Date Published: June 19, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Alex Brenchat, Maria Rocasalbas, Daniel Zamanillo, Michel Hamon, José Miguel Vela, Luz Romero.
No study has ever examined the effect of 5-HT7 receptor agonists on nociception by using 5-HT7 receptor knockout mice. Basal sensitivity to noxious heat stimuli and formalin-induced nociception in both phase I and II of the formalin test did not differ in 5-HT7 receptor knockout mice and paired wild-type controls. Similarly, there was no significant difference in basal body temperature between both genotypes. Subcutaneous administration of 5-HT7 receptor agonists AS-19 (10 mg/kg), E-57431 (10 mg/kg), and E-55888 (20 mg/kg) significantly reduced formalin-induced licking/biting behavior during the phase II of the test in wild-type but not in 5-HT7 receptor knockout mice. At these active analgesic doses, none of the three 5-HT7 receptor agonists modified the basal body temperature neither in wild-type nor in 5-HT7 receptor knockout mice. However, a significant decrease in body temperature was observed at a higher dose (20 mg/kg) of AS-19 and E-57431 in both genotypes. Our data strongly suggest that the 5-HT7 receptor agonists AS-19, E-57431, and E-55888 produce antinociception in the formalin test by activating 5-HT7 receptors. These results also strengthen the idea that the 5-HT7 receptor plays a role in thermoregulation, but by acting in concert with other receptors.
The 5-HT7 receptor has been cloned from different genomes and its binding profile is consistent across species and between cloned and native receptors [1, 2]. In recent years, considerable efforts have focused on the development of selective 5-HT7 receptor agonists and antagonists. To date, the search for 5-HT7 receptor antagonists has led to the discovery of LY215840 , SB-258719 , DR4004 , SB-269970 , and SB-656104-A . Regarding 5-HT7 receptor agonists, AS-19 [8, 9], MSD-5a , LP-44 , LP-211 , E-55888 , and E-57431  have been developed. However, most of these agonists display rather modest selectivity because their affinity for the 5-HT7 type is only 11-fold higher than for 5-HT1D in case of AS-19 , 28.6-fold higher than for 5-HT1A in case of MSD-5a , and 33-fold higher than for dopamine D2 receptor , and 5-14-fold higher than for 5-HT1B, 5-HT2B, 5-HT2C, and 5-HT5A in case of LP-211 . Indeed, among 5-HT7 receptor agonists, only E-55888 and E-57431 seem to have a satisfactory selectivity with affinity for the 5-HT7 receptor 280-fold higher than for 5-HT1A and 112.7-fold higher than for 5-HT1D, respectively  (see Table 1). When tested in a functional assay, 5-HT7 receptor agonists concentration dependently increased cAMP formation in HEK-293F/h5-HT7 cells. AS-19 has been found to behave as a potent (EC50 = 9 ± 1 nM) but partial 5-HT7 receptor agonist, with a maximal effect reaching 77% of that of 5-HT . However, E-55888 and E-57431 behave as full agonists, with efficacies (Emax = 99 ± 1% and 94.5 ± 1%, resp.) and potencies (EC50 = 16 ± 1 nM and 21.5 ± 1 nM) similar to those of 5-HT, as previously described [13, 14].
In this study, the in vivo target-specific effects of the 5-HT7 receptor agonists AS-19, E-57431, and E-55888 on nociception (i.e., formalin-induced nociception) and thermoregulation were examined using 5-HT7 receptor knockout mice. These 5-HT7 receptor agonists exerted antinociceptive effects in phase II of the formalin test in wild-type but not in 5-HT7 receptor knockout mice, suggesting that their analgesic effect is actually 5-HT7 receptor mediated. Analgesic doses of 5-HT7 receptor agonists did not change body temperature neither in 5-HT7 receptor knockout nor in wild-type mice. However, a reduction in body temperature was observed in both genotypes when the dose of the agonists were increased up to levels exceeding their selectivity window.
Data obtained in this study strengthen the notion that 5-HT7 receptors play a role in nociceptive control in pain conditions involving central sensitization and add further support to their fine-tuning effects in body temperature homeostasis through possible actions in concert with other molecular targets. In addition, this study provides evidence that formalin-induced nociceptive behaviors and body temperature in 5-HT7 receptor knockout mice are useful models and relatively simple approaches to assess in vivo specificity of 5-HT7 receptor agonists.