Convergence of hippocampal and dopaminergic input onto identified neurons in the nucleus accumbens of the rat

The hippocampal input to the nucleus accumbens was interrupted by an electrolytic lesion of the fimbria-fornix. Boutons degenerating as a result of this lesion were found in asymmetric synaptic contact with dendritic spines and shafts in the medial part of the nucleus accumbens. Dopaminergic fibres and terminals in this area, identified using an antibody to tyrosine hydroxylase, established symmetrical synaptic contacts with dendritic shafts, spines and somata. In material where neurons in the nucleus accumbens had been Golgi-impregnated, it was found that the hippocampal and dopaminergic inputs converge onto the same neurons, and that the post-synaptic targets could be either spiny or aspiny neurons. It has been suggested that hippocampal dysfunction is involved in schizophrenia and this convergence of input from the hippocampus onto the same neurons that are post-synaptic to the dopaminergic input, which presumably originates from neurons in the ventral tegmental area, may provide an anatomical basis for the therapeutic effects of neuroleptic drugs which are dopamine antagonists.     

Ultrastructure of Golgi-impregnated and gold-toned spiny and aspiny neurons in the monkey neostriatum

Summary Golgi-impregnated, gold-toned spiny and aspiny neurons in the monkey neostriatum were deimpregnated and examined at the electron microscope level.Spiny type I neurons have relatively large nuclei with few indentations and aggregates of chromatin under the nuclear membrane which in some regions give the appearance of a dark rim. The small quantity of surrounding cytoplasm is poor in organelles.Aspiny type I neurons have eccentric, highly indented nuclei. The relatively large proportion of cytoplasm is rich in organelles especially Golgi apparatus and rough endoplasmic reticulum which often appears in stacks.Synapses with symmetric membrane densities are common on the somata of spiny type I neurons. Those on the proximal and distal dendritic shafts are few in number and asymmetric, and those on spines more frequent and primarily asymmetric. Aspiny type I neurons have few synapses on their cell bodies. Proximal and distal dendrites, however, are contacted by numerous profiles which contain small round vesicles and make both symmetric and asymmetric synapses. The same axon terminals also synapse with dendritic spines of spiny neurons, indicating that an input, most likely of afferent origin, is shared by both cell types. Other less frequently occurring profiles forming symmetric membrane densities also contact the dendrites of aspiny and spiny neurons. The axon hillocks and initial segments of both neuronal types receive a synaptic input, which is more common on spiny cells.Results offer unequivocal evidence for the differences in the ultrastructure of these two most common categories of medium-size neostriatal neurons, which may help in their proper identification in standard material, as well as information on the types and distributions of synaptic inputs onto these neurons. Moreover, the findings clarify some controversies in the literature probably originating from observations on a mixed population of cells of medium size.     

The postsynaptic targets of substance P-immunoreactive terminals in the rat neostriatum with particular reference to identified spiny striatonigral neurons

Summary Substance P-immunoreactive boutons were examined in the electron microscope in sections of the rat neostriatum that contained retrogradely labelled striatonigral neurons and/or Golgi-impregnated medium-size densely spiny neurons. The postsynaptic targets of the immunoreactive boutons were characterized on the basis of ultrastructural features, their projection to the substantia nigra and/or their somato-dendritic morphology. Substance P-immunoreactive axonal boutons formed symmetrical synaptic specializations. Of a total of 233 randomly identified synaptic boutons 72.5% made contact with dendritic shafts, 15% with dendritic spines and 10.7% with perikarya. The ultrastructural characteristics of some of the postsynaptic neuronal perikarya were consistent with their identification as striatal interneurons. Similarly, the observation of some of the substance P-containing terminals in contact with spines, spine-bearing dendritic shafts and perikarya with the ultrastructural characteristics of medium-size densely spiny neurons suggested that one of the targets of substance P-positive terminals are striatal projection neurons. Direct evidence for this was obtained in sections from rats that had received injections of horseradish peroxidase conjugated with wheatgerm agglutinin in the substantia nigra. The perikarya of retrogradely labeled striatonigral neurons were found to receive symmetrical synaptic input from substance P-positive boutons. Ultrastructural analysis of Golgi-impregnated medium-size densely spiny neurons, some of which were also retrogradely labeled from the substantia nigra, demonstrated directly that this class of neuron was postsynaptic to the substance P-immunoreactive boutons. The combination of Golgi-impregnation with substance P-immunocytochemistry made it possible to study the pattern or topography of the substance P-positive input to medium size densely spiny neurons. The substance P-containing boutons made contact predominantly with perikarya and dendritic shafts. This pattern of input is markedly different from that of other identified inputs to medium-size densely spiny neurons.     

