Expression of alpha 2 adrenoceptors during rat brain development--I. Alpha 2A messenger RNA expression.
ABSTRACT The distribution of alpha 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 alpha 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 alpha 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 alpha 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 alpha 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 alpha 2A receptors in the fetal brain is in marked contrast to the postnatal development of the alpha 2C receptor type.
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ABSTRACT: Several lines of evidence indicate that the antinociception produced by intrathecal administration of the α2-adrenoceptor agonists dexmedetomidine or ST-91 is mediated by different subtypes of the α2-adrenoceptor. We recently provided additional pharmacologic evidence for this idea, as well as for differences in the function of these receptors between Harlan and Sasco rats, two widely-used outbred substrains of Sprague–Dawley rat. The present study used isobolographic analysis to further characterize the receptors at which intrathecally administered ST-91 and dexmedetomidine act in these two substrains. The rationale for these studies derives from the assumption that if dexmedetomidine and ST-91 act as agonists at the same receptor then they should interact in an additive manner. However, if they interact in a supra-additive manner, then they must act at different subtypes of the α2-adrenoceptor. In the tail-flick test, the dose–effect relationship for a 1:3 mixture of dexmedetomidine and ST-91 was shifted significantly to the left of the theoretical dose-additive line in both Harlan and Sasco Sprague-Dawley rats. A similar finding was made in the hot-plate test despite the fact that the dose–response characteristics of the agonists were different in this test. Thus, in Harlan rats, in which ST-91 is a full agonist and dexmedetomidine is essentially inactive, the dose–effect relationship for the mixture of dexmedetomidine and ST-91 was shifted far to the left of the dose-additive line. Similarly, in Sasco rats, in which ST-91 is a partial agonist and dexmedetomidine is inactive, co-administration of the two agonists also shifted the dose–response relationship to the left of the dose-additive line. The consistent finding that these two α2-adrenoceptor agonists interact in a supra-additive manner provides strong evidence that dexmedetomidine and ST-91 produce antinociception by acting at different α2-adrenoceptor subtypes in the spinal cord. This conclusion is consistent with the earlier proposal that dexmedetomidine acts predominantly at α2A-adrenoceptors whereas ST-91 acts predominantly at non-α2A-adrenoceptors. Recent anatomical evidence indicates that these non-alpha2A adrenoceptors may be of the α2C type. The synergistic combination of an α2A- and an α2C-adrenoceptor agonist may provide a unique and highly effective drug combination for the treatment of pain without the sedation produced by an equianalgesic dose of a single α2-adrenoceptor agonist.Pain 01/2000; 85(1):135-143. · 5.64 Impact Factor
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ABSTRACT: A variety of data suggest that noradrenaline and acetylcholine may interact in the basal forebrain, however no morphological studies have addressed whether indeed cholinergic neurons express adrenergic receptors. We have investigated the presence of alpha-adrenergic receptor subtype α2A -AR in cholinergic neurons of the basal forebrain. Cholinergic neurons were identified with an antibody against choline acetyltransferase and the receptor with a polyclonal antibody raised against a 47 amino acid fragment of the third intracellular loop of the α2A-AR. For double labeling at the light microscopic level the Ni-DAB/DAB technique was used, and for electron microscopy an immunoperoxidase/immunogold method was applied. We detected the α2A-AR protein in cholinergic as well as in non-cholinergic neurons. Almost half of all cholinergic neurons contained this adrenergic receptor. Double-labeled neurons were distributed throughout the rostro-caudal extent of the basal forebrain cholinergic continuum, including the medial septum, vertical and horizontal diagonal band nuclei, pallidal regions, substantia innominata and the internal capsule. Non-cholinergic neurons that expressed the α2A-AR outnumbered cholinergic/α2A-AR neurons by several factors. Electron microscopy confirmed the presence of α2A-AR in cholinergic neurons in the medial septum, vertical and horizontal diagonal band nuclei. Gold particles (10 nm) indicative of α2A-AR were diffusely distributed in the cytoplasm and accumulated in cytoplasmic areas near the Golgi complex and cysterns of the endoplasmic reticulum and were associated with the cellular membranes at synaptic and non-synaptic locations. Since many of the α2A-AR+/non-cholinergic neurons we detected are likely to be GABAergic cells, our data support the hypothesis that noradrenaline may act via basal forebrain cholinergic and non-cholinergic neurons to influence cortical activity.Journal of Neurocytology 01/2004; 33(3). · 1.94 Impact Factor
- Journal of Neurochemistry 08/2001; 78(4):685-693. · 3.97 Impact Factor