What is the neurological basis of bipolar disorder? Many sources claim that bipolar disorder is associated with problems in the homeostasis of excitatory neurotransmitters.

Is this true? What are the leading theories about neurological causes for bipolar disorder?
The exact cause of bipolar disorder is unknown (source: NHS). It is believed that there are a number of interacting physical, environmental and social factors that may cause bipolar disorder to develop.


Chemical imbalance in the brain: Bipolar disorder is believed to be the result of chemical imbalances in the brain, and noradrenaline has been implicated (Young et al., 1994). Noradrenalin (norepinephrine) in the central nervous system regulates arousal and modulates reward circuitry and positive reinforcement. Many antidepressant drugs target noradrenergic projections to alleviate symptoms of depression (Trevino et al, 2012). There is evidence that episodes of mania may occur when levels of noradrenaline are too high, and episodes of depression may be the result of noradrenaline levels becoming too low (source: NHS). Reduced serotonin levels have been found too, which is a general hallmark of depression and not specifically related to bipolar disorder (Young et al., 1994).
Genetics: bipolar disorder tends to run in families (Lichtenstein et al., 2009. The family members of a person with the condition have an increased risk of developing it themselves. However, no single gene is responsible for bipolar disorder. Instead, a number of genetic and environmental factors are thought to act as triggers (sources: NHS and Mayo Clinic).
Triggers: A stressful circumstance or situation often triggers the symptoms of bipolar disorder, e.g., physical, sexual or emotional abuse, or
the death of a close family member. These life-altering events can cause episodes of depression at any time in a person's life. Bipolar disorder may also be triggered by physical illness, sleep disturbances, overwhelming problems in everyday life (sources: NHS and Mayo Clinic).


References
- Lichtenstein et al., Lancet (2009); 373(9659): 234–9
- Trevino et al, Encyclopedia of Human Behavior (2012), 2nd ed.
- Young et al., Biol Psychiatry (1994); 35(2): 121–7