Modulation of mu-mediated antitussive activity in rats by a delta agonist

[Recent Findings on the Mechanism of Cough Hypersensitivity as a Cause of Chronic Cough]

An increasing number of patients complain to medical institutions about a cough that persists for more than 8 weeks, namely chronic cough. The cough observed in patients with chronic cough is not responsive to conventional antitussive agents such as dihydrocodeine and dextromethorphan, and this is a major clinical problem. The most common pathology of chronic cough in Japan is dry cough. Two causes of dry cough are increased sensitivity of cough receptors (cough hypersensitivity) and increased contraction of bronchial smooth muscle. Among these, the mechanisms of cough hypersensitivity are diverse, and understanding these mechanisms is important for the diagnosis and treatment of chronic cough. In this paper I will review the regulatory mechanisms of cough hypersensitivity, especially the regulation of Aδ fiber excitability by C fibers. Furthermore, the central mechanisms involved cough reflex are discussed in relation to central acting antitussives.

Effect-site concentration of remifentanil for preventing cough during emergence in elderly patients undergoing nasal surgery: a comparison with adult patients

Purpose

Prevention of cough during emergence after nasal surgery is important for avoiding surgical site bleeding. We investigated the remifentanil effect-site concentration in 50% (EC50) of the elderly patients undergoing nasal surgery for smooth emergence without cough and compared it with that of adult patients.

Methods

Twenty-two elderly (aged 65–80 years) and 25 adult patients (aged 20–60 years) with an American Society of Anesthesiologists physical status I/II undergoing nasal surgery were enrolled. Anesthesia was maintained with sevoflurane and remifentanil. Remifentanil EC50 and EC95 for preventing cough were determined using the modified Dixon’s up-and-down method and isotonic regression with bootstrapping approach. Recovery profiles were also recorded.

Results

With Dixon’s up-and-down method, the EC50 of remifentanil in elderly patients (2.40±0.25 ng/mL) was not significantly different from that of adults (2.33±0.30 ng/mL) (P=0.687). With isotonic regression, the EC95 of remifentanil in elderly patients (3.32 [95% confidence interval: 3.06–3.38] ng/mL) was not significantly different from that of adults (3.30 [95% confidence interval: 2.96–3.37] ng/mL). However, eye opening time (14.1±3.8 vs 12.0±2.9 seconds), extubation time (17.2±4.1 vs 14.0±3.0 seconds), and postanesthesia care unit duration (44.5±7.6 vs 38.7±3.4 minutes) in elderly patients were significantly longer than those in adults (P<0.05).

Conclusion

Remifentanil EC50 for preventing cough after nasal surgery with sevoflurane anesthesia did not differ between elderly and adult patients. However, delayed awakening and respiratory adverse events may warrant attention in elderly patients.

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.

Systems approaches and polypharmacology for drug discovery from herbal medicines: an example using licorice

ETHNOPHARMACOLOGICAL RELEVANCE

Licorice, one of the oldest and most popular herbal medicines in the world, has been widely used in traditional Chinese medicine as a cough reliever, anti-inflammatory, anti-anabrosis, immunomodulatory, anti-platelet, antiviral (hepatitis) and detoxifying agent. Licorice was used as an example to show drug discovery from herbal drugs using systems approaches and polypharmacology.

AIM OF THE STUDY

Herbal medicines are becoming more mainstream in clinical practice and show value in treating and preventing diseases. However, due to its extreme complexity both in chemical components and mechanisms of action, deep understanding of botanical drugs is still difficult. Thus, a comprehensive systems approach which could identify active ingredients and their targets in the crude drugs and more importantly, understand the biological basis for the pharmacological properties of herbal medicines is necessary.

MATERIALS AND METHODS

In this study, a novel systems pharmacology model that integrates oral bioavailability screening, drug-likeness evaluation, blood-brain barrier permeation, target identification and network analysis has been established to investigate the herbal medicines.

