Immunocytochemical localization of serotonin-containing neurons in the myelencephalon, brainstem and diencephalon of the sheep.

Using immunocytochemistry, morphological characteristics and distribution of serotonin-containing neurons and fibers of the sheep myelencephalon, brainstem and diencephalon were studied, employing highly specific antibodies to serotonin. The immunocytochemical procedure described here allowed the visualization of endogenous, and thus presumably physiological, pools of serotonin, because no pharmacological treatments (colchicine, inhibitors of monoamine oxidase or 5-hydroxytryptophan) were used to increase the endogenous amount of antigen. The distribution of serotonin cell bodies observed in the study is in agreement with that described by other authors in the rat using a similar method. The present work also shows more numerous groups than the formaldehyde-induced fluorescence method, because five additional groups were revealed, designated S1 to S5. Compared with those in the rat, sheep serotonergic structures exhibit striking specific characteristics: (1) greater scattering of cell bodies within the different groups visualized, (2) absence of group B4 and hypothalamic groups, (3) only a weak serotonergic innervation of the suprachiasmatic nuclei area.