Opium

Opium produces its effects by activating specific G protein-coupled receptors in the brain, spinal cord, and peripheral nervous system.‍​‍‍‍​‍‍‍‍‍‍‍‍​‍ There are three main classes of opioid receptors: δ-opioid, κ-opioid, and μ-opioid. Opium generates agonist activity that subsequently opens potassium channels and prevents the opening of voltage-dependent calcium channels. This activity causes a reduction in neuronal excitability and inhibits the release of pain neurotransmitters. The addictive nature of opium is related to binding to μ-opioid receptors, which activate dopaminergic neurons in the ventral tegmental area of the midbrain, thereby increasing dopamine release in the nucleus accumbens. This mechanism involves the reward activity of the mesolimbic dopaminergic pathway.