Opioid substances mimic the effects of endogenous opioids within the brain, binding to opioid receptors.[1] A key goal for research is to better understand whether common pathways underpin all substance addictions, in order to design therapies for substance-use disorders.[1]
Reference:
[1] Nestler EJ. Is there a common molecular pathway for addiction? Nat Neurosci 2005; 8 (11): 1445–1449.