MMP9 and schizophrenia: The present study found that psychosocial stress that was induced in the resident-intruder paradigm and low levels of MMP-9 were collectively required to evoke negative-like symptoms of schizophrenia in mice, indicated by: (i) an increase in immobility in the tail suspension test, reflecting by a decrease in the ability of stressed heterozygous mice to cope with the adverse conditions; and (ii) a shorter interaction time with a gender-matched conspecific mouse, reflecting social withdrawal.