RESULTS

The comprehensive systems approach effectively identified 73 bioactive components from licorice and 91 potential targets for this medicinal herb. These 91 targets are closely associated with a series of diseases of respiratory system, cardiovascular system, and gastrointestinal system, etc. These targets are further mapped to drug-target and drug-target-disease networks to elucidate the mechanism of this herbal medicine.

CONCLUSION

This work provides a novel in silico strategy for investigation of the botanical drugs containing a huge number of components, which has been demonstrated by the well-studied licorice case. This attempt should be helpful for understanding definite mechanisms of action for herbal medicines and discovery of new drugs from plants.

The clinical effective dose of alfentanil for suppressing cough during emergence from desflurane anesthesia

Background

The administration of short-acting opioids before emergence is useful for preventing emergence cough induced by an endotracheal tube. This study examined the clinically effective dose of alfentanil for suppressing cough during emergence from desflurane anesthesia.

Methods

Twenty-nine adult patients undergoing elective oral surgery were enrolled in this study. During emergence from anesthesia, the patients received alfentanil diluted in 10 ml normal saline when the end-tidal vol% of desflurane decreased to 3%. The initial alfentanil dose was 16 µg/kg. The alfentanil dose for consecutive patients, determined by Dixon's up-and-down method, increased or decreased by 2 µg/kg according to a previous patient's result.

Results

The 50% effective dose (ED₅₀) of alfentanil for suppressing cough during emergence from desflurane anaesthesia was 9.3 ± 1.5 µg/kg according to Dixon's up-and-down method. Isotonic regression revealed an ED₅₀ and ED₉₅ (95% confidence interval) of alfentanil 10.0 µg/kg (6.8-13.2 µg/kg) and 14.0 µg/kg (7.7-19.4 µg/kg), respectively.

Conclusions

The ED₉₅ of alfentanil for suppressing emergence cough was 14.0 µg/kg. A single bolus administration of alfentanil during emergence from anesthesia was useful for suppressing emergence cough.

Opioids in preclinical and clinical trials

Since 1952, when Gates determined the stereo structure of morphine, numerous groups have focused on discovering a nonnarcotic opioid drug. Although several natural, semisynthetic, and synthetic opioid ligands (alkaloids and peptides) have been developed in clinical studies, very few were nonnarcotic opioid drugs. One of the most important studies in the opioid field appeared in 1976, when Martin and colleagues established types of opioid receptors (these are now classified into mu, delta, and kappa types). Later, Portoghese discovered a highly selective mu type opioid receptor antagonist, beta-funaltrexamine. This led to the finding that the mu type opioid receptor was correlated to drug dependence. Consequently, delta, and particularly kappa, opioid agonists were expected to lead to ideal opioid drugs. Moreover, opioid antagonists were evaluated for the treatment of symptoms related to undesirable opioid system activation. In this chapter, we provide a short survey of opioid ligands in development and describe the discovery of the two most promising drugs, TRK-851 and TRK-820 (nalfurafine hydrochloride).

Depression of cough reflex by microinjections of antitussive agents into caudal ventral respiratory group of the rabbit

We have previously shown that the caudal nucleus tractus solitarii is a site of action of some antitussive drugs and that the caudal ventral respiratory group (cVRG) region has a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. These findings led us to suggest that the cVRG region, and possibly other neural substrates involved in cough regulation, may be sites of action of antitussive drugs. To address this issue, we investigated changes in baseline respiratory activity and cough responses to tracheobronchial mechanical stimulation following microinjections (30-50 nl) of some antitussive drugs into the cVRG of pentobarbital-anesthetized, spontaneously breathing rabbits. [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) and baclofen at the lower concentrations (0.5 mM and 0.1 mM, respectively) decreased cough number, peak abdominal activity, and peak tracheal pressure and increased cough-related total cycle duration (Tt). At the higher concentrations (5 mM and 1 mM, respectively), both drugs abolished the cough reflex. DAMGO and baclofen also affected baseline respiratory activity. Both drugs reduced peak abdominal activity, while only DAMGO increased Tt, owing to increases in expiratory time. The neurokinin-1 (NK(1)) receptor antagonist CP-99,994 (10 mM) decreased cough number, peak abdominal activity, and peak tracheal pressure, without affecting baseline respiration. The NK(2) receptor antagonist MEN 10376 (5 mM) had no effect. The results indicate that the cVRG is a site of action of some antitussive agents and support the hypothesis that several neural substrates involved in cough regulation may share this characteristic.

