Cough In Guinea Pigs Research Articles

Characterization of increased cough sensitivity after antigen challenge in guinea pigs.

Increased sensitivity of cough reflex is a fundamental feature of bronchodilator resistant non-productive cough associated with eosinophilic tracheobronchitis. Our hypothesis is that cough sensitivity is increased by airway allergic reaction characterized by airway eosinophilic inflammation. The aim of this study was to elucidate the hypothesis and clarify the characteristics of the increased cough sensitivity. Number of coughs elicited by inhalation of increasing concentrations of capsaicin (10-8, 10-6 and 10-4 M) was counted 24 h after an aerosolized antigen or saline in actively sensitized or non-sensitized (naive) conscious guinea pigs and then bronchoalveolar lavage was performed. The cough response was also measured 1 day before and 1, 2, 3, 5 and 7 days after an aerosolized antigen challenge in sensitized or naive animals. In addition, effect of procaterol (0.1 mg/kg), atropine (1 or 10 mg/kg), phosphoramidon (2.5 mg/kg) given intraperitoneally 30 min before the capsaicin challenge or capsaicin desensitization on the cough response was examined. Furthermore, the thromboxane A2 (TXA2) receptor antagonist S-1452 in a dose of 0.01 or 0.1 mg/kg or vehicle (saline) was given intraperitoneally at 24 and 1 h before the measurement of cough response. Number of coughs caused by capsaicin was extremely increased 24 h after an antigen challenge in sensitized guinea pigs compared with a saline or an antigen challenge in naive animals or a saline challenge in sensitized animals. The increased cough response disappeared at 3-7 days after the antigen challenge. Eosinophils in bronchoalveolar lavage fluid obtained after the measurement of capsaicin-induced coughs, which was performed 24 h after the antigen challenge, were significantly increased in sensitized guinea pigs. The eosinophil count was significantly correlated to the number of capsaicin-induced coughs. Procaterol or atropine did not alter the antigen-induced increase of cough sensitivity, whereas atropine did reduce the cough response in naive animals. Phosphoramidon increased the number of capsaicin-induced coughs in naive guinea pigs but not in sensitized and antigen-challenged animals. Capsaicin desensitization decreased the cough response in both antigen-challenged sensitized guinea pigs and naive animals. S-1452 reduced the antigen-induced increase of cough response in sensitized guinea pigs, but not in naive animals. Airway allergy accompanied with airway eosinophilia induces transient increase in cough sensitivity, which is not mediated by bronchoconstriction. The increased cough sensitivity may result in part from inactivation of neutral endopeptidase and TXA2, one of the inflammatory mediators.

Nociceptin inhibits cough in the guinea-pig by activation of ORL(1) receptors.

We studied the central and peripheral antitussive effect of ORL(1) receptor activation with nociceptin/orphanin FQ in conscious guinea-pigs. In guinea-pig cough studies, nociceptin/orphanin FQ (10, 30, and 90 microg) given directly into the CNS by an intracerebroventricular (i.c.v.) route inhibited cough elicited by capsaicin exposure by approximately 23, 29 and 52%, respectively. The antitussive activity of nociceptin/orphanin FQ (90 microg, i.c.v.) was blocked by the selective ORL(1) antagonist [Phe(1)gamma(CH(2)-NH)Gly(2)]nociceptin-(1-13)-NH(2) (180 microg, i.c.v.) and J113397 (10 mg kg(-1), i.p.) but not by the opioid antagonist, naltrexone (3 mg kg(-1), i.p.). Furthermore, intravenous (i.v.) nociceptin/orphanin FQ (1.0 and 3.0 mg kg(-1)) also inhibited cough approximately by 25 and 42%, respectively. These findings indicate that selective ORL(1) agonists display the potential to inhibit cough by both a central and peripheral mechanism, and potentially represent a novel therapeutic approach for the treatment of cough.

Participation of thromboxane A(2) in the cough response in guinea-pigs: antitussive effect of ozagrel.

