Impaired hippocampal–prefrontal synchrony in a genetic mouse model of schizophrenia

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Abstract
The 22q11.2 microdeletion is one of the most reliable known genetic risk factors for schizophrenia. Mice with a disruption in the equivalent chromosomal region have problems with working memory, one feature of schizophrenia. Sigurdsson et al. show that these mice also have disruptions in synchronous firing between prefrontal cortex and hippocampal neurons, a phenomenon that has been linked to learning and memory, and which is also disrupted in schizophrenia patients. These findings suggest that disruption of communication between these brain regions may underlie schizophrenia, and efforts to ameliorate this disruption may lead to novel treatments. A deletion on human chromosome 22 (22q11.2) is one of the largest genetic risk factors for schizophrenia. Mice with a corresponding deletion have problems with working memory, one feature of schizophrenia. It is now found that these mice also show disruptions in synchronous firing between neurons of the prefrontal cortex and of the hippocampus, an electrophysiological phenomenon that has been linked to learning and memory and which is also thought to be disrupted in schizophrenia patients. Abnormalities in functional connectivity between brain areas have been postulated as an important pathophysiological mechanism underlying schizophrenia1,2. In particular, macroscopic measurements of brain activity in patients suggest that functional connectivity between the frontal and temporal lobes may be altered3,4. However, it remains unclear whether such dysconnectivity relates to the aetiology of the illness, and how it is manifested in the activity of neural circuits. Because schizophrenia has a strong genetic component5, animal models of genetic risk factors are likely to aid our understanding of the pathogenesis and pathophysiology of the disease. Here we study Df(16)A+/– mice, which model a microdeletion on human chromosome 22 (22q11.2) that constitutes one of the largest known genetic risk factors for schizophrenia6. To examine functional connectivity in these mice, we measured the synchronization of neural activity between the hippocampus and the prefrontal cortex during the performance of a task requiring working memory, which is one of the cognitive functions disrupted in the disease. In wild-type mice, hippocampal–prefrontal synchrony increased during working memory performance, consistent with previous reports in rats7. Df(16)A+/– mice, which are impaired in the acquisition of the task, showed drastically reduced synchrony, measured both by phase-locking of prefrontal cells to hippocampal theta oscillations and by coherence of prefrontal and hippocampal local field potentials. Furthermore, the magnitude of hippocampal–prefrontal coherence at the onset of training could be used to predict the time it took the Df(16)A+/– mice to learn the task and increased more slowly during task acquisition. These data suggest how the deficits in functional connectivity observed in patients with schizophrenia may be realized at the single-neuron level. Our findings further suggest that impaired long-range synchrony of neural activity is one consequence of the 22q11.2 deletion and may be a fundamental component of the pathophysiology underlying schizophrenia.