Striatal dopamine release shapes responses to reward stimulation

The release of neurotransmitters in the brain allows for the transfer of information that can be associated with different functions. Dopamine is one such neurotransmitter that is used to reinforce actions through a reward stimulus. A paper by Li and Jasanhoff, published in 2020, specifically explores how dopamine release in the striatum changes responses. Here, rats were implanted with a cannula in the ventral striatum and the Lateral hypothalamus (LH), injected with dopamine-sensitive and non-dopamine-sensitive contrast agents, and scanned during LH stimulation to determine responses. The scanning methods used were multi-gradient echo MRI, functional Magnetic Resonance Imaging (fMRI), and blood-oxygen-level-dependent (BOLD) contrast responses. Additionally, some rats were injected with a mix of SCH 23390 and eticlopride, antagonists of the dopamine D1 and D2 receptors instead of a dopamine sensor, to verify non‑dopaminergic activity or postsynaptic effects of dopamine on haemodynamic signals that might account for discrepancies between BOLD and dopamine signals. 

Some key findings: 

• In two groups of rats, one was injected with a dopamine-sensitive protein-based MRI contrast agent, BM3h-9D7, and the other was injected with a control agent, BM3h-WT, which lacks dopamine sensitivity. Here we see that the haemodynamic responses, or the rate of blood flow and oxygen, as determined by BOLD, were suppressed with both agents. A map of peak stimulus-dependent dopamine release for the BM3h-9D7 rats was determined in the nucleus accumbens, medial caudate putamen, olfactory tubercle, and lateral septal area. Rats with the BM3h-WT control group had no independent dopamine signals. 
• With the rats injected with dopamine blockers, we see more correlation between BOLD and Dopamine signals. The fMRI responses here also showed an alteration in time, and no changes in the spatiotemporal properties of dopamine release. This shows us that the post-synaptic effects of dopamine contribute to the discrepancies between BOLD and dopamine signals. Further exploring striatal dopamine release in the brain using brain-wide fMRI signals, it is found that striatal dopamine can indirectly modulate reward-evoked activation in the cortex.  

Overall, these results show increased connectivity between striatal release response in various brain regions contributing to motivated action, and an increase in postsynaptic activity when dopaminergic neurons are stimulated. This allows for a better understanding of dopamine signaling in learning and memory, as well as explaining related dopamine or reward-based neuroimaging results. 

~ Alaina Jeeson

Reference: 

Li, Nan, and Alan Jasanoff. “Local and Global Consequences of Reward-Evoked Striatal Dopamine Release.” Nature, vol. 580, no. 7802, Apr. 2020, pp. 239–244, https://doi.org/10.1038/s41586-020-2158-3. Accessed 13 Apr. 2020.