Zeroing in on what actually motivates — learning and motivation

Dopamine is a neurotransmitter molecule that influences brain pathways — involved in motivation, movement, reasoning/perception, and reward-driven learning. How dopamine contributes to such seemingly-unrelated varied behaviors is the topic of this GEITP email. In fact, understanding the biochemical and genetic pathways as to how learning, memory and motivation actually “work,” is one of the biggest (still outstanding) challenging areas of future research. Environmental signals — that lead to activation of genetic pathways (responsible for motivation, movement, reasoning/perception, and reward-driven learning) — falls within the purview of gene-environment interactions (at least, in the mind of these GEITP pages). 😉

Authors [see attached article & editorial] elucidate how dopamine release is regulated (in rat brain) to accomplish these different functions. Dopamine is produced by neurons located in the midbrain — in regions known as the ventral tegmental area (VTA) and substantia nigra

pars compacta. The long efferent (outgoing) axons of these neurons extend to other parts of the brain — including the nucleus accumbens, dorsal striatum, and prefrontal cortex. Within these target sites, the axons branch extensively, like a “tree,” to form a structure known as an “arbor.”

The textbook description of dopamine-signaling suggests that activation of dopamine-producing neurons in the midbrain generates electrical signals that travel along these axons to their target regions, where they cause dopamine release — which is then transmitted throughout the regions covered by the axonal arbors. This concept is fundamental to current ideas as to how reward-based learning occurs: an unexpected reward leads to increased activity of dopamine neurons that is assumed to transmit a dopamine signal throughout the target regions to facilitate learning.

However, dopamine release in the target regions is more complicated than the textbook description (e.g. dopamine release can be regulated locally by neurotransmitters and other molecules). Furthemore, studies of dopamine neuron activity in animals (using an imaging approach to monitor dopamine neuron activity, or a microelectrode method to assess dopamine release) indicate that an unexpected reward can cause the predicted increased activity of the axonal arbor, as well as dopamine release in the nucleus accumbens.

Dopamine is famously associated with “reward” — but how, exactly? Authors compared spiking of VTA dopamine cells with nucleus accumbens dopamine release, during the same decision-making task. Cues — which would indicate an upcoming reward — were found to increase both the spiking and the release. However, nucleus accumbens core dopamine release also co-varied with dynamically evolving reward expectations, without corresponding changes in VTA dopamine cell-spiking. These intriguing data suggest that there is a fundamental difference in how dopamine release is regulated to achieve these two distinct functions: transmitted burst signals promote learning, whereas local control signals drive motivation. 😊


Nature 6 June 2o19; 570: 65-70 & News’N’Views pp 40-42

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