Expression of α2 adrenoceptors during rat brain development—I. α2A messenger RNA expression

Abstract

The distribution of α 2A adrenoceptor messenger RNA expression in developing rat brain was characterized using in situ hybridization with 35S-labeled riboprobes. Intense hybridization signal was detected as early as embryonic day 14 in several areas adjacent to the forebrain and hindbrain germinal zones and in central noradrenergic neurons. A marked increase in messenger RNA expression was observed throughout the brain during late prenatal development, consistent with the migration and maturation of neurons in developing brain structures. In embryonic brain, there was a temporal and spatial correspondence in the appearance of α 2A messenger RNA expression and binding sites labeled with [ 3H]idazoxan or p-[ 125I]iodoclonidine, indicating translation into receptor protein at an early stage of development. Whereas the presynaptic expression remained constant throughout development, there was an early postnatal decline of α 2A receptor expression in many brain regions, including the olfactory bulb, cortex, caudate–putamen, hippocampus, thalamus, hypothalamus and medulla. Thereafter, messenger RNA expression increased, establishing an adult-like pattern during the second postnatal week, but remained low in areas such as the caudate–putamen, thalamus and hippocampus, which do not exhibit extensive expression in the adult. The transient perinatal expression of this α 2 adrenoceptor type, which coincides with a period of hyperreactivity to sensory stimuli in the locus coeruleus, may indicate a specific functional role for the α 2A receptor in the developing rat brain. The early and intense expression in olfactory structures suggests an involvement in early olfactory learning. The pattern of widespread, transient expression of α 2A receptors in the fetal brain is in marked contrast to the postnatal development of the α 2C receptor type.