Interaction of naloxone with mu- and delta-opioid agonists on the respiration of rats

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

In vivo profiling of seven common opioids for antinociception, constipation and respiratory depression: no two opioids have the same profile

BACKGROUND AND PURPOSE

For patients experiencing inadequate analgesia and intolerable opioid-related side effects on one strong opioid analgesic, pain relief with acceptable tolerability is often achieved by rotation to a second strong opioid. These observations suggest subtle pharmacodynamic differences between opioids in vivo. This study in rats was designed to assess differences between opioids in their in vivo profiles.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats were given single i.c.v. bolus doses of morphine, morphine-6-glucuronide (M6G), fentanyl, oxycodone, buprenorphine, DPDPE ([D-penicillamine(2,5) ]-enkephalin) or U69,593. Antinociception, constipation and respiratory depression were assessed using the warm water tail-flick test, the castor oil-induced diarrhoea test and whole body plethysmography respectively.

KEY RESULTS

These opioid agonists produced dose-dependent antinociception, constipation and respiratory depression. For antinociception, morphine, fentanyl and oxycodone were full agonists, buprenorphine and M6G were partial agonists, whereas DPDPE and U69,593 had low potency. For constipation, M6G, fentanyl and buprenorphine were full agonists, oxycodone was a partial agonist, morphine produced a bell-shaped dose-response curve, whereas DPDPE and U69,593 were inactive. For respiratory depression, morphine, M6G, fentanyl and buprenorphine were full agonists, oxycodone was a partial agonist, whereas DPDPE and U69,593 were inactive. The respiratory depressant effects of fentanyl and oxycodone were of short duration, whereas morphine, M6G and buprenorphine evoked prolonged respiratory depression.

CONCLUSION AND IMPLICATIONS

For the seven opioids we assessed, no two had the same profile for evoking antinociception, constipation and respiratory depression, suggesting that these effects are differentially regulated. Our findings may explain the clinical success of 'opioid rotation'.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

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.

S-Methadone augments R-methadone induced respiratory depression in the neonatal guinea pig

Methadone is administered as a racemic mixture, although its analgesic and respiratory effects are attributed to R-isomer activity at the mu opioid receptor (MOP). Recently, we observed a four-fold increase in inspiratory time in 3-day-old guinea pigs following an injection of racemic methadone. We hypothesized that this effect was due to augmentation of R-methadone induced respiratory depression by the S-methadone isomer. In the current longitudinal study, we injected 3-, 7-, and 14-day-old neonatal guinea pigs with saline, R-methadone, S-methadone, or R- plus S-methadone in order to characterize the roles of the individual isomers, as well as the synergistic effects of co-administration. Using plethysmography, we measured respiratory parameters while breathing room air and during a 5% CO(2) challenge. S-Methadone alone had no respiratory effects. However, the R- plus S-methadone group showed greater respiratory depression and increased inspiratory time than the R-methadone group in the youngest animals, suggesting that the respiratory effects of R-methadone are augmented by S-methadone in early development.

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.

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.

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.

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.

Developmental changes of (3)H-labelled mu-opioid receptors in brainstems of intra-uterine growth-restricted rats

The opioid mu-system is involved in brainstem-mediated respiratory control. Infants with intra-uterine growth restriction (IUGR) have more respiratory disorders in the early postnatal period. Using [(3)H]DAGO, a mu-selective ligand, and a computer-based image analysis of autoradiography, we compared the ontogeny and distribution of mu-opioid binding sites in the brainstem of IUGR and control rats in utero (E21), at birth (P0) and on postnatal days 1 (P1), P7, P10, P14 and P21. The ontogeny pattern was found to be similar in both groups. The density of the binding sites, which was low in E21, increased at P0, slightly declined at P1 and remained relatively constant thereafter. The distribution of DAGO-binding sites, also similar in both groups, was heterogeneous and was much denser in the dorsal areas of medulla and pons. In particular, binding sites were highly concentrated in nuclei involved in the cardio-respiratory function. However, DAGO-binding density was higher at all ages (except for P0 and P1) in IUGR than in control rats. Taken together, these results give at least a partial explanation for the effects of IUGR which lowers the Apgar score at birth and raises the incidence of respiratory disorders in infants.

