Detection of plasma proteins in CNS neurons: conspicuous influence of tissue-processing parameters and the utilization of serum for blocking nonspecific reactions

Despite the presence of a blood-brain barrier (BBB), plasma proteins have been detected intraneuronally in regions with axonal projections confined to the CNS. This finding raises the question of whether plasma proteins are taken up from the brain interstitium or whether the results are due to experimental artifact. We examined the effect of various protocols for tissue processing on the intraneuronal distribution of plasma proteins using immunohistochemistry. The detection level of plasma proteins decreased after prolonged fixation, irrespective of the fixative and embedding method employed. In cryostat sections, attempts to block nonspecific staining by serum protein caused considerable nonspecific staining in itself. When nonspecific staining was blocked with a serum-free buffer, specifically labeled neuronal perikarya were found in cryostat sections of brains fixed by perfusion with paraformaldehyde without postfixation. Albumin and IgG occurred predominantly in neurons having projections beyond the BBB but also sparsely in neurons having projections confined to the CNS. Transferrin was evenly distributed within neuronal somata, irrespective of the orientation of projections. The immunoreaction product of the three plasma proteins exhibited a specific intraneuronal localization in the differently projecting neurons. In circumventricular organs, plasma proteins were observed extracellularly and in projecting fibers. In conclusion, plasma proteins are present in neurons with projections confined to the CNS and are probably taken up from the brain interstitium.