E-6801, a 5-HT6 receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission in the rat

In rats, 5-hydroxytryptamine(6) (5-HT6) receptor antagonists improve learning and memory, but the effects of agonists are poorly defined. This study investigated the effects of 5-HT6 receptor agonists and antagonists on a rodent model of recognition memory. Selective 5-HT6 receptor agonists and antagonists were administered either alone, after a scopolamine-induced impairment, or combined with sub-effective doses of the acetylcholinesterase inhibitor, donepezil, or the glutamate NMDA receptor antagonist, memantine, in a novel object discrimination paradigm in adult rats. After a 4-h inter-trial delay to induce natural forgetting, vehicle-treated rats spent an equivalent time exploring novel and familiar objects during the choice trial. The 5-HT6 receptor agonists, E-6801 (1.25-10 mg/kg i.p.) and EMD-386088 (5-10 mg/kg i.p.), and antagonists, SB-271046 and Ro 04-6790 (5 and 10 mg/kg), along with donepezil (0.1-3 mg/kg) and memantine (5-20 mg/kg) all produced significant and mostly dose-dependent increases in novel object exploration, indicative of memory enhancement. Furthermore, sub-effective doses of E-6801 (1 mg/kg) when co-administered with either SB-271046 (3 mg/kg), donepezil (0.1 mg/kg) or memantine (5 mg/kg), and EMD-386088 (2 mg/kg) co-administered with SB-271046 (3 mg/kg) also significantly enhanced object-recognition memory. Additionally, using a 1-min inter-trial delay, E-6801 (2.5 and 5 mg/kg) was as effective as donepezil (0.3 and 1 mg/kg) in reversing a scopolamine-induced (0.5 mg/kg) impairment in object recognition. This is the first study to demonstrate that E-6801, a potent 5-HT6 receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission.