Distribution of mRNA encoding three alpha 2-adrenergic receptor subtypes in the developing mouse embryo suggests a role for the alpha 2A subtype in apoptosis.

alpha 2-Adrenergic receptors (alpha 2-ARs) respond to norepinephrine and epinephrine to mediate diverse physiological effects. Using in situ hybridization, the expression pattern of the mRNA encoding the three alpha 2-AR subtypes (alpha 2A, alpha 2B, and alpha 2C) 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 alpha 2A-AR (coincident with norepinephrine availability), 11.5 d.p.c. for the alpha 2B-AR, and 14.5 d.p.c. for the alpha 2C-AR subtype. The mRNA encoding the alpha 2A-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 alpha 2A-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 alpha 2A-AR might contribute to the apoptotic events underlying this process. To test whether alpha 2A-AR can signal apoptotic events, the alpha 2A-AR subtype was introduced into two mouse mesenchymal cell lines, C3H/10t1/2 and NIH-3T3; expression of the alpha 2A-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 alpha 2A-AR subtype, the alpha 2B-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 alpha 2C-AR mRNA is detected in the nasal cavity and cerebellar primordium only at > or = 14.5 d.p.c. These studies represent the first characterization of the temporal and spatial expressions of the alpha 2A-AR, alpha 2B-AR, and alpha 2C-AR subtypes during embryogenesis and provide important insights concerning the loci and possible roles of alpha 2-AR-mediated regulation of physiological processes during the developmental program.