Distribution, ontogeny and projections of cholecystokinin-8, vasoactive intestinal polypeptide and gamma-aminobutyrate-containing neuron systems in the rat spinal cord: an immunohistochemical analysis

The distribution, ontogeny and fiber projections of cholecystokinin-8, vasoactive intestinal polypeptide and gamma-aminobutyrate-containing neuronal systems in the rat spinal cord were investigated by means of immunocytochemistry. Immunoreactive fibers to cholecystokinin-8, vasoactive intestinal polypeptide and glutamate decarboxylase (gamma-aminobutyrate-synthesizing enzyme, used as a marker of gamma-aminobutyrate) were widely distributed in the spinal cord, being particularly concentrated in the superficial dorsal horn, suggesting a close relationship to the pain transmission system. Cholecystokinin-8-containing neurons were mostly distributed in the dorsal laminae and glutamate decarboxylase-containing neurons were distributed in both the dorsal and ventral horns. Vasoactive intestinal polypeptide-containing neurons were detected in the lateral spinal nucleus and the lamina X. Cholecystokinin-8 and vasoactive intestinal polypeptide immunoreactive structures first appeared on gestational day 17-18. Although no substantial change in immunoreactive structures was observed during the fetal period, they increased markedly after birth. On the other hand, glutamate decarboxylase-positive structures appeared at gestational day 16 and those in the grey matter reached a maximum content at birth; both groups were present in adult animals. Transection of the upper cervical cord resulted in accumulations of cholecystokinin-8 and glutamate decarboxylase rostral to the lesion, revealing the presence of supraspinal projections of cholecystokinin-8 and glutamate decarboxylase to the spinal cord. The same experimental procedure demonstrated the existence of vasoactive intestinal polypeptide-mediating neuronal projections to the supraspinal level, as the accumulating fibers occurred in the area caudal to the lesion.