I quickly skimmed some info and found this, which should at least give you an idea. I'm really hoping Birdgirl can clear things up a bit, also.

Advances in neurobiology have provided a biological model for
explaining substance abuse.[19, 21, 33] Drugs of abuse affect the
production of dopamine, a neurotransmitter associated with response and
reward and now held to explain the compulsive self-administration of
drugs.[10, 19, 21, 33]

However, cannabinoid receptors are not located on dopamine
producing neurons.[17] In vivo microdialysis has shown that unlike other
drugs[10] THC, the active chemical in marijuana, does not affect striatal
dopamine levels.[55] (Earlier research that had indicated a cannabinoid
effect on dopamine was found to be strain-specific, and has not been
replicated in other animal studies.[1, 17, 33])

Heroin (schedule I), cocaine (schedule II) and amphetamines (schedule III) all affect dopamine production. Marijuana, cannabinoids,
and THC do not. Where shall we schedule marijuana under existing
provisions of controlled substances act?

- Izenwasser, S., Kornetsky, C. (1992) "Brain-Stimulation Reward: A
Method For Assessing the Neurochemical Bases Of Drug-Induced Euphoria." In Watson, R.A. (1992) Drugs of Abuse and Neurobiology. Boca Raton: CRC -Press.