The hippocampal formation is one of the most extensively studied regions of the brain, with well-described anatomy and basic physiology; moreover, aspects of human memory mediated by the hippocampus are well characterized. In schizophrenia, alterations in hippocampal anatomy, perfusion, and activation are consistently reported; impairments in declarative memory function, especially in the flexible use of event memories (e.g., in the service of memory-based inference), are common. Postmortem molecular changes suggest a selective reduction in glutamate transmission in the dentate gyrus and in its efferent fibers, the mossy fiber pathway. A reduction in dentate gyrus glutamatergic output and in its information processing functions could generate two co-occurring outcomes in the hippocampus: 1) a change in homeostatic plasticity processes in cornu ammonis 3 (CA3), accompanied by increased activity due to reduced afferent stimulation from the dentate gyrus onto CA3 neurons, a process that could increase the pattern completion functions of CA3, and 2) the loss of mnemonic functions specific to the dentate gyrus, namely pattern separation, a change that could increase the prevalence of illusory pattern completion and reduce discrimination between present and past experiences in memory. The resulting increase in "runaway" CA3-mediated pattern completion could result in cognitive "mistakes," generating psychotic associations and resulting in memories with psychotic content. Tests of this model could result in novel approaches to the treatment of psychosis and declarative memory alterations and in novel animal preparations for basic schizophrenia research.

Post mortem human brain tissue is used for the study of many different brain diseases. A key factor in conducting postmortem research is the quality of the tissue. Unlike animal tissue, whose condition at death can be controlled and influenced, human tissue can only be collected naturalistically. This introduces potential confounds, based both on pre-and postmortem conditions, that may influence the quality of tissue and its ability to yield accurate results. The traditionally recognized confounds that reduce tissue quality are agonal factors (e.g., coma, hypoxia, hyperpyrexia at the time of death), and long postmortem interval (PMI). We measured tissue quality parameters in over 100 postmortem cases collected from different sources and correlated them with RNA quality (as indicated by the RNA Integrity Number (RIN)) and with protein quality (as measured by the level of representative proteins). Our results show that the most sensible indicator of tissue quality is RIN and that there is a good correlation between RIN and the pH. No correlation developed between protein levels and the aforementioned factors. Moreover, even when RNA was degraded, the protein levels remained stable. However, these correlations did not prove true under all circumstances (e.g. thawed tissue, surgical tissue), that yielded unexpected quality indicators. These data also suggest that cases whose source was a Medical Examiner's office represent high tissue quality.

Hippocampal dysfunction in schizophrenia is widely acknowledged, yet the mechanism of such dysfunction remains debated. In this study we investigate the excitatory and inhibitory hippocampal neurotransmission using two complementary methodologies, proton magnetic resonance spectroscopy (MRS) and tissue biochemistry, sampling individuals with schizophrenia in vivo and postmortem hippocampal tissue in vitro. The results show significantly lower glutamate concentrations in hippocampus in schizophrenia, an in vivo finding mirrored by lower GluN1 protein levels selectively in the dentate gyrus (DG) in vitro. In a mouse model with a DG knockout of the GRIN1 gene, we further confirmed that a selective decrease in DG GluN1 is sufficient to decrease the glutamate concentrations in the whole hippocampus. Gamma-aminobutyric acid (GABA) concentrations and GAD67 protein were not significantly different in hippocampus in schizophrenia. Similarly, GABA concentrations in the hippocampi of mice with a DG knockout of the GRIN1 gene were not significantly different from wild type. These findings provide strong evidence implicating the excitatory system within hippocampus in the pathophysiology of schizophrenia, particularly indicating the DG as a site of pathology.

Altered markers of cortical GABA neurotransmission are among the most consistently observed abnormalities in postmortem studies of schizophrenia. The altered markers are particularly evident between the chandelier class of GABA neurons and their synaptic targets, the axon initial segment (AIS) of pyramidal neurons. For example, in the dorsolateral prefrontal cortex of subjects with schizophrenia immunoreactivity for the GABA membrane transporter is decreased in presynaptic chandelier neuron axon terminals, whereas immunoreactivity for the GABAA receptor α2 subunit is increased in postsynaptic AIS. Both of these molecular changes appear to be compensatory responses to a presynaptic deficit in GABA synthesis, and thus could represent targets for novel therapeutic strategies intended to augment the brain’s own compensatory mechanisms. Recent findings that GABA inputs from neocortical chandelier neurons can be powerfully excitatory provide new ideas about the role of these neurons in the pathophysiology of cortical dysfunction in schizophrenia, and consequently in the design of pharmacological interventions.

No abstract