Ultrastructure and synaptic relations of neural elements containing glutamic acid decarboxylase (GAD) in the perigeniculate nucleus of the cat. A light and electron microscopic immunocytochemical study

GABAergic and non-GABAergic projections of accessory optic nuclei, including the visual tegmental relay zone, to the nucleus of the optic tract and dorsal terminal accessory optic nucleus in rat

This study examines the non-gamma-amino butyric acid (GABA)ergic (group I neurons) and GABAergic neurons (group II neurons) of the accessory optic system projecting to the nucleus of the optic tract (NOT)/dorsal terminal nucleus (DTN) of the accessory optic system in rat. These nuclei include the dorsal (MTNd) and ventral (MTNv) divisions of the medial terminal nucleus, the lateral terminal nucleus, the interstitial nucleus of the superior fasciculus, the posterior fibers, and the visual tegmental relay zone. GABAergic neurons of these nuclei that do not target the NOT/DTN (group III neurons) have also been observed. The fluorescent retrograde tracer fluoro-gold was injected into the pretectum, targeting the NOT/DTN and the tissue prepared immunocytochemically to reveal neurons containing the neurotransmitter GABA. Three groups of neurons (groups I, II, and III neurons) were examined in terms of their distribution, density, and percentage present. Group I neurons are single-labeled with fluoro-gold and represent non-GABAergic neurons projecting to the NOT/DTN. These neurons are of the highest density in the lateral terminal nucleus (204 neurons/mm2). Their densities are also substantial in the MTNv (120 neurons/mm2), interstitial nucleus of the superior fasciculus, posterior fibers (96 neurons/mm2), and visual tegmental relay zone (93 neurons/mm2). Group II neurons are double-labeled with fluoro-gold and GABA. They form a system of GABAergic neurons projecting to the NOT/DTN, which are exceedingly dense in the MTNd (78 neurons/mm2) but are also dense in both the visual tegmental relay zone (49 neurons/mm2) and MTNv (33 neurons/mm2). Group III neurons are GABAergic neurons that do not target the NOT/DTN but must project to other brain nuclei and/or be interneurons. These are of extremely high concentration in the visual tegmental relay zone (316 neurons/mm2) and are also of substantial densities in the MTNd (77 neurons/mm2), lateral terminal nucleus (72 neurons/mm2), and MTNv (44 neurons/mm2). The MTNd has the highest percentage of GABAergic neurons projecting to the NOT/DTN (72%). GABAergic neurons also form significant percentages of the projections to the NOT/DTN from the visual tegmental relay zone (34%) and MTNv (21%). The percentage of the total GABAergic neurons that project to the NOT/DTN is the highest in the MTNd (50%) and MTNv (42%). The described GABAergic afferents to the NOT/DTN may function to process information concerned with the compensation for retinal slip.

Noradrenergic innervation of the thalamic reticular nucleus: a light and electron microscopic immunohistochemical study in rats

Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.

Evidence of GABA-immunopositive neurons in the dorsal part of the lateral geniculate nucleus of reptiles: morphological correlates with interneurons

The distribution and staining pattern of gamma-aminobutyric acid immunoreactivity have been examined by both light and electron microscopy in the dorsal part of the lateral geniculate nucleus of three reptilian species: the turtle Chinemys reevesi, the lizard Ophisaurus apodus and the snake Vipera aspis. After perfusion of the animals with 1% paraformaldehyde and 1% glutaraldehyde and polyethyleneglycol embedding of the brains, the analysis of sections processed immunocytochemically with an anti-GABA antiserum has revealed a moderate-to-dense labeling of the neurons of the dorsal part of the lateral geniculate complex in these species. Labeled cell bodies are small-sized, either rounded or fusiform and the GABA-positive dendrites emerging from them are not preferentially oriented in any particular direction. Quantitative studies in Vipera indicate that GABA-positive neurons make up about 14% of the population of neurons of the dorsal part of the lateral geniculate nucleus. Electron microscopy of specimens treated by either pre- or post-embedding techniques has confirmed that these cells corresponded to neurons. No glial cells were ever observed to be immunopositive. These GABA-positive neurons, characterized by the presence of pleiomorphic synaptic vesicles localized either in their perikaryon or more often in presynaptic dendrites, established symmetrical synaptic contacts. In this case, the latter were involved both pre- and postsynaptically in serial and, more rarely, in triadic arrangements, a synaptic organization specific to interneurons. The involvement of such GABA-positive neurons in local circuits is discussed.

Preferential innervation of immunoreactive choline acetyltransferase synapses on relay cells of the cat's lateral geniculate nucleus: a double-labelling study

Relationships between cholinergic axons and GABA cells in the lateral geniculate and the perigeniculate nuclei were quantitatively assessed by combining pre-embedding immunocytochemical visualization of choline acetyltransferase with post-embedding immunogold labelling of GABA at the electron microscopic level. In the lateral geniculate nucleus, vesicle-containing profiles immunoreactive for choline acetyltransferase made synapses exclusively with dendritic profiles both within and outside synaptic glomeruli. Only 7.0% (seven of 100 dendrites) of the target dendrites tested were positive for GABA. Of these, the majority (85.7%; 6/7) were dendritic profiles containing synaptic vesicles. This is in contrast with the overall population of geniculate synapses where 21.3% of the targets were GABA-immunopositive, a three-fold significant difference. In the perigeniculate nucleus, immunoreactive choline acetyltransferase synapses targeted mainly dendritic profiles (93.9%; 31/33) and to a lesser extent somata (6.1%). In this nucleus, all unequivocally defined targets were GABA-immunopositive. Cholinergic inputs thus show a preference to target geniculate relay neurons without a strong innervation of GABA-immunoreactive interneurons. In the perigeniculate, however, cholinergic synapses provide a powerful input to GABAergic cells. This suggests that the facilitatory effects of acetylcholine on the excitability of relay cells would be mediated through (i) a direct innervation of the geniculate relay cells, and (ii) a cholinergic inhibitory innervation of the recurrent inhibition to the lateral geniculate nucleus via the perigeniculate nucleus.

A quantitative study of synaptic contacts on interneurons and relay cells of the cat lateral geniculate nucleus

The relative proportions of synapses made by retinal and extraretinal terminals on interneurons and relay cells in lamina A of the dorsal lateral geniculate nucleus (LGN) of the cat were estimated quantitatively in a sample of 4003 synapses. Processes of interneurons or relay cells were identified by presence or absence of GABA immunoreactivity, respectively, in thin sections treated with post-embedding anti-GABA immunogold. On the basis of ultrastructural features, synaptic terminals were interpreted as belonging to retinal axons, cortical axons or axon collaterals of relay cells. GABAergic terminals were positively identified by being immunoreactive. GABA(-) terminals with heterogeneous and poorly defined characteristics, which could not be identified in the above classes, were grouped together in an "undetermined" category. Among the total synaptic inputs to interneurons, the following relative percentages of synapses from different terminals were obtained: retinal 25%, cortical 37%, GABAergic 26%, axon collaterals 2%, undetermined 6%. The vast majority of retinal terminals synapse on dendritic appendages of interneurons rather than on their dendritic trunks (about 20:1). By contrast, the majority of cortical terminals synapse on dendrites rather than on dendritic appendages (about 5:1). Virtually all axon-collaterals synapses were established on dendritic appendages. 17% of the dendritic profiles of interneurons contain synaptic vesicles; many of these profiles were seen in postsynaptic relation to cortical axons and in presynaptic relation with relay dendrites. Given the extensive electrotonic lengths of these cells observed by others, and the expected high electric resistance of the slender stalks that are known to connect the dendritic appendages to interneurons, these results suggest that microcircuits involving the interneuronal dendritic appendages with dendrites of relay cells are under predominantly retinal control. The microcircuits established by presynaptic dendritic trunks with relay dendrites, are under predominantly cortical control. The axonal (spiking) output of interneurons would be under control of the few retinal synapses on proximal dendrites of these cells. Among the total synaptic inputs to relay cells, the following relative percentages of different synapses were obtained: retinal 12%, cortical 58%, GABAergic 24%, axon collaterals 0.3%, undetermined 5%. Relay cells receive twice the number of cortical synapses than interneurons, suggesting that direct cortical excitatory influences on relay cells are more preponderant than cortico-interneuron mediated inhibition on these cells. The observed proportions of dendritic profiles of relay cells and interneurons (80% and 20%, respectively) in the geniculate neuropil are similar to the known proportions of somata of both types of cells in the A-laminae.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium binding proteins as molecular markers for cat geniculate neurons

