Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The convergence of axon terminals from the mediodorsal thalamic nucleus and ventral tegmental area on pyramidal cells in layer V of the rat prelimbic cortex

We investigated the ultrastructural basis of the synaptic convergence of afferent fibres from the mediodorsal thalamic nucleus (MD) and the ventral tegmental area (VTA) on the prefrontal cortical neurons of the rat by examining the synaptic relationships between thalamocortical or tegmentocortical terminals labelled with anterograde markers [lesion-induced degeneration or transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP)] and randomly selected unlabelled apical dendrites of layer V pyramidal cells in the prelimbic cortex. WGA-HRP-labelled terminals from the VTA ranged in diameter from 0.7 to 2.8 microm and established synaptic contacts with large dendritic profiles, i.e. proximal segments of apical dendritic shafts and spines from layer V pyramidal cells. Symmetrical synapses, i.e. inhibitory synapses, were more often seen than asymmetrical ones. Degenerating terminals from the MD formed asymmetrical synapses on dendritic spines or occasionally on small dendritic shafts of apical dendrites from layer V pyramidal cells, which received tegmentocortical synapses, mostly within layer III. Thalamocortical synapses were more distally distributed over common apical dendrites than tegmentocortical synapses, although some of them overlapped. The numerical density of direct synaptic inputs from the MD and VTA was low. These results suggest that fibres from the VTA exert their inhibitory effects directly on pyramidal cells in layer V via synaptic junctions with apical dendrites of these pyramidal cells, and that the tegmentocortical fibres are in an ideal anatomical position to modulate the reverberatory circuits between the MD and the prelimbic cortex.

Prefrontal cortex inputs of the nucleus accumbens-nigro-thalamic circuit

The functional organization of the cortico-nucleus accumbens-substantia nigra pars reticulata circuit was investigated in the rat using combined anatomical and electrophysiological approaches. The nucleus accumbens neurons which project to the substantia nigra pars reticulata are located in a circumscribed region of the core immediately adjacent and extending dorsally to the anterior commissure. As shown by retrograde and anterograde transports of wheatgerm agglutinin conjugated to horseradish peroxidase, the region of the nucleus accumbens related to the substantia nigra was found to receive bilateral inputs from restricted areas of the medial and lateral prefrontal cortex, i.e., prelimbic/medial orbital and dorsal agranular insular areas. The electrical stimulation of these medial and lateral prefrontal cortical areas induced excitatory responses in nucleus accumbens neurons projecting to the dorsomedial substantia nigra pars reticulata. Interestingly, an important proportion (61%) of the nucleus accumbens-nigral cells responding to the stimulation of the lateral prefrontal cortex were also excited by the stimulation of the medial prefrontal cortex, demonstrating the existence of a convergent influence of these cortical areas on single nucleus accumbens cells. Furthermore, the present data also show that the stimulation of the medial prefrontal cortex results in a powerful inhibition of the tonic firing of the substantia nigra pars reticulata neurons. In conclusion, this study reveals the existence of a functional link between the prefrontal cortex (prelimbic/medial orbital and agranular insular areas) and the nucleus accumbens neurons which innervate the dorsomedial region of the substantia nigra pars reticulata. Since the dorsomedial region of substantia nigra pars reticulata is known to project to subfields of the mediodorsal and ventromedial thalamic nuclei related to the prefrontal cortex, the present data further demonstrate the existence of a prefrontal-nucleus accumbens-thalamo-cortical circuit involving the substantia nigra pars reticulata.

Glutamate and aspartate immunoreactive neurons of the rat basolateral amygdala: colocalization of excitatory amino acids and projections to the limbic circuit

The basolateral amygdala has projections to several structures that take part in the limbic cortico-striato-pallido-thalamic circuit, including the prefrontal cortex, ventral striatum, and mediodorsal thalamic nucleus. The present investigation used a technique that combines retrograde tract tracing with immunohistochemistry for glutamate and aspartate to determine if amygdaloid neurons projecting to different targets in the limbic circuit can be distinguished on the basis of their content of excitatory amino acids. Cell counts revealed that at least 85-95% of the neurons in the basolateral nucleus projecting to the prefrontal cortex or ventral striatum were pyramidal cells that exhibited glutamate or aspartate immunoreactivity. Colocalization studies indicated that 94-100% of aspartate-immunoreactive neurons in the basolateral nucleus were also glutamate positive and that 92-94% of glutamate-immunoreactive neurons were also aspartate positive. A small number of glutamate-positive pyramidal neurons in the anterior subdivision of the cortical nucleus were found to project to the mediodorsal thalamic nucleus. However, the great majority of amygdaloid neurons with projections to the mediodorsal nucleus did not exhibit glutamate or aspartate immunoreactivity. The absence of glutamate and aspartate immunoreactivity in these cells suggests that these neurons do not use excitatory amino acids as neurotransmitters. The finding of high levels of glutamate and aspartate in basolateral amygdaloid neurons projecting to the prefrontal cortex and ventral striatum is consistent with previous reports indicating that these neurons may use excitatory amino acids as neurotransmitters, but is not a definitive criterion for this determination.