A second type of striatonigral neuron: a comparison between retrogradely labelled and Golgi-stained neurons at the light and electron microscopic levels

In a light and electron microscopic examination of the neostriata of rats that had received injections of horseradish peroxidase into the ipsilateral substantia nigra, two morphologically distinct types of horseradish peroxidase-labelled neurons were observed. In confirmation of previous findings, one type was of medium-size and was characterized by Golgi-staining and gold-toning as the densely spinous type. The second type of neuron was in contrast, larger, had an indented nucleus and numerous cytoplasmic organelles. The synaptic input to the perikarya of the latter neurons consisted of numerous boutons containing large round and oval vesicles. The boutons formed symmetrical synaptic contacts and were similar to those of the local axon collaterals of medium-size densely spiny striatonigral neurons.In an attempt to establish what type of Golgi-impregnated neuron the second type of horseradish peroxidase-labelled neuron was, seventeen Golgi-stained or gold-toned neurons were examined in the electron microscope. Three of them were very similar in their ultrastructural features and synaptic input to the horseradish peroxidase-labelled neurons. All three were of a similar morphological appearance in the light-microscope and characteristically had long (up to 700 μm), essentially smooth dendrites. Both the large horseradish peroxidase-labelled neurons and the Golgi-impregnated neurons with long dendrites have so far only been found in the most ventral regions of the neostriatum.It is concluded that there are at least two morphologically distinct types of striatonigral neurons.     

Evidence of a breakdown of corticostriatal connections in Parkinson's disease

Dendritic spines are important structures which receive synaptic inputs in many regions of the CNS. The goal of this study was to test the hypothesis that numbers of dendritic spines are significantly reduced on spiny neurones in basal ganglia regions in Parkinson's disease as we had shown them to be in a rat model of the disease [Exp Brain Res 93 (1993) 17]. Postmortem tissue from the caudate and putamen of patients suffering from Parkinson's disease was compared with that from people of a similar age who had no neurological damage. The morphology of Golgi-impregnated projection neurones (medium-sized spiny neurones) was examined quantitatively. The numerical density of dendritic spines on dendrites was reduced by about 27% in both nuclei. The size of the dendritic trees of these neurones was also significantly reduced in the caudate nucleus from the brains of PD cases and their complexity was changed in both the caudate nucleus and the putamen. Dendritic spines receive crucial excitatory input from the cerebral cortex. Reduction in both the density of spines and the total length of the remaining dendrites is likely to have a grave impact on the ability of these neurones to function normally and may partly explain the symptoms of the disorder.     

Striatal grafts in rats with unilateral neostriatal lesions--I. Ultrastructural evidence of afferent synaptic inputs from the host nigrostriatal pathway

Tyrosine hydroxylase immunocytochemistry, in combination with Golgi impregnation, has been used to study the dopaminergic afferent input to striatal suspension grafts implanted into the previously ibotenic acid-lesioned striatum in adult recipient rats. The rats were perfused for combined light- and electron microscopy at 10-11 months after transplantation, at the end of a series of behavioural experiments and a study of in vivo GABA release, reported in the two accompanying papers. A tyrosine hydroxylase-positive fibre network occurred within the grafts in all eight specimens analysed. The tyrosine hydroxylase-positive fibres had a distinct "patchy" distribution, throughout the graft tissue, and within these patches the terminal density was similar to that of the normal intact striatum. Ultrastructurally, the tyrosine hydroxylase-positive fibres were seen to make abundant synaptic contacts with neuronal elements within the grafts. As in the normal striatum, they were all of the symmetric type and dendritic shafts and spines were the most usual postsynaptic targets. Sections from three of the grafted animals were taken for combined Golgi-impregnation and immunostaining. Only cells of the medium-sized densely spiny type were impregnated in this material. Six of them, which had portions extending into the immunostained neuropil, were drawn using a camera lucida and processed for electron microscopy. Tyrosine hydroxylase-positive boutons were seen to make symmetrical synaptic contacts onto the shafts and spines of the impregnated dendrites, and in one case also with the perikaryon. The results indicate that the medium-sized densely spiny neuron type (which is a predominant target for the dopaminergic afferents in the normal striatum) is abundant in the grafted tissue, and that these neurons represent a synaptic target also for the tyrosine hydroxylase-positive innervation of the striatal grafts.     

