Venous blood levels of prostaglandin E₂ (PGE₂) are elevated in migraineurs during an attack (Satchielli et al, 2000), suggestive of a role for PGE₂ in the pathophysiology of migraine. We have previously shown that PGE₂, acting via EP₄ receptors mediates vasodilatation of human cerebral arteries (Davis et al, 2004). The aim of the present study was to use the selective EP₄ antagonist, EP₄A (Machwate et al, 2001), to ascertain whether PGE₂-induced relaxation in human middle meningeal arteries (HMMA) is mediated via EP₄ receptors.

HMMA were obtained at autopsy with the informed consent of the donors’ next of kin and with the approval of the local ethics committee. HMMA were dissected from the dura mater, and intact rings (internal diameter <2mm) were mounted under isometric conditions in 10ml tissue baths under an initial tension of 5mN. All tissue rings were allowed to equilibrate ­for 60 min in Krebs solution at 37°C containing indomethacin (3 µM) and gassed with 95% O₂/ 5% CO₂. A cumulative concentration-effect curve (CEC) to phenylephrine (PE) (0.01 µM to 100 µM) was generated in all tissues. After washout, tissues were pre-incubated (60 min) with either EP₄A (0.03 µM, 0.1 µM or 1 µM) or vehicle (0.1% DMSO), and then pre-contracted with an ~EC₇₀ concentration of PE (10 µM). Once a stable PE contraction was obtained, a cumulative CEC to PGE₂ (0.01 µM to 100 µM) was constructed. Prostacyclin (1 µM) was added at the end of the CEC to determine an approximate maximum relaxatory response of each tissue.

Results: Cumulative addition of PE to isolated rings of HMMA caused reproducible concentration-dependent contractions (pEC₅₀ of 5.8 ±0.3, (n=4)). On PE pre-contracted HMMA rings, PGE₂ caused concentration-dependent relaxations exhibiting a pEC₅₀ of 8.0 ± 0.2 (n=10) (Figure 1A). Pre-treatment with the selective EP₄ antagonist, EP₄A (0.03 µM, 0.1 µM or 1 µM; n=3-7) caused a rightward surmountable shift of the PGE₂ CEC. Schild analysis generated a plot EP₄A with a slope not significantly different from unity (0.8 ± 0.1) and a pK_B for of 8.2 (Figure 1B).

Figure 1. (A)Concentration-response curves to PGE₂ in the absence and presence of various concentrations of the EP₄ receptor antagonist, EP₄A. (B)Schild analysis shown.

In the present study, we have demonstrated for the first time that EP₄ receptors mediate PGE₂-induced dilatation of pre-contracted HMMA. These data are consistent with the hypothesis that EP₄ antagonists may be of potential clinical benefit in the treatment of migraine.

Davis, R.J. et al. (2004) Br. J. Pharmacol. 141, 580-585
Machwate, M. et al. (2001) Mol. Pharmacol. 60, 36-41
Satchielli, P. et al. (2000) Cephalalgia 20, 907-918