Some improvement in tissue preparation and colloidal-gold immunolabeling for electron microscopy

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

Enzyme histochemistry and immunohistochemistry with freeze-dried or freeze-substituted resin-embedded tissue

Freeze-drying or freeze-substitution, combined with low-temperature resin-embedding, represents a new approach to the optimum preservation of tissue for enzyme histochemistry and immunohistochemistry. This method, which avoids tissue fixation, combines excellent tissue morphology with the preservation of enzyme activity and immunoreactivity and allows high-resolution enzyme histochemical and immunohistochemical studies to be performed. The activity of a wide range of enzymes can be demonstrated in sections of freeze-dried or freeze-substituted resin-embedded tissue. Enzymes are retained in situ with high activity, accurate localization and no diffusion. Immunohistochemical studies can also be performed on resin sections, and antigens--especially labile antigens--are immobilized in situ without denaturation and can be demonstrated with high sensitivity and accurately localized. This method allows the localization and distribution of enzymes and antigens to be studied in relation to excellent histological and cytological detail.

Neuronal imaging with colloidal gold

Colloidal gold is easily prepared, and readily adsorbs to a number of immunoreagents and other proteins for a wide variety of uses for neuronal visualization. Gold probes serve a role as immunolabels for both light and electron microscopy. As an ultrastructural immunocytochemical marker for detection of proteins, peptides or amino acids, gold can be used for immunostaining thick or thin sections prior to embedding, or for immunostaining ultrathin sections after embedding tissue in conventional or unusual embedding matrices. By virtue of its particulate nature, gold as an immunolabel facilitates a semi-quantitative analysis of relative antigen densities on ultrathin sections. Various combinations of different size gold particles or dual immunolabelling with enzymatic immunolabels together with colloidal gold or silver-intensified gold serve well for ultrastructural immunocytochemical localization of two antigens in the same tissue section. Colloidal gold can be detected with light microscopy, transmission and scanning electron microscopy, and with confocal laser microscopy. Silver intensification allows detection of gold at both the light and electron microscope level, and increases the sensitivity of immunogold procedures. Colloidal gold is useful as a tracer for physiological studies of transport and internalization in neurons in vivo and in vitro; computer-assisted video imaging techniques allow detection and tracking of single gold particles in living cells.

Comparison of LR white resin, Lowicryl K4M and Epon postembedding procedures for immunogold staining of actin in the testis

The efficiency of various postembedding procedures for actin immunogold detection was compared using testicular tissue as a model. Whatever the fixative, testes embedded in LR White resin or in Lowicryl K4M showed few differences as regard ultrastructural preservation and gave similar actin antigenicity preservation. A purified polyclonal antibody (IgG) and a monoclonal antibody (IgM) visualized with gold secondary antibody yielded high labeling intensity whereas the IgG-protein-A gold association was less efficient. Crude antisera gave a low specific staining/background ratio. Samples of testes, fixed in different conditions, were also embedded in Epon, omitting propylene oxide and lowering polymerization temperature to 40 degrees-50 degrees C. This slight modification improved ultrastructural preservation which was better than with hydrophilic resins, as well as made possible immunogold detection of actin though antigenicity preservation was lesser than with these resins. Thus, in Epon embedded samples actin labeling, using IgG antiactin-gold secondary antibody, was similar to that observed after hydrophilic resin-protein-A gold procedures. In addition to actin labeling of various somatic cells it was confirmed that actin is a consistent component of the subacrosomal space of spermatids during the greater part of spermiogenesis in rat.

Immunohistochemical approach to the functional morphology of the hypothalamic-hypophysial system

Immunohistochemical studies at the light and electron microscopic levels have provided much information on functional morphology in the hypothalamic-hypophysial system. The present paper describes the immunohistochemical techniques available at present and their use to determine the localizations of neurons containing hypophysiotrophic substances, the co-storage of plural signals in these neurons, and the synaptic regulation of these neurons in rats.

Light- and electron-microscopic evidence of costoring of immunoreactive enkephalins and substance P in dorsal horn neurons of rat

The topographical localization of substance P (SP) and methionine-enkephalin-octapeptide (Enk-8) was examined immunohistochemically in the surface layer of the dorsal horn of rat cervical spinal cord. Although a few neurons were immunoreactive for Enk-8 in the intact animals, after an intracisternal administration of colchicine, immunoreactive Enk-8 neurons were numerous, and half of them indicated immunoreactivity also for SP. Some immunoreactive SP neurons appeared to show no immunoreactivity for Enk-8. Immuno-reactive nerve fibers, on the other hand, were numerous, and many of them contained both peptides. Electron-microscopic examination of the nerve fibers in tissue prepared by a freeze-drying procedure and stained by a postembedding procedure, revealed the costoring of both peptides in the same cored vesicles. The physiological significance of this costoring is discussed.

Intragranular colocalization of arginine vasopressin and methionine-enkephalin-octapeptide in CRF-axons in the rat median eminence

Ultrastructural appearances of axonal terminals containing corticoliberin (CRF) were examined in the rat median eminence prepared by a freeze-drying procedure. Immunolabeling was performed by using 5-, 8-, or 15-nm gold-antibody complexes for CRF, arginine vasopressin (VP) and methionine-enkephalin-octapeptide (Enk-8), singly or in combination. In intact animals, the CRF-containing secretory granules were only slightly labeled with gold-anti-VP or -Enk-8. In adrenalectomized rats, granules within single axons appeared to be labeled with all the immunogold complexes. This intragranular colocalization of the three antigens was confirmed by using three neighboring sections of the same axon terminals which were stained separately with each one of the antibodies and visualized with the avidin-biotin-peroxidase complex method. The granules labeled for CRF had decreased 9 days after adrenalectomy but had increased again by day 21, while those labeled for VP steadily increased after adrenalectomy. However, this did not correspond with the appearances of cell bodies in the paraventricular nucleus; the cell bodies labeled for both CRF and VP steadily increased in number and in stainability. By contrast, Enk-8 immunoreactivity in the axonal terminals and cell bodies was not affected by adrenalectomy. These findings suggest that although the three peptides could be released simultaneously from the axonal terminals, VP may play some special role in the expression of CRF activity.