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.

Effects of naloxone and morphine on acute hypoxic survival in mice

OBJECTIVE

To investigate a role of the opiate system during acute hypoxic hypoxia, the effects of naloxone and morphine on hypoxic survival rate were investigated in awake adult mice.

DESIGN

Prospective, randomized, animal trial.

SETTING

University research laboratory.

SUBJECTS

Male dd-Y mice (n = 864 in experiment I, n = 144 in experiment II, n = 30 in experiment III).

INTERVENTIONS

The animals were placed in an airtight plastic chamber into which a continuous flow of 8 L/min 5% oxygen-95% nitrogen was passed.

MEASUREMENTS AND MAIN RESULTS

One and 5 mg/kg naloxone had no significant effect on the survival rate of mice subjected to acute hypoxic hypoxia, whereas 10 mg/kg naloxone decreased the survival rate. On the other hand, 2 and 5 mg/kg morphine was shown to have a protective action against acute hypoxic hypoxia. The protective effects of 5 mg/kg morphine against hypoxia was even antagonized by 5 mg/kg naloxone, which did not itself show any significant effect on the survival rate. The oxygen consumption in the morphine-treated (5 mg/kg) mice was significantly (p < .05) lower (87.0% +/- 4.6%; mean +/- SE) than that in the saline-treated animals.

CONCLUSIONS

The present study suggests that the endogenous opiate system does not play a significant role on the pathophysiology caused by acute hypoxic hypoxia and that the improved survival of the hypoxic animals by morphine is at least partly attributable to its depressant effect on oxygen consumption.

Effect of chronic resistive loading on hypoxic ventilatory responsiveness

Depression of ventilation mediated by endogenous opioids has been observed acutely after resistive airway loading. We evaluated the effects of chronically increased airway resistance on hypoxic ventilatory responsiveness shortly after load imposition and 6 wk later. A circumferential tracheal band was placed in 200-g rats, tripling tracheal resistance. Sham surgery was performed in controls. Ventilation and the ventilatory response to hypoxia were measured by using barometric plethysmography at 2 days and 6 wk postsurgery in unanesthetized rats during exposure to room air and to 12% O2-5% CO2-balance N2. Trials were performed with and without naloxone (1 mg/kg i.p.). Room air arterial blood gases demonstrated hypercapnia with normoxia in obstructed rats at 2 days and 6 wk postsurgery. During hypoxia, a 30-Torr fall in PO2 occurred with no change in PCO2. Hypoxic ventilatory responsiveness was suppressed in obstructed rats at 2 days postloading. Naloxone partially reversed this suppression. However, hypoxic responsiveness at 6 wk was not different from control levels. Naloxone had a small effect on ventilatory pattern at this time with no overall effect on hypoxic responsiveness. This was in contrast to previously demonstrated long-term suppression of CO2 sensitivity in this model, which was partially reversible by naloxone only during the immediate period after load imposition. Endogenous opioids apparently modulate ventilatory control acutely after load imposition. Their effect wanes with time despite persistence of depressed CO2 sensitivity.

Explanation at the opioid receptor level for differing toxicity of morphine and morphine 6-glucuronide

The radiolabelled opioid receptor binding affinities of morphine and its active metabolite morphine 6-glucuronide at the total mu, mu 1, mu 2 and delta receptors were determined. Morphine 6-glucuronide was found to have a 4-fold lower affinity for the mu 2 receptor (IC50 17 nM and 82 nM for morphine and morphine 6-glucuronide respectively, P = 0.01), the receptor postulated to be responsible for mediating the respiratory depression and gastrointestinal effects after morphine. This provides a possible explanation for the reduced respiratory depression and vomiting seen following morphine 6-glucuronide in man. A similar reduction in affinity of morphine 6-glucuronide was seen at the total mu receptor whilst there was no significant difference seen at the mu 1 or delta receptor. Hence the increased analgesic potency of morphine 6-glucuronide over morphine remains unexplained.