1. The purpose of this study was to investigate the involvement of thromboxane A(2) (TXA(2)) in the cough response in a guinea-pig cough model. Here, we describe results obtained using a selective TXA(2) synthetase inhibitor, ozagrel, and a selective TXA(2) agonist, U-46619. 2. Guinea-pigs were anaesthetized and exposed to an aerosol of capsaicin (100 microM) to elicit coughing. The number of coughs was 20.0+/-5.8 during capsaicin provocation (5 min), but only 2. 8+/-0.4 during a 5-min inhalation of phosphate-buffered saline (PBS) (P:<0.05). 3. TXB(2) levels in BAL were 101.4+/-8.0 and 58.4+/-8.7 pg ml(-1) following capsaicin and PBS inhalation, respectively (P:<0. 01), but there was no intergroup difference in the cell populations in BAL. 4. Inhalation of U-46619 did not induce a cough response by itself at concentrations of 100 ng ml(-1) to 10 microg ml(-1). However, it caused a 2 fold increase in the number of capsaicin-induced coughs. 5. To explore the source of the TXA(2), BAL cells were stimulated with capsaicin and the supernatants collected for analysis. The TXB(2) concentration in BAL was increased dose-dependently, indicating that TXA(2) is released from BAL cells in response to capsaicin. 6. Ozagrel was administered orally 1 h before a 5 min capsaicin provocation and the number of coughs was counted during the capsaicin inhalation. Ozagrel decreased the number of coughs dose-dependently (ED(50) value, 26.3 mg kg(-1)). 7. These results show that TXA(2) modulates the capsaicin-induced cough response by increasing capsaicin-sensitivity.

Antitussive effect of Adhatoda vasica extract on mechanical or chemical stimulation-induced coughing in animals

The antitussive activity of Adhatoda vasica (AV) extract was evaluated in anaesthetized guinea pigs and rabbits and in unanaesthetized guinea pigs. AV was shown to have a good antitussive activity. Intravenously, it was 1/20–1/40 as active as codeine on mechanically and electrically induced coughing in rabbits and guinea-pigs. After oral administration to the guinea-pig the antitussive activity of AV was similar to codeine against coughing induced by irritant aerosols.

Involvement of tachykinin NK3 receptors in citric acid-induced cough and bronchial responses in guinea pigs.

Aerosolized citric acid induces several pulmonary effects including bronchoconstriction, airway inflammation, and cough. Evidence from the use of tachykinin NK1 and NK2 receptor antagonists, as well as chronic treatment with high doses of capsaicin, have suggested that these effects are mediated through the release of tachykinins from sensory nerve endings. In the present study, we have investigated the effects of a tachykinin NK3 receptor antagonist, SR 142801 (osanetant), on cough, bronchoconstriction, and bronchial hyperresponsiveness induced by aerosolized citric acid (0.4 M) in guinea pigs. SR 142801, at 0.3 and 1 mg . kg-1 by intraperitoneal route, significantly inhibited cough in conscious guinea pigs by 57 +/- 3 and 62 +/- 10% (n = 8), respectively. In anaesthetized guinea pigs, it failed to inhibit the bronchoconstriction induced by citric acid when given alone but abolished it when combined with the tachykinin NK2 receptor antagonist, SR 48968 (saredutant). In guinea pigs pretreated with thiorphan (1 mg . kg-1), aerosolized citric acid (0.4 M, 1 h) induced airway hyperresponsiveness 24 h later, displayed by an exaggerated response to the bronchoconstrictor effect of acetylcholine. A microvascular leakage hypersensitivity also occurred and was demonstrated by a potentiation of the plasma protein extravasation from bronchial vessels induced by histamine. When given once intraperitoneally at 1 mg . kg-1 30 min before the citric acid exposure, SR 142801 inhibited both hyperresponsiveness to acetylcholine and the potentiation of histamine-induced increase in microvascular permeability. The results suggest that tachykinin NK3 receptors are involved in citric acid-induced effects on airways.

P-621 - Antitussive effect of moguiestine on allergic cough in guinea pig

P-621 - Antitussive effect of moguiestine on allergic cough in guinea pig

Effect of 5-HT A receptor agonist, 8-OH-DPAT, on cough responses in the conscious guinea-pig