Immunocytochemistry revealed that in the cat dorsal lateral geniculate nucleus (dLGN) almost all parvalbumin-positive cells are GABAergic and about 56% of the calbindin D-28K (calbindin-immunoreactive neurons are also GABA-positive. On the other hand, in the same nucleus, almost all GABAergic neurons contain parvalbumin, and about 89% of the GABA-immunoreactive neurons contain calbindin. Double-labeling with calbindin and parvalbumin revealed that approximately 50% of the immunoreactive neurons are double-stained. In the PGN, virtually all neurons are GABA and parvalbumin-positive. Only a few scattered cells were also calbindin-immunoreactive. These results show that GABAergic geniculate cells can be differentiated on the basis of their calcium-binding protein immunoreactivity. Four types of immunoreactive cells are described here: (1) cells positive for GABA, parvalbumin and calbindin, (2) cells positive for GABA and parvalbumin, but negative for calbindin, (3) cells negative for GABA and parvalbumin, but positive for calbindin, (4) cells negative for GABA, parvalbumin and calbindin.

A role for GABAB receptors in excitation and inhibition of thalamocortical cells

Gamma-aminobutyric acid (GABA) in the thalamus has mainly been associated with the inhibitory modulation of the sensory and cortical flow of information via a 'classical', chloride-dependent, GABAA receptor-mediated action. However, the discovery of a late, long-lasting potassium-dependent inhibitory postsynaptic potential (IPSP) mediated by GABAB receptors present on thalamocortical cells, has allowed new insights into our understanding of the physiological role of this neurotransmitter. In particular, work on the dorsal lateral geniculate nucleus indicates that together with a relatively weak inhibition, GABAB receptor-mediated IPSPs 'prepare' thalamocortical cells for burst firing by activating low-threshold calcium potentials. Thus, GABA in the thalamus can no longer be viewed only as a 'classical' inhibitory transmitter but also as a neuromodulator with a 'priming' role for burst firing excitation. This dual role of GABAB receptors in inhibition and excitation of thalamocortical cells might allow different interpretations of earlier findings in animals and humans, both in healthy and pathological conditions. It will also help to identify new functions for postsynaptic GABAB receptors in other parts of the central nervous system.

Effect of monocular enucleation or impulse blockage on gamma-aminobutyric acid and cytochrome oxidase levels in neurons of the adult cat lateral geniculate nucleus

Much attention has been paid to the effect of various types of neonatally induced retinal insults on neurons of the cat lateral geniculate nucleus (LGN). Little is known about cellular adjustment to functional alterations commencing in the adult. The present study was aimed at examining the effect of monocular enucleation or retinal impulse blockade on mature neurons of the cat LGN and perigeniculate nucleus. In addition to labeling the relay neurons with cytochrome oxidase (CO) histochemistry and immunohistochemistry, and presumed interneurons with GABA immunohistochemistry, the two markers were also combined in the same section. The results showed that GABA-immunoreactive neurons in the LGN can be subdivided into two major groups: highly immunoreactive neurons (Hir) and moderately immunoreactive neurons (Mir). These two groups differed slightly in their size and CO levels. With monocular enucleation or TTX treatment, there was a reduction in the numerical density of Hir and a concomitant increase in Mir in the affected laminae. However, there was no evidence of a reduction in GABA immunoreactivity in neurons of the perigeniculate nucleus. With regard to relay cells our data were in agreement with our previous findings (Kageyama & Wong-Riley, 1985) that there was a statistically significant positive correlation between cell size and CO levels, so that neurons with large cross-sectional areas were predominately darkly reactive for CO (DCO), while medium-to-small neurons were mainly moderate to lightly reactive (M/LCO). After monocular enucleation or TTX injections, the numerical density and size of DCO were significantly reduced, while those of M/LCO were proportionally increased in the affected laminae, indicating a conversion of some DCO to M/LCO. The results indicate that the maintenance of CO levels in predominately the large relay cells and GABA levels in a subclass of small neurons of the mature cat LGN are activity-dependent and are highly sensitive to retinal insults.

A GABAergic projection from the pretectum to the dorsal lateral geniculate nucleus in the cat

To study the projection from the pretectum to the lateral geniculate nucleus, we placed wheat-germ agglutinin conjugated to horseradish peroxidase into the lateral geniculate nuclei of six cats, allowed this marker to be retrogradely transported by afferent axons to their parent somata in the pretectum, and revealed the label in these cells with stabilized tetramethylbenzidine histochemistry. In three cases we made large pressure injections that completely infiltrated the lateral geniculate nucleus and extended into neighboring thalamic nuclei; in the other three we made smaller iontophoretic injections largely confined to the A- and C-laminae of the lateral geniculate nucleus. In both types of injection we found labeled pretectal cells mainly in the nucleus of the optic tract but also found some cells labeled in the olivary pretectal nucleus and the posterior pretectal nucleus. After one of the larger injections we analysed both sides of the pretectum and found that 11% of the labeled cells were located contralaterally and were distributed in the same three nuclei. We analysed only the ipsilateral side in the remaining five cats. In those five experiments we also immunohistochemically stained the pretectal sections with an antibody directed against the neurotransmitter, GABA. Of the retrogradely labeled pretectal cells, 40% were also labeled for GABA, and those were similar in soma size (350 microns 2 in cross-sectional area) to those labeled only with the retrograde marker (331 microns 2). GABA-positive cells not labeled by retrograde transport were smaller (246 microns 2) than either of these other cells populations. These results indicate that at least 40% of the cells involved in the projection from the pretectum to the lateral geniculate nucleus are GABAergic. We suggest that this extrathalamic projection may serve to inhibit thalamic GABAergic cells. This, in turn, would disinhibit geniculate relay cells, thereby facilitating the geniculate relay of retinal information to cortex.

Light and electron microscopic evidence for a GABAergic projection from the caudal basal forebrain to the thalamic reticular nucleus in rats

Neurons in the magnocellular nucleus of the caudal basal forebrain extend an axonal projection which arborizes within the reticular nucleus of the thalamus. The present study addresses the ultrastructure and neurochemistry of this projection in rats. Many labeled terminals are apparent within the thalamic reticular nucleus following Phaseolus vulgaris leucoagglutinin injections into the caudal basal nucleus; anterogradely labeled axon terminals most commonly contact both somata and dendrites of reticular nucleus neurons with symmetric membrane specializations. Thus, the majority of the labeled terminals examined contrast with choline acetyltransferase positive terminals which have been previously identified as contacting dendrites and forming asymmetric synapses within this nucleus. Many of the neurons within the caudal basal nucleus which are retrogradely labeled following tracer injections into the thalamic reticular nucleus are gamma-aminobutyric acid (GABA) immunoreactive. In addition, following injections of Phaseolus vulgaris leucoagglutinin or fluoro-ruby into the caudal basal forebrain, some of the labeled axonal swellings and boutons within the thalamic reticular nucleus also contain glutamic acid decarboxylase. These results indicate that a significant component of the projection is GABAergic. These anatomical observations suggest that the projection from the caudal basal nucleus onto the thalamic reticular nucleus could facilitate the relay of information through the dorsal thalamus by inhibiting reticular nucleus neurons, and thus, in turn, disinhibiting thalamic relay neurons.

