Intracellularly injected cobaltous ions accumulate at synaptic densities

Ultrastructural cytochemical visualization of chromium in the skin of sensitized guinea pigs

Using a modified sulfide silver method for electron microscopy, the intraepidermal and intracellular localization of epicutaneously applied potassium dichromate was investigated at varying times in sensitized and nonsensitized guinea pigs. The hapten penetrated rapidly into the epidermis. There was a homogeneous extra- and intracellular staining of the keratinocytes in the upper epidermis. The basal and suprabasal cells, by contrast, exhibited a predominant extracellular and plasma membrane localization of the silver grains. This membrane staining pattern was also observed in the Langerhans cells showing cellular and endocytotic activation in the sensitized animals. No specific cellular uptake of the hapten by the Langerhans cells was found. These results demonstrate that the epicutaneous application of chromate resulted in a characteristic intraepidermal distribution which may be related to the epidermal conversion of the hexavalent chromate to the immunogenic trivalent form. Moreover, the absent intracellular localization of the hapten in the activated Langerhans cells supports the notion that contact allergens can be presented to T cells without prior intracellular processing.

Neuronal structure and synaptic distribution of a uropod closer motor neuron in the crayfish terminal ganglion

One of the uropod closer muscles in the crayfish, the adductor exopodite, is innervated by two large identified motor neurons. They were injected intracellularly with horseradish peroxidase or nickel chloride to reveal the structure and distribution of the input and output synapses using electron microscopy. The development of nickel with rubeanic acid greatly improved the tissue preservation at the ultrastructural level compared with ammonium sulphide. Cell bodies of the motor neurons lying in the ventro-lateral cortex of the ganglion are extensively invaginated by glial cells. Input synapses occur directly upon the primary neurite within the neuropil or upon the major anterior neurite. They are most abundant, however, upon the numerous smaller neurites of the motor neuron. The primary neurite in the dorsal region of the neuropil, upon which no synapses were made, is wrapped with glial cells. Occasionally, these two adductor exopodite motor neurons were found as adjacent postsynaptic profiles at the same synapse when both cells were stained simultaneously in the same preparation. In the present study we could not locate any sites of synaptic output which strictly fulfil the structural criteria of a synapse on the processes of the motor neuron. This result is inconsistent with physiological evidence which suggests that spikeless interactions occur between the two adductor exopodite motor neurons and their synergists. This might be the result of two possible features of the interaction: the sites of synaptic output may be limited to a few restricted branches, and the interaction between these motor neurons may depend largely upon electrical synapses.

Functional morphology of insect neuronal cell-surface/glial contacts: the trophospongium

Ultrastructural studies were carried out on the surfaces of insect nerve cell bodies. Some of the neurons were identified, by using physiological criteria, before filling with dye. Their surface patterns were compared, to provide data needed for understanding dynamic relationships with glial cells, in the trophospongium. The data are also needed in connection with interpretation of electrical signals recorded from the somata and of their roles in integration and in learning and memory. The surfaces were found to be extremely complex and also varied, even for neurons of comparable size and function, as well as for different regions of the same neuron, suggesting that the surface is constantly changing as the neuron receives food and loses waste. There is a variety of cytoplasmic types of invagination of neuron somata by glial processes. The invaginations were classified into four easily recognized types: regular, chunky, filigree, and ridge (present only in axon hillock regions). Motor neurons also make reciprocal invaginations into the glial cells that surround them. Some of these extend for distances up to 40 microns from the surface. The effective surface area is increased, compared with that calculated for a smooth surface, as a result of the invaginations, by from as little as 5% for a small interneuron to as much as 12-fold for a large motor neuron. The axon hillock region of all types of neurons is heavily invaginated.

Glial cells of an insect ganglion

The rapid development of the study of insect neurobiology, which is currently occurring principally because individual neurons can be re-identified and because their activities can be recorded in situ and related to behavior, is generating a demand for more knowledge concerning insect glial cells and their functional relationships with neurons. This study examines the ultrastructure of glial cells in locust metathoracic ganglia in relation to general locale within the ganglion and also to specific identified neurons and neuron types. Seven major types of glial cell form are recognized, with subdivisions, requiring a new scheme for classification. Glial invaginations into neurons are of four different kinds: regular, chunky, filigree, and ridge (found only at axon hillocks). They also range from only intrusive to fully reciprocal. In addition, some neurons make projections of various lengths into surrounding glia and between neighboring neuron somata, and some glia make long, branched projections into other glial cells. The differences show that insect glial cells develop highly specific functional specializations; they may not be interchangeable. The complexity and intimacy of relationships of glia with neurons suggest that some glial cells may have roles other than that of nursemaids, possibly in modulation of behavior-determining neural activity, and in learning and other adaptive acts.

