Effects of 5-HT 4 Receptor Agonists and Antagonists in Learning

Abstract

In the present work, the effects of pre- or post-training (ip) injection of BIMU1 and BIMU8 (5-HT 4 agonists) were figured out in the autoshaping learning task. Furthermore, the post-training effects of these agonists after treatment with SDZ 205-557 and GR 125487D (5-HT 4 antagonists) or p-Chloroamphetamine (PCA) were also explored. Animals were individually trained in a lever-press response on the autoshaping task and 24 hours later were tested. The results showed that pre-training injection of BIMU1 (5–20 mg/Kg) or BIMU8 (20 mg/Kg) increased the CR; in contrast, the post-training administration of BIMU1 (10–20 mg/Kg) or BIMU8 (5 and 20 mg/Kg) decreased it. Further experiments revealed that the post-training injections of SDZ 205-557 (1.0–10.0 mg/Kg) or GR 125487D (0.39–1.56 mg/Kg) by themselves did not alter the CR. When BIMU1 or BIMU8 was administered to rats pretreated with SDZ 205-557 (10 mg/Kg) or GR 125487D (0.78 mg/Kg), the decrement induced by 5-HT 4 the agonists was reversed; in contrast, the administration of PCA failed to modify the CR or the agonist-induced responses. The findings showed that the pre-training stimulation of 5-HT 4 receptors enhanced the acquisition of CR, while, post-training activation of 5-HT 4 receptors, impaired the consolidation of learning. The latter effect was not altered by PCA pretreatment. The data show that 5-HT 4 receptors are involved in the acquisition and consolidation of learning. It seems that postsynaptic 5-HT 4 receptors are involved in the latter effect.