Heterogeneous properties of central lateral and parafascicular thalamic synapses in the striatum.

To understand the principles of operation of the striatum it is critical to elucidate the properties of the main excitatory inputs from cortex and thalamus, as well as their ability to activate the main neurons of the striatum, the medium spiny neurons (MSNs). As the thalamostriatal projection is heterogeneous, we set out to isolate and study the thalamic afferent inputs to MSNs using small localized injections of adeno-associated virus carrying fusion genes for channelrhodopsin-2 and YFP, in either the rostral or caudal regions of the intralaminar thalamic nuclei (i.e. the central lateral or parafascicular nucleus). This enabled optical activation of specific thalamic afferents combined with whole-cell, patch-clamp recordings of MSNs and electrical stimulation of cortical afferents, in adult mice. We found that thalamostriatal synapses differ significantly in their peak amplitude responses, short-term dynamics and expression of ionotropic glutamate receptor subtypes. Our results suggest that central lateral synapses are most efficient in driving MSNs to depolarization, particularly those of the direct pathway, as they exhibit large amplitude responses, short-term facilitation and predominantly express postsynaptic AMPA receptors. In contrast, parafascicular synapses exhibit small amplitude responses, short-term depression and predominantly express postsynaptic NMDA receptors, suggesting a modulatory role, e.g. facilitating Ca(2+)-dependent processes. Indeed, pairing parafascicular, but not central lateral, presynaptic stimulation with action potentials in MSNs, leads to NMDA receptor- and Ca(2+)-dependent long-term depression at these synapses. We conclude that the main excitatory thalamostriatal afferents differ in many of their characteristics and suggest that they each contribute differentially to striatal information processing.

Differential localization of GABA(A) receptor subunits in relation to rat striatopallidal and pallidopallidal synapses.

As a central integrator of basal ganglia function, the external segment of the globus pallidus (GP) plays a critical role in the control of voluntary movement. The GP is composed of a network of inhibitory GABA-containing projection neurons which receive GABAergic input from axons of the striatum (Str) and local collaterals of GP neurons. Here, using electrophysiological techniques and immunofluorescent labeling we have investigated the differential cellular distribution of α1, α2 and α3 GABA(A) receptor subunits in relation to striatopallidal (Str-GP) and pallidopallidal (GP-GP) synapses. Electrophysiological investigations showed that zolpidem (100 nm; selective for the α1 subunit) increased the amplitude and the decay time of both Str-GP and GP-GP IPSCs, indicating the presence of the α1 subunits at both synapses. However, the application of drugs selective for the α2, α3 and α5 subunits (zolpidem at 400 nm, L-838,417 and TP003) revealed differential effects on amplitude and decay time of IPSCs, suggesting the nonuniform distribution of non-α1 subunits. Immunofluorescence revealed widespread distribution of the α1 subunit at both soma and dendrites, while double- and triple-immunofluorescent labeling for parvalbumin, enkephalin, gephyrin and the γ2 subunit indicated strong immunoreactivity for GABA(A) α3 subunits in perisomatic synapses, a region mainly targeted by local axon collaterals. In contrast, immunoreactivity for synaptic GABA(A) α2 subunits was observed in dendritic compartments where striatal synapses are preferentially located. Due to the kinetic properties which each GABA(A) α subunit confers, this distribution is likely to contribute differentially to both physiological and pathological patterns of activity.

Cholinergic modulation of midbrain dopaminergic systems.

Dopamine neurons in the midbrain respond to behavioral events and environmental stimuli. Their different patterns of activation in turn modulate the activity of forebrain regions and modulate the expression of selective behavioral responses. However, their activity is closely dependent on the cholinergic systems in the brainstem. Ascending cholinergic projections from the pedunculopontine and laterodorsal tegmental nuclei target dopaminergic neurons in the substantia nigra compacta and ventral tegmental area following a topographical gradient. These projections, by means of the activation of acetylcholine receptors, influence the firing of dopamine neurons and therefore their responsiveness, ultimately affecting the release of dopamine in their forebrain targets. Brainstem cholinergic neurons are thus in a position to critically influence the activity of dopaminergic neurons in the midbrain, and thereby have a critical role in the expression of behavior.

