Da "feel-good" chemical in yer brain dat temporarily lifts yer mood whenever something positive/exciting/enjoyable happens, but den ya feel like a complete dope after da "high" wears off.
I loved ridin' da roller-coaster a whole bunch, but den afterwards I had a brain-chemical letdown --- sounds like a classic case of "Dopeamine crash" to me.
A word similar to the slang "Dope" meaning Cool or Great. It is often used for emphasis.
Frank: Hey Jimmy, Golf Course party tonight!
Jimmy: Dopeamine!
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