Central mechanisms II: pharmacology of brainstem pathways

Following systemic administration, centrally acting antitussive drugs are generally assumed to act in the brainstem to inhibit cough. However, recent work in humans has raised the possibility of suprapontine sites of action for cough suppressants. For drugs that may act in the brainstem, the specific locations, types of neurones affected, and receptor specificities of the compounds represent important issues regarding their cough-suppressant actions. Two medullary areas that have received the most attention regarding the actions of antitussive drugs are the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral respiratory column. Studies that have implicated these two medullary areas have employed both microinjection and in vitro recording methods to control the location of action of the antitussive drugs. Other brainstem regions contain neurones that participate in the production of cough and could represent potential sites of action of antitussive drugs. These regions include the raphe nuclei, pontine nuclei, and rostral ventrolateral medulla. Specific receptor subtypes have been associated with the suppression of cough at central sites, including 5-HT1A, opioid (mu, kappa, and delta), GABA-B, tachykinin neurokinin-1 (NK-1) and neurokinin-2, non-opioid (NOP-1), cannabinoid, dopaminergic, and sigma receptors. Aside from tachykinin NK-1 receptors in the NTS, relatively little is known regarding the receptor specificity of putative antitussive drugs in particular brainstem regions. Our understanding of the mechanisms of action of antitussive drugs would be significantly advanced by further work in this area.

Modulation of the cough reflex by antitussive agents within the caudal aspect of the nucleus tractus solitarii in the rabbit

We have previously shown that ionotropic glutamate receptors in the caudal portion of the nucleus tractus solitarii (NTS), especially in the commissural NTS, play a prominent role in the mediation of tracheobronchial cough and that substance P potentiates this reflex. This NTS region could be a site of action of some centrally acting antitussive agents and a component of a drug-sensitive gating mechanism of cough. To address these issues, we investigated changes in baseline respiratory activity and cough responses to tracheobronchial mechanical stimulation following microinjections (30-50 nl) of centrally acting antitussive drugs into the caudal NTS of pentobarbitone-anesthetized, spontaneously breathing rabbits. [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and baclofen decreased baseline respiratory frequency because of increases in the inspiratory time only at the higher concentration employed (5 mM and 1 mM, respectively). DAMGO (0.5 mM) and baclofen (0.1 mM) significantly decreased cough number, peak abdominal activity, peak tracheal pressure, and increased cough-related total cycle duration. At the higher concentrations, these agents suppressed the cough reflex. The effects of these two drugs were counteracted by specific antagonists (10 mM naloxone and 25 mM CGP-35348, respectively). The neurokinin-1 (NK1) receptor antagonist CP-99,994 (10 mM) abolished cough responses, whereas the NK2 receptor antagonist MEN 10376 (5 mM) had no effect. The results indicate that the caudal NTS is a site of action of some centrally acting drugs and a likely component of a neural system involved in cough regulation. A crucial role of substance P release in the mediation of reflex cough is also suggested.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

Phrenic and iliohypogastric nerve discharges during tussigenic stimulation in paralyzed and decerebrate guinea pigs and rats

