Donald C. Bolser

Peripheral and central sites of action of GABA-B agonists to inhibit the cough reflex in the cat and guinea pig.

1 The GABA‐B receptor agonists baclofen and 3‐aminopropylphosphinic acid (3‐APPi) have antitussive activity in the cat and guinea pig. The purpose of this study was to investigate the sites of action of these GABA‐B receptor agonists to inhibit the cough reflex. 2 Single intracerebroventricular (i.e.v.) cannulas were placed in the lateral ventricles of anaesthetized guinea pigs. Approximately 1 week later, the animals were exposed to aerosols of capsaicin (0.3 μm) to elicit coughing. Coughs were detected with a microphone and counted. 3 Cough was produced in anaesthetized cats by mechanical stimulation of the intrathoracic trachea and was recorded from electromyograms of respiratory muscle activity. Cannulas were placed for intravenous (i.v.) or, in separate groups of animals, intravertebral arterial (i.a.) administration of baclofen, 3‐APPi, the centrally active antitussive drug codeine or the peripherally active antitussive drug BW443c. Dose‐response relationships for i.v. and i.a. administration of each drug were generated to determine a ratio of i.v. ED50 to i.a. ED50, known as the effective dose ratio (EDR). The EDR will be 20 or greater for a centrally acting drug. 4 In the guinea pig, baclofen (3 mg kg−1, s.c.) and 3‐APPi (10 mg kg−1, s.c.) inhibited capsaicin‐induced cough by 50% and 35% respectively. The antitussive activity of baclofen was completely blocked by i.e.v. administration of the GABA‐B receptor antagonist CGP 35348 (10 μg). Conversely, the antitussive effect of 3‐APPi was unaffected by i.e.v. CGP 35348. However, systemic administration of CGP 35348 (30 mg kg−1, s.c.) completely blocked the antitussive activity of 3‐APPi (10 mg kg−1, s.c). In separate experiments baclofen alone (1 μg, i.c.v.) inhibited capsaicin‐induced cough by 78%. 3‐APPi (10 and 100 μg, i.c.v.) had no effect on capsaicin‐induced cough in the guinea pig. 5 In the cat, potencies (ED50) of the standards and GABA‐B agonists by the i.v. route were: codeine (0.34 mg kg−1), BW443C (0.17 mg kg−1), baclofen (0.63 mg kg−1) and 3‐APPi (2.3 mg kg−1). Potencies of these drugs by the i.a. route were: codeine, 0.013 mg kg−1; BW443C, 0.06 mg kg−1; baclofen, 0.016 mg kg−1; and 3‐APPi, 0.87 mg kg−1. The EDRs for each drug were: codeine, 26; BW443C, 3; baclofen, 39; and 3‐APPi, 3. 6 We conclude that in both the cat and guinea pig baclofen inhibits cough by a central site of action, while 3‐APPi inhibits cough by a peripheral site of action.

GABAB receptors in the lung

gamma-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in the mammalian CNS, is also found in peripheral tissues, including the lung. Recent pharmacological studies using selective ligands for GABAA and GABAB receptors demonstrate that of these two, the GABAB receptor is the important receptor subtype controlling lung functions. GABAB agonists inhibit a variety of responses in the airways, including neuronally induced cholinergic- and tachykinin-mediated smooth muscle contraction, microvascular leakage, anaphylactic bronchospasm and cough. Because these conditions are seen in certain respiratory diseases, such as asthma, a selective GABAB agonist may have therapeutic potential for the treatment of this respiratory disorder.

Antitussive effects of GABAB agonists in the cat and guinea-pig.

1 GABAB agonists inhibit neuronal processes which are important in the pathogenesis of airway disease, such as bronchospasm. Cough is a prominent symptom of pulmonary disease, but the effects of GABAB agonists on this airway reflex are unknown. Experiments were conducted to determine the antitussive effect of GABAB receptor agonists in comparison to the known antitussive agents, codeine and dextromethorphan. 2 Unanaesthetized guinea‐pigs were exposed to aerosols of 0.3 mm capsaicin to elicit coughing, which was detected with a microphone and counted. Cough also was produced in anaesthetized cats by mechanical stimulation of the intrathoracic trachea and was recorded from electromyograms of respiratory muscle activity. 3 In guinea‐pigs, the GABAB agonists baclofen and 3‐aminopropyl‐phosphinic acid (3‐APPi) produced dose‐dependent inhibition of capsaicin‐induced cough when administered by subcutaneous or inhaled routes. The potencies of baclofen and 3‐APPi compared favourably with codeine and dextromethorphan. 4 The GABAB antagonist, CGP 35348 (0.3– 30 mg kg−1, s.c.) inhibited the antitussive effect of baclofen (3.0 mg kg−1, s.c). However, CGP 35348 (10 mg kg−1, s.c.) had no effect on the antitussive activity of codeine (30 mg kg−1, s.c). The antitussive effect of baclofen was not influenced by the GABAA antagonist, bicuculline (3 mg kg−1, s.c.) or naloxone (0.3 mg kg−1, s.c). 5 In the cat, baclofen (0.3–3.0 mg kg−1, i.v.) decreased mechanically‐induced cough in a dose‐dependent manner. In this model, baclofen (ED50 = 0.63 mg kg−1) was less potent than either codeine or dextromethorphan. The antitussive effect of baclofen in the cat was antagonized by the GABAB antagonists, CGP 35348 (10 mg kg−1, i.v.) and 3‐aminopropylphosphonic acid (3 mg kg−1, i.v.). 6 We show that baclofen and 3‐APPi have antitussive effects in the guinea‐pig and cat and these effects are mediated by GABAB receptors.

