Distribution of mRNA Encoding Three a2-Adrenergic Receptor Subtypes in the Developing Mouse Embryo Suggests a Role for the a2A Subtype in Apoptosis

a2-Adrenergic receptors (a2-ARs) respond to norepinephrine and epinephrine to mediate diverse physiological effects. Using in situ hybridization, the expression pattern of the mRNA encoding the three a2-AR subtypes (a2A, a2B, and a2C) was examined in the mouse embryo. The mRNA encoding the three subtypes was first detected at stage 9.5 days postcoitus (d.p.c.) for the a2A-AR (coincident with norepinephrine availability), 11.5 d.p.c. for the a2B-AR, and 14.5 d.p.c. for the a2C-AR subtype. The mRNA encoding the a2A-AR subtype shows both the earliest and the most widespread expression pattern, including developing stomach and cecum, many craniofacial regions and areas in the central nervous system. Strikingly, the a2A-AR mRNA is expressed in the interdigital mesenchyme between stage 12.5 and 14.5 d.p.c. in parallel with digit separation, raising the possibility that the a2A-AR might contribute to the apoptotic events underlying this process. To test whether a2A-AR can signal apoptotic events, the a2A-AR subtype was introduced into two mouse mesenchymal cell lines, C3H/10t1⁄2 and NIH-3T3; expression of the a2A-AR correlated with accelerated apoptosis, as detected both by the TUNEL assay and the loss of cell viability. In contrast to the wide distribution of mRNA encoding the a2A-AR subtype, the a2B-AR mRNA was detected only in the developing liver and was most readily detectable between 11.5 and 14.5 d.p.c., when the liver is the principal site of hematopoiesis. The a2C-AR mRNA is detected in the nasal cavity and cerebellar primordium only at $14.5 d.p.c. These studies represent the first characterization of the temporal and spatial expressions of the a2A-AR, a2B-AR, and a2C-AR subtypes during embryogenesis and provide important insights concerning the loci and possible roles of a2-AR-mediated regulation of physiological processes during the developmental program.