The distribution of substance P and met-enkephalin in the periaqueductal gray matter of the rat.

The distribution in the periaqueductal gray matter (PAG) of substance P (SP) and Met-enkephalin (Met-enk), two neuropeptides related to pain modulation, was investigated. The study was carried out at different mesencephalic levels, with immunocytochemical methods in both "normal" and colchicine treated rats. The SP immunoreactive structures are made up of uniformly distributed fibers and neurons. The latter are particularly numerous in the ventral PAG and their number increases in a caudo-cranial direction. Along the midbrain axis, however, no significant variation of the Met-enk elements was observed. The Met-enk fibers are most numerous in lateral PAG, together with the few positive cells, which are also present. The comparison between the present results and others reported in the literature suggests that there may be species differences in the distribution of the two neuropeptides. The distribution of immunostructures has been discussed in the light of the physiological and cytoarchitectural features of PAG.