Prenatal development of retinogeniculate projections in the rabbit: an HRP study

The prenatal development of the rabbit's retinal projections to the dorsal lateral geniculate nucleus (dLGN) was studied by using anterograde axonal transport of HRP injected intraocularly. Further, the ontogenesis of the dLGN's alpha and beta sectors was studied. Fetuses aged embryonic day 18 (E18) to E29 were examined. Gestation in the rabbit is 30-31 days. On E18 the future dorsal lateral and medial geniculate nuclei appear as a continuous strip of cells along the lateral margin of the dorsal thalamus. On E21 labelled retinal fibers are invading the lateral margin of the dLGN contralateral, but not ipsilateral, to an injected eye. At this age the dorsal lateral and medial geniculate nuclei are separating. By E23 contralateral fibers occupy the entire presumptive alpha sector, while ipsilateral fibers are invading the caudal half of the sector, overlapping the contralateral fibers. At this age the alpha and beta sectors begin to differentiate. On E25 contralateral fibers are more densely distributed throughout the alpha sector and the ipsilateral fibers are concentrated dorsally within the caudal three-quarters of the sector. By E27 contralateral fibers begin to withdraw from a medial zone of the alpha sector, while ipsilateral fibers remain densest in this zone and begin to withdraw from more lateral and caudal aspects of the sector; contralateral fibers, but not ipsilateral fibers, invade the beta sector. At this age the alpha and beta sectors acquire an adult-like appearance. By E29 the contralateral fibers vacate the beta sector and the medial zone of the dLGN and the ipsilateral fibers are restricted to this zone. Thus, 1 or 2 days before birth, the locations of the ipsilateral and contralateral retinal projections to the dLGN resemble those seen in the adult. The early overlapping projections of ipsilateral and contralateral retinal fibers within the dLGN and their eventual segregation in the fetal rabbit are consistent with the development of these projections in other mammalian orders. Further, the brief invasion of the beta sector by the contralateral fibers resembles the transient occupation of the carnivores' perigeniculate nucleus by developing retinal fibers. In addition, direct comparisons of temporal and spatial events during retinal innervation of the dLGN and the superior colliculus indicate several developmental differences between the two nuclei.

N-methyl-D-aspartate receptors contribute to excitatory postsynaptic potentials of cat lateral geniculate neurons recorded in thalamic slices

Neurons of the cat's dorsal lateral geniculate nucleus were recorded intracellularly to study the contribution of N-methyl-D-aspartate (NMDA) receptors to excitatory postsynaptic potentials (EPSPs) and low-threshold calcium spikes. EPSPs were evoked by stimulation of retinogeniculate axons in the optic tract and/or corticogeniculate axons in the optic radiations; EPSPs from both sources were similar. These EPSPs had one or two components, and the second component had several characteristics of NMDA receptor-mediated events. For example, EPSP amplitude decreased when neurons were hyperpolarized and increased when stimulus frequency was increased; these EPSPs could also be blocked reversibly by application of the selective NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). We also studied the influence of NMDA receptors on low-threshold calcium spikes, which are large, voltage- and calcium-dependent depolarizations that are often accompanied by high-frequency action potential discharge. APV blocked synaptically activated low-threshold calcium spikes, but APV had no effect on low-threshold calcium spikes that were elicited by current injection. Therefore, APV does not appear to have a direct effect on the T-type calcium channel that is involved in generation of low-threshold calcium spikes. The voltage and frequency dependence of the NMDA receptor-mediated component of the EPSPs, as well as its ability to trigger low-threshold calcium spikes, provide for complex signal processing in the lateral geniculate nucleus.

Cholinergic synapses in the central nervous system: studies of the immunocytochemical localization of choline acetyltransferase

Cholinergic synapses can be identified in immunocytochemical preparations by the use of monoclonal antibodies and specific antisera to choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh) and a specific marker for cholinergic neurons. Electron microscopic studies demonstrate that the fibers and varicosities observed in light microscopic preparations of many brain regions are small-diameter unmyelinated axons and vesicle-containing boutons. The labeled boutons generally contain clear vesicles and one or more mitochondrial profiles. Many of these boutons form synaptic contacts, and the synapses are frequently of the symmetric type, displaying thin postsynaptic densities and relatively short contact zones. However, ChAT-labeled synapses with asymmetric junctions are also observed, and their frequency varies among different brain regions. Unlabeled dendritic shafts are the most common postsynaptic elements in virtually all regions examined although other neuronal elements, including dendritic spines and neuronal somata, also receive some cholinergic innervation. ChAT-labeled boutons form synaptic contacts with several different types of unlabeled neurons within the same brain region. Such findings are consistent with a generally diffuse pattern of cholinergic innervation in many parts of the central nervous system. Despite many similarities in the characteristics of ChAT-labeled synapses, there appears to be some heterogeneity in the cholinergic innervation within as well as among brain regions. Differences are observed in the sizes of ChAT-immunoreactive boutons, the types of synaptic contacts, and the predominant postsynaptic elements. Thus, the cholinergic system presents interesting challenges for future studies of the morphological organization and related function of cholinergic synapses.

GABAergic neural elements in the rat basilar pons: electron microscopic immunochemistry

Previous light microscopic immunoperoxidase studies of glutamic acid decarboxylase (GAD)-immunoreactive neural elements in the rat basilar pontine nuclei revealed immunocytochemical reaction product in neuronal somata and axon terminals. In the present study, pre-embedding immunoperoxidase labeling of GAD or gamma-aminobutyric acid (GABA) and postembedding immunogold labeling of GABA allowed the ultrastructural visualization of these neural elements in the basilar pontine nuclei of colchicine-treated animals. At the electron microscopic level, immunolabeled neuronal somata exhibited smoothly contoured nuclei, whereas some dendrites also contained reaction product after immunocytochemical treatment and were postsynaptic to both immunoreactive and nonimmunoreactive axon terminals. Synaptic boutons immunoreactive for GAD or GABA exhibited cross-sectional areas that ranged from 0.1 to 3.8 microns 2 and generally appeared round or elongated in most sections. The majority (95%) of immunolabeled boutons contained pleomorphic synaptic vesicles and formed symmetric synapses at their postsynaptic loci; however, boutons exhibiting round vesicles and boutons forming asymmetric synapses (5%) were also immunopositive. Small (less than 1.5 microns 2) GAD- or GABA-labeled axon terminals formed synaptic contact mainly with small dendritic profiles, dendritic spines, and neuronal somata, whereas large labeled boutons (greater than 1.5 microns 2) formed synapses with all sizes of dendritic profiles. Occasionally, a single immunolabeled bouton formed synaptic contact with two separate postsynaptic dendrites. It is suggested that the immunolabeled neuronal somata and dendrites observed in the rat basilar pontine nuclei represent a population of pontine local circuit neurons; however, it is known that GABAergic cell groups extrinsic to the pontine gray provide afferent projections to the basilar pons, and therefore at least some immunoreactive axon terminals present in the pontine nuclei are derived from these extrinsic sources. The ultrastructural observation of GABAergic neural elements in the rat basilar pontine nuclei confirms previous light microscopic findings and provides an anatomical substrate through which GABAergic neurons, whether arising from an intrinsic or extrinsic source, might exert an inhibitory influence on target cells within the pontine nuclei.

Quantitative immunogold analysis reveals high glutamate levels in synaptic terminals of retino-geniculate, cortico-geniculate, and geniculo-cortical axons in the cat

A postembedding immunogold procedure was used to estimate quantitatively, at the electron-microscopical level, the intensity of glutamate (GLU) immunoreactivity in different identifiable profiles of the lateral geniculate nucleus (LGN) and perigeniculate nucleus (PGN) of the cat. Synaptic terminals of retinal and cortical origins in the LGN, and of axon collaterals of geniculo-cortical relay cells in the PGN, were identified by previously determined ultrastructural features. Processes of interneurons or relay cells were identified by being immunoreactive or non-immunoreactive, respectively, in serial thin section reacted with a GABA antibody. The results showed that synaptic terminals of geniculo-cortical relay cells in the PGN have significantly higher levels of GLU immunoreactivity than their parent somata or dendrites in the LGN; this suggests transmitter storage of this amino acid in these terminals. By contrast, synaptic terminals of interneurons did not show enrichment of GLU relative to their parent somata. This argues against the possibility that the relative enrichment of GLU in relay cells terminals is due to factors other than presynaptic storage. In addition, axon collateral terminals of relay cells in the pGN, as well as retinal and cortical terminals in the LGN, showed significantly higher GLU immunoreactivity than GABAergic terminals. These immunocytochemical results suggest that GLU is a neurotransmitter in the retino-geniculate, cortico-geniculate, and geniculo-cortical pathways in the cat.