Stereological estimates of dopaminergic, GABAergic and glutamatergic neurons in the ventral tegmental area, substantia nigra and retrorubral field in the rat.

Midbrain dopamine neurons in the ventral tegmental area, substantia nigra and retrorubral field play key roles in reward processing, learning and memory, and movement. Within these midbrain regions and admixed with the dopamine neurons, are also substantial populations of GABAergic neurons that regulate dopamine neuron activity and have projection targets similar to those of dopamine neurons. Additionally, there is a small group of putative glutamatergic neurons within the ventral tegmental area whose function remains unclear. Although dopamine neurons have been intensively studied and quantified, there is little quantitative information regarding the GABAergic and glutamatergic neurons. We therefore used unbiased stereological methods to estimate the number of dopaminergic, GABAergic and glutamatergic cells in these regions in the rat. Neurons were identified using a combination of immunohistochemistry (tyrosine hydroxylase) and in situ hybridization (glutamic acid decarboxylase mRNA and vesicular glutamate transporter 2 mRNA). In substantia nigra pars compacta 29% of cells were glutamic acid decarboxylase mRNA-positive, 58% in the retrorubral field and 35% in the ventral tegmental area. There were further differences in the relative sizes of the GABAergic populations in subnuclei of the ventral tegmental area. Thus, glutamic acid decarboxylase mRNA-positive neurons represented 12% of cells in the interfascicular nucleus, 30% in the parabrachial nucleus, and 45% in the parainterfascicular nucleus. Vesicular glutamate transporter 2 mRNA-positive neurons were present in the ventral tegmental area, but not substantia nigra or retrorubral field. They were mainly confined to the rostro-medial region of the ventral tegmental area, and represented approximately 2-3% of the total neurons counted ( approximately 1600 cells). These results demonstrate that GABAergic and glutamatergic neurons represent large proportions of the neurons in what are traditionally considered as dopamine nuclei and that there are considerable heterogeneities in the proportions of cell types in the different dopaminergic midbrain regions.

Basal ganglia macrocircuits.

This is the introductory chapter to an edited volume comprising 18 chapters written by 38 specially selected authors covering the anatomy, physiology, biochemistry/pharmacology and behavioral aspects of GABA in the basal ganglia. In this chapter the various nuclei of the basal ganglia are defined and their cellular structure, connections and function reviewed in brief in order to provide an orientation for the subsequent 17 chapters.

The neural network of the basal ganglia as revealed by the study of synaptic connections of identified neurones.

The study of synaptic connections in the electron microscope has established an 'elementary' circuit for the neostriatum which consists of a pathway from cortical areas (neocortex, hippocampus, amygdala) to medium spiny neurones of the striatum that also receive converging synaptic input from midbrain dopamine neurones. The striatal medium spiny neurones are projection neurones and they form synaptic contacts with output neurones in the globus pallidus and substantia nigra reticulata. In this way, dopaminergic afferents can directly modulate the flow of information from cortical areas through the striatum to the 'premotor' areas of the brainstem and to the thalamus. It is proposed that certain parts of the striatum can themselves control the activity of midbrain dopamine neurones and so one part of the striatum can 'gate' the flow of information through another part.

Advances in the molecular understanding of GABA(B) receptors.

The molecular nature of the metabotropic GABA(B) receptor was for some time a mystery, however it was recently discovered that two related G-protein-coupled receptors have to heterodimerize to form the functional GABA(B) receptor at the cell surface. This review discusses the most recent findings in the rapidly expanding field of GABA(B) receptor research, and includes a summary of all splice variants of both receptor subunits identified to date. It also evaluates emerging evidence that certain splice variants might play a role in determining pharmacologically distinguishable receptors, and reviews receptor localization at the sub-cellular level and involvement in neuronal development.

Relationship of activity in the subthalamic nucleus-globus pallidus network to cortical electroencephalogram.