Opioid-induced respiratory depression and analgesia may be mediated by different subreceptors

Use of selective delta opioid antagonists provide evidence that the delta receptor within the brain seems an integrated part in the mediation of respiratory depression induced by a potent analgesic like fentanyl. Low doses of the delta antagonists RX-8008M (3-6 micrograms/kg) as well as ICI 174,864 (3-6 micrograms/kg) reversed fentanyl-related respiratory depression (arterial blood gases) in the unanesthetized canine. Opioid-induced blockade of afferent sensory nerve volleys (amplitude height of the somatosensory-evoked potential) could be reversed only by a high dose (9 micrograms/kg) of RX-8008M. Depression of amplitude height of the SEP could not be reversed by ICI 174,864 over the whole dose range (3-6-9 micrograms/kg). In comparison, naloxone (1-5-10 micrograms/kg) not only reversed depression of PaO2, it also reversed the blockade of afferent sensory nerve impulses in the low (5-micrograms/kg)-dose range. A highly selective delta antagonist may have a therapeutic value in reversing opioid-related respiratory depression, resulting in little or no attenuation of analgesia.

Differential roles of opioid receptors in respiration, respiratory disease, and opiate-induced respiratory depression

In summary, these findings indicate the importance of designing future experiments that delineate between opioid and nonopioid forms of respiratory disease and dysfunction, and the need to identify means of diagnosing them in order to achieve successful recovery. Apparently there is great diversity between animal species in terms of contributions of endogenous opioids to tonic control of ventilation, and future work should strive to identify which species is most appropriate as a model of human ventilatory control and disease. Certain opioid receptor types appear to be linked to independent respiratory functions. For instance, mu receptors in the brain stem produce strong inhibitory actions on respiratory parameters, including RR, VT, VE, and CO2 sensitivity. These effects have been observed in vivo and by electrophysiologic recordings in vitro. Delta receptors may also exert some inhibitory effect on respiration, especially in the NTS. In the CNS, the ventral surfaces of the medulla and pons, especially the NTS and NA, seem to be important sites for opioid-induced inhibition of respiration, whereas the spinal cord probably is not involved in opioid-mediated ventilatory depression. Kappa receptors appear to be devoid of respiratory depressant activity, whereas sigma receptors may stimulate some ventilatory parameters. Morphine and similar pure mu agonists, such as fentanyl and oxymorphine, probably produce their analgesic and respiratory depressant effects through stimulation of mu receptors. Mixed agonists/antagonists that have mu antagonist (or partial agonist) activity plus kappa agonist and/or sigma agonist activity show a ceiling effect for respiratory depression. Future tests need to determine which opioid receptor may be responsible for the ceiling effect. In addition, the effects of mu, delta, kappa, and sigma selective agonists on hypoxic drive should also be determined, as a drug that stimulates hypoxic sensitivity in the face of hypercapnic depression may produce less overall respiratory depression due to counteractive effects. In the future, clinically optimal opiates should have more specificity of action than those available now. This may be achieved by creating drugs selective for single receptors or by creating drugs with desirable combinations of receptor selectivities. The combinations of mixed agonists/antagonists with pure mu agonists currently in use today are promising, as they provide analgesia with reduced respiratory depression. In the early days of opiate research and development, combination drug regimens were thoroughly tested to determine the "ideal ratios" that would retain analgesic properties but not the other undesirable effects such as respiratory depression (196).(ABSTRACT TRUNCATED AT 400 WORDS)

Spinal administration of receptor-selective drugs as analgesics: new horizons

The processing at the spinal cord levels of sensory information is subject to modulation by a number of local receptor systems, including opioids: alpha 2 adrenergic; and to a lesser extent serotonin, GABAB, neuropeptide Y, cholinergic, adenosine, and the NMDA-glutamate site. The functional utility of these multiple systems are only partially understood, but it appears that (a) they may act individually to alter different aspects of the nociceptive sensory message (b) they could be used synergistically to reduce the incidence of side effects by reducing the dose of agents required to yield analgesic effects, and (c) they may function variably in animals made tolerant to classes of receptor agonists.

