Electron microscopic study of the substantia nigra and the strio-nigral projection in the rat

Modified viral-genetic mapping reveals local and global connectivity relationships of ventral tegmental area dopamine cells

Dopamine cells in the ventral tegmental area (VTADA) are critical for a variety of motivated behaviors. These cells receive synaptic inputs from over 100 anatomically defined brain regions, which enables control from a distributed set of inputs across the brain. Extensive efforts have been made to map inputs to VTA cells based on neurochemical phenotype and output site. However, all of these studies have the same fundamental limitation that inputs local to the VTA cannot be properly assessed due to non-Cre-dependent uptake of EnvA-pseudotyped virus. Therefore, the quantitative contribution of local inputs to the VTA, including GABAergic, DAergic, and serotonergic, is not known. Here, I used a modified viral-genetic strategy that enables examination of both local and long-range inputs to VTADA cells in mice. I found that nearly half of the total inputs to VTADA cells are located locally, revealing a substantial portion of inputs that have been missed by previous analyses. The majority of inhibition to VTADA cells arises from the substantia nigra pars reticulata, with large contributions from the VTA and the substantia nigra pars compacta. In addition to receiving inputs from VTAGABA neurons, DA neurons are connected with other DA neurons within the VTA as well as the nearby retrorubal field. Lastly, I show that VTADA neurons receive inputs from distributed serotonergic neurons throughout the midbrain and hindbrain, with the majority arising from the dorsal raphe. My study highlights the importance of using the appropriate combination of viral-genetic reagents to unmask the complexity of connectivity relationships to defined cells in the brain.

The residence of synaptically released dopamine on D2 autoreceptors

Neuromodulation mediated by synaptically released endogenous transmitters acting in G-protein-coupled receptors (GPCRs) is slow primarily because of multistep downstream signaling. What is less well understood is the spatial and temporal kinetics of transmitter and receptor interaction. The present work uses the combination of the dopamine sensor, dLight, to detect the spatial release and diffusion of dopamine and a caged form of a D2-dopamine receptor antagonist, CyHQ-sulpiride, to rapidly block the D2 autoreceptors. Photoactivation of the CyHQ-sulpiride blocks receptors in milliseconds such that the time course of dopamine/receptor interaction is mapped onto the downstream signaling. The results show that highly localized release, but not dopamine diffusion, defines the time course of the functional interaction between dopamine and D2 autoreceptors, which determines downstream inhibition.

Desensitized D2 autoreceptors are resistant to trafficking

Dendritic release of dopamine activates dopamine D2 autoreceptors, which are inhibitory G protein-coupled receptors (GPCRs), to decrease the excitability of dopamine neurons. This study used tagged D2 receptors to identify the localization and distribution of these receptors in living midbrain dopamine neurons. GFP-tagged D2 receptors were found to be unevenly clustered on the soma and dendrites of dopamine neurons within the substantia nigra pars compacta (SNc). Physiological signaling and desensitization of the tagged receptors were not different from wild type receptors. Unexpectedly, upon desensitization the tagged D2 receptors were not internalized. When tagged D2 receptors were expressed in locus coeruleus neurons, a desensitizing protocol induced significant internalization. Likewise, when tagged µ-opioid receptors were expressed in dopamine neurons they too were internalized. The distribution and lack of agonist-induced internalization of D2 receptors on dopamine neurons indicate a purposefully regulated localization of these receptors.

Localization of substance P in neuronal pathways

The main neuronal systems containing substance P are summarized on the basis of immunohistochemical evidence. The substance P striatonigral projection is one of the most conspicuous of these. Electron microscopic studies using the peroxidase-antiperoxidase technique reveal some heterogeneity in the substance P-immunostained material in the substantia nigra. Immunoreactivity for the peptide is found in terminals establishing both symmetrical and asymmetrical synapses with substantia nigra dendrites. Substance P immunoreactivity in the substantia gelatinosa of the trigeminal nerve and in the skin of the trigeminal territory was found to be depleted after sensory denervation. Electron microscopy showed that in this area of the rat brain substance P-immunoreactive elements are largely associated with dendrites and establish asymmetrical axo-dendritic synapses. Substance P-immunoreactive terminals synapsing with presynaptic dendrites were also observed (i.e. dendrites that themselves are presynaptic to other dendrites). The origin of substance P-containing fibres in the prevertebral ganglia has been investigated in the guinea-pig by combining surgical procedures and immunohistochemistry. Only procedures which disconnected dorsal root ganglia from prevertebral ganglia depleted substance P immunofluorescence in the latter. This substance P-immunoreactive material disappeared after administration of capsaicin. Electron microscopic studies in prevertebral ganglia show that substance P-immunoreactive varicosities establish axodendritic contacts with the sympathetic neurons. These observations provide strong evidence for direct synaptic sensory-autonomic interactions in the prevertebral ganglia involving substance P-containing collaterals of peripheral sensory nerve fibres.

The pars reticulata of the substantia nigra: a window to basal ganglia output

Together with the internal segment of the globus pallidus (GP(i)), the pars reticulata of the substantia nigra (SNr) provides a main output nucleus of the basal ganglia (BG) where the final stage of information processing within this system takes place. In the last decade, progress on the anatomical organization and functional properties of BG output neurons have shed some light on the mechanisms of integration taking place in these nuclei and leading to normal and pathological BG outflow. In this review focused on the SNr, after describing how the anatomical arrangement of nigral cells and their afferents determines specific input-output registers, we examine how the basic electrophysiological properties of the cells and their interaction with synaptic inputs contribute to the spatio-temporal shaping of BG output. The reported data show that the intrinsic membrane properties of the neurons subserves a tonic discharge allowing BG to gate the transmission of information to motor and cognitive systems thereby contributing to appropriate selection of behavior.

Basal somatodendritic dopamine release requires snare proteins

Dopaminergic neurons have the capacity to release dopamine not only from their axon terminals, but also from their somatodendritic compartment. The actual mechanism of somatodendritic dopamine release has remained controversial. Here we established for the first time a rat primary neuron culture model to investigate this phenomenon and use it to study the mechanism under conditions of non-stimulated spontaneous firing (1-2 Hz). We found that we can selectively measure somatodendritic dopamine release by lowering extracellular calcium to 0.5 mm, thus confirming the previously established differential calcium sensitivity of somatodendritic and terminal release. Dopamine release measured under these conditions was dependent on firing activity and independent of reverse transport through the plasma membrane. We found that treatment with botulinum neurotoxins A and B strongly reduced somatodendritic dopamine release, thus demonstrating the requirement for SNARE proteins SNAP-25 and synaptobrevin. Our work is the first to provide such direct and unambiguous evidence for the involvement of an exocytotic mechanism in basal spontaneous somatodendritic dopamine release.