One of the functions of the excitatory subthalamic nucleus (STN) is to relay cortical activity to other basal ganglia structures. The response of the STN to cortical input is shaped by inhibition from the reciprocally connected globus pallidus (GP). To examine the activity in the STN-GP network in relation to cortical activity, we recorded single and multiple unit activity in STN and/or GP together with cortical electroencephalogram in anesthetized rats during various states of cortical activation. During cortical slow-wave activity (SWA), STN and GP neurons fired bursts of action potentials at frequencies that were similar to those of coincident slow ( approximately 1 Hz) and spindle (7-14 Hz) cortical oscillations. Spontaneous or sensory-driven global activation was associated with a reduction of SWA and a shift in STN-GP activity from burst- to tonic- or irregular-firing. Rhythmic activity in STN and GP neurons was lost when the cortex was inactivated by spreading depression and did not resume until SWA had recovered. Although rhythmic STN-GP activity was correlated with SWA, the phase relationships of activities of neurons within the STN and GP and between the nuclei were variable. Even when neurons displayed synchronous bursting activity, correlations on the millisecond time scale, which might indicate shared synaptic input, were not observed. These data indicate that (1) STN and GP activity is intimately related to cortical activity and hence the sleep-wake cycle; (2) rhythmic oscillatory activity in the STN-GP network in disease states may be driven by the cortex; and (3) activity of the STN-GP network is regulated in space in a complex manner.

GABA(B) receptors at glutamatergic synapses in the rat striatum.

Although multiple effects of GABA(B) receptor activation on synaptic transmission in the striatum have been described, the precise locations of the receptors mediating these effects have not been determined. To address this issue, we carried out pre-embedding immunogold electron microscopy in the rat using antibodies against the GABA(B) receptor subunits, GABA(B1) and GABA(B2). In addition, to investigate the relationship between GABA(B) receptors and glutamatergic striatal afferents, we used antibodies against the vesicular glutamate transporters, vesicular glutamate transporter 1 and vesicular glutamate transporter 2, as markers for glutamatergic terminals. Immunolabeling for GABA(B1) and GABA(B2) was widely and similarly distributed in the striatum, with immunogold particles localized at both presynaptic and postsynaptic sites. The most commonly labeled structures were dendritic shafts and spines, as well as terminals forming asymmetric and symmetric synapses. In postsynaptic structures, the majority of labeling associated with the plasma membrane was localized at extrasynaptic sites, although immunogold particles were also found at the postsynaptic specialization of some symmetric, putative GABAergic synapses. Labeling in axon terminals was located within, or at the edge of, the presynaptic active zone, as well as at extrasynaptic sites. Double labeling for GABA(B) receptor subunits and vesicular glutamate transporters revealed that labeling for both GABA(B1) and GABA(B2) was localized on glutamatergic axon terminals that expressed either vesicular glutamate transporter 1 or vesicular glutamate transporter 2. The patterns of innervation of striatal neurons by the vesicular glutamate transporter 1- and vesicular glutamate transporter 2-positive terminals suggest that they are selective markers of corticostriatal and thalamostriatal afferents, respectively. These results thus provide evidence that presynaptic GABA(B) heteroreceptors are in a position to modulate the two major excitatory inputs to striatal spiny projection neurons arising in the cortex and thalamus. In addition, presynaptic GABA(B) autoreceptors are present on the terminals of spiny projection neurons and/or striatal GABAergic interneurons. Furthermore, the data indicate that GABA may also affect the excitability of striatal neurons via postsynaptic GABA(B) receptors.

Topographical and Synaptic Organization of the GABA-Containing Pallidosubthalamic Projection in the Rat.