Although effects of antitussive drugs have been examined in inbred small animals using a whole body plethysmography, neuronal mechanisms underlying the cough reflex are not fully understood. The present study analyzed the reflex discharge patterns of the phrenic (PN) and iliohypogastric nerves (IHN) evoked in decerebrate and paralyzed guinea pigs and rats. In guinea pigs, electrical stimulation of the superior laryngeal nerve, chemical stimulation with capsaicin and mechanical stimulation to the intratracheal mucosa equally produced a serial PN-IHN response. This response was characterized by an increased PN discharge and following spindle-shaped burst of the IHN. The evoked discharges overlapped for 20 ms. In rats, by contrast, mechanical stimulation was without effect while capsaicin and electrical stimulation produced two types of responses, both of which differed from that observed in guinea pigs. The first type consisted of an augmented burst of the IHN that was immediately followed by an increased PN discharge. The second type was a large spindle-shaped burst of the IHN that occurred 80 ms after the end of the preceding PN discharge. Codeine (3 mg/kg i.v.) depressed all types of responses evoked in guinea pigs and rats. The present study demonstrated that the fictive cough comparable with those induced in other experimental animals was produced consistently in guinea pigs, but not in rats. Therefore, guinea pigs are suitable for investigation of the neuronal mechanisms underlying the cough reflex and assessment of antitussive drugs.

Delta-opioid receptor antagonists as a new concept for central acting antitussive drugs

Our recent findings indicated that mu- and kappa-opioid receptors enhance each other's antitussive processes. However, delta-opioid receptors played an inhibitory role in antitussive processes mediated by the mu- and kappa-opioid receptors. We also concluded that delta(1)-opioid receptors may play an inhibitory role, whereas delta(2)-opioid receptors may play a synergistic role, in antitussive processes mediated by mu-opioid receptors. Furthermore, we clearly demonstrated that delta-opioid receptor antagonists, such as naltrindole and 7-benzylidenenaltrexone, produced potent antitussive effects. These delta-opioid receptor-mediated antitussive effects may be mediated by the antagonism of delta(1)-, but not delta(2)-opioid receptors. In this review, we study the possibility of the delta-opioid receptor antagonist as a new concept for central acting antitussive drugs.

[Coughing model by microinjection of citric acid into the larynx in guinea pig]

Many studies of cough were performed under the restrained or anesthetized condition, and coughs were evoked by inhalation of capsaicin or citric acid. Inhalation of irritants induced by "diving response" with apnea and coughs, and these responses induced a change of tidal volume. As a result, respiratory responses are dependent on the inhalation volume. Therefore we developed a new coughing model, and coughs were evoked by microinjection of citric acid into the larynx in the unanesthetized unrestrained guinea pig. Microinjection of 7.5% citric acid (2 microliters/30 s, 5 min) induced coughs (27.03 +/- 4.03 coughs/10 min), and citric acid-induced responses were stable independent of the inhalation volume. In the inhalation studies, animals were exposed to citric acid only once because induced-responses were remarkably decreased by repeated administration at an interval of 24 h. However in our coughing model it was possible to repeatedly challenge the animals by microinjection of citric acid at intervals of 24 h. Microinjection of citric acid into the larynx induced coughs in Sprague-Dawley rats, but inhalation of citric acid did not induce cough. These results indicate that this coughing model is highly sensitive and correctly assessed cough responses.

Neurochemical regulation of cough response to capsaicin in guinea-pigs

1. Although monumental efforts have been made to define the action sites of cough, the importance of neurotransmitter systems in the cough reflex has received limited attention. We studied the roles for four major neurotransmitters [acetylcholine, histamine, serotonin (5-hydroxytryptamine, 5-HT) and dopamine] in the modulation of the cough reflex. 2. Atropine (muscarinic cholinergic blocking agent), pyrilamine maleate (PM, histamine H1 blocker), cimetidine (histamine H2 blocker), 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, specific 5-HT1A receptor agonist) and SCH-23390 (selective dopamine D1 receptor antagonist) were examined on the cough response to inhaled capsaicin in conscious guinea-pigs. 3. All the drugs significantly decreased the number of capsaicin-induced coughs in a dose-dependent manner. To compare the sensitivity of these drugs on cough response, we calculated the effective doses for 50% inhibition of cough (ED50) when the animals were exposed to 3 x 10-4 m capsaicin. The ED50 values were 0.03 microm kg-1 for atropine, 0.2 microm kg-1 for 8-OH-DPAT, 6.2 microm kg-1 for SCH-23390, 8.5 microm kg-1 for PM and 13.9 microm kg-1 for cimetidine. 4. These findings indicated that all these four neurotransmitters may be involved in the regulation of the cough reflex. Multiple changes of these neurotransmitters in disorders of the central nervous system might synergically affect the cough reflex.