Differential subcellular localization of mGluR1a and mGluR5 in the rat and monkey Substantia nigra

Neurons in the rat substantia nigra (SN) are enriched in group I metabotropic glutamate receptor (mGluR) subtypes and respond to group I mGluR activation. To better understand the mechanisms by which mGluR1 and mGluR5 mediate these effects, the goal of this study was to elucidate the subsynaptic localization of these two receptor subtypes in the rat and monkey substantia nigra. At the light microscope level, neurons of the SN pars reticulata (SNr) displayed moderate to strong immunoreactivity for both mGluR1a and mGluR5 in rats and monkeys. However, mGluR1a labeling was much stronger in monkey than in rat SN pars compacta (SNc) neurons, whereas a moderate level of mGluR5 immunoreactivity was found in both species. At the electron microscope level, the immunoreactivity for both group I mGluR subtypes was primarily expressed postsynaptically, although light mGluR1a labeling was occasionally seen in axon terminals in the rat SNr. Immunogold studies revealed a striking difference in the subcellular distribution of mGluR1a and mGluR5 immunoreactivity in SNr and SNc neurons. Although the bulk of mGluR1a was attached to the plasma membrane, >80% of mGluR5 immunoreactivity was intracellular. Plasma membrane-bound immunoreactivity for group I mGluRs in both SNc and SNr neurons was mostly extrasynaptic or in the main body of symmetric, putative GABAergic synapses. On the other hand, asymmetric synapses either were nonimmunoreactive or displayed perisynaptic labeling. These data raise important questions about the trafficking, internalization, and potential functions of group I mGluRs at extrasynaptic sites or symmetric synapses in the substantia nigra.

Electrophysiological and immunocytochemical characterization of GABA and dopamine neurons in the substantia nigra of the rat

Neurons in the substantia nigra pars reticulata and pars compacta of the rat were studied using a combination of intracellular electrophysiological recording in in vitro and subsequent immunocytochemical double and triple labelling techniques. The neurons recorded in the pars reticulata were identified as either GABA or dopamine neurons: neurons were considered to be GABA neurons if they were immunopositive for glutamate decarboxylase, whereas those neurons which were immunopositive for tyrosine hydroxylase were considered to be dopaminergic. The GABA neurons had short duration action potentials (0.45+/-0.03 ms halfwidth), no apparent rectifying currents, no low threshold calcium spikes, were spontaneously active (7.4+/-3.7 Hz), and could maintain high firing rates. The dopamine neurons had long duration action potentials (1.49+/-0.10 ms), displayed both anomalous inward and transient outward rectifying currents, and more than half (12/17 neurons) displayed a low threshold calcium spike. Their spontaneous firing rate was lower than that of the GABA neurons (2.3+/-1.0 Hz), and they displayed strong frequency adaptation. Morphological reconstruction of neurobiotin-filled neurons revealed that the pars reticulata GABA neurons had more extensive local dendritic arborization than the dopamine neurons from either the pars reticulata or the pars compacta. All of the neurons recorded from the pars compacta were dopamine neurons; they were found not to be different either electrophysiologically or morphologically from pars reticulata dopamine neurons. The electrophysiology of the GABA neurons suggests that input activity is translated linearly to spike frequency. These GABA neurons probably represent the projection neurons of the pars reticulata, and it is thus likely that this basal ganglia output is frequency coded. The close similarity between the dopamine neurons in the pars compacta, which give rise to the nigrostriatal pathway, and those in the pars reticulata supports the notion that the dopamine neurons in these two regions are part of the same neuronal population.

Midbrain dopaminergic neurons in the mouse: computer-assisted mapping

The dopaminergic (DA) neurons in the midbrain play a role in cognition, affect and movement. The purpose of the present study was to map and quantify the number of DA neurons in the midbrain, within the nuclei that constitute cell groups A8, A9 and A10, in the mouse. Two strains of mice were used; the C57BL/6 strain was chosen because it is commonly used in neurobiological studies, and the FVB/N strain was chosen because it is used frequently in transgenic studies. DA neurons were identified, in every fifth 20-microns-thick coronal section, using an antibody against tyrosine hydroxylase. Cell locations were entered into a computer imaging system. The FVB/N strain has 42% more midbrain DA neurons than the C57BL/6 strain; on one side of the brain there were 15,135 +/- 356 neurons (mean +/- S.E.M.) in the FVB/N strain, and 10,645 +/- 315 neurons in the C57BL/6 strain. In both strains, approximately 11% of the neurons were located in nucleus A8 (the DA neurons in the retrorubral field), 38% in nucleus A9 (the DA neurons in the substantia nigra pars compacta, pars reticulata, and pars lateralis), and 51% in nucleus A10 (the DA neurons in midline regions such as the ventral tegmental area, central linear nucleus, and interfascicular nucleus). The number of midbrain DA cells, and their distribution within the three nuclear groups, is discussed with respect to findings in other species.

Cytoarchitecture of the substantia nigra pars lateralis in the opossum (Didelphis virginiana): a correlated light and electron microscopic study

BACKGROUND

The substantia nigra has been divided into three subdivisions. However, the cytoarchitecture of one of these subdivisions, the pars lateralis (SNl), has not been previously examined in detail at the light and electron microscopic levels in any species. In the adult opossum, the three nigral subdivisions can be easily distinguished as distinct, rostrocaudally oriented cell groups separated by neuron-free zones. Thus it was possible to determine the boundaries of the SNl unambiguously. This report covers the results of an examination of the morphology and organization of the SNl in the opossum.

METHODS

Material from 13 opossums was used for this study. Eight of the animals had been previously stained for Nissl substance (n = 4) or impregnated by the Golgi technique (n = 4). The remaining five animals were prepared for electron microscopic studies using standard procedures.

RESULTS

Two cell types were identified on the basis of morphological differences, small and medium-large neurons. Small neurons (10-18 microns long axis) have large nuclei with moderate amounts of heterochromatin and a thin rim of cytoplasm. They have long (up to 500 microns), spine-free dendrites. Medium-large neurons (18-54 microns long axis) have rounded nuclei with electron-lucent nucleoplasm. Few indentations of the nuclear envelope were observed. The surrounding cytoplasm has dense arrays of organelles. Nissl bodies are particularly prominent in the form of pyramids with their bases at juxtanuclear positions and their apices directed toward emerging dendrites. Dendrites of medium-large neurons are long (some > 1 mm in length), are primarily oriented in the frontal plane, and extend along the dorsal surface of or into the cerebral peduncle. Some cells have dendrites that are moderately spinous, whereas other neurons possess sparsely spinous dendrites. Relatively few synaptic profiles are observed to contact somata and proximal dendrites.

CONCLUSION

This report provides added morphological support for the idea that the SNl is a distinct subdivision of the substantia nigra, a distinction previously made on the basis of the physiologically characterized relationship between the lateral substantia nigra and orienting behaviors and seizure-related function.

Extrapyramidal symptoms with selective serotonin reuptake inhibitors

BACKGROUND

Several case reports in the literature suggest that selective serotonin reuptake inhibitors can produce extrapyramidal symptoms.