The anterograde transport of Phaseolus vulgaris-leucoagglutinin (PHA-L) was combined with postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) to study the topography, the synaptic organization and the neurotransmitter content of the pallidosubthalamic projection in the rat. After injections of PHA-L in different parts of the globus pallidus a rich plexus of anterogradely labelled fibres and terminals was found in the ipsilateral subthalamic nucleus. The immunoreactive elements were distributed according to a mediolateral and rostrocaudal topography. Injections of PHA-L restricted to the lateral two-thirds of the globus pallidus gave rise to a massive anterograde labelling confined to the lateral half of the subthalamic nucleus. On the other hand, injections of PHA-L strictly confined to the medial part of the globus pallidus resulted in anterograde labelling that occupied the ventromedial pole of the subthalamic nucleus. In some cases a few retrogradely labelled cells were found in the subthalamic nucleus after PHA-L injections in the globus pallidus. The perikarya and the primary dendrites of these labelled cells were sometimes surrounded by anterogradely labelled terminals suggesting a close reciprocal connection between the globus pallidus and the subthalamic nucleus. Electron microscopic analysis of the PHA-L-labelled terminals revealed that they contain many mitochondria, numerous small round or slightly pleomorphic vesicles and occasionally one or two large dense core vesicles. They form symmetrical synaptic contacts predominantly with the proximal dendrites (39%) and less frequently with the perikarya (31%) and the distal dendrites (30%) of the subthalamic cells. Quantitative measurements showed that the pallidosubthalamic varicosities have a diameter ranging from 0.7 to 4.5 microm and a mean cross-sectional area of 0.79 +/- 0.26 microm2 (Mean +/- SD). Postembedding immunocytochemistry for GABA revealed that the PHA-L-immunoreactive pallidosubthalamic axon terminals display GABA immunoreactivity. The results of our study demonstrate that the pallidosubthalamic projection is organized according to a mediolateral and rostrocaudal topography and that the proximal dendrites of the subthalamic cells are the major targets of the GABA-immunoreactive pallidosubthalamic terminals. This suggests that the globus pallidus exerts a powerful control over the subthalamic cells through an inhibitory GABAergic pathway.

Host Corticostriatal Fibres Establish Synaptic Connections with Grafted Striatal Neurons in the Ibotenic Acid Lesioned Striatum.

The connections between host corticostriatal afferents and neurons in intrastriatal grafts of foetal striatal tissue have been studied with electron microscopic immunocytochemistry using Phaseolus vulgaris leucoagglutinin (PHA-L) as a label of the host corticostriatal fibres. Adult rats with unilateral ibotenic acid lesions of the head of the caudate putamen received foetal cell suspension grafts from E14-15 rat embryos into the lesioned striatal area. Ten months after transplantation, multiple iontophoretic injections of PHA-L were made into the host frontal cortex. These injections labelled large numbers of corticostriatal fibres which extended across the graft - host border to form a rich axonal network mainly in the peripheral portions of the grafts. At the ultrastructural level a total of 134 PHA-L-labelled terminals were identified to form asymmetric synaptic contacts with neurons within the grafts. Of these contacts, 83% were in contact with dendritic spines, 12% with dendritic shafts, and 5% with small shafts or spines. The synaptic contacts were similar to those identified in intact regions of the host striatum that were spared by the lesion. However, the synapses in the host striatum were almost exclusively in contact with spines (98%). These results demonstrate that the corticostriatal projection, which constitutes a major source of afferent control in the normal striatum, not only extends axons into the intrastriatal striatal grafts, but also establishes synaptic connections with the implanted neuronal elements.

Sites of [3H]taurine Uptake in the Rat Substantia Nigra in Relation to the Release of Taurine from the Striatonigral Pathway.

The autoradiographic localization of radiolabelled taurine taken up in the rat substantia nigra in vivo together with conditions of release of the [3H]taurine taken up into brain slices were studied to determine whether they are consistent with the hypothesis that taurine may act as a neurotransmitter in the striatonigral pathway. At the light microscopic level the main cellular elements that became radiolabelled following the injection of [3H]taurine into the substantia nigra could be identified as glial cells. Electron microscope autoradiography confirmed that a subpopulation of glial cells including astrocytes, pericytes, and oligodendrocytes were radiolabelled and that neuronal perikarya were not radiolabelled. In addition, axonal elements including both terminal and preterminal boutons were found to have silver grains overlying them and were thus considered to be radiolabelled. This was supported by a quantitative analysis of the distribution of the silver grains; whereas glial elements had a significantly higher number of grains associated with them than with any other structure, axonal elements had a significantly greater number of grains than dendritic structures. Release of the preloaded [3H]taurine from superfused slices of substantia nigra occurred in response to veratridine, was calcium-dependent and was sensitive to inhibition by high magnesium concentrations or tetrodotoxin. Following the destruction of neurons in the striatum by ibotenic acid injections, although the weight of the ipsilateral substantia nigra was reduced, the uptake of [3H]taurine was not altered. In contrast to this, the veratridine-stimulated release was markedly attenuated, implying that the destruction of striatal neurons causes the loss of sites in the substantia nigra from which exogenous taurine is released. These results add further support to previous suggestions that taurine might act as a neurotransmitter or neuromodulator in the striatonigral pathway.

