Decreased numerical density of CA3 hippocampal mossy fiber synapses in schizophrenia

The CA3 region of the hippocampus is unique in its connectivity, its role in cognitive maintenance, and its great vulnerability in schizophrenia. The down regulation of the expression and binding activity of glutamate receptors was revealed in the CA3 hippocampal region and may be attributed to cognitive disturbances in schizophrenia. Our previous study demonstrated that only schizophrenics with predominantly positive (but not predominantly negative) symptoms had smaller-sized branched spines (thorny excrescences) of CA3 pyramidal neurons and fewer synaptic contacts formed by dentate mossy fiber terminals (MFT-synapses). In the present study, we used an unbiased stereological physical dissector method to verify whether the numerical density of MFT-synapses is altered in schizophrenia. A morphometric study was performed in 10 normal controls and eight age-matched cases with chronic schizophrenia, including five cases with predominantly positive and three with predominantly negative symptoms. Schizophrenic cases had a significantly reduced numerical density of MFT-synapses (-25%, P < 0.01) compared with the control group. The decrease was similar in schizophrenic subgroups with predominantly positive and predominantly negative symptoms. No effects of postmortem delay, age, duration of disease, and neuroleptic exposure were found. Taken together with our previous results, the data suggest that the decrease of numerical density of MFT-synapses may be the result of different mechanisms in schizophrenics with predominantly positive and predominantly negative symptoms.