Failure to demonstrate retrograde labelling of cerebellar Purkinje cells after injection of [3H]GABA in Deiters' nucleus

Unorthodox view of the functioning of a GABAA synapse

1. Studies about the permeation of labelled chloride and GABA across single plasma membranes microdissected from vestibular Deiters' neurons have yielded two unexpected results: (a) intracellular GABA stimulates chloride permeation in an asymmetric fashion (efflux being favoured); (b) under certain conditions GABA permeates by a diffusion mechanism in the out-->in direction across these plasma membranes. 2. These two main results have been obtained over many years together with a host of other indications about the fine mechanism of these events. Thus, a picture has emerged of their physiological meaning within the context of the functioning of the GABAA synapses between the Purkinje cells and the Deiters' neurons. 3. In short, it is proposed that at these synapses GABA accumulates into the postsynaptic neuron after its release and activation of the postsynaptic receptors. GABA accumulated in the Deiters' neurons is involved in the process of chloride extrusion to build an inward directed electrochemical gradient for chloride.

GABA and chloride permeate via the same channels across single plasma membranes microdissected from rabbit Deiters' vestibular neurones

The permeation of labelled gamma-aminobutyric acid (GABA) across single microdissected Deiters' membranes has been studied in a microchamber system. The GABA permeation is via pores which are blocked by 4,4'-diisothiocyanato stilbene-2-2'disulphonic acid (DIDS). As this substance blocks as well chloride permeation across these membranes we tested whether GABA and chloride permeate across the same pores. Membrane pre-treatment with different doses of corticotropin releasing factor (CRF), a membrane permeant cyclic AMP analogue and phalloidin parallelly block the permeation of the two substances. Thus, it is most probable that GABA and chloride pass across the same pores. These pores may be swelling activated ones, opened by the mechanical stress on the membranes in the microchamber system. The passage of GABA across these pores may be of physiological importance in the termination of GABA inhibitory action on the vestibular Deiters' neurones.

GABA uptake by rabbit restiform body homogenates

Restiform body (inferior cerebellar peduncle) preparations were obtained from rabbit brain stem slices and homogenized. When challenged with labelled GABA, these homogenates took it up briskly. We have characterized pharmacologically this uptake which resulted almost equally neuronal and glial. The neuronal component of the GABA uptake might be due to the adjacent cochlear nuclei coming along in the preparations, whereas the glial component probably belongs to the restiform body proper. Another possibility is that actually both components are due to the myelinated fibers and glia which make up the restiform body.

Chloride permeation across the Deiters' neuron plasma membrane: activation by GABA on the membrane cytoplasmic side

Single plasma membranes were microdissected from Deiters' neurons freshly obtained from the lateral vestibular nucleus of the rabbit and their chloride permeability was studied in a microchamber system. The basal in-->out 36Cl- permeation initially found was brought to zero by Zn2+, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid and iodide. GABA on the membrane cytoplasmic side resulted in a measurable in-->out 36Cl- passage, which was blocked by the GABA(A) antagonists bicuculline and picrotoxin. This effect peaked at 1 microM GABA on the inner side of the membrane. At higher GABA concentrations, a strong desensitization of the effect was found. Stimulation of Cl- permeability by GABA on the extracellular side of the membrane peaked at much higher GABA concentrations, 10-100 microM. This excludes an effect due to passage of the neurotransmitter from the inner to the outer compartment in our microchamber device. Moreover, this possibility is also dismissed by the fact that 1 microM GABA on the membrane outside did not evoke any 36Cl- in-->out permeation. In addition, pentobarbitone by itself could also stimulate 36Cl- in-->out permeation when added on the cytoplasmic side of Deiters' membrane. On these bases and in agreement with our previous reports, we propose that structures behaving pharmacologically as GABA(A) receptors respond to low levels of GABA on the cytoplasmic side of these neurons' membranes. We suggest that these structures are devices that, at the expense of ATP consumed in their phosphorylation, extrude Cl- after postsynaptic GABA uptake into the Deiters' neuron.

