Primates and rodents, which descended from a common ancestor around 90 million years ago 1 , exhibit profound differences in behaviour and cognitive capacity; the cellular basis for these differences is unknown. Here we use single-nucleus RNA sequencing to profile RNA expression in 188,776 individual interneurons across homologous brain regions from three primates (human, macaque and marmoset), a rodent (mouse) and a weasel (ferret). Homologous interneuron types-which were readily identified by their RNA-expression patterns-varied in abundance and RNA expression among ferrets, mice and primates, but varied less among primates. Only a modest fraction of the genes identified as 'markers' of specific interneuron subtypes in any one species had this property in another species. In the primate neocortex, dozens of genes showed spatial expression gradients among interneurons of the same type, which suggests that regional variation in cortical contexts shapes the RNA expression patterns of adult neocortical interneurons. We found that an interneuron type that was previously associated with the mouse hippocampus-the 'ivy cell', which has neurogliaform characteristics-has become abundant across the neocortex of humans, macaques and marmosets but not mice or ferrets. We also found a notable subcortical innovation: an abundant striatal interneuron type in primates that had no molecularly homologous counterpart in mice or ferrets. These interneurons expressed a unique combination of genes that encode transcription factors, receptors and neuropeptides and constituted around 30% of striatal interneurons in marmosets and humans.Vertebrate brains contain many specialized brain structures, each with its own evolutionary history. For example, the six-layer neocortex arose in mammals about 200 million years ago 2 , whereas distinct basal ganglia were already present in the last common ancestor of vertebrates more than 500 million years ago 3 .Brain evolution may often be driven by adaptive changes in cellular composition or molecular expression within conserved structures [4][5][6] . Examples of modifications to specific cell types within larger conserved brain systems include hindbrain circuits that control species-specific courtship calls in frogs 7 , the evolution of trichromatic vision in primates 8 , and neurons that have converted from motor to sensory processing to produce a new swimming behaviour in sand crabs 4 . Evolution can modify brain structures through diverse means, such as by increasing or reducing the production or survival of cells of a given type, altering the molecular and cellular properties of shared cell types, reallocating or redeploying cell types to new locations, losing a cell type 9 or inventing a new cell type (Fig. 1a).Genome and RNA sequencing (RNA-seq) analyses have allowed molecular inventories to be compared across species 10,11 , and single-cell RNA-seq now enables the detailed comparison of cell types and expression patterns between homologous brain structures 8,10,11 . A recent study compared n...

Recent success in identifying gene regulatory elements in the context of recombinant adeno-associated virus vectors have enabled cell type-restricted gene expression. However, within the cerebral cortex these tools are presently limited to broad classes of neurons. To overcome this limitation, we developed a strategy that led to the identification of multiple novel enhancers to target functionally distinct neuronal subtypes. By investigating the regulatory landscape of the disease gene Scn1a, we identified enhancers that target the breadth of its expression, including two that are selective for parvalbumin and vasoactive intestinal peptide cortical interneurons. Demonstrating the functional utility of these elements, we found that the PV-specific enhancer allowed for the selective targeting and manipulation of these neurons across species, from mice to humans. Finally, we demonstrate that our selection method is generalizable to other genes and characterize four additional PV-specific enhancers with exquisite specificity for distinct regions of the brain. Altogether, these viral tools can be used for cell-type specific circuit manipulation and hold considerable promise for use in therapeutic interventions.Large-scale transcriptomic studies are rapidly revealing where and when genes associated with neuropsychiatric disease are expressed within specific cell types (1-4). Approaches for understanding and treating these disorders will require methods for targeting and manipulating specific neuronal subtypes. Thus, gaining access to these populations in non-human primates and humans has become paramount. AAVs are the method of choice for gene delivery in the nervous system but have a limited genomic payload and are not intrinsically selective for particular neuronal populations (5). We and others have identified short regulatory elements capable of restricting viral expression to broad neuronal classes. In addition, systematic enhancer discovery has been accelerated by the recent development of technologies allowing for transcriptomic and epigenetic studies at single-cell resolution (6-12). Despite these advances, the search space for enhancer selection remains enormous and to date success has been limited. To focus our enhancer selection, we chose to specifically examine the regulatory landscape of Scn1a, a gene expressed in distinct neuronal populations and whose disruption is associated with severe epilepsy (13).Combining single-cell ATAC-seq data with sequence conservation across species, we nominated ten candidate regulatory sequences in the vicinity of this gene. By thoroughly investigating each of these elements for their ability to direct viral expression, we identified three enhancers that collectively target the breadth of neuronal populations expressing Scn1a. Among these, one particular short regulatory sequence was capable of restricting viral expression to parvalbumin-expressing cortical interneurons (PV cINs). To fully assess the utility of this element beyond reporter expression, we validated it in a v...

Background-Thalamic deep brain stimulation (DBS) is proven therapy for essential tremor, Parkinson's disease, and Tourette's Syndrome. We tested the hypothesis that high-frequency electrical stimulation results in local thalamic glutamate release.

26Primates and rodents, which descended from a common ancestor more than 90 million years 27 ago, exhibit profound differences in behavior and cognitive capacity. Modifications, 28 specializations, and innovations to brain cell types may have occurred along each lineage. We 29 used Drop-seq to profile RNA expression in more than 184,000 individual telencephalic 30 interneurons from humans, macaques, marmosets, and mice. Conserved interneuron types 31 varied significantly in abundance and RNA expression between mice and primates, but varied 32 much more modestly among primates. In adult primates, the expression patterns of dozens of 33 genes exhibited spatial expression gradients among neocortical interneurons, suggesting that 34 adult neocortical interneurons are imprinted by their local cortical context. In addition, we found 35 that an interneuron type previously associated with the mouse hippocampus-the "ivy cell", which 36 has neurogliaform characteristics-has become abundant across the neocortex of humans, 37 macaques, and marmosets. The most striking innovation was subcortical: we identified an 38 abundant striatal interneuron type in primates that had no molecularly homologous cell population 39 in mouse striatum, cortex, thalamus, or hippocampus. These interneurons, which expressed a 40 unique combination of transcription factors, receptors, and neuropeptides, including the 41 neuropeptide TAC3, constituted almost 30% of striatal interneurons in marmosets and humans. 42Understanding how gene and cell-type attributes changed or persisted over the evolutionary 43 divergence of primates and rodents will guide the choice of models for human brain disorders and 44 mutations and help to identify the cellular substrates of expanded cognition in humans and other 45 primates. 46 47 53 54 Brain structures, circuits, and cell types have acquired adaptations and new functions along 55 specific evolutionary lineages. Numerous examples of modifications to specific cell types within 56 larger conserved brain systems have been discovered, including hindbrain circuits that control 57 species-specific courtship calls in frogs 3 , the evolution of trichromatic vision in primates 4 , and 58 neurons that have converted from motor to sensory processing to produce a novel swimming 59 behavior in sand crabs 5 . Evolution can modify brain structures through a wide range of 60 mechanisms, including increasing or reducing production of cells of a given type, altering the 61 molecular and cellular properties of shared cell types, reallocating or redeploying cell types to 62 new locations in the brain, or inventing entirely new cell types (Fig. 1a). 63 64 Single-cell RNA sequencing, which systematically measures gene expression in thousands of 65 individual cells, has recently enabled detailed comparisons of cell types and expression patterns 66 between homologous brain structures separated by millions of years of evolution 4,6,7 (non-single 67 cell approaches have also yielded important insights in this domain, e.g. 8 ). For example...