METHODS

Computerised literature searches were used to identify reports on extrapyramidal symptoms and serotonin reuptake inhibitors. Subsequently, manual searches were made for articles in which there was any indication of the mechanisms responsible for these extrapyramidal symptoms.

RESULTS

Only a few reports could be identified in which serotonin reuptake inhibitors were implicated in extrapyramidal symptoms in some patients.

CONCLUSIONS

Evidence is discussed from preclinical and clinical studies suggesting the interaction between serotoninergic and dopaminergic neurotransmitter system, as a possible mechanism for production of extrapyramidal symptoms.

Convergent synaptic input from the neostriatum and the subthalamus onto identified nigrothalamic neurons in the rat

The two major afferents of the substantia nigra pars reticulata are the subthalamic nucleus and the striatum. Stimulation of these afferents has opposing physiological effects on the output neurons of the substantia nigra pars reticulata. In order to better understand the role of these afferents in the flow of information through the basal ganglia and to better understand the ways in which they might interact, experiments have been performed to test the possibility that single-output neurons of the substantia nigra pars reticulata receive convergent synaptic input from the subthalamic nucleus and the neostriatum. To address this, rats received iontophoretic deposits of the anterograde tracer Phaseolus vulgaris leucoagglutinin in the subthalamic nucleus, injections of the anterograde tracer biocytin in the neostriatum and injections of the retrograde tracer horseradish peroxidase conjugated to wheat-germ agglutinin in the ventral medial nucleus of the thalamus. Following appropriate survival times the animals were perfusion-fixed and sections of the substantia nigra were processed to reveal the transported tracers and prepared for electron microscopy. Light microscopic examination revealed that the substantia nigra contained rich plexuses of anterogradely labelled subthalamic and striatal terminals, as well as many retrogradely labelled nigrothalamic neurons. The anterogradely labelled terminals were often seen apposed to the retrogradely labelled neurons. In the electron microscope the subthalamic terminals were seen to form asymmetrical synaptic contacts (subthalamic type 1) with the identified nigrothalamic neurons as well as unlabelled perikarya and both proximal and distal dendrites. In confirmation of previous findings, the striatal terminals made symmetrical synaptic contact with the nigrothalamic neurons as well as unlabelled neurons. In areas of overlap between the two classes of terminals, identified nigrothalamic neurons and unlabelled nigral neurons were found to receive convergent synaptic input from the subthalamic nucleus and the neostriatum. In addition to the anterogradely labelled subthalamic terminals that formed asymmetrical synaptic specializations, a second, much rarer class was also observed (subthalamic type 2). These terminals were much larger and formed symmetrical synapses; several lines of evidence suggest that they originated not in the subthalamic nucleus but in the globus pallidus. These terminals were found to make synaptic contacts with identified nigrothalamic neurons and non-labelled neurons and to form convergent synaptic contacts with subthalamic type 1 terminals and striatal terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Synaptic organization of GABAergic inputs from the striatum and the globus pallidus onto neurons in the substantia nigra and retrorubral field which project to the medullary reticular formation

Anatomical tract-tracing and immunohistochemical techniques involving correlated light and electron microscopy were used to determine whether the descending striatal and pallidal afferents to the substantia nigra pars reticulata converge onto individual neurons projecting to the pontomedullary and medullary reticular formation in the rat. Injections of biocytin into the ventrolateral region of the striatum and Phaseolus vulgaris-leucoagglutinin into the ventrolateral and caudal regions of the globus pallidus led to overlapping anterogradely labelled terminal fields within the dorsolateral substantia nigra pars reticulata. These terminal fields were punctuated by neurons which had been retrogradely labelled following injections of wheatgerm agglutinin conjugated to horseradish peroxidase into the lateral pontomedullary reticular formation. The anterogradely labelled striatal and pallidal terminals displayed different morphological characteristics; the striatal terminals were small and diffusely distributed throughout the neuropil without any particular neuronal association whereas the pallidal terminals were large and formed pericellular baskets around the perikarya of retrogradely and non-retrogradely labelled nigral neurons. In areas of the substantia nigra where there was an overlap between the two terminal fields, individual retrogradely labelled nigroreticular neurons were found to be apposed by both sets of anterogradely labelled terminals. Electron microscopic analysis revealed that the striatonigral and pallidonigral terminals displayed different ultrastructural features, the striatal terminals were small, contained few mitochondria and formed symmetric synaptic contacts predominantly with the distal dendrites of nigroreticular neurons whereas the pallidal terminals were large, contained numerous mitochondria and formed symmetric synaptic contacts preferentially with perikarya and proximal dendrites of nigroreticular neurons. Post-embedding immunohistochemical staining revealed that both striatonigral and pallidonigral terminals, some which formed synaptic contact with nigroreticular neurons, displayed GABA immunoreactivity. Examination of twelve retrogradely labelled neurons in the electron microscope revealed that all received synaptic inputs from both sets of anterogradely labelled terminals. In addition to the substantia nigra pars reticulata, neurons of the retrorubral field were also retrogradely labelled following injections of wheatgerm agglutinin conjugated to horseradish peroxidase into pontomedullary reticular formation. These retrorubroreticular neurons were part of a continuum of labelled cells which extended from the dorsolateral substantia nigra pars reticulata caudally into the retrorubral field. When combined with anterograde tracing methods it was found that the retrorubroreticular neurons received synaptic inputs from pallidal terminals which were morphologically similar to the pallidonigral terminals and formed symmetric synapses with the neuronal somata and proximal dendrites. In contrast to nigroreticular neurons, the stratonigral terminals were not seen in contact with retrorubroreticular cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Nigrostriatal dopaminergic cell activity is under control by substantia nigra of the contralateral brain side: electrophysiological evidence

The biochemical balance between right- and left-ascending DA systems is an essential factor to regulate behavioral lateralization. However, there is no electrophysiological evidence for the regulation of interhemispheric DA systems. In the present paper we report electrophysiological evidence supporting the hypothesis that the A9 DA cells are under control of the contralateral substantia nigra. The activity of more than 80% of the A9 cells recorded was affected by contralateral SN stimulation. This is a very high proportion because the previously reported response of A9 cells to ipsilateral caudate stimulation is proportionally lower. The potency of a stimulus, estimated as the number of action potentials induced or inhibited by each electrical stimulation of the contralateral substantia nigra or by the percentage of modification in the number of action potentials induced or inhibited in relation to the spontaneous potential expected, was also high. The response to the contralateral stimulation was complex. Fifty-four percent of all the DA cells studied showed more than a single response. Forty-four percent showed at least one stimulation and at least one inhibition. Because 1) the percentage of cells with at least one stimulation (70%) was higher than the percentage of cells with at least one inhibition (56%), and 2) the potency of stimulations was higher than the inhibition potency, the present data provide evidence that contralateral control of A9 cells is mainly excitatory. The percentage of cells activated by contralateral stimulation was high, between 30 ms and 220 ms and between 400 ms and 700 ms. The probability of inhibition was higher than the probability of activation between 10 ms and 30 ms and between 230 and 380 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Convergence of synaptic inputs from the striatum and the globus pallidus onto identified nigrocollicular cells in the rat: a double anterograde labelling study