Stimulation of chloride in-->out permeation across the Deiters' neuron membrane by pentobarbital on the cytoplasmic side: additional evidence of GABA(A) receptors acting as chloride extrusion pumps

Pentobarbital stimulates 36Cl- permeation across single Deiters' membranes in a microchamber system, acting on classical, extracellularly facing, GABA(A) receptors. However, when applied on the membrane cytoplasmic side it activates per se labeled chloride in-->out permeation. No effect was found on chloride out-->in permeation. Similarly, at lower concentrations it facilitates the increase of 36Cl- in-->out permeation by application of GABA on the membrane inside, again via asymmetric chloride channels allowing in-->out but not out-->in passage. These data confirm that on the Deiters' membrane cytoplasmic side there are structures behaving pharmacologically as GABA(A) receptors whose function is that of a Cl- extrusion pump. This mechanism involves a cycle of activation-phosphorylation/desensitization-reactivation of the receptor complexes.

Regional distribution of GABA transporter 1 (GAT1) mRNA in the rat brain: comparison with glutamic acid decarboxylase67 (GAD67) mRNA localization

The mRNA expression of GABA transporter 1 (GAT1) was investigated in the rat brain by in situ hybridization histochemistry using an oligonucleotide probe specific to the mRNA. The pattern of GABA transporter 1 mRNA expression was well coincident with that of GABAergic terminals detected by using glutamic acid decarboxylase (GAD) as a marker. In addition, we compared the GAT1 mRNA expression with that of glutamic acid decarboxylase67 (GAD67) mRNA positive neurons. Although the expression patterns of both mRNAs were similar in most nuclei, the number of GABA transporter 1 mRNA positive cells were higher than that of GAD67 mRNA positive cells in some nuclei, such as the inferior colliculus, cerebellar nuclei and cerebral cortex. Furthermore the subthalamic nucleus and inferior olive nucleus, which lacked GAD67 mRNA expression, showed intense GABA transporter 1 mRNA expression. These data suggest that GABA transporter 1 mRNA is expressed not only in GABAergic neurons but also in non-GABAergic neurons and/or glial cells which are involved in GABAergic neurotransmission.

Anatomical evidence for glutamate and/or aspartate as neurotransmitters in the geniculo-, claustro-, and cortico-cortical pathways to the cat striate cortex

Data obtained by using various experimental approaches suggest that in the mammalian brain, most neurons within the visual system projecting to the striate cortex employ excitatory amino acids as transmitters. In order to investigate further the neurotransmitter phenotype of the ipsilateral afferents to area 17 of the cat, we have injected D-[3H]-aspartate, a retrograde tracer which selectively reveals putative glutamatergic and/or aspartatergic pathways, into this area. Retrogradely labelled neurons were observed in the dorsal lateral geniculate nucleus, visual claustrum, cortical areas 18, 19, 21a, and in both posteromedial and posterolateral parts of the suprasylvian areas but not in other known thalamic afferents such as the lateral posterior-pulvinar complex and the intralaminar nuclei. The distribution and localization of the labelled cells in all these regions were similar to that observed by using the non-selective tracer horseradish peroxidase conjugated to wheat germ agglutinin, though the number of cells was higher with the latter. Our findings provide additional evidence for the presence of excitatory amino acids as neurotransmitters in the major afferents to the cat striate cortex.