Physiological differentiation within the auditory part of the thalamic reticular nucleus of the cat

Spike trains of 153 single units were recorded in the caudoventral part of the thalamic reticular nucleus (RE) of 7 nitrous oxide anaesthetized cats. Functional properties defined by spontaneous activity pattern, studied by mean of auto renewal density histograms, were used to subdivide the units into 4 groups. Types I (18%), II (56%) and III (15%) were defined by an increasing bursting activity and Type IV (11%) by firing no bursts spontaneously. The responses to auditory stimuli confirmed that the caudoventral part of RE is tightly related to central auditory pathways. Responses to white noise bursts (200 ms duration) significantly let appear that Type I units responded in a high proportion (greater than 70%) until 80 ms after the stimulus onset, Type II units where mostly affected during the entire stimulus duration, and Type III units showed preferentially late responses. The units responsive to high frequencies (greater than 8 kHz) were mostly located in the dorsal and the units responsive to low frequencies (less than 2 kHz) in the anteroventral sector of auditory RE. However, only a loosely tonotopy is supported by this study. The neuronal circuitry within RE was shown to be stable when white noise bursts were delivered. Cross-correlograms indicated a large proportion of interconnected units (64%) and signs of mutual inhibition between neighboring RE units (11%). The hypothesis is discussed that the auditory RE exerts a fine control on the time-dependent analysis of the incoming auditory input to the cerebral cortex. The complex intranuclear connectivity suggests that the cell types correspond to distinct patterns of functional connections.

Double labeling of cytochrome oxidase and gamma-aminobutyric acid in central nervous system neurons of adult cats

The relationship between the levels of cytochrome oxidase and gamma-aminobutyric acid (GABA) was investigated within single neurons by double labeling the 2 markers in the same section. Double staining was equally effective when immunogold-silver staining of GABA was followed by indirect immunoperoxidase labeling of cytochrome oxidase, or when cytochrome oxidase histochemistry was followed by immunogold-silver staining of GABA. Neurons in the perigeniculate nucleus (PGN) and basket cell terminals in the cerebellum were GABA positive and rich in cytochrome oxidase. Interneurons of the lateral geniculate nucleus (LGN) as well as stellate and Golgi cells of the cerebellum were GABA-rich but poor in cytochrome oxidase. These results demonstrate that there is no consistent relationship between the levels of cytochrome oxidase and GABA in neurons.

Organization of cholinergic synapses in the cat's dorsal lateral geniculate and perigeniculate nuclei

In the preceding article, we showed that cholinergic fibers originating from the brainstem reticular formation provide a dense innervation of the lateral geniculate nucleus. In this report we describe the ultrastructure of these fibers and their relations with other elements in the neuropil of the lateral geniculate nucleus. Cholinergic fibers were labeled with an antibody to choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh). In the A-laminae of the lateral geniculate nucleus, ChAT + profiles are small and contain tightly packed, mostly round vesicles. Some end in encapsulated synaptic zones where they form asymmetrical synaptic contacts with processes of both projection cells and interneurons. Others form synapses upon the shafts of dendrites. Of the four classical types of vesicle-containing profiles identified by Guillery (Z. Zellforsch. Mikrosk. 96:1-38, '69; Vision Res. [Suppl.] 3:211-227, '71), ChAT + profiles most closely resemble RSD profiles (Round vesicles, Small profile, Dark mitochondria). However, as a population, ChAT + profiles can be distinguished from the unlabeled population of RSD profiles because they are larger in size, contain more mitochondria, and make synapses with smaller postsynaptic membrane specializations. Each of these differences is statistically significant and together they indicate that ChAT + profiles are a distinct morphological type of synaptic profile. ChAT + profiles in the perigeniculate nucleus resemble those found in the lateral geniculate nucleus; they also make synapses with obvious postsynaptic thickenings.

The binocular input to cells in the feline dorsal lateral geniculate nucleus (dLGN)

1. Cells in the A laminae of the dorsal lateral geniculate nucleus receive their primary innervation from either the contralateral (A) or ipsilateral (A1) eye. This paper provides evidence concerning the responses they give to visual stimulation of what is commonly regarded as the ineffective or non-dominant eye. It also examines the contribution of the corticofugal input to these responses. 2. Cells were identified and classified according to their responses to stimulation of the dominant eye receptive field. This was then occluded and the corresponding location in the non-dominant eye field stimulated by a moving bar. Out of fifty-seven cells examined forty-three (75%) gave a response to stimulation of the non-dominant eye. There was no obvious difference between the effects on X and Y cells in these experiments. 3. In most cases (thirty-seven) the response involved an inhibition of the resting discharge level, but three cells gave a mixed excitatory and inhibitory response and three a pure excitatory response. All the responses were weak and only revealed by prolonged periods of averaging (20-100 trials). 4. Ionophoretic application of the GABA antagonist N-methyl-bicuculline (NMB) blocked the visually elicited inhibitory effects and in most cases (twenty-seven out of thirty-two tested) revealed an excitatory response. Out of a further eight cells previously unresponsive to the non-dominant eye, NMB application revealed excitatory responses in three. 5. Increasing background discharge levels and cell excitability by ionophoretic application of either acetylcholine or the excitatory amino acid, quisqualate, did not eliminate inhibitory responses and did not reveal excitatory responses. We suggest that the visually driven non-dominant eye suppression of the background discharge involves a GABA-mediated inhibitory input which masks an underlying excitatory input. 6. An excitatory non-dominant eye response could potentially derive from the influence of the corticofugal projection. However, removal of the corticofugal input by aspiration of areas 17 and 18 did not reduce either the excitatory or the inhibitory components of the response. 7. In the absence of corticofugal input all cells tested (fourteen) exhibited a non-dominant eye response and all studied during NMB application (eleven) gave an excitatory response. The primary effect of removing the corticofugal input appeared to involve the loss of a 'damping' influence on the excitatory and inhibitory responses, such that they were more easily revealed. The significance of these findings is discussed.

Axonal arborizations of a magnocellular basal nucleus input and their relation to the neurons in the thalamic reticular nucleus of rats

A dense axonal plexus, arising in a portion of the magnocellular basal nucleus, was identified in the thalamic reticular nucleus in adult rats. The details of these axonal arbors as well as their relation to the neurons of the reticular nucleus were investigated by using Phaseolus vulgaris leucoagglutinin injections into the basal nucleus and intracellular injections of Lucifer yellow into reticular nucleus neurons. Axons arising in the caudal basal nucleus at the medial margin of the globus pallidus do not enter the dorsal thalamus but are confined to the reticular nucleus, where they arborize widely and densely. Neurons in the reticular nucleus are large, with sparsely spined and beaded dendrites, which radiate within the plane of the nucleus. Bouton-like swellings along basal nucleus axons are often found apposed to the somata of reticular nucleus neurons, although many are also apposed to dendrites. These morphological observations suggest a second potentially significant route, in addition to its well-known direct cortical projection, through which the magnocellular basal nucleus could influence cortical function: it may, by strategically modulating the excitability of reticular nucleus neurons, alter the general state of the thalamus and hence affect the initial transmission of information to the cortex.

Effects of bicuculline on signal detectability in lateral geniculate nucleus relay cells

The lateral geniculate nucleus conveys the center-surround organized retinal receptive fields to the cortex in a way that does not significantly alter their spatial structure. However, non-retinal influences may change the 'strength' (detectability) of the signal under conditions of anesthesia, arousal and attention. A previous analysis of receiver operating characteristic curves in cat suggests that a reduction in signal detectability occurs in lateral geniculate nucleus (LGN) relay cells in anesthetized animals in comparison to the retinal afferents. In the present study, it was found that antagonism of GABAA receptors with bicuculline (BIC) increased signal detectability in LGN relay cells in the tree shrew (Tupaia belangeri). This change is consistent with the hypothesis that feedforward and/or feedback GABAergic circuits in the LGN differentially affect the retinogeniculate transfer ratio for visually driven activity versus maintained (spontaneous) activity. Under conditions of arousal or attention, signal detectability may be increased by brainstem activation, thus increasing the flow of information in the visual system.

