People used to attribute addiction to a combination of factors. These include early exposure to a drug, early lifeemotional trauma, physical or sexual abuse, and certain psychiatric conditions.   42   　　Individuals who are genetically predisposed to any type of addiction have a defective gene in the part of the brainresponsible for the manufacture of dopamine, the neurotransmitter involved with drugs. Cocaine, heroin, nicotine,amphetamines, and other addictive drugs alter the brain’s pleasure circuit, producing a feel-good sensation. The pleasurecircuit communicates in the chemical language of dopamine, and this neurotransmitter zips from neuron to neuron in thebrain like a molecular happy face to produce feelings of mild happiness to euphoria.   43   The most important is areduction in the number of dopamine receptors.   44   Having fewer dopamine receptors means that less passing dopamine gets caught, so the pleasure circuit calmsdown. With fewer dopamine receptors, a hit that used to produce pleasure no longer does. In order to achieve a high thatequals the original, the addict must increase the dose. 　　Researchers recently identified “hot spots” or regions of chromosomes linked to a risk for addiction. Now they arezeroing in on the actual genes.   45  
【題組】42
(A) Current thinking points to genetics as the single strongest risk factor.
(B) Addictions have been shown to be anywhere from 67 percent to 80 percent hereditary.
(C) They hope these genes will be targets for designer drugs that one day will strike at addiction with precision.
(D) But chronic use of these addictive drugs produces enduring changes in the brain.