Effects and interactions of opioids on plasma exudation induced by cigarette smoke in guinea pig bronchi

The effects of opioids on cigarette smoke-induced plasma exudation were investigated in vivo in the main bronchi of anesthetized guinea pigs, with Evans blue dye as a plasma marker. Acute inhalation of cigarette smoke increased plasma exudation by 216% above air control values. Morphine, 0.1-10 mg/kg but not 30 mg/kg, inhibited the exudation but had no significant effect on substance P-induced exudation. Both 10 and 30 mg/kg of morphine increased exudation in air control animals, an effect inhibited by antihistamines but not by a tachykinin neurokinin type 1-receptor antagonist. Naloxone inhibited all morphine responses. Cigarette smoke-induced plasma exudation was inhibited by a mu-opioid-receptor agonist (DAMGO) but not by agonists at delta (DPDPE)- or kappa (U-50488H)-receptors. None of these agonists affected exudation in air control animals. DPDPE prevented the inhibition by DAMGO of cigarette smoke-induced plasma exudation, and the combination of DAMGO and DPDPE increased exudation in air control animals. Prevention of inhibition and the combination-induced increase were inhibited by antihistamines or the mast cell-stabilizing drug sodium cromoglycate. U-50488H did not alter the response to either DAMGO or DPDPE. We conclude that, in guinea pig main bronchi in vivo, mu-opioid-receptor agonists inhibit cigarette smoke-induced plasma exudation via a prejunctional mechanism. Plasma exudation induced by mu- and delta-receptor interactions is due to endogenous histamine release from mast cells.

Antagonistic effect of buprenorphine on the antitussive effect of morphine is mediated via the activation of mu 1-opioid receptors

The effect of buprenorphine on the antitussive effect of morphine was examined in mice. Buprenorphine at doses of 0.1, 0.3 and 1 mg/kg given i.p. alone have no effects on the % inhibition in the number of capsaicin-induced coughs. However, pretreatment with the same doses of buprenorphine for 2 hr significantly attenuated the antitussive effect of morphine (3 mg/kg, i.p.). Naloxonazine, a selective mu 1-opioid receptor antagonist, had no effect of buprenorphine on antitussive effect of morphine. These results suggest that buprenorphine antagonizes the antitussive effect of morphine via the activation of mu 1-opioid receptors.

Antitussive effect of dihydroetorphine in mice

The present study examined the opioid receptors involved in the antitussive effect of dihydroetorphine in mice. Dihydroetorphine suppressed coughs dose dependently at doses between 0.1-1 micrograms/kg i.p. Blockade of mu-opioid receptors by pretreatment with beta-funaltrexamine significantly reduced the antitussive effect of dihydroetorphine. Furthermore, the antitussive effect of dihydroetorphine was also antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. However, pretreatment with naltrindole, a delta-opioid receptor antagonist, did not affect the antitussive effect of dihydroetorphine. These results indicate that the antitussive effect of dihydroetorphine is mediated by the activation of mu-opioid receptors and of kappa-opioid receptors, but not delta-opioid receptors.

Antitussive effect of [Met5]enkephalin-Arg6-Phe7 in mice

We examined the effect of [Met5]enkephalin-Arg6-Phe7 (MEAP) on the capsaicin-induced cough reflex in mice. Intracerebroventricular administration of MEAP significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of MEAP was blocked by nor-binaltorphimine, a selective kappa-opioid receptor antagonist. However, beta-funaltrexamine, a mu-opioid receptor antagonist, had no effect on the antitussive effect of MEAP. On the other hand, the antinociceptive effect of MEAP, as determined in the tail-flick test, was blocked by both nor-binaltorphimine and beta-funaltrexamine. Naltrindole, a delta-opioid receptor antagonist, had no effect on either the antitussive effect or the antinociceptive effect of MEAP. These data suggest that MEAP exerts its antitussive effect in mice through the stimulation of kappa-opioid receptors, whereas the antinociceptive effect of MEAP is mediated through the simulation of both kappa- and mu-opioid receptors.

