IN-VITRO AUTORADIOGRAPHIC LOCALIZATION OF THE CALCITONIN RECEPTOR ISOFORMS, C1A AND C1B, IN RAT-BRAIN

In this study the distribution of the calcitonin receptor isoforms, C1 a and C1b, were mapped in rat brain using in vitro autoradiography and manipulation of their different pharmacological specificities. While salmon calcitonin binds to both receptors with high affinity, only the C1a receptor interacts with human calcitonin. Thus, the distribution of C1a specific binding sites was mapped using [I-125]human calcitonin . The C1b receptors were mapped using [I-125]salmon calcitonin in the presence of unlabelled human calcitonin and rat amylin, displacing bin ding of [I-125]salmon calcitonin to C1a and C3 (amylin) sites, respect ively. The distribution of C1a and C1b receptors was found to predomin antly overlap. Brain regions displaying C1a, but little or no C1b, bin ding sites included the nucleus of the solitary tract, area postrema a nd the intermediate lobe of the pituitary. Although there were no nucl ei expressing exclusively C1b receptors, parts of the mesencephalic an d pontine reticular formation, and the thalamic paraventricular nucleu s were enriched in C1b receptors relative to the density of C1a recept ors in other brain regions. These data indicate that the relative expr ession of the two receptor isoforms, although predominately parallel, is not uniform in the rat brain.