It is well known that histamine, cysteinyl leukotriene (cysLTs), thromboxanes, prostaglandins (PGs) and cytokines are responsible for allergic rhinitis. 1,2) There are two phases of allergic rhinitis response, named "early phase" and "late phase." Early phase response occurs within minutes of exposure to the allergen and is characterized by sneezing, rhinorrhea and nasal congestion. Late phase response occurs 4 to 8 h after allergen exposure and is characterized mainly by nasal congestion.
3)Histamine H 1 receptor antagonists, such as chlorpheniramine and astemizol, are widely used to treat allergic rhinitis clinically, 4,5) however, these H 1 receptor antagonists are not completely effective for nasal congestion. [6][7][8] In an animal model, we have reported that although H 1 receptor antagonists inhibited the increase of enhanced pause (Penh) in early phase induced by toluene 2,4-diisocyanate (TDI) challenge, 9) a relatively high dose was needed. On the other hand, we reported that cys-LTs receptor antagonists (pranlukast, zafirlukast) and thromboxane A 2 (TXA 2 ) receptor antagonists (seratrodast, ramatroban) also caused an inhibitory effect on the increase of Penh in early and late phases after TDI challenge in sensitized rats. 10) Therefore, it seems likely that different mediator is involved in early and late phase responses in nasal congestion.Prostaglandin E 2 (PGE 2 ) is arachidonic acid metabolite similar to LTs and TXA 2 , and there are four different receptors designated EP 1 , EP 2 , EP 3 and EP 4 . Kunikata et al. 10) reported that mice lacking EP 3 receptor developed potent allergic inflammation compared with wild-type mice or mice deficient in other prostaglandin E 2 receptor subtypes. On the other hand, Gomi et al.11) described that PGE 2 selectively enhanced the immunoglobulin E (IgE)-mediated production of interleukin-6 (IL-6). These actions may produce by the receptors other than EP 3 receptor.In a previous study, we established a nasal congestion model using Brown Norway (BN) rats by intranasal immunization with toluene 2,4-diisocyanate (TDI) as an antigen.
8)In this model, the animals showed not only sneezing but also nasal congestion in the early and late phases.In the present study, in order to clarify the role of PGE 2 receptor (EP 1 , EP 2 , EP 3 and EP 4 ) in the development of allergic rhinitis symptoms, we investigated the effects of each PGE 2 receptor agonists with a nasal congestion model in BN rats.
MATERIALS AND METHODSAnimals Six-week-old male BN rats were obtained from Kyudo Co., Ltd. (Saga, Japan). The animals were housed in an air-conditioned room, maintained at 24Ϯ2°C, with relative humidity of 55Ϯ10%. The rats were given standard laboratory rodent food (Oriental Yeast, Tokyo, Japan) and water ad libitum. All procedures involving animals were conducted in accordance with the Guidelines for Animal Experiments at Okayama University Advanced Science Research Center.Materials The following reagents were obtained from the sources shown in parentheses: TDI (Wako, Tokyo, Japan)...