The termination of callosal fibres in the auditory cortex of the rat. A combined Golgi--electron microscope and degeneration study

When the corpus callosum of the rat is sectioned, the callosal fibres in the cerebral cortex undergo degeneration. In the auditory cortex (area 41) the degenerating axon terminals form asymmetric synapses, and the vast majority of them synapse with dendritic spines. Some other synapse with the shafts of both spiny and smooth dendrites, and a few with the perikarya of non-pyramidal cells. The degenerating axon terminals are contained principally within layer II/III, in which they aggregate in patches. Using a technique in which neurons within the cortex are Golgi-impregnated, then gold-toned and examined in the electron microscope, it has been shown that the dendritic spines of pyramidal neurons with cell bodies in different layers receive the degenerating callosal afferents. The spines arise from the main apical dendritic shafts and their branches, from the dendrites of the apical tufts, and in some cases from the basal dendrites of the pyramidal neurons. The shafts of some pyramidal cell apical dendrites also form asymmetric synapses with callosal afferents. Since we have encountered no spiny non-pyramidal neurons in Golgi preparations of rat auditory cortex, and because other types of non-pyramidal cells have few dendritic spines, it is concluded that practically all of the dendritic spines synapsing with callosal afferents originate from pyramidal neurons.     

Synaptic relationships of a type of GABA-immunoreactive neuron (clutch cell), spiny stellate cells and lateral geniculate nucleus afferents in layer IVC of the monkey striate cortex

The precise stimulus specificity of striate cortical neurons is strongly influenced by processes involving gamma-aminobutyric acid (GABA). In the visual cortex of the monkey most afferents from the lateral geniculate nucleus terminate in layer IVC. We identified a type of smooth dendritic neuron (clutch cell) that was immunoreactive for GABA, and whose Golgi-impregnated dendrites and axon were largely restricted to layer IVC beta. The slightly ovoid somata were 8-12 micron by 12-15 micron and the dendritic field was often elongated, extending 80-200 micron in one dimension. The axonal field was 100-150 micron in diameter and densely packed with large bulbous boutons. Although mainly located in IVC beta both the dendritic and axonal processes entered IVC alpha. Fine structural features of GABA-immunoreactive and-impregnated clutch cells and impregnated spiny stellate cells were compared. Clutch cells had more cytoplasm, more densely packed mitochondria and endoplasmic reticulum, and made type II as opposed to type I synapses. A random sample of 159 elements postsynaptic to three clutch cells showed that they mainly terminated on dendritic shafts (43.8-58.5%) and spines (20.8-46.3%), rather than somata (10-17%). The majority of the postsynaptic targets were characteristic of spiny stellate cells. This was directly demonstrated by studying synaptic contacts between an identified GABA positive clutch cell and the dendrites and soma of an identified spiny stellate cell. The termination of clutch cells mainly on dendrites and spines of spiny stellate cells suggests that they interact with other inputs to the same cells. Following an electrolytic lesion in the ipsilateral lateral geniculate nucleus we examined the distribution of degenerating terminals on three identified spiny stellate neurons in layer IVC beta. Out of eight synapses from the lateral geniculate nucleus one was on a dendritic shaft, the rest on spines. Only a small fraction of all synapses on the cells were from degenerating boutons. A clutch cell within the area of dense terminal degeneration was not contacted by terminals from the lateral geniculate nucleus. The results show that layer IVC in the monkey has a specialized GABAergic neuron that terminates on spiny stellate cells monosynaptically innervated from the lateral geniculate nucleus. Possible functions of clutch cells may include inhibitory gating of geniculate input to cortex; maintenance of the antagonistic subregions in the receptive fields; and the creation from single opponent of double colour opponent receptive fields.     

