Immunocytochemical analysis of noradrenaline, substance P and enkephalin axonal contacts on serotonin neurons in the caudal raphe nuclei of the cat

The present study investigates the anatomical basis for interactions between serotonin immunoreactive neurons in nuclei raphe magnus and pallidus, and either noradrenaline, substance P (SP) or enkephalin immunoreactive axonal varicosities. Using a double-label immunocytochemical method, we found that each of these neurochemicals could be localized to axons which contacted serotonin immunoreactive neurons. The frequency and location of these inputs differed in nuclei raphe magnus and pallidus. SP immunoreactive varicosities formed the greatest number of contacts. These findings suggest that serotonin-containing neurons in the caudal raphe nuclei receive input from multiple putative neurotransmitters.

Widespread distribution of the c-src gene product in nerve cells and axon terminals in the adult rat brain

The regional and cellular distribution of the proto-oncogene product pp60c-src, a member of the family of membrane-associated tyrosine-specific protein kinases, was analysed in adult rat brain. High-resolution SDS-PAGE allowed analysis of both the 'fibroblast' 60-kDa form and a variant, 61-kDa neuron-specific form of the c-src gene product which is encoded by an alternately processed c-src mRNA. Studies of microdissected brain regions showed that all CNS regions contained both forms of the enzyme, the 61-kDa form predominating in most regions with high content of gray matter and high density of synapses. Lesion-induced degenerations of specific neuronal elements in the basal ganglia decreased the level of both forms of the c-src gene product both in regions where cell bodies had been lesioned and in regions where nerve terminals had degenerated. The 61-kDa form of the enzyme appeared somewhat more sensitive to the effects caused by these lesions than the 60-kDa form. These results indicate that, within the mature mammalian brain, both cell body regions and nerve terminals of many, and possibly all, nerve cells contain both forms of the c-src gene product, the 61-kDa form being most highly enriched in the nerve cells. These results suggest that the enzyme may be involved in pleiotropic functions, including signal transduction in nerve terminals.

Types and spatial distribution of vasoactive intestinal polypeptide (VIP)-containing synapses in the rat visual cortex

In the rat visual cortex vasoactive intestinal polypeptide (VIP)-containing structures were studied by means of light and electron microscopy and image analysis. VIP-immunoreactive axon terminals were found to form symmetric synapses with small dendritic shafts, dendritic spines and somata of pyramidal cells and interneurons. VIP-terminals often occurred in pairs with VIP-negative, asymmetric synapses on the same postsynaptic structure. VIP-immunostained dendrites and perikarya were contacted by a purely asymmetric and a mixed population of VIP-negative terminals, respectively. Synaptic connections between two VIP-neurons are seldom as compared to the other types of VIP-synapses. Quantitative studies obtained by the image analysis of VIP-stained boutons and dendritic particles in light microscopic preparations suggest a distinct laminar distribution. Dendritic particles are most frequent in layers I-II, whereas axonal boutons have three laminar accumulations: at the border of layers I-II, in layer IV and layer VI. Together with previous results, the present findings argue for a non-random spatial distribution of VIP-boutons.

Thalamic branched and unbranched axons to feline polysensory cortex

Thalamic afferents to polysensory neocortex of the suprasylvian, anterior lateral, and pericruciate gyri were demonstrated by retrograde axonal transport of wheat germ agglutinin lectin-bound horseradish peroxidase (HRP) and nuclear yellow (NY). Marker injections were placed in areas identified as responsive to auditory stimuli through the use of click-evoked potentials. Both HRP and NY labeled neurons were found in close proximity or in overlapping regions of the rostral intralaminar nuclei and the adjacent ventral lateral nucleus of the thalamus. While thalamic neurons projecting to different polysensory neocortical areas were often in overlapping fields, double labeling was rare. These results indicate that similar auditory responses seen in different cortical polysensory areas are mediated by mainly separate but topographically related populations of thalamocortical neurons. Similarities in auditory response patterns seen across widely separated areas of polysensory neocortex may be due to common inputs relayed through overlapping or adjacent areas of the thalamus.

