REGIONAL SUBDIVISIONS IN THE MIDBRAIN PERIAQUEDUCTAL GRAY OF THE CAT REVEALED BY INVITRO RECEPTOR AUTORADIOGRAPHY

Abstract

The periaqueductal gray (PAG) is a midline structure that encircles the mesencephalic aqueduct. While appearing homogeneous by some neuroanatomical criteria (Mantyh, 1982), recent studies in the rat of cell clustering and cytochrome oxidase activity have suggested the existence of four subdivisions in the PAG with different neuronal diameters and neuronal packing densities (Beitz, 1985; Conti et al., 1988). [Earlier studies described somewhat different subdivisions of the PAG in the cat (Hamilton, 1973)]. These regions also have differential connectivities with descending (hypothalamic, cortical and limbic) and ascending (medullary and spinal) afferents and reciprocal efferent connections, a..