I am wondering if euphoric states are among the signs of manic depression.

Before resorting to a psychiatric diagnosis, you might want to consider how much these mood swings are interfering with your life. Many very successful and creative people have alternating high and low periods (read Kay Redfield Jamison's Touched With Fire for more on this). Technically, many could be diagnosed with a mental illness, but should they be? The tendency toward high and low moods becomes a significant problem when life is affected by inappropriate or dangerous behavior, suicidal tendencies, social withdrawal, and so on. But not everyone who feels euphoric sometimes and depressed other times experiences much negative impact. And even those that do can often learn to be more accepting of their mood states, and less inclined to act them out. Although some people clearly suffer from mental imbalance that looks very ill, the psychiatric system has become much too quick to diagnose people who deviate from the norm as mentally unhealthy. Maybe they are just different.

I speak from experience, as someone who once took numerous psychiatric medications to control mood swings, and now stays comfortable with none. The trick (which meditation teaches) is learning to understand one's moods and to resist translating feelings into impulsive behavior. A tendency toward moodiness, comfortably managed, is not an illness. Often, it can be a source of inspiration.

A person is suffering from manic depression means suffering from alternating periods of exitement and melancholic( low sprited ) depression.Thus euphoric states i.e.states of well being & pleasent exitement may be visible as a symptom.