Epigenetic and Neurochemical Correlates of Bipolar Disorder

Bipolar disorder (BD) is a chronic and multifactorial psychiatric condition characterized by recurrent episodes of mania and depression, affecting approximately 2-3% of the global population. Despite extensive research, the precise pathophysiology of BD remains incompletely understood. Studies have shown that the pathophysiology of BD involves a combination of genetic, neurobiological, and environmental factors.

Classification of Bipolar Spectrum Disorders

They are generally divided into:

• BDI: This condition is characterized by alternating manic and depressive episodes.
• BDII: This condition is characterized by alternating depressive and hypomanic episodes.
• Cyclothymia: This condition is characterized by the alternation of mild depressive episodes and mild hypomanic episodes.
• BD not otherwise specified: in this condition, patients have symptoms of mania or hypomania that are too low or too brief to meet the diagnostic criteria for a syndrome.

Research on biomarkers for BD is ongoing, but no consensus has been reached on clinically diagnostic biomarkers. Recent findings also suggest that biomarkers may vary between early and late stages of BD. Neurochemical studies have shown that various neurotransmitters, such as serotonin, dopamine, glutamate (Glu) and GABA, may be involved in the pathophysiology of BD.

Epigenetics of BD: DNA Methylation of IL-1B (an Inflammatory Cytokine) Epigenetics involves hereditable reversible changes in gene function without altering the DNA sequence, such as DNA methylation, histone modification, and non-coding RNA regulation. While initially studied in normal development, epigenetic processes are now linked to various diseases including neurological and psychiatric disorders. These mechanisms reveal how gene-environment interactions contribute to the pathogenesis and comorbidity of such disorders.

DNA methylation is an epigenetic mechanism where a methyl group is added to the C5 position of cytosine, forming 5-methylcytosine. While gene expression in eukaryotes is regulated through various mechanisms, DNA methylation a common epigenetic tool used by cells to silence genes, effectively locking them in the "off" position.

IL 1B is a proinflammatory cytokine and has been implicated in the pathophysiology of BD. A meta-analysis revealed significantly higher peripheral IL-1β levels in patients with BD compared to controls, which supports inflammatory hypothesis of mood disorders.

Studies have found an inverse correlation between DNA methylation of the IL1B, IL6, and IL8 gene promoters and their corresponding mRNA levels, with this inverse correlation being most significant for IL1B.

Serotonin Receptors Serotonin (5-hydroxytryptamine, 5-HT) is a widely known monoamine neurotransmitter that regulates neural activity and various neuropsychological processes. Research on serotonin and its receptors continues to provide new biological insights with medical relevance across various organ systems. Serotonin plays a crucial role in regulating nearly all brain functions, and disruptions in the serotonergic system have been implicated in the pathogenesis of numerous psychiatric and neurological disorders.

The role of serotonin in the pathogenesis of mood disorders, particularly major (unipolar) depression, has been extensively studied, with most research indicating decreased central serotonergic function in major depressive disorder. However, the role of serotonin in the pathogenesis of BD has received comparatively less attention. The presence of manic episodes in BD suggests different or additional neurochemical abnormalities and highlights the need for studies of BD.

The serotonin 2A receptor (5-HT2A) is one of several subtypes of 5-HT receptors and has been implicated in mental disorders with complex and still poorly understood etiologies. It plays a role in cognitive processes, such as learning and memory, as well as neurogenesis.

Given the widespread distribution of this receptor in brain regions responsible for cognitive functions and social interaction, it is involved in disorders where these functions are impaired. Conditions such as schizophrenia, depression, obsessive-compulsive disorder (OCD), and attention-deficit hyperactivity disorder (ADHD) have been linked to alterations in the 5-HT2A receptor. Numerous drugs are designed to target this receptor.

Glutamate Glutamate is the primary excitatory neurotransmitter in the nervous system, with its pathways closely linked to those of other neurotransmitters. Glutamate receptors are present throughout the brain and spinal cord, in both neurons and glia. Dysfunction in glutamate signaling has significant effects in disease and injury.

Glutamate system dysfunction has been implicated in various mood disorders, with several studies highlighting reduced glutamate levels in certain brain regions of patients with major depressive disorder (MDD). In contrast, research on BD has yielded mixed results, with some studies showing altered glutamate activity while others report no significant changes.