DISTRIBUTION, QUANTIFICATION, AND MORPHOLOGICAL-CHARACTERISTICS OF SEROTONIN-IMMUNOREACTIVE CELLS OF THE SUPRALEMNISCAL NUCLEUS (B9) AND PONTOMESENCEPHALIC RETICULAR-FORMATION IN THE RAT

In their initial report on the rat, Dahlstrom and Fuxe ([1964] Acta Ph ysiol. Scand. 62:1-55) identified nine brainstem serotonin-containing cell groups, which they termed B1-B9. B9 has received considerably les s attention than other serotonergic nuclei (B1-B8) due in part to the fact that its precise location and extent have not been well documente d in subprimates. B9 (supralemniscal nucleus; SLN) has been viewed as a minor serotonergic cell group. In addition, 5-hydroxytryptamine (5-H T)-containing cells have been shown to be only sparsely distributed th roughout the pontomeseneephalic reticular formation (PMRF). By using 5 -HT immunohistochemical techniques, we examined the distribution and m orphological characteristics of SLN and PMRF 5-HT neurons of the ponto mesencephalic tegmentum. We showed that 5-HT cells of both the SLN and the PMRF extend rostrocaudally from the rostral midbrain to the midpo ns. 5-HT SLN cells are located within or dorsal to the medial lemniscu s (ML); those of the PMRF are widely distributed throughout the PMRF. The mean numbers of 5-HT containing cells in the SLN, PMRF, dorsal rap he, and median raphe nuclei were 4,571, 1,948, 15,191, and 4,114, resp ectively. The SLN (B9) contains more 5-HT neurons than any serotonergi c group other than the dorsal raphe nucleus. The dendrites of both SLN and PMRF 5-HT cells are primarily oriented mediolaterally and general ly extend for long distances (75-300 mu m), running perpendicular to t he fibers of the ML (SLN) or, to those coursing through the brainstem (PMRF). The present anatomical delineation of SLN and PMRF shows that they are major 5-HT-containing cell groups in the rat and provides the foundation for the further examination of their properties and functi ons. (C) 1997 Wiley-Liss, Inc.