We have studied the role for 5-hydroxytryptamine (5-HT) in the modulation of the cough reflex by examining the effect of a selective 5-HT 1A receptor agonist, 8-hydroxy-2-(di- n-propylamino) tetralin (8-OH-DPAT) on cough and respiratory rate in conscious guinea pigs. Animals were placed in a box and exposed to the tussive agent citric acid (0.5 M) for 10 min, 3 min after terbutaline (0.05 mg/kg i.p.) was administered to prevent bronchoconstriction. 8-OH-DPAT inhibited at low doses (0.008 and 0.016 mg/kg) but potentiated at high doses (0.25 and 0.5 mg/kg) the citric acid-induced number of coughs, but dose-dependently increased respiratory rate. Methysergide (0.05–5 mg/kg), a 5HT 1 and 5HT 2 receptor antagonist, and ketanserin (0.05 mg/kg), a 5HT 2 receptor antagonist, had no effects on cough or respiratory rate. Methysergide inhibited the increased cough responses and respiratory rate induced by 8-OH-DPAT at high doses, while ketanserin was without effect. These results suggest that 8-OH-DPAT may induce both an inhibition and activation of the cough reflex, the latter involving central 5HT 1-receptor activation.

Effects of specific tachykinin receptor antagonists on citric acid-induced cough and bronchoconstriction in unanesthetized guinea pigs

We compared the effects of a tachykinin NK 1 receptor antagonist, FK888 ( N 2-[(4 R)-4-hydroxy-1-(1-methyl-1 H-indol-3-yl)carbonyl- l-prolyl]- N-methyl- N-phenylmethyl-3-(2-naphtyl)- l-alaninamide), and a tachykinin NK 2 receptor antagonist, SR48968 (( S)- N-methyl- N[4-(4-acetylamino-4-phenyl-piperidino)-2-(3,4-dichlorophenyl)butyl]benzamide]), on citric acid-induced cough and bronchoconstriction in conscious guinea pigs. FK888 and SR48968 inhibited the cough dose dependently. Combination of FK888 and SR48968 showed a small additive effect compared with that of FK888 or SR48968 alone. SR48968 but not FK888 inhibited the bronchoconstriction dose dependently. These results indicate that tachykinin NK 1 receptors as well as tachykinin NK 2 receptors are involved in the citric acid-induced cough response. The antitussive activity of the tachykinin NK 1 receptor antagonist appeared not to depend on the anti-bronchoconstrictor effects.

Capsazepine inhibits cough induced by capsaicin and citric acid but not by hypertonic saline in guinea pigs.

Acidic solutions mimick many of the effects of capsaicin (Cap), including pain, bronchoconstriction, cough, and sensory neuropeptide release. Evidence from the use of the Cap antagonist capsazepine suggests that in some cases protons act at the Cap receptor. In the present study, we have investigated whether cough evoked by Cap and citric acid (CA) is mediated specifically via the Cap receptor on airway sensory nerves. We have examined the effects of capsazepine on Cap-, CA-, and hypertonic saline-induced cough and also on CA-induced nasal irritation in awake guinea pigs. Capsazepine was nebulized for 5 min before cough challenges with Cap for 5 min and CA for 10 min. Control animals were pretreated with vehicle alone. Capsazepine (100 microM) inhibited the cough response to 30 microM Cap from 0.77 +/- 0.14 coughs/min in control animals to 0.23 +/- 0.08 coughs/min (P < 0.05) and to 80 microM Cap from 1.4 +/- 0.23 to 0.3 +/- 0.11 coughs/min (P < 0.01). There was no effect, however, of lower concentrations of capsazepine (5 and 10 microM) against Cap-evoked cough. At a concentration of 100 microM, capsazepine also inhibited the coughing induced by 0.25 M CA from 1.8 +/- 0.26 to 0.93 +/- 0.31 coughs/min (P < 0.05) but not that induced by 0.5 M CA. Nasal irritation induced by 0.25 M CA, but not by 0.5 M CA, was also inhibited by capsazepine from 2.47 +/- 0.37 to 0.75 +/- 0.31 nose wipes/min (P < 0.05). This inhibitory effect of capsazepine did not appear to reflect a nonspecific suppression of the cough reflex, since cough evoked by exposure to hypertonic (7%) saline for 10 min was unaffected by pretreatment with capsazepine (100 microM). These data show that capsazepine is a specific inhibitor of Cap- and CA-induced cough in guinea pigs. Moreover, they suggest that low pH stimuli evoke cough and nasal irritation by an action at the Cap receptor, either directly or through the release of an intermediate agent.

Antitussive and antibronchoconstriction actions of fenspiride in guinea-pigs.

