Neurons in the lateral sacral cord of the cat project to periaqueductal gray, but not to thalamus

Previous work of our laboratory has shown that neurons in the lateral sacral cord in cat project heavily to the periaqueductal gray (PAG), in all likelihood conveying information from bladder and genital organs. In humans this information usually does not reach consciousness, which raises the question of whether the lateral sacral cell group projects to the thalamus. After wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections into the sacral cord, anterogradely labeled fi bers were found in the thalamus, specifi cally in the ventral anterior and ventral lateral nuclei, the medial and intralaminar nuclei, the lateral ventrobasal complex/ ventroposterior lateral nucleus, and the nucleus centre médian, lateral to the fasciculus retrofl exus. Much denser projections were found to the central parts of the PAG, mainly to its dorsoand ventrolateral parts at caudal levels and lateral parts at intermediate levels. In a subsequent retrograde tracing study, injections were made in those parts of the thalamus that received sacral fi bers, as found in the anterograde study. Labeled neurons were observed in the sacral cord, but not in the lateral sacral cell group. In contrast, a small control injection in the caudal PAG resulted in many labeled neurons in the lateral sacral cord. These results suggest that afferent information regarding micturition and sexual behavior is relayed to the PAG, rather than to the thalamus. INTRODUCTION The lateral sacral cord gray matter in cats contains a specifi c group of neurons that projects to the periaqueductal gray (PAG; VanderHorst et al., 1996; Mouton and Holstege, 2000). Cells in this group are relatively small, predominantly fusiform or oval shaped, and located on the dorsolateral and lateral border of the dorsal horn of S1S2, in the lateral lamina V in the caudal S1 and further caudally in the medial lamina VI and lamina X in S2-S3. According to Mouton and Holstege (2000) this cell group contains approximately 10% of all spino-PAG neurons. Functionally, this group has been associated with the transmission of information from visceral structures to the PAG, because the location of these cells corresponds with the termination area of the primary afferents of genitals and bladder (Morgan et al., 1981; Nadelhaft et al., 1983; Ueyama et al., 1984; Thor et al., 1989). The PAG needs this information, because it plays a crucial role in the control of micturition and mating behavior (Sakuma and Pfaff, 1979; Blok and Holstege, 1994; Blok and Holstege, 1996; VanderHorst and Holstege, 1996; VanderHorst et al., 2000a). For example, with respect to micturition, the PAG is the main receiver of the afferent viscerosensory information from the bladder, because strong sacral projections reach the PAG, but not, at least in cat, the pontine micturition center (Blok et al., 1995).