Cholecystokinin (CCK) is now recognized as one of the most abundant peptides in the mammalian central nervous system. We have previously used immunohistochemistry to localize CCK in the adult and developing Brazilian opossum brain. However, little is known about the distribution of CCK binding sites in the developing mammalian brain. Therefore, to further our knowledge of the sites of action for CCK during development, we initiated a series of studies to localize CCK binding sites in the adult and developing Brazilian opossum. This species was chosen because pups are born in a fetus‐like state. Receptor autoradiography was performed on coronally sectioned brains of 1 to 60 day postnatal (PN) animals and adults with 125I‐Bolton Hunter‐CCK‐8 as the radioligand. Binding is evident in the 1PN opossum brainstem and is observed in the developing forebrain by 5PN. Region‐specific binding increases during development, and binding in the 35PN brain resembles the adult pattern. Binding is evident prior to the detection of CCK‐like immunoreactivity in many areas. The facial motor nucleus is identifiable and exhibits high levels of binding in Brazilian opossum pups of 10 to 35 days of age. However, binding is undetectable in the facial motor nucleus of 45 and 60PN pups. In general, the binding patterns for CCK in the adult opossum resemble those of other mammals and likely mediate similar physiological functions. However, some cholecystokininergic pathways appear to be unique to neonatal mammals. © 1993 Wiley‐Liss, Inc.
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