Expression of alpha 2 adrenoceptors during rat brain development--II. Alpha 2C messenger RNA expression and [3H]rauwolscine binding.

The distributions of alpha 2C adrenoceptor messenger RNA and high-affinity [3H]rauwolscine binding sites were characterized in developing rat brain. Using in situ hybridization with 35S-labeled riboprobes directed against the third intracellular loop, alpha 2C messenger RNA expression appeared in an adult-like pattern during the first and second postnatal weeks, in the anterior olfactory nucleus, caudate-putamen, olfactory tubercles, islands of Calleja and hippocampus, following the time-course of maturation of these structures. Only in the cerebellum was alpha 2C messenger RNA transiently expressed during the critical period of granule cell development. High-affinity [3H]rauwolscine binding sites were detected using receptor autoradiography and revealed a similar spatial and temporal time-course of appearance during rat brain development. The highest numbers of binding sites were detected in the olfactory tubercles and islands of Calleja, and moderate numbers in the anterior olfactory nucleus, caudate-putamen and hippocampus. Like alpha 2C messenger RNA expression, high-affinity [3H]rauwolscine binding sites were transiently expressed in the cerebellum. In some areas (e.g., the substantia nigra), [3H]rauwolscine binding sites were detected even though alpha C2 messenger RNA expression was absent. The strong spatial and temporal correspondence between messenger RNA expression and radioligand binding supports the conclusion that [3H]rauwolscine selectively labels alpha 2C adrenoceptors in the rat brain. The developmental pattern which was observed is in marked contrast to the early, transient expression of the alpha 2A adrenoceptor. Thus, the alpha 2A and alpha 2C receptor types may serve distinct functional roles in the developing brain.