Tyrosine hydroxylase-immunoreactive boutons in synaptic contact with identified striatonigral neurons, with particular reference to dendritic spines

Tyrosine hydroxylase-immunoreactive fibres in the rat neostriatum were studied in the electron microscope in order to determine the nature of the contacts they make with other neural elements. The larger varicose parts of such fibres contained relatively few vesicles and rarely displayed synaptic membrane specializations; however, thinner parts of axons (0.1-0.4 micron) contained many vesicles and had symmetrical membrane specializations, indicative of en passant type synapses. By far the most common postsynaptic targets of tyrosine hydroxylase-immunoreactive boutons were dendritic spines and shafts, although neuronal cell bodies and axon initial segments also received such input. Six striatonigral neurons in the ventral striatum were identified by retrograde labelling with horseradish peroxidase and their dendritic processes were revealed by Golgi impregnation using the section-Golgi procedure. The same sections were also developed to reveal tyrosine hydroxylase immunoreactivity and so we were able to study immunoreactive boutons in contact with the Golgi-impregnated striatonigral neurons. Each of the 280 immunoreactive boutons examined in the electron microscope displayed symmetrical synaptic membrane specializations: 59% of the boutons were in synaptic contact with the dendritic spines, 35% with the dendritic shafts and 6% with the cell bodies of striatonigral neurons. The dendritic spines of striatonigral neurons that received input from immunoreactive boutons invariably also received input, usually more distally, from unstained boutons that formed asymmetrical synaptic specializations. A study of 87 spines along the dendrites of an identified striatonigral neuron showed that the most common type of synaptic input was from an individual unstained bouton making asymmetrical synaptic contact (53%), while 39% of the spines received one asymmetrical synapse and one symmetrical immunoreactive synapse. It is proposed that the spatial distribution of presumed dopaminergic terminals in synaptic contact with different parts of striatonigral neurons has important functional implications. Those synapses on the cell body and proximal dendritic shafts might mediate a relatively non-selective inhibition. In contrast, the major dopaminergic input that occurs on the necks of dendritic spines is likely to be highly selective since it could prevent the excitatory input to the same spines from reaching the dendritic shaft. One of the main functions of dopamine released from nigrostriatal fibres might thus be to alter the pattern of firing of striatal output neurons by regulating their input.     

Neuronal and synaptic organization of the centromedian nucleus of the monkey thalamus: a quantitative ultrastructural study, with tract tracing and immunohistochemical observations

Summary The ultrastructure of the centromedian nucleus of the monkey thalamus was analysed qualitatively and quantitatively and projection neurons, local circuit neurons, and synaptic bouton populations identified. Projection neurons were mostly medium-sized, with oval-fusiform or polygonal perikarya, few primary dendrites, and frequent somatic spines; local circuit neurons were smaller. Four basic types of synaptic boutons were distinguished: (1) Small- to medium-sized boutons containing round vesicles (SR) and forming asymmetric contacts, identified as corticothalamic terminals. (2) Heterogeneous medium-sized boutons with asymmetric contacts and round vesicles, similar to the so-called large round (LR) boutons, which were in part of cortical origin. (3) Heterogeneous GAD-positive small- to medium-sized boutons, containing pleomorphic vesicles and forming symmetric contacts (F1 type), which included pallidothalamic terminals. (4) Presynaptic profiles represented by GAD-positive vesicle-containing dendrites of local circuit neurons. Complex synaptic arrangements, serial synapses and triads with LR and SR boutons engaging all parts of projection neuron dendrites and somata, were seen consistently, whereas classical glomeruli were infrequent. LR and SR boutons also established synapses on dendrites of local circuit neurons. F1 boutons established synapses on projection neuron somata, dendrites and initial axon segments. Compared to other previously studied motor-related thalamic nuclei, differences in synaptic coverage between proximal and distal projection neuron dendrites were less pronounced, and the density of synapses formed by local circuit dendrites on projection neuron dendrites was lower. Thus, compared to other thalamic nuclei, the overlap of different inputs was higher on monkey centromedian cells, and centromedian inhibitory circuits displayed a different organization.     