On the properties and origin of the GABAB inhibitory postsynaptic potential recorded in morphologically identified projection cells of the cat dorsal lateral geniculate nucleus

Intracellular recordings were performed from projection cells of the cat dorsal lateral geniculate nucleus in vitro to investigate the properties and origin of optic tract evoked inhibitory postsynaptic potentials mediated by GABAB receptors and their relationship to the physiologically different cell classes present in this nucleus. In all three main laminae of the dorsal lateral geniculate nucleus, stimulation of the optic tract evoked an excitatory postsynaptic potential followed by two inhibitory postsynaptic potentials. The first is a GABAA receptor mediated inhibitory postsynaptic potential since it was blocked by bicuculline, reversed in polarity following intracellular Cl- injection and had a reversal potential similar to the bicuculline sensitive hyperpolarizing effect of GABA. The second is a GABAB receptor mediated inhibitory postsynaptic potential. Its amplitude was not linearly related to membrane potential (maximal amplitude at -60 mV), it decreased when using frequencies of stimulation higher than 0.05 Hz and it was reversibly increased by addition of bicuculline to the perfusion medium. The reversal potential of GABAB inhibitory postsynaptic potentials was dependent on the extracellular K+ concentration but did not change in the presence of bicuculline or when recording with Cl- filled microelectrodes. While GABAA inhibitory postsynaptic potentials always abolished repetitive firing of projection cells, GABAB inhibitory postsynaptic potentials were able to block weak firing but unable to decrease strong activation of projection cells evoked by direct current injection. Optic tract evoked GABAB (as well as GABAA) inhibitory postsynaptic potentials could be recorded in slices which did not include the perigeniculate nucleus, thus indicating that they are generated by the interneurons of the dorsal lateral geniculate nucleus. Using intracellular injection of horseradish peroxidase, we have found that the GABAB inhibitory postsynaptic potentials are present in projection cells showing many different types of neuronal morphologies. In conclusion, GABA released from interneurons in the dorsal lateral geniculate nucleus is capable of evoking an early, short-lasting GABAA and a late, long-lasting GABAB inhibitory postsynaptic potential in projection cells with diverse morphology, indicating that the late inhibition in the dorsal lateral geniculate nucleus can no longer be associated exclusively with the recurrent inhibitory pathway through the perigeniculate nucleus.

Reticulo- and trigemino-hypoglossal connections: a quantitative comparison of ultrastructural substrates

Axon terminals were identified and characterized by electron microscopy after injections of horseradish peroxidase (HRP) into the spinal V nucleus (SPVN) or the medullary reticular formation adjacent to the XIIth nucleus. The synaptic organization and topology of these two different populations of hypoglossal afferents (T-XII and R-XII respectively) were determined by quantitative comparisons. Significant differences were obtained in the ratios of morphological types of terminals, sizes of axonal endings and their location on postsynaptic structures. Axon terminals containing spherical vesicles (S-terminals) and those with flattened/pleomorphic vesicles (F-terminals) were anterogradely labeled with HRP from both injection sites. However, the S/F ratio for R-XII terminals was 1.2:1 compared to 2.6:1 for T-XII afferents. Asymmetrical membrane densities (Gray Type I) were the predominant form of junctional specialization for S-terminal synapses. Asymmetrical densities with subjunctional dense bodies/bars (S-Taxi) were associated with a higher proportion of T-XII synapses than R-XII synapses. Almost all of the F-terminals from both sources had symmetrical densities (Gray Type II). The average diameter of R-XII terminals was greater than that of T-XII terminals. R-XII-F terminals were the largest terminals. The majority of axon terminals from both sources formed axodendritic synapses. However, R-XII terminals had a higher incidence (10% vs. 3%) of axosomatic contacts. The proportion of R-XII-F-terminals decreased from the central toward the distal dendrites, whereas the opposite was found for T-XII-F and T-XII-S-terminals. In contrast to these findings, R-XII-S-terminals were more uniformly distributed on dendrites of all sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic organization of the thalamic reticular nucleus

This study describes the synaptic organization of the thalamic reticular nucleus (TRN) in the rat, cat, and monkey using electron microscopy combined with immunocytochemistry, degeneration, or horseradish peroxidase (HRP) tracing methods. Three morphological types of terminals are described in the TRN of the rat: a small terminal with densely packed spherical vesicles (D-terminal), which originates from the cortex; a large terminal with loosely packed spherical vesicles (L-terminal), which originates in the dorsal thalamus; and a terminal containing flattened synaptic vesicles (F-terminal) that is probably a TRN recurrent collateral. The cortical input to the TRN has been shown by double-labeling studies to terminate directly upon TRN projection neurons. Similar classes of terminals are found in the TRN of cat and monkey, but there is in addition a large terminal with spherical synaptic vesicles that is invaginated by dendritic spines. Also present in the cat and monkey, but not in the rat, are vesicle-containing dendrites and dendritic appendages. In the rat, degeneration experiments indicate that the terminals of TRN projection neurons in the dorsal thalamus are F-terminals. These terminals contain flattened synaptic vesicles and exhibit GABA immunoreactivity.

Effects of EEG synchronization on visual responses of the cat's geniculate relay cells: a comparison among Y, X and W cells

Single unit activities of Y, X and W cells were recorded in the cat lateral geniculate nucleus to study how their responses to stationary light spots change with the cortical EEG. The shift from desynchronized to synchronized EEG drastically suppressed W cell activities, depressed both transient and sustained components of X cell responses, but did not affect the transient component of Y cell responses. This was ascribed to different inhibitory circuitries in the three parallel pathways and their differential modulation by the ascending brainstem activities.

GABA immuno-electron microscopic study of the nucleus of the optic tract in the rabbit

The pretectal nucleus of the optic tract (NOT) was investigated immunocytochemically with an antiserum against gamma aminobutyric acid (GABA) employing the pre-embedding peroxidase antiperoxidase technique at the light microscopic level and the postembedding colloidal gold technique at the electron microscopic level. GABA immunoreactivity was observed in cell bodies of different sizes and as punctate structures in the neuropil. In the electron microscope, besides immunoreactive dendrites, four different types of terminals were found to be GABA-immunopositive; three types of terminals with clustered and flattened vesicles (F-profile) and a fourth type with pleomorphic vesicles, presumably of dendritic origin (P-profile). Both P- and F-profiles formed symmetrical synapses with dendritic profiles arranged in clusters ensheathed by glial elements. GABA-immunopositive terminals were observed in synaptic contact with somata and retinal terminals (R-profiles) that were always GABA-immunonegative. Some GABA-immunopositive somata showed presynaptic contacts with dendrites. The presence of GABA in numerous distinct elements in the NOT and the diversity in labeled somata and terminals demonstrate the importance of the inhibitor neurotransmitter in the NOT and suggest that its function is not limited to interneurons.

