Evidence for altered excitatory and inhibitory tone in the post-mortem substantia nigra in schizophrenia

Abstract

Objectives: The substantia nigra (SN) receives glutamatergic and GABAergic inputs that regulate dopaminergic neuronal activity. Imaging studies have shown hyperactivity of the SN in schizophrenia (SZ) patients. We examined neurochemically defined inputs to the SN, synaptic density, and neuromelanin content that might contribute to or reflect this hyperexcitability.

Methods: Glutamatergic axon terminals were identified by the immunohistochemical localisation of vGLUT1 and vGLUT2; GABA inputs were identified by the immunohistochemical localisation of GAD67. Neuromelanin granules are visible in unstained sections and thus were assessed in unstained sections. Optical densitometry was measured to assess the density of vGLUT1, vGLUT2 or GAD67 immunolabelled axon terminals and neuromelanin granules. Electron microscopy was used to quantify synaptic and mitochondrial density.

Results: Compared to controls, SZ subjects had nonsignificant trends toward a decrease in vGLUT1, and an increase in both vGLUT2 and GAD67. vGLUT1 was negatively correlated with GAD67 in normal controls (NCs) and positively correlated in SZ subjects. A correlation of coefficient analysis showed a significant difference between the negative correlation in NCs and the positive correlation in SZ subjects. Frequency histograms showed the distribution of neuromelanin density was different in SZ subjects compared to NCs. Synaptic density data showed a decrease in inhibitory synapses in SZ subjects. Mitochondrial density was normal in SZ subjects.

Conclusions: Synaptic density alterations and the lack of a positive correlation between GAD67 and vGLUT1 could contribute to hyperactivity in the SN.

Keywords:

• Ultrastructure
• synapses
• mitochondria
• dopamine
• neuromelanin

Acknowledgements

The authors thank the Alabama Brain Collection and Maryland Brain Collection for brain tissue and diagnoses. We also thank Joy Roche for technical support with the electron microscopy.

Compliance with ethical standards

Our study used de-identified human post-mortem brain samples and was thus deemed ‘Not human Subjects’ by the University of Alabama Institutional Review Board. However, as former director of the Maryland Brain Collection and current Director of the Alabama Brain Collection, Dr. Roberts has been involved with getting informed consent from and phone interviews with family member of the donors. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

This manuscript does not contain any studies with animals performed by any of the authors.

Statement of interest

None to declare.

Additional information

Funding

Dr. Roberts received support from NIMH R01 MH66123. Dr. Roberts is a member of the Scientific Advisory Board of the Tourette’s Association of America, and serves as their Brain Tissue Coordinator. Mr. Mabry received the President’s Summer Research Scholarship from UAB. Dr. McCollum received an NRSA from NIMH (5F31MH098566-02). Ms. Bloom received the Summer in Biomedical Sciences award from UAB.