Size-related properties of vestibular afferent fibers in the frog: uptake of and immunoreactivity for glycine and aspartate/glutamate

Vestibular afferent fibers and their somata in the ganglion of Scarpa colocalize glutamate and glycine in a size-related manner. In this study tritiated aspartate, glycine or GABA was injected in the vestibular nuclear complex of frogs to investigate the uptake by afferent fibers and the retrograde transport of these amino acids to the cell bodies in the ganglion by autoradiographical methods. Ganglion cells were labeled by [3H]aspartate or [3H]glycine but not by [3H]GABA. The intensity of labeling with [3H]glycine increased and the intensity of labeling with [3H]aspartate decreased with cell size. On consecutive semithin sections the immunoreactivity of the same neurons was investigated with antibodies against glutamate or glycine. The results of this combined study showed that smaller, strongly glutamate immunopositive ganglion cells exhibited only weak or no labeling with [3H]glycine whereas larger, less strongly glutamate immunopositive ganglion cells were more intensely labeled with [3H]glycine. A similar size-related labeling pattern was observed in ganglion cells for [3H]aspartate and glycine-immunoreactivity. Both glycine uptake and glutamate immunoreactivity, as well as aspartate uptake and glycine-immunoreactivity, tended to be inversely correlated with the size of a given ganglion cell. These results provide evidence for a specific, size-related uptake of aspartate and glycine and are compatible with our hypothesis that the two amino acids are coreleased by thick but not by thin vestibular afferents. In an accompanying paper [Straka H. et al. (1995) Neuroscience 70, 697-707], we provide evidence for a size-related, monosynaptic activation of different glutamate receptors by vestibular afferent fibers.

Evidence for excitatory amino acid neurotransmitters in forward and feedback corticocortical pathways within rat visual cortex

It is a commonly accepted notion that cells which make projections between the multiple cortical areas found in the mammalian visual system are excitatory, but there is little direct evidence that this is the case. Here we demonstrate using retrograde tracing with D-[3H]aspartate that connections in the rat which project from lower to higher visual areas (i.e. forward) and those which project from higher to lower areas (i.e. feedback) may use excitatory amino acid neurotransmitters. Following injection into the primary visual cortex, clusters of retrogradely labelled cells were found in several extrastriate areas within the cytoarchitectonic subdivisions 18a ('areas' LM, AL, PX, FLX, RL, AX) and 18b ('area' MX), and in the retrosplenial cortex. In all of these areas D-[3H]aspartate-labelled cells were surrounded by diffuse label which may represent anterograde labelling of axon terminals. This suggests that both legs of reciprocal intracortical circuits have similar chemospecificity. To directly demonstrate excitatory amino acid localization in forward projections, D-[3H]aspartate was injected into extrastriate area LM. As expected, the results revealed retrogradely labelled neurons within area 17. Outside area 17, LM injections labelled neurons in AL, PX, FLX, RL, AX and MX. Taken in the context of the hierarchy of areas in rat cerebral cortex (Coogan and Burkhalter, J. Neurosci., 13, 3749-3772, 1993), these results show that D-[3H]aspartate labels: (1) forward connections from area 17 to LM, AL, PX, RL, AX and MX, (2) feedback connections from LM, AL, FLX, PX, RL, AX and MX to area 17, (3) feedback connections from AL, PX, RL, AX and MX to LM, and (4) lateral connections between FLX and LM. These findings strongly indicate that both forward and feedback connections as well as lateral connections at several different levels of the cortical hierarchy use excitatory amino acid neurotransmitters.

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Excitatory amino acid transmitters and their receptors in neural circuits of the cerebral neocortex

