A quantitative morphological study of osmotically induced swelling and shrinkage in nervous tissue

Fixation strategies for retinal immunohistochemistry

Immunohistochemical and ex vivo anatomical studies have provided many glimpses of the variety, distribution, and signaling components of vertebrate retinal neurons. The beauty of numerous images published to date, and the qualitative and quantitative information they provide, indicate that these approaches are fundamentally useful. However, obtaining these images entailed tissue handling and exposure to chemical solutions that differ from normal extracellular fluid in composition, temperature, and osmolarity. Because the differences are large enough to alter intercellular and intracellular signaling in neurons, and because retinae are susceptible to crush, shear, and fray, it is natural to wonder if immunohistochemical and anatomical methods disturb or damage the cells they are designed to examine. Tissue fixation is typically incorporated to guard against this damage and is therefore critically important to the quality and significance of the harvested data. Here, we describe mechanisms of fixation; advantages and disadvantages of using formaldehyde and glutaraldehyde as fixatives during immunohistochemistry; and modifications of widely used protocols that have recently been found to improve cell shape preservation and immunostaining patterns, especially in proximal retinal neurons.

Moniliform deformation of retinal ganglion cells by formaldehyde-based fixatives

Protocols for characterizing cellular phenotypes commonly use chemical fixatives to preserve anatomical features, mechanically stabilize tissue, and stop physiological responses. Formaldehyde, diluted in either phosphate-buffered saline or phosphate buffer, has been widely used in studies of neurons, especially in conjunction with dyes and antibodies. However, previous studies have found that these fixatives induce the formation of bead-like varicosities in the dendrites and axons of brain and spinal cord neurons. We report here that these formaldehyde formulations can induce bead formation in the dendrites and axons of adult rat and rabbit retinal ganglion cells, and that retinal ganglion cells differ from hippocampal, cortical, cerebellar, and spinal cord neurons in that bead formation is not blocked by glutamate receptor antagonists, a voltage-gated Na(+) channel toxin, extracellular Ca(2+) ion exclusion, or temperature shifts. Moreover, we describe a modification of formaldehyde-based fixatives that prevents bead formation in retinal ganglion cells visualized by green fluorescent protein expression and by immunohistochemistry.

CNS energy metabolism as related to function

Large amounts of energy are required to maintain the signaling activities of CNS cells. Because of the fine-grained heterogeneity of brain and the rapid changes in energy demand, it has been difficult to monitor rates of energy generation and consumption at the cellular level and even more difficult at the subcellular level. Mechanisms to facilitate energy transfer within cells include the juxtaposition of sites of generation with sites of consumption and the transfer of approximately P by the creatine kinase/creatine phosphate and the adenylate kinase systems. There is evidence that glycolysis is separated from oxidative metabolism at some sites with lactate becoming an important substrate. Carbonic anhydrase may play a role in buffering activity-induced increases in lactic acid. Relatively little energy is used for 'vegetative' processes. The great majority is used for signaling processes, particularly Na(+) transport. The brain has very small energy reserves, and the margin of safety between the energy that can be generated and the energy required for maximum activity is also small. It seems probable that the supply of energy may impose a limit on the activity of a neuron under normal conditions. A number of mechanisms have evolved to reduce activity when energy levels are diminished.

Volume regulatory decrease in UMR-106.01 cells is mediated by specific alpha1 subunits of L-type calcium channels

An early cellular response of osteoblasts to swelling is plasma membrane depolarization, accompanied by a transient increase in intracellular calcium ([Ca2+]i), which initiates regulatory volume decrease (RVD). The authors have previously demonstrated a hypotonically induced depolarization of the osteoblast plasma membrane, sufficient to open L-type Ca channels and mediate Ca2+ influx. Herein is described the initiation of RVD in UMR-106.01 cells, mediated by hypotonically induced [Ca2+]i transients resulting from the activation of specific isoforms of L-type Ca channels. The authors further demonstrate that substrate interaction determines which specific alpha1 Ca channel subunit isoform predominates and mediates Ca2+ entry and RVD. Swelling-induced [Ca2+]i transients, and RVD in cells grown on a type I collagen matrix, are inhibited by removal of Ca from extracellular solutions, dihydropyridines, and antisense oligodeoxynucleotides directed exclusively to the alpha1C isoform of the L-type Ca channel. Ca2+ transients and RVD in cells grown on untreated glass cover slips were inhibited by similar maneuvers, but only by antisense oligodeoxynucleotides directed to the alpha1S isoform of the L-type Ca channel. This represents the first molecular identification of the Ca channels that transduce the initiation signal for RVD by osteoblastic cells.

