or 2 doses of 500 μmol/kg, IV). Taken together, these data suggest that D-CYSee may exert its beneficial effects via entry into cells that mediate the deleterious effects of opioids on breathing and gas exchange. Whether D-CYSee acts as a respiratory stimulant or counteracts the inhibitory actions of µ-opioid receptor activation remains to be determined. In conclusion, D-CYSee and related thiol esters may have clinical potential for the reversal of the adverse effects of opioids on breathing and gas exchange, while largely sparing antinociception and sedation.

S-Nitroso-L-Cysteine Stereoselectively Blunts the Deleterious Effects of Fentanyl on Breathing While Augmenting Antinociception in Freely-Moving Rats

Endogenous and exogenously administered S-nitrosothiols modulate the activities of central and peripheral systems that control breathing. We have unpublished data showing that the deleterious effects of morphine on arterial blood-gas chemistry (i.e., pH, pCO2, pO2, and sO2) and Alveolar-arterial gradient (i.e., index of gas exchange) were markedly diminished in anesthetized Sprague Dawley rats that received a continuous intravenous infusion of the endogenous S-nitrosothiol, S-nitroso-L-cysteine. The present study extends these findings by showing that unanesthetized adult male Sprague Dawley rats receiving an intravenous infusion of S-nitroso-L-cysteine (100 or 200 nmol/kg/min) markedly diminished the ability of intravenous injections of the potent synthetic opioid, fentanyl (10, 25, and 50 μg/kg), to depress the frequency of breathing, tidal volume, and minute ventilation. Our study also found that the ability of intravenously injected fentanyl (10, 25, and 50 μg/kg) to disturb eupneic breathing, which was measured as a marked increase of the non-eupneic breathing index, was substantially reduced in unanesthetized rats receiving intravenous infusions of S-nitroso-L-cysteine (100 or 200 nmol/kg/min). In contrast, the deleterious effects of fentanyl (10, 25, and 50 μg/kg) on frequency of breathing, tidal volume, minute ventilation and non-eupneic breathing index were fully expressed in rats receiving continuous infusions (200 nmol/kg/min) of the parent amino acid, L-cysteine, or the D-isomer, namely, S-nitroso-D-cysteine. In addition, the antinociceptive actions of the above doses of fentanyl as monitored by the tail-flick latency assay, were enhanced by S-nitroso-L-cysteine, but not L-cysteine or S-nitroso-D-cysteine. Taken together, these findings add to existing knowledge that S-nitroso-L-cysteine stereoselectively modulates the detrimental effects of opioids on breathing, and opens the door for mechanistic studies designed to establish whether the pharmacological actions of S-nitroso-L-cysteine involve signaling processes that include 1) the activation of plasma membrane ion channels and receptors, 2) selective intracellular entry of S-nitroso-L-cysteine, and/or 3) S-nitrosylation events. Whether alterations in the bioavailability and bioactivity of endogenous S-nitroso-L-cysteine is a key factor in determining the potency/efficacy of fentanyl on breathing is an intriguing question.

Systemic 8-OH-DPAT challenge causes hyperventilation largely via activating pre-botzinger complex 5-HT1A receptors

Systemic 8-OH-DPAT (a 5-HT_1A receptor agonist) challenge evokes hyperventilation independent of peripheral 5-HT_1A receptors. Though the pre-Botzinger Complex (PBC) is critical in generating respiratory rhythm and activation of local 5-HT_1A receptors induces tachypnea via disinhibition of local GABA_A neurons, its role in the respiratory response to systemic 8-OH-DPAT challenge is still unclear. In anesthetized rats, 8-OH-DPAT (100 μg/kg, iv) was injected twice to confirm the reproducibility of the evoked responses. The same challenges were performed after bilateral microinjections of (S)-WAY-100135 (a 5-HT_1A receptor antagonist) or gabazine (a GABA_A receptor antagonist) into the PBC. Our results showed that: 1) 8-OH-DPAT caused reproducible hyperventilation associated with hypotension and bradycardia; 2) microinjections of (S)-WAY-100135 into the PBC attenuated the hyperventilation by ˜60 % without effect on the evoked hypotension and bradycardia; and 3) the same hyperventilatory attenuation was also observed after microinjections of gabazine into the PBC. Our data suggest that PBC 5-HT_1A receptors play a key role in the respiratory response to systemic 8-OH-DPAT challenge likely via disinhibiting local GABAergic neurons.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

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

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

Depressed GABA and glutamate synaptic signaling by 5-HT1A receptors in the nucleus tractus solitarii and their role in cardiorespiratory function