Antitussive effects of naltrindole, a selective delta-opioid receptor antagonist, in mice and rats

The effects of naltrindole, a selective delta-opioid receptor antagonist, on the capsaicin-induced cough reflex in mice and rats were studied. Intraperitoneal administration of naltrindole decreased the number of coughs both in mice and rats dose dependently. The cough-depressant effects reached a peak 15 min after the administration of naltrindole and lasted more than 120 min. Pretreatment with [D-Pen2,D-Pen5]enkephalin, a selective delta-opioid receptor agonist, partially but significantly reduced the antitussive effect of naltrindole. Blockade of kappa-opioid receptors by pretreatment with nor-binaltorphimine also partially antagonized the antitussive effect of naltrindole. However, the antitussive effect of naltrindole was not antagonized by beta-funaltrexamine, a selective mu-opioid receptor antagonist. Thus, it is possible that the antitussive effect of naltrindole may be mediated, in part, by kappa-opioid receptors. The present results provide evidence for the development of delta-opioid antagonists, especially naltrindole, for use as antitussive drugs.

Effects of rimcazole, a specific antagonist of sigma sites, on the antitussive effects of non-narcotic antitussive drugs

We examined the effects of rimcazole, a specific antagonist of sigma sites, on the antitussive effects of dextromethorphan and noscapine in mice. Intraperitoneal injection of rimcazole, in doses from 1 to 10 mg/kg, significantly and dose dependently antagonized the cough depressant effect of N,N'-di(orthotolyl)guanidine (DTG), a sigma ligand. The cough depressant effects of dextromethorphan (3 mg/kg i.p.) and noscapine (10 mg/kg i.p.) were also significantly and dose dependently reduced by pretreatment with rimcazole. However, rimcazole (10 mg/kg i.p.) did not have a significant effect on the antitussive effect of morphine (3 mg/kg i.p.). Furthermore, rimcazole by itself (10 and 30 mg/kg i.p.) had no significant effect on the number of coughs. These results suggest that sigma sites may be involved in the antitussive mechanism of centrally acting non-narcotic antitussive drugs.

The role of the mu 2-opioid receptor in the antitussive effect of morphine in mu 1-opioid receptor-deficient CXBK mice

The effect of morphine on the capsaicin-induced cough reflex was studied in mu 1-opioid receptor-deficient CXBK mice. There was no significant difference between the morphine-induced antitussive effect in CXBK mice and C57BL/6 mice, a progenitor strain. Furthermore, the antitussive effects of morphine in both the CXBK and C57BL/6 mice were antagonized by pretreatment with either naloxone or beta-funaltrexamine, a mu-opioid receptor antagonist, whereas pretreatment with naltrexonazine, a selective mu 1-opioid receptor antagonist, had no effect. Moreover, naltrindole, a selective delta-receptor antagonist, also had no significant effect on the antitussive effects of morphine in either CXBK or C57BL/6 mice. These results support our previous hypothesis that mu 2- rather than mu 1-opioid receptors are involved in morphine-induced antitussive effects.

