Kana Taniguchi

The prostaglandin E2 EP1 receptor mediates pain perception and regulates blood pressure

The lipid mediator prostaglandin E2 (PGE2) has diverse biological activity in a variety of tissues. Four different receptor subtypes (EP1-4) mediate these wide-ranging effects. The EP-receptor subtypes differ in tissue distribution, ligand-binding affinity, and coupling to intracellular signaling pathways. To identify the physiological roles for one of these receptors, the EP1 receptor, we generated EP1-deficient (EP1-/-) mice using homologous recombination in embryonic stem cells derived from the DBA/1lacJ strain of mice. The EP1-/- mice are healthy and fertile, without any overt physical defects. However, their pain-sensitivity responses, tested in two acute prostaglandin-dependent models, were reduced by approximately 50%. This reduction in the perception of pain was virtually identical to that achieved through pharmacological inhibition of prostaglandin synthesis in wild-type mice using a cyclooxygenase inhibitor. In addition, systolic blood pressure is significantly reduced in EP1 receptor-deficient mice and accompanied by increased renin-angiotensin activity, especially in males, suggesting a role for this receptor in cardiovascular homeostasis. Thus, the EP1 receptor for PGE2 plays a direct role in mediating algesia and in regulation of blood pressure.

Antinociceptive activity of CP-101,606, an NMDA receptor NR2B subunit antagonist.

1 The analgesic activity of CP‐101,606, an NR2B subunit‐selective N‐methyl‐D‐aspartate (NMDA) receptor antagonist, was examined in carrageenan‐induced hyperalgesia, capsaicin‐ and 4β‐phorbol‐12‐myristate‐13‐acetate (PMA)‐induced nociceptive tests in the rat. 2 CP‐101,606 30 mg kg−1, s.c., at 0.5 and 2.5 h after carrageenan challenge suppressed mechanical hyperalgesia without any apparant alternations in motor coordination or behaviour in the rat. 3 CP‐101,606 also inhibited capsaicin‐ and PMA‐induced nociceptive responses (licking behaviour) with ED50 values of 7.5 and 5.7 mg kg−1, s.c., respectively. 4 These results suggest that inhibition of the NR2B subunit of the NMDA receptor is effective in vivo at modulating nociception and hyperalgesia responses without causing the behavioural side effects often observed with currently available NMDA receptor antagonists.

CJ-023,423, a Novel, Potent and Selective Prostaglandin EP4 Receptor Antagonist with Antihyperalgesic Properties

The prostaglandin (PG) EP4 receptor subtype is expressed by peripheral sensory neurons. Although a potential role of EP4 receptor in pain has been suggested, a limited number of selective ligands have made it difficult to explore the physiological functions of EP4 or its potential as a new analgesic target. Here, we describe the in vitro and in vivo pharmacology of a novel EP4 receptor antagonist, N-[({2-[4-(2-ethyl-4,6-dimethyl-1H-imidazo [4,5-c] pyridin-1-yl) phenyl]ethyl}amino) carbonyl]-4-methylbenzenesulfonamide (CJ-023,423). In vitro, CJ-023,423 inhibits [3H]PGE2 binding to both human and rat EP4 receptors with Ki of 13 ± 4 and 20 ± 1 nM, respectively. CJ-023,423 is highly selective for the human EP4 receptor over other human prostanoid receptor subtypes. It also inhibits PGE2-evoked elevation in intracellular cAMP at the human and rat EP4 receptors with pA2 of 8.3 ± 0.03 and 8.2 ± 0.2 nM, respectively. In vivo, oral administration of CJ-023,423 significantly reduces thermal hyperalgesia induced by intraplantar injection of PGE2 (ED50 = 12.8 mg/kg). CJ-023,423 is also effective in models of acute and chronic inflammatory pain. CJ-023,423 significantly reduces mechanical hyperalgesia in the carrageenan model. Furthermore, CJ-023,423 significantly reverses complete Freunds adjuvant-induced chronic inflammatory pain response. Taken together, the present data indicate that CJ-023,423, a highly potent and selective antagonist of both human and rat EP4 receptors, produces antihyperalgesic effects in animal models of inflammatory pain. Thus, specific blockade of the EP4 receptor signaling may represent a novel therapeutic approach for the treatment of inflammatory pain.

