HTR1A and Arthritis: However, the involvement of 5-HT1A and 5-HT3 receptors in EA inhibition of arthritis-induced pain warrants further study, since an i.t. injection of the 5-HT3 receptor agonist induced significant anti-nociceptive effects assessed with paw pressure test [21], and an i.t. injection of the selective 5-HT1A receptor antagonist blocked the anti-nociceptive action of 5-HT in formalin-induced pain [23].