Tolerance to the respiratory actions of opiates: withdrawal tolerance and asymmetrical cross-tolerance

Cross-tolerance to the respiratory depression induced by i.c.v. [D-Ala2,D-Leu5]enkephalin (DADLE) and by s.c., i.c.v. and i.v. morphine was studied in anesthetized rats that had been rendered tolerant to s.c. sufentanil (4 micrograms/h per 7 days). Tolerance induced by i.c.v. sufentanil was also compared during withdrawal and its absence. Tolerance was evaluated by estimation of the changes in both the maximum attained reduction of respiratory frequency (Emax) and the area under the time-course curve for the effects (AUC). In contrast to the failure to develop tolerance, as estimated with the Emax, after i.c.v. sufentanil in the absence of abstinence (tolerance index: 1.1), rats displayed significant degrees of cross-tolerance to i.c.v. morphine (3.6), i.c.v. DADLE (4.3) and s.c. morphine (6.6). No tolerance, however, was obtained to i.v. morphine. Tolerance to i.c.v. sufentanil in non-abstinent rats was not present when the Emax was considered but was when AUC was considered. During withdrawal animals showed tolerance as estimated with both the Emax and AUC. These results complement previous findings and show that the appearance of dissimilar degrees of cross-tolerance to different agonists depends not only on their intrinsic activities but also on their pharmacokinetic properties. These observations, along with the critical influence of withdrawal upon the manifestation of tolerance, support the need to distinguish between the development of the tolerance state and the degree of tolerance that can be expressed under specific conditions.

Comparative pharmacological and biochemical studies between butorphanol and morphine

A number of in vivo and in vitro studies were undertaken to compare the pharmacological and biochemical effects of the partial agonist, butorphanol, with that of morphine. Both compounds were equipotent antinociceptive agents in the rat tail withdrawal test. In the acetic acid writhing test butorphanol had approximately 3.5 times the antiwrithing activity on a molar basis than morphine. In a study of the effects of these compounds on body temperature, butorphanol as well as morphine produced hyperthermia after acute dosing. Additionally, butorphanol produced a profound diuresis and decrease in urine osmolality after acute administration. In contrast, morphine produced an antidiuresis throughout most of the study period with no significant changes in urine osmolality from control. Butorphanol administration had no effect on respiratory rate, while morphine markedly decreased respiratory rate. In in vitro radioligand displacement studies, butorphanol was a potent competitor against 3H-DAGO, 3H-DPDPE, and 3H(-)-EKC binding, exhibiting 3, 10, and 30 times more activity, respectively, than morphine. Both compounds were weak inhibitors of 3H-(+)-SKF 10047 binding, yielding IC50 values of in excess of 1 microM. The results indicate that butorphanol has multiple actions on the opioid receptor system, and shares similarities as well as differences in its mechanism(s) of actions with morphine.

Ventilatory responses of dystrophic and control hamsters to naloxone

In this study we determined if endogenous opioid peptides may contribute to the depression of ventilation seen in dystrophic hamsters. Ventilation of control and dystrophic awake hamsters was determined prior to either naloxone (1 mg/kg) or saline administration and then 5, 15 and 30 minutes postinjection. Subsequently, animals were exposed to a hypercapnic challenge (7% CO2 in O2). Control hamsters increased ventilation significantly (p less than 0.01) after naloxone compared to saline treatment. In contrast, dystrophic hamsters showed no difference in ventilation when they received either naloxone or saline. Both groups increased ventilation significantly (p less than 0.05) after hypercapnic challenge, whether they had received naloxone or saline. Although dystrophic hamsters can respond to a ventilatory stimulant (CO2), naloxone did not increase ventilation, possibly indicating that endogenous opioids are not responsible for their depressed ventilation.

Mechanisms involved in the respiratory depressant actions of nicotine in anesthetized rats

In the urethane-pentobarbital anesthetized rat, the respiratory depressant and lethal effects of intravenously infused (-)-nicotine (120 micrograms/kg/min) or (+)-nicotine (600 micrograms/kg/min) were effectively prevented by pretreatment with the opioid antagonist, naltrexone, whereas the lethal effect of (-)-nicotine (120 micrograms/kg/min) was not altered by bilateral adrenalectomy. Further, pretreatment with either the nicotinic ganglion-blocker, mecamylamine, a secondary amine, or the quarternary nicotinic ganglion-blocker, hexamethonium, completely prevented the lethal effects of (-)-nicotine (120 micrograms/kg/min). These data suggest that central opioidergic and nicotinic processes are involved in nicotine's respiratory depressant and lethal effects.

