Autoradiographic visualization and characteristics of [ 125I]bradykinin binding sites in guinea pig brain

Abstract

The present study was undertaken to localize and characterize bradykinin (BK) binding sites in 10 μm serial sections of guinea pig brain by in vitro quantitative receptor autoradiography. Specific binding of [ 125I-Tyr 8]bradykinin ([ 125I]BK) was localized in the medulla oblongata to the regions of the nucleus of the solitary tract (nTS), the area postrema (AP), the dorsal motor nucleus of the vagus (X) and the caudal subnucleus of the spinal trigeminal nucleus. No significant specific [ 125I]BK binding was seen in other brain regions. The specific binding (85–90% of total binding) was of high affinity and saturable with a K D of73.5±9.9pM and a B max of27.8± 1.9 amol per mm 2 of tissue. In competition studies, the rank order of potencies was:BK>Met-Lys-BK>Lys-BK> >Des-Arg 9-BK. The B 2 receptor antagonist d-Arg 0-Hyp 3-Thi 5,8- d-Phe 7-BK inhibited [ 125I]BK binding with a K i value of3.5±1.5nM while Des-Arg 9-[Leu 8]-BK, a B 1 receptor antagonist did not significantly inhibit [ 125I]BK binding in concentrations up to 10μM. Our finding of specific high affinity [ 125I]BK binding sites in the nTS, AP and the X is important because these brain areas are known to be involved in central cardiovascular regulation. Moreover, our results suggest that the specific [ 125I]BK binding sites in the guinea pig medulla are of the bradykinin B 2 receptor type.