Antiinflammatory and analgesic activity of a non-peptide substance P receptor antagonist

CP-96,345, a potent non-peptide antagonist of the substance P (SP) receptor, inhibited SP-, neurokinin A (NKA)- and neurokinin B-induced plasma extravasation in guinea pig dorsal skin. The inhibition was specific for the three tachykinins; CP-96,345 was not active against plasma leakage caused by histamine, bradykinin, platelet-activating factor or leukotriene D4. CP-96,345 inhibited capsaicin-induced plasma extravasation in the ureter, an inflammatory response caused by neuropeptides released from afferent C-fibers. Thus, the NK1 receptor appears to play a major role in vascular permeability increases induced by exogenous and endogenous tachykinins. In contrast, CP-96,345 was inactive against SP- and NKA-induced contraction of guinea pig ureter, suggesting that the smooth muscle contraction is not NK1-mediated. CP-96,345 exhibited analgesic activity in acetic acid-induced abdominal stretching in mice, indicating for the first time that SP plays a critical role in this model. The results of these studies support a pathophysiological role of SP and NK1 receptor under acute neurogenic inflammatory conditions and in pain.

Non-specific activity of (+/-)-CP-96,345 in models of pain and inflammation.

The non‐peptide NK1 receptor antagonist, CP‐96,345, and its 2R,3R enantiomer CP‐96,344, which is not an NK1 receptor antagonist (IC50 > 10 μm), were evaluated for antinociceptive and anti‐inflammatory activities in several classical models of pain and inflammation in the rat. Both CP‐96,345 and CP‐96,344 reduced carrageenin‐induced paw oedema and hyperalgesia, and attenuated the second phase of formalin‐induced paw licking with equal potency. These results indicate that NK1 antagonism is not responsible for the activity of (±)‐CP‐96,345 in the above animal models.

Effects of the selective EP4 antagonist, CJ‐023,423 on chronic inflammation and bone destruction in rat adjuvant‐induced arthritis

Prostaglandin E2 (PGE2) produced by cyclooxygenase (COX) is a potent pro‐inflammatory mediator. We have recently discovered CJ‐023,423, a highly selective antagonist of EP4 receptors, one of the PGE2 receptors. This agent is suitable for exploring the effects of blocking EP4 receptors following oral administration in rats. In this study, CJ‐023,423 was used in rats with adjuvant‐induced arthritis (AIA) to investigate the role of the EP4 receptor in chronic inflammation and bone destruction. These effects were compared with those of rofecoxib, a selective COX‐2 inhibitor. CJ‐023,423 had significant inhibitory effects on paw swelling, inflammatory biomarkers, synovial inflammation and bone destruction in AIA rats. In particular, the inhibitory effect on paw swelling in AIA rats was comparable to that of rofecoxib. These results suggest that PGE2 acting via the EP4 receptor is involved in the development of chronic inflammation and bone destruction, particularly with respect to oedema in AIA rats. This is the first study to confirm the in‐vivo effects of EP4 receptor blockade on inflammation and bone destruction in AIA rats with a small‐molecule compound.

Non-specific activity of (±)CP-96,345 in models of pain and inflammation

The non-peptide NK1 receptor antagonist, CP-96,345, and its 2R,3R enantiomer CP-96,344, which is not an NK1 receptor antagonist (IC50 > 10 microM), were evaluated for antinociceptive and anti-inflammatory activities in several classical models of pain and inflammation in the rat. Both CP-96,345 and CP-96,344 reduced carrageenin-induced paw oedema and hyperalgesia, and attenuated the second phase of formalin-induced paw licking with equal potency. These results indicate that NK1 antagonism is not responsible for the activity of (+/-)-CP-96,345 in the above animal models.