A new method for determination of in vivo pA2 using infusions of naloxone to steady-state blood concentrations

A method is described for the measurement of pA2 in vivo for the opioid antagonist naloxone, infused to predicted steady-state blood concentrations in rats. The method has been applied to antagonism of the respiratory depressant effects of opioid agonists. The basis of the technique rests upon determination of the plasma clearance of the antagonist whence appropriate loading doses and zero order infusion rate constants are calculated. The predicted concentrations of naloxone were verified by direct plasma measurement. Using the infusion protocol pA2 determinations by Schild analysis were significantly different from those obtained by bolus injection. The approach described provides more accurate estimates of in vivo pA2 and could be applied to other antagonists, species, and biological responses, overcoming many of the problems inherent in the use of bolus antagonist dosing, which is currently widely employed.

Effects of the enkephalin analogue FK33-824 on rectal temperature and respiratory rate in male mice

Subcutaneous administration of the enkephalin analogue FK33-824 (FK) elicited a dose-related decrease in rectal temperature and respiratory rate in male ddY strain mice. Naloxone and 3 days' implantation of morphine pellet decreased the effects of FK, suggesting the involvement of opioid receptors and cross-tolerance with morphine to both effects of FK. A positive correlation was found between the FK-induced decrease in rectal temperature and that in respiratory rate among the 6 strains of inbred mice including BALB/c, C3H, A/J, CBA, C57BL/6 and DBA/2. The degree of hypothermia elicited by FK was different among strains, whereas marginal strain difference was seen in the respiratory depression induced by FK. The strain difference in the FK responses may be due to the difference in the opioid receptor subtypes in the brain.

Etorphine inhibition of pancreatic exocrine secretion in rats: comparison with methadone

The effects of etorphine, a potent opiate agonist without preferential affinity for mu, delta or kappa receptors, on exocrine pancreatic secretion were studied in rats fitted with chronic or acute pancreatic fistulas and compared to those of methadone, a well-documented mu agonist. In conscious rats etorphine (3 micrograms/kg s.c.) inhibited basal pancreatic secretion by about 50% for volume and bicarbonate output and by 70% for protein output. Pancreatic secretion returned to its basal level within 2 h. Methadone (5 mg/kg s.c.) was about equipotent but the inhibition lasted longer. The effects of both etorphine and methadone were completely antagonized by naloxone (1 mg/kg s.c.) and to a lesser extent by diprenorphine (10 microgram/kg s.c.). Yohimbine did not suppress the inhibitory effect of etorphine on protein output but showed some antagonism against the effects of etorphine on water and bicarbonate output. In anaesthetized rats etorphine (3 micrograms/kg) inhibited the pancreatic secretion stimulated by 2-deoxy glucose, a centrally acting vagal stimulatory agent, by 50-60% for volume and bicarbonate output and totally for protein output. The same dose of etorphine did not inhibit the pancreatic secretion evoked by vagal electrical stimulation, a peripheral stimulus. Methadone (5 mg/kg) inhibited the pancreatic secretion stimulated by 2-deoxy glucose to the same extent, but for a longer time than etorphine, and at the same dose did not suppress the pancreatic pancreatic response to vagal electrical stimulation. The inhibitory effects of etorphine and methadone in anaesthetized rats were completely suppressed by naloxone (1 mg/kg s.c.) and only reduced by diprenorphine (10 micrograms/kg s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of mu- and delta-opioid receptors in cat respiratory areas: an autoradiographic study

Autoradiography after in vitro binding with selective ligands for either mu ([3H](Tyr-D-Ala-Gly-(NMePhe)-Gly-ol] or delta ([3H](Tyr-D-Thr-Gly-Phe-Leu-Thr)) opioid receptor types revealed the presence of variable amounts of radioactive labeling in the cat brainstem. Areas involved in the respiratory rhythmogenesis were among the most prominently labeled structures. The pneumotaxic center, including the nucleus parabrachialis medialis and the Kolliker-Füse nucleus, contains a very high density of delta binding sites while the dorsal respiratory nucleus which corresponds to the nucleus tractus solitarius, is more heavily labeled by the mu ligand. The neuroanatomical differences in the distribution of opioid receptors correlates well with the pharmacological responses induced by administration of specific mu- or delta ligands.