Localisation of parvalbumin-immunoreactive structures in primate caudate-putamen.

To investigate the morphology, distribution, and connections of parvalbumin-containing neurones in the caudate-putamen of primates, perfuse-fixed sections were stained to reveal parvalbumin immunoreactivity. In agreement with previous observations, the caudate-putamen was rich in parvalbumin-positive neurones and neuropil. The neuropil staining was uneven such that the dense background staining was interspersed with zones of relatively weak staining. The distribution corresponded to the striosome/matrix system as defined by substance P or met-enkephalin immunostaining in adjacent sections. Because parvalbumin-positive neurones are present in regions known to project to the caudate-putamen and the majority of parvalbumin-positive terminals in the matrix formed asymmetric synapses, it is concluded that the uneven staining is probably due to afferents of the neostriatum. The morphology of the parvalbumin-immunoreactive neurones varied between the striosomes and matrix; those in the matrix were smaller and possessed dendritic arborisations that were relatively uniform, whereas those in the striosomes were generally more extensively stained and possessed a greater variation in their dendritic branching patterns. The dendrites frequently crossed the boundary between the striosomes and matrix. A population of giant parvalbumin-immunoreactive neurones was also observed in the putamen. Electron microscopic analysis revealed that, in addition to terminals forming asymmetric synapses, a smaller population formed symmetric synaptic specialisations and are presumed to be derived from the local parvalbumin-immunoreactive neurones. Terminals of the latter group formed synapses with medium-sized spiny neurones. Because parvalbumin-positive neurones receive input from the cortex, they may transmit cortical information to spiny neurones.

Cholinergic synaptic input to different parts of spiny striatonigral neurons in the rat.

The postsynaptic targets of cholinergic boutons in the rat neostriatum were assessed by examination in the electron microscope of boutons that were immunoreactive for choline acetyltransferase, the synthetic enzyme for acetylcholine. These boutons formed symmetrical synaptic specializations with neostriatal neurons. Of 209 immunoreactive synaptic boutons observed in random searches of the neostriatum, 45% made contact with dendritic shafts, 34% with dendritic spines, and 20% with neuronal perikarya. Many of the postsynaptic structures had ultrastructural characteristics of the most common type of striatal neuron, the medium-size densely spiny neuron. This was confirmed by the examination in the electron microscope of Golgi-impregnated medium-size spiny neurons from sections that had also been immunostained for choline acetyltransferase. Immunoreactive boutons formed symmetrical synaptic specializations with all parts of the neurons examined, i.e., perikarya, proximal and distal dendritic shafts, and dendritic spines. Two of the Golgi-impregnated medium-size spiny neurons that received input from the cholinergic boutons were also retrogradely labelled with horseradish peroxidase that had been injected into the substantia nigra, they were thus further characterized as striatonigral neurons. Similarly, seven retrogradely labelled perikarya of striatonigral neurons were found to receive input from the cholinergic boutons. It is concluded that cholinergic boutons in the neostriatum form synaptic specializations and that one of their major targets is the medium-size densely spiny neuron that projects to the substantia nigra. The topography of the cholinergic afferents of these cells is distinctly different from that of other boutons derived from local neurons and from boutons that form asymmetrical synaptic specializations, but it is similar to that of the dopaminergic boutons originating from neurons in the substantia nigra.

The striatum and the globus pallidus send convergent synaptic inputs onto single cells in the entopeduncular nucleus of the rat: a double anterograde labelling study combined with postembedding immunocytochemistry for GABA.

