The Mysterious Motivational Functions of Mesolimbic Dopamine

Nucleus accumbens dopamine is known to play a role in motivational processes, and dysfunctions of mesolimbic dopamine may contribute to motivational symptoms of depression and other disorders, as well as features of substance abuse. Although it has become traditional to label dopamine neurons as “reward” neurons, this is an overgeneralization, and it is important to distinguish between aspects of motivation that are differentially affected by dopaminergic manipulations. For example, accumbens dopamine does not mediate primary food motivation or appetite, but is involved in appetitive and aversive motivational processes including behavioral activation, exertion of effort, approach behavior, sustained task engagement, Pavlovian processes, and instrumental learning. In this review, we discuss the complex roles of dopamine in behavioral functions related to motivation.

Brain study reveals the roots of chocolate temptations

Researchers have new evidence in rats to explain how it is that chocolate candies can be so completely irresistible. The urge to overeat such deliciously sweet and fatty treats traces to an unexpected part of the brain and its production of a natural, opium-like chemical, according to a report published online on September 20th in Current Biology, a Cell Press publication.

"This means that the brain has more extensive systems to make individuals want to overconsume rewards than previously thought," said Alexandra DiFeliceantonio of the University of Michigan, Ann Arbor. "It may be one reason why overconsumption is a problem today."

DiFeliceantonio’s team made the discovery by giving rats an artificial boost with a drug delivered straight to a brain region called the neostriatum. Those animals gorged themselves on more than twice the number of M&M chocolates than they would otherwise have eaten. The researchers also found that enkephalin, the natural drug-like chemical produced in that same brain region, surged when rats began to eat the candy-coated morsels, too.

It’s not that enkephalins or similar drugs make the rats like the chocolates more, the researchers say, but rather that the brain chemicals increase their desire and impulse to eat them.

The findings reveal a surprising extension of the neostriatum’s role, as DiFeliceantonio notes that the brain region had primarily been linked to movement. And there is reason to expect that the findings in rats can tell us a lot about our own binge-eating tendencies.

"The same brain area we tested here is active when obese people see foods and when drug addicts see drug scenes," she says. "It seems likely that our enkephalin findings in rats mean that this neurotransmitter may drive some forms of overconsumption and addiction in people."

The researchers now hope to unravel a related phenomenon that some of us might wish we could do more to control: what happens in our brains when we pass by our favorite fast food restaurant and feel that sudden desire to stop.

Though the seconds may tick by on the clock at a regular pace, our experience of the ‘fourth dimension’ is anything but uniform. When we’re waiting in line or sitting in a boring meeting, time seems to slow down to a trickle. And when we get caught up in something completely engrossing – a gripping thriller, for example – we may lose sense of time altogether.

But what about the idea that time flies when we’re having fun? New research from psychological science suggests that the familiar adage may really be true, with a caveat: time flies when we’re have goal-motivated fun.

3D representation of motivational system (striatum ventral) activated during physical or mental effort. Credit: Copyright M. M. Pessiglione, Inserm)

Is there a general motivation center in the depths of the brain?

Mathias Pessiglione and his team from Inserm unit 975 “Centre de recherche en neurosciences de la Pitié-Salpêtrière” examined whether mental and physical efforts are driven by a motivation ‘centre’ or whether they are conducted by different parts of the brain. The researchers studied the neural mechanisms resulting from activities that combine both action and cognition.

Using images obtained from the MRI scans taken during the test, Mathias Pessiglione and his team identified a general motivational system in the depths of the brain, i.e. a structure capable of activating any effort type, both mental (concentrating on the task in hand) or physical (lifting a load). The researchers observed that the ventral striatum was activated in proportion to the amount of money involved: the higher the degree of motivation, the higher the activation level. Furthermore, the ventral striatum is connected to the median part of the striatum (the caudate nucleus) when the task to be performed is cognitively difficult (when the physical size and the numerical value of the numbers did not correspond). This ventral region solicits the lateral part of the striatum (the putamen) when the difficulty is motor-related (when the handle had to be squeezed very tightly).

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