How much people want or like food, drugs, sex and other rewarding stimuli appears to depend significantly on individual differences in how they process the neurotransmitter dopamine. According to several recent brain-imaging studies, these same differences in dopamine processing also help to determine people’s chances of being impulsive or drawn to risky behaviors.
“What are starting to emerge now are data on the specific aspects of the [dopamine] system that relate to different personality traits,” says David Zald, a researcher at Vanderbilt University who led one of the studies.
Zald and his colleagues examined 34 people using positron emission tomography (PET) to measure the density of a particular set of brain-cell receptors in a midbrain region that produces dopamine. The receptors, known as dopamine autoreceptors, serve as part of a feedback loop to inhibit the excess flow of dopamine to other reward-sensitive regions in the brain. Zald’s group found that people with a lower density of the receptors, who thus presumably had a lower capacity to restrain midbrain dopamine production, tended to be “novelty-seeking,” as measured by a standard psychological test.
A similar result comes from Karen Berman’s laboratory at the National Institutes of Health in Bethesda, Md. Berman and her colleagues recruited 27 young male and female subjects, noting for each one the variations, or alleles, of the gene for the enzyme COMT (catechol-O-methyltransferase). COMT snips dopamine molecules into pieces and thus helps to regulate their levels around nerve synapses in certain reward-sensitive areas of the brain.
For 22 of the subjects, Berman and her colleagues were also able to obtain information on the alleles for DAT1, the dopamine transporter protein, which also regulates dopamine levels (by sucking nearby dopamine molecules in from the synaptic space).
Berman and her colleagues found that those subjects with alleles for less-efficient forms of COMT and DAT1—which leave more dopamine around synapses, available for signaling—had higher functional magnetic resonance imaging (fMRI) responses in key reward-signaling areas of their brains, both when cued to anticipate a monetary reward and when actually receiving such a reward unexpectedly.
A related study was conducted at the University of Pittsburgh in 2007. Researchers in the laboratory of Ahmad Hariri measured the variability, among 89 people, of the fMRI response to rewards in a reward-sensitive region known as the ventral striatum. They found that the variations in fMRI reward response was significantly determined by variations in the genes for DAT1 and two kinds of brain-cell receptor that also serve as regulators of dopamine levels in reward circuits. Previously Hariri and colleagues had found that people with greater ventral striatum responses to rewards were more likely to be rated as impulsive on a standard psychological test.
What such research seems to show is that people who—for genetic or other reasons—have a greater capacity to pump dopamine through their reward systems are also more likely to be impulsive and novelty-seeking. Both impulsivity and novelty seeking are known to correlate with the risk for addiction behaviors.
Many strands to brain’s reward systems
Yet the relationship between dopamine and behavior may be more complicated than it seems. As Hariri points out, not all studies in this area have separated out “anticipatory reward” (wanting) from “consummatory reward” (liking). When these are separated, different variations in dopamine-regulating genes appear to have different impacts on reward-system responsivity. In particular, “the COMT effect seems to predominate during the anticipation of reward” rather than reward delivery, Hariri says.
Moreover, as Zald notes, some animal studies hint that impulsivity and proneness to addiction can also stem from a relatively weak dopamine response—for which extreme novelty seeking and drug taking would serve as compensation, not result.
Zald and some other researchers suspect that the answer to this conundrum might lie in the difference between anticipatory and consummatory rewards. Impulsivity, novelty seeking and related traits might arise, Zald says, from a disconnect “or even an inversion of the strength of the anticipatory and the consummatory rewards,” so that, for example, one can’t get enough enjoyment out of consuming something to stop wanting it.
Zald would like to be able to tell a relatively straightforward story about dopamine and personality, he says, “but my belief, at least right now, is that the story is fairly complicated.”