Comparative analyses of dopaminergic signalling in the mouse brain
Keywords: dLight
nucleus accumbens
ventral striatum
head-fixed behaviour
midbrain
photometry
fast-scan cyclic voltammetry
dopamine
dopamine neurons
reward prediction error
locomotion
dorsolateral striatum
dopamine release dynamics
mouse
juxtacellular recording

Despite dopamine being one of the most extensively studied neurotransmitters in the mammalian brain, there remains a great deal of debate as to the dynamics and function of dopaminergic signalling in the midbrain and striatum. Here, I report on a series of in vivo experiments in both anaesthetised and awake mice that were designed to explore the nature of dopaminergic signalling, both within and across midbrain-striatal circuits.
Simultaneous use of single-unit electrophysiology in concert with fast-scan cyclic voltammetry (FCV) would be a powerful approach to addressing questions about the nature of the relationship between midbrain dopaminergic neuron firing and striatal dopamine release. I developed and demonstrated a method of achieving this in anaesthetised mice. The data reported a positive linear relationship between optogenetically evoked dopaminergic neuron firing and striatal dopamine release, which suggests a close relationship between these two modes of dopaminergic signalling. However, the combination of electrophysiology and FCV presented unique challenges that might be avoided with the use of optical (photometric) approaches for recording striatal dopamine release.
To compare the signalling dynamics of the novel genetically encoded fluorescent dopamine sensor dLight with FCV, I recorded electrically evoked striatal dopamine transients with both dLight photometry and FCV, across a broad range of midbrain stimulation parameters, in anaesthetised mice. Furthermore, to investigate whether dLight might be a useful tool for investigating striatal heterogeneity in dopaminergic signalling, transients were recorded from both dorsolateral striatum (DLS) and ventral striatum (VS). My findings revealed that: (1) dLight and FCV transients both reflected the intensity of midbrain stimulation; (2) photometric dLight transients were consistently faster than dopamine transients recorded with FCV; and (3) dopaminergic signalling in DLS occurred at faster timescales than in VS. This faster timescale in DLS dopamine signals may reflect differences in the local modulation of striatal dopamine release and uptake. As FCV and dLight displayed similar dynamics in response to midbrain stimulation, I concluded that dLight photometry would be a suitable tool for reading out dopaminergic signalling accompanying behaviour.
To explore the functional heterogeneity of dopamine release in the dorsal and ventral striatum, I recorded photometric dLight transients from the DLS and VS of mice performing treadmill-based locomotion, with and without reward contingency, and in response to instrumental cues. Dopaminergic signalling around locomotion, reward collection and cue presentation were broadly similar in both striatal regions. Dopamine signals decreased around the initiation of locomotive bouts (‘peri-initiation trough’) but increased after bout termination (‘post-termination’) and reward delivery. Furthermore, instrumental cues were represented in both striatal regions, but the valence of the cue response in the dopamine signal was dependent on the mouse’s behavioural performance. However, dopamine signals from each striatal region had dissociable dynamics in some cases, with more phasic fluctuations in the DLS and delayed transients in VS. Overall, these findings suggest that both DLS and VS dopamine encode movement and reward, and thus that a binary functional distinction between dorsal and ventral striatum is oversimplistic.
In conclusion, there is considerable heterogeneity in the dynamics of dopaminergic signalling in different striatal regions. I have made some important steps towards characterising the dynamics of the dLight fluorescent sensor in vivo, including in relation to behaviour. Furthermore, I have demonstrated a novel approach for exploring diversity in different modes of dopaminergic signalling, namely midbrain neuronal firing and striatal release. This thesis lays groundwork for future explorations of the nature and heterogeneity of dopamine signalling in the brain.