Cl- - and K+-dependent inhibitory postsynaptic potentials evoked by interneurones of the rat lateral geniculate nucleus

1. Hyperpolarizing potentials evoked by electrical stimulation of the optic tract were studied in projection cells of the rat dorsal lateral geniculate nucleus (LGN) in vitro. In the same cells the effects of gamma-amino butyric acid (GABA), baclofen and acetylcholine (ACh) were also investigated. 2. In the majority of cells a short- (SHP) (34 ms) and a long-lasting (LHP) (240 ms) hyperpolarizing potential could be recorded in the presence and in the absence of a preceding EPSP. They were blocked by tetrodotoxin (1 microM) and were more sensitive than the monosynaptic EPSP to a low-Ca2+-high-Mg2+ solution. 3. The SHP was associated with a marked decrease (75%) in input resistance, was blocked by bicuculline (1-100 microM) and its reversal potential (-67 mV) was dependent on the extracellular Cl- concentration. 4. The LHP was associated with a smaller decrease (45%) in input resistance and its reversal potential (-76 mV) was dependent on the extracellular K+ concentration. It was increased by bicuculline (100% at 50 microM) and nipecotic acid (30% at 10 microM), blocked by Ba2+ (1 mM), and unaffected by eserine (1-10 microM), neostigmine (1-10 microM) or by recording with EGTA-filled electrodes. In the presence of bicuculline, a single LHP was able to evoke, as a rebound response, a low-threshold Ca2+ spike that was, however, not followed by another LHP (or any other long-lasting hyperpolarization). 5. Ionophoretic applications of GABA evoked in the same cell a Cl- -dependent hyperpolarization (reversal potential: -65 mV) and/or depolarization, both of which were associated with a marked decrease (91%) in input resistance and abolished by bicuculline. GABA was also able to evoke a bicuculline-insensitive, K+-dependent hyperpolarization that had a reversal potential of -75 mV and was associated with a smaller decrease (43%) in input resistance. 6. Baclofen, applied by ionophoresis, pressure ejection or in the perfusion medium (1-100 microM), produced a hyperpolarization that had a reversal potential of -79 mV and was associated with a decrease (45%) in input resistance. 7. In the majority of cells (thirty-seven out of forty) ACh evoked a slow depolarization and only in three cells a hyperpolarization which had a reversal potential of -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar organization and ultrastructure of GABA-immunoreactive neurons and processes in the dorsal lateral geniculate nucleus of the tree shrew (Tupaia belangeri)

The distribution and ultrastructure of neurons and neuropil labeled by an antiserum to gamma-aminobutyric acid (GABA) were examined in the lateral geniculate nucleus (LGN) of the tree shrew (Tupaia belangeri). The LGN of this species segregates center type and cell class into three distinct pairs of laminae: a medial pair (laminae 1 and 2) containing ON-center cells, a more lateral pair (4, 5) containing OFF-center cells, and 2 laminae (3, 6) containing W-like cells. The relationship between this laminar segregation and the distribution of GABA immunoreactivity was investigated in the present study. GABA-immunoreactive neurons and neuropil were present in all six of the laminae. However, both the density of labeled cells (adjusted for neuronal density across laminae) and the density of labeled neuropil showed a medial-to-lateral gradient. The adjusted density of labeled cells was higher laterally than medially, and the density of labeled neuropil was significantly greater in the more lateral OFF-center laminae and W-like laminae than in the medial two ON-center laminae. Thus, inhibitory, GABAergic influences may modulate to different degrees the visual signals in the ON, OFF, and W pathways. Labeled cells had a mean cross-sectional area (107 microns 2) approximately one-half that of unlabeled cells (216 microns 2). They constitute 16-34% of the neurons in the LGN. At the electron microscope level, three different kinds of labeled profile were observed. Vesicle containing profiles like the F2 profiles of cat were postsynaptic to retinal terminals and presynaptic to conventional dendrites. F1 axon terminals with dense clusters of vesicles were also labeled as were some myelinated axons. Another labeled profile, which we suggest should be called an F3 process, was a large dendrite of irregular caliber with punctate groups of vesicles near the synapse. Our results suggest that GABAergic circuitry is an important part of the functional organization in the LGN of the tree shrew.

The beta sector of the rabbit's dorsal lateral geniculate nucleus

The beta sector of the rabbit's dorsal lateral geniculate nucleus is a small region of nerve cells scattered among the fibres of the geniculocortical pathway. In its topographical relations it resembles the perigeniculate nucleus of carnivores, which contains neurons driven by geniculate and visual cortical neurons and which sends inhibitory fibres back into the geniculate relay. We have traced retinogeniculate, geniculocortical and corticogeniculate pathways in rabbits by using horseradish peroxidase or radioactively labelled proline and have found that the beta sector resembles the perigeniculate nucleus in receiving no direct retinal afferents, sending no efferents to the visual cortex (V-I), and receiving afferents from the visual cortex. The corticogeniculate afferents are organized so that the visual field map in the beta sector and the main part of the lateral geniculate relays are aligned, as are the maps in the cat's perigeniculate nucleus and the main part of the geniculate relay of carnivores. Electron microscopical studies show similar types of axon terminals in the rabbit and the cat for the main part of the geniculate relay on the one hand and for the beta sector and the perigeniculate nucleus on the other. Earlier observations that the proportion of putative inhibitory terminals (F-type terminals) is lower in the rabbit's than the cat's geniculate region are confirmed. A major difference between the beta sector and the perigeniculate nucleus has been revealed by immunohistochemical staining for GABA. Whereas almost all of the cat's perigeniculate cells appear to be GABAergic, the proportion in the beta sector is much lower, and not significantly different from that found in the main part of the rabbit's geniculate relay. It is concluded that the beta sector shares many of the organizational features of the perigeniculate nucleus. A common developmental origin seems probable, but the functional differences remain to be explored.

Structural correlates of functionally distinct X-cells in the lateral geniculate nucleus of the cat

In the companion paper (Humphrey and Weller, '88), we demonstrated 2 physiologically different groups of X-cells (XL and XN) in the A-laminae of the cat lateral geniculate nucleus. In order to investigate their possible morphological correlates, we iontophoresed horseradish peroxidase intracellularly into physiologically identified XL- and XN-cells and examined their light microscopic appearance. The 11 HRP-labeled XL-cells constituted the smallest relay neurons in the A-laminae, and were similar morphologically. All had small somata (mean soma size = 236 micron2), very thin (less than 1.0 micron) axons, few primary dendrites, and narrow, sinuous distal dendrites, which usually formed trees that were oriented perpendicular to laminar borders. The dendrites could be smooth or display beadlike varicosities, hairlike appendages, and/or occasional complex stalked appendages, but their most consistent feature was numerous clusters of grapelike dendritic appendages located at or near dendritic branch points. The 14 labeled XN-cells were structurally more heterogeneous, and they included relay neurons and interneurons. Eight of 11 XN-relay cells differed markedly from the XL-cells. These XN-cells were multipolar neurons with medium to large somata (mean soma size = 365 micron2), small to medium-size axons (1.0-2.0 micron), numerous primary dendrites, and straight distal dendrites that formed radially symmetric trees. The dendrites of the cells were largely smooth, except for occasional spines and/or hairs, and they were devoid of grapelike and other complex appendages. The three other XN-relay neurons had morphologies either similar to XL-cells or intermediate between XL-cells and more simple, multipolar XN-relay cells, but two of these cells had larger somata and axons than most XL-cells. Finally, three XN-cells were intrageniculate interneurons, which possessed small somata (mean soma size = 174 micron2), fine sinuous dendrites covered with beadlike varicosities on stalked appendages, and no obvious axon. These results reveal that, despite minor overlap, there are marked structural differences between XL- and XN-cells. Among the relay cells, these differences relate to soma and axon diameter, dendritic orientation, and the presence or absence of grapelike dendritic appendages. Our finding that interneurons were strongly excited at short latencies by spot onset supports the hypothesis (Mastronarde, '87a; Humphrey and Weller, '88) that such interneurons provide the major inhibitory input to XL-cells, and that this input is important in generating the spot-induced early dips in XL-cell discharge.

Light and electron microscopic immunocytochemical localization of parvalbumin in the dorsal lateral geniculate nucleus of the cat: evidence for coexistence with GABA

The coexistence in individual neurons of parvalbumin and gamma-aminobutyric acid (GABA) was studied in the dorsal lateral geniculate nucleus (dLGN) of the cat using pre- and postembedding immunocytochemical methods. PV(+) cell bodies and processes were found in the perigeniculate nucleus (PGN) and throughout all laminae of the dLGN. PV(+) neurons were relatively small and had circular to fusiform shapes. Electron microscopy revealed PV(+) reaction product within the perikarya, axons, and dendrites of labeled cells. It was associated preferentially with microtubules, postsynaptic densities, and intracellular membranes. PV(+) presynaptic boutons were identified on the basis of their synaptic relations and ultrastructure as retinal terminals (RLP) and as profiles originating from inhibitory interneurons (F1 and F2). Immunopositive somata and dendrites received asymmetric synaptic contacts from labeled RLP and non-identified, non-immunoreactive synaptic boutons. Moreover, PV(+) dendrites were postsynaptic to labeled F profiles. In the PGN all neurons were both PV(+) and GABA-immunoreactive and in the dLGN the vast majority of PV(+) neurons showed GABA-immunoreactivity. It is suggested that the high incidence of PV in GABAergic neurons is related to the particular activation patterns of these neurons and the resulting demand for calcium buffer systems.

