discovery of dopamine as a neurotransmitter

[cyble]

i'm afraid that i can't help on this one, but i have an additional 
question: are levels of various neurotransmitters hereditary? in other 
words, have there been any studies suggesting that normal/abnormal 
levels of chemicals like dopamine and serotonin have a genetic basis? 
i'm doing a paper on the hereditary nature of bipolar disorder and 
schizophrenia, and i'm intrigued by this part.
thanks!

brooke