Ontogeny of cholecystokinin-8 containing neuron system of the rat: an immunohistochemical analysis--II. Lower brain stem.

The ontogeny of the cholecystokinin-8 neuron system in the lower brain stem of the rat was investigated by means of indirect immunofluorescence. Cholecystokinin-8-like immunoreactive structures first appeared in the primordium of the ventral tegmental area, ventral tegmental nucleus of Gudden and the area lateral to the superior central nucleus of the fetus on gestational day 17. Although cholecystokinin-8-like immunoreactive structures appeared gradually in various lower brain stem areas from this day, and developed markedly from postnatal day 6, thereafter, the ontogeny of cells and fibers was different. In general, the fibers increased in number as the rats grew and reached the maximum in adulthood, whereas the cells reached a maximum number between postnatal days 6-12, after which they decreased in number. Colchicine treatment, however, demonstrated the presence of cholecystokinin-8-like immunoreactive cells in various areas of the lower brain stem. These findings suggest that the cholecystokinin-containing neuron system is active in the adult and that cholecystokinin-containing cell somata cannot store a sufficient amount of the peptide for visualization in the absence of colchicine. Thus, it might be concluded that cholecystokinin may play an important role in the brain function of the adult as a neurotransmitter or neuromodulator, though its exact function is now open to discussion.