A pharmacological study on respiratory rhythm in the isolated brainstem-spinal cord preparation of the newborn rat

An in vitro brainstem-spinal cord preparation of the newborn rat was used to examine the effects of neurotransmitters and transmitter candidates on respiratory frequency. Spontaneous periodic depolarization of the spinal ventral roots of the 4th or 5th cervical segment was observed at a frequency of 5-15 min-1 constantly for more than 5 h. The frequency of this depolarization was monitored as an index of the respiratory frequency. An elevation of the concentration of Ca2+ or Mg2+ caused a decrease in the respiratory frequency, whereas an elevation of K+ concentration caused an increase. The frequency was also increased by a reduction of pH. The highest frequency was observed at 27-28 degrees C. Dopamine, 5-hydroxytryptamine, histamine, acetylcholine, glutamic acid, substance P, and thyrotropin releasing hormone accelerated the respiratory frequency when applied by perfusion to the brainstem, whereas noradrenaline, gamma-aminobutyric acid, glycine, and [Met5] enkephalin and [Leu5] enkephalin slowed the frequency. Experiments with antagonists suggested that the stimulant effect of acetylcholine on respiratory frequency was mediated mainly by muscarinic receptors and the depressant effect of noradrenaline was mediated by alpha-adrenoceptors.

Differential respiratory patterns induced by opioids applied to the ventral medullary and dorsal pontine surfaces of cats

The purpose of the present study was to make a functional dissection of the respiratory action of opioids, by their restricted application to the ventral surface of the medulla oblongata and to the rostro-dorsal surface of the pons in cats. The effects were compared to those induced after intracerebroventricular (i.c.v.) injection. Two mu-agonists, morphine and D-Ala2-Me-Phe4-Met(O)ol5-enkephalin (FK-33824), and the delta-agonist D-Ala2-D-Leu5-enkephalin (DADLE) were used. When applied to the ventral medullary surface, the opioids selectively depressed the generating mechanisms for tidal volume and the response to CO2, whereas the frequency was increased. The application to the rostral dorsal surface of the pons was followed by a selective depression of the respiratory frequency. By intracerebroventricular administration, the opioids depressed both the tidal volume and frequency generating mechanisms. The effects were always reversed by naloxone. The pontine structures were more sensitive to the action of the opioids than were the medullary centres. These findings suggest that the opioids can interact with different populations of respiratory neurones and that the respiratory output differs depending on the group of neurones selectively affected and the function they subserve in regulating respiratory activity.

Effects of enkephalin and 5-hydroxytryptamine on solitary tract neurones involved in respiration and respiratory reflexes

Both enkephalin and 5-hydroxytryptamine (5-HT) have been implicated in neural mechanisms underlying the central control of respiration. In view of the role of the nuclei tractus solitarii (NTS) in respiratory regulation, we carried out a study in artificially ventilated, chloralose-anaesthetized cats of the effects of the microiontophoretic application of [D-Ala2, Met5]-enkephalinamide (DAME) and 5-HT on functionally identified NTS neurones implicated in the control of respiration and respiratory tract reflexes. The neurones examined belonged primarily to two groups: respiratory neurones having a rhythmic activity in phase with the simultaneously recorded phrenic nerve rhythm (viz inspiratory neurones), and presumed reflex interneurones which had no rhythmic activity but which could be orthodromically excited by vagus nerve (X) or superior laryngeal nerve (SLN) stimulation. Application of DAME produced a slow, prolonged depression in 9 of 20 respiratory neurones and 4 of 5 reflex interneurones tested: the remaining neurones showed no change in activity with DAME. The DAME-induced depression was reversed by the intravenous administration of naloxone in the 4 neurones tested. The predominant effect of 5-HT on the 26 respiratory neurones examined was a facilitatory effect of slow onset and long duration; this was noted in 15 neurones. A small number (n = 2) showed a prolonged depression, and the remaining 9 were unaffected. With the reflex interneurones, in contrast, facilitation was not seen with the application of 5-HT; instead, 4 were depressed and 4 unaffected. These findings on functionally identified NTS neurones provide support for the view that both endogenous 5-HT and opiate-related mechanisms are involved in the control of respiration and respiratory tract reflexes. Our findings suggest that they may operate, at least in part, by differing modulatory actions on neurones in the NTS involved in these respiratory-related functions, with enkephalin exerting depressive actions and 5-HT having both facilitatory and depressive effects.

