Although millions of people use drugs in the US every year, relatively few users (< 3%) develop addiction. The relatively low number of individuals who become addicted has led researchers to focus efforts on identifying brain regions that drugs target, how drugs influence neural activity, and why only 3% of drug users become addicted. Drug addiction is a chronically relapsing disorder characterized by a loss of control over drug use. One of the most common theories among addiction researchers is that people develop addiction because drugs of abuse ‘hijack’ the reward circuitry of the brain. By taking over the controls, drugs steer the addict toward uncontrollable drug-seeking and drug use and away from natural rewards (e.g., food, water, sex, etc.). By observing an addicts behavior this theory appears true, but do drugs actually take over brain areas that encode natural rewards?
Both drugs and natural rewards target the brain’s reward pathway (i.e. the mesolimbic pathway). This pathway becomes activated when animals engage in pleasurable activities. For example, neurons in the reward pathway respond when a thirsty rat takes a sip of water. Likewise, neurons in the reward pathway respond when the same laboratory rat takes a hit of cocaine. However, the majority of neurons that respond to water don’t respond to cocaine. Thus, it appears that there are distinct ‘cocaine neurons’ and ‘water neurons’ in the brain. The brain actually contains neurons that respond to drugs and other neurons that respond to natural rewards. If indeed drugs hijack the natural reward circuitry, then neurons that respond to water theoretically should begin to respond to cocaine, and only to cocaine. This is not the case. Neurons that respond to water don’t fall victim to the drug’s hijacking demands -they continue to respond to water despite repeated cocaine use.
The fact that the brain contains unique drug responsive neurons and others that respond only to natural rewards begs all sorts of evolutionary questions. Why are there neurons that respond to drugs like cocaine? How would this class of neuron evolve? What/ how many species have drug responsive neurons? There are many just-so stories that one could conjure up; however, I will leave this enigma for evolutionary biologists to ponder. Returning the focus to drug addiction, how is it that an addict becomes so preoccupied with drugs like cocaine? Do they have more ‘drug neurons’ than other individuals? Do their drug neurons possess a more powerful neurochemical arsenal? Do cocaine neurons respond to other drugs? Neuropsychopharmacological researchers may soon provide answers to these perplexing questions. Regardless, it appears that drugs have secured their neural niche within the brain’s reward pathway. Not by way of hijacking, but instead, via the evolutionary express.
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