Rats have the ability to learn and perform sophisticated behavioral tasks, making them very useful for investigating neural circuit functions. In contrast to the extensive mouse genetic toolkit, the paucity of recombinase-expressing rat models has limited the ability to monitor and manipulate molecularly-defined neural populations in this species. Here we report the generation and validation of two knock-in rat strains expressing either Cre or Flp recombinase under the control of Parvalbumin (Pvalb), a gene expressed in the critical "fast-spiking" subset of inhibitory interneurons (FSIs). These strains were generated with CRISPR-Cas9 gene editing and show highly specific and penetrant labeling of Pvalb-expressing neurons, as demonstrated by in situ hybridization and immunohistochemistry. We validated these models in both prefrontal cortex and striatum using both ex vivo and in vivo approaches, including whole-cell recording, optogenetics, extracellular physiology and photometry. Our results demonstrate the utility of these new transgenic models for a wide range of neuroscience experiments. IntroductionThe extensive library of recombinase driver mouse models [1,2] has enabled precise functional and structural dissection of neural circuits by providing specific and reliable genetic access to molecularly defined neural populations. When used in combination with recombinase-dependent reporter models or viruses, reporter genes can be expressed in specific neural populations [3]. For complex behavioral and large-scale in vivo electrophysiological experiments, rats are the model of choice. Yet their usefulness is restricted by the limited number of transgenic recombinase driver models to label molecularly defined neural populations. Thus, reliable rat recombinase driver models have great potential for accelerating progress in neuroscience.One important application of recombinase driver models is to suppress local brain activity in real time. This can be effectively achieved by optogenetic stimulation of local inhibitory interneurons, as demonstrated in mice using inhibitory neuron-specific Cre models [2,4]. In both cortical [5] and subcortical [6] regions, Parvalbumin (Pvalb) is expressed by GABAergic interneurons that provide perisomatic inhibition to projection neurons [7]. A rat recombinase driver model that directs transgene expression to Pvalb+ interneurons will therefore be a powerful tool for temporally-precise control of local circuit function.Until recently, it has been difficult to insert exogenous genes of interest into specific genomic locations in rats, primarily due to the lack of pluripotent stem cells for in vitro genomic modification [8]. To overcome this limitation, genetically modified rats have been predominantly generated using Bacterial Artificial Chromosome (BAC) transgenic technology [9][10][11]. This involves inserting a large chunk of chromosomal DNA (200-300kb) into the genome that includes the promoter of interest, coupled with an additional coding sequence for the desired recombinase. Expres...
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