Endogenous opiates: 1983

This paper is the sixth in an annual series of reviews of research involving the endogenous opiates, each installment being restricted to work published during the previous year. Although the early articles in the series attempted to be comprehensive and cover the complete range of research with the opiate peptides, in the last two years we have limited our coverage to non-analgesic and behavioral work due to the enormous number of articles published in the field. The specific areas discussed here include stress, tolerance and dependence, consummatory responses, other gastrointestinal functions, interactions with alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, neurological disorders, activity, and miscellaneous other topics. As in previous years, we have attempted to present a relatively complete review of the subjects covered only for the previous year and generally have not tried to evaluate their contributions relative to those of past years.

A comparative study in rats of the respiratory depression and analgesia induced by mu- and delta-opioid agonists

A comparison was made in awake rats between the analgesic and the respiratory depressant actions induced by the mu-opiate agonists morphine and Tyr-D-Ala-Gly-N-Me-Phe-Met-(O)-ol (FK-33824), and the delta-agonists Tyr-D-Ala-Gly-Phe-D-Leu ( DADLE ) and Tyr-D-Ser-Gly-Phe-D-Leu-Thr (D-Ser2- Thr6 ), injected into the cerebral ventricles. The four opioids caused a dose-dependent analgesia (tail-flick); FK-33824 was the most potent, followed by morphine, DADLE and D-Ser2- Thr6 , and the duration of the analgesic effect of D-Ser2- Thr6 was very short. Respiratory frequency was dose-dependently depressed by FK-33824 and DADLE ; dose-response curves with morphine and D-Ser2- Thr6 could not be obtained for technical reasons. The in vivo apparent pA2 values for naloxone against the mu-agonist FK-33824 and the delta-agonist DADLE were similar in analgesia suggesting an interaction with the same type of receptor. On the other hand, in respiration studies the pA2 value for the interaction of naloxone with DADLE was significantly higher than with FK-33824. The ratio between the ED50 required to induce respiratory depression and analgesia was 1,500 times higher for FK-33824 than for DADLE . It was concluded that agonist interaction with mu-receptors can result in antinociceptive effect in the tail-flick test, whereas respiratory depression seems to require a prominent, but non-exclusive, interaction with delta-receptors.

A comparative study of the respiratory depressant and analgesic effects of bremazocine, a kappa-agonist

The respiratory depression and analgesia induced by a kappa-agonist, bremazocine, were studied by the ED50 ratios and the apparent pA2 values of the interaction with naloxone. The ratio between the ED50 in respiratory depression and analgesia (i.c.v.) was very high compared to that of other mu- and delta-agonists, indicating its small activity on the respiratory center. pA2 values were 7.33 and 7.20 for analgesia and respiratory depression, respectively, after i.c.v. administration. After s.c. injection, the pA2 value for analgesia was 6.16. The results confirm the involvement of different opiate receptors in the mediation of supraspinal and spinal analgesia. The results suggest that bremazocine may induce analgesia by interacting with different types of receptors located at different levels of the CNS. This property and the low activity on the respiratory center make drugs like bremazocine good candidates in the search for safer opiate analgesics.

Participation of pontine structures in the respiratory action of opioids

The respiratory action of morphine, D-Ala2-D-Leu5-enkephalin, and D-Ala2-Me-Phe4-Met(O)ol5-enkephalin, restrictively applied to the dorso-rostral surface of the pons, was studied in anesthetized cats. Frequency was selectively and dose-dependently depressed, down to apnea, whereas tidal volume and its response to CO2 either remained unchanged or were increased. Similar effects were observed in vagotomized and decerebrate cats. From these and previous (1) results, it can be concluded that the medullary and pontine structures related to respiration are differentially affected by opioids. Pontine nuclei are more sensitive to opioid depression and account for changes in frequency, whereas medullary depression results in reduction of tidal volume and CO2 sensitivity.