Immunocytochemistry with internally labeled monoclonal antibodies.

One of the advantages of the production of monoclonal antibodies by tissue culture methods is that they can be internally labeled by using appropriate radioactive amino acid in the culture fluid. Thus, radioactive immunological probes of high specific activity can be prepared. Here we report applications of these internally labeled monoclonal antibodies for the direct localization of immunoreactive sites in the central nervous system of the rat at both light and electron microscopic levels (radioimmunocytochemistry). We explored the combined use of radioimmunocytochemistry and immunoenzymatic methods for the simultaneous detection of two antigenic sites: substance P and serotonin or substance P and enkephalin. In neurobiology this procedure could help to clarify certain aspects of transmitter-specific synaptic interactions and the coexistence of neuroactive substances in single neuronal cell bodies or nerve terminals. We also describe the application of radioimmunocytochemistry with internally labeled monoclonal antibodies to quantify the immunoreactions in discrete microscopic areas.