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, Cla and Clb, 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 Cla receptor interacts with human calcitonin. Thus, the distribution of Cla specific binding sites was mapped using [ ¹²⁵ I]human calcitonin. The Clb receptors were mapped using [ ¹²⁵ I]salmon calcitonin in the presence of unlabelled human calcitonin and rat amylin, displacing binding of [ ²⁵ I]salmon calcitonin to Cla and C3 (amylin) sites, respectively. The distribution of Cla and Clb receptors was found to predominantly overlap.Brain regions displaying Cla, but little or no Clb, binding sites included the nucleus of the solitary tract, area postrema and the intermediate lobe of the pituitary. Although there were no nuclei expressing exclusively Clb receptors, parts of the mesencephalic and pontine reticular formation, and the thalamic paraventricular nucleus were enriched in C16 receptors relative to the density of Cla receptors in other brain regions. These data indicate that the relative expression of the two receptor isoforms, although predominately parallel, is not uniform in the rat brain.