title: Role of prelimbic excitatory neurons and secondary motor cortex in response inhibition
Creators: name: Deubner, Jan
Publisher: name: Albert-Ludwigs-Universität Freiburg
Year: 2020
Subjects: subject: 570

description: Withholding inappropriate actions in response to external cues is one of the most crucial mechanisms to survive in an ever-changing environment. Majorly involved in behavioural inhibition is the prefrontal cortex (PFC) (Hardung et al., 2017), yet the precise mechanisms and neuronal circuits underlying this complex behaviour remain poorly understood. In the present thesis, we investigated the role of excitatory PL neurons in this phenomenon and their impact on secondary motor cortex (M2), a major relay for movement-related processing, in a response preparation task, adequate to distinguish reactive and proactive behaviour. We were able to show that rats reliably engage in this task and employ a cue-based strategy, rapidly switching to internal timing mechanisms when cues are omitted. After an error, animals and humans slow their reaction time (RT) to retain high levels of accuracy, a phenomenon called post-error slowing (PES). Surprisingly we found different characteristics of PES, depending on the type of error made in the previous trial: premature release errors elicited less slowing than late responses. This dependency has not been reported before and does demand further investigation. Additionally, we found a significant increase of late responses upon optogenetic inhibition of excitatory PL neurons in short delay trials. Contradicting previous results which adjudicated inhibitory functions to dorsal mPFC regions in response inhibition, the current findings, based on cell-type specific manipulation, might hint towards more intricate mechanisms in these brain region. Electrophysiological recordings in M2 showed a majority of neurons to be modulated during one or multiple time points of the task, closely resembling previous observations in PFC. Inhibiting excitatory PL neurons, however, only showed a slight and slow impact on activity in M2, hinting at more complex circuitry, possibly including thalamic and striatal structures, to be involved rather than direct connections between those two areas. To disentangle neural mechanisms of response inhibition, it is necessary to target specific cell populations to identify their role in complex behaviour. We used a design ankyrin repeat proteins (DARPin)-based approach utilising vectors equipped with artificial antibody-mimetic proteins, to selectively transfect parvalbumin-positive interneurons (PV)-positive neurons. We found functional expression of channelrhodopsin (ChR2) in various cortical and subcortical brain areas. Unfortunately, histological analysis revealed extensive cell degeneration and cell death, probably caused by overexpression. Revealing unique roles of specific neuronal subpopulations in response inhibition, this study serves as a general foundation for more detailed investigations into intricate mechanisms and inter-area synergies involved in complex behaviour. Functional expression of optogenetic tools with a new targeting strategy, even though further ameliorations are still required, pave the way for methodological avenues to facilitate precise research of highly intricate neuronal interactions and complex behaviour in rats.