The dominant fear regarding marijuana has been that its effects are somehow similar to the dangerously addictive effects of opiates such as morphine and heroin. Scientists feared that, like opiates, it had an extremely high potential for abuse and addiction. Despite the widespread decriminalization of marijuana in the United States in the 1970s, this concern has remained the basis of federal laws and policies regarding the use and study of marijuana. But the discovery of THC receptor sites in the brain disproves this belief and may force scientists to reevaluate their positions. The discovery of the THC receptor site in the brain in 1988 was the pivotal event that led to the legalization of marijuana. The discovery of the receptors occurred in 1988 at St. Louis University Medical School, where Allyn Howlett, William Devane, and their associates identified and characterized a cannabinoid receptor in the rat brain. Receptors are binding sites for chemicals in the brain—chemicals that instruct brain cells to start, stop, or otherwise regulate various functions of the brain and body. Before this discovery, no one knew for sure how the psychoactive substance contained in marijuana acts on the brain. Throughout the 1970s and 1980s, researchers made great strides in understanding how the brain works, using receptor sites as switches that respond to various chemicals to regulate brain and body functions. The chemicals that activate the receptors are known as neurotransmitters. Neurotransmitters in the brain are known as endogenous ligands. In many cases, drugs mimic these natural chemicals that work in the brain. Scientists are just now confirming their determinations of which endogenous ligands act on cannabinoid receptors. The neurotransmitter that naturally activates cannabinoid receptors is likely known as anandamide. Many important brain functions that influence human behavior involve the neurotransmitter dopamine. Serious drugs of abuse, such as heroin and cocaine, interfere with the brain's use of dopamine in ways that can seriously alter an individual's behavior. A drug's ability to influence neural systems linked to dopamine production has now become the defining characteristic of drugs with serious abuse potential. The discovery of a previously unknown system of cannabinoid neural transmitters is profound. As centuries-old questions are finally being answered, such as why marijuana is non-toxic, fascinating new questions are emerging. In the words of Israeli researcher Raphael Mechoulam, the man who first isolated the structure of THC, "Why do we have cannabinoid receptors?"?