The section-Golgi-impregnation procedure--3. Combination of Golgi-impregnation with enzyme histochemistry and electron microscopy to characterize acetylcholinesterase-containing neurons in the rat neostriatum

Three morphologically distinct types of neuron that contain acetylcholinesterase have been distinguished by Golgi-impregnation of sections of the rat neostriatum that had been incubated to reveal acetylcholinesterase activity. The neuron that stained most intensely for acetylcholinesterase was a large cell, with smooth or sparsely spiny dendrites; the axon of one these neurons was partially impregnated by the Golgi stain and had local axon collaterals (type 1). Another acetylcholinesterase-containing neuron had a small to medium-size cell body with long sparsely spiny dendrites emerging from opposite poles (type 2). The third type of neuron that contained acetylcholinesterase was medium to large size and had many primary, sparsely spiny dendrites that branched frequently (type 3). Examination of the same Golgi-impregnated, acetylcholinesterase-stained neurons that had been studied in the light microscope by electron microscopy allowed us to distinguish several other differences between the three types of neuron. Whereas all three types had acetylcholinesterase reaction product in the endoplasmic reticulum and along the nuclear envelope, only neurons of type 1 displayed reaction product in the Golgi apparatus. All three types of neuron received synaptic input, mainly along their dendrites. It is concluded that the combination of Golgi-impregnation with histochemical procedures that demonstrate endogenous enzyme activity can be applied to reveal the morphological characteristics, synaptic input and local synaptic output of neurons with specific biochemical properties.     

Synaptic replacement in the dentate gyrus after unilateral entorhinal lesion: electron microscopic analysis of the extent of replacement of synapses by the remaining entorhinal cortex

Summary In response to a unilateral entorhinal lesion the input from the contralateral entorhinal cortex to the dentate gyrus appears to increase. We have studied this crossed projection by electron microscopy in normal animals and in animals one year or more after a unilateral entorhinal lesion. In normal animals few degenerating boutons are found after a contralateral entorhinal lesion. However, when the contralateral lesion was made one year after an ipsilateral entorhinal lesion, degenerating boutons were readily identified. The boutons were relatively few in number, but formed an abnormally large number of synaptic contacts. These results support the previous conclusion that fibres from the contralateral entorhinal cortex form additional synapses when their ipsilateral homologues are removed. However, these new cortical synapses probably account for only a small portion of those formed in response to the lesion. Thus an anatomically homologous input does not, in this case, selectively capture most of the newly available synaptic sites.     

The section-Golgi impregnation procedure. 2. Immunocytochemical demonstration of glutamate decarboxylase in Golgi-impregnated neurons and in their afferent synaptic boutons in the visual cortex of the cat

Sections of the cat's visual cortex were stained by an antiserum to glutamate decarboxylase using the peroxidase-antiperoxidase method; they were then impregnated by the section Golgi procedure and finally the Golgi deposit was replaced by gold. Neurons containing glutamate decarboxylase immunoreactivity were found in all layers of the visual cortex, without any obvious pattern of distribution. Fifteen immunoreactive neurons were also Golgi-impregnated and gold-toned, which enabled us to study the morphology and synaptic input of identified GABAergic neurons. These neurons were found to be heterogeneous both with respect to the sizes and shapes of their perikarya and the branching patterns of their dendrites. All the immunoreactive, Golgi-impregnated neurons had smooth dendrites, with only occasional protrusions. The synaptic input of glutamate decarboxylase-immunoreactive neurons was studied in the electron microscope. Immunoreactive neurons received immunoreactive boutons forming symmetrical synapses on their cell bodies. The Golgi-impregnation made it possible to study the input along the dendrites of immunoreactive neurons. One of the large neurons in layer III whose soma was immunoreactive was also Golgi-impregnated: it received numerous non-immunoreactive asymmetrical synaptic contacts along its dendrites and occasional ones on its soma. The same neuron also received a few boutons forming symmetrical synaptic contacts along its Golgi-impregnated dendrites; most of these boutons were immunoreactive for glutamate decarboxylase. Glutamate decarboxylase-immunoreactive boutons were also found in symmetrical synaptic contact with non-immunoreactive neurons that were Golgi-impregnated. A small pyramidal cell in layer III was shown to receive several such boutons along its somatic membrane. It is concluded that the combination of immunoperoxidase staining and Golgi impregnation is technically feasible and that it can provide new information. The present study has shown that there are many morphologically distinct kinds of aspiny GABAergic neurons in the visual cortex; that the predominant type of synaptic input to the dendrites of such neurons is from boutons forming asymmetrical synapses, but that some of the GABAergic neurons also receive a dense symmetrical synaptic input on their cell bodies, and occasional synapses along their dendrites, from the boutons of other GABAergic neurons. These findings provide a morphological basis, firstly, for a presumed powerful excitatory input to GABAergic interneurons and, secondly, for the disinhibition which has been postulated from electrophysiological studies to occur in the cat's visual cortex.     

