Glia and immune cell signaling in bipolar disorder: insights from neuropharmacology and molecular imaging to clinical application

Abstract

Bipolar disorder (BD) is a debilitating mental illness characterized by severe fluctuations in mood, sleep, energy and executive functioning. Pharmacological studies of selective serotonin reuptake inhibitors and the monoamine system have helped us to clinically understand bipolar depression. Mood stabilizers such as lithium and valproic acid, the first-line treatments for bipolar mania and depression, inhibit glycogen synthase kinase-3 beta (GSK-3β) and regulate the Wnt pathway. Recent investigations suggest that microglia, the resident immune cells of the brain, provide a physiological link between the serotonin system and the GSK-3β/Wnt pathway through neuroinflammation. We review the pharmacological, translational and brain imaging studies that support a role for microglia in regulating neurotransmitter synthesis and immune cell activation. These investigations provide a model for microglia involvement in the pathophysiology and phenotype of BD that may translate into improved therapies.

Figure 1

Proposed role of activated microglia in the modulation of inflammatory mediators, neurotransmitter synthesis and synaptic plasticity in bipolar disorder (BD). CRP, C-reactive protein; IDO, indoindoleamine 2,3-dioxygenase; 5-HT-serotonin; RNS, reactive nitrogen species; ROS, reactive oxygen species.

Figure 2

The Wnt/GSK- 3β pathway.

Figure 3

Microglia express TSPO and Beta-catenin that connect monoamine synthesis and the Wnt pathway. (5-HTT, serotonin transporter; CRP, C-reactive protein; 5-HT, serotonin; TSPO, translocator protein).