THE JOURNAL OF COMPARATIVE NEUROLOGY 308:505-512 (1991) Musculotopic Organization of the Hypoglossal Nucleus in the Grass Frog, zy Rand pipiens ALAN J. SOKOLOFF zyxwv Biological Anthropology, Harvard University, Cambridge, MA 02138 ABSTRACT Recent neural tracer studies in several mammalian species have demonstrated a similar musculotopic organization of the hypoglossal motoneurons which innervate individual tongue muscles. The distribution of this musculotopic organization in nonmammalian tetrapods, however, has not received detailed investigation. As part of an ongoing study on the comparative organization of the vertebrate hypoglossal nucleus, the musculotopic organization of the hypoglossal nucleus of zyxwvutsr Rana pipiens was studied by injection of lectin-conjugated horseradish peroxidase into four distinct tongue muscles and the geniohyoid muscle. Injections into the hyoglossus muscle label neurons in dorsal regions of the hypoglossal nucleus in middle and rostral nucleus levels. Injections into the genioglossus basalis muscle label neurons in ventral and lateral regions of the hypoglossal nucleus in caudal nucleus levels. Injections into the genioglossus medialis muscle label neurons in dorsal regions in caudal levels, throughout the nucleus in middle levels, and in ventral regions in more rostral levels. Injections into the geniohyoid muscle label neurons in the ventral tip of the hypoglossal nucleus and in the ventromedial corner of the medullary gray matter in middle and rostral nucleus levels. These results demonstrate that the organization of the hypoglossal nucleus in Rana pipiens is more complex than previous tracer studies indicated. Similarities in the musculotopic organization of the amphibian and mammalian hypoglossal nuclei suggest an evolutionary conservatism of the motor system controlling tongue movement. Key words: tongue musculature, brainstem, retrograde transport, amphibian The second spinal nerve in adult ranids originates from two nuclei in the medulla, a ventrolateral nucleus (VLNj merging caudally with the anterior horn of the spinal cord and a dorsomedial nucleus (DMN) located immediately ventral to the floor of the fourth ventricle (Stuesse et al., '83). Neurons innervating tongue musculature are located primarily in the dorsomedial nucleus, and this column of motoneurons is considered homologous to the hypoglossal nucleus of mammals (Black, '17; Matesz and Szekely, '77; Stuesse et al., '83; Oka et al., '87). Despite the documentation of a "true" hypoglossal nu- cleus in frogs there is disagreement on the details of anuran hypoglossal nucleus organization. Cytoarchitectonic stud- ies have suggested dorsal and ventral subdivisions within the dorsomedial nucleus (Barnard, '40; Senn, '72; Stuesse et zyxwvutsr al., '83), although some workers dispute the histological evidence for hypoglossal nucleus subdivisions (Opdam et al., '76). Studies of the innervation of tongue muscles in the common toad (Weerasuriya and Ewert, '81) and the Japa- nese toad (Satou et al., '85; Takei et al., '87) have shown that neurons innervating the hyoglossus muscle are located rostrally, while neurons innervating the genioglossus mus- cle are located caudally in the hypoglossal nucleus. This segregation, however, is not complete, and there is an extensive overlap of hyoglossus and genioglossus motoneu- rons at middle nucleus levels (Weerasuriya and Ewert, '81; Takei et al., '87). Retrograde tracer experiments on a number of mammals have demonstrated a discrete musculotopic segregation of hypoglossal motoneurons in the mediolateral and dorsoven- tral planes. In the rat, cat, and monkey motoneurons in dorsal or dorsolateral regions of the hypoglossal nucleus innervate tongue retrusor muscles (styloglossus and hyoglos- sus) while motoneurons in ventral or medial regions inner- vate the major tongue protrusor (genioglossus) (Krammer et al., '79; Uemura et al., '79; Uemura-Sumi et al., '81). The geniohyoid muscle usually is innervated by a separate motoneuron group lying ventral to the main body of the hypoglossal nucleus (Gammer et al., '79; Uemura et al., '79; Kitamura et al., '83, '85; Sokoloff, '89). The distribution of hypoglossal motoneurons has not been fully described in ranid frogs. In order to ascertain the Accepted March 18, 1991. A.J. Sokoloffs current address is Section of Population Biology, Morphol- zy ogy and Genetics, Brown University, Providence, RI 02912. o 1991 WILEY-LISS, INC.