The entopeduncular nucleus is one of the major output stations of the basal ganglia. In order to better understand the role of this structure in information flow through the basal ganglia, experiments have been performed in the rat to examine the chemical nature, morphology, and synaptology of the projections from the globus pallidus and striatum to the entopeduncular nucleus. In order to examine the morphology and synaptology of pallidoentopeduncular terminals and striatoentopeduncular terminals, rats were subjected to a double anterograde labelling study. The globus pallidus was injected with Phaseolus vulgaris-leucoagglutinin (PHA-L), and on the same side of the brain, the striatum was injected with biocytin. The entopeduncular nuclei of these animals were then examined for anterogradely labelled pallidal and striatal terminals. Rich plexuses of PHA-L-labelled pallidal terminals and biocytin-labelled striatal terminals were identified throughout the entopeduncular nucleus. At the electron microscopic level, the pallidal boutons were classified as two types. The majority (Type 1), were large boutons that formed symmetrical synapses with the dendrites and perikarya of neurones in the entopeduncular nucleus. Type 2 PHA-L-labelled terminals were much rarer, slightly smaller, and formed asymmetrical synapses. It is suggested that the Type 2 boutons are not derived from the globus pallidus but from the subthalamic nucleus. The biocytin-labelled terminals from the striatum had the typical morphological features of striatal terminals and formed symmetrical synapses. The distribution of the postsynaptic targets of the pallidal terminals and the striatal terminals differed in that the pallidal terminals preferentially made synaptic contact with the more proximal regions of the neurones in the entopeduncular nucleus, whereas the striatal terminals were located more distally on the dendritic trees. Examination in the electron microscope of areas where there was an overlap of the two sets of anterogradely labelled boutons revealed that terminals from the globus pallidus and the striatum made convergent synaptic contact with the perikarya and dendrites of individual neurones in the entopeduncular nucleus. In order to examine the chemical nature of the input to the entopeduncular nucleus from the globus pallidus and the striatum, ultrathin sections were immunostained by the postembedding method to reveal endogenous GABA. Three classes of GABA-containing terminals were identified; two of them formed symmetrical synapses and one rare type formed asymmetrical synapses. The combination of the GABA immunocytochemistry and anterograde labelling revealed that both the striatal and pallidal afferents that make symmetrical synapses with neurones in the entopeduncular nucleus, including those involved in convergent inputs, are GABAergic.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of glutamate receptor subunits at neurochemically characterized synapses in the entopeduncular nucleus and subthalamic nucleus of the rat.

Glutamatergic neurotransmission in the subthalamic nucleus (STN) and in the output nuclei of the basal ganglia is critical in the expression of basal ganglia function, and increased glutamate transmission in these nuclei has been implicated in the pathology of Parkinson's disease. In order to determine the precise spatial relationship of subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) glutamate receptors to nerve terminals enriched in glutamate or gamma-aminobutyric acid (GABA) in one of the output nuclei, the entopeduncular nucleus (EP), and the STN, postembedding immunolabelling for glutamate receptor subunits and for glutamate and GABA was carried out in the rat. Immunolabelling for the AMPA glutamate receptor subunits 1, 2/3, and 4 (GluR1, GluR2/3, and GluR4) and the NMDA receptor subunit 1 (NR1) was localized predominantly within asymmetrical synapses in both the EP and STN. Quantitative analysis revealed that, on average for the whole population, each of the receptor subunits was evenly distributed along the synaptic specialization. Multiple AMPA receptor subunits and the GluR2/3 and NMDA (NR1) subunits were co-localized within individual synapses. The combination of immunolabelling for glutamate and GABA with the receptor immunolabelling revealed that the majority of axon terminals presynaptic to the receptor-immunoreactive synapses were enriched in glutamate immunoreactivity and were GABA-immunonegative. However, at some NR1- and GluR2/3-positive synapses, the level of glutamate immunoreactivity was low in the presynaptic terminal and, in the STN, some of them were GABA-immunopositive. It is concluded that glutamatergic transmission at individual synapses of different origins in the EP and STN is mediated by a combination ofAMPA and NMDA glutamate receptors.

