Monoamine receptors in the amygdaloid complex of the tree shrew (Tupaia belangeri).

Although it is well known that the mammalian amygdala comprises a heterogeneous complex of cytoarchitectonically and histochemically distinct nuclei, the association of these nuclei with different monoamine systems has not been described in detail. We therefore investigated the pattern of receptors for monoamines in the amygdala of the tree shrew (Tupaia belangeri). Binding sites for the alpha 2-adrenoceptor ligand (3H)rauwolscine, the alpha 1-adrenoceptor ligand (3H)prazosin, the beta-adrenoceptor ligand (125I)iodocyanopindolol, and the serotonin1A-receptor ligand (3H)8-hydroxy-2(di-n-propylamino)tetralin were visualized by in vitro autoradiography, and anatomically localized by comparing the autoradiograms to Nissl- and acetylcholinesterase-stained sections. To characterize binding of the radioligands pharmacologically, displacement experiments with different specific competitors were performed. Whereas the highest number of alpha 2-adrenergic binding sites was detected in the medial and the central nucleus as well as in the intercalated nuclei, the majority of serotonin1A binding sites was found in the magnocellular basal nucleus and the accessory basal nucleus, demonstrating a clear difference in the anatomy of the alpha 2-adrenergic and the serotonin1A receptor systems. In contrast, the pattern of alpha 1-adrenoceptor binding partially overlaps with that of both former receptor types. While the number of alpha-adrenergic and serotonin1A binding sites is relatively high in the tree shrew amygdala, there is a low number of beta-adrenergic binding sites in most nuclei. However, in the cortical nuclei, moderate to high numbers of binding sites for all radioligands are present. Therefore, according to our data on the tree shrew amygdala, which is anatomically similar to the amygdala of cats and primates, alpha 2-adrenoceptors cover primarily the medial part of the amygdaloid formation and serotonin1A-receptors predominantly occupy the basal nuclei, whereas alpha 1-adrenoceptors are present in both parts of the formation.