Suprachiasmatic nucleus neurons immunoreactive for vasoactive intestinal polypeptide have synaptic contacts with axons immunoreactive for neuropeptide Y: an immunoelectron microscopic study in the rat

An electron microscopic study showed by using a dual immunolabeling technique that in the suprachiasmatic nucleus of the rat, axon terminals immunoreactive for neuropeptide Y (NPY) made synaptic contacts upon neurons immunoreactive for vasoactive intestinal polypeptide (VIP). Diaminobenzidine (DAB)-labeled NPY axon terminals made synaptic contacts on silver-gold-labeled VIP perikarya and dendritic processes. The presynaptic NPY terminals contained many small clear vesicles and a few cored vesicles labeled with DAB chromogen. At the synaptic portion, a symmetrical thickening of the pre- and post-synaptic membranes was evident.

Neurofilament phosphorylation in axons and perikarya: immunofluorescence study of the rat spinal cord and dorsal root ganglia with monoclonal antibodies

Rat dorsal root ganglia and spinal cord were stained with 12 monoclonal antibodies reacting with phosphorylated epitopes of two neurofilament proteins (NF 150K and NF 200K). Three monoclonal antibodies were axon-specific in both locations; neuronal perikarya were not stained. Nine monoclonal antibodies stained a subpopulation of neurofilament-positive sensory neurons, as indicated by double labeling experiments with polyclonal antibodies reacting with phosphorylated and dephosphorylated forms of the neurofilament protein triplet. Of these nine antibodies, two stained motor neuron perikarya in the spinal cord, while the remaining seven antibodies were axon-specific in this location. Subpopulations of stained and unstained motor neurons were not observed. With all 12 antibodies, the staining pattern in the lumbar dorsal root ganglia and spinal cord remained unchanged following sciatic nerve crush and ligature. The findings suggest that, in the neurofilament, some phosphorylated epitopes are axon specific, while other phosphorylated epitopes are present in both axons and perikarya. Furthermore, they suggest that differences exist between neuronal populations as to the presence of phosphorylated epitopes in perikaryal neurofilaments. It remains to be seen whether phosphorylation events in perikarya and axons have similar or different effects on neurofilament structure and function.

Intrinsic connections of the retrohippocampal region in the rat brain: III. The lateral entorhinal area

This paper describes the retrohippocampal projections of individual layers of the lateral entorhinal area as studied by the method of anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L) in the rat. As in the medial entorhinal area (EA), (Köhler, '86a) PHA-L injections restricted to individual layers of the lateral EA resulted in labeling of sparse projections to the subicular complex (e.g., subiculum, pre- and parasubiculum), whereas projections to the perirhinal area and piriform cortex were prominent. All PHA-L injections resulted in the labeling of axons projecting longitudinally within the entorhinal area, in both dorsal and ventral directions, albeit the ventral projections were the most prominent ones. PHA-L injections into layers 2a and 2b resulted in labeling of axons that could be followed into layers 2a, 2b, and layer 1 on both sides of the injection site. Whereas numerous axons appeared to terminate in layer 2, most fibers ascended into layer 1, where they ran in a medial direction, passing the medial EA, around the parasubiculum to the presubiculum. Numerous axons were found to take a lateral route running past the lateral aspect of the lateral EA to the piriform cortex. The axons running medial in layer 2 did not enter the medial EA. After PHA-L injections into layer 3, a large number of axons left the labeled cells on both sides of the injection site, in addition to massive projections that ascended into layers 2b, 2a and 1, just above the injection. Few axons entered layers 2-6 of the medial EA, but numerous axons innervated layer 1, where they were found to run in the outer half of this layer. The axons running in a medial direction reached layer 1 of the presubiculum, whereas the laterally oriented ones innervated the molecular layer of the piriform cortex. PHA-L injections into layer 4 resulted in massive labeling of projections to all superficially located layers. Layers 1, and 2b through 5 were innervated lateral to, and layer 4 medial to, the injection site. After a PHA-L injection into layer 5, ascending projections were found innervating layers 1 through 4. The terminal fields were found to be particularly dense in the deep parts of layer 3 and in layer 1. This projection expanded laterally, but few projections reached into the medial sector of the lateral EA or into the medial EA.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of axonal branching using the retrograde transport of fluorescent latex microspheres

