¹Wakayama Medical University, School of Pharmaceutical Sciences, Wakayama, Japan, ²Niigata University Brain Research Institute, Department of System Pathology for Neurological Disorders, Niigata, Japan

Background: Auditory hallucinations are a pathological hallmark of schizophrenia; however, the biological mechanisms underlying these symptoms remain to be elucidated. Brain-imaging studies in patients suggest that impaired frontotemporal connectivity and abnormal tonotopic organization in the auditory cortex may contribute to auditory hallucinations. To gain insight into the etiology and neuropathology of schizophrenia, we have been characterizing animal models established by neonatal administration of inflammatory cytokines, such as epidermal growth factor (EGF), based on the cytokine hypothesis of schizophrenia (Watanabe Y et al., Psychiatry Clin Neurosci 64:217, 2010).

Aims & Objectives: Using 3D whole-brain imaging of neural circuits and live macro-scale calcium imaging, we investigated the mechanisms underlying auditory dysfunction in a rat model of schizophrenia. This model, generated by neonatal EGF treatment, is known to exhibit multiple pathophysiological deficits in the auditory system, including abnormalities in mismatch negativity (MMN) for tone and duration, auditory steady-state responses (ASSR), P300, and acoustic startle responses (ASR) (Inaba H et al., Schizophr Bull Open 2:sgaa070, 2021).

Method: In adulthood, one cohort of EGF-treated rats as neonates received injections of the viral tracer AAV-tdTomato into multiple cortical regions. Following tissue clearing, three-dimensional mapping of axonal projections from each cortical region was carried out and compared with that of control rats. A second cohort was established using the transgenic rats expressing the GCaMP8 calcium sensor in pyramidal neurons. Under anesthesia, we recorded the temporal and spatial patterns of neuronal activity across the auditory cortex in response to a range of sound stimuli and compared these responses with those of control rats.

Results: Whole-brain clearing and imaging revealed impaired axonal projections originating from the auditory cortex—but not from the frontal or visual cortices—in EGF-treated rats compared with controls. Calcium imaging demonstrated that the primary auditory cortex (AI), but not the anterior auditory field (AAF) or suprarhinal auditory field (SRAF), exhibited reduced responsiveness to low-frequency tones (1–3 kHz), consistent with findings in patients with schizophrenia (Gaëlle E et al., NPJ Schizophrenia 5:16, 2019). In addition, spatial response patterns in the auditory cortex evoked by white-noise stimulation were markedly distorted in this model.

Discussion & Conclusions: The reasons why the circuit development of the brain auditory system is particularly vulnerable to the cytokine remain to be elucidated. However, these imaging findings suggest that inflammatory cytokine activity during prenatal and perinatal development disrupts normal axon guidance and impairs intra- and inter-cortical connectivity within the auditory system, ultimately leading to auditory dysfunction and hallucination-like phenotypes.
Acknowledgements; This study is supported by KAKENHI grants (23K23991, 21K18242) from JSPS as well as by the collaborative research projects of Niigata University BRI.