Effect of light deprivation upon the morphology of axon terminals in the dorsal lateral geniculate nucleus of mouse: an electron microscopical study using serial sections

Two populations of morphologically different large axon terminals have been observed electron microscopically in the dorsal lateral geniculate nucleus of mice raised in complete darkness from birth up to 19 days of age. One population includes larger terminals indistinguishable from the large terminals present in control animals, i.e. they have round synaptic vesicles, rather pale mitochondria, membrane saculae, coated vesicles, and asymmetric contacts with encrusted dendritic spines of portions of dendrites of geniculo-cortical relay neurons. The other population includes large terminals which also have asymmetric contacts with encrusted dendritic spines or portions of dendrites of geniculo-cortical relay neurons, but they show darker mitochondria, absence of both membrane saculae and coated vesicles, and significantly higher synaptic vesicle density and smaller size than the large control ones. We suggest that the latter population of terminals could be inactive due to the absence of visual input.     

Nitrergic neurons make synapses on dual-input dendritic spines of neurons in the cerebral cortex and the striatum of the rat: implication for a postsynaptic action of nitric oxide

Pre-embedding electron microscopic immunocytochemistry was used to examine the ultrastructure of neurons containing nitric oxide synthase and to evaluate their synaptic relationships with target neurons in the striatum and sensorimotor cerebral cortex. Intense nitric oxide synthase immunoreactivity was found by light and electron microscopy in a type of aspiny neuron scattered in these two regions. The intensity of the labeling was uniform in the soma, dendrites and axon terminals of these neurons. In both forebrain regions, nitric oxide synthase-immunoreactive neurons received synaptic contacts from unlabeled terminals, which were mostly apposed to small-caliber dendrites. The unlabeled symmetric contacts were generally about four times as abundant as the unlabeled asymmetric contacts on the nitric oxide synthase-immunoreactive neurons. Terminals labeled for nitric oxide synthase were filled with synaptic vesicles and were observed to contact unlabeled neurons. Only 54% (in the cerebral cortex) and 44.3% (in the striatum) of the nitric oxide synthase-immunoreactive terminals making apposition with the target structures were observed to form synaptic membrane specializations within the plane of the randomly sampled sections. The most common targets of nitric oxide synthase-immunoreactive terminals were thin dendritic shafts (54% of the immunoreactive terminals in the cortex and 75.7% of the immunoreactive terminals in the striatum), while dendritic spines were a common secondary target (42% of the immunoreactive terminals in the cortex and 20.6% of the immunoreactive terminals in the striatum). The spines contacted by nitric oxide synthase-immunoreactive terminals typically also received an asymmetric synaptic contact from an unlabeled axon terminal.These findings suggest that: (i) nitric oxide synthase-immunoreactive neurons in the cortex and striatum preponderantly receive inhibitory input; (ii) nitric oxide synthase-containing terminals commonly make synaptic contact with target structures in the cortex and striatum; (iii) spines targeted by nitric oxide synthase-containing terminals in the cortex and striatum commonly receive an asymmetric contact as well, which may provide a basis for a synaptic interaction of nitric oxide with excitatory input to individual spines.     

Distribution of DARPP-32 in the basal ganglia: an electron microscopic study

Summary DARPP-32, a dopamine and cyclic AMP-regulated phosphoprotein, has been studied by light and electron microscopical immunocytochemistry in the rat caudatoputamen, globus pallidus and substantia nigra. In the caudatoputamen, DARPP-32 was present in neurons of the medium-sized spiny type. Immunoreactivity for DARPP-32 was present in dendritic spines, dendrites, perikaryal cytoplasm, most but not all nuclei, axons and a small number of axon terminals. Immunoreactive axon terminals in the caudatoputamen formed symmetrical synapses with immunolabelled dendritic shafts or somata. Neurons having indented nuclei were never immunoreactive. In the globus pallidus and substantia nigra pars reticulata, DARPP-32 was present in myelinated and unmyelinated axons and in axon terminals. The labelled axon terminals in these regions formed symmetrical synaptic contacts on unlabelled dendritic shafts or on unlabelled somata. These data suggest that DARPP-32 is present in striatal neurons of the medium-sized spiny type and that these DARPP-32-immunoreactive neurons form symmetrical synapses on target neurons in the globus pallidus and substantia nigra. The presence of DARPP-32 in these striatal neurons and in their axon terminals suggests that DARPP-32 mediates part of the response of medium-size spiny neurons in the striaturn to dopamine D-l receptor activation.     

