BDNF and major depressive disorder: Brain-derived neurotrophic factor (BDNF), a protein complex and part of the neurotrophin family, is synthesised and secreted by the central nervous system and plays a major role in brain plasticity and survival of the developing neurons.10 The neurotrophic hypothesis of depression states that during times of stress, BDNF is down-regulated, especially in the limbic areas, influencing emotional responses.7 It is suggested that the downregulation is caused by corticosterone.11