Brain peptides are neurotransmitter or neuromodulators in the adult brain. In the embryonic brain, they may play a role in the organization and development of neurons and synaptogenesis. We studied the ontogeny of cholecystokinin (CCK) and enkephalin in the rat brain from day 12 prenatal to adulthood by measuring the peptides and the mRNA levels in parallel in the same tissues. CCK-immunoreactive peptides (CCK-IR) and enkephalin immunoreactive peptides (enkephalin-IR) were measured by using a specific radioimmunoassay. Messenger RNA levels were measured by blot hybridization of poly(A)+ RNA with the corresponding cDNA probes. We also studied the ontogeny of tyrosine hydroxylase (TH), the limiting enzyme in the synthesis of noradrenaline and dopamine. The monoamines, their metabolites and the mRNA levels were measured comparatively in substantia nigra and locus ceruleus of the rat during development.