Convergence of cortical and thalamic input to direct and indirect pathway medium spiny neurons in the striatum

The major afferent innervation of the basal ganglia is derived from the cortex and the thalamus. These excitatory inputs mainly target the striatum where they innervate the principal type of striatal neuron, the medium-sized spiny neurons (MSNs), and are critical in the expression of basal ganglia function. The aim of this work was to test directly whether corticostriatal and thalamostriatal terminals make convergent synaptic contact with individual direct and indirect pathway MSNs. Individual MSNs were recorded in vivo and labelled by the juxtacellular method in the striatum of BAC transgenic mice in which green fluorescent protein reports the expression of dopamine D1 or D2 receptors. After recovery of the neurons, the tissue was immunolabelled for vesicular glutamate transporters type 1 and 2, as markers of cortical and thalamic terminals, respectively. Three of each class of MSNs were reconstructed in 3D and second-order dendrites selected for electron microscopic analysis. Our findings show that direct and indirect pathway MSNs, located in the matrix compartment of the striatum, receive convergent input from cortex and thalamus preferentially on their spines. There were no differences in the pattern of innervation of direct and indirect pathway MSNs, but the cortical input is more prominent in both and synaptic density is greater for direct pathway neurons. The 3D reconstructions revealed no morphological differences between direct and indirect MSNs. Overall, our findings demonstrate that direct and indirect pathway MSNs located in the matrix receive convergent cortical and thalamic input and suggest that both cortical and thalamic inputs are involved in the activation of MSNs.     

Thalamic input to parvalbumin-immunoreactive GABAergic interneurons: organization in normal striatum and effect of neonatal decortication.

The neocortex and thalamus send dense glutaminergic projections to the neostriatum. The neocortex makes synaptic contact with spines of striatal projection neurons, and also targets a distinct class of GABAergic interneurons immunoreactive for the calcium-binding protein parvalbumin. We determined whether the parafascicular thalamic nucleus also targets striatal parvalbumin-immunoreactive interneurons. The anterograde tracer biotinylated dextranamine was injected into the parafascicular nucleus of adult rats. Double-labeled histochemistry/immunohistochemistry revealed overlapping thalamic fibers and parvalbumin-immunoreactive neurons in the neostriatum. Areas of overlap within the sensorimotor striatum were analysed by electron microscopy. Of 311 synaptic boutons originating from the parafascicular nucleus, 75.9% synapsed with unlabeled dendrites, 22.5% with unlabeled spines, and 1.3% had parvalbumin-immunoreactive dendrites as a postsynaptic target. Only 4% of all asymmetric synapses on parvalbumin-immunoreactive dendrites were derived from the parafascicular nucleus. A separate group of animals underwent bilateral neocortical deafferentation on the third postnatal day, prior to injection of anterograde tracer into the parafascicular nucleus of adult animals. These experiments were performed with the dual purpose of (i) reducing the possibility that thalamic inputs to parvalbumin-immunoreactive neurons are the result of transsynaptic uptake of tracer by a thalamo-cortico-striatal route, and (ii) determining whether competitive interactions between developing corticostriatal and thalamostriatal fibers may account for the relatively sparse thalamic input onto parvalbumin-immunoreactive interneurons. In decorticates, 219 striatal synaptic contacts derived from the parafascicular nucleus, out of which 77.2% were on unlabeled dendrites, 20.9% were upon unlabeled spines, and 0.9% targeted parvalbumin-immunoreactive dendrites. We conclude that the thalamic parafascicular nucleus indeed sends synaptic input to parvalbumin-immunoreactive striatal neurons. Parafascicular nucleus inputs to striatal parvalbumin-immunoreactive interneurons are sparse in comparison to other asymmetric inputs, most of which are likely to be of cortical origin. The synaptic profile of thalamostriatal inputs to parvalbumin-immunoreactive neurons and unlabeled elements is unchanged following neonatal decortication. This suggests that competitive interaction between developing thalamostriatal and corticostriatal projections is not a major mechanism determining synaptic input to striatal subpopulations.     

