Localization of neurophysin in the rat supraoptic nucleus. I. Ultrastructural immunocytochemistry using the post-embedding technique

Electron microscopic and immunocytochemical study of rapidly frozen, freeze-substituted neural lobes of rats

Rapid freezing of freshly dissected or incubated neural lobes was explored as a means of obtaining ultrastructural preservation of the more natural state of this tissue. A quantitative assessment of the region of good fixation was made in order to determine the relative fractions occupied by axons, pituicytes and the extracellular space. The immunocytochemical distributions of neurophysins and the glycopeptide portion of the vasopressin precursor were evaluated using the immunogold technique in order to determine the relative numbers of oxytocin and vasopressin fibre types in the fixed region, and the subcellular localization of these antigens. The uncut surface of rat neural lobes was rapidly frozen against a highly polished copper plug and freeze-substituted in osmium-acetone either immediately after dissection (approximately 2 min), or after a 15 min incubation period in vitro in an oxygenated, balanced salt solution. Substituted neural lobes were prepared for either conventional electron microscopy, or for immunogold labelling of neurophysins and the glycopeptide precursor to vasopressin. Membranes, subcellular organelles and extracellular matrix were well preserved 10 microns deep to the contacted surface. The extracellular space accounted for approximately 30% of the cross-sectional area of the neuropil and could be divided into two domains: an extended perivascular space (28-29% of total area); and a narrow (approximately 24 nm; approximately 1% of total) space between closely apposed neurosecretory processes or between these processes and pituicytes. Pituicytes accounted for about 30% of the area and axons 20-25%. Pituicytes occupied close to 60% of the basal lamina at the neurohaemal contact zone, while axons occupied approximately 20%. There were no differences between neural lobes frozen immediately after dissection and those incubated for 15 min in any of these measures, suggesting minimal fluid redistribution. Gold particles were specifically localized over large (100-200 nm) dense core vesicles, and less frequently over multivesicular bodies and lysosomes. No etching of the plastic or reduction of osmium was necessary to achieve labelling. Specific labelling of one set of terminals and axons (about 80%) was observed with the monoclonal antibody previously shown to be specific for oxytocin-neurophysin, while in neighbouring sections the remaining 20% of the processes were labelled with the antiserum to the vasopressin precursor, or with non-specific antibodies to neurophysins. In conclusion, ultrarapid freezing preserves a large extracellular space in the neural lobe and provides for high resolution morphological and immunocytochemical studies of neurohypophysial structure.

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.

The dopaminergic innervation of the supraoptic and paraventricular nucleus. A light and electron microscopical study

An antiserum that has been raised against glutaraldehyde-conjugated dopamine was used to demonstrate specifically dopamine in the rat hypothalamus. This dopamine antiserum permitted an optimal fixation with glutaraldehyde and therefore enabled the simultaneous light and electron microscopic immunocytochemical localization of dopamine. It was demonstrated that the paraventricular and supraoptic nuclei of the hypothalamus were innervated by thin dopaminergic fibers, in contrast to the suprachiasmatic nucleus, which hardly received any dopaminergic input. Ultrastructural observations revealed that the dopamine fibers terminated synaptically on the magnocellular neurons and their processes. It is concluded that the present results may explain the effect of centrally injected dopamine on vasopressin and oxytocin release. In the dopamine-containing terminals the reaction product was frequently observed in 90 nm dense core vesicles and around clear vesicles.

Horseradish peroxidase: a tool for study of the neuroendocrine cell and other peptide-secreting cells