Pharmacological studies of allergic cough in the guinea pig

The pharmacological mechanisms of allergic cough in the guinea pig were studied. Actively sensitized guinea pigs were exposed to aerosols of antigen to elicit coughing. In separate experiments, naive guinea pigs were exposed to aerosols of capsaicin to elicit coughing. Both allergic and capsaicin-induced cough were inhibited by loratadine (0.3-10 mg kg-1 p.o.) and chlorpheniramine (0.1-3.0 mg kg-1 p.o.). Neither cimetidine (10 mg kg-1 s.c.), nor thioperamide (3-10 mg kg-1 s.c.), inhibited allergic or capsaicin-induced cough. Codeine (3-30 mg kg-1 p.o.), salbutamol (0.003-3.0 mg kg-1 s.c.) and ipratropium (0.03-1.0 mg kg-1 s.c.) inhibited both allergic and capsaicin-induced cough. Hexamethonium (10 and 30 mg kg-1 s.c.) inhibited allergic, but not capsaicin-induced cough. Allergic and capsaicin-induced cough were unaffected by phenidone (5.0 and 10.0 mg kg-1 s.c.). Indomethacin (5.0 and 10.0 mg kg-1 s.c.) had no effect on allergic cough but slightly inhibited capsaicin-induced cough. We conclude that allergic and capsaicin-induced cough are modulated by histamine H1 receptor and cholinergic mechanisms. Histamine H2 or histamine H3 receptor mechanisms, and lipoxygenase and cyclooxygenase products of arachidonic acid metabolism do not influence allergic and capsaicin-induced cough. Ganglionic mechanisms play a minor role in the production of allergic cough and no role in capsaicin-induced cough.

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.

Antitussive Action of Antihistamines Is Independent of Sedative and Ventilation Activity in the Guinea Pig

We studied the oral actions of antihistamines from six chemical classes, namely: the ethanolamines (ENA, diphenhydramine and clemastine); ethylenediamines (EDA, pyrilamine and tripelennamine); piperidines (PPD, terfenadine and astemizole); piperazines (PPZ, hydroxyzine and cetirizine); phenothiazines (PTZ, promethazine), and the alkylamines (ALA, chlorpheniramine and bromopheniramine) on cough reflexes, pentobarbital-induced sedation and minute ventilation in the conscious guinea pig. Antihistamines of the ENA class had minimal effects on capsaicin-induced cough although both diphenhydramine (30 and 100 mg/kg p.o.) and clemastine (30 and 100 mg/kg p.o.) increased sedation time (ST). The PPZ class demonstrated both antitussive and sedating activity. The minimum effective oral antitussive dose (MED) of cetirizine and hydroxyzine was 30 and 10 mg/kg, respectively. The EDA did not exhibit antitussive activity. Tripelennamine (10, 30 and 100 mg/kg p.o.) but not pyrilamine enhanced ST. The MED for the PTZ, promethazine, was 10 mg/kg, and at 100 mg/kg promethazine increased ST. The ALA group displayed antitussive activity but only chlorpheniramine (10 mg/kg p.o.) had any effects on ST. The MED for chlorpheniramine and bromopheniramine was 3 and 10 mg/kg p.o., respectively. The PPD antihistamines, namely terfenadine and astemizole, inhibited cough (MED 30 and 10 mg/kg p.o.) without sedative effects. Of the antihistamines tested only promethazine (100 mg/kg p.o.) depressed ventilation responses; however, this dose of promethazine was associated with adverse behavioral effects. The present findings indicate that the antitussive actions of antihistamines are not directly related to histamine H1-receptor blockade because several antihistamines did not antagonize capsaicin-induced cough. In addition, the antitussive actions of antihistamines are independent of their sedative or ventilation effects.