In 1954, L-glutamate (Glu) and L-aspartate (Asp) were first suggested as being excitatory synaptic transmitters in the cerebral cortex. Since then, evidence has mounted steadily in favor of the view that Glu and Asp are major excitatory transmitters in the neocortex. Many of the experimental studies which reported how Glu/Asp came to satisfy the criteria for transmitters in the neocortex are reviewed here, according to the methods employed. Since the question of which particular synaptic sites in cortical neural circuits Glu/Asp operate as excitatory transmitters has not previously been reviewed, particular attention is given to efferent, afferent and intrinsic neural circuits of the visual and somatosensory cortices, where circuitry is relatively clearly delineated. Recent studies using chemical assays of released amino acids, high-affinity uptake mechanisms of Glu/Asp from nerve terminals, the direct micro-iontophoretic administration of Glu/Asp antagonists, and immunocytochemical techniques have demonstrated that almost all corticofugal efferent projections employ Glu/Asp as excitatory synaptic transmitters. Evidence indicating that thalamocortical afferent projections, including geniculocortical projections and some intrinsic connections are glutamatergic, is also reviewed. Thus, the results highlighted here indicate that the main framework of neocortical circuitry is operated by Glu/Asp. Pharmacological studies indicate that synaptic receptors for Glu/Asp can be classified into a few subtypes, including N-methyl-D-aspartate (NMDA) and quisqualate/kainate (non-NMDA) types. Some evidence indicating the sites of operation of NMDA and non-NMDA receptors in neocortical circuitry is reviewed, and the distinct, functional significance of these two types of Glu/Asp receptors in information processing in the neocortex is proposed.

Afferents to the median raphe nucleus of the rat: retrograde cholera toxin and wheat germ conjugated horseradish peroxidase tracing, and selective D-[3H]aspartate labelling of possible excitatory amino acid inputs

Afferents to the median-paramedian raphe nuclear complex, which contains the B8 serotonergic cell group, were investigated in the rat with neuroanatomical and transmitter-selective retrograde labelling techniques. Injection of sensitive retrograde tracers, cholera toxin genoid or wheat germ agglutinin conjugated horseradish peroxidase into the median raphe resulted in labelling of neurons in a large number of brain regions. Projections from 26 of these regions are supported by available orthograde tracing data; the cingulate cortex, bed nucleus of stria terminalis, medial septum and diagonal band of Broca, ventral pallidum, medial and lateral preoptic areas, lateral hypothalamus, dorsomedial nucleus of hypothalamus, lateral habenula, interpeduncular nucleus, substantia nigra, central (periaqueductal) gray, and laterodorsal tegmental nucleus seem to represent major sources of afferents to the median-paramedian raphe complex. Retrogradely labelled cells were also observed in a number of regions for which anterograde tracing data are not available, including the perifornical hypothalamic nucleus, ventral premammillary nucleus, supramammillary and submammillothalamic nuclei and the B9 area. Possible excitatory amino acid afferents were identified with retrograde D-[3H]aspartate labelling. Microinjection of D-[3H]aspartate at a low concentration, 10(-4) M in 50 nl, resulted in retrograde labelling of a limited number of median raphe afferents. The most prominent labelling was observed in the lateral habenula and the interpeduncular nucleus, but retrogradely labelled cells were also noted in the medial and lateral preoptic areas, lateral and dorsal hypothalamus, ventral tegmental area, laterodorsal tegmental nucleus, medial parabrachial nucleus, and the pontine tegmentum. After injections of 10(-3) M D-[3H]aspartate selective labelling also appeared in more distant afferent regions, including cells in cingulate cortex, and in some regions located at shorter distances, such as the supramammillary nucleus. Injections of D-[3H]aspartate at high concentration, 10(-2) M, resulted in the appearance of weakly to moderately labelled cells in most afferent areas which were devoid of labelled cells after injections of lower concentrations, suggesting that this labelling may be non-specific. It was concluded that the median-paramedian raphe receives afferents from a large number of forebrain and hypothalamic regions, while relatively few brain stem regions project to this nuclear complex. The selectivity of retrograde labelling with D-[3H]aspartate was found to be concentration dependent, and it is suggested that the connections showing high affinity for D-[3H]aspartate may use excitatory amino acids as transmitters. Excitatory amino acid inputs from lateral habenula and interpeduncular nucleus may play predominant roles in the control of ascending serotonergic and non-serotonergic projections originating in the median and paramedian raphe nuclei.

