Quantitative immunocytochemistry of tyrosine hydroxylase in rat brain. I. Development of a computer assisted method...

We sought to develop a quantitative immunocytochemical procedure using the peroxidase-antiperoxidase (PAP) technique to analyze amounts of neurotransmitter biosynthetic enzyme tyrosine hydroxylase (TH) within the nucleus locus coeruleus (LC) of rat brain. Rats were perfused with 4% paraformaldehyde. The brains were embedded in paraffin, sectioned at 5 micrometers in the sagittal plane and immunocytochemically stained with antibodies to bovine adrenal TH. Staining intensity, measured by a TV image analysis system was reproducible within +/- 5%. Reaction conditions required so that the intensity of the PAP reaction product was directly and linearly related to the amount of TH enzyme protein in tissue were obtained by reacting tissues with saturating concentrations (2.5 mM) of diaminobenzidine (DAB) substrate, a constant dilution of antibody, and incubation time adjusted so that the darkest elements in tissue were below saturation. Variations in staining intensity of serial sections through the LC were found to be insignificant when compared to variations in staining between animals. In order to increase the amount of immunoreactive TH in the LC, groups of rats were treated with reserpine. Immunocytochemical and biochemical analyses were performed in parallel groups of control and reserpine animals allowed to survive 1-3 days following a single injection (10 mg/kg s.c.) of reserpine. A close correlation was found to exist between the amount of TH enzyme protein determined biochemically and the density of staining for TH. The maximal increase in TH measured immunocytochemically was 2.2-fold which was about 80% of the maximal induction determined biochemically. We conclude that the PAP method can be used for quantitative immunocytochemistry of brain TH providing that optimal reaction conditions are established.