The regional and subcellular development of cholecystokinin immunoreactivity in vertebrate brain

Abstract

We have previously demonstrated that the development of CCK in rat brain occurs during the first postnatal month. In order to determine whether the appearance of CCK is associated with specific aspects of brain histogenesis, we examined the development of brain CCK immunoreactivity in both precocial and altricial mammals and birds. The two precocial species (guinea pig and chicken) were found to achieve adult CCK concentrations prenatally, while the altricial species (zebra finch and rat) manifested adult brain CCK concentrations only after several weeks of postnatal development. In adulthood, both mammals showed relatively high forebrain CCK concentrations, while the two species of birds manifested much lower forebrain levels. Brainstem levels of CCK were similar in all species studied. In each species, the development of CCK followed a common time course across all major brain areas, although adult brainstem levels of CCK were generally attained shortly before adult forebrain levels. Correlation of our comparative ontogenetic data with known patterns of brain histogenesis indicated that CCK development follows regional neuroblastic proliferation, migration and differentiation, and occurs during or soon after local synaptogenesis. In the rapidly developing precocial chicken brain, CCK production precedes the postnatal gliogenic and myelinogenic increases in brain weight, suggesting that neurogenic production of CCK occurs independently of these non-neuronal maturation events. Subcellular fractionation of developing chicken brain revealed that a substantial fraction of brain CCK is localized in synaptosomes relatively early in embryogenesis; this synaptosomal localization becomes even more pronounced with further brain maturation. This early appearance of CCK in synaptic terminals indicates a correspondingly precocial maturation for the intraneuronal mechanisms subserving peptide cleavage, axonal transport and vesicular insertion, and suggests that CCK may be available for neurotransmission quite early in development. In an analysis of the molecular forms of CCK, gel filtration disclosed no differences between species or different brain areas in the form of CCK present. CCK-8 always predominated in brain, with smaller void volume (pro-CCK) peaks, and negligible amounts of CCK-33. Finally, duodenal CCK (largely CCK-33) appeared much earlier than brain CCK in all species examined, suggesting that the gut and brain CCK systems develop independently of one another.