Distribution of B/K protein in rat brain

B/K protein is a recently isolated member of the double C2-1ike-domain prot ein family, which is highly abundant in rat brain. We generated high-titer rabbit polyclonal antibodies with specificity to the 55-kDa rat B/K protein , and examined the expression pattern of B/K protein in rat brain using an immunohistochemical staining method. Immunoreactivity to B/K protein was wi dely found in distinct regions of rat brain: strongly in the hypothalamus, most of the circumventricular organs, the locus coeruleus, the A5 neurons o f the pens, and the anterior pituitary; moderately in the anterior olfactor y nucleus, the raphe nucleus, the subfornical organ, and the median eminenc e; and faintly in the olfactory bulb, the telencephalon, the substantia nig ra pars compacta, and the ventral tegmental area. In contrast, immunoreacti vity to B/K protein was not observed in the thalamus, the cerebellum, the p osterior pituitary, or the spinal cord. In most of the B/K-expressing neuro ns, immunoreactivity was expressed mainly in soma but not in nerve fibers. B/K was also expressed in nonneuronal cells such as the tanycytes and the s ubcommissural organ. In the vasopressin-secreting supraoptic and paraventri cular nuclei of the hypothalamus, the site where B/K cDNA was originally is olated from, all of the neurons showing vasopressin immunoreactivity also e xpressed B/K protein, suggesting an overlap of their expression patterns.