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The influence of neuromodulators on medial prefrontal cortical microcircuits during working memory

Final Report Summary - PFC-DOPA (The influence of neuromodulators on medial prefrontal cortical microcircuits during working memory)

Publishable Summary Report

1.1 Background information

Working memory (WM) is the capacity to temporarily maintain and manipulate task-relevant information, and has long been thought to crucially depend upon medial prefrontal cortex (mPFC) function. Investigations employing many different approaches have identified neuronal processes strongly linked with, and necessary for, working memory tasks. These processes can be modified by levels of neuromodulators such as dopamine (DA), in keeping with the proposed role of this neurotransmitter in modulating working memory function. However a mechanistic explanation for this cortical activity, and precisely how neuromodulators can influence it, remains lacking.

The aim of this project is to 1) characterise the dynamics of patterns of mPFC activation at the level of neural populations and, 2) determine how the neuromodulator DA influences these representations. To do this, I will combine advanced in vivo electrophysiological techniques (high density multi-site population recordings and whole-cell patch clamp) with optogenetic manipulations to directly assess the role of DA on patterns of neocortical activity and WM.


1.2 Main Results

1.2.1 DA innervation of PFC
The project has probed the influence of DA on prefrontal cortical circuit activity on both short and long timescales. Firstly, in order to understand precisely the anatomical connectivity between the DA VTA neurons and the PFC, we have used the selective labeling of the DA mesocortical pathway to quantify the layer-and subregion- specificity of the projections to the PFC (Figure 1).

By selectively labeling the DA system, and using previous descriptions of the parcellation of mouse prefrontal cortex (1, 2)(Figure 1a and b), we can the approximated borders of cortical and layer subregions, and quantify the intensity of DA axons within(Figure 1c).

we have found that the intensity of DA axons is heterogeneous within different prefrontal areas, and also between cortical layers (Figure 1d). This provides and anatomical basis for potential inter-regional and inter-layer difference in DA responses.

1.2.2 in vivo modulation of PFC neurons by DA activation
In order to probe the responses to PFC neuron of DA activation, we have used optogenetics couple with a transgenic mouse line to activate the DA system (Figure 2a,b). Interestingly, the most prominent responses in PFC to activation of the DA system have been in long term changes in neural activity. This is in contrast to relatively little response of PFC neurons to DA stimulation on a shorter timescale (data not shown). Notably, we have seen interesting modulations of PFC neuron activity relative to underlying cortical oscillations, notably in the theta band (Figure 2c-f). This is particularly important, as theta oscillations have been shown to increase in power during execution of delayed-response tasks(3). This may point to a role for dopamine in modulating the incoming input from the hippocampus to prefrontal cortex during these types of behavioural tasks.

1.2.3 In vitro in vivo modulation of PFC neurons by DA activation
To further characterize this response, we have collaborated with Prof. Wisden in Imperial College probe this response at a more mechanistic level. In vitro, it appears that we indeed also see relatively little by way of a short-latency DA response. Instead, a longer term decrease in excitability, characterized by decreased numbers of action potentials to depolarizing current injections, was observed (Figure 3a,b). This effect is reliant on the D1 receptor(Figure 3c,d), which is expressed in some, but not all, PFC pyramidal neurons(4). Interestingly, this DA response only appears present in those neurons lacking a hyperpolarization-activated Ih current (Figure 3e,f). This current has also been used to identify projection-specific populations of PFC neurons(4), and therefore suggests that this DA effect is projection-specific.

1.3 Conclusions

In conclusion, we have identified a long-term DAergic influence on neural populations in the mouse prefrontal cortex. This effect appears to be mediated by the D1 receptor, and modulates firing patterns relative to the ongoing theta oscillation, which is prominent in tasks related to attention and working memory. Work is ongoing to further characterize this response in vivo and in vitro, and to identify the possible behavioural correlates of this DAergic modulation using attentive and working memory tasks.

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2. H. J. J. M. Van De Werd, H. B. M. Uylings, Comparison of (stereotactic) parcellations in mouse prefrontal cortex. Brain Struct Funct 219, 433–459 (2014).
3. S. Fujisawa, G. Buzsáki, A 4 Hz oscillation adaptively synchronizes prefrontal, VTA, and hippocampal activities. Neuron 72, 153–165 (2011).
4. H. J. Seong, A. G. Carter, D1 Receptor Modulation of Action Potential Firing in a Subpopulation of Layer 5 Pyramidal Neurons in the Prefrontal Cortex. Journal of Neuroscience 32, 10516–10521 (2012).