Different modes of expression of AMPA and NMDA receptors in hippocampal synapses.

Postembedding immunogold labeling was used to determine the relationship between AMPA and NMDA receptor density and size of Schaffer collateral-commissural (SCC) synapses of the adult rat. All SCC synapses expressed NMDA receptors. AMPA and NMDA receptors were colocalized in at least 75% of SCC synapses; the ratio of AMPA to NMDA receptors was a linear function of postsynaptic density (PSD) diameter, with AMPA receptor number dropping to zero at a PSD diameter of approximately 180 nm. These findings indicate that 'silent' SCC synapses are smaller than the majority of SCC synapses at which AMPA and NMDA receptors are colocalized. Thus synapse size may determine important properties of SCC synapses.

A multisomatic axon in the central nervous system of the leech.

There is one particularly large axon in the medial bundle of the nerve cord of the leech. It extends along the entire length of the cord and is connected to a single cell body in each ganglion. The cell bodies in adjacent ganglia are tightly electrically coupled, and dye injected into one cell body can diffuse along the axon and into the cell body of the next ganglion. If the nerve cord is cut between two ganglia, neither end of the axon degenerates. Repair of the axon appears to occur by end-to-end fusion.

Gamma-aminobutyrate-like immunoreactivity in the thalamus of the cat.

Serial sections of the cat's thalamus were incubated with a purified antiserum raised against gamma-aminobutyric acid conjugated to bovine serum albumin by distilled glutaraldehyde. This serum has been extensively characterized and appears to react selectively with fixed gamma-aminobutyric acid in brain tissue treated with glutaraldehyde. Adjoining sections were stained with thionin and served as invaluable guides for a correct evaluation of the immunolabelling pattern. In the neuropil the intensity of the immunostaining varies considerably between thalamic nuclei and even between nuclear subdivisions. The neuropil staining appears particularly dense in the nuclei parataenialis, periventricularis, centralis medialis, reuniens, rhomboideus, habenularis lateralis, centrum medianum, parafascicularis, subparafascicularis, submedius, dorsal and ventral parts of the lateral geniculate body, the dorsal part of the medial geniculate body, the posterior complex, suprageniculate nucleus, pulvinar and parts of the lateral posterior nucleus. The pulvinar/lateralis posterior complex shows a particularly well-differentiated staining pattern which closely matches Updyke's [Updyke (1983) J. comp. Neurol. 219, 143-181] parcellation of this region. In several thalamic nuclei or subareas--and notably in those relay nuclei which are known to project upon non-primary sensory cortical areas--the immunostained neuropil is characterized by many puncta encircling an unstained profile. With few exceptions all thalamic nuclei displayed immunoreactive nerve cell bodies. Several examples were found of a mismatch between the number of such cells and the staining intensity of the neuropil. Thus the nuclei periventricularis, parafascicularis, subparafascicularis, parataenialis, limitans and centrum medianum although being very rich in neuropil staining have practically no immunostained perikarya. Rough estimates were made of the size and the proportion of gamma-aminobutyric acid labelled neurons in all major--and some minor--thalamic nuclei and their subdivisions. In some thalamic nuclei, notably the nuclei reticularis, anterior dorsalis, lateralis dorsalis, centralis lateralis, ventralis posterior and the dorsal lateral geniculate body, the population of immunoreactive neurons is distinctly heterogeneous with regard to soma size. The findings are discussed with regard to previous immunocytochemical studies of the distribution of gamma-aminobutyric acid and its synthesizing enzyme in the thalamus. Particular emphasis is put on the great species differences which appear to exist in this respect.

Terminals of subthalamonigral fibres are enriched with glutamate-like immunoreactivity: an electron microscopic, immunogold analysis in the cat.

Wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) histochemistry was combined with post-embedding immunogold cytochemistry in order to establish whether the subthalamic nucleus (STN) gives origin to glutamate (Glu)-enriched nerve terminals in substantia nigra, pars reticulata (SNr). Two adult cats served as normal controls and in two other animals crystalline WGA-HRP had been implanted bilaterally in STN. In all four animals ultrathin sections from SN were subjected to an immunogold procedure using antiserum raised against either Glu or gamma-aminobutyric acid (GABA). In some experiments the sections were subjected to consecutive incubations with both GABA and Glu antisera. These two antisera label two morphologically distinct types of boutons in SNr. The GABA antiserum labels boutons with pleomorphic vesicles, and they establish symmetrical synaptic contacts, mainly with dendritic shafts and spines, and occasionally with cell bodies. The Glu antiserum labels boutons with vesicles which are smaller and more uniform with regard to size and shape than those seen in the GABA-labelled boutons. The Glu-labelled boutons are engaged in asymmetrical synaptic contacts mainly with dendritic shafts and more rarely with cell bodies. The number of GABA-labelled boutons in SNr greatly exceeds the number of Glu-labelled ones. In the experimental material a considerable number of boutons in SNr are labelled with WGA-HRP reaction product. Several of these boutons are enriched in Glu-like immunoreactivity (Glu-LI), but not in GABA-LI. It is concluded that the subthalamonigral projection in the cat is likely to use Glu as a transmitter. The findings are briefly discussed with respect to the role played by STN in movement disorders and the involvement of excitatory amino acids in SN for the propagation of epileptic seizures and development of neurotoxicity.

The arrangement of glutamate receptors in excitatory synapses.

Electron microscopic immunogold analyses have revealed a highly differentiated arrangement of glutamate receptors at excitatory synapses in the central nervous system. Studies focused on the hippocampus and cerebellum have shown that the postsynaptic specialization is the preferential site of NMDA and AMPA receptor expression, and that the delta 2 receptor is similarly concentrated at this site. In cases of colocalization (AMPA and NMDA, or AMPA and delta 2) the two receptor types appear to be intermingled rather than segregated to separate parts of the membrane. The different groups of metabotropic receptor exhibit distinct distributions at the synapse: group I receptors occur in membrane domains lateral to the postsynaptic specialization; group II receptors are expressed in preterminal membranes or extra-synaptically; whereas group III receptors are found in, or close to, the presynaptic active zone consistent with their roles as autoreceptors. The differentiated distribution of glutamate receptors reflects their functional heterogeneity and explains why some receptors are activated only at high firing frequencies.

Studies on the cerebellar projections from the main and external cuneate nuclei in the cat by means of retrograde axonal transport of horseradish peroxidase.

The cerebellar projections from the main and external cuneate nuclei in the cat have been studied by means of retrograde axonal transport of horseradish peroxidase. The main projection from the external cuneate nucleus (ECN) is to the intermediate and, possibly, the small lateral part of lobule V and to the paramedian lobule on the ipsilateral side. The projection from the ECN to the cerebellar regions mentioned is topographically organized. Cells in the caudal part of the ECN send their axons to the caudal parts of lobule V and to the rostral part of the paramedian lobule. Cells in the rostral part of the ECN project to the rostralmost part of lobule V and to the folia in the caudal part of the paramedian lobule. The experimental study also shows that cells in the main cuneate nucleus (MCN) send their axons to the cerebellum. These axons, like those from the ECN, terminate in the intermediate part of lobule V of the anterior lobe and in the paramedian lobule. However, the axons of the cells in the MCN terminate only in the superficial parts of the folia, whereas those from the ECN terminate in the depth of the folia in these two cerebellar areas. The present study also gives evidence that cells in the ventral part of the gracile nucleus send their axons to lobules I and II of the anterior lobe vermis. The observations referred to here are to our knowledge the first anatomical findings demonstrating a projection from the main cuneate and gracile nuclei onto the cerebellar cortex. The observations confirm previous physiological studies.

Branched projections from the nucleus prepositus hypoglossi to the oculomotor nucleus and the cerebellum. A retrograde fluorescent double-labeling study in the cat.