Fenspiride is a nonsteroidal anti-inflammatory agent, which we have previously shown to have an in vivo antibronchoconstrictor action in guinea pigs. We have currently studied this action using the constrictors Substance P, neurokinin A, citric acid and capsaicin in anaesthetized guinea-pigs. Fenspiride has also been reported to produce a subjective improvement in cough in patients. We have used a conscious guinea-pig model of cough as a more definitive method to study the effect of fenspiride on capsaicin- and citric acid-induced cough. Aerosolized fenspiride (1 mg.mL-1) caused a 58% reversal of capsaicin-induced bronchoconstriction; and i.v. fenspiride (1mg.kg-1) a 45% reversal of citric acid induced bronchoconstriction. Substance P- and neurokinin A-induced bronchoconstriction were unaffected by 1 mg.kg-1 i.v. fenspiride. Aerosolized fenspiride (1, 3 and 10 mg.mL-1) administered for 4 min reduced citric acid (300 mM) induced cough, but 0.1 mg.mL-1 was without effect. Pretreatment with aerosolized fenspiride (10 mg.mL-1) caused a shift in the citric acid dose response curve to the right. For citric acid-induced cough, the duration of action of aerosolized fenspiride (10 mg.mL-1) was found to be 5 and 15 min post-treatment. Aerosolized capsaicin (30 microM) induced cough was also reduced by 3 and 10 mg.mL-1 aerosolized fenspiride, but no significant effect was found with 1 mg.mL-1. We conclude that aerosolized fenspiride reduces capsaicin- and citric acid-induced bronchoconstriction as well as induced cough in guinea-pigs in vivo. Whether a pathway common to both cough and bronchoconstriction is the site of action of fenspiride remains to be established. We postulate that fenspiride, acting as an antitussive and antibronchoconstrictor agent, would be beneficial in the clinical situation for those patients with hyperresponsive airways.

Cough--diversity and the peripheral mechanisms of production

The physiology and pharmacology of the cough reflex were reviewed from the perspective that the cough response and its peripheral mechanisms have diversity, taking reported findings together with the results obtained from our own studies. It is clear that there was a remarkable difference in the magnitude of expiration and sound in coughing between the two types of coughs in guinea pigs: one is caused by mechanical irritation to the airway mucosa or citric acid inhalation and the other caused by inhalation of pharmacological agents such as capsaicin and substance P. Four types of stimulation, i.e., mechanical, physicochemical, chemical and pharmacological stimulation, were discussed with respect to the site and the mechanisms of action in the airway. Mechanical stimulants and chemical stimulants such as citric acid seem to act mainly on A delta-fibers. However, it is unclear whether pharmacological agents act on C-fibers to produce cough. As to the difference in distribution of cough receptors in the airway, pharmacological differences were found between coughs caused by mechanical irritation on the laryngeal sites and the site of bifurcation of the trachea. Furthermore, capsaicin, applied by a topical spraying method newly developed by us, produced cough-like forced expiration when it was sprayed around the site of the bifurcation of the trachea. This response was not depressed by codeine, but depressed by ophiopogonin, a Chinese herbal antitussive; mephenesine; and a neurokinin A antagonist. Mechanisms of cough augmentation in bronchitic guinea pigs were also described briefly. In conclusion, the site of action of cough stimulants and the mechanisms of cough production are still controversial. To solve these problems, we need to develop new methods and strategies for studying the cough reflex.

Moguisteine: a novel peripheral non-narcotic antitussive drug

1. The antitussive effects of moguisteine have been compared with codeine in several experimental models of cough in guinea-pigs and dogs. 2. Moguisteine and codeine dose-dependently (respective ED50 values are given in parentheses) inhibited cough induced in guinea-pigs by 7.5% citric acid aerosol (25.2 and 29.2 mg kg-1, p.o.), by 30 microM capsaicin aerosol (19.3 and 15.2 mg kg-1, p.o.), by mechanical stimulation (22.9 and 26.4 mg kg-1, p.o.) and by tracheal electrical stimulation (12.5 and 13.9 mg kg-1, p.o.). 3. Moguisteine was effective against cough induced by tracheal electrical stimulation in dogs (ED50 17.2 mg kg-1, p.o.); codeine was not tested because of its emetic effect. 4. After repeated dosing (12-15 days), moguisteine did not induce tolerance in either guinea-pigs or dogs. 5. Moguisteine did not interact with opiate receptors, since it did not show affinity for [3H]-naloxone binding sites and furthermore naloxone (5 mg kg-1, s.c.) did not antagonize its antitussive effects. 6. Moguisteine had no antitussive effect after i.c.v. administration (20 micrograms), whilst codeine (2-10 micrograms) and dextromethorphan (2.5-20 micrograms) were highly effective. 7. Our findings demonstrate that moguisteine is a novel peripherally acting non-narcotic antitussive agent, the mode of action of which remains to be elucidated fully.

