They're saying that memory loss is partly caused by downregulation of glutamate receptors. Receptors downregulate when there is a surfeit of the substance they link to. For example androgen receptors downregulate in athletes taking steroids, and opioid receptors down regulate in people taking heroin or opioid medication, so that more is needed for the same effect. Medically it is related to tachyphylaxis.
Now what if a person consumes high levels of monosodium glutamate (MSG). Does that cause downregulation and memory impairment?