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

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.

Function-related plasticity in hypothalamus

Physiological activation of the magnocellular hypothalamo-neurohypophysial system induces a coordinated astrocytic withdrawal from between the magnocellular somata and the parallel-projecting dendrites of the supraoptic nucleus. Neural lobe astrocytes release engulfed axons and retract from their usual positions along the basal lamina. Occurring on a minutes-to-hours time scale, these changes are accompanied by increased direct apposition of both somatic and dendritic membrane, the formation of dendritic bundles, the appearance of novel multiple synapses in both the somatic and dendritic zones, and increased neural occupation of the perivascular basal lamina. Reversal, albeit with varying time courses, is achieved by removing the activating stimuli. Additionally, activation results in interneuronal coupling increases that are capable of being modulated synaptically via second messenger-dependent mechanisms. These changes appear to play important roles in control and coordination of oxytocin and vasopressin release during such conditions as lactation and dehydration.

A simple method of rapid freezing adequately preserves brain tissue for immunocytochemistry, light and electron microscopic examination

A simple and reproducible method for cryopreservation of brain tissue from patients with Alzheimer's disease is described. Fresh brain slices (1 cm thick) obtained less than 6 h postmortem are placed in sealed plastic bags, sandwiched between 0.3-cm-thick aluminium sheets, and frozen by placing the entire "sandwich" between layers of dry ice pellets. The frozen brain slices are stored at -85 degrees C. Specific anatomic areas can be retrieved at any time for light and electron microscopic, immunocytochemical, autoradiographic and neurochemical studies.

A new technique for post-embedding immunogold localization of vasopressin in the rat median eminence

A modified cryosubstitution method is described which simultaneously preserves the ultrastructure and antigenicity of the osmium-sensitive neuropeptide vasopressin in rat median eminences. The organs were aldehyde-fixed, osmicated, and embedded in Epon. Thin sections were no longer needed to be etched and the titer of primary antiserum was near to the light microscopic level. Furthermore ice crystallization could not be observed.

Freeze substitution after fast-freeze fixation in preparation for immunocytochemistry

As compared to classical chemical fixation, the physical immobilization of ultrastructures by fast-freeze fixation (FFF) and the subsequent exchange of water in its solid state by freeze substitution (FS) improve the preparation procedure for immunogold labeling (IGL). FFF-FS results in a morphological preservation of unchallenged quality, as well as in a better preservation of antigenic reactivity, thus allowing remarkable precision of labeling on sections. However, FFF, particularly over a cooled metal plate, requires a heavy and expensive machine. It is not suitable for all biological specimens and in the best conditions, which remain difficult to standardize, the thickness of the well-preserved portion of the specimen does not exceed a few microns for compact tissues, and exceptionally 30-40 microns for isolated cells. The FS procedure is long and must be adjusted empirically for every new specimen and antigenic detection. The preservation of a given antigen's reactivity in the presence of fixative agents and embedding resins remains unpredictable. The action of fixative agents is different and milder in FS than when they are used classically in chemical fixation. By chance, one of the best FS procedures for the preservation of both ultrastructure and antigenicity appears to be by using acetone alone, together with a molecular sieve to improve the water exchange process. A large choice of embedding resins usually allows us to find a compromise between ultrastructural and antigenic preservation.

A method for fixation of previously fresh-frozen human adult and fetal brains that preserves histological quality and immunoreactivity

A method is described that enables fixation of previously fresh-frozen and stored adult and fetal human or animal brains. The method involves fixing during thawing under controlled, cryoprotected conditions and is compatible with good histological quality and the preservation of enzymatic activity and immunoreactivity of many neural antigens. It offers considerable advantages for the storage of large amounts of tissue from which multiple samples can be taken and processed under fixation and other conditions that can be optimized for a variety of methods, many of which may be incompatible if the whole brain is fixed in a single fixative prior to storage.

Arginine vasopressin mobilises intracellular calcium via V1-receptor activation in astrocytes (pituicytes) cultured from adult rat neural lobes

An extremely close association exists between the membranes of the neurosecretory endings and the resident astrocytes (pituicytes) of the neurohypophysis. Indeed, synaptoid contacts involving neurosecretory vesicle-containing axons contacting pituicytes have been observed, suggesting pituicytes as targets of the products released from neurosecretory axons. We have investigated the effects of various neural lobe peptides on pituicytes in primary culture from adult neurohypophyses. Using Fura-2 loaded cells and dynamic ratio imaging, we have determined that arginine vasopressin (AVP) or V1- but not V2-receptor agonists, mobilise pituicyte intracellular Ca2+ ([Ca2+]i) in the absence of extracellular Ca2+. AVP was consistently effective at concentrations of 10 nM or higher in elevating [Ca2+]i by 200-1000 nM. These responses could be blocked by V1-antagonists and were shown to be associated with accumulation of phosphoinositides. Oxytocin was also found to mobilise [Ca2+]i but was effective only at higher concentrations than for AVP. Oxytocin-evoked [Ca2+]i elevations were also blocked by V1-antagonists. Raising [K+]0 was ineffective in changing [Ca2+]i suggesting that these cells lack voltage-gated Ca2+ channels. We conclude that pituicytes possess V1-receptors, activation of which mobilises [Ca2+]i, possibly functioning to initiate a Ca(2+)-activated K+ conductance which could contribute to further depolarisation of secretory terminals and facilitate exocytosis.

Separate ultrastructural distributions of neurophysin and C-terminal glycopeptide within dense core vesicles in rat neural lobe

The distribution of the neurohypophysial prohormone products neurophysin and C-terminal glycopeptide within dense core vesicles were investigated at the ultrastructural level using quantitative immunogold labeling of rapidly frozen rat neural lobes. In vasopressin vesicles, gold particles after labeling for the glycopeptide were found significantly closer to membrane (chi = 16 nm) than were particles after labeling for neurophysin (chi = 32 nm). This distribution supports a previous histochemical localization of glycopeptide to the rim of vasopressin vesicles and suggests there is morphological segregation of prohormone products within the vesicle.

Elevated extracellular potassium is associated with a reduced extracellular space in rat neural lobe in vitro

Increased neural activity of neurosecretory cells is accompanied by large increases in extracellular K+. The possibility that elevations of this ion might involve fluid redistribution and thus affect the size of the extracellular space and the relationship between pituicytes and axons in the rat neural lobe was explored using rapid freezing and freeze-substitution. Neural lobes were incubated for 15 min before freezing either in a normal medium or one containing a 10 mM increase in KCl (high KCl), a 10 mM increase in KCl balanced by an equimolar reduction in NaCl (high KCl-low NaCl), or only a 10 mM reduction in NaCl (low NaCl). A quantitative assessment of the region of good fixation was made to determine the relative fractions occupied by axons, pituicytes and the extracellular space near the neurohaemal contact zone. In addition, the percentage of basal lamina contacted by pituicytes and axons was calculated, as was the degree of enclosure of axons by pituicytes. In neural lobes incubated in normal medium, 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 occupied almost 60% of the basal lamina at the neurohaemal contact zone, while axons occupied approximately 20%.(ABSTRACT TRUNCATED AT 250 WORDS)