Comparison of gene expression for two distinct bombesin receptor subtypes in postnatal rat central nervous system

The two mammalian bombesin-like peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB), are structurally related neuropeptides reported to elicit a variety of responses in different cell types, including modulation of neuronal activity and growth. Two distinct bombesin receptor subtypes have recently been characterized: a mouse GRP-preferring receptor (GRP-R) derived from Swiss 3T3 fibroblasts and a rat esophagus NMB-preferring bombesin receptor (NMB-R). In this study, we report the cloning and structural characterization of the rat GRP-R cDNA. The rat GRP-R and NMB-R cDNAs were used to map the regional distribution of these two bombesin receptor subtypes in rat CNS using in situ hybridization. Throughout postnatal development, cells expressing either receptor mRNA were found within the forebrain, brain stem, and spinal cord. The most distinct expression patterns were observed in the forebrain, where GRP-R mRNA was prominent in the hypothalamic region but NMB-R mRNA expression was strongest in olfactory and thalamic regions. In other brain regions, including dentate gyrus and nucleus ambiguus, both receptor mRNAs were detected. In the majority of brain regions, the adult pattern of expression was seen at birth. However, in several CNS regions, GRP-R and NMB-R mRNA showed striking changes during postnatal development (PN1, PN9, and PN16). In the dentate gyrus, GRP-R and NMB-R mRNA expression increased, while in the caudate-putamen and lateral cerebellar nucleus, expression decreased. These regional changes in receptor expression during postnatal life may be significant for both normal brain growth and development.