Two major sources of afferent synaptic inputs to projection neurons in the rat substantia nigra reticulata are the striatum and the globus pallidus. In order to understand better the functional relationships between these two afferents in the control of the activity of nigrofugal neurons, experiments have been performed to test the possibility that single nigrofugal cells receive convergent synaptic inputs from the striatum and the globus pallidus. To address this question we have used two different approaches. First, we have developed a double anterograde labelling technique suitable for both light and electron microscopy and combined this procedure with the retrograde transport of lectin-conjugated horseradish peroxidase in order to retrogradely label the nigrocollicular cells. Second, we have combined the anterograde transport of Phaseolus vulgaris-leucoagglutinin from the globus pallidus and immunocytochemistry for DARPP-32 as a marker for the striatal terminals, with the retrograde transport of lectin-conjugated horseradish peroxidase from the superior colliculus. In the double anterograde labelling experiment, biocytin was injected in the striatum, Phaseolus vulgaris-leucoagglutinin in the globus pallidus and lectin-conjugated horseradish peroxidase in the superior colliculus. Following these injections, rich plexuses of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals were found in the ventral two-thirds of the substantia nigra. The biocytin-positive terminals (striatonigral) were generally small and formed rich plexuses without any apparent neuronal association whereas the Phaseolus vulgaris-leucoagglutinin-labelled terminals (pallidonigral) were much larger and formed baskets around the perikarya of retrogradely and non retrogradely labelled cells in the substantia nigra reticulata. In areas of the substantia nigra reticulata where the fields of biocytin- and Phaseolus vulgaris-leucoagglutinin-labelled terminals overlapped, the perikarya and the proximal dendrites of retrogradely and non retrogradely labelled cells were found to be apposed by numerous Phaseolus vulgaris-leucoagglutinin-immunoreactive pallidonigral terminals and a few biocytin-labelled striatonigral terminals. In the sections prepared for electron microscopy, the biocytin was localized using 3,3'-diaminobenzidine tetrahydrochloride whereas Phaseolus vulgaris-leucoagglutinin was localized using benzidine dihydrochloride. It was thus possible to distinguish the biocytin- from the Phaseolus vulgaris-leucoagglutinin-labelled terminals in the electron microscope by the texture of the reaction product associated with them.4+ Examination of 231 biocytin-labelled (striatonigral) terminals and 105 Phaseolus vulgaris-leucoagglutinin-immunoreactive (pallidronigral) terminals revealed that the striatonigral terminals were generally small, contained few mitochondria and formed symmetric synapses predominantly with the distal dendrites (77%) and far less frequently with the perikarya (3%) of substantia nigra reticulata cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The GABA and substance P input to dopaminergic neurones in the substantia nigra of the rat

In order to examine the synaptic input to dopaminergic neurones in the substantia nigra from GABAergic terminals and terminals that contain substance P, double and triple immunocytochemical studies were carried out at the light and electron microscopic levels in the rat. In a first series of experiments sections of the substantia nigra were incubated to reveal axon terminals containing either substance P or glutamate decarboxylase and then incubated to reveal dopaminergic neurones using tyrosine hydroxylase immunocytochemistry. Examination of this material in the light microscope revealed that many substance P- and glutamate decarboxylase-immunoreactive boutons were associated with the dopaminergic cells. In the electron microscope it was found that the perikarya and dendrites of the dopaminergic neurons received symmetrical synaptic input from terminals that displayed immunoreactivity for substance P or glutamate decarboxylase. A small proportion of the substance P-positive boutons formed asymmetrical synapses. In a second series of experiments sections of the substantia nigra were processed by the pre-embedding immunocytochemical technique for tyrosine hydroxylase and then the post-embedding immunogold technique for gamma-aminobutyric acid (GABA). Examination in the electron microscope revealed that the tyrosine hydroxylase-positive neurons received symmetrical synaptic input from many GABA-positive terminals. Quantitative analyses demonstrated that a minimum of 50-70% of all boutons afferent to the dopaminergic neurones display glutamate decarboxylase or GABA immunoreactivity. Triple immunocytochemical studies i.e. pre-embedding immunocytochemistry for tyrosine hydroxylase and substance P, combined with post-embedding immunogold staining for GABA, revealed that some of the substance P-immunoreactive boutons that were in contact with the dopaminergic neurones also displayed GABA immunoreactivity. In a third series of experiments the combination of anterograde transport of lectin-conjugated horseradish peroxidase or biocytin with post-embedding GABA immunocytochemistry demonstrated that at least one of the sources of GABA-containing terminals in the substantia nigra is the striatum. The results of the present study: (1) demonstrate that dopaminergic neurones in the substantia nigra receive symmetrical synaptic input from GABAergic and substance P-containing terminals, (2) show that a proportion of these terminals contain both substance P and GABA and (3) suggest that the major synaptic input to dopaminergic neurones is from GABAergic terminals and that a part of this innervation is derived from the striatum.

Two different types of dynorphin-A-immunoreactive terminals in rat substantia nigra

The opioid peptide dynorphin A (1-17) is the third transmitter identified in the striatonigral projection, the other two being gamma-aminobutyric acid (GABA) and substance P. The ultrastructural features of the dynorphinergic terminals in substantia nigra/pars reticulata were studied using pre-embedding immunocytochemistry with the classical peroxidase-antiperoxidase-diaminobenzidine-method; these features were compared with GABAergic boutons visualized with an immunogold method. Two distinct types of dynorphin-A-immunoreactive boutons could be identified: (1) type A (81%) possessing characteristics similar to the GABAergic nerve endings in this region, i.e., large pleomorphic vesicles and symmetric synaptic contacts; (2) type B (19%) displaying asymmetric synaptic zones and small, mostly round vesicles. These results are in agreement with physiological studies suggesting a dual action of dynorphin A in substantia nigra.

