Prostaglandin E2 mediates the tussive response via the EP3 receptor; implications for future therapy

Sarah Maher, Mark Birrell, Maria Belvisi

Imperial College, London, UK

Prostaglandin E2 (PGE2) may provide a novel therapy for the treatment of inflammatory airway diseases as it has been shown to confer both bronchodilator and anti-inflammatory activity in asthmatic subjects (Melillo et al., 1994; Gauvreau et al., 1999). However, PGE2 can cause irritancy of the upper airway resulting in a reflex cough. The aim of this study was to identify which of the 9 prostanoid receptors (EP1, EP2, EP3, EP4, DP, FP, IP, TP and CRTh2) mediates the PGE2 induced cough. The cough reflex is under the control of sensory afferent nerve fibres that innervate the lungs via the vagus nerve. Using our in vitro model of vagal sensory nerve depolarisation (mV), gene and protein expression techniques and our guinea-pig cough system, we investigated the prostanoid receptor(s) responsible for the PGE2-induced depolarisation of the vagus nerve and cough.

The effect of prostanoid receptor antagonists on the depolarisation induced by PGE2 (10μM) was investigated in mouse (male C57bl/6j, 18-20g) and guinea-pig (male Dunkin Hartley, 300-350g) isolated vagal nerve preparations. We have previously shown that antagonists at the EP1, EP2, EP4, DP, FP, IP and TP receptors had no effect on the response to PGE2 whereas an EP3 receptor antagonist (0.2μM) significantly inhibited PGE2-induced depolarisation of the vagus nerve in the guinea-pig and mouse by 79.5 ± 8.8% (p<0.05) and 64.8 ± 2.7% (p<0.05) respectively. Furthermore, we used isolated vagus nerves from prostanoid receptor KO mice (male, all C57bl/6j except EP4; 129Ola/C57bl6j, 18-20g) and showed that the response to PGE2 (10μM) was significantly reduced in EP3KO mice (n=4-6, p<0.05; Kruskal-Wallis, Dunns post-hoc test) (Maher et al., 2007). We have shown that the EP3 antagonist inhibited PGE2-induced depolarisation in human isolated vagus nerves by 80% providing human proof of concept data (n=1). Expression of the EP3 receptor in guinea-pig was confirmed in the kidney (positive control) and in cell bodies of the vagal nerve fibres contained in the nodose and jugular ganglia using Taqman RT-PCR and immunohistochemistry. Moreover, the EP3 antagonist (300mg kg-1) inhibited cough induced by PGE2 (300μg ml-1) in the guinea-pig (Vehicle control 45.6 ± 8.5 coughs, antagonist group 23.2 ± 3 coughs. n=5-6, p<0.05, Mann-Whitney U test).

In conclusion, PGE2-induced depolarisation of mice, guinea-pig and human vagus nerves can be inhibited by the EP3 antagonist and the response to PGE2 was virtually abolished in isolated nerves from EP3KO mice. It has been shown that the EP3 receptor is expressed on vagal nerve ganglia and furthermore, the EP3 antagonist inhibits PGE2-induced cough. If the receptor that is responsible for the anti-inflammatory and bronchodilator effects of PGE2 is different, then a therapy for asthma could be developed that is devoid of the tussive side effects.

Gauvreau et al. (1999) American Journal of Respiratory and Critical Care Medicine, 159, p31-36

Maher et al. (2007) From Proceedings of the British Pharmacological Society at http://www.pa2online.org/abstracts/Vol5Issue2abst081P.pdf

Melillo et al. (1994) American Journal of Respiratory and Critical Care Medicine, 149, p1138-1141