Vortioxetine dose-dependently reverses 5-HT depletion-induced deficits in spatial working and object recognition memory: A potential role for 5-HT_1A receptor agonism and 5-HT₃ receptor antagonism

We previously reported that the investigational multimodal antidepressant, vortioxetine, reversed 5-HT depletion-induced memory deficits while escitalopram and duloxetine did not. The present report studied the effects of vortioxetine and the potential impact of its 5-HT_1A receptor agonist and 5-HT₃ receptor antagonist properties on 5-HT depletion-induced memory deficits. Recognition and spatial working memory were assessed in the object recognition (OR) and Y-maze spontaneous alternation (SA) tests, respectively. 5-HT depletion was induced in female Long-Evans rats using 4-cholro-DL-phenylalanine methyl ester HCl (PCPA) and receptor occupancies were determined by ex vivo autoradiography. Rats were acutely dosed with vortioxetine, ondansetron (5-HT₃ receptor antagonist) or flesinoxan (5-HT_1A receptor agonist). The effects of chronic vortioxetine administration on 5-HT depletion-induced memory deficits were also assessed. 5-HT depletion reliably impaired memory performance in both the tests. Vortioxetine reversed PCPA-induced memory deficits dose-dependently with a minimal effective dose (MED) ≤0.1mg/kg (~80% 5-HT₃ receptor occupancy; OR) and ≤3.0mg/kg (5-HT_1A, 5-HT_1B, 5-HT₃ receptor occupancy: ~15%, 60%, 95%) in SA. Ondansetron exhibited a MED ≤3.0μg/kg (~25% 5-HT₃ receptor occupancy; OR), but was inactive in the SA test. Flesinoxan had a MED ≤1.0mg/kg (~25% 5-HT_1A receptor occupancy; SA); only 1.0mg/kg ameliorated deficits in the NOR. Chronic p.o. vortioxetine administration significantly improved memory performance in OR and occupied 95%, 66%, and 9.5% of 5-HT₃, 5-HT_1B, and 5-HT_1A receptors, respectively. Vortioxetine's effects on SA performance may involve 5-HT_1A receptor agonism, but not 5-HT₃ receptor antagonism, whereas the effects on OR performance may involve 5-HT₃ receptor antagonism and 5-HT_1A receptor agonism.