Objective:  In order to identify whether the mechanisms associated with neurotransmitter release are involved in the pathologies of bipolar disorder and schizophrenia, levels of presynaptic [synaptosomal‐associated protein‐25 (SNAP‐25), syntaxin, synaptophysin, vesicle‐associated membrane protein, dynamin I] and structural (neuronal cell adhesion molecule and alpha‐synuclein) neuronal markers were measured in Brodmann's area 9 obtained postmortem from eight subjects with bipolar I disorder (BPDI), 20 with schizophrenia and 20 controls.

Methods:  Determinations of protein levels were carried out using Western blot techniques with specific antibodies. Levels of mRNA were measured using real‐time polymerase chain reaction.

Results:  In BPDI, levels of SNAP‐25 (p < 0.01) and synaptophysin (p < 0.05) increased. There were no changes in schizophrenia or any other changes in BPDI. Levels of mRNA for SNAP‐25 were decreased in BPDI (p < 0.05).

Conclusion:  Changes in SNAP‐25 and synaptophysin in BPDI suggest that changes in specific neuronal functions could be linked to the pathology of the disorder.