Pathological Basis for Deficient Excitatory Drive to Cortical Parvalbumin Interneurons in Schizophrenia

Abstract

Objective Deficient excitatory drive to parvalbumin-containing cortical interneurons is proposed to serve as a key neural substrate for altered gamma oscillations and cognitive dysfunction in schizophrenia. However, a pathological entity producing such a deficit has not yet been identified. In this study, we tested the hypothesis that cortical parvalbumin interneurons receive fewer excitatory synaptic inputs in individuals with schizophrenia. Method Fluorescent immunohistochemistry, confocal microscopy and post-image processing techniques were used to quantify the number of vesicular glutamate transporter 1-positive (VGlut1+)/postsynaptic density protein 95-positive (PSD95+) puncta (putative excitatory synapses) per surface area of parvalbumin-positive (PV+) or calretinin-positive (CR+) neurons in the dorsolateral prefrontal cortex from schizophrenia subjects and matched unaffected comparison subjects. Results Mean density of VGlut1+/PSD95+ puncta on PV+ neurons was significantly 18% lower in schizophrenia subjects. This deficit was not influenced by methodological confounds or schizophrenia-associated comorbid factors, not present in monkeys chronically exposed to psychotropic medications, and not present in CR+ neurons. The mean density of VGlut1+/PSD95+ puncta on PV+ neurons predicted the activity-dependent expression levels of parvalbumin and glutamic acid decarboxylase 67 (GAD67) in schizophrenia subjects but not in comparison subjects. Conclusions: Here we demonstrate for the first time that the density of excitatory synapses is lower selectively on parvalbumin interneurons in schizophrenia and predicts the activity-dependent down-regulation of parvalbumin and GAD67 levels. Because the activity of parvalbumin interneurons is required for the generation of cortical gamma oscillations and working memory function, these findings reveal a novel pathological substrate for cortical dysfunction and cognitive deficits in schizophrenia.

Developmental pruning of excitatory synaptic inputs to parvalbumin interneurons in monkey prefrontal cortex

“…These results might seem contradictory with prior findings that (i) PV expression is activity dependent (20,38,39), and (ii) the density of excitatory synapses on PV neurons positively predicts PV immunoreactivity levels in mouse hippocampus (44) and human DLPFC (28). However, we also found that the excitatory synapses remaining after pruning have higher apparent levels of VGlut1 and PSD95 proteins.…”

“…After washing three times in PBS, sections were mounted in Prolong Gold Antifade reagent (Life Technologies) and stored at 4°C until imaging. The specificity of these antibodies has been described previously (28).…”

“…Pre-and postsynaptic elements of excitatory synapses were defined as VGlut1+ and PSD95+ puncta, respectively. Masking of VGlut1+ and PSD95+ puncta and PV cell body was performed using the previously described method (SI Methods) (28). Excitatory synapses on PV interneurons were defined by the overlap of VGlut1+ and PSD95+ (VGlut1+/PSD95+) puncta within PV cell bodies (Fig.…”

“…Lastly, our findings from the primate DLPFC are from female monkeys and might not generalize to male monkeys. However, neither the density of excitatory inputs to PV neuron (28), the development of parvalbumin expression (57), nor the trajectory of synaptic pruning (8) differs between sexes in the DLPFC.…”

“…Deficits in cortical PV interneuron maturation have been linked to several neuropsychiatric disorders, including schizophrenia (58). In the DLFPC of individuals with schizophrenia, a pathogenic shift in ErbB4 splicing from the JM-b to JM-a variants has been associated with fewer excitatory synaptic inputs to PV interneurons (26,28), which could reflect an exaggerated pruning process in these neurons. Therefore, findings from our study support the view that schizophrenia is a neurodevelopmental disorder with disturbances in the normal maturation process of prefrontal inhibitory circuits (59).…”