Sub-chronic vortioxetine (but not escitalopram) normalizes brain rhythm alterations and memory deficits induced by serotonin depletion in rats

Major depressive disorder (MDD) is a chronic and disabling psychiatric disorder characterized by a wide range of signs/symptoms, including cognitive dysfunction. Vortioxetine (VOR) is a multimodal antidepressant drug with pro-cognitive actions in animal models and MDD patients. The VOR-mediated blockade of 5-HT-R in a subpopulation of GABA interneurons enhances pyramidal neuron activity in rat medial prefrontal cortex, an effect possibly underlying its pro-cognitive action. Brain oscillations are involved in regulation of cognitive function. We therefore examined VOR effects on oscillatory activity in four brain areas of freely-moving rats (prelimbic cortex, PrL; nucleus accumbens, NAc; dorsal hippocampus, dHPC; paraventricular thalamic nucleus, PVA), in standard and in serotonin-depleted rats showing recognition memory deficits. 4-chloro-DL-phenylalanine (pCPA) markedly reduced low frequency oscillations (LFO, mainly 1 Hz oscillations) and enhanced theta oscillations in PrL and NAc. It also reduced gamma and high frequency oscillations (HFO) in PVA. Subchronic VOR and escitalopram (ESC) treatments had little effect on oscillatory activity in standard rats. However, VOR –but not ESC- prevented recognition memory deficits in 5-HT-depleted rats, and normalized LFO and theta powers in PrL and NAc. In parallel, VOR –but not ESC- prevented the deficit in PrL-dHPC gamma coherence, but not the decrease in gamma and HFO powers in PVA. Overall, this supports a prominent role of serotonergic neurotransmission on brain oscillatory activity, particularly in cortico-striatal pathways linked to short-term recognition memory. Further, VOR prevented pCPA-induced cognitive deficits by normalizing oscillatory activity at lower frequencies in the PrL-NAc pathway, also normalizing the PrL-dHPC coherence at gamma frequencies.