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)

Cholinergic somata and terminals in the rat substantia nigra: an immunocytochemical study with optical and electron microscopic techniques

The topographical distribution, histochemical characteristics, and anatomical relationships of the cellular elements containing choline acetyltransferase (ChAT) immunoreactivity, demonstrated with specific monoclonal antibodies to ChAT following the unlabelled antibody peroxidase-antiperoxidase (PAP) procedure at the optical and electron microscopic levels, were investigated in the rat substantia nigra (SN). Scarce, large (20-30 microns in maximum soma extent) cholinergic cell bodies and processes were found within the boundaries of the SN, in the borders of the pars compacta and pars reticulata, principally at caudal levels. Occasionally, cholinergic neurons were also found at intermediate levels of the SN, in the borders of the pars reticulata and pars lateralis. Cytologically, these large cells resembled ChAT-positive neurons localized in other areas of the central nervous system (CNS) of the rat--for example, the pontomesencephalotegmental (PMT) cholinergic complex (Ch5-Ch6) and the nucleus basalis of Meynert (nbM) (Ch4). Histochemically, ChAT-positive cells in the SN were characterized by their ability to utilize the reduced cofactor nicotinamide adenine dinucleotide phosphate (NADPH). Identified ChAT-positive neurons in the light microscope were subsequently studied in the electron microscope. All cholinergic neurons in the SN share essentially the same ultrastructural characteristics. The copious cytoplasm was rich in organelles with large lipofuscin granules. The synaptic input onto cell bodies and their dendrites was studied in serial sections. Synaptic contacts onto the perikarya and proximal dendrites were sparse and of asymmetric type. Both symmetric and asymmetric synaptic specializations onto ChAT-positive distal dendrites were detected. Asymmetric synaptic contacts onto cell bodies and dendrites were often defined by the presence of subjunctional dense bodies associated with the postsynaptic membrane. The pattern of the synaptic input to these cells differs strikingly from that onto unlabelled neighboring neurons. The perikarya and dendrites of the latter were characteristically covered with synaptic boutons. Scarce immunoreactive terminals in asymmetric synaptic contact with unlabelled dendritic profiles were also detected in portions of SN compacta with no ChAT-positive cells. Extranigrally located ChAT-positive cells of the PMT cholinergic complex were also examined in the electron microscope for comparison purposes. These cells exhibited, on the basis of their morphology and synaptic input pattern, very similar characteristics to those shown by SN cholinergic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron microscopic localization of cholinergic terminals in the rat substantia nigra: an immunocytochemical study

The presence of cholinergic terminals in the substantia nigra (SN) of the rat was investigated under the electron microscope using a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine (ACh)-synthesizing enzyme, following the unlabelled antibody peroxidase-antiperoxidase (PAP) procedure. ChAT-immunoreactive terminals were found making synaptic contacts with unlabelled dendrites in the SN pars compacta (SNC). Synaptic contacts established between cholinergic boutons and immunonegative dendrites were observed in serial sections to be of asymmetric type. The unlabelled postsynaptic dendrites to immunoreactive terminals displayed similar morphological aspects to typical dopamine-containing dendrites of the SN. Results of this study provide fine ultrastructural neurochemical support for the existence of a cholinergic innervation of the rat SNC and are consistent with the reported excitatory action of ACh on SNC dopaminergic neurons.

Human ventral mesencephalic xenografts to the catecholamine-depleted striata of athymic rats: ultrastructure and immunocytochemistry

On the basis of animal studies, grafts of fetal human dopaminergic cells have been suggested as a therapy for Parkinson's disease. The purpose of this study was to characterize the ultrastructure and immunocytochemistry of human ventral mesencephalic xenografts placed into the catecholamine-depleted striata of athymic "nude" rats. Human fetal tissue was obtained from tissue fragments derived from elective abortions during the first trimester of pregnancy. Small pieces of the basal mesencephalon were grafted into the catecholamine-depleted striata of four athymic nude rats. The rats were allowed to survive from 3 to 6 months after grafting; following fixation, the striatal tissue containing the grafts was labeled with antibodies against tyrosine hydroxylase and serotonin. Immunocytochemistry revealed tyrosine-hydroxylase-like-immunoreactive (THLI) and serotoninlike-immunoreactive (5HTLI) cell bodies within the human grafts. Both 5HTLI and THLI fibers crossed the graft-host interface and innervated the previously lesioned striatum. Both types of fibers also entered the host cortex from the adjacent human graft. At the ultrastructural level, THLI and 5HTLI fibers and synaptic terminals were observed in the host neuropil. THLI and 5HTLI dendrites and axon terminals were also observed in the neuropil of the grafts themselves. THLI axon terminals are not normally present in the substantia nigra. The results of our study indicate that human xenografts can survive in the neuropil of the host striatum and form morphologically appropriate synapses within the host brain.

Immunocytochemical demonstration of GABAergic synaptic connections in rat substantia nigra after different lesions of the striatonigral projection

Vibratome sections of rat substantia nigra (SN) topically injected with colchicine were processed for glutamate decarboxylase (GAD) immunocytochemistry to reveal GABAergic neurons using electronmicroscopic procedures. Both, GAD-immunoreactive neurons and non-immunoreactive neurons receive a dense innervation of GAD-immunoreactive nerve terminals. In an attempt to clarify the origin(s) of this GABAergic input, the projections between caudate-putamen (CP) and SN were lesioned by circumscribed ibotenic acid injections in CP, or by complete hemitransection either at the level of the globus pallidus or at the frontal pole of SN. In addition, the axonal transport was blocked by local injection of colchicine. After survival times from 40 h to 7 days, the interrelations of GAD-immunoreactive neurons and of unstained neurons with degenerated and preserved boutons were investigated. Striatal terminals contact GAD-negative (presumably dopaminergic) neurons, and, at least as frequently, the GABAergic pars reticulata neurons. Numerous GAD-immunoreactive boutons are apparently intact after the different types of lesions; also the spared GABAergic boutons synapse on both, GAD-positive and GAD-negative neurons. Thus, at the level of SN the striatonigrostriatal loop as well as the striatonigrothalamic (-tectal) projections are under the control of inhibitory axon collaterals of the GABAergic pars reticulata neurons.

The cholinergic innervation of the rat substantia nigra: a light and electron microscopic immunohistochemical study

Putative cholinergic axons and synaptic endings were demonstrated in the substantia nigra (SN) of the rat by light and electron microscopy on the basis of the localization of choline acetyltransferase (ChAT) immunoreactivity. The distribution of ChAT immunoreactivity in the SN as demonstrated by light microscopy revealed a modest network of ChAT-immunoreactive beaded axons in the SNc, in comparison to a relatively sparse distribution in the SNr. These axonal profiles were most dense in the middle of the rostral-caudal extent of the SNc and appeared to be concentrated in the middle third of the medial-lateral extent. By electron microscopy, unmyelinated, small diameter (0.25 micron) ChAT-immuno-reactive axons were observed interspersed among numerous other non-immunoreactive axons in the SNc. ChAT-immunoreactive synaptic endings were observed in juxtaposition to small caliber (0.5 micron) non-immunoreactive dendrites, and contained numerous spheroidal synaptic vesicles and occasional mitochondria. Synaptic contact zones were characterized by an accumulation of synaptic vesicles along the presynaptic membrane, and a prominent postsynaptic densification producing an asymmetrical pre-/postsynaptic membrane profile typical of excitatory synapses. These findings provide direct evidence for a cholinergic innervation of the SN, and suggest that this input may have an excitatory effect on neuronal elements in the SNc.