Cholecystokinin innervation of rat thalamus, including fibers to ventroposterolateral nucleus from dorsal column nuclei

The distribution of cholecystokinin octapeptide immunoreactive fibers and puncta in the adult rat thalamus was studied using immunocytochemical methods. Small to moderate numbers of immunoreactive fibers were present in the lateral habenular nucleus, ventral lateral geniculate nucleus, zona incerta, parataenial, mediodorsal, medioventral, and submedial nuclei, the rhomboid, paracentral, central lateral and parafascicular nuclei, and in the medial geniculate and dorsal lateral geniculate nuclei. Moderate to large numbers of cholecystokinin (CCK)-positive fibers were present in the paraventricular nuclei, the reticular nucleus, the anteroventral, anteromedial, and central medial nuclei, and in the rostral extension of the internal medullary lamina between the parataenial and anteroventral nuclei. Dense concentrations of immunoreactive fibers were also found in a principal sensory relay nucleus, the ventroposterolateral nucleus (VPL), of the ventrobasal complex. The number of CCK-positive fibers in VPL showed a marked unilateral decrease in rats which had received lesions of the contralateral gracile and cuneate nuclei. The results of this study demonstrate that CCK-immunoreactive fibers and puncta are widely distributed in the rat thalamus, and that the source of these fibers in VPL is probably the dorsal column nuclei.

The action of the corticofugal pathway on sensory thalamic nuclei: a hypothesis

The N-methyl-D-aspartate receptor has recently attracted great interest due to its nonlinear current-voltage behavior. In order to evoke a large depolarizing postsynaptic current, the synaptic-induced conductance change must be paired with a postsynaptic depolarization. This temporally tuned AND gate could underlie a number of different operations throughout the nervous system. We propose that the synapses made by the optical nerve onto projection cells in the mammalian dorsal lateral geniculate nucleus are of the N-methyl-D-aspartate type. [In this Commentary, we have pooled data regarding sensory thalamic nuclei from a number of different mammalian species. Unless otherwise mentioned, we have referred to the dorsal division of the cat lateral geniculate nucleus.] About half of all synapses in these cells--located almost exclusively in the peripheral two-thirds of the dendritic tree--are associated with axons originating in layer VI of visual cortex. It then follows that the massive corticogeniculate pathway controls the gain of the retinogeniculate pathway via its action on the N-methyl-D-aspartate receptors. Thus, near-simultaneous activation of the retinal and the cortical input will transiently enhance the geniculate cell response. Generalizing to other thalamic sensory nuclei, afferent information will be routed through the thalamus and on to the cortex as long as cortical activity is congruent with sensory input to the thalamus. Experimental evidence argues for such a mechanism to control the gain of the somatosensory input to the ventrobasal thalamic nucleus.

Ultrastructural identification of synaptic terminals from the axon of type 3 interneurons in the cat lateral geniculate nucleus

Synaptic terminals from the axons of type 3 neurons in the A-laminae of the cat LGN impregnated with the Golgi gold-toning procedure were examined at light and electron microscopic levels. The axons were identified by their somatic origin, thin diameter, and, in one of these cells, by dense undercoating beneath the axolemma, which is a known characteristic of the axon initial segment. The axon of one of the analyzed cells was profusely branched and extended throughout most of lamina A within the dendritic domains of the cell, and both types of processes were oriented along projection lines in LGN. This suggests that the dendrites and axons of type 3 cells receive inputs and exert effects, of probably inhibitory nature, within restricted retinotopic regions of LGN. The vast majority of the axon terminals of these cells were distributed in series along axonal branches. In one of the type 3 cells, however, a dense cluster of terminals arising from a secondary axonal branch was observed. Ultrastructurally, the analyzed synaptic terminals of the type 3 cells contained flattened or pleomorphic synaptic vesicles, dark mitochondria, and established synapses that appeared to be of symmetrical type when the membranes were perpendicularly cut. On the basis of these characteristics these terminals are classified as F boutons, following Guillery's (Z. Zellforsch. 96:1-38, '69), nomenclature. The postsynaptic elements to the axon terminals were dendrites of small to medium size, which received "en passant" synaptic contacts in extraglomerular regions of the geniculate neuropil by the terminals distributed in series. The axon terminals located in clusters, however, made synapses with dendrites in glomerular regions of the neuropil, where they were not seen postsynaptic to retinal or other types of terminals. This is in contrast to the postsynaptic nature of F2 boutons in the same glomeruli, which have been identified as dendritic appendages of the GABA positive type 3 neurons in the cat LGN (Montero: J. Comp. Neurol. 254:228-245, '86). On the other hand, the axonal F terminals differ from F1 boutons in terms of synaptic relations and ultrastructure, since the latter have been shown to be presynaptic to F2s and somata and to contain crowded populations of flat synaptic vesicles which give them a characteristic dark appearance. Terminals equivalent to F1 boutons have been shown to originate from perigeniculate cells in the rat LGN. From these observations it is suggested that the geniculate GABAergic interneurons support two morphologically and functionally different type of inhibitory terminals synapsing the dendrites of relay cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of gamma-aminobutyric acid in the hypoglossal nucleus of the macaque monkey, Macaca fuscata: a light and electron microscopic study

The hypoglossal nucleus of the macaque monkey Macaca fuscata was investigated with light and electron microscopic immunocytochemistry with an antibody directed against gamma-aminobutyric acid (GABA). At the light microscopic level, GABA immunoreactivity was present in small neurons, punctate structures, and thin, fiberlike structures. These GABA-positive elements were distributed throughout the hypoglossal nucleus at rostrocaudal levels. There was no immunoreactivity in the hypoglossal motoneurons. The GABA-positive small neurons were fusiform or ovoid (15 X 9 micron) and extended a few proximal dendrites from both poles. At the ultrastructural level, these small neurons were characterized by a markedly invaginated nucleus and a scanty cytoplasm in which cisternae of rough endoplasmic reticulum were not organized into extensive lamellar arrays as seen in the motorneurons. The GABA-positive punctate structures at the light microscopic level were identified as vesicle-containing axon boutons at the electron microscopic level. These GABA-positive axon terminals made synaptic contacts mainly with the dendrites of the motoneurons and infrequently with the somata. The majority of them made symmetric synapses and they contained pleomorphic synaptic vesicles. However, a small number of GABA-positive terminals (7%) formed asymmetric synapses with the dendrites of motoneurons, and these contacts exhibited postsynaptic dense bars or Taxi bodies lying beneath the postsynaptic membranes. There were no GABA-positive boutons that contacted the cell bodies of the small neurons. Although GABA-positive myelinated and unmyelinated axons were seen as thin, fiberlike structures, these myelinated and unmyelinated axons rarely gave rise to boutons on the motoneurons. The present study suggests that GABAergic inhibition in the monkey hypoglossal nucleus occurs mainly on the dendrites of the motoneurons and to some extent on the somata.