Effect of codeine on the inspiratory and expiratory burst pattern during fictive cough in cats

Experiments were conducted to study the effect of the opioid, codeine, on different components of the cough motor pattern. Midcollicular decerebrate cats were paralyzed and artificially ventilated by a pump triggered by the phrenic neurogram. Inspiratory (phrenic) and expiratory (cranial iliohypogastric) neurograms were recorded. Fictive cough was produced by mechanical stimuli applied to the intrathoracic trachea. Codeine (0.03-1.0 mg.kg-1, i.v.) decreased cough frequency (average number of coughs per stimulus trial), expiratory burst amplitude, and inspiratory burst amplitude in a dose-dependent manner. The maximum reduction in cough frequency and expiratory amplitude produced by codeine was 80-90% for both parameters. However, codeine was more potent in reducing cough frequency (ED50 = 0.1 mg.kg-1) than expiratory burst amplitude (ED50 = 0.35 mg.kg-1). The maximum observed reduction of inspiratory burst amplitude elicited by codeine was approximately 40%. There was a positive linear relationship between phrenic and cranial iliohypogastric burst amplitudes during fictive cough (r = 0.82, P < 0.001). Codeine destabilized the motor pattern during fictive cough by disrupting this relationship between inspiratory and expiratory burst amplitudes. We conclude: (a) the central pattern generator for cough is functionally organized into a cough frequency generator, an expiratory burst amplitude generator and an inspiratory burst amplitude generator, each of which have different sensitivities to codeine (b) there exists a specific codeine-sensitive neural mechanism matching the relative magnitude of central drive to inspiratory and expiratory motoneurons during cough.

Respiratory effects of baclofen and 3-aminopropylphosphinic acid in guinea-pigs.

1 The effects of the GABAB receptor agonists, baclofen and 3‐aminopropylphosphinic acid (3‐APPi) given by the subcutaneous or intracerebroventricular (i.c.v.) route were examined on minute ventilation (), tidal volume (VT) and respiratory rate (f) due to room air and carbon dioxide (CO2)‐enriched gas hyperventilation in conscious guinea‐pigs. 2 Baclofen (0.3–10 mg kg−1, s.c.) produced a dose‐dependent inhibition of and f due to room air and CO2 inhalation. The maximum inhibition of room air breathing was 85% ± 3 and f was 74% ± 3 at 10 mg kg−1, s.c. The maximum effects on CO2‐induced hyperventilation were 68% ± 9 and 51% ± 6, for and f respectively. Only the highest dose of baclofen studied (10 mg kg−1) produced a significant inhibition of VT due to room air breathing (46% ± 6) and CO2 breathing (38% ± 11). 3 3‐aPPi (0.3–100 mg kg−1, s.c.) did not affect , VT or f due to room air breathing or CO2 inhalation at any dose tested. Also, i.c.v. administration of 3‐aPPi (100 μg) did not affect ventilatory responses due to room air breathing or CO2 inhalation. 4 Pretreatment with the GABAB antagonist, CGP 35348 3‐aminopropyl‐(diethoxymethyl) phosphinic acid (3–30 mg kg−1, s.c.) blocked the respiratory depressant effects of baclofen (3 mg kg−1, s.c.) in a dose‐related fashion. 5 Intracerebroventricular (i.c.v.) administration of CGP 35348 (50 μg) blocked the respiratory depressant effects of baclofen. CGP 35348 given alone either i.c.v. or s.c. had no effects on respiration due to room air or CO2 inhalation. 6 Pretreatment with either the GABAA antagonist bicuculline (30 mg kg−1, s.c.) or the opioid antagonist, naloxone (1 mg kg−1, s.c.) had no effect on the respiratory depressant action of baclofen (3 mg kg−1, s.c). 7 These results show that baclofen inhibits ventilation due to room air breathing, and attenuates the hyperventilation response to CO2 inhalation. The peripherally acting GABAB agonist, 3‐APPi had no effect on ventilation. These findings demonstrate that the respiratory depressant effects of baclofen are due to activation of CNS GABAB receptors and indicates that only GABAB receptor agonists that penetrate into the CNS may cause respiratory depression.

Evan's blue dye blocks capsaicin-induced cough and bronchospasm in the guinea pig

The influence of Evans blue dye on capsaicin-induced bronchoconstrictor and cough responses was investigated in the guinea pig. Evans blue (30 mg kg-1 i.v.) pretreatment shifted the bronchoconstrictor dose-response to capsaicin (0.3-100 micrograms kg-1 i.v.) to the right by 10-fold, but had no effect on the bronchospasm elicited by neurokinin A (0.3-10 micrograms kg-1 i.v.). Evans blue (0.3-30 mg kg-1 s.c.) also inhibited capsaicin-induced cough in a dose-dependent manner. Evans blue blocked capsaicin responses by the intravenous, subcutaneous, or inhaled routes of administration. We conclude bronchoconstrictor responses and cough in vivo.

Intracardiac phenylbiguanide causes excitation of spinal neurons by activation of cardiac sympathetic afferents

The responses of spinothalamic, spinoreticular, and unidentified spinal neurons to intracardiac administration of phenylbiguanide, a 5-HT3 receptor agonist, were examined in anesthetized cats and monkeys. Eighteen neurons were excited, 5 were inhibited, and 12 were unresponsive to this stimulus. Results suggest that cardiac sympathetic afferents mediate the excitatory responses produced by phenylbiguanide, because bilateral cervical vagotomy failed to block these responses, and aortic injections of phenylbiguanide had little effect on cell activity.