The role of schizophrenia susceptibility genes in associative learning
Schizophrenia is highly heritable, indicating that a large proportion of one’s susceptibility to developing the disorder is attributable to genetics. Recent large-scale genomic studies have revealed that genetic variants in patients with schizophrenia affect genes involved in synaptic plasticity processes, which are required for learning and memory, including genes encoding protein complexes associated with the NMDA receptor and the postsynaptic density. Further evidence suggests that associative learning may be particularly affected, although it is unclear which components of this cognitive process are implicated in schizophrenia.\ud The present studies investigated the relationship between particular phases of associative learning, represented by the consolidation, retrieval and extinction of contextual fear memory in rats, with genetic variants, psychoactive drugs and postsynaptic density proteins associated with schizophrenia. I tested associative learning-related gene expression datasets for enrichment in genetic copy number variants from a large cohort of patients with schizophrenia and demonstrated that only genes associated with extinction learning are enriched in patient variants (Chapter 3). I report that fear extinction in rats was impaired by administration of the NMDA antagonist and psychotomimetic, ketamine (Chapter 4). The expression of activity- induced, postsynaptic density products of the Homer1 gene, which has been linked to psychiatric disease, was differentially regulated in specific hippocampal subregions following extinction learning (Chapter 5), and the effect of a partial knockdown of these genes during different phases of associative learning was investigated (Chapter 6).\ud These results build on clinical studies linking abnormalities in associative and, specifically, extinction learning with schizophrenia and support the notion that genetic variants associated with the disorder impact particular cognitive domains. My findings are consistent with the theory that altered inhibitory-type learning processes contribute to the manifestation of schizophrenia.