A method is described for the quantitative analysis of double retrograde labelled neuronal cell bodies following labelling of branched axonal projections. This exploits the known ability of retrograde translocator proteins to transport latex microspheres following their uptake at nerve terminals. Conditions necessary for uptake and transport include small bead diameter (0.05-micron) and carboxylation of the latex particle. Using coumarin- and rhodamine-labelled microspheres a reliable, sensitive, rapid method has been developed, which results in double retrograde cell labelling in branched axonal pathways from the frontal cortex, basal forebrain, and brainstem. The technique has several advantages over currently available double retrograde labelling methods and yields repeatable quantitative estimates of populations of neurones bearing branched axons.

Effects of the external ions and metabolic poisoning on the constriction of the squid giant axon after axotomy

After transecting the squid giant axon in the presence of an artificial external medium, which was composed of the ions normally present in squid blood, the cut ends of the axon constrict. This constriction could be completely blocked by cutting the axon in the presence of an artificial internal medium composed of the ions normally present inside the axon. By interchanging the ions in the internal medium with those in the external medium, it was determined that constriction was stimulated by the high concentrations of calcium, chloride, and magnesium ions present in the external medium and inhibited by the high concentrations of potassium ion in the internal medium. Constriction could also be inhibited by 2,4-dinitrophenol and cyanide. Softening of axoplasm and elution of the axoplasmic proteins at the cut end of the axon also occurred in the external medium. This softening and elution may be necessary for constriction since constriction occurred only in those media that also induced axoplasmic softening and elution. Softening and elution are not sufficient for constriction, however, since high potassium, 2,4-dinitrophenol, and cyanide inhibited constriction without inhibiting the softening or elution of axoplasm.

Distribution of albumin-like immunoreactivity in rat sciatic nerve after nerve crush injury

To understand better the role of local factors in the response of peripheral nerve to crush injury, we studied the distribution of albumin-like immunoreactivity (A-LI) in the rat sciatic nerve from one day to eight weeks (wk) after a crushing injury; we used electron microscopic immunocytochemistry. In the nerve distal to the crush degenerating axons demonstrated intra-axonal A-LI, and by one wk most of the Schwann cells also showed A-LI. As regenerating sprouts entered the distal nerve, those Schwann cells in contact with sprouts lost their A-LI, while those cells not in contact with axons retained immunoreactivity up to eight wk after injury. Proximal to the nerve crush many axons showed intra-axonal A-LI from one to two wk after injury, despite appearing normal ultrastructurally. This immunoreactivity diminished as the distance from the crush site increased. Many Schwann cells proximal to the crush also showed A-LI from one to four wk after injury. These findings suggest that an albumin-like protein may play a role in the response of Schwann cells and axons to injury.

Immunoelectron microscopic localization of neural cell adhesion molecules (L1, N-CAM, and myelin-associated glycoprotein) in regenerating adult mouse sciatic nerve

