NRN1 and major depressive disorder: Activation of their downstream signaling pathways was shown to enhance maturation and dendritic development.48 Induction of neuritin and PSD95 are also indicative of an increased synapse formation and function.37 However, additional antidepressant mechanisms may be involved, resulting from the enhanced 5-HT neurotransmission in other brain areas implicated in MDD, such as the ventromedial prefrontal cortex.