Final answer:
Acute and chronic stress decrease Brain-derived neurotrophic factor (BDNF) levels, while SSRIs increase it, which is important for the growth and survival of neurons and is linked with the therapeutic effects on depression.
Step-by-step explanation:
Acute and chronic stress decrease the level of the neurotrophin known as Brain-derived neurotrophic factor (BDNF), while Selective Serotonin Reuptake Inhibitors (SSRIs) increase it. BDNF plays a critical role in the survival and growth of neurons, and is thus important for long-term memory and other cognitive functions. Stress, via the action of glucocorticoids, downregulates BDNF expression, which can be counteracted by SSRIs. SSRIs enhance serotonergic transmission and lead to an increase in BDNF levels, which is believed to contribute to their therapeutic effects in the treatment of major depressive disorder (MDD).