The localization of the neural cell adhesion molecules L1, N-CAM, and the myelin-associated glycoprotein was studied by pre- and postembedding staining procedures at the light and electron microscopic levels in transected and crushed adult mouse sciatic nerve. During the first 2-6 d after transection, myelinated and nonmyelinated axons degenerated in the distal part of the proximal stump close to the transection site and over the entire length of the distal part of the transected nerve. During this time, regrowing axons were seen only in the proximal, but not in the distal nerve stump. In most cases L1 and N-CAM remained detectable at cell contacts between nonmyelinating Schwann cells and degenerating axons as long as these were still morphologically intact. Similarly, myelin-associated glycoprotein remained detectable in the periaxonal area of the degenerating myelinated axons. During and after degeneration of axons, nonmyelinating Schwann cells formed slender processes which were L1 and N-CAM positive. They resembled small-diameter axons but could be unequivocally identified as Schwann cells by chronical denervation. Unlike the nonmyelinating Schwann cells, only few myelinating ones expressed L1 and N-CAM. At the cut ends of the nerve stumps a cap developed (more at the proximal than at the distal stump) that contained S-100-negative and fibronectin-positive fibroblast-like cells. Most of these cells were N-CAM positive but always L1 negative. Growth cones and regrowing axons expressed N-CAM and L1 at contact sites with these cells. Regrowing axons of small diameter were L1 and N-CAM positive where they made contact with each other or with Schwann cells, while large-diameter axons were only poorly antigen positive or completely negative. 14 d after transection, when regrowing axons were seen in the distal part of the transected nerve, regrowing axons made L1- and N-CAM-positive contacts with Schwann cells. When contacting basement membrane, axons were rarely found to express L1 and N-CAM. Most, if not all, Schwann cells associated with degenerating myelin expressed L1 and N-CAM. In crushed nerves, the immunostaining pattern was essentially the same as in the cut nerve. During formation of myelin, the sequence of adhesion molecule expression was the same as during development: L1 disappeared and N-CAM was reduced on myelinating Schwann cells and axons after the Schwann cell process had turned approximately 1.5 loops around the axon. Myelin-associated glycoprotein then appeared both periaxonally and on the turning loops of Schwann cells in the uncompacted myelin.(ABSTRACT TRUNCATED AT 400 WORDS)

Both adult and juvenile tau microtubule-associated proteins are axon specific in the developing and adult rat cerebellum

Several antibodies directed against the heterogeneous microtubule-associated protein group tau have been used to determine the immunocytochemical localization of these proteins in the developing rat cerebellum. Immunoblot analysis of brain extracts showed that both monoclonal and polyclonal anti-tau antibodies revealed not only the adult tau proteins (50,000-70,000 mol. wt) but also the immature (48,000 mol. wt) tau form. Immunocytochemical studies showed that, whatever the stage of development, anti-tau antibodies stained several types of axonal fibres. The Purkinje cell bodies and their dendrites were never significantly labelled. This means that immature tau is, as adult tau, localized essentially in axons. Axonal labelling seems to follow the cerebellar developmental pattern. For instance, the climbing fibres which reach the cerebellum during the embryonic life were stained soon after birth by the anti-tau antibodies. In contrast, the parallel fibres, that begin to develop perinatally, do not express tau at early (5 days) postnatal stages; a clear labelling of the deeper parallel fibres (which are more mature than the superficial ones) was seen at day 10 after birth in the vicinity of the developing dendrites of the Purkinje cells. This suggests that (1) the appearance of tau immunoreactivity reflects a certain stage of maturity of the parallel fibre; (2) both immature and mature tau microtubule-associated proteins seem to be axon specific in the developing rat cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotoninergic innervation of the cat cerebral cortex

Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.

Propriospinal fibers in the white matter of the cat sacral spinal cord

The propriospinal system, which consists of those neurons completely contained within the spinal cord, is important because it underlies much spinal behavior. To provide quantitative data on this system, the present study determines numbers of axons in the isolated S2 cat spinal cord and compares these figures with the normal. The conclusion is that 60% of the fibers in the spinal cord at this location are propriospinal. Findings of particular interest are that the great majority of unmyelinated propriospinal axons are found in the dorsal part of the lateral funiculus, and that there are large numbers of descending myelinated fibers in the dorsal funiculi. These data will serve as a basis for evaluating axon numbers that follow various experimental regimens purporting to result in neural sprouting.

