Where Do Different Levels of Motivation Come From?
One source of differences may depend on dopamine activity in the nucleus accumbens. Researchers at McGill University measured dopamine levels as female rats licked and groomed their pups; the output paralleled licking/grooming activity in duration and intensity. But what was really interesting was that the dopamine release preceded the onset of the maternal behavior. In addition, mothers that displayed the highest levels of licking and grooming behavior had more dopamine receptors than the low licking/grooming mothers. The researchers proposed that this dopamine release is triggered by the release of oxytocin in the medial preoptic nucleus; besides maternal behavior, oxytocin is important in sexual behavior and social bonding. Of course, further research must be done to determine whether these findings apply to other forms of motivated behavior or to humans. Journal of Neuroscience, Vol 24, 4113-4123.
A Possible Neural Basis for Sensation Seeking
According to page 158, arousal theory and sensation seeking behavior have helped shift psychologists' emphasis from drives as tissue deficits to drives as brain states. Now there is evidence for neural differences that could explain why some people get a kick out of jumping out of an airplane and others don't. A questionnaire was used to categorize 20 volunteers according to novelty seeking (agreeing to statements like "I like to try new things just for fun.") or reward dependence (agreeing to statements like "I'd rather stay home than go out."). Novelty seekers tend to be high in impulsivity, exploratory drive, and excitability, while those who are reward dependent are high in sociability and need for social approval and tend to repeat actions associated with rewards. The researchrs then used a form of MRI called diffusion tensor imaging to quantify white matter connections. The novelty seekers had stronger connections from the striatum to the hippocampus and amygdala; in rats the striatum and hippocampus are involved with novelty detection and novelty seeking. The reward dependent volunteers had stronger connections between the striatum and prefrontal cortex. In Chapter 3 you learned that the prefrontal cortex plays a role in impulse control and adjusting behavior in response to rewards and punishment. These findings are significant beyond the importance of the behaviors themselves, because they imply that there are discernible correlates between brain structure and personality. Nature Neuroscience, Vol 12, 32-34.
What Do You Really Lose When You Lose Weight?
Researchers have assumed that we neither make new fat cells nor lose them, because the number of fat cells is essentially constant after adulthood. Testing this assumption would ordinarily involve "labeling" fat cells as they are produced with a radioactive isotope; fat cells do not divide after developing, so researchers could determine when the cells were "born" by measuring the deterioration of the radioactive isotope. This procedure can be toxic, so it is not used with humans, but Swedish researchers came up with an ingenious alternative. Because above-ground nuclear tests spewed carbon-14 into the atmosphere from 1955 to 1963, any fat cells produced during that time would naturally carry a radioactive label proportionate to the level of the isotope in the air at the time. After studying 700 adults, the researchers were able to conclude that the number of fat cells increases from birth until the early twenties and remains constant after that. The cells have a limited life span of about 10 years, but as the cells die they are replaced almost one-for-one. As an adult you're stuck with the number of fat cells you already have; even marked weight loss just reduces the volume of those cells. Nature, Vol 453, 783-787.
How Peptide YY Reduces Appetite
Peptide YY (PYY) is released in the digestive tract in response to the presence of food, and earlier research by Rachel Batterham demonstrated that PYY acts in the hypothalamus to reduce subsequent calorie intake by about a third (see p. 171). Now we know why PYY reduces appetite. Using fMRI, Batterham's team found that PYY increases activity not only in the hypothalamus but, surprisingly, in the left orbital frontal cortex, which is involved in reward and pleasure. On days when the volunteers received only saline in their intravenous drip the number of calories they consumed later was predicted by activity in the hypothalamus (presumably indicating their level of hunger after their 14-hour fast); but following the PYY infusion, activity in the orbital frontal cortex predicted consumption. This result has implications both for our understanding of feeding regulation and, potentially, for the treatment of obesity. Nature, Vol 450, 106-111.
