Microglial activation in the rat brain following chronic antipsychotic treatment at clinically relevant doses

Neuroinflammation is increasingly implicated in the pathogenesis of Schizophrenia (SCZ). In addition, there is increasing evidence for a relationship between the dose and duration of antipsychotic drug (APD) treatment and reductions in grey matter volume. The potential contribution of microglia to these phenomena is however not yet defined. Adult rats were treated with a common vehicle, haloperidol (HAL, 2 mg/kg/day) or olanzapine (OLZ, 10 mg/kg/day) for 8 weeks via an osmotic mini-pump implanted subcutaneously. Microglial cells, identified by their Iba-1 immunoreactivity, were quantified in four regions of interest chosen based on previous neuroimaging data: the hippocampus, anterior cingulate cortex, corpus striatum, and secondary somatosensory cortex. Those cells were also analysed according to their morphology, providing an index of their activation state. Chronic APD treatment resulted in increased density of total microglia in the hippocampus, striatum, and somatosensory cortex, but not in the ACC. Importantly, in all brain regions studied, both APD tested led to a dramatic shift towards an amoeboid, reactive, microglial morphology after chronic treatment compared to vehicle-treated controls. These data provide the first in vivo evidence that chronic APD treatment at clinically relevant doses leads to microglial proliferation and morphological changes indicative of activated microglia in the naïve rat brain. Although caution needs to be exerted when extrapolating results from animals to patients, these data suggest a potential contribution of antipsychotic medication to markers of brain inflammation. Further investigation of the links between antipsychotic treatment and the immune system are warranted.