ABSTRACT

Present knowledge about the biological mechanism of normal mood has made much progress in recent years, although it is not yet fully understood. Decades ago, it was assumed that decreased norepinephrine or serotonin in the synaptic cleft is the major, and possibly the only, factor involved in inducing major depressive disorder (MDD). However, recent evidence demonstrates that in mood disorders, and especially in MDD, there is regional reduction in central nervous system (CNS) volume, probably secondary to reductions in number and/or size of glia and neurons (especially in number and morphology of dendrites) in discrete brain areas (e.g. the hippocampus). Although the precise cellular mechanism underlying these morphometric changes remain to be fully elucidated, data suggest a predominant role of altered neuronal plasticity and cellular resilience. The concept of neuroplasticity refers to the capacity of the CNS to adapt itself to changing external stimuli through appropriate signal transduction, consequent gene expression, and the production of various neurotrophic factors responsible for normal cell connectivity.