The output neurones and the dopaminergic neurones of the substantia nigra receive a GABA-containing input from the globus pallidus in the rat.

One of the major pathways of information flow through the basal ganglia is the pallidonigrofugal system. In order to better understand this system in the rat, experiments have been performed to study the topography, synaptic organization, and neurotransmitter content of the pallidonigral projection and to determine whether the pallidonigral neurones make direct synaptic contacts with nigrofugal cells. This was achieved by combining the anterograde transport of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) with the retrograde transport of lectin-conjugated horseradish peroxidase (WGA-HRP), postembedding immunocytochemistry for gamma-aminobutyric acid (GABA), and pre-embedding immunocytochemistry for tyrosine hydroxylase (TH). Following injections of PHA-L in different regions of the lateral part of the globus pallidus, a substantial number of immunoreactive fibres and terminals occurred in the ipsilateral substantia nigra reticulata (SNr). The immunoreactive elements were distributed according to a rostral to medial and caudal to lateral topography. Injections that were restricted to the medial tip of the globus pallidus led to the anterograde labeling of a small number of fibres that were sparsely distributed in the SNr. The most characteristic feature of the pallidonigral fibres was the presence of large varicosities that were often grouped to form pericellular baskets. Injections of WGA-HRP in the ventromedial thalamic nucleus, superior colliculus, or midbrain tegmentum, including the pedunculopontine nucleus, showed that the perikarya and primary dendrites of the output cells of the SNr were often surrounded by the large pallidonigral varicosities. The number of varicosities surrounding a single cell varied from 2-12. Electron microscopic analysis showed that the varicosities contained round or slightly pleomorphic vesicles and numerous mitochondria and that they established symmetrical synaptic contacts. Quantitative measurements revealed that the varicosities had a maximum diameter varying from 0.5 to 2.5 microns and a mean cross-sectional area of 0.76 +/- 0.25 microns 2 (N = 237, mean +/- S.D.). The postsynaptic structures of the pallidonigral varicosities included perikarya (48%), large dendrites (38%), and small dendrites (14%). A large proportion of these postsynaptic targets were retrogradely labeled after injection of WGA-HRP in the ventromedial thalamic nucleus, superior colliculus, or midbrain tegmentum. Postembedding immunocytochemistry was used to show that the pallidonigral axons and terminals in contact with nigrofugal neurones displayed GABA immunoreactivity. The use of a double immunocytochemical method revealed, that in addition to the nondopaminergic SNr output neurones, the dendrites and perikarya of the substantia nigra pars compacta (SNc) receive an input from the globus pallidus.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of synaptic terminals of thalamic or cortical origin in contact with distinct medium-size spiny neurons in the rat neostriatum.

In order to determine what types of neurons in the striatum receive direct synaptic input from corticostriatal and thalamostriatal fibres and whether these afferents converge on individual striatal neurons, double anterograde labelling of axon terminals was combined with Golgi impregnation at both the light and electron microscopic levels. The area of the central neostriatum that receives input from both the parafascicular nucleus of the thalamus and the somatosensory cortex was identified by retrograde transport of a conjugate of horseradish peroxidase and wheat germ agglutinin (HRP-WGA). The same region of the neostriatum was studied in rats that had received multiple electrolytic lesions in the somatosensory cortex and also an injection of HRP-WGA in different parts of the parafascicular nucleus. Sections of this part of the neostriatum were impregnated by the single-section Golgi procedure after revealing anterogradely transported HRP-WGA. Twelve Golgi-impregnated spiny neurons were recovered and examined in the light and electron microscope after gold-toning. Ten of these neurons were typical very densely spiny medium-size neurons and they were all found to receive asymmetric synaptic input on dendritic spines from degenerating corticostriatal boutons. However, even though numerous boutons labelled anterogradely by HRP-WGA from the parafascicular nucleus were found within the dendritic fields of neurons that received cortical input, none of the terminals from the thalamus made synaptic contact with these neurons. Instead, all 96 thalamostriatal boutons studied were found in asymmetric synaptic contact with dendritic shafts of other neurons. Two such neurons that received input from the parafascicular nucleus were Golgi-impregnated and appeared to be medium-size spiny neurons, but they had a lower density of spines than the typical very densely spiny neurons. An independent confirmation that the targets of thalamostriatal neurons originating in the parafascicular nucleus are dendritic shafts was provided by studying the boutons labelled following electrolytic lesioning or injection of the lectin Phaseolus vulgaris-leucoagglutinin (PHA-L) into this nucleus: these boutons were also found to form asymmetric synaptic contacts with dendritic shafts within the neostriatum. It is concluded that although afferents from the somatosensory cortex and from the parafascicular nucleus converge upon the same part of the neostriatum, they probably do not converge upon the same spiny neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Monosynaptic innervation of trigeminal motor neurones involved in mastication by neurones of the parvicellular reticular formation.

