Localization of some neuropeptide- and serotonin-like immunoreactivities in the vegetative network of guinea pig spinal cord.

The localization of Substance P(SP)-, Methionine-Enkephalin(met-Enk)-, Somatostatin(SOM)- Serotonin(SER)-, Cholecystokinin(CCK)-, and Vasoactive intestinal polypeptide (VIP)-like immunoreactivity (-LIR) has been determined immunocytochemically in the thoracic spinal cord intermediate zone of male and female guinea pigs. All neuroactive substances studied are exclusively localized in nerve fibre varicosities and terminals building up the vegetative network of the thoracic spinal cord intermediate zone. This network is situated dorsally to the central canal as a longitudinal plate of approximate thickness of 90-100 microns. Immunoreactive fibres are observed in the two Fasciculi longitudinales laterales and the two Fasciculi longitudinales mediales which are interconnected by transverse and oblique peptide-containing bundles (the terminology used by Petras and Cummings 1972; Galabov and Davidoff 1976). All these bundles interconnect the nuclei intermediolaterales principales and funiculares, the nuclei intercalates spinales and the nuclei intercalates paraependimales in ipsi- and contralateral as well as in rostral and caudal direction. The neurones of these nuclei are surrounded by immunoreactive varicosities and terminals. The quantity of the immunoreactive structures and intensity of the staining varied for the different neuroactive substances. As to the origin of the vegetative network immunoreactive fibres three main possibilities exists: a). From primary afferent neurones situated in the dorsal root ganglia, which send their axons via the dorsal roots (mainly for SP and perhaps for CCK); b). From supraspinal neurones which send their axons descending in the white matter funiculi and in the fasciculi longitudinales laterales and mediales and c). From intrinsic spinal cord neurones, which send their neurites in ascending and descending directions, ipsi- and contralaterally and interconnect the spinal cord segments. The different origin of the vegetative network immunoreactive fibres as well as the complex innervation of the preganglionic sympathetic nerve cells in the intermediate zone of the spinal cord suggests that this network may play an important role in the integration of the central and peripheral vegetative nervous system as well as probably in the integration of the somatic and the vegetative nervous system.