What is a manic state?
What triggers it?
What changes in the brain during a manic state?
Is it physically possible to maintain such a state for extended periods of time or indefinetly?
All of the above but for hypomania.
I’m afraid I cant answer all your questions and im certainly no doctor but I can provide a starting point -
a manic state is a set of extreme behaviours that can manifest when someone is suffering from bipolar depression. A mani episode manifests itself by the person switching from being depressed and withdrawn to irritable and/or hyperactive.
I believe (though I am not 100% sure) that to be medically diagnosed as being in a manic state you have to have been exhibiting the symptoms for an extended period of time (over a week), though many people who feel they have experienced such a state will argue that its a very sporadic thing, happening for a few days at a time before returning to their depressed state.
hypomania is a more mild version of manic state. someone having a manic episode is experiencing such extreme emotions and expressing such extreme behaviour patterns that they often find it hard to interact at a comfortable level with those around them. People with hypomania however can function with others much beter, it is often described as a pleasurable experience (though technically people experiencing it dont realise it at the time, they tend to just assume that they have gotten over their depression and are acting ‘normal’).
I do not know how these states affect the brain I am afraid. In terms of triggers I am not sure either im afraid, I’ve always assumed a greatly stressful occurence but have no medical proof to back that up.
Manic state would be hard to physically maintain indefinately, it would be rather like trying to keep a child on a permanent sugar rush. Hypomania would be more maintainable but if left untreated tends to accelerate into a manic episode or dissipates back into depression sooner or later…
hope someone comes on with some proper knowledge to help you further 
From the DSM-IV:
A distinct period of abnormally and persistently elevated, expansive, or irritable mood, lasting at least 1 week (or any duration if hospitalization is necessary).
During the period of mood disturbance, three (or more) of the following symptoms have persisted (four if the mood is only irritable) and have been present to a significant degree:
- Inflated self-esteem or grandiosity.
- Decreased need for sleep (e.g., feels rested after only 3 hours of sleep).
- More talkative than usual or pressure to keep talking.
- Insomnia or hypersomnia nearly every day.
- Psychomotor agitation or retardation nearly every day (observable by others, not merely subjective feelings of restlessness or being slowed down).
- Flight of ideas or subjective experience that thoughts are racing.
- Distractibility (i.e., attention too easily drawn to unimportant or irrelevant external stimuli).
- Increase in goal-directed activity (either socially, at work or school, or sexually) or psychomotor agitation.
- Excessive involvement in pleasurable activities that have a high potential for painful consequences (e.g., engaging in unrestrained buying sprees, sexual indiscretions, or foolish business investments)
The mood disturbance is sufficiently severe to cause marked impairment in occupational functioning or in usual social activities or relationships with others, or to necessitate hospitalization to prevent harm to self or others, or there are psychotic features.
The symptoms are not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication, or other treatment) or a general medical condition (e.g., hyperthyroidism).
[quoteWhat triggers it?[/quote]
Nobody knows.
Scientists aren’t sure. There are guesses about neurotransmitter levels, but no concrete answers.
Extended periods of time, yes. Manic states lasting for many months are not uncommon among those with bipolar disorder. Indefinitely? Some bipolar people have a tendency to be more manic than depressive, but I have never heard of or read about the possibility of being persistently manic.
There are documented cases in the literature of people dying from exhaustion after extended manic episodes. These would be the people who don’t sleep, neglect to eat and have seemingly boundless energy- it’s simply not possible to keep that up for more than a week or two.