Tracing specific transmitter pathways in the rat CNS: combination of [3H]serotonin retrograde labelling with immunocytochemical detection of endogenous transmitters

Selective retrograde labelling with [3H]serotonin ([3H]5-HT) can be used to identify serotonergic cell bodies after specific [3H]5-HT uptake by the corresponding nerve terminals. In the present study, we demonstrate that autoradiography of this [3H]5-HT radiolabelling can be combined with immunocytochemical detection of endogenous serotonin, GABA or substance P on the same tissue section. The midbrain raphe serotonergic projections to the olfactory bulb and the spinal projections of medullary serotonergic nuclei were investigated. The specificity of retrograde labelling with [3H]5-HT was confirmed by immunoreactivity of the radiolabelled cells for serotonin, using an antiserum specific for formaldehyde-fixed serotonin. After spinal injections of [3H]5-HT, many retrogradely labelled cells in the medullary raphe were immunopositive for substance P, and a few for GABA. These results are in agreement with the available information on the co-existence of putative transmitters in the spinal projections of caudal raphe neurons. Therefore, autoradiography of [3H]5-HT retrograde labelling combined with immunocytochemistry offers a possibility to test the specificity of transmitter-selective retrograde labelling, to identify transmitter-defined neuronal interactions and to investigate the projection fields of multitransmitter containing neurons.

Distribution and ultrastructure of neurons in opossum piriform cortex displaying immunoreactivity to GABA and GAD and high-affinity tritiated GABA uptake

GABAergic neurons have been identified in the piriform cortex of the opossum at light and electron microscopic levels by immunocytochemical localization of GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase and by autoradiographic visualization of high-affinity 3H-GABA uptake. Four major neuron populations have been distinguished on the basis of soma size, shape, and segregation at specific depths and locations: large horizontal cells in layer Ia of the anterior piriform cortex, small globular cells with thin dendrites concentrated in layers Ib and II of the posterior piriform cortex, and multipolar and fusiform cells concentrated in the deep part of layer III in anterior and posterior parts of the piriform cortex and the subjacent endopiriform nucleus. All four populations were well visualized with both antisera, but the large layer Ia horizontal cells displayed only very light 3H-GABA uptake, thus suggesting a lack of local axon collaterals or lack of high-affinity GABA uptake sites. The large, ultrastructurally distinctive somata of layer Ia horizontal cells receive a very small number of symmetrical synapses; the thin, axonlike dendrites of small globular cells are exclusively postsynaptic and receive large numbers of both symmetrical and asymmetrical synapses, in contrast to somata which receive a small number of both types; and the deep multipolar and fusiform cells receive a highly variable number of symmetrical and asymmetrical synapses on somata and proximal dendrites. Labeled puncta of axon terminal dimensions were found in large numbers in the neuropil surrounding pyramidal cell somata in layer II and in the endopiriform nucleus. Moderately large numbers of labeled puncta were found in layer I at the depth of pyramidal cell apical dendrites with greater numbers in layer Ia at the depth of distal apical segments than in layer Ib. High-affinity GABA uptake was demonstrated in the termination zone of the projection from the anterior olfactory nucleus to the anterior piriform cortex. Cell bodies of origin of this projection displayed heavy retrograde labeling with 3H-GABA. Matching neuropil and cellular labeling was demonstrated with the GABA-BSA antiserum but not with the GAD antiserum, thus suggesting that GABA is normally present in these cells but is taken up from the neuropil rather than synthesized. No comparable high-affinity GABA uptake was demonstrated in the association fiber systems that originate in the piriform cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Possible excitatory amino acid afferents to nucleus raphe dorsalis of the rat investigated with retrograde wheat germ agglutinin and D-[3H]aspartate tracing

