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NEUROSCIENCE
The neurotransmitter dopamine facilitates learning, motivation and movement. Evidence of its release independently of the activity of dopamine-producing neurons in rat brains forces a rethink of dopamine regulation.
Dopamine is a neurotransmitter molecule that influences brain pathways that are involved in motivation, movement, cognition and reward-driven learning. How it contributes to such varied behaviours is the subject of ongoing investigation. Writing in Nature, Mohebi et al.1 shed light on how dopamine release is regulated in the rat brain to accomplish different functions.
Dopamine is produced by neurons located in the midbrain, in regions known as the ventral tegmental area (VTA) and the substantia nigra pars compacta. The long axons of these neurons extend to other parts of the brain, including the nucleus accumbens, the dorsal striatum and the prefrontal cortex. Within these target sites, the axons branch extensively2 to form a structure known as an arbor. The textbook description of dopamine signalling suggests that the activation of dopamine-producing neurons in the midbrain generates electrical signals that travel along their axons to their target regions, where they cause a dopamine release that is 'broadcast' throughout the territories covered by the axonal arbors. This concept is fundamental to current ideas about how reward-based learning occurs: an unexpected reward leads to an increase in the activity of dopamine neurons that is assumed to transmit a dopamine signal throughout the target regions to facilitate learning3,4.
Yet dopamine release in the target regions is more complicated than its textbook description. For example, it can be regulated locally by neurotransmitters and other molecules5. Moreover, studies in animals of the activity of dopamine neurons using an imaging approach to monitor the activity of dopamine neurons or a microelectrode method to assess dopamine release indicate that an unexpected reward can cause the predicted increase in the activity of the axonal arbor, and dopamine release in the nucleus accumbens6,7. However, these features are absent in the neighbouring dorsal striatum6,7, providing an argument against a role for dopamine as a universal broadcast signal. In addition to reward-dependent increases in dopamine release, dopamine release in the nucleus accumbens ramps up slowly8-13 as an animal approaches a reward site, before it obtains a reward. The amount of dopamine that is released during this ramping-up phase...