Antitussive effects of mu- and kappa-agonists in diabetic rats

We evaluated the antitussive effect of morphine and U-50,488 in diabetic and non-diabetic rats. The antitussive potency of morphine (0.3 mg/kg, i.p.) in diabetic rats was significantly reduced as compared to the results in non-diabetic rats. The antitussive effect of U-50,488, a kappa-agonist, was also significantly lower in diabetic rats than in non-diabetic rats. When naltrindole (0.03 mg/kg, i.p.), a delta-antagonist, was administered 15 min before morphine or U-50,488, there was no difference between the antitussive potencies of these two opioid agonists in non-diabetic rats and in diabetic rats. Furthermore, naltrindole produces a reduction of the number of coughs in diabetic rats, but not in non-diabetic rats. It is possible that the enhancement of the antitussive potency of morphine and U-50,488 in naltrindole-treated diabetic rats is the result of the antitussive synergy produced by these opioid agonists and naltrindole. It seems likely, therefore, that delta-receptor-mediated endogenous inhibitory systems in mu- and kappa-receptor-mediated antitussive processes may be activated under diabetic conditions.

Differential modulation of mu-opioid receptor-mediated antitussive activity by delta-opioid receptor agonists in mice

We examined the effect of [D-Ala2]deltorphin II, a selective delta 2-opioid receptor agonist, on the antitussive effect of [D-Ala2, MePhe4,Gly-ol5]enkephalin (DAMGO), a selective mu-opioid receptor agonist. [D-Ala2]deltorphin (3 nmol i.c.v.) had no significant effect on the number of coughs. However, upon i.c.v. pretreatment with [D-Ala2]deltorphin II (3 nmol) the antitussive activity of DAMGO (0.03 nmol) was significantly enhanced. The enhancement of the antitussive activity of DAMGO caused by [D-Ala2]deltorphin II was prevented by a benzofuran derivative of naltrindole (0.1 mg/kg s.c.), a selective delta 2-opioid receptor antagonist. These results suggest that delta 2-opioid receptors may play a synergistic role in antitussive processes that are mediated by mu-opioid receptors.

Antitussive effect of beta-endorphin is mediated by mu-opioid receptors, but not by kappa- or epsilon-opioid receptors

The present study examined the opioid receptors involved in the antitussive effect of beta-endorphin in mice. beta-Endorphin injected i.c.v. depresses coughs dose dependently in doses from 0.1 to 1 microgram. Blockade of mu-opioid receptors by pretreatment with beta-funaltrexamine significantly reduced the antitussive potency of i.c.v. beta-endorphin. However, the antitussive effect of beta-endorphin was not antagonized by nor-binaltorphimine, a kappa-opioid receptor antagonist. Moreover, i.c.v. injection of beta-endorphin-(1-27), an epsilon-opioid receptor antagonist, did not affect the antitussive effect of beta-endorphin. The results indicate that the antitussive effect of beta-endorphin is mediated by activation of mu-opioid receptors, but not of kappa- or epsilon-opioid receptors.

Possible involvement of mu 2-mediated mechanisms in mu-mediated antitussive activity in the mouse

The effect of pretreatment with naloxonazine on mu-opioid agonist-mediated antitussive effects was studied in mice. The antitussive effects of [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAMGO) and morphine were significantly antagonized by naloxone pretreatment, 1 mg/kg given i.p. 5 min earlier, but not by naloxonazine pretreatment, 35 mg/kg given s.c. 24 h earlier. In contrast, the antinociceptive effects of these mu agonists, as determined by the tail-flick method, were significantly reduced by pretreatment with both naloxone and naloxonazine. These results suggest that mu 2 rather than mu 1 mechanisms are involved in mu-mediated antitussive effects.

Involvement of haloperidol-sensitive sigma-sites in antitussive effects

The effects of selective sigma-ligands on the capsaicin-induced cough reflex in rats were studied. Intraperitoneal injection of (+)-N-allylnormetazocine ((+)-SKF-10,047) and N,N'-di(ortho-tolyl)guanidine (DTG) in doses that ranged from 0.3 to 3.0 mg/kg decreased the number of coughs dose dependently. The antitussive effects of these sigma-ligands were significantly attenuated by pretreatment with haloperidol. Pretreatment with haloperidol also markedly reduced the antitussive effects of (+/-)-pentazocine and dextromethorphan. These results suggest that haloperidol-sensitive sigma-sites may be involved in the regulation of coughs.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.