Evidence for excitatory amino acid afferents to nucleus raphe dorsalis (NRD) has been found with retrograde tracing techniques. For neuroanatomical definition of afferent sources to NRD, rats received stereotaxic injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or implantations of crystal WGA-HRP in glass micropipettes. Retrogradely transported WGA-HRP was visualized with the tetramethyl-benzidine method, and afferents to NRD were identified from 20 different brain regions. Large numbers of labeled cells appeared in the lateral hypothalamus, lateral habenular nucleus, ventral tegmental area, periaqueductal gray, parabrachial nuclei and nucleus raphe magnus. Important inputs were also noted from dorsomedial hypothalamus and the area surrounding the perihypoglossal nucleus. Smaller numbers of WGA-HRP labeled cells appeared in bed nucleus of stria terminalis, diagonal band of Broca, cuneiform nucleus, superior vestibular nucleus, pontine periventricular gray, and some hypothalamic and reticular areas. Another group of rats received microinjections of D-[3H]aspartate (D[3H]Asp) and autoradiography consistently revealed retrograde labeling of cell bodies in 4 of the regions indicated by the WGA-HRP experiments as afferents to NRD. The most prominent aggregation of D-[3H]Asp-labeled cells was found in the lateral habenular nucleus, indicating that this input operates with an excitatory amino acid as transmitter. Significant numbers of D-[3H]Asp-labeled cells were also found in substantia nigra, periaqueductal and pontine periventricular gray. After large D-[3H]Asp injections involving NRD as well as surrounding areas, labeled cells were observed in several additional areas. Some of these areas were considered as afferents to surrounding periaqueductal gray or dorsal tegmental nuclei, while others may represent NRD afferents with relatively lower affinity for D-[3H]Asp. Several afferents to NRD failed to label with D-[3H]Asp, including diagonal band of Broca, hypothalamic areas, ventral tegmental area, parabrachial nuclei, locus coeruleus and reticular areas.

Selectivity of the uptake of glutamate and GABA in two morphologically distinct insect neuromuscular synapses

The common inhibitor (CI) and slow excitor tibiae (SETi) innervated slow muscles 135cd of the locust Schistocerca gregaria were incubated under high-affinity uptake conditions either in [3H]GABA or in [3H]glutamate. [3H]GABA is accumulated in the glia of the nerve endings of the CI as well as the SETi; however, it is accumulated only in the terminal axons of the CI, not in the terminal axons of the SETi. The grain densities above the glia and above the CI terminal axons are approximately 2 grains/micron2. After incubation in [3H]glutamate the grain densities above the CI terminal axons and the SETi terminal axons are approximately 4 grains/micron2; the grain densities above the glia of both types of nerve endings are approximately 17 grains/micron2. The relatively high labeling (3 grains/micron2) of the muscles after incubation in the presence of glutamate is ascribed to the high metabolic requirements of slow muscles. The conclusion is drawn that a high-affinity uptake system for GABA is present in the CI terminal axons and in the glia of both the CI and SETi nerve endings. However, while the glutamate uptake in the CI and SETi nerve endings of the slow 135cd is comparable to the high-affinity uptake of glutamate in the fast excitor tibiae (FETi) nerve endings of the fast retractor unguis muscle, a high-affinity uptake of glutamate was only demonstrated in the glia of both types of nerve endings. A high-affinity uptake in the terminal axons of the CI and SETi may be masked by an extensively low-affinity uptake of glutamate by the muscles.

Organization of the rostral thalamus in the rat: evidence for connections to layer I of visual cortex

The present study demonstrates the organization of a thalamocortical projecting system which terminates within layer I of the visual cortex in the hooded rat. Horseradish peroxidase (HRP) injections restricted to layer I resulted in retrograde labeling of large and medium-sized multipolar and fusiform neurons that are located within the ventromedial (VM) nucleus and a dorsomedial subunit of the ventral anterolateral nucleus (VAL). Retrograde cellular labeling also occurs within the anteromedial nucleus (AM) following these injections. After restriction of HRP injections to layer I, peroxidase labeling was not found within neurons of the classically defined intralaminar system, i.e., central medial, paracentral, and central lateral nuclei, or within the rostral continuations of the intralaminar system. Since the VM, dorsomedial VAL, and AM nuclei are directly adjacent to portions of the internal medullary lamina, we refer to this amalgam of rostral thalamic nuclei that project to layer I as the "paralaminar" system. We also provide cytoarchitectonic criteria that can be used to distinguish three separate subdivisions within the VAL complex, including that portion of the VAL which is part of the "paralaminar" system. In contrast, when control injections of WGA-HRP are placed within either the cellular supragranular or infragranular layers of the visual cortex, no appreciable number of neurons are labeled within the VM, VAL, or AM.