Effect of clonidine on induced cough and bronchoconstriction in guinea pigs and healthy humans

We examined the effects of the alpha 2-receptor agonist clonidine, administered orally and by inhalation, on citric acid- and capsaicin-induced reflexes in guinea pigs and healthy human subjects. In groups (n = 8-10) of conscious guinea pigs, oral clonidine (10 and 100 micrograms/kg) was without effects, whereas inhaled clonidine (10-1,000 microM) caused a concentration-dependent inhibition of citric acid-induced cough (coughs during 3 min: control, 6.5 +/- 0.9; 1,000 microM clonidine, 1.7 +/- 1.0; P < 0.05) and reflex bronchoconstriction (time to onset of bronchoconstriction: control, 191 +/- 24 s; 1,000 microM clonidine, 317 +/- 33 s; P < 0.05). The inhibitory effect of inhaled clonidine on both reflexes was completely reversed by pretreatment with yohimbine but not with prazosin. In 12 healthy human volunteers, oral clonidine (150 mg) caused a significant fall in supine and erect systolic blood pressure and a significant increase in drowsiness as measured on a visual analogue scale 1 and 2 h after administration. Despite these effects, oral clonidine had no effect on capsaicin-induced cough or reflex bronchoconstriction in humans. In contrast to the effects in guinea pigs, inhaled clonidine (281 microM) had no effect on capsaicin-induced cough or reflex bronchoconstriction in humans. These data suggest that peripheral alpha 2-receptors exert an inhibitory effect on sensory neurotransmission in the guinea pig but not in the healthy human airway, indicating an important difference between the two species.

Evidence for substance P as an endogenous substance causing cough in guinea pigs.

We examined an endogenous substance causing cough in awake guinea pigs. An intraperitoneal injection of phosphoramidon, a selective inhibitor of neutral endopeptidase (E.C. 3.4.24.11), caused cough in a dose-dependent fashion for approximately 40 min. At a dose of 3 x 10(-3) mol/kg, phosphoramidon caused a total of 11.6 +/- 1.4 coughs in 40 min. Phosphoramidon (3 x 10(-3) mol/kg)-induced cough was significantly inhibited by systemic pretreatment with capsaicin (p < 0.01). Aerosols of FK 888 (1 min), a specific inhibitor of substance P (NK1) receptor, inhibited phosphoramidon (3 x 10(-3) mol/kg)-induced cough in a dose-dependent fashion with complete inhibition at a dose of 10(-5) M. Likewise, aerosols of FK 224 (10(-5) M; 1 min), another inhibitor of NK1 and NK2 receptors, or lidocaine (4%, 1 min) significantly inhibited phosphoramidon (3 x 10(-3) mol/kg)-induced cough (p < 0.01). Furthermore, aerosols of FK 888 (10(-5) M; 1 min) significantly inhibited cough induced by cigarette smoke in awake guinea pigs (p < 0.01). These results suggest that substance P released from sensory nerves in the airway may be an endogenous substance causing cough and the substance P antagonist may be the drug for treatment of cough in respiratory disease.

Neurokinin A, but not neurokinin B and substance P, induces codeine-resistant coughs in awake guinea-pigs

Neurokinin A, but not neurokinin B and substance P, induces codeine-resistant coughs in awake guinea-pigs

Effects of inhaled α 2-adrenoceptor and GABA B receptor agonists on citric acid-induced cough and tidal volume changes in guinea pigs

The effects of α 2-adrenoceptor and GABA receptor agonists on citric acid-induced cough and increased tidal volume were investigated in conscious guinea pigs. Inhalation of low doses of B-HT 920 (5-allyl-2-amino 5,6,7,8-tetrahydro-4H-thiazolo[4.5-d]azepine dihyrochloride), and xylazine significantly inhibited citric acid-induced cough and tidal volume increases. Intraperitoncal administration of higher doses of B-HT 920 than those given by acrosol were ineffective. The inhibitory effects of B-HT 920 were antagonised by prior intraperitoneal administration of yohimbinc, but not atropine. Inhalation of GABA or baclofen inhibited tidal volume increases, but had no effect on cough. Inhaled α 2-adrenoceptor or GABA agonists had no effect on the reduced respiratory rate after citric acid inhalation. It is concluded that α 2-adrenoceptor agonists inhibit cough via a mechanism which may not be related to their ability to reduce citric acid-induced tidal volume increases, since GABA and baclofen inhibited tidal volume increases but not cough. We suggest that α 2-adrenoceptor agonists may have therapeutic potential in the treatment of cough.

