Adenosine 5'-triphosphate (ATP) has been shown to be an important extracellular signaling
molecule that mediates various physiological activities via the P2 (P2X and P2Y) receptors.
However, information on the expression patterns of the P2 receptors during mammalian
embryogenesis is limited. We therefore examined the expression patterns of different P2
receptor subtypes in rat embryos. In the hindbrain neural tube, the P2X3 receptor was
transiently expressed at embryonic day E11 in the cranial motor neurons and the outgrowing
axons. ATP significantly inhibited neurite outgrowth from neural tube explants. P2X3
receptors were also prominently expressed in sensory ganglia at this early stage and were
coexpressed with P2X2 receptors in E16.5 embryos. Other P2X receptor subtypes were
observed in different brain regions such as subventricular zones, the site of postnatal
neurogenesis. In addition, the P2Y receptor expression was detected in the somites and
subsequently in the developing skeletal muscle but was downregulated as development
proceeded. While the P2Y1 receptor was no longer expressed in the adult skeletal muscle, the
expression of P2Y2 receptor was present, although restricted in the satellite cells and the P2Y4
receptor showed reduced expression in adult skeletal muscle. Likewise, the expression of the
P2Y receptors was initially expressed throughout the myocardium (E12) but was gradually
restricted to the trabeculated myocardium (E14-18). Studies on Ca2+ influx showed that particular P2 receptor subtypes of P2X2, P2X4, P2Yi, P2Y2, P2Y4 and P2Y6 receptors
responded to nucleotides in E14 cardiomyocytes. P2X7 receptor expression was detected in
developing pancreatic islet cells and later coexpressed with glucagon in a-cells. In addition,
transient expression of the P2X7 receptor in insulin-expressing cells was observed in the
embryonic, but not in adult, islet cells. Together, the results indicated that widespread and
dynamic expression of P2 receptors was found in the three-germ layer-derived embryonic
tissues, particularly in some transient embryonic structures during development, which
suggested they may be important in embryonic organogenesis