Yes, MDD can affect one's memories/concentration because of dopamine and other neurochemicals involved with the info recall.
There are other conditions that too can interfere such as - sleep disorders, ADD, neurological (organic) ones, thyroid and hormonal changes, low levels of B vitamin to name a few.
In order to address the memory problems, if in fact the depression is the main cause behind, one's brain chemistry/mood would have to be balanced generally with psychotropic medication as well as CBT. There are also natural options such as Pregnenolone or Piracetam 800, Omega fatty acids, B vitamin shots or supplements (depending on level of B vitamin through full blood panel test). Of course, it won't hurt to rule out other causes as some of those mentioned above in order to receive the proper treatment matching the cause for this.
Another option that does not involve medication is called brain wave synchronization.
Gamma brain waves have been found to be connected to increase in concentration/recall. Changing one's brain waves can lead to physiological/psychological changes.
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Feel free to clarify if this is what you were inquiring about by using reply. As far as recalling forgotten or suppressed/repressed memories, hypnosis and NLP (neurolinguistic programming can be used).