Peripheral site of action of levodropropizine in experimentally-induced cough: Role of sensory neuropeptides

The mechanism of action of levodropropizine has been investigated in different models of experimentally-induced cough in guinea-pigs. In particular it has been demonstrated that the antitussive drug has a peripheral site of action by injecting the drug intracerebroventricularly (i.c.v.). In these experiments levodropropizine (40 μg/50 μl i.c.v.) did not prevent electrically-induced cough. On the other hand, codeine (5 μg/50 μl i.c.v.) markedly prevented coughing. A difference in the potency ratio of levodropropizine and codeine has been demonstrated in capsaicin-induced cough; after oral administration, codeine was about two to three times more potent than levodropropizine. However, after aerosol administration the two compounds were equipotent. These data might suggest a peripheral site of action for levodropropizine which is related to sensory neuropeptides. Further support for the role of sensory neuropeptides in the mechanism of action of levodropropizine comes from the results obtained in capsaicin-desensitized animals. In this experimental model levodropropizine failed to prevent the vagally elicited cough in neuropeptide-depleted animals, whereas codeine did not differentiate between control and capsaicin-treated animals. In conclusion, our results support the suggestion that levodropropizine has a peripheral site of action. In addition, the interference with the sensory neuropeptide system may explain, at least in part, its activity in experimentally-induced cough.

Selective inhibition of cough and bronchoconstriction in conscious guinea pigs.

In guinea-pigs citric acid-induced cough and bronchoconstriction were inhibited by beta 2-agonist and xanthine drugs. Lidocaine inhibited only cough. Cromoglycate and ipratropium bromide inhibited only bronchoconstriction. We conclude that cough and bronchoconstriction in guinea-pigs are distinct reflexes and that the inhibitory pharmacology of these airway reflexes may agree in many respects, with that observed in asthmatic subjects.

Ruthenium red decreases capsaicin and citric acid-induced cough in guinea pigs

The influence of aerosols of Ruthenium red (RR) on capsaicin- and citric acid-induced cough was investigated in guinea pigs. Aerosols of RR (0.3, 1, 3%) reduced capsaicin-induced cough in dose-dependent manner. Inhalation of RR also reduced cough produced by low, (200 mM) but not high (550 mM), concentrations of citric acid. These data suggest that RR is not a specific capsaicin antagonist and that citric acid and capsaicin share a common mechanism for activation of airway C-fibers that is RR sensitive. Furthermore, high concentrations of citric acid can elicit cough through a RR-insensitive mechanism.

Evidence for peripheral mechanisms mediating the antitussive actions of opioids in the guinea pig

1. Comparisons were made between the doses required of aerosol and intraperitoneally administered morphine, dextromethorphan, codeine and the specific peripherally acting μ-receptor agonist DALDA (H-Tyr-D-Arg-Phe-Lys-NH 2) to suppress citric acid-induced coughing in conscious guinea pigs. 2. Estimated ID 50s for inhibition of numbers of coughs induced by an aerosol of 5% citric acid were 1.0 and 2.4 mg/kg for intraperitoneally administered morphine and dextromethorphan, respectively. 3. The estimated ID 50s after inhalation of morphine and dextromethorphan as aerosols were ≈ 2.2 and ≈ 12 μg/kg, respectively. 4. Aerosilized codeine (≈ 72 μg/kg, n = 5) significantly inhibited coughing by 62 ± 23% whereas 3 mg/kg, i.p. was required to significantly reduce coughing by a similar degree (60 ± 6%, n = 7). 5. Inhalation of DALDA (≈ 7.2 μg/kg, n = 7) also significantly inhibited coughing. 6. The antitussive effect of inhaled morphine (≈ 7.2 μg/kg, n = 11) was inhibited after administration of 3 mg/kg of either naloxone hydrochloride or naloxone methylbromide intraperitoneally. 7. The results support the hypothesis that effects at a peripheral site can make a major contribution to the antitussive actions of these drugs.