The phasic release of dopamine in the striatum determines various aspects of reward and action selection, but the dynamics of the dopamine effect on intracellular signalling remains poorly understood. We used genetically encoded FRET biosensors in striatal brain slices to quantify the effect of transient dopamine on cAMP or PKA-dependent phosphorylation levels, and computational modelling to further explore the dynamics of this signalling pathway. Medium-sized spiny neurons (MSNs), which express either D or D dopamine receptors, responded to dopamine by an increase or a decrease in cAMP, respectively. Transient dopamine showed similar sub-micromolar efficacies on cAMP in both D1 and D2 MSNs, thus challenging the commonly accepted notion that dopamine efficacy is much higher on D than on D receptors. However, in D2 MSNs, the large decrease in cAMP level triggered by transient dopamine did not translate to a decrease in PKA-dependent phosphorylation level, owing to the efficient inhibition of protein phosphatase 1 by DARPP-32. Simulations further suggested that D2 MSNs can also operate in a 'tone-sensing' mode, allowing them to detect transient dips in basal dopamine. Overall, our results show that D2 MSNs may sense much more complex patterns of dopamine than previously thought.

Although it is known that protein kinase A (PKA) in the nucleus regulates gene expression, the specificities of nuclear PKA signaling remain poorly understood. Here, we combined computational modeling and live-cell imaging of PKA-dependent phosphorylation in mouse brain slices to investigate how transient dopamine signals are translated into nuclear PKA activity in cortical pyramidal neurons and striatal medium spiny neurons. We observed that the nuclear PKA signal in striatal neurons featured an ultrasensitive responsiveness, associated with fast all-or-none responses, which is not consistent with the commonly accepted theory of a slow and passive diffusion of catalytic PKA in the nucleus. Our numerical model suggests that a positive feed-forward mechanism inhibiting nuclear phosphatase activity - possibly mediated by DARPP-32 (also known as PPP1R1B) - could be responsible for this non-linear pattern of nuclear PKA response, allowing for a better detection of the transient dopamine signals that are often associated with reward-mediated learning.

The influx of amyloid-β peptide (Aβ) across the blood-brain barrier is partly mediated by the receptor for advanced glycation end products (RAGE). But other transporters, like Oatp (organic anion transporter polypeptide, SLC21) transporters, could also be involved. We used in situ brain perfusion to show that rosuvastatin and taurocholate, two established Oatp1a4 substrates, decreased (5-fold) the Clup of [3H]Aβ while L-thyroxine increased it (5.5-fold). We demonstrated an interaction between Aβ and Oatp1a4 by co-immunoprecipitation and western blotting experiments, supporting the hypothesis that the rosuvastatin- and taurocholate-sensitive transporter was Oatp1a4. In conclusion, our results suggest that, in mice, the brain uptake of Aβ is partly mediated by Oatp1a4 and that L-thyroxine may play a crucial role in the inhibition of brain Aβ clearance.