Serotonin (5-HT), and its 5-HT1A receptor (5-HT1AR) subtype, is a powerful modulator of the cardiorespiratory system and its sensory reflexes. The nucleus tractus solitarii (nTS) serves as the first central station for visceral afferent integration and is critical for cardiorespiratory reflex responses. However, the physiological and synaptic role of 5-HT1ARs in the nTS is relatively unknown. In the present study, we examined the distribution and modulation of 5-HT1ARs on cardiorespiratory and synaptic parameters in the nTS. 5-HT1ARs were widely distributed to cell bodies within the nTS but not synaptic terminals. In anesthetized rats, activation of 5-HT1ARs by microinjection of the 5-HT1AR agonist 8-OH-DPAT into the caudal nTS decreased minute phrenic neural activity via a reduction in phrenic amplitude. In brain stem slices, 8-OH-DPAT decreased the amplitude of glutamatergic tractus solitarii-evoked excitatory postsynaptic currents, and reduced overall spontaneous excitatory nTS network activity. These effects persisted in the presence of GABAA receptor blockade and were antagonized by coapplication of 5-HT1AR blocker WAY-100135. 5-HT1AR blockade alone had no effect on tractus solitarii-evoked excitatory postsynaptic currents, but increased excitatory network activity. On the other hand, GABAergic nTS-evoked inhibitory postsynaptic currents did not change by activation of the 5-HT1ARs, but spontaneous inhibitory nTS network activity decreased. Blocking 5-HT1ARs tended to increase nTS-evoked inhibitory postsynaptic currents and inhibitory network activity. Taken together, 5-HT1ARs in the caudal nTS decrease breathing, likely via attenuation of afferent transmission, as well as overall nTS network activity.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Isoflurane inhibits bronchopulmonary C-fiber-mediated apneic response to phenylbiguanide by depressing 5-HT3 receptor function in anesthetized rats

A previous study by the authors has shown that isoflurane (ISO), a commonly used volatile anesthetic, has an excitatory effect on bronchopulmonary C-fibers (PCFs). Since selective stimulation of PCFs by action on local 5-HT3 receptors could evoke an apnea, this current study addresses whether inhalation of ISO would facilitate the PCF 5-HT3 receptor-mediated apneic response and, if so, how. In anesthetized and spontaneously breathing rats, inhalation of 5% ISO markedly inhibited the apneic response to intra-atrium injection of phenylbiguanide (PBG, 25 μg/kg), a 5-HT3 receptor agonist, which was contrary to the hypothesis. Extracellular recording of the nodose ganglion neurons in anesthetized, paralyzed and ventilated rats revealed that ISO attenuated the PBG-elicited excitation of pulmonary C neurons. Furthermore, using the patch clamp technique, it was found that ISO depressed the PBG-induced inward current of the pulmonary C neurons labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) instilled previously into the lungs. These results suggest that ISO inhibits PCF 5-HT3 channel functions, and thereby attenuates PCF excitatory response to PBG, likely contributing to the diminution of the PBG-induced apnea by ISO in rats.

Perivagal antagonist treatment in rats selectively blocks the reflex and afferent responses of vagal lung C fibers to intravenous agonists

The terminals of vagal lung C fibers (VLCFs) express various types of pharmacological receptors that are important to the elicitation of airway reflexes and the development of airway hypersensitivity. We investigated the blockade of the reflex and afferent responses of VLCFs to intravenous injections of agonists using perivagal treatment with antagonists (PAT) targeting the transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors in anesthetized rats. Blockading these responses via perivagal capsaicin treatment (PCT), which blocks the neural conduction of C fibers, was also studied. We used capsaicin, α,β-methylene-ATP, and phenylbiguanide as the agonists, and capsazepine, iso-pyridoxalphosphate-6-azophenyl-2',5'-disulfonate, and tropisetron as the antagonists of transient receptor potential vanilloid 1, P2X, and 5-HT(3) receptors, respectively. We found that each of the PATs abolished the VLCF-mediated reflex apnea evoked by the corresponding agonist, while having no effect on the response to other agonists. Perivagal vehicle treatment failed to produce any such blockade. These blockades had partially recovered at 3 h after removal of the PATs. In contrast, PCT abolished the reflex apneic response to all three agonists. Both PATs and PCT did not affect the myelinated afferent-mediated apneic response to lung inflation. Consistently, our electrophysiological studies revealed that each of the PATs prevented the VLCF responses to the corresponding agonist, but not to any other agonist. PCT inevitably prevented the VLCF responses to all three agonists. Thus these PATs selectively blocked the stimulatory action of corresponding agonists on the VLCF terminals via mechanisms that are distinct from those of PCT. PAT may become a novel intervention for studying the pharmacological modulation of VLCFs.