Ontogeny of the leucine-enkephalin neuron system of the rat: immunohistochemical analysis. I. Lower brainstem.

Ontogeny of the leucine-enkephalin (L-Enk) neuron system in the lower brainstem of the rat was investigated by means of indirect immunofluorescence. L-Enk-containing cells first appear in the primordium of the medullary reticular formation just medial to the n. tractus spinalis nerve trigemini at the level of the the rostral half of the inferior olivary nucleus, in the n. cuneiformis, and in the mesencephalic reticular formation of the fetus at gestational day 16 (14-15-mm embryos). From that time onward, L-Enk-containing cells appear in various areas of the lower brainstem one after another until birth. After birth, although L-Enk-containing cells decrease slightly in number as the rats grow, L-Enk-containing fibers continue to increase in number.