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and layer III pyramidal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and Golgi electron microscopic technique was designed to examine the distribution and density of axon terminals from the mediodorsal thalamic nucleus (MD) over layer III pyramidal cells in the prelimbic cortex of the rat. The reconstructive analysis of serial ultrathin sections of gold-toned apical and basal dendrites of layer III pyramidal cells showed that degenerating thalamocortical axon terminals from MD formed asymmetrical synaptic contacts predominantly with dendritic spines of the identified basal dendrites as well as apical dendrites. There was little difference in the numerical density of thalamocortical synapses from MD per unit length of both apical and basal dendrites.

Ventral striatopallidothalamic projection: IV. Relative involvements of neurochemically distinct subterritories in the ventral pallidum and adjacent parts of the rostroventral forebrain

Retrograde and anterograde tract-tracing studies were carried out to determine whether the capacity of the nucleus accumbens to influence the thalamic mediodorsal nucleus via ventral striatopallidothalamic connections disproportionately favors the shell over the core subterritory. After injections of Fluoro-Gold into the mediodorsal thalamic nucleus, retrogradely labeled neurons were detected in sections also processed for calbindin-D 28-kD and neurotensin immunoreactivities to facilitate identification of subterritories in the ventral pallidum. Fluoro-Gold-labeled cells were counted in series of sections cut through the ventral pallidum, rostral globus pallidus, nucleus of the vertical limb of the diagonal band, preoptic region, lateral hypothalamus, and the sublenticular gray region, including parts of the extended amygdala. Data were expressed as cells/unit area and as percentages of all labeled forebrain cells. Mediodorsal nucleus-projecting rostroventral forebrain neurons were most numerous in the ventromedial part of the subcommissural ventral pallidum and pallidal parts of the olfactory tubercle. Few were observed in the dorsolateral part of the subcommissural ventral pallidum. In addition, following injections into the ventral pallidum, anterogradely transported biotinylated dextran amine was evaluated in sections processed for calbindin or tyrosine hydroxylase immunoreactivities. Injection into the ventromedial part of the subcommissural ventral pallidum resulted in robust anterograde labeling of the medial segment of the mediodorsal nucleus and ventral tegmental area and weak labeling of the substantia nigra and subthalamic nucleus. Conversely, after injection into the dorsolateral part of the subcommissural ventral pallidum, anterograde labeling was weak in the mediodorsal nucleus and ventral tegmental area, but robust in the substantia nigra and subthalamic nucleus. The results are consistent with a predominant accumbens shell influence on the mediodorsal nucleus and with cortico-ventral striatopallidal-thalamocortical pathways that begin and end in different parts of the frontal lobe.

The mediodorsal nucleus of the thalamus in rats--I. forebrain gabaergic innervation

The aim of this study was to determine whether forebrain neurons projecting to the mediodorsal nucleus of the thalamus in rats express glutamate decarboxylase messenger RNA as a marker for GABAergic neurons. Forebrain glutamate decarboxylase messenger RNA-containing neurons that project to the mediodorsal nucleus were identified using a combination of retrograde tracing with Fluoro-Gold and in situ hybridization for the messenger RNA encoding the 67,000 molecular weight synthetic enzyme for GABA. Glutamate decarboxylase messenger RNA-containing afferents to the mediodorsal nucleus were observed in the olfactory tubercle, vertical limb of the diagonal band of Broca, ventral pallidum, sublenticular substantia innominata, globus pallidus, lateral preoptic area, bed nucleus of the stria terminalis and reticular nucleus of the thalamus. The largest proportions of glutamate decarboxylase messenger RNA-containing afferents to the mediodorsal nucleus were observed in the vertical limb of the diagonal band, ventral pallidal parts of the olfactory tubercle and the reticular nucleus of the thalamus. Somewhat fewer glutamate decarboxylase messenger RNA-containing, retrogradely labeled neurons were observed in the subcommissural ventral pallidum and sublenticular substantia innominata. These data suggest that a GABAergic projection from the basal forebrain to the mediodorsal nucleus of the thalamus can influence the function of this nucleus.

Excitatory responses evoked in prefrontal cortex by mediodorsal thalamic nucleus stimulation: influence of anaesthesia

The prefrontal cortex and the mediodorsal thalamic nucleus are reciprocally connected through excitatory amino acid pathways. Cortical excitatory responses resulting from activation of either the mediodorsal thalamic nucleus-prefrontal cortex pathway (short latency) or the recurrent collaterals of prefrontal cortex-mediodorsal thalamic nucleus neurons (long latency) can be discriminated mainly by their latency. The present study was undertaken to compare the effects of halothane and ketamine anaesthesia on these cortical excitatory