Distribution and morphology of serotonin-immunoreactive neurons in the brainstem of the New Zealand white rabbit.

The aim of the present study was to demonstrate the morphology and distribution of the serotonergic neurons in the brainstem of the New Zealand white rabbit by using a highly specific immunocytochemical procedure. It was possible to divide the serotonergic neurons into a rostral group, which is situated in the mesencephalon and the rostral part of the pons containing four serotonergic nuclei, and a caudal group, which is located in the medulla and the caudal part of the pons containing five serotonergic nuclei. The localization of the serotonergic neurons is presented in a detailed brainstem atlas, and the distribution of the serotonergic neurons is in accordance with results obtained by other authors in different species. Special emphasis was given to the fact that many of the serotonergic neurons were distributed in more lateral parts of the brainstem. The laterally orientated neurons, which were large and multipolar, were morphologically different from the serotonergic neurons in the midline, which were mostly small and relatively nonpolar. The serotonergic system of the New Zealand white rabbit has undergone a major lateralization, like the serotonergic system of man and higher primates, and it may therefore be excellently suited for experimental procedures directed towards the serotonergic system. The difference between serotonergic neurons localized in the midline and those situated laterally may reflect functional differences based on dissimilarity in connectivity and morphology, and this possible subspecialization of the serotonergic system is discussed in the context of present knowledge of serotonergic anatomy and function.