Simple, noninvasive method to measure the antibronchoconstrictor activity of drugs in conscious guinea pigs

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.

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 (V), 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 V and f due to room air and CO2 inhalation. The maximum inhibition of room air breathing V 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 V 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 V, VT or f due to room air breathing or CO2 inhalation at any dose tested. Also, i.c.v. administration of 3-APPi (100 micrograms) 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 micrograms) 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.