Synaptic connections of neurones identified by Golgi impregnation: characterization by immunocytochemical, enzyme histochemical, and degeneration methods

Advances in thin tissue Golgi-Cox impregnation: fast, reliable methods for multi-assay analyses in rodent and non-human primate brain

In 1873 Camillo Golgi discovered a staining technique that allowed for the visualization of whole neurons within the brain, initially termed 'the black reaction' and is now known as Golgi impregnation. Despite the capricious nature of this method, Golgi impregnation remains a widely used method for whole neuron visualization and analysis of dendritic arborization and spine quantification. We describe a series of reliable, modified 'Golgi-Cox' impregnation methods that complement some existing methods and have several advantages over traditional whole brain 'Golgi' impregnation. First, these methods utilize 60-100μm thick brain sections, which allows for fast, reliable impregnation of neurons in rats (7-14 days) and non-human primates (NHP) (30 days) while avoiding the pitfalls of other 'rapid Golgi' techniques traditionally employed with thin sections. Second, these methods employ several common tissue fixatives, resulting in high quality neuron impregnation in brain sections from acrolein, glutaraldehyde, and paraformaldehyde perfused rats, and in glutaraldehyde perfused NHP brain tissue. Third, because thin sections are obtained on a vibratome prior to processing, alternate sections of brain tissue can be used for additional analyses such as immunohistochemistry or electron microscopy. This later advantage allows for comparison of, for example, dendrite morphology in sections adjacent to pertinent histochemical markers or ultrastructural components. Finally, we describe a method for simultaneous light microscopic visualization of both tyrosine hydroxylase immunohistochemistry and Golgi impregnation in the same tissue section. Thus, the methods described here allow for fast, high quality Golgi impregnation and conserve experimental subjects by allowing multiple analyses within an individual animal.

Simultaneous Golgi-Cox and immunofluorescence using confocal microscopy

Visualization of neuronal elements is of fundamental importance in modern neuroscience. Golgi-Cox impregnation is a widely employed method that provides detailed information about morphological characteristics of neurons, but none regarding their neurochemical features. Immunocytochemical procedures, on the other hand, can provide a high degree of biochemical specificity but poorer morphological details, in particular if compared to Golgi-Cox impregnation. Hence, the combined use of these two approaches is highly desirable, especially for confocal microscopy that can exploit the advantages of both methods simultaneously. Here we show an innovative procedure of perfusion and fixation of brain tissue, that allows, by applying Golgi-Cox impregnation and immunofluorescence in the same histological section, to obtain high-quality histological material, with a very simple and inexpensive method. This procedure is based on three simple fixation steps: (1) a paraformaldehyde perfusion followed by a standard post-fixation to stabilize the subsequent immunofluorescence reaction; (2) the classical Golgi-Cox impregnation and (3) an immunofluorescence reaction in previously impregnated material. This combination allows simultaneous visualization of (a) the structural details (Golgi-Cox impregnated neurons), (b) the antigens' characterization, (c) the anatomical interactions between discrete neuronal elements and (d) the 3D reconstruction and modeling. The method is easy to perform and can be reproducibly applied by small laboratories and expanded through the use of different antibodies. Overall, the method presented in this study offers an innovative and powerful approach to study the nervous system, especially by using confocal microscopy.

Growth and pruning of mushroom body Kenyon cell dendrites during worker behavioral development in the paper wasp, Polybia aequatorialis (Hymenoptera: Vespidae)

Adult workers of some social insect species show dramatic behavioral changes as they pass through a sequence of task specializations. In the paper wasp, Polybia aequatorialis, female workers begin adult life within the nest tending brood, progress to maintaining and defending the nest exterior, and ultimately leave the nest to forage. The mushroom body (MB) calyx neuropil increases in volume as workers progress from in-nest to foraging tasks. In other social Hymenoptera (bees and ants), MB Kenyon cell dendrites, axons and synapses change with the transition to foraging, but these neuronal effects had not been studied in wasps. Furthermore, the on-nest worker of Polybia wasps, an intermediate task specialization not identified in bees or ants, provides the opportunity to study pre-foraging worker class transitions. We asked whether Kenyon cell dendritic arborization varies with the task specialization of Polybia workers observed in the field near Monteverde, Costa Rica. Golgi-impregnated arbors in the lip and collar calyces, which receive a predominance of olfactory and visual input, respectively, were quantified using Sholl's concentric circles and a novel application of virtual spherical probes. Arbors of the lip varied in a manner reminiscent of honeybees, with foragers having the largest and in-nest workers having the smallest arbors. In contrast, arbors of the collar were largest in foragers but smallest in on-nest workers. Thus, progression through task specializations in P. aequatorialis involves subregion specific dendritic growth and regression in the MB neuropil. These results may reflect the sensitivity of Kenyon cell dendritic structure to specialization dependent social and sensory experience.

