Developing novel therapeutics to treat the core cognitive deficits (e.g. working memory) in individuals with schizophrenia (SZ) remains a major unmet need. Working memory depends, in part, on properly timed activation of layer 3 pyramidal neurons in the dorsolateral prefrontal cortex (DLPFC). In particular, this DLPFC microcircuit appears to be crucial for filtering out distracting stimuli during working memory tasks. Convergent experimental and computational data suggest that successful filtering of distractors occurs through inhibition onto pyramidal neuron dendrites by somatostatin (SOM)-expressing GABA neurons. Thus, the widely replicated findings of lower SOM mRNA expression in gray matter samples from SZ subjects implicate altered GABA signaling from SOM neurons as potential contributors to the neural basis for working memory dysfunction in the illness. The few studies which have been conducted on SOM neurons in SZ consistently demonstrate lower SOM mRNA expression in total gray matter DLPFC samples as well as a lower density of neurons positive for SOM mRNA. These prior findings raise key questions about these alterations: first, does a lower density of SOM-expressing neurons in the DLPFC of SZ subjects reflect fewer total SOM neurons in the illness or a failure of a subset of these neurons to express SOM mRNA? Second, do SOM neurons exhibit a diminished capacity to synthesize GABA, indexed by lower expression of the principal GABA synthesizing enzyme, GAD67? We propose to address these questions utilizing novel multiplex fluorescent in-situ hybridization, relying on the use of Sox6 as a proxy marker for SOM neurons while measuring SOM and GAD67 mRNA levels in a cell-type specific manner. We predict that the density of SOM neurons is unaltered in SZ and that SOM and GAD67 mRNA levels per neuron are lower (Aim 1). Third, upstream factors which contribute to lower SOM mRNA in SOM neurons residing in layers 2-superficial 3, but not those not deep 6-white matter, are unknown, but likely reflect microcircuit abnormalities. Both SOM and GAD67 mRNA expression levels are positively associated with activity levels. Layer 3 pyramidal neurons, which furnish the majority of excitatory inputs onto SOM neurons in the superficial layers, but not those in deeper layers or white matter, are hypoactive in the illness. Thus, we will test the hypothesis that SOM neuron abnormalities in SZ are secondary to weaker excitatory drive from layer 3 pyramidal neurons. We will perform RNAseq on SOM neurons in the superficial gray matter layers and superficial white matter and measure levels of activity-associated transcripts, predicting lower expression of these transcripts in SOM neurons from the superficial gray matter (Aim 2). A primate model of the DLPFC microcircuitry will be employed to provide direct experimental evidence that lower activity of layer 3 pyramidal neurons contributes to the observed SOM alterations in SZ (Aim 3). The results of these studies will offer insight into the nature of SOM neuron abnormalities and the basis for these alterations in SZ, guiding future novel therapeutic strategies to improve SOM neuron function and working memory in the illness.
The ability to successfully perform key cognitive functions, such as working memory, are thought to rely on the ability of somatostatin interneurons in layer 2/superficial 3 of the dorsolateral prefrontal cortex to successfully filter out potential distracting stimuli. Thus, the reported alterations in these somatostatin neurons in schizophrenia might represent the neural substrate for impaired distractor filtering and deficits in working memory. In this proposal, we will investigate the nature and basis of somatostatin neuron alterations in schizophrenia, providing key insights into novel therapeutic strategies to improve the function of these neurons and working memory in the illness.