The retrograde transport of fluorescent substances was used in order to investigate divergent axon collaterals of neurons in the nucleus prepositus hypoglossi (Ph). Fast blue (FB) was injected into the flocculus, paraflocculus and/or the vermis, while nuclear yellow (NY) was injected into the oculomotor nucleus alone or combined with injections in the nucleus of Darkschewitsch, the interstitial nucleus of Cajal and the medial longitudinal fascicle. Within optimal survival time, separate populations of single-labeled neurons of both dyes were found in Ph in all cases. Double-labeled neurons were seen in the rostral Ph following FB injections into the flocculus and the paraflocculus and NY injections restricted to the oculomotor nucleus. The present findings demonstrate that many neurons in the rostral Ph give collateral branches to the cerebellum and to the oculomotor nucleus.

Mesencephalic and diencephalic afferents to the superior colliculus and periaqueductal gray substance demonstrated by retrograde axonal transport of horseradish peroxidase in the cat.

The mesencephalic and diencephalic afferent connections to the superior colliculus and the central gray substance in the cat were examined by means of the retrograde transport of horseradish peroxidase (HRP). After deep collicular injections numerous labeled cells were consistently found in the parabigeminal nucleus, the mesencephalic reticular formation, substantia nigra pars reticulata, the nucleus of posterior commissure, the pretectal area, zona incerta, and the ventral nucleus of the lateral geniculate body. A smaller number of cells was found in the inferior colluculus, the nucleus of the lateral lemniscus, the central gray substance, nucleus reticularis thalami, the anterior hypothalamic area, and, in some cases, in the contralateral superior colliculus, Forel's field, and the ventromedial hypothalamic nucleus. Only the parabigeminal nucleus and the pretectal area showed labeled cells following injections in the superficial layers of the superior colliculus. In the cats submitted to injections in the central gray substance, labeled cells were consistently found in the contralateral superior colliculus, the mesencephalic reticular formation, substantia nigra parts reticulata, zona incerta and various hypothalamic areas, especially the ventromedial nucleus. In some cases, HRP-positive cells were seen in the nucleus of posterior commissure, the pretectal area, Forel's field, and nucleus reticularis thalami. A large injection in the mediodorsal part of the caudal mesencephalic reticular formation, which included the superior colliculus and the central gray substance, resulted in numerous labeled cells in nucleus reticularis thalami. The findings are discussed with respect to the suggested functional division of the superior colliculus into deep and superficial layers. Furthermore, the possible implications of labeled cells in zona incerta and the reticular thalamic nucleus are briefly discussed.

Thalamic commissural connections in the cat.

In order to identify possible connections between the two thalami injections of wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) were made unilaterally in the thalamus of adult cats. Except for the ventral lateral geniculate body, the nucleus reticularis thalami (R) is the only thalamic structure which shows labeled cells bilaterally following multiple unilateral thalamic injections of WGA-HRP. Furthermore, it appears that the nucleus ventralis medialis and adjoining ventralis lateralis is one main - and possibly sole - recipient area of a bilateral input from R.

Demonstration of a reciprocal connection between the periaqueductal gray matter and the reticular nucleus of the thalamus.

Lectin-conjugated horseradish peroxidase was either injected or implanted in crystalline form in various parts of the periaqueductal gray substance in the cat. After survival times ranging between 24 and 48 h the animals were fixed, and the mesencephalon and thalamus were sectioned and processed for peroxidase histochemistry, using tetramethylbenzidine as the chromogen. Light microscopic examination of the sections revealed that there exists a prominent reciprocal connection between the ventral and lateral parts of the periaqueductal gray matter on one hand, and the reticular nucleus of the thalamus on the other. The connections are mainly ipsilateral, and involve the entire rostrocaudal extent of the thalamic reticular nucleus, but mainly the ventrolateral sector of its caudal two thirds. There is a differential labelling within the thalamic reticular nucleus.