Efferent projections of the subthalamic nucleus in the rat: light and electron microscopic analysis with the PHA-L method

Efferent projections of rat subthalamic nucleus were studied by use of the axonal transport of phaseolus vulgaris-leucoagglutinin (PHA-L), and the results were analyzed with light and electron microscopes. PHA-L injections in the subthalamic nucleus (STH) resulted in heavy labeling of fiber plexus with en passant boutons and terminals in the pallidal complex, i.e., the entopeduncular nucleus (EP), the globus pallidus (GP) and the ventral pallidum (VP), and the substantia nigra pars reticulata (SNR). Labeling in GP was characterized by two distinct bands of labeled terminals oriented dorsoventrally, whereas labeling in SNR was patchy. STH efferents to the pallidum and SNR displayed a mediolateral topographic organization. With regard to dorsoventral organization, projections to GP were inverted, but those to SNR were not. There were moderate projections to the neostriatum and sparse projections to the frontal cortex, substantia innominata, substantia nigra pars compacta (SNC), pedunculopontine tegmental nucleus, ventral part of the central gray matter including the dorsal raphe nucleus, and the mesencephalic and pontine reticular formation. PHA-L injections in the zona incerta and the lateral hypothalamic area resulted in fiber and terminal labelings in many structures, including the basal forebrain, EP, SNC, and other brainstem areas that overlap with some of the terminal sites of STH projections. Ultrastructural observations of PHA-L labeled processes in GP and SNR revealed that STH terminals in both structures contained small pleomorphic vesicles and formed asymmetrical contacts. These contacts were mainly on dendritic shafts, but some were on somata. It also was observed that the myelinated axons of STH neurons lost their myelin after reaching their target areas and the synaptic boutons arose from relatively thin unmyelinated axons.

The striatonigral projection and nigrotectal neurons in the rat. A correlated light and electron microscopic study demonstrating a monosynaptic striatal input to identified nigrotectal neurons using a combined degeneration and horseradish peroxidase procedure

In a light and electron microscopic study of the substantia nigra of the rat, the distribution and morphology of nigrotectal neurons and the pattern of termination of striatonigral fibres have been examined following the placement of horseradish peroxidase injections in the superior colliculus and kainic acid lesions in the dorsal striatum. In confirmation of previous findings, nigrotectal neurons which had been identified by the retrograde transport of horseradish peroxidase from the superior colliculus had mainly medium sized somata, varied from fusiform to stellate in shape and were found in mainly ventral regions of the rostral two-thirds of the substantia nigra pars reticulata. On electron microscopic examination, single and multiple (from two to six) degenerating striatonigral boutons were found in synaptic contact with the soma, proximal mainstem dendrites and small dendrites (but mainly on small dendrites) of labelled nigrotectal and unlabelled nigral neurons in the ventral region of the pars reticulata. In addition, a small number of degenerating striatonigral boutons formed axoaxonic synapses with degenerating or normal boutons which were presynaptic to nigral dendrites. Almost all of the identified striatonigral synapses were of the symmetrical type, although a few degenerating boutons established asymmetrical synaptic contacts on unlabelled dendrites. These findings provide evidence of a monosynaptic input from the dorsal striatum to nigrotectal projection neurons in the substantia nigra and thus demonstrate the existence of a bineuronal pathway from the striatum through the substantia nigra to the superior colliculus. The possible significance of the pattern of termination of striatonigral fibres in the substantia nigra is discussed with reference to the known dendritic arborization of nigral neurons.

Cytoarchitectonics of substantia nigra grafts: a light and electron microscopic study of immunocytochemically identified dopaminergic neurons and fibrous astrocytes

Maturation of dopaminergic (DA) neurons and astroglia was studied in transplants of the substantia nigra grown for up to 7 months in the brain of rats. The investigation had three specific aims. The first was to observe effects of different transplant positions on the longevity of DA neurons. Second, the grafts were examined for changes of synaptic interactions and associations between DA neurons and astroglia. Third, an answer was sought to the question whether transplanted DA neurons migrate into the adjacent host brain. The grafts were taken from the ventral mesencephalon of rat embryos of different ages (day 14 to 18 of gestation) and placed into the cerebral cortex, tectum, cerebellum, or ventricles of newborn host animals. Following different times of survival the immunocytochemical localization of tyrosine hydroxylase (TH) and of glia filament protein (GFA) in the transplants were observed. In all of the transplantation sites, except for one, neurons of different morphologies that contained TH were found in the grafts. The cerebellar white matter of the host brain failed to support the long-term survival of DA neurons. The overall structure of mature substantia nigra grafts had some resemblance to intact substantia nigra (SN). On the ultrastructural level, it was found that morphological expression of some immature features of DA neurons, such as glial sheaths, somatic spines, and lack of oligodendroglia, persisted in mature grafts. Specific associations of DA neurons and astroglia in the grafts suggested that the cytoarchitectonic appearance of a given brain region may be related to the existence of particular neuron glia relationships. In contrast to intact SN, transplants revealed deficiencies in unlabeled pleomorphic boutons and contained some TH-immunoreactive terminals. Migration of DA neurons and their processes into the adjacent host brain was rarely observed.

Electron microscopic analysis of the mesencephalic ventromedial tegmentum in the cat

We have studied the normal ultrastructure of the ventral mesencephalic tegmentum (VMT) in the cat, particularly the morphology and distribution of presynaptic terminals and the types of synaptic junctions. The following subnuclei of the region were examined: n. linearis rostralis (LR), n. paranigralis (PN), and n. interfascicularis (IF). The qualitative and quantitative data revealed significant ultrastructural differences between these subnuclei. Each subnucleus had a characteristic dendritic structure. In LR the dendrites were nonspinous and cylindrical and had presynaptic terminals randomly distributed over their surface. In PN we observed varicose dendrites with spines; the presynaptic terminals formed clusters on the narrow segments of the dendrites and around the spines. Dendrodendritic synapses were also observed in this nucleus. In IF, there was an internal division regarding dendritic structure: in the rostral part of the nucleus there were cylindrical dendrites while in the caudal part irregularly shaped dendrites bearing long spines were found. In IF and LR some of the cylindrical dendrites were seen to be in direct contact with the basal lamina of blood vessels. Four types of presynaptic terminals were distinguished by the morphology of their vesicles, and the proportion of each type in the total terminal population was determined. On this basis the compositions of the presynaptic terminal population in the three subnuclei were found to be very similar. Most terminals contained clear, round vesicles (62.6%), or both clear and dense-cored vesicles (35.1%). Few terminals were seen with dense-cored vesicles only (1.4%) or with pleomorphic vesicles (0.9%). The majority of synapses in the VMT were found to have symmetrical densities. LR had twice as many asymmetrical synapses as the other two subnuclei. Eighty percent of the terminals formed synapses with dendrites, although axosomatic and axoaxonic synapses were also seen. The density of the terminals was significantly different for each subnucleus: 191/1,000 micrometers 2 in IF, 120/1,000 micrometers 2 in PN, and 81/1,000 micrometers 2 in LR. These data indicate that while the subnuclei of the VMT receive morphologically similar afferents, each has a unique way of processing the information provided by them, through a different internal circuitry.