The versatility of horseradish peroxidase is its usefulness both as an antigenic marker and as a probe molecule. We have demonstrated in the neuroendocrine cell that an HRP-bound antibody offers a high order of resolution for determining in which cellular compartment an antigen is located and where it is not. When native peroxidase is applied as an intracellular probe, it labels organelles associated with endocytosis in retrograde axonal transport and with the lysosomal system in both retrograde and orthograde axonal transport. The investigation that remains is the application of lectin-bound HRP to determine the pathways of membrane flow at the time when the neuroendocrine cell is stimulated to synthesize, transport, and secrete its peptide. For example, we are interested to know (1) whether internalized axon terminal membrane tagged with wheat germ agglutinin-HRP is channeled to all Golgi saccules engaged in the production of secretory granules in salt stimulated supraoptic neurons; and (2) if internalized cell membrane of the supraoptic cell body is tagged with wheat germ agglutinin-HRP and channeled to GERL, will this membrane be transferred from GERL to secretory granules, lysosomes in the cell body and axon, the axonal endoplasmic reticulum, and to autophagic/crinophagic vacuoles in axon terminals of salt-stressed supraoptic neurons? These additional studies should provide a more comprehensive, morphological picture of membrane flow in a neuroendocrine cell that is responding to the metabolic demands placed upon it.

Golgi apparatus, GERL, and secretory granule formation within neurons of the hypothalamo-neurohypophysial system of control and hyperosmotically stressed mice

The vasopressin-producing neurons of the hypothalamo-neurohypophysial system are a particularly good model with which to consider the relationship between the Golgi apparatus nd GERL and their roles in secretory granule production because these neurons increase their synthesis and secretion of vasopressin in response to hyperosmotic stress. Enzyme cytochemical techniques for acid phosphatase (AcPase) and thiamine pyrophosphatase (TPPase) activities were used to distinguish GERL from the Golgi apparatus in cell bodies of the supraoptic nucleus from normal mice, mice hyperosmotically stressed by drinking 2% salt water, and mice allowed to recover for 5-10 d from hyperosmotic stress. In nonincubated preparations of control supraoptic perikarya, immature secretory granules at the trans face of the Golgi apparatus were frequently attached to a narrow, smooth membrane cisterna identified as GERL. Secretory granules were occasionally seen attached to Golgi saccules. TPPase activity was present in one or two of the trans Golgi saccules; AcPase activity appeared in GERL and attached immature secretory granules, rarely in the trans Golgi saccules, and in secondary lysosomes. As a result of hyperosmotic stress, the Golgi apparatus hypertrophied, and secretory granules formed from all Golgi saccules and GERL. Little or no AcPase activity could be demonstrated in GERL, whereas all Golgi saccules and GERL-like cisternae were TPPase positive. During recovery, AcPase activity in GERL returned to normal; however, the elevated TPPase activity and secretory granule formation seen in GERL-like cisternae and all Golgi saccules during hyperosmotic stress persisted. These results suggest that under normal conditions GERL is the predominant site for the secretory granule formation, but during hyperosmotic stress, the Golgi saccules assume increased importance in this function. The observed cytochemical modulations in Golgi saccules and GERL suggest that GERL is structurally and functionally related to the Golgi saccules.

Cellular characteristics of immunolabeled luteinizing hormone releasing hormone (LHRH) neurons

This study utilized the preembedding immunocytochemical technique in order to identify LHRH-containing neurons in rat brain and define their ultrastructural characteristics. LHRH-containing neurons in the vertical limb of the diagonal band of Broca, medial septum, triangular nucleus of the septum and other regions were studied by taking ultrathin serial sections. These neurons had scant cytoplasm surrounding a centrally-located, spheroid, euchromatic nucleus. Neurosecretory granules were evenly distributed throughout the cell, but many tended to lie directly under the plasmalemma. The cytoplasm was organized in such a way that the most extensive portion of the rough endoplasmic reticulum was polar opposite to areas having high concentrations of Golgi complex, lysosome-like bodies, smooth endoplasmic reticulum and ribosomes. The perikarya had no axosomatic synapses but functional interaction via unspecialized appositions to the plasmalemma cannot be discounted. Many of the perikarya bore at least one cilium. Processes from immunonegative cells were occasionally observed to penetrate the cytoplasm of the LHRH perikaryon or its processes. At their points of origin, dendrites were found to be broadened processes containing many elements common to the cytoplasm: ribosomes, smooth endoplasmic reticulum, cristal and lamellar mitochondria, neurotubules, and an occasional alveolate caveola. Infrequently, some of the LHRH axons were partially myelinated. This method of studying serial-sectioned immunocytochemically-identified cells is suggested as a means of describing the cellular and subcellular characteristics of other specific peptide-containing cells.