All the known drugs of abuse activate reinforcing brain circuitries (Koob and Volkow, 2010). These drugs, however, recruit distinct molecular targets in the brain and show notable differences in their pharmacological actions, which has led researchers and physicians to classify them into distinct groups. Opiates, acting directly at opioid receptors, produce sedative effects in addition to euphoria, and are therefore known as narcotics. In contrast, psychostimulants that include cocaine, amphetamine and methamphetamine, provide immediate euphoria with a feeling of intellectual and physical power, and indifference to pain and fatigue, mainly via direct stimulation of dopaminergic transmission. Nicotine, a major component of tobacco, is also considered a mild stimulant and α-nicotinic receptors constitute their molecular target. Relaxing and euphoric sensations searched by marijuana users arise from the stimulation of CB1 receptors by cannabinoids, including the most active component delta9-tetrahydrocannabinol (THC). Finally, a most widely abused licit drug is alcohol, targeting several receptors and ion channels in the brain and representing a major health problem (Hyman, 2008). It is now well established that the endogenous opioid system plays an important role in
