A hypothesis of monoamine (5-HT) - Glutamate/GABA long neural circuit: Aiming for fast-onset antidepressant discovery

Major depressive disorder has become a global public health problem of serious concern. Most of the clinical antidepressants are developed under the classic "monoamine hypothesis (strategy)". These drugs generally have such deficiencies including slow onset and limited efficiency, cognitive impairment and suicidal tendency. Therefore, it is the direction to break through the classic monoamine strategy framework for developing antidepressants that have fast-acting, lower side effects, and cognitive enhancement, to satisfy the major clinical needs. In 2019, the launch of fast-acting antidepressants such as S-ketamine(S-Ket) and brexanolone into market by FDA has opened up new prospects for non-monoamine strategy mainly based on the N-methyl-d-aspartate (NMDA) and γ-aminobutyric acid type A (GABA_A) receptors. There are two main trends in the development of fast-onset antidepressants: the optimized multi-target monoamine strategy (modern monoamine strategy) and the non-monoamine strategy based on glutamate(Glu)-GABA balance modulation. Based to the research conducted by foreign peers and our lab, we propose a hypothesis of "monoamine (5-HT)- Glu/GABA long neural circuit", which holds the view that both monoaminergic mechanisms (such as 5-HT neurons located in raphe nucleus) and non-monoaminergic mechanisms (Glu/GABA neurons located in prefrontal cortex) are all part of the rapid-acting antidepressant mechanisms, and both of them form a long neural circuit mediating the fast synaptogenesis of the brain regions including prefrontal cortex. Based on this, it is proposed that fast launch and activation of this circuit may be an important mechanism for fast-onset of antidepressant, in which Glu/GABA (excitation/ inhibition, E/I) rebalance should be the critical rate-limiting step for the onset speed. Therefore, five potential strategies are proposed for fast-acting antidepressant based on this circuit: 1) Achieve the rapid E/I balance by relieving the inhibition of GABA interneurons on glutamatergic pyramidal neurons or directly activating pyramidal neurons; 2) Simultaneously modulate 5-HT neuronal activity and Glu/GABA balance by 5-HT transporter combining with some receptors such as 5-HT_1A/1B (namely simultaneous enhancement of the 5-HT and Glu/GABA links); 3) Directly activate mammalian rapamycin target protein complex 1 (mTORC1) and rapidly enhance brain-derived neurotrophic factor (BDNF) -mTOR pathway; 4) Stimulate rapid release of BDNF in the brain; 5) Positive allosteric modulator of synaptic and extrasynaptic GABA_A receptors. It is hoped that these ideas will provide possible strategies for the further development of a new generation of antidepressants and provide a useful reference for the further discovery of fast-onset antidepressant candidate targets.