Background Prefrontal dopamine D1 receptor (D1R) mediates behavior related to anxiety, reward and memory, and is involved in inflammatory processes, all of which are affected in bipolar disorder. Interleukin-6 (IL-6), a pro-inflammatory cytokine, is increased in patients with bipolar disorder in plasma samples, imaging studies and postmortem tissue and is an indicator for an inflammatory state. We could previously show that lentiviral overexpression of D1R in the medial prefrontal cortex (mPFC) of male adult rats and its termination induces bipolar disorder-like behavior. The purpose of this study was to investigate anxiety and the role of the immune system, specifically IL-6 positive neurons in this animal model. Due to its high density of inflammatory mediator receptors and therewith sensibility to immune activation, the hippocampus was investigated. Methods Expression of the gene for D1R in glutamatergic neurons within the mPFC of male, adult rats was manipulated through an inducible lentiviral vector. Animals over-expressing the gene (mania-like state), after termination of the expression (depressive-like) and their respective control groups were investigated. Anxiety behavior was studied in the elevated plus maze and marble burying test. Furthermore, IL-6-positive cells were counted within several subregions of the hippocampus. Results D1R manipulation in the mPFC had only mild effects on anxiety behavior in the elevated plus maze. However, subjects after termination buried more marbles compared to D1R over-expressing animals and their respective control animals indicating elevated anxiety behavior. In addition, animals in the depressive-like state showed higher numbers of IL-6 positive cells reflecting an elevated pro-inflammatory state in the hippocampus, in the CA3 and dentate gyrus. Consistently, inflammatory state in the whole hippocampus and anxiety behavior correlated positively, indicating a connection between anxiety and inflammatory state of the hippocampus. Conclusions Behavioral and neurobiological findings support the association of manipulation of the D1R in the mPFC on anxiety and inflammation in the hippocampus. In addition, by confirming changes in the inflammatory state, the proposed animal model for bipolar disorder has been further validated.
Background - Prefrontal dopamine D1 receptor (D1R) mediates behavior related to anxiety, reward and memory, and is involved in inflammatory processes, all of which are affected in bipolar disorder. Patients with bipolar disorder show alternations in the dopamingergic and immune system. Interleukin-6 (IL-6), a pro-inflammatory cytokine, is increased in plasma samples, imaging studies and post mortem tissue. Manipulation of the D1R in the medial prefrontal cortex (mPFC) e.g. results in BD-like behavior. The purpose of the study is the investigation of the influence of D1R over-expression and its termination on the immune system and anxiety behavior in rats.Methods – Expression of the gene for D1R in glutamatergic neurons within the mPFC of male, adult rats was manipulated through an inducible (Tet.On) lentiviral vector. Anxiety behavior was studied in the elevated plus maze and marble burying test in ‘ON’ (D1R over-expression) and ‘OFF’ (termination of D1R over-expression) states. IL-6-positive cells were counted to identify the inflammatory state within several subregions of the hippocampus.Results - D1R ‘OFF’ subjects buried more marbles compared to D1R ‘ON’ and their respective control animals indicating an increased anxiety behavior. D1R ‘OFF’ animals reflected an elevated pro-inflammatory state in the hippocampus, in the CA3 and dentate gyrus especially. Consistently, inflammatory state in the whole hippocampus and anxiety behavior correlated positively, indicating a connection between anxiety and inflammatory state of the hippocampus.Conclusions – Behavioral and molecular findings support the association of D1R’s impact on anxiety and inflammation. In addition, by confirming an involvement of IL-6, the new animal model for bipolar disorder has been further validated.
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