Both electrical and chemical transmission between the 'lobula giant movement detector' and the 'descending contralateral movement detector' neurons of locusts are supported by electron microscopy

Conventional electron microscopy combined with cobalt staining techniques has revealed chemical synapses and gap junction-like areas denoting specific regions of contact between two large, uniquely identifiable visual interneurons in the brain of the locust Schistocerca gregaria. The morphological demonstration of chemical synapses suggests that one of the two neurons, the 'descending contralateral movement detector', receives a chemically mediated input from its main presynaptic element, the 'lobula giant movement detector'. This observation supports recent electrophysiological studies demonstrating synaptic delays between the two cells, characteristic of chemical synapses. However, regions with the appearance of gap junctions are also observed. This corroborates earlier work which suggested that these two neurons are coupled electrically.

Synaptic connections between the hindwing stretch receptor and flight motor neurones in the locust revealed by double cobalt labelling for electron microscopy

Synaptic interactions between sensory and motor neurones in the locust flight system have been investigated by using intracellular labelling with cobalt and nickel for electron microscopy. Simultaneous axonal filling of two neurones with different concentrations of metal ions produces differential labelling, so that contacts between them in the central nervous system can be recognized. We have investigated the connectivity of the hindwing stretch receptor neurone (SR) with a direct hindwing depressor motor neurone (MN 127) known from physiological experiments to receive monosynaptic input from the SR, and an indirect hindwing depressor motor neurone (MN 112/1), for which no monosynaptic connection with the SR has been reported. We have found no direct synapses between the SR and MN 112/1, although some of their branches lie close together in the neuropile. We have, however, found some evidence for polysynaptic connections between them. There are many synapses of conventional dyadic morphology from both the lateral and mediolateral branches of the SR to MN 127; the medial branch was not examined. Those from the lateral branch contact the motor neurone on branches close to the neuropilar segment, while those from the mediolateral branch contact long, thin distal twigs. We estimate that there are about 600 anatomical synapses between these two neurones. Our results suggest that a large number of widely distributed anatomical synapses constitute the physiological synaptic connection between the SR and MN 127. The dyadic arrangement of these synapses provides an anatomical correlate for the physiologically established divergence of SR outputs onto interneurones and motor neurones.

Competition controls the growth of an identified axonal arborization

The shape of the axonal arborization was studied in an identified insect sensory neuron. The distribution of presynaptic varicosities within an axonal arbor was shown to be modulated by the density of neighboring terminals. Removal of neighbors near one portion of the axon terminal increased the growth rate in the denervated region and caused a compensatory retraction in other regions. The results support the hypothesis that the size of an axonal arbor is determined intrinsically, whereas the distribution of varicosities within the terminal is determined extrinsically by neighboring terminals. These findings provide a direct demonstration of the effects of competition on an identified nerve cell, as well as one of the first examples of competitive interactions in an invertebrate central nervous system.

Cytology of cobalt-filled neurons in flies: cobalt deposits at presynaptic and postsynaptic sites, mitochondria and the cytoskeleton

Combined light and electron microscopy of identified neurons requires an intracellular marker that is both photon opaque and has electron scattering properties. We describe results using cobalt chloride block intensified with silver as an intracellular label. The novelty of the method is its integration in tissue fixation, prior to dehydration, resulting in fine grain precipitates that resolve certain intracellular structures. Filled neurons are clearly distinguishable from unfilled profiles by cobalt-silver precipitates. Energy dispersive X-ray analysis confirms that silver is specifically deposited onto cobalt sulphide cores which are characteristically associated with microtubules, mitochondria, presynaptic and postsynaptic specializations and gap junction-like membrane appositions.

The effects of salines and fixatives upon the size of an identified neuron

Because accurate neuronal dimensions are essential for mathematical modeling of neuronal properties, the effects of a number of salines and fixative procedures on neuronal size were compared, including the non-chemical, freeze substitution method. Using an identified neuron we compared diameters and found some of the fixative-saline combinations caused shrinkage by as much as a factor of four from our best estimates of the in vivo size from the quick frozen preparations. A glutaraldehyde based fixation procedure was found which gives results in good agreement with the frozen tissue.

Structural comparison of a homologous neuron in gryllid and acridid insects

The fast extensor tibiae (FETi) motor neuron is responsible for exciting the extensor tibiae muscle to produce most of the force for jumping in acridids. Because of its relatively large size and crucial role in jumping, FETi has been studied in an ever-increasing number of orthopteran species. Here we describe the structure of the metathoracic FETi neuron in six species of acridids and in two species of gryllids. The morphology of FETi within the respective groups is essentially equivalent, but marked differences are apparent between acridid and gryllid FETis. There are similarities in the size and location of the cell body and the course of the neurite through the ganglion. Differences are found in the number of large branches, density of branching, and the volume of neuropil receiving branches. We propose that the gryllid FETi is an intermediate form between slow extensor tibiae motor neurons involved in walking and acridid fast extensor tibiae motor neurons specialized for jumping.