Ultrastructural localization of acetylcholinesterase in substantia nigra: a comparison between rat and guinea pig

The distribution and ultrastructural localization of acetylcholinesterase (AChE) was examined in the substantia nigra of rat and guinea pig. Although the pars compacta, in both species, is clearly defined when stained with thionin, there is an apparent discrepancy in the distribution of AChE at the light microscope level. In the rat substantia nigra the enzyme appears to be concentrated mainly in the pars compacta, whereas in the guinea pig the AChE seems homogeneous throughout the entire substantia nigra. Ultrastructural studies, however, reveal a close correspondence in the morphology of rat and guinea pig substantia nigra. The subcellular localization of AChE is also similar. The between-species discrepancy seen with the light microscope was attributed to relative differences in level of intensity of staining for AChE. In both rat and guinea pig, the enzyme is localized in two types of neuron and in the extracellular space. The ultrastructural distribution of AChE is discussed with reference to neurochemical studies on its release from nigral neurons.

The ultrastructural morphology of the subthalamic-nigral axon terminals intracellularly labeled with horseradish peroxidase

The labeled axons of neurons intracellularly injected with horseradish peroxidase (HRP) in the rat subthalamic nucleus (STH) were studied with electron microscopy. The main axons and the efferent daughter branches were all myelinated. The morphology of the intrinsic axon terminals within STH was obscured by the dark HRP reaction products, but the labeled efferent STH terminals in the substantia nigra (SN) were revealed to contain small oval vesicles and formed asymmetrical synapses with dendrites of SN neurons.

Comparative morphology of the substantia nigra and ventral tegmental area in the monkey, cat and rat

Four types of neurons were identified in the substantia nigra (SN) of the monkey, cat, and rat. The compacta-type neurons, characterized by unevenly distributed and intensely stained Nissl substance, display many shapes and sizes. The reticulata-type neurons, characterized by the presence of discrete Nissl bodies, are triangular or round. The intermediary-type neurons contain less intensely stained but more diffusely distributed Nissl substance. These triangular or fusiform neurons have thinner processes than the compacta- and reticulata-type cells. The globular-type neurons, characterized by a high nuclear/cytoplasmic ratio, are much smaller than the three other types of SN neurons. The total number of neurons of the SN, which is much greater in the macaque (n=73,508) than in the cat (n=38,366) and the rat (n=22,532), is comprised mainly of the compacta type neurons (n=62,624; 22,323; and 9.925 in the three species, respectively). The reticulata-type neurons are more abundant in the cat, and the intermediary and globular types are more numerous in the rat. The compacta-type neurons have a particular distribution in each species. The ventral tegmental area (VTA) contains numerous globular-type neurons and a number of compacta-like or transitional type neurons which constitute the foyer pédiculaire of the central linear nucleus and the paranigral nucleus. The rostral linear nucleus is unique to the cat brain.

The cytology of dopaminergic and nondopaminergic neurons in the substantia nigra and ventral tegmental area of the rat: a light- and electron-microscopic study

The results of this study support the conclusion that dopaminergic cells can be distinguished from non-dopaminergic cells, at both the light- and electron-microscopic level, by cytological features, and particularly by the pattern of Nissl substance. In both the substantia nigra and the ventral tegmental area, two main categories of cell type can be identified in Nissl preparations: (1) dark-staining, basophilic cells with large masses of Nissl substance and (2) light-staining cells with more translucent cytoplasm. The following findings provide evidence that the basophilic cells of both substantia nigra and ventral tegmental area are the dopaminergic cells. (1) There is a good correlation between the topographic distribution of basophilic cells and that of dopaminergic cells mapped by both histofluorescence and immunohistochemical methods. (2) After unilateral destruction of the dopaminergic neurons by intracerebral injection of 6-hydroxydopamine in the dopaminergic pathway, the basophilic cells in the substantia nigra and ventral tegmental area disappeared on the lesion side, while the lighter-staining cells appeared unaffected. (3) In normal rats, and in rats with unilateral 6-hydroxydopamine lesions, intraventricular injection of [3H]norepinephrine was used for specific labeling of dopaminergic neurons. In autoradiograms of semithin sections, such labeling was observed only in dark-staining and not in light-staining cells, and in cases of unilateral 6-hydroxydopamine lesion was totally absent on the lesion side. Electron-microscopy showed much of the cytoplasm of the basophilic dopaminergic cells to be densely filled with free ribosomes associated with large, well organized complexes of rough endoplasmic reticulum. The cytoplasm of the light, non-dopaminergic cells contains only sparse free ribosomes and small, widely spaced aggregates of rough endoplasmic reticulum. Both cell types occur in a similar variety of size and shape.

A theory of the functional organization of the neostriatum and the neostriatal control of voluntary movement

A theory of the intrinsic, functional organization of the neostriatum and the neostriatal control of voluntary movement is presented. The cell types of neostriatum are described in accordance with the classification scheme of Pasik, Pasik and DiFiglia (ref. 204) and their suspected neurotransmitters are identified. The functional relations between the afferent projections to neostriatum and the intrinsic elements of neostriatum are described. A division of the neostriatal efferents into two functional cell systems is conceived, in which one efferent system, the Spiny I cell matrix, is thought to be represented by a lateral inhibitory network comprised of the common Spiny I neurons which inhibit their targets, while another is termed the Spiny II cell cluster and consists of the far less numerous excitatory efferents of neostriatum (the Spiny II cells) and is represented by clusters of neurons, the members of each cluster consisting of one excitatory efferent neuron and three interneurons. The implications of this view for neostriatal input--output relations are discussed, and the theory is developed by reference to a variety of converging lines of empirical evidence, and is used to interpret the effects of various pharmacologic strategies which have been employed to achieve symptom management in disorders of voluntary movement. In the course of this review, there emerges an integrative theory of the principles governing the neostriatal control of voluntary movement.