Optimization of Golgi methods for impregnation of brain tissue from humans and monkeys

Golgi impregnation is unique in its ability to display the dendritic trees of large numbers of individual neurons. However, its reputation for inconsistency leaves many investigators reluctant to embrace this methodology, particularly for the study of formalin-fixed human brain tissue. After reviewing the literature, testing a variety of technical variations, and discussing the procedure with experienced practitioners, we have concluded that much of the unpredictability can be removed by matching the Golgi technique to the conditions that were used for fixation of the tissue. Briefly fixed tissues worked best with the rapid Golgi technique, which includes osmium during the initial chromation step, and with the Golgi-Cox method, which includes mercuric chloride during chromation. For tissues that have been fixed for several years or even for several decades, superior results are obtained with the Golgi-Kopsch technique, using multiple changes of a chromation solution that contains paraformaldehyde. In the Golgi-Kopsch technique, pH should be used to monitor the reduction of Cr6+ to Cr3+, which is a crucial determinant of successful chromation. With any Golgi technique, agitation throughout the impregnation helps to avoid precipitates and to improve the quality of impregnation. When the appropriate method is chosen, Golgi impregnation is a useful technique for the neuropathologist.

Cortical interneurons: from Cajal to 2001

After the collective work of many investigators, beginning with the early studies of Cajal, the following main [figure: see text] conclusions may be drawn regarding the morphology, biochemical characteristics and synaptic connections of interneurons: 1. Interneurons show a great variety of morphological, biochemical and physiological types. They constitute approximately 15-30% of the total population of neurons. 2. Because of the heterogeneity of interneurons and the lack of consensus as to which characteristics are essential for an individual neuron to be considered a member of a given cell type, there is no definitive classification of interneurons. Nevertheless, certain interneurons can be readily recognized by their unique morphological characteristics, or they can be more generally divided into subgroups on the basis of their biochemical characteristics, patterns of axonal arborization, or synaptic connections with pyramidal cells. 3. All interneurons have a more or less dense axonal arborization distributed near the cell body, mainly within the area occupied by their dendritic field. However, some interneurons may display, in addition, prominent long, horizontal or vertical axonal collaterals. [figure: see text] 4. Most interneurons form symmetrical synapses with both pyramidal cells and other interneurons, with the exception of chandelier cells, which only form synapses with the axon initial segment of pyramidal cells. Furthermore, interneurons are not only connected by chemical synapses (unidirectional connections), but they may also form electrical synapses through gap junctions (bidirectional) in a specific manner. 5. With the exception of chandelier cells, other types of interneurons include among their synaptic targets more than one type of postsynaptic element. But the degree of preference for these postsynaptic elements varies markedly between different types of interneurons. 6. The number of synapses made by a single axon originating from a given interneuron on another neuron is on the order of ten or less. Since, in general, cortical neurons receive many more interneuronal (symmetrical) synapses (on the order of a few hundred or thousand), a considerable convergence of various types of interneurons to pyramidal cells and interneurons appears to occur. 7. Most interneurons are GABAergic and also express a number of different neurotransmitters (or their synthesizing enzymes), neuropeptides and calcium-binding proteins. Thus, interneurons are, biochemically, widely heterogeneous. 8. Some of the morphologically identifiable neurons can be characterized by their particular biochemical characteristics, and some biochemically definable subgroups of interneurons display a particular morphology. However, different morphological types of GABAergic neurons may share one or several neurotransmitters, neuroactive substances and/or other molecular markers. Therefore, a great variety of subgroups of morphologically and biochemically identifiable neurons exist. 9. Some interneurons appear to be common to all species and, therefore, may be considered as basic elements of cortical circuits, whereas others may represent evolutionary specializations which are characteristic of particular mammalian subgroups and, thus, cannot be taken as essential, or general, features of cortical organization. 10. Given the complexity of cortical circuits and the areal and species differences, it is impossible to draw a "sufficiently" complete basic diagram of cortical microcircuitry that is valid for all cortical areas and species.

