Opium

Opium produces its effects by activating specific G protein-coupled receptors in the brain, spinal cord, and peripheral nervous system. There are three major classes of opioid receptors being δ-opioid, κ-opioid and μ-opioid. Opium will generate an agonist activity which will later open the potassium channels and prevent the opening of voltage-gated calcium channels. This activity causes a reduction in neuronal excitability and inhibits the release of pain neurotransmitters.

The addictive character of opium is related to the binding to the μ-opioid receptors, which will activate dopaminergic neurons in the ventral tegmental area of the midbrain and thus, enhance the dopamine release in the nucleus accumbens. This mechanism involves the reward activity of the mesolimbic dopaminergic pathway.