Contributions of inhibitory mechanisms to the shift responses of X and Y cells in the cat lateral geniculate nucleus

1. Adult cats were anaesthetized with a mixture of halothane, nitrous oxide and oxygen to record from single neurones of the dorsal lateral geniculate nucleus (d.l.g.n.) with five-barrel glass micro-electrodes. Periphery effects (shift effects) were elicited by large-field phase-reversing gratings presented in the visual field outside the conventional receptive field area. 2. A range of transient excitatory responses was found in X and Y cells. Y cells had phasic shift effects with significantly higher amplitudes and shorter durations (mean 52 impulses/s, 135 ms) than those observed in the tonic shift effects of X cells (mean 24 impulses/s, 169 ms). All Y cells and most X cells responded to stimulation of remote retinal regions. About 7% of the X cells displayed no shift effect. 3. Micro-ionophoresis of the gamma-aminobutyric acid (GABA) antagonist bicuculline, acetylcholine (ACh) and L-glutamate specifically influenced the shift effects of X and Y cells. 4. During continuous application of the GABA antagonist bicuculline the differences in maximal response rates and amplitudes of X and Y cells were eliminated. The maintained activity raised predominantly in X cells and the early peak rates increased more in X- than in Y-cell shift effects, leading to equal average peak rates of 100 and response amplitudes of about 85 impulses/s in both cell classes. The characteristic time courses of X- and Y-cell responses were not affected. 5. Micro-ionophoretic application of ACh caused a combination of excitatory and disinhibitory effects. Maintained activity as well as early parts of stimulus-evoked responses were similarly raised in X and Y cells. In addition, the Y-cell shift effects became less phasic by elevation of the late response part. Sodium pentobarbitone, used to block ACh excitation, suppressed the ACh-induced effects in the early phase of the X- and Y-cell shift effects and the increase of maintained activity in Y-cells, while the effect on the late part of Y-cell responses persisted. Elevation of background activity partially remained in X cells, and the X-cell responses became tonically prolonged at the same time. 6. L-Glutamate increased the activity of X and Y cells without changing the characteristic shift-effect properties of both cell classes. 7. It is concluded that different short- and long-lasting inhibitory mechanisms shape the responses of d.l.g.n. neurones to stimulation outside the conventional receptive field.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The nigrotectal projection in the cat: an electron microscope autoradiographic study

Recent evidence indicates that the nigrotectal tract plays an important role in regulating the premotor responses of cells in the in the intermediate gray layer of the superior colliculus. The purpose of the present study was to characterize the ultrastructure of nigrotectal terminals and of their postsynaptic targets in the intermediate gray layer. Nigrotectal terminals were identified in the electron microscope by labeling them autoradiographically, following injections of tritiated proline into the substantia nigra pars reticulata. The majority of nigrotectal terminals contain a high proportion of pleomorphic vesicles and form symmetrical synaptic contacts. Most of these terminals synapse with small dendritic profiles (2.00 micron +/- 0.83 SD), which may be the distal dendrites of neurons in the intermediate gray layer. Less than 10% of the labeled contacts are made with cell bodies or initial axonal segments.

The serotoninergic fibers in the dorsal lateral geniculate nucleus of the cat: distribution and synaptic connections demonstrated with immunocytochemistry

The distribution, morphology, and synaptic contacts of serotoninergic fibers were studied with immunocytochemical methods in the lateral geniculate complex of the cat. The serotonin-immunoreactive fibers are diffusely distributed throughout the main laminae of the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN) and reach a particular density in the ventral lateral geniculate nucleus (vLGN). The labeled fibers are in most cases very thin and sometimes varicose. There is no obvious order in their distribution pattern except that they sometimes partially encircle the unlabeled cell bodies of the dLGN. The synaptic connections of the serotoninergic fibers were investigated mainly in the A laminae of the dLGN. Few synaptic complexes were found, most of them with asymmetric morphology. The postsynaptic elements were small dendritic profiles. Perisomatic serotoninergic fibers were seen, but no convincing synaptic contacts were found between labeled fibers and cell somata. In the dLGN, serotoninergic profiles were almost exclusively confined to the extraglomerular neuropile. In the PGN serotoninergic fibers also contacted dendritic profiles and formed asymmetrical synapses, but as in the geniculate, synaptic specializations were very rare.

Light and electron microscopical immunocytochemistry of 5'-nucleotidase in rat cerebellum

5'-Nucleotidase in nervous tissue has so far not been localised at the ultrastructural level using immunocytochemical techniques. We have now applied monoclonal antibodies and a polyclonal antiserum raised against this ecto-enzyme and describe the distribution of 5'-nucleotidase antigenicity in rat cerebellum both at the light and electron microscopic levels. Within all cerebellar layers, 5'-nucleotidase immunoreactivity was found on plasma membranes of glial elements, i.e. Bergmann glial cell processes crossing the molecular layer, astrocytic end-feet around blood vessels and glial cell extensions surrounding single Purkinje cells. In the granular layer, 5'-nucleotidase immunoreactivity was present on glial membranes interposed between granule cells. Neuronal cells or processes were devoid of immunoreactivity. The immunocytochemical results were compared with conventional 5'-nucleotidase histochemistry. Both techniques showed the same ecto-localisation of the enzyme and favour the view of 5'-nucleotidase being predominantly situated at glial plasma membranes.

Localization of gamma-aminobutyric acid (GABA) in type 3 cells and demonstration of their source to F2 terminals in the cat lateral geniculate nucleus: a Golgi-electron-microscopic GABA-immunocytochemical study

Postembedding immunocytochemistry with a gamma-aminobutyric acid (GABA) antiserum was done on semithin sections of cat lateral geniculate nucleus (LGN) previously processed with the rapid-Golgi and gold-toning procedures, to determine which of the three main morphological types (1, 2,3) of neurons in the A-laminae show immunoreactivity and are, therefore, presumably GABAergic. Only type 3 cells were found to be GABA positive. These cells were characterized by small somata and few, scarcely branched dendrites bearing almost exclusively appendages with long slender stalks. Some of these cells have extensive filiform "axonlike" processes originating from different regions of dendrites and having appendages similar to those originating directly from dendrites. Many of these Golgi gold-toned impregnated dendritic appendages of type 3 cells were analyzed in the electron microscope and were identified as typical F2 terminals by their content of pleomorphic synaptic vesicles; by being postsynaptic to retinal (RLP), cortical (RSD), and perigeniculate (F1) terminals; and by being presynaptic to dendrites. In addition, since it was previously demonstrated that glutamic acid decarboxylase (GAD) and GABA-positive cells are not retrogradely labeled with horseradish peroxidase (HRP) from the visual cortex, the present results, by showing that GABA-positive cells have type 3 morphology, provide supporting evidence for the interneuronal nature of type 3 cells in cat LGN.

GABA immunoreactivity in the thalamic reticular nucleus of the rat. A light and electron microscopical study

The thalamic reticular nucleus (TRN) of the rat has been studied immunocytochemically using an antiserum against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Combined light and electron microscopic investigations by means of peroxidase-antiperoxidase and immunogold labeling show that this nucleus contains a homogeneous population of GABA-immunoreactive neurons receiving extensive GABAergic connections suggestive of self-inhibitory inputs.

The proportion and size of GABA-immunoreactive neurons in the magnocellular and parvocellular layers of the lateral geniculate nucleus of the rhesus monkey

Neurons containing GABA-immunoreactivity in LGN of the macaque monkey were analyzed quantitatively in semithin (1 micron) sections. The percentage of GABA(+) cells per unit area of the sections was 26% in the magnocellular layers and 19% in the parvocellular layers. However, the percentage of GABA(+) cells in a unit volume of LGN, calculated by a stereological method that takes into account the observed difference in size of labeled and unlabeled somata, was 35% in the magnocellular layers and 25% in the parvocellular layers. GABA(+) somata in the magnocellular layers were significantly larger than those in the parvocellular layers. The possible role of GABAergic cells in inhibitory mechanisms of receptive fields of parvo- and magnocellular neurons are discussed in the light of current knowledge of the physiology and neural circuits of macaque LGN.

Evidence for two types of GABA-containing interneurons in the A-laminae of the cat lateral geniculate nucleus: a double-label HRP and GABA-immunocytochemical study

Neurons containing GABA immunoreactivity were analyzed in the A-laminae of normal cat LGN and of LGN retrogradely labeled with HRP from the visual cortex. In contrast to retrograde labeling of relay cells, GABA+ cells were devoid of HRP label, providing additional evidence for the interneuronal nature of GABAergic cells in the cat LGN. Cell body area measurements showed that the population of GABA+ cells is composed of a large proportion of small (beta) cells and a smaller proportion of medium size (alpha) cells. The proportion of alpha GABA+ cells increases from medial to lateral parts of the A-laminae, resembling a similar medio-lateral increase of physiologically defined Y cells and of morphologically defined type 1 cells in these laminae. This suggests that the alpha and beta GABAergic cells are related to the Y and X geniculo-cortical relay cells, respectively.