What evidence exists on long-term damage to the central nervous system caused by ketamine (street name k) use?

Couple of studies: This study shows the effect in humans: http://www.Ncbi.Nlm.Nih.Gov/pubmed/21161184 and this one in mice http://www.Ncbi.Nlm.Nih.Gov/pmc/articles/pmc3119682/, and changes in gene and protein expression in chronic ketamine usage. As experimentation in humans with this potentially lethal drug is not a good idea, we will likely have to depend on animal models for study.

Verbal anmemory loss: Not much research has been done on the long term use of this drug. However, what seems to exist is that there are affects on the verbal and short term memory systems. Other research seems to point to urinary problems.