Cobalt mapping of the nervous system: how to avoid artifacts

Infusion of cobalt ions into cut axons is an established method for tracing neuron projections in the central nervous system. Artifacts, where unintended neurons are stained, however, have been reported, leading to difficulties in interpretation. Experiments in the locust Schistocerca gregaria Forskål show that such artifacts can be induced through damage to axons caused by cutting peripheral nerves and by using high cobalt chloride concentrations (0.4M and above). Mixtures of cobalt and nickel chlorides and nickel chloride alone were introduced into different branches of the same nerve and developed with rubeanic acid to give precipitates of different colors in the two sets of axons. Preparations were examined with the light microscope, where mixing of ions would appear as intermediate colors, and by x-ray probe microanalysis. No evidence for leakage of metal ions from the filled axons or for ion uptake by other axons could be detected, provided that low concentrations of cobalt and nickel chlorides were used and nerve cutting was reduced to a minimum by making preparations in vivo. If extreme conditions are avoided when making the preparation, the risk of producing artifacts is minimized, thus enabling the cobalt method to be used with greater confidence for describing neuronal projections.

Optical sectioning of HRP-stained molluscan neurons

The use of high-resolution differential interference contrast(DIC) microscopy on cleared whole-mounts of the circumesophageal nervous system from Hermissenda crassicornis permits visualization of neuronal morphology in detail without the need for physical sectioning. Such optical sectioning, when preceded by intracellular iontophoresis of horseradish peroxidase (HRP) permits rapid and accurate examination of the arborization of electrically characterized neurons. Details such as varicosities and terminal swellings can readily be resolved. This method has revealed new morphological features of neurons implicated in training-specific behavioral modification in Hermissenda, and promises to be of further general use for the quantitative morphometry of electrically identified neurons.

Locust local nonspiking interneurons which tonically drive antagonistic motor neurons: physiology, morphology, and ultrastructure

Local nonspiking interneurons have been implicated in the control of behavior. We have characterized the physiology of two local nonspiking interneurons in the locust and subsequently examined the neurons in the light and electron microscopes. Physiologically the two interneurons have opposite effects upon antagonistic motor neurons and are tonically releasing transmitter at their "resting potentials." This combination of tonic release and reciprocal driving of antagonistic motor neurons by single interneurons provides a hitherto undescribed means of controlling posture. One interneuron (DCVII, 4) excites flexor tibiae and inhibits the slow extensor tibiae motor neurons when depolarized. The other interneuron (DCVII, 5) inhibits the flexor tibiae and excites the slow extensor tibiae motor neurons when depolarized. In both cases, when the interneurons are hyperpolarized, they have the opposite effects upon the same motor neurons. Intracellular staining of these neurons confirms that they are local interneurons. Furthermore, an examination of sectioned material shows that the neurons are unique and can be identified as such in a population of locust neurons. Ultrastructurally, we find synapses only on the smaller (less than 2 micrometers) branches. These neurons may form the presynaptic element in either of two configurations, these being the discrete density (one presynaptic) and the dense bar (one presynaptic, two postsynaptic) type of configurations. The functional implications of these findings for neurons controlling posture are discussed.

Unique, identifiable local nonspiking interneurons in the locust mesothoracic ganglion

The hypothesis that local nonspiking interneurons are unique and identifiable has been tested rigorously for a neuron in the mesothoracic ganglion of the locust. Neurons were physiologically characterized and subsequently stained with cobalt ions. The resulting preparations were examined in whole mounts and serial sections. It is concluded that at least three neurons are unique, based upon a combination of their function, gross morphology, and the location and size of their main processes relative to other neurons. It is strongly suggested that there are other local nonspiking interneurons that are unique and identifiable. A classification system for local nonspiking interneurons is proposed. The implications of this finding for future neuroethological studies are discussed.

Organization of motor neurons to a multiply innervated insect muscle

Nine excitatory motor neurons have been identified as innervating the locust metathoracic flexor tibiae. The anatomical organization of the flexor motor neurons within the ganglion was examined with both light and electron microscopy. Flexor motor neurons were physiologically identified prior to intracellular staining with Procion or cobalt. Some of the cobalt-stained neurons were then silver intensified. The reliability of soma location and variability of neurite branching were examined. While the position of a soma could vary within its cluster by up to one radius, the anterior, posterior, and lateral soma clusters bore a consistent relationship to each other. The density of neurite branching varied greatly for any particular flexor. The ultrastructure of the tract containing the flexor neurites revealed the individual neurites to be glial wrapped, while the tract itself was isolated from the neuropil by additional glia. The hypothesis that subsets of the flexor motor neuron pool are recruited for different behaviors is discussed in light of the last two findings.