In order to determine whether neurones in the parvicellular reticular formation are in direct synaptic contact with motor neurones innervating masticatory muscles, a combined retrograde and anterograde transport study was carried out in the rat at both light and electron microscopic levels. The animals received injections of the retrograde tracers wheat germ agglutinin conjugated to horseradish peroxidase or cholera toxin B conjugated to horseradish peroxidase into the masticatory muscles and of the anterograde tracer biocytin into the ipsilateral parvicellular reticular formation. The trigeminal motor nucleus was then examined for both anterograde and retrograde labelling in the light and electron microscopes. Retrogradely labelled motor neurones were identified in the trigeminal motor nucleus. They were large and their locations within the nucleus depended on the muscle injected. In addition, terminals anterogradely labelled with the biocytin that was injected in the parvicellular reticular formation were identified throughout the motor nucleus. At the electron microscopic level, the retrogradely labelled cells were found to receive input both from distinct types of unlabelled terminals and from terminals that were anterogradely labelled from the parvicellular reticular formation. The labelled terminals comprised one of the four classes of afferent terminals, being 1-2 microns in diameter and densely packed with spherical vesicles. They formed mostly asymmetrical but also symmetrical synapses with the labelled perikarya and dendrites. Anterogradely labelled terminals were also observed to form both symmetrical and asymmetrical synaptic contacts with unlabelled structures in the motor nucleus. It is concluded that neurones in the parvicellular reticular formation form direct synaptic contact with motor neurones of masticatory muscles. This pathway may represent the anatomical substrate by which the reticular formation exerts at least part of its influence on mastication. Since the parvicellular reticular formation receives input from the substantia nigra pars reticulata, it is possible that this pathway represents a system whereby the basal ganglia directly influence orofacial movement.

Monosynaptic cortical input and local axon collaterals of identified striatonigral neurons. A light and electron microscopic study using the Golgi-peroxidase transport-degeneration procedure.

Following the injection of horseradish peroxidase into the ipsilateral substantia nigra, 36 retrogradely labelled neurons in the striatum were characterized (in three rats) by Golgi staining and gold toning: each neuron was of the medium-size, densely spinous type. Prior to the injection of horseradish peroxidase, two of the rats had had lesions placed in the ipsilateral motor cortex, the third rat had had a lesion placed in the ipsilateral frontal and prefrontal cortex. In the electron microscope, degenerating boutons of cortical neurons were found in asymmetrical synaptic contact with the spines of proximal and distal dendrites of all six of the identified striatonigral neurons that were studied. Some of the degenerating boutons were small (diameter 0.1-0.3 micron), while others were larger (1-2 microns). An individual dendrite of a striatonigral neuron was in symaptic contact with very few degenerating boutons. Local axon collaterals in the striatum could be traced from two of the identified striatonigral neurons that received degenerating cortical boutons. These were studied in the electron microscope; their boutons formed symmetrical synapses with spines or dendritic shafts of other striatal neurons. The synaptic boutons contained large, clear, round and pleomorphic vesicles. The postsynaptic targets of these boutons morphologically resemble the dendrites of medium-size spiny neurons. It is concluded that afferents from the cortex make monosynaptic contact with the dendritic spines of medium-size spiny striatonigral neurons and that such neurons have local axon collaterals in the striatum that form synapses with other spiny neurons.