The distribution and cytological characteristics of the serotonincontaining cell bodies in the brain stem of the squirrel monkey (Suimiri sciureus) are described using the fluorescence histochemical technique of Falck and Hillarp for the demonstration of monoamines. Eight groups of serotonin-containing neurons were found, located predominantly in the midline raphe region of the brain stem within defined nuclei, such as the nucleus raphe obscurus, nucleus raphe pallidus, nucleus raphe magnus, nucleus raphe pontis, nucleus raphe dorsalis and nucleus centralis superior. Portions of some of the groups also extend laterally in the brain stem, particularly at the levels of the upper pons and midbrain. The appearance and distribution of the serotonin-containing groups of cell bodies in the squirrel monkey were found to be somewhat similar to that described for the rat, but differed in at least one major respect from that reported for the cat. This paper dealing with the distribution and cytological characteristics of serotonincontaining cells is the third in a series of reports concerning the histochemical localization and identification of monoaminecontaining cell bodies in the brain stem of the squirrel monkey (Saimiri sciureus) by the gas-phase formaldehyde-induced fluorescence technique (Carlson, Falck and Hillarp, '62; Falck, '62; Falck, Hillarp, Thieme and Torp, '62). In the next and final paper in this series, the distribution of the serotonin-and catecholaminecontaining groups of cell bodies will be indicated in an atlas of Nisd and myelinstained frontal sections of the brain stem of the squirrel monkey.Although the distribution of the serotonin-containing cells has been described in lower mammals, particularly in the rat (Dahlstrom and Fuxe, '65) and cat (Pin, Jones and Jouvet, '68), no detailed study in a primate has been reported. In view of the importance of serotonin to particular functions of the brain, and in consideration of the evolutionary differences between these lower mammals and man, this work was undertaken in an animal p h y b genetically more closely related to man. A New World primate, the squirrel monkey, is ideal for this research because of its relatively small size and increasingly frequent use in behavioral and neurophysiological research. MATERIALS AND METHODSThe procedures for this study were detailed in the MATERIALS AND METHODS section of the first paper in this series, which dealt with the locus caeruleus of the squirrel monkey (Hubbard and Di Carlo, '73). Briefly, the brain stems of young squirrel monkeys were rapidly removed, isolated, and cut into sections 2 mm thick in a frontal plane to facilitate stereotaxic correlations. The thin blocks of tissue were frozen in liquid propane cooled by liquid 1 Supported in part by a U.S.P.H.S. GRS grant to Vincenzo Di Carlo. This work constituted a portion of the dissertation submitted by Jack E. Hubbard to the

In this second paper of the series on the monoamine-containing cell bodies found in the brain stem of the squirrel monkey (Saimiri sciureus) by the fluorescence histochemical technique, a description is presented of the catecholaminergic neurons. The first paper was a report on the locus caeruleus ( a norepinephrine containing group), which, in consideration of its physiological importance, was the object of a particularly detailed study. The other groups of catecholqminergic cell bodies were found to have a topographic distribution and general morphological characteristics quite similar to those described for the rat. In particular, in the squirrel monkey brain stem homologous groups of neurons were observed for each group described in the rat, with the exception of the weakly fluorescing group A3. In comparing the findings on the squirrel monkey with the description of the catecholaminergic cell groups in the cat some significant differences were noticed.Utilization of the fluorescence histochemical technique for monoamines (Carlsson, Falck and Hillarp, '62; Falck, '62; Falck, Hillarp, Thieme and Torp, '62) in studies on lower mammals, especially in the rat (Dahlstrom and Fuxe, '65) has allowed other workers to reveal that the majority of the monoamine-containing cell bodies in the central nervous system are located within fairly circumscript cell groups in the brain stem. In view of the significance of the monoamines as probable neurotransmitters as well as their probable clinical significance in neurology and psychiatry, and in consideration of the large phylogenetic gap between the lower mammals and man, a systematic study of the monoamine-containing cell bodies in the brain stem of a primate was undertaken. This report on the catecholaminecontaining groups of cells is the second in a series of papers dealing with the localization and identification of monoamine-containing groups of cell bodies in the brain stem of the squirrel monkey (Saimiri sciureus). The serotonincontaining groups of cells will be considered in the next paper and, in the final paper, an atlas will be presented showing the position of these groups in frontal sections of the brain stem. MATERIALS AND METHODSThe procedures for this study were described in the MATERIALS AND METHODS section of the first paper, which dealt with the locus caeruleus of the squirrel monkey (Hubbard and Di Carlo, '73). Briefly, the brain stems were rapidly removed, isolated, and cut into sections 2 mm thick in a frontal plane to facilitate stereotaxic correlations. The thin blocks of tissue were frozen in liquid propane cooled by liquid nitrogen, freeze-dried, treated with hot vapors from standardized paraformaldehyde, vacuum embedded in paraffin, sectioned, and mounted. Groups of catecholamine-containing cell bodies identified by

The topography of the various groups of catecholamine-and serotonin-containing cell bodies in the squirrel monkey (Saimiri sciureus ), as described in the preceding three papers of this series, is presented in an atlas of Nissl and myelin stained frontal sections. The major findings of this study are reviewed and discussed. These include : ( 1 ) evidence of fundamental similarities in the topographic distribution of the cell groups between the primate and the rat and of significant differences between the primate and the cat; ( 2 ) identification as the catecholaminergic cells of the locus caeruleus of a cellular group incorrectly identified in the literature as nucleus tracti mesencephalici nervi trigemini; (3 ) consolidation of several catecholamine-containing cell groups as belonging respectively to the locus caeruleus (C4, C6, C7) and the substantia nigra (C8 and C9); ( 4 ) predominant distribution of the serotoninergic cell groups in the midline raphe region (with lateral extensions in some cases) and location within well defined nuclei, such as the nucleus raphe obscurus, nucleus raphe pallidus, nucleus raphe magnus, nucleus raphe pontis, nucleus raphe dorsalis and nucleus centralis superior.The purpose of this atlas is to provide the reader with a pictorial presentation of the distribution of the groups of catecholamine-and serotonin-containing cell bodies found in the brain stem of the squirrel monkey, as reported in the first three papers of this series. The procedures for the study were described in detail in the MATERIALS AND METHODS section of the first paper, which dealt with the locus caeruleus (Hubbard and Di Carlo, '73). Photographs were taken of the Nissl and myelin stained sections which were adjacent to those which were to be used for fluorescence observations. Those regions which contained serotoninergic or catecholaminergic cell bodies were marked on the photographs of the stained sections. This atlas represents a compilation of those photographic records.The atlas is composed of two parts: the first portion (figs. 1, 2) presents macroscopic views of the fixed whole brain as well as the brain stem. In the second part ( figs. 4-25 2 The results of the study on monoamine-containing cell bodies in the brain stem of the squirrel monkey described in the present series of papers were summarized in a brief report at the meeting of the American Society of Zoologists in Philadelphia (December 26-31, 1971). An abstract of such report was pub. lished in Am. Zoologist, 11: 704; 1971.