Dendro-dendritic synapses in substantia nigra: descriptions based on analysis of serial sections

Dendro-dendritic synapses were studied in serial sections of rat substantia nigra in subjects pretreated with intraventricularly administered 5-hydroxydopamine, a monoaminergic synaptic label. These synapses had symmetrical membrane thickenings and small clusters of heavily labeled pleomorphic vesicles in the presynaptic dendrites, which were assumed to extend from the dopaminergic cells of pars compacta. Dendro-dendritic synapses were seen in the pars compacta of the substantia nigra, but were not found in blocks trimmed to include only the nondopaminergic pars reticulata. The presynaptic dendrites did not receive other synapses near the sites of dendro-dendritic synapses. These dendrites were frequently apposed to other dendrites for long distances with no intervening glial processes. In many cases, several adjacent dendro-dendritic contacts were made by a single presynaptic dendrite onto several different postsynaptic dendrites. Presynaptic dendrites did not participate in reciprocal synapses, serial synapses, or dendro-axonic synapses. Presynaptic and postsynaptic dendrites engaged in dendro-dendritic synaptic contact were of similar appearance and both had cross-sectional diameters of 0.23-1.9 microns. In several cases, label could also be detected in the postsynaptic dendrite in cisternae of smooth endoplasmic reticulum, providing evidence for dendro-dendritic synapses between dopaminergic neurons. Release of dopamine from dopaminergic dendrites and their role in the control of neuronal activity in substantial nigra are discussed.

Noradrenaline and 5-hydroxytryptamine modulation of brain dopamine function: implications for the treatment of Parkinson's disease

1 Dopamine deficiency in the brain is the prime biochemical deficit in Parkinson's disease, but loss of noradrenaline and 5HT also may contribute. 2 In rats, 5HT-containing neurones originating from the dorsal and median raphe nuclei innervate forebrain dopamine-containing areas so as to impose an inhibitory regulatory tone on dopamine function. However, this interaction between brain dopamine and 5HT-containing neuronal systems is complex, and the effect produced appears dependent on the relative activity of each system. 3 Anatomical evidence for innervation of dopamine-containing brain regions by noradrenaline fibres in the rat is scanty, but functional studies suggest the existence of inputs which facilitate dopamine function. 4 Drug therapy designed to increase or decrease brain 5HT function has had no consistent effect in Parkinson's disease. 5 Manipulation of brain noradrenergic activity in Parkinson's disease had little effect, although the noradrenaline precursor L-threo-DOPS may reduce freezing attacks. 6 Until more specific drug molecules are available the role of brain noradrenergic and 5HT mechanisms in Parkinson's disease remains uncertain.

The postnatal development of the substantia nigra: a light and electron microscopy study

The early postnatal development of neurons, dendrites and synaptic connectivity in kitten substantia nigra (SN) was studied by light and electron microscopy. The compact and reticular divisions of the SN are present at birth but boundaries are indistinct. Most nigral neurons stain deeply in routine histological sections and their diameters increase slightly with age. Ultrastructurally, cell bodies are characterized by eccentrically located and often invaginated nuclei surrounded by cytoplasm rich in well-formed organelles. Axosomatic synapses are infrequent and cell surfaces are enveloped by glial processes. Immature dendritic features, including growth cones and filiform processes, are commonly observed during the first 10 days. Gradually the dendritic profiles elongate and thicken and contours become smoother, retaining only scattered spinelike appendages. Clear examples of the three synaptic types described in cat are found in newborn kittens, but immature terminals contain fewer synaptic vesicles and mitochondria. Approximately 90% of synapses present at birth in both nigra subdivisions are Type I, which contain large pleomorphic vesicles and contact dendrites symmetrically. Asymmetrical contacts characterize most of the remaining definable synapses. The postnatal increase in synaptic connectivity, which was estimated from random photographs of pars reticulata neuropil, is twofold during the first 50 days of life. Initially young dendrites are enveloped by glia and then gradually become ensheathed by axon terminals. Synaptogenesis in pars reticulata reflects the postnatal increase of neostriatal inputs to this subdivision and can be correlated with functional changes in strionigral connectivity.

Neuronal and synaptic organization of the lateral geniculate nucleus of the tree shrew, Tupaia glis

The ultrastructural study of the lateral geniculate nucleus (LGN) of the tree shrew (Tupaia glis) revealed two types of neurons: (1) a large thalamocortical relay cell (TCR), which may bear cilia, and (2) a small Golgi type-II interneuron (IN) with an invaginated nucleus. The narrow rim of pale cytoplasm of the IN contains fewer lysosomes and fewer Nissl bodies than the cytoplasm of the TCR. The IN perikarya, which in some cases establish somatosomatic contacts, frequently contain flattened or pleomorphic synaptic vesicles. The ratio of TCR to IN is 3:1. Three types of axon terminals were observed in the LGN. Two of them contain round synaptic vesicles but differ in size. The large RL boutons undergo dark degeneration after enucleation; they are the terminals of retino-geniculate fibers. The smaller RS boutons show dark degeneration after ablation of the visual cortex; they are the terminals of the cortico-geniculate fibers. The third type of bouton (F1) does not degenerate after either intervention. The boutons of this type are filled with flattened vesicles and are believed to be intrageniculate terminals. F2-profiles were interpreted as presynaptic dendrites of the IN. The characteristic synaptic glomeruli found in the LGN contain in their center an optic terminal. These optic terminals establish synaptic contacts with dendrites or spine-like dendritic protrusions of TCRs as well as with presynaptic dendrites. Synaptic triads were also seen. The distribution of the individual types of synaptic contacts in layers 3 and 4 were determined. Layer 4 contains only one third of the retino-geniculate synapses and of the synaptic contacts of F1-terminals.

Monosynaptic input from the nucleus accumbens--ventral striatum region to retrogradely labelled nigrostriatal neurones

After placement of lesions (either electrolytic or by injection of kainic acid) in an area including the nucleus accumbens and part of the ventral striatum in the rat, the ipsilateral substantia nigra was studied in the electron microscope. Degenerating axons and nerve terminals were found mainly in the zona reticulata and in the ventral layer of the zona compacta. Degenerating synaptic boutons were found in contact with cell bodies (symmetric synapses) and dendrites (mainly symmetric, but a few asymmetric). The postsynaptic target of some of the afferent fibres from the accumbens-ventral striatum was established by demonstrating degenerating synaptic boutons of the above types in contact with nigrostriatal neurones which had been identified by the retrograde transport of horseradish peroxidase (HRP) from the main body of the striatum. Some of the HRP-labelled cells were also impregnated by the Golgi stain and degenerating boutons were found in contact with their distal dendrites. We also observed two types of HRP-containing boutons (presumably labelled anterogradely) in the subsantia nigra after injection of HRP into the main body of the striatum : type 1 boutons contained large spherical vesicles, and formed symmetrical synapses mainly on dendritic shafts in the zona reticulata and in one case the dendrite was from a nigrostriatal neurone; type 2 boutons had pleomorphic and flattened vesicles and formed symmetrical synapses with perikarya and proximal dendrites, especially in the zona compacta. The latter type of HRP-labelled bouton was frequently found in synaptic contact with the cell bodies of nigrostriatal neurones and the same neurones sometimes also received degenerating boutons originating from neurones in the nucleus accumbens-ventral striatum. It is concluded that part of the striato-nigro-striatal circuit includes a monosynaptic link between neurones in the ventral striatum-accumbens and some nigrostriatal neurones. The possible convergence of input from different regions of the striatum on to single nigrostriatal neurones is also suggested.