P2X4 Receptor Reporter Mice: Sparse Brain Expression and Feeding-Related Presynaptic Facilitation in the Arcuate Nucleus.

UNLABELLED: P2X4 receptors are ATP-gated cation channels that are widely expressed in the nervous system. To identify P2X4 receptor-expressing cells, we generated BAC transgenic mice expressing tdTomato under the control of the P2X4 receptor gene (P2rx4). We found sparse populations of tdTomato-positive neurons in most brain areas with patterns that matched P2X4 mRNA distribution. tdTomato expression within microglia was low but was increased by an experimental manipulation that triggered microglial activation. We found surprisingly high tdTomato expression in the hypothalamic arcuate nucleus (Arc) (i.e., within parts of the neural circuitry controlling feeding). Immunohistochemistry and genetic crosses of P2rx4 tdTomato mice with cell-specific GFP reporter lines showed that the tdTomato-expressing cells were mainly AgRP-NPY neurons and tanycytes. There was no electrophysiological evidence for functional expression of P2X4 receptors on AgRP-NPY neuron somata, but instead, we found clear evidence for functional presynaptic P2X4 receptor-mediated responses in terminals of AgRP-NPY neurons onto two of their postsynaptic targets (Arc POMC and paraventricular nucleus neurons), where ATP dramatically facilitated GABA release. The presynaptic responses onto POMC neurons, and the expression of tdTomato in AgRP-NPY neurons and tanycytes, were significantly decreased by food deprivation in male mice in a manner that was partially reversed by the satiety-related peptide leptin. Overall, we provide well-characterized tdTomato reporter mice to study P2X4-expressing cells in the brain, new insights on feeding-related regulation of presynaptic P2X4 receptor responses, and the rationale to explore extracellular ATP signaling in the control of feeding behaviors. SIGNIFICANCE STATEMENT: Cells expressing ATP-gated P2X4 receptors have proven problematic to identify and study in brain slice preparations because P2X4 expression is sparse. To address this limitation, we generated and characterized BAC transgenic P2rx4 tdTomato reporter mice. We report the distribution of tdTomato-expressing cells throughout the brain and particularly strong expression in the hypothalamic arcuate nucleus. Together, our studies provide a new, well-characterized tool with which to study P2X4 receptor-expressing cells. The electrophysiological studies enabled by this mouse suggest previously unanticipated roles for ATP and P2X4 receptors in the neural circuitry controlling feeding.