A combined light and electron microscopic immunocytochemical method for the simultaneous localization of multiple tissue antigens

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Abstract
In order to study the morphological interrelationships between immunocytochemically identified neuronal systems, a double labelling procedure — suitable for correlative light and electron microscopic observations — is introduced. The technique is based on the consecutive use of the silver-gold (SG) intensified and non-intensified forms of the oxidized 3,3′-diaminobenzidine (DAB) chromogen in the framework of the peroxidase-antiperoxidase complex (PAP) indirect immunocytochemical procedure. The first tissue antigen is detected by the SG intensified DAB chromogen, which has a black color and high electron density. The structures containing the second antigen are visualized by the non-intensified DAB-endproduct, which is less electron dense than the silver-gold amplified form and is brown. The metallic shield that forms around the labeled antibody sequences associated with the first antigen prevents non-specific binding of immunoglobulins used for the detection of the second tissue antigen. The application of this method for the simultaneous detection of tyrosine hydroxylase (TH)- and corticotropin releasing factor (CRF)-immunoreactive structures revealed that black colored TH-immunopositive fibers contacted brown colored CRF-synthesizing neurons in the hypothalamic paraventricular nucleus. The juxtaposition of TH-and CRF-containing elements was apparent in both thick vibratome (40 μm) and semithin (1 μm) sections. At the ultrastructural level, TH-positive terminals — labeled by silvergold grains — were observed to establish asymmetric synapses with both CRF- and TH-immunoreactive neurons. The former finding indicates a direct, TH-immunopositive, catecholaminergic influence upon the hypothalamic CRF system, while the latter demonstrates the existence of intrinsic connections between TH-positive elements.

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