A light and electron microscopic study of NADPH-diaphorase-, calretinin- and parvalbumin-containing neurons in the rat nucleus accumbens

The rat nucleus accumbens contains medium-sized, spiny projection neurons and intrinsic, local circuit neurons, or interneurons. Sub-classes of interneurons, revealed by calretinin (CR) or parvalbumin (PV) immunoreactivity or reduced nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, were compared in the nucleus accumbens core, shell and rostral pole. CR, PV and NADPH-diaphorase-containing neurons are shown to form three non-co-localising populations in these three areas. No significant differences in neuronal population densities were found between the subterritories. NADPH-diaphorase-containing neurons could be further separated morphologically into three sub-groups, but CR- and PV-immunoreactive neurons form homogeneous populations. Ultrastructurally, NADPH-diaphorase-, CR- and PV-containing neurons in the nucleus accumbens all possess nuclear indentations. These are deeper and fewer in neurons immunoreactive for PV than in CR- and NADPH-diaphorase-containing neurons. CR-immunoreactive boutons form asymmetrical and symmetrical synaptic specialisations on spines, dendrites and somata, while PV-immunoreactive boutons make only symmetrical synaptic specialisations. Both CR- and PV-immunoreactive boutons form symmetrical synaptic specialisations with medium-sized spiny neurons and contact other CR- and PV-immunoreactive somata, respectively. A novel non-carcinogenic substrate for the peroxidase reaction (Vector Slate Grey, SG) was found to be characteristically electron-dense and may be distinguishable from the diaminobenzidine reaction product. We conclude that the three markers used in this study are localised in distinct populations of nucleus accumbens interneurons. Our studies of their synaptic connections contribute to an increased understanding of the intrinsic circuitry of this area.     

The integrative properties of spiny distal dendrites.

The dendritic spines of many central neurons are generally thought to modulate the ability of individual synaptic conductances to depolarize the dendritic shaft. A compartmental analysis using typical spine dimensions shows that spine neck resistances are probably far too low to support such a function, because low conductance synapses act as time-varying current sources. However, the collective presence of all spines on a dendrite significantly modifies the electrical properties of the branch in ways which have previously been overlooked. In particular, they lower its input impedance and length constant, reducing the amplitude of the unitary excitatory postsynaptic potential as well as the strength of spatial summation. This enables a dendrite to integrate large numbers of synaptic inputs while occupying minimal volume. In this way, dendritic spines are analogous to axonal myelin, which also alters transcellular impedance in order to maximize neurite function and minimize volume. Unlike membrane resistance changes, spines have little effect on the membrane time-constant so they maintain a long window for temporal summation. Though spine shape and neck resistance do not significantly affect dendritic potentials, spine area does. Therefore, while changes in spine morphology probably do not directly potentiate the strength of individual synapses, changes in spine density can regulate the synaptic excitability of an entire dendrite. The shortened length-constant of the spiny dendrite requires excitable membranes to be located in distal dendrites. These, in turn, eliminate many of the electrotonic nonlinearities associated with summation in long, thin processes, and make all distal synapses equipotent. The short length-constant also enhances the sensitivity of dendritic spikes to local impedance changes while decreasing the sensitivity to distant impedance changes. This would enable a neuron to effectively use inhibitory synapses or branch points to regulate propagation through its spiny dendritic tree. A model neuron is developed in which dendritic spines, excitable membranes, and dendritic branching combine to form a two-stage filter, which serves as a synaptic input coincidence detector with adjustable gain. Gain is regulated by potassium conductances which modulate branch point safety factor. The model is consistent with the notion of functional independence of distal dendrites and demonstrates that certain aspects of dendritic spiking which have previously been thought to require membrane hot-spots can also result from geometrical properties. It is suggested that the activation of spiny neurons may depend as much on the density as on the number of active synapses, and that spiny neurons may tend to have discrete output states whereas nonspiny neurons may be more continuous.