Two Parts of the Nucleus Prepositus Hypoglossi Project to Two Different Subdivisions of the Dorsolateral Periaqueductal Gray in Cat ESTHER MARIJE KLOP, 1 LEONORA J. MOUTON, 1 THOMAS EHLING, 2 AND GERT HOLSTEGE 1 * 1 Department of Anatomy and Embryology, University Medical Center Groningen, University of Groningen, 9700 AD Groningen, The Netherlands 2 GGZ Drenthe, 9411 PA Beilen, The Netherlands ABSTRACT The dorsolateral column of the mesencephalic periaqueductal gray (PAG) is a separate part of the PAG. Its afferent sources, efferent targets, and neurochemical properties differ from the adjacent PAG columns. The dorsolateral PAG is thought to be associated with aversive behaviors, but it is not yet understood how these behaviors are brought about. To elucidate the function of the PAG further, in the present study we investigated which brainstem regions project to the dorsolateral PAG. Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections involving the dorsolateral PAG, but extending into the lateral part, resulted in many retrogradely labeled cells in the pontine and medullary tegmentum bilaterally. However, it was concluded that these neurons were labeled from the lateral PAG, because no anterograde labeling was found in the dorsolateral PAG after a large injection into the tegmentum. Retrogradely labeled cells were also found in the nucleus prepositus hypoglossi (PPH), mainly contralaterally. Injections of [ 3 H]leucine or WGA-HRP in the PPH resulted in anterogradely labeled fibers in the dorsolateral PAG. Two separate distribution patterns were found. The caudal and intermediate PPH projected to a small region on the dorsolateral edge of the dorsolateral column, whereas the supragenual PPH distributed labeled fibers to all other parts of the dorsolateral PAG, except the area on the dorsolateral edge. These separate PPH projections suggest that two subdivisions exist within the dorsolateral PAG. The present findings suggest a role for the dorsolateral PAG in the oculomotor system. J. Comp. Neurol. 492:303–322, 2005. © 2005 Wiley-Liss, Inc. Indexing terms: nucleus supragenualis; gaze; saccade; oculomotor; mesencephalon; emotional motor system The mesencephalic periaqueductal gray (PAG) plays a pivotal role in the output of the limbic system. It is well known for its role in nociception control (Mayer et al., 1971; Liebeskind et al., 1973; Vanegas and Schaible, 2004) and emotional motor output (Holstege et al., 2004). Stim- ulation of the PAG in animals can elicit several integrated defensive behaviors such as aggression or freezing (Ban- dler and Depaulis, 1991; Carrive, 1993) but also more specific motor functions such as vocalization (Kanai and Wang, 1962), adopting mating posture (Sakuma and Pfaff, 1979), and micturition (Blok and Holstege, 1996). The PAG receives many afferents from major parts of the prefrontal cortex (Shipley et al., 1991) but also from the hypothalamus (Mantyh, 1982; Berk and Finkelstein, 1982; Holstege, 1987; Shipley et al., 1991), amygdala (Hopkins and Holstege, 1978; Mantyh, 1982), bed nucleus of the stria terminalis (Holstege et al., 1985), and several other limbic structures. Other afferents originate from large parts of the brainstem and spinal cord (Mantyh, 1982; Bjorkeland and Boivie, 1984; Bandler and Tork, *Correspondence to: Gert Holstege, Department of Anatomy and Embry- ology, University Medical Center Groningen, University of Groningen, Antonius Deusinglaan 1, Bldg. 3215, P.O. Box 196, 9700 AD Groningen, The Netherlands. E-mail: g.holstege@med.umcg.nl Received 24 January 2005; Revised 15 April 2005; Accepted 18 June 2005 DOI 10.1002/cne.20728 Published online in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 492:303–322 (2005) © 2005 WILEY-LISS, INC.