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 behavioural and physiological responses to potentially threatening or stressful stimuli. Underlying these behaviours is the diverse interconnectivity of this region, and it is possible that neurochemical subdivisions within the FAG reflect the functional properties of the different FAG regions. In this study, using in situ hybridization, we have investigated the distribution in the rat FAG of the messenger ribonucleic acids (mRNAs) encoding seven neuropeptides: enkephalin (ENK), substance P (SP), somatostatin (SST), galanin (GAL), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP). Each peptide mRNA had a distinct topographical distribution in the FAG. Preproenkephalin A (ENK) mRNA-expressing cells were found at all levels of the FAG in three distinct longitudinal columns. Preprotachykinin A (SP)-expressing cells were found at all levels of the FAG, principally in the Edinger-Westphal nucleus and the lateral and dorsal FAG. There was a column of neurones producing mRNA-encoding somatostatin that extended along the rostrocaudal extent of the ventrolateral FAG; there were also labelled cells in the dorsal and dorsolateral subdivisions at some levels of the FAG. Galanin mRNA-producing neurones were limited to the dorsal raphe nucleus and to a second population in the ventral border of the aqueduct. VIP mRNA-producing neurones were found in very localized regions of the FAG, including the cell-sparse region immediately ventral to the aqueduct and the ventral part of the dorsal raphe nucleus. NPY mRNA-producing neurones were localized mainly in some cells of the Edinger-Westphal nucleus and dorsal raphe nucleus. CGRP mRNA-expressing neurones were limited to the oculomotor and trochlear nucleus. The results showed a topographical distribution of neuropeptides over the rostrocaudal extent of the FAG that is compatible with the emerging theory that the anatomical and functional specificity of the FAG is expressed in the form of longitudinally arranged neuronal columns that extend for varying distances along the rostrocaudal axis of the midbrain FAG. (C) 1994 Wiley-Liss, Inc.