Ontogeny of cholecystokinin binding sites in the hindbrain of the laboratory rat

Studies in our laboratory have revealed a robust, transient expression of cholecystokinin binding sites in the facial motor nucleus during development in the Brazilian opossum, Monodelphis domestica. To investigate the ubiquity of this phenomenon, we have performed receptor autoradiography on the hindbrains of embryonic and neonatal rat pups. Cholecystokinin binding sites are present at very low levels in the embryonic day-16 rat hindbrain, but binding sites are abundant prior to birth. The greatest increase in labelled nuclei occurs prior to 5 days of postnatal age. Binding levels are heavy in the nucleus of the solitary tract, medial vestibular nucleus, posterior dorsal tegmental nucleus, area postrema, and caudal spinal trigeminal nucleus by 30 days postnatal. Both A-type and B-type receptors are present in the neonatal brainstem, although most labelled areas appear to be B-type. A-type binding sites are present in the ventral cochlear nucleus, the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, the area postrema, the spinal nucleus of the trigeminal, and the cuneate and gracile nuclei by 5 days postnatal. As reported for the Brazilian opossum, cholecystokinin binding sites are expressed in the facial motor nucleus of neonatal rats and are transient. In this study of the brainstem in laboratory rats, a transient expression is also observed in the rubrospinal tract, parvocellular reticular nucleus, raphe obscurus, cuneate and gracile nuclei, and the ventral median fissure of the spinal cord. As vasopressin binding sites and estrogen receptors have also been shown to be expressed transiently in the laboratory rat facial motor nucleus, the physiological and developmental significance of transient binding site expression remains to be elucidated.