Cell swelling exacerbates hypoxic neuronal damage in rat hippocampal slices

In the present study we investigated the effect of acute cell swelling on the sensitivity of rat hippocampal slices to hypoxia. Hippocampal slices were exposed to different degrees of hypo- or hyperosmolality 15 min prior to and during a 15-min hypoxia followed by reoxygenation under isosmotic (293 mOsm) conditions. Recovery of neuronal function (an electrically evoked population spike) after hypoxia was significantly diminished in slices exposed to hyposmotic conditions as 57% of control (isosmotic) slices showed recovery compared with 51%, 35%, and 13% recovery rate in slices made hyposmotic (273, 253, and 233 mOsm, respectively). Of slices exposed to a medium made hyperosmotic by the addition of 20, 40, 60, and 80 mM mannitol, only those exposed to the most hyperosmotic treatment (373 mOsm) exhibited a recovery rate significantly greater than control (70% vs. 57%). The competitive NMDA antagonist CGS-19755 (50 microM) completely protected both isosmotic and hyposmotic (233 mOsm) slices against hypoxic damage. However, a threshold dose (15 microM) of the antagonist provided no protection to isosmotic slices (51% vs. 57% recovery rate) while affording substantial protection to hyposmotic slices (233 mOsm), as 54% of the treated slices recovered their neuronal function after hypoxia compared to 13% recovery rate of the untreated slices. These results suggest an increase in activation of the NMDA receptor under hyposmotic conditions. We conclude that acute osmotic swelling of neuronal tissue predisposes it to hypoxic damage, possibly by activation of NMDA receptors that are not usually activated by hypoxia alone.

The use of continuous density gradients for the assessment of islet and exocrine tissue densities and islet purification

The purification of large numbers of human pancreatic islets remains one of the limiting factors in islet transplantation. This paper describes and validates a method for accurately and reproducibly determining the density of islets and exocrine tissue in pancreatic digest on the basis of their isopycnic distribution on linear continuous density gradients. The use of this data to analyse and compare the purity of a standard 60% islet yield is described. The results obtained using such gradients will enable factors responsible for the variation in yield between pancreases to be determined and optimized, improving the results and reliability of islet purification.

Dimensional changes in cells and tissues during specimen preparation for the electron microscope

Studies on dimensional changes incurred during preparation of tissue specimens for the transmission and scanning electron microscopes are reviewed, with emphasis on quantitative measurements pertinent to morphometry and three-dimensional reconstruction. The scope of the review includes fixation, dehydration, plastic embedment, critical-point drying, and freeze-drying. Recommendations are presented for monitoring dimensional changes; a strategy for the choice of method of specimen preparation is outlined.

Seizure and acute osmotic change: clinical and neurophysiological aspects

There are a number of clinical situations where overhydration may occur. If the reduction in plasma osmolality is acute, passive water influx swells brain cells, shrinking the extracellular space around them. It is during this time that susceptibility to generalized tonic-clonic seizure dramatically increases. Common clinical examples include hastened rehydration therapy, the dialysis disequilibrium syndrome, compulsive polydipsia, the syndrome of inappropriate ADH secretion (SIADH) and post-TURP syndrome. Treatments that tend to restore normal cellular volume (dehydration, mannitol infusion) help protect against this form of seizure. Support for a correlation between plasma osmolality and seizure susceptibility is scattered amongst the literature of several medical disciplines and spans almost 70 years. However a cellular basis to explain how overhydration might promote epileptiform activity has been examined only recently. The neocortical and hippocampal brain slice preparations permit an examination of how acute osmotic change alters cortical excitability independent of vascular damage, brain compression or other factors secondary to brain swelling. Electrophysiological evidence indicates that hyposmolality promotes epileptiform activity by strengthening both excitatory synaptic communication in neocortex and field effects among the entire cortical population. Moreover there is little evidence that associated hyponatremia in itself leads to increased CNS excitability. Such findings help in understanding how rapid lowering of plasma osmolality in clinical situations can promote the hyperexcitability associated with generalized tonic-clonic seizure.

Preparation of fetal rat brains for light and electron microscopy

To study cellular shapes, growth patterns, and fine structure during early stages of CNS development in rat embryos, preparative procedures were evaluated and modified to meet two criteria: 1) Coronal semithin sections should reveal undeformed telencephalic hemispheres that were symmetrically expanded on both sides of midline structures and were surrounded by contiguous mesenchyme. 2) In electron micrographs, cells should have intact, undistorted surface membranes, evenly distributed nucleoplasm and well preserved cytoplasmic organelles. To meet these criteria, 378 fetuses with a gestational age of 11-20 days (E11-E20) were used to test and modify procedures for anesthesia, embryo removal and handling, dissection, fixation, dehydration, and embedding of the embryonic CNS. Most specimens were in an early stage of development (E11-E13), which, in case of the neopallial wall, is the preneural period. The tests produced methods that met the above criteria and identified the most common artifacts and their causes. Deformities of the cerebral hemispheres and separations between the brain and its coverings were usually caused by trauma during embryo removal and during handling before fixation. Changes in cellular volumes, especially swelling during fixation and dehydration, were the most important causes of histological artifacts. The procedures and methods that consistently produced the best light and electron microscopic preservation of the E11-E13 rat CNS are described. Fixation was best when the brains were treated with glutaraldehyde and s-collidine buffer, followed by osmium tetroxide in s-collidine buffer. A surprisingly beneficial effect of sodium chloride in the dehydrating alcohol was noted.

Seizure susceptibility and the osmotic state

In some unknown manner, water uptake by brain cells (hyposmolality) promotes generalized seizure in humans and experimental animals, whereas cell dehydration (hyperosmolality) protects against it. We have replicated both scenarios in slices of hippocampus undergoing electrographic seizures. Surprisingly, a shift in osmolality does not change the excitability of individual neurons but rather, it alters the degree to which neurons interact. Hyposmolality enhances both excitatory synaptic transmission in neocortex and field (ephaptic) effects, the latter arising when cortical cells fire as a population. We propose that these increased excitatory interactions promote the synchrony that characterizes epileptiform activity.