Effects of area 17 ablation on neurotransmitter parameters in efferents to area 18, the lateral geniculate body, pulvinar and superior colliculus in the cat

The result of unilateral ablation of visual cortical area 17 in adult cats was consistent with glutamate-aspartate being the neurotransmitter in efferents to the lateral geniculate body, the pulvinar and the visual part of superior colliculus but not in efferents to area 18 and the non-visual strata of superior colliculus. Furthermore, the distribution of glutamatergic, GABAergic and cholinergic markers within the various subdivisions of the cat visual system complied well with observations made previously with biochemical, neurophysiological, histochemical and immunohistochemical methods, in this and other mammalian species.

Selective retrograde labelling of the rat olivocerebellar climbing fiber system with D-[3H]aspartate

Selective retrograde labelling of the olivocerebellar climbing fiber system with D-[3H]aspartate has been observed in the rat, and the results have implications for the identification of a transmitter candidate as well as the neuroanatomical understanding of these cerebellar afferents. Microinjections of D-[3H]aspartate (50 nl, ca 10-2 M) were made into various parts of cerebellar cortex. Survival times were 6, 12 or 24 h. Pronounced diffusion of the tracer resulted in large injection sites. Within the zone of injection, glial elements were labelled over background. Most granule cells exposed to the tracer were unlabelled; the small numbers demonstrating labelling were believed to have been injured by the micropipette penetration. Beneath injection sites, large numbers of well-labelled nerve fibers appeared in the white matter and could be followed through the brainstem to the contralateral inferior olive, where labelled perikarya were found. After the inferior olivary neurons had been effectively destroyed with 3-acetylpyridine, evidence of cerebellar afferent labelling with D-[3H]aspartate was missing. Retrograde labelling of the olivocerebellar system was also observed after superfusion of the vermis with D-[3H]aspartate at concentrations in the range of Km for high affinity uptake (10(-5) or 10(-4) M, for 2 h). Mossy fiber or monoaminergic afferents to the cerebellum were never labelled with D-[3H]aspartate. The distribution of labelled cells in the olivary subnuclei after injections in different cerebellar areas was in line with the olivocerebellar organization previously described in the cat. Moreover, it was demonstrated that fibers from the different subnuclei follow different routes through the brainstem towards the cerebellum. Labelling of climbing fiber collaterals in uninjected parts of cerebellum indicated that some of the retrogradely migrating D-[3H]aspartate was directed in anterograde direction at axonal branching points. Collaterals were demonstrated in all deep cerebellar and Deiters' nuclei, and the results of intranuclear injections suggested that virtually every olivary neuron sends collaterals to these nuclei. Intracortical collaterals were organized in sagittal zones. Midline injections into the anterior lobe and VI lobule labelled collaterals in several zones of the posterior lobe spinal area and uninjected parts of the anterior lobe vermis. Hemispheral injection into copula pyramidis labelled collaterals in two prominent bundles in the anterior lobe.(ABSTRACT TRUNCATED AT 400 WORDS)

Retrograde transport of D-[3H]aspartate in thalamocortical neurones

D-[3H]aspartate (D-Asp) injected into the sensorimotor cortex of the rat resulted in retrograde labelling of thalamic neurones, mainly in the intralaminar and ventromedial nuclei, and the posterior complex. Few cells in the ventrolateral nucleus and ventrobasal complex were labelled. The results show that certain thalamocortical neurones can take up and axonally transport the false transmitter D-Asp, indicating that they might use glutamate or aspartate as transmitter(s).