Generation, description and storage of dendritic morphology data

It is generally assumed that the variability of neuronal morphology has an important effect on both the connectivity and the activity of the nervous system, but this effect has not been thoroughly investigated. Neuroanatomical archives represent a crucial tool to explore structure-function relationships in the brain. We are developing computational tools to describe, generate, store and render large sets of three-dimensional neuronal structures in a format that is compact, quantitative, accurate and readily accessible to the neuroscientist. Single-cell neuroanatomy can be characterized quantitatively at several levels. In computer-aided neuronal tracing files, a dendritic tree is described as a series of cylinders, each represented by diameter, spatial coordinates and the connectivity to other cylinders in the tree. This 'Cartesian' description constitutes a completely accurate mapping of dendritic morphology but it bears little intuitive information for the neuroscientist. In contrast, a classical neuroanatomical analysis characterizes neuronal dendrites on the basis of the statistical distributions of morphological parameters, e.g. maximum branching order or bifurcation asymmetry. This description is intuitively more accessible, but it only yields information on the collective anatomy of a group of dendrites, i.e. it is not complete enough to provide a precise 'blueprint' of the original data. We are adopting a third, intermediate level of description, which consists of the algorithmic generation of neuronal structures within a certain morphological class based on a set of 'fundamental', measured parameters. This description is as intuitive as a classical neuroanatomical analysis (parameters have an intuitive interpretation), and as complete as a Cartesian file (the algorithms generate and display complete neurons). The advantages of the algorithmic description of neuronal structure are immense. If an algorithm can measure the values of a handful of parameters from an experimental database and generate virtual neurons whose anatomy is statistically indistinguishable from that of their real counterparts, a great deal of data compression and amplification can be achieved. Data compression results from the quantitative and complete description of thousands of neurons with a handful of statistical distributions of parameters. Data amplification is possible because, from a set of experimental neurons, many more virtual analogues can be generated. This approach could allow one, in principle, to create and store a neuroanatomical database containing data for an entire human brain in a personal computer. We are using two programs, L-NEURON and ARBORVITAE, to investigate systematically the potential of several different algorithms for the generation of virtual neurons. Using these programs, we have generated anatomically plausible virtual neurons for several morphological classes, including guinea pig cerebellar Purkinje cells and cat spinal cord motor neurons. These virtual neurons are stored in an online electronic archive of dendritic morphology. This process highlights the potential and the limitations of the 'computational neuroanatomy' strategy for neuroscience databases.

Intracellular injection in fixed slices in combination with neuroanatomical tracing techniques and electron microscopy to determine multisynaptic pathways in the brain

Intracellular Lucifer Yellow filling in fixed tissue has been recently introduced as a novel neuroanatomical approach to reveal the detailed morphology of individual neurons in isolated preparations of the central nervous system. Since dye injections are performed under visual control, the method is characterized by a high degree of inherent staining selectivity, thus circumventing the element of randomness often considered to be the crux of classical golgi-impregnation techniques. Moreover, the opportunity to optically monitor the injection procedure renders fixed slice preparations highly advantageous to be used in combination with retrograde fluorescent tracing. Subsequently, dye-filled neurons may be subjected to a simple photoconversion procedure leading to the intracellular formation of a stable polymer thus obtaining permanent specimens for light microscopy purposes. Due to the osmiophilic nature of the precipitate the photoconverted material is equally suitable for correlated electron microscopy, thus enabling the analysis of neuronal microcircuitry. At the ultrastructural level, sources of afferent input to identified projection neurons may be revealed by lesion-induced anterograde degeneration of synaptic terminals, therefore enabling the direct demonstration of multisynaptic links. Finally, morphologically identified neurons may be immunocytochemically characterized at the pre- and postembedding levels. It is therefore suggested that their methodological versatility and relative technical ease render intracellular fixed-slice injections a promising complement to the catalogue of anatomical techniques.

Determining the neuronal connectivity of Golgi-impregnated neurons: ultrastructural assessment of functional aspects

The combined light and electron microscopic analysis of Golgi-impregnated neural tissue is a potent tool for determining the connectivity of neural networks within the brain. In the experimental paradigms commonly applied in these studies, the Golgi-impregnated neurons are typically examined as the postsynaptic neuronal components. The structural characteristics and the pattern of distribution of their synaptic connections with other groups of identified neurons are analyzed. Due to the high power of resolution of the Golgi-electron microscopic technique, the ultrastructural analysis of Golgi-impregnated neurons can be expanded to elucidate activity-dependent structural alterations in their cytoarchitecture. These structural alterations can then be correlated under different physiological conditions with changes in the functional efficacy of the subcellular neuronal components.

Quantitative aspects of synapses on Golgi-impregnated neurons

With the classical Golgi techniques, numerous types of neurons can be distinguished in the cerebral cortex, each with a specific dendritic geometry and pattern of axonal ramifications. In the present review we describe two techniques which allow quantification of synapses on identified neurons: (1) Golgi-rapid impregnation-gold toning-electron microscopy, and (2) Golgi-Kopsch impregnation-gold toning-electron microscopy in combination with staining of the tissue with ethanolic phosphotungstic acid (E-PTA). Both techniques were applied on neurons in the visual cortex of young and adult rabbits. By means of rotating and tilting specimens in the electron microscope, the nondistinctive ultrastructure of obliquely sectioned synapses can be circumvented, leading to precise estimates of asymmetrical vs. symmetrical synapses without complete reconstruction of the neuron.

Opening the grey box

The single neurone has been the guiding light for generations of neuroscientists. Now there are signs from experimental and theoretical work on the neocortex that we are on the threshold of a revolution in which the hegemony of the single neurone will be replaced by much more circuit-oriented concepts. We consider here why traditional views of the significance of single neurones are fading in power, and consider the problem of deciding on the form of a new order.