INNERVATION OF THE DURA-MATER-ENCEPHALI OF CAT AND RAT - ULTRASTRUCTURE AND CALCITONIN-GENE-RELATED PEPTIDE-LIKE AND SUBSTANCE P-LIKE IMMUNOREACTIVITY

Ultrastructural, immunocytochemical, and immunoelectron microscopical examinations are reported that describe the morphology of putative sen sory nerve endings in the dura mater encephali of the rat and the cat. Morphometrical measurements and reconstructions showed that in the ca t the mean diameter of axons, the bare area of axolemma, and the conte nt of mitochondria and vesicles are highly variable in dural nerve end ings. Nerve fibers with a high volume density of mitochondria are thou ght to be sensory, while nerve fibers containing many small vesicles a re considered autonomic. There is, however, a broad overlap of mitocho ndria-rich and vesicle-rich nerve fibers in the dura, so that discrimi nation between sensory and autonomic endings by these characteristics frequently fails. Whole-mount preparations treated cytochemically for detection of substance P- and calcitonin gene-related peptide-like imm unoreactivity in the rat and the cat showed a network of immunopositiv e nerve fibers in the vicinity of dural blood vessels. Most of these p eptidergic and probably sensory nerve fibers were found terminating in the dural connective tissue far from vessels. Calcitonin gene-related peptide-positive nerve fibers were much more abundant than substance P-positive fibers. Immunoelectron microscopic preparations revealed th at calcitonin gene-related peptide- and substance P-like immunoreactiv ity is found in a small proportion of generally thin unmyelinated nerv e fibers. These proportions were very similar in the rat and the cat. Summarizing the recent literature, the morphological characteristics o f putative sensory nerve fibers in the dura mater are discussed in rel ation to their possible functional significance for neurogenic inflamm ation and nociception.