IMMUNOLOCALIZATION OF TYPE-I OR TYPE-II ACTIVIN RECEPTORS IN THE RAT-BRAIN

We have studied immunolocalization of activin receptors in the central nervous system using polyclonal antibodies (IgG) to type I (50-55 kDa , ActRI), type II (70-75 kDa, ActRII) or a subtype of type II known as type IIB (ActRIIB) receptors of activin, A total of I antisera to rat activin receptors was generated, i.e. 3 kinds of antisera to the extr acellular domain (ActRI(81-89), ActRII(91-100), or ActRIIB(90-99)) and 4 antisera to the kinase domain (ActRI(323-333), ActRII(307-319), Act RII(407-420) or ActRIIB(306-319)). The region of aa 407-420 of ActRII is identical with that of ActRIIB. At first, we characterized these an tibodies by Western blot analysis using ovarian proteins fractionated by preparative SDS-PAGE, All antibodies to ActRII and ActRIIB specific ally reacted with 75 kDa-proteins which could also bind to activin-A. Anti-ActRII(91-100) antibody also reacted with 62 kDa-proteins which w ere capable of binding with activin-A, Although no positive reactions to anti-ActRI(81-89) antibody were seen in ovarian proteins, a positiv e reaction was detected at 52 kDa only when the proteins were deglycos ylated. By use of these antibodies, immunolocalization of activin rece ptors was examined in the rat brain, The patterns of expression of act ivin type I and type II receptors were different. Positive reactions t o anti-ActRII(91-100) antibody were detected in neurons of the cerebra l cortex, hippocampus, medial amygdala and thalamus. In the hypothalam us, some neurons of the supraoptic nucleus were weakly stained, and wi dely scattered neurons of the lateral hypothalamic area were moderatel y stained, On the contrary, the most intense reactions to anti-ActRI(8 1-89) antibody were detected in neurons of the lateral hypothalamic ar ea. In addition, many neurons of the cerebral cortex were also stained , but neurons of the hippocampus and the amygdala were not stained. Th ese results suggest that activin may have physiological roles not only for hypothalamic neuroendocrinological and feeding-related systems as suggested previously but may also have functions in cortical and limb ic pathways as a neuromodulator or for maintenance of neurons.