Distinct distribution of two bombesin receptor subtypes in the rat central nervous system

We have previously demonstrated the presence of two distinct bombesin receptor subtypes in the rat CNS and distinguished them as bombesin/gastrin-releasing peptide (BBS/GRP) and neuromedin B (NMB)-preferring binding sites. In the present study, we conducted a complete evaluation of the distribution of these binding sites throughout the rat brain using in vitro receptor autoradiography. The BBS/GRP-preferring binding sites were characterized as those that bound ¹²⁵I-(Tyr⁴)BBS but not ¹²⁵I-(d-Tyr⁰)NMB. At these sites ¹²⁵I-(Tyr⁴)BBS binding was inhibited in the presence of 100 nM BBS but not by the same concentration of NMB. In contrast, NMB-preferring sites bound both radioligands and binding at these sites was inhibited in the presence of 100 nM NMB. Our results indicate that the distributions of BBS/GRP and NMB-preferring binding sites are widespread and distinct at all levels of the rat brain suggesting these peptides mediate separate functions in the rat central nervous system.