Specific binding of 125I-salmon calcitonin to rat brain: Regional variation and calcitonin specificity.

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Abstract
Rat brain particulate fraction contained binding sites for 125I-Salmon Calcitonin-I (125I-SCT). Maximum binding occurred in the physiological pH range of 7.25-7.5. The binding reaction proceeded in a temperature-dependent manner. Binding sites were broadly distributed among the various rat brain regions and considerable regional differences existed in the affinity and density as detected by Scatchard analysis. The highest affinity was recorded in the case of the hypothalamus and the lowest in the case of the cerebellum. The Kd (nM) and Bmax [maximum number of binding sites] (pmol/mg protein) estimated for the binding to 4 regions were as follows: hypothalamus: 1.4 and 0.19; midbrain, hippocampus plus striatum: 1.5 and 0.08; pons plus medulla oblongata: 3.0 and 0.15; and cerebellum: 8.3 and 0.20. Using a particulate fraction of rat brain void of cerebellum and cortices, a binding assay for calcitonins was developed. Binding of 125I-SCT was inhibited by unlabeled salmon, [Asu1,7]-eel [synthetic eel calcitonin [1,7-aminosuberic acid]-calcitonin]] and porcine calcitonins in a dose-dependent manner and the IC50 [median inhibitory concentration] were 2.0, 8.0 and 30 nM, respectively. The IC50 were comparable to those estimated using a kidney particulate fraction. Human calcitonin, .beta.-endorphin and substance P were weak inhibitors of the binding. Other peptides, drugs and putative neurotransmitters tested (totally 23 substances) failed to inhibit the binding at concentrations of 1.0 .mu.M. The physiological significance of brain binding sites for calcitonin, with the possibility that the brain may possess endogenous ligands for these sites, is discussed.