Bipolar Disorder: The Role of the Kynurenine and Melatonergic Pathways

Bipolar disorder (BD) is a long-recognized severe and common psychiatric disorder, with a complex and often diverse range of presentations. BD is a heterogenous disorder that has traditionally, if rather simply, been defined by the recurrences of manic and depressive episodes, and presents with numerous immune-inflammatory and circadian/sleep abnormalities. A number of different lines of research have investigated the biological underpinnings of BD and demonstrate a heritability of about 80-90%. This genetic contribution is thought to be mediated by a wide array of genetic factors, rather than being strongly influenced by a couple of genes. In this context, a clearer formulation of the biological underpinnings of BD is needed in order to encompass the diverse effects of multiple susceptibility genes. The biological underpinnings of BD includes work that has focussed on the role played by increased immune inflammatory activity, particularly changes in pro-inflammatory cytokines, as measured both centrally and systemically. Changes in immune- inflammatory activity are intimately associated with alterations in levels of oxidative and nitrosative stress (O&NS), which are increased in BD. Many of the neuroregulatory changes driven by O&NS and immune-inflammatory activity are mediated by the tryptophan catabolite (TRYCAT) pathways, with changes in TRYCATs being evident both centrally and peripherally. A consequence of increased pro-inflammatory cytokines, is their induction of indoleamine 2,3-dioxygenase (IDO), which takes tryptophan away from serotonin, Nacetylserotonin and melatonin synthesis, driving it to the synthesis of neuroregulatory TRYCATs. Most work exploring such changes has emphasized the role of TRYCATs in enhancing or decreasing neuronal activity. However, a relatively overlooked consequence of cytokine induced IDO and TRYCAT pathway activation is the impact that this has on aryl hydrocarbon receptor (AhR) activation and in decreasing melatonergic pathway activity. Melatonin is classically associated with night-time synthesis by the pineal gland, in turn regulating circadian rhythms. However, melatonin is produced by many, if not all mitochondria containing cells, with consequences for gut regulation, as well as glia and immune cell reactivity. The melatonergic pathways are genetic susceptibility factors for BD. Interactive changes in O&NS, immune-inflammatory activity, TRYCATs and the melatonergic pathways form an emerging biological perspective on the etiology, course and management of BD. Here, we review such changes in BD, and how this better integrates the diverse array of BD presentations and comorbidities, including addiction and cardiovascular disorders as well as decreased life-expectancy. We then look at the future directions such research may take.