THE EFFERENT PROJECTIONS OF THE PERIAQUEDUCTAL GRAY IN THE RAT - A PHASEOLUS-VULGARIS-LEUKOAGGLUTININ STUDY .2. DESCENDING PROJECTIONS

The descending projections of the periaqueductal gray (FAG) have been studied in the rat using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The tracer was injected into the dorsolateral or ventrolateral subdivisions of the FAG at rostral or caudal sites. It was found that the patterns of the descending projections of the rostral and caudal parts of the dorsolateral FAG were the same and that the patterns of the descending projections of the rostral and caudal parts of the ventrolateral FAG were the same. However, the patterns of projections of the dorsolateral and ventrolateral FAG subregions were substantially different. These results suggest that the dorsolateral and ventrolateral parts of the FAG are organized into longitudinal columns that extend throughout the length of the FAG. The axons of FAG neurons descended through the pens and medulla via two routes. A small fiber bundle was present in the periaqueductal gray and in the periventricular area. This bundle distributed fibers and terminals locally within the periaqueductal gray and in the locus coeruleus and Barrington's nucleus. A larger bundle had a diffuse arrangement in the pontine reticular formation, however, and it had a more restricted distribution in the medulla, where it occupied a position dorsolateral to the pyramid. This bundle supplied structures in the pontine and medullary tegmentum. The dorsolateral column preferentially supplied the locus coeruleus, subcoeruleus, the gigantocellular nucleus pars alpha, the rostral part of the paragigantocellular nucleus, and the region of the A5 noradrenergic cell group. The ventrolateral column preferentially supplied the nucleus raphe magnus, the caudal part of the lateral paragigantocellular nucleus, and the rostroventrolateral reticular nucleus. (C) 1995 Wiley-Liss, Inc.