Brain and General Sections Whereabouts in the brain gets triggered/switched on when an addictive substance is ingested/taken. I'm doing an addiction study with a couple of colleagues and would like some help with it.
It depends on the type of addictive substances (i.e., opioid, amphetamine, LSD, etc). Different types bind to different sets of receptors of neurotransmitters (i.e., dopamine, serotonin, GABA, etc.) in the nervous system. And different receptors have different distribution in the nervous system. So the affected regions are different for different addictive substances.

The easy-to-read, entry-level references that you can use are probably How Drugs Affect the Brain and Central Nervous System and Drugs and the Brain. For in-depth references, you can search for the literature on each specific drug at NCBI.