Migratory pathways and neuritic differentiation of inferior olivary neurons in the rat embryo. Axonal tracing study using the in vitro slab technique

The use of the HRP retrograde tracing method, applied in vitro to embryonic (E15-E20) cerebellum-brainstem slabs, has allowed the identification of single young postmitotic olivary neurons. Labeled neurons move within two migratory streams: one superficial, under the pia (the marginal stream), and the other, of earlier onset, deeper in the medullary parenchyma (the submarginal stream). All neurons in the latter converge to the inferior olive ipsilaterally to their proliferation site; whereas, most neurons within the marginal stream cross the midline and bypass the olivary domain. Only a few HRP-labeled neurons leave the marginal stream towards the olivary territory, on their proliferation side. Hence, contrary to previous reports, the submarginal stream provides almost all the olivary neurons (95% at least), while the contribution of the marginal stream is very small (5% at the most). Axonogenesis is the earliest event in neuritic differentiation. By E15, 48 h after proliferation, the axons at the front of the migrating neurons have already crossed the interolivary commissure, and reached at least the site of HRP application, while the cell bodies have not yet penetrated their terminal domain. An ipsilateral component of this axonal tract was never detected. Hence, the olivocerebellar projection is formed very early, and is entirely crossed from its onset. Dendritogenesis was also analyzed during intra-uterine life; olivary neurons evolve from a fusiform shape (typical of migrating neurons) to a stellate form, with long and straight dendrites (once arrived at their ultimate location). Thus the acquisition of their mature spherical 'ball of wool' shape is a postnatal event, most probably concomitant with the major synaptogenetic phase.

The morphology and distribution of rat serotoninergic intraspinal neurons: an immunohistochemical study

An immunohistochemically derived morphological description of a diverse population of rat lamina VII and X intraspinal 5HT neurons is provided. These bipolar or multipolar neurons occur most frequently in lamina X, dorsal or dorsolateral to the central canal, in thoracolumbar, sacral, and coccygeal spinal segments. These 5HT intraspinal neurons are found in normal rat spinal cords as well as in spinal cords that have been hemisected or transected 60 days prior to serotonin immunostaining. Therefore, 5HT intraspinal neurons are the probable source of the biochemically detectable 5HT that remains in the spinal cord distal to a spinal transection. In the rat, serotonin intraspinal neurons are most often associated with spinal autonomic nuclei but it is unknown if they are preganglionic in nature.

Biochemical composition and dynamics of the axonal cytoskeleton in the corticospinal system of the adult hamster

The corticospinal system is an important central nervous system (CNS) pathway that is implicated in debilitating diseases such as amyotrophic lateral sclerosis and in traumatic injuries to the spinal cord. This study characterizes some of the fundamental biochemical and kinetic properties of normal corticospinal axons, establishing an important reference for studies that aim to elucidate the cellular modifications that result during pathological conditions of these axons. Slow axonal transport which conveys the axonal cytoskeleton as well as cytomatrix constituents, such as many of the metabolic enzymes and regulatory proteins, has been examined. For these studies, [35S]methionine was injected into the sensorimotor cortex of adult male Golden hamsters, and labeled, transported proteins present in corticospinal axons at 1-42 days after injection were assessed using one- and two-dimensional gel electrophoresis/fluorography. The complex group of slow component b (SCb) proteins (including clathrin, actin, enolase, creatine phosphokinase, and many others) was observed to move at a rate of approximately 2 mm/day in adult corticospinal axons. The slow component a (SCa) proteins (tubulins, neurofilament proteins, and actin) were transported at a substantially slower rate of approximately 0.4 mm/day. The biochemical and kinetic properties of slow transport in corticospinal axons were very similar to those previously described in another CNS pathway, axons of retinal ganglion cells, and substantially different from those documented in large, peripheral sensory or motor axons. These findings suggest that some of the basic properties of axonal transport which determine many of the structural and functional properties of axons may be different in the CNS compared to the peripheral nervous system.

Demonstration of serotoninergic axon terminals on somatostatin-immunoreactive neurons of the anterior periventricular nucleus of the rat hypothalamus

Using a combination of electron microscopic autoradiography and immunocytochemistry, the connections between serotoninergic axons and somatostatin neurons of the anterior periventricular nucleus of the rat hypothalamus were examined. The serotoninergic elements were identified after selective uptake of tritiated serotonin and the somatostatin neurons with immunocytochemistry. Synaptic connections between labeled serotoninergic nerve endings and somatostatin-immunoreactive neurons were observed. This finding provides morphological evidence for a direct influence of serotoninergic elements on somatostatin neurons of the anterior periventricular nucleus projecting to the median eminence of the hypothalamus.

Organotypic culture of central histamine neurons

Organotypic cultures of histaminergic tuberomammillary (TM) neurons were grown using explants obtained from newborn rats. The cultures were examined after immunohistochemical localization of the histamine synthetic enzyme, L-histidine decarboxylase (HDC). The morphological properties of the somata, dendrites and axons of HDC-immunoreactive TM neurons in organotypic culture were virtually indistinguishable from those seen in situ. Extensive plexuses of HDC-immunopositive axons, including growth cones, were seen within the hypothalamus, the plasma surrounding the explant and co-cultured hippocampus. Organotypic cultures of TM histamine neurons, and co-cultures with their targets, provide a useful model system for studying several aspects of central histaminergic neurobiology.

Morphological and electrophysiological identification of gigantocellular tegmental field neurons with descending projections in the cat: I. Pons

Two different descending projections from the pontine gigantocellular tegmental field (PFTG) were defined by the use of intracellular recording and intracellular horseradish peroxidase (HRP) techniques in the cat. Type I neurons (reticulospinal neurons) had antidromic spike potentials produced by stimulation of the ipsilateral medial longitudinal fasciculus (MLF) and sent axons to the ipsilateral MLF. Most type I neurons had large ellipsoidpolygonal somata (mean, 59.7 microns), thick axons (average diameter, 3.33 microns), and slightly oblate large dendritic fields. The mean anteroposterior extent of the dendritic field was 1,492 microns, the mean mediolateral extent was 1,784 microns, and the mean dorsoventral extent was 1,562 microns. There were no type I neurons with axon collaterals. In contrast, type II neurons (reticuloreticular neurons) had antidromic spike potentials produced by stimulation of the bulbar reticular formation (BRF) and sent axons directly to the BRF. In comparison with type I neurons, most type II neurons had smaller ellipsoidpolygonal somata (mean, 40.2 microns), thinner axons (average diameter, 2.32 microns), and smaller, slightly oblate dendritic fields. The mean anteroposterior extent of the dendritic field was 1,264 microns; the mean mediolateral extent was 1,511 microns; and the mean dorsoventral extent was 1,226 microns. Also in contrast to type I neurons, 36% of type II neurons had axon collaterals.

Distribution of axons exhibiting both enkephalin- and serotonin-like immunoreactivities in the lumbar cord segments: an immunohistochemical study in the cat

Axons exhibiting both enkephalin- and serotonin-like immunoreactivities were observed by the double immunofluorescence method in the lumbar cord segments of the cat. Double-labeled axons were seen most frequently in laminae I, IIa and the lateral part of lamina V. They were also distributed in other parts of the dorsal horn and lamina X (especially in the dorsal part), but rarely found in laminae VII, VIII and IX. After cervical hemicordotomy the vast majority of double-labeled axons disappeared from the spinal gray ipsilateral to the lesion.

Enigmatic bipolar cell of rat visual cortex

Our earlier Golgi-electron microscopic study of bipolar cells in the rat visual cortex showed the axons of these neurons as forming asymmetric synapses (Peters and Kimerer; J. Neurocytol, 10:921-946, '81) in which the most common postsynaptic elements were dendritic spines. This result was unexpected, since Parnavelas (Parnavelas, Sullivan, Lieberman, and Webster: Cell Tissue Res. 183:499-517, '77) had earlier shown a bipolar cell from the same cortex to have an axon forming symmetric synapses with dendritic shafts. Here then was an enigma, strengthened by examination of neuronal components labelled by antibodies to two compounds in particular--namely, vasoactive intestinal polypeptide (VIP) and choline acetyltransferase (ChAT). Antibodies to these compounds preferentially label bipolar cells in the rat cerebral cortex, and the labelled axon terminals form symmetric synapses. Against this background the present study was performed, and it has been shown that the resolution to the enigma is that there are two different populations of bipolar cells in the rat visual cortex. Thus some Golgi-impregnated bipolar cells examined by electron microscopy after gold toning have been found to possess axons forming asymmetric synapses, and others have been found to have axons forming symmetric synapses. The axons of the bipolar cells forming asymmetric synapses most commonly synapse with dendritic spines (67%), although other terminals synapse with dendritic shafts (33%). In contrast, the bipolar cells with axons forming symmetric synapses preferentially synapse with dendritic shafts (100%). The population of bipolar cells that form symmetric synapses includes the ones that label with antibodies to vasoactive intestinal polypeptide (VIP), for the axons of VIP-labelled bipolar cells have been traced to labelled terminals forming symmetric synapses. However, examination of the population of VIP-labelled axon terminals shows that in addition to dendritic shafts, some of the labelled terminals synapse with the cell bodies of pyramidal and nonpyramidal cells. This includes bipolar cells, some of which receive large numbers of VIP-labelled axon terminals. It is also shown that some VIP-positive bipolar cells have myelinated axons. Analysis of tissue labelled with VIP antibody reveals that about 50% of the total population of bipolar cells in the rat visual cortex is VIP positive. These results are discussed in the light of information about labelling of bipolar cells with antibodies to gamma-aminobutyric acid (GABA) and to other peptides, and it is suggested that most VIP-positive bipolar cells also contain GABA.

Immunoreactivity for laminin in the developing ventral longitudinal pathway of the brain

The first long tract to form in the brain of a vertebrate embryo is the ventral longitudinal pathway. In order to investigate what chemical cues may guide nerve growth cones along this pathway, affinity-purified antibodies to laminin and collagen type IV were used to stain sections of mouse embryos from Embryonic Days 8 through 17. A monoclonal anti-neurofilament antibody was used to show the development of the ventral longitudinal pathway in relationship to immunoreactivity for laminin and collagen type IV. At Day 8 fluorescent immunoreactivity for laminin is bright in the external limiting membrane of the neural tube, but the neuroepithelium does not show bright laminin or neurofilament immunoreactivity. At E9 the ventral longitudinal pathway is forming and punctate immunoreactivity for laminin is present on the surfaces of neuroepithelial cells in the marginal zone, through which axons of the ventral pathway extend. Punctate immunofluorescence for laminin remains concentrated in the marginal zone on Days E10 through E14, but on E16 punctate immunofluorescence was much reduced, although immunoreactivity for laminin remained bright in the maturing pial and arachnoid membranes and on blood vessels in the brain. Immunoreactivity for collagen type IV was strong in the external limiting membrane and on blood vessels, but never showed concentrated punctate immunofluorescence in the marginal zone. These results indicate that laminin may be available on cell surfaces and in extracellular spaces as an adhesive ligand for growth cones during the formation of the ventral longitudinal pathway.

Organization of glutamate-like immunoreactivity in the rat superficial dorsal horn: light and electron microscopic observations

Glutamate has been shown to be a neurotransmitter in the central nervous system of vertebrates, and it has been hypothesized that glutamate is functional as a neurotransmitter in the spinal cord dorsal horn. A monoclonal antibody to fixative-modified glutamate was used in this study to examine the light microscopic and ultrastructural profiles of glutamate-like immunoreactivity in the superficial dorsal horn of the rat spinal cord. Glutamate-like immunoreactivity was observed in neurons, fibers, and terminals of both laminae I and II. Marginal zone immunoreactive neurons ranged from 10 to 30 micron in diameter and received many nonimmunoreactive somatic synapses. In substantia gelatinosa, immunoreactive neurons were observed in both inner and outer layers, ranged 5 to 10 micron in diameter, and received few nonimmunoreactive somatic synapses. Glutamate-like immunoreactive dendrites were observed in both laminae and were contacted primarily by nonimmunoreactive synaptic terminals that generally contained small clear vesicles. Both myelinated and unmyelinated immunoreactive axons were observed in Lissauer's tract. Immunoreactive terminals contained small (40 nm) clear vesicles and generally formed simple synaptic contacts with nonimmunoreactive dendrites in laminae I and II. The results of this study corroborate the importance of glutamate as a neurotransmitter in spinal sensory mechanisms.