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Key points Imbalances in the activity of the D1‐expressing direct pathway and D2‐expressing indirect pathway striatal projection neurons (SPNs) are thought to contribute to many basal ganglia disorders, including early‐onset neurodevelopmental disorders such as obsessive–compulsive disorder, attention deficit hyperactivity disorder and Tourette's syndrome. This study provides the first detailed quantitative investigation of development of D1 and D2 SPNs, including their cellular properties and connectivity within neural circuits, during the first postnatal weeks. This period is highly dynamic with many properties changing, but it is possible to make three main observations: many aspects of D1 and D2 SPNs progressively mature in parallel; there are notable exceptions when they diverge; and many of the defining properties of mature striatal SPNs and circuits are already established by the first and second postnatal weeks, suggesting guidance through intrinsic developmental programmes. These findings provide an experimental framework for future studies of striatal development in both health and disease. Abstract Many basal ganglia neurodevelopmental disorders are thought to result from imbalances in the activity of the D1‐expressing direct pathway and D2‐expressing indirect pathway striatal projection neurons (SPNs). Insight into these disorders is reliant on our understanding of normal D1 and D2 SPN development. Here we provide the first detailed study and quantification of the striatal cellular and circuit changes occurring for both D1 and D2 SPNs in the first postnatal weeks using in vitro whole‐cell patch‐clamp electrophysiology. Characterization of their intrinsic electrophysiological and morphological properties, the excitatory long‐range inputs coming from cortex and thalamus, as well their local gap junction and inhibitory synaptic connections reveals this period to be highly dynamic with numerous properties changing. However it is possible to make three main observations. Firstly, many aspects of SPNs mature in parallel, including intrinsic membrane properties, increases in dendritic arbours and spine densities, general synaptic inputs and expression of specific glutamate receptors. Secondly, there are notable exceptions, including a transient stronger thalamic innervation of D2 SPNs and stronger cortical NMDA receptor‐mediated inputs to D1 SPNs, both in the second postnatal week. Thirdly, many of the defining properties of mature D1 and D2 SPNs and striatal circuits are already established by the first and second postnatal weeks, including different electrophysiological properties as well as biased local inhibitory connections between SPNs, suggesting this is guided through intrinsic developmental programmes. Together these findings provide an experimental framework for future studies of D1 and D2 SPN development in health and disease.

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A dysfunctional striatum is thought to contribute to neurodevelopmental disorders such as ADHD, Tourette's syndrome and OCD. Insight into these disorders is reliant on an understanding of the normal development of the striatal cellular and circuit properties. Here we combined whole-cell patch-clamp electrophysiology and anatomical reconstructions of D1 and D2 striatal projection neurons (SPNs) in brain slices to characterize the development of the electrophysiological and morphological properties as well as their long-range and local inputs during the first three postnatal weeks. Overall, we find that many properties develop in parallel but we make several key observations. Firstly, that the electrophysiological properties of young D1 SPNs are more mature and that distinctions between D1 and D2 SPNs become apparent in the second postnatal week. Secondly, that dendrites and spines as well as excitatory inputs from cortex develop in parallel with cortical inputs exhibiting a prolonged period of maturation involving changes in postsynaptic glutamate receptors. Lastly, that initial local connections between striatal SPNs consist of gap junctions, which are gradually replaced by inhibitory synaptic connections. Interestingly, relative biases in inhibitory synaptic connectivity seen between SPNs in adulthood, such as a high connectivity between D2 SPNs, are already evident in the second postnatal week. Combined, these results provide an experimental framework for future investigations of striatal neurodevelopmental disorders and show that many of the cellular and circuit properties are established in the first and second postnatal weeks suggesting intrinsic programs guide their development. Significance StatementNormal brain development involves the formation of neurons, which develop correct electrical and morphological properties and are precisely connected with each other in a neural circuit. In neurodevelopmental disorders these processes go awry leading to behavioral and cognitive problems later in life. Here we provide for the first time a detailed quantitative description of the cellular and circuit properties of the two main neuron types of the striatum during the first postnatal weeks. This can form an experimental framework for future studies into neurodevelopmental disorders. We find that most of the properties for both types of striatal neuron develop in parallel and are already established by the second postnatal week suggesting a key role for intrinsic programs in guiding their development.

The diversity of cell types and states can be scalably measured and defined by expressed RNA transcripts. However, approaches to programmably sense and respond to the presence of specific RNAs within living biological systems with high sensitivity are lacking. RNA sensors that gate expression of reporter or cargo genes would have diverse applications for basic biology, diagnostics and therapeutics by enabling cell-state specific control of transgene expression. Here, we engineer a novel programmable RNA-sensing technology, Reprogrammable ADAR Sensors (RADARS), which leverages RNA editing by adenosine deaminases acting on RNA (ADAR) to gate translation of a protein payload on the presence of endogenous RNA transcripts. In mammalian cells, we engineer RADARS with diverse payloads, including luciferase and fluorescent proteins, with up to 164-fold activation and quantitative detection in the presence of target RNAs. We show RADARS are functional either expressed from DNA or as synthetic mRNA. Importantly, RADARS can function with endogenous cellular ADAR. We apply RADARS to multiple contexts, including RNA-sensing induced cell death via caspases, cell type identification, and in vivo control of synthetic mRNA translation, demonstrating RADARS as a tool with significant potential for gene and cell therapy, synthetic biology, and biomedical research.