DISTRIBUTION OF MESSENGER-RNAS ENCODING ENKEPHALIN, SUBSTANCE-P, SOMATOSTATIN, GALANIN, VASOACTIVE INTESTINAL POLYPEPTIDE, NEUROPEPTIDE-Y, AND CALCITONIN-GENE-RELATED PEPTIDE IN THE MIDBRAIN PERIAQUEDUCTAL GREY IN THE RAT

The midbrain periaqueductal grey matter (PAG) has numerous functional roles that include mediating nociceptive inhibition and integrating be havioural and physiological responses to potentially threatening or st ressful stimuli. Underlying these behaviours is the diverse interconne ctivity of this region, and it is possible that neurochemical subdivis ions within the FAG reflect the functional properties of the different FAG regions. In this study, using in situ hybridization, we have inve stigated the distribution in the rat FAG of the messenger ribonucleic acids (mRNAs) encoding seven neuropeptides: enkephalin (ENK), substanc e P (SP), somatostatin (SST), galanin (GAL), vasoactive intestinal pol ypeptide (VIP), neuropeptide Y (NPY), and calcitonin gene-related pept ide (CGRP). Each peptide mRNA had a distinct topographical distributio n in the FAG. Preproenkephalin A (ENK) mRNA-expressing cells were foun d at all levels of the FAG in three distinct longitudinal columns. Pre protachykinin A (SP)-expressing cells were found at all levels of the FAG, principally in the Edinger-Westphal nucleus and the lateral and d orsal FAG. There was a column of neurones producing mRNA-encoding soma tostatin that extended along the rostrocaudal extent of the ventrolate ral FAG; there were also labelled cells in the dorsal and dorsolateral subdivisions at some levels of the FAG. Galanin mRNA-producing neuron es were limited to the dorsal raphe nucleus and to a second population in the ventral border of the aqueduct. VIP mRNA-producing neurones we re found in very localized regions of the FAG, including the cell-spar se region immediately ventral to the aqueduct and the ventral part of the dorsal raphe nucleus. NPY mRNA-producing neurones were localized m ainly in some cells of the Edinger-Westphal nucleus and dorsal raphe n ucleus. CGRP mRNA-expressing neurones were limited to the oculomotor a nd trochlear nucleus. The results showed a topographical distribution of neuropeptides over the rostrocaudal extent of the FAG that is compa tible with the emerging theory that the anatomical and functional spec ificity of the FAG is expressed in the form of longitudinally arranged neuronal columns that extend for varying distances along the rostroca udal axis of the midbrain FAG. (C) 1994 Wiley-Liss, Inc.