Measurement by ELISA of complement factor 4 (C4) in the rat brain: necessity for removal of cerebrovascular proteins.

Assessment of complement 4 (C4) levels in experimental animals is used as a marker for activation of the classical complement pathway. The objective of this study was to develop a method for measuring C4 concentrations in the rat brain. An ELISA (sensitivity = 0.5 ng C4/ml) was used to measure C4 in regional brain homogenates from Fischer rats cardiac-perfused with phosphate buffered saline to remove cerebrovascular contents, and from sham-perfused rats. Ventral midbrain C4 levels were increased (p < 0.001) versus frontal cortex and striatum in sham-perfused rats, whereas after perfusion there were no differences between brain regions. Removal of cerebrovascular contents decreased C4 by 43% in striatum, 52% in frontal cortex, and 69% in ventral midbrain (all p < 0.01 versus sham-perfused means). These results indicate that C4 in the rat brain can be measured quantitatively by ELISA provided that cerebrovascular proteins are removed by perfusion.