A formal model of mood disorders based on the neural circuit dynamics of the triple network model

Psychiatric diagnoses are based on consensus and are not related to pathophysiology, leading to confusion in treatment and in basic and clinical psychiatric research. The pathology of mood disorders arises from the intrinsic function and interactions between key neural circuits of the triple network. These circuits are the central executive network composed of the dorsolateral prefrontal cortex and posterior parietal cortex; the default mode network consisting of the dorsal medial prefrontal cortex, posterior cingulate/precuneus and angular gyrus and the salience network made up of the anterior insula, dorsal anterior cingulate cortex associated with subcortical limbic nodes including the amygdala. In this work, we develop a formal model using nonlinear dynamics and network theory, which captures the dynamic interactions of these three brain networks, allowing us to illustrate how various mood disorders can arise. Recurrent circuit dynamics are modeled on the physio-dynamics of a single neural component and is dependent on a balance of total input (feedforward and feedback) and the sensitivity of activation of its neural components. We use the average percentage of maximal firing rate frequency as a measure of network activity over long periods, which corresponds to fMRI activity. While the circuits function at moderate rates in euthymia, depressive symptoms are due to hypoactivity of the CEN and SN and hyperactivity of the DMN. Mania arises from a hyperactive SN with hypofunction of the CEN and moderate to high activity of the DMN. Functional abnormalities arise from genetic or epigenetic changes, affecting either the weight of neural interconnections or the sensitivity of activation of neurons comprising the network. Decreased excitation in unipolar depressive states is caused by diminished dendritic branches and decreased density of AMPA and NMDA receptors or a decrease in glutamate released by presynaptic neurons. All bipolar states result from heightened neural sensitivity due to altered sodium, calcium, or potassium channel conductance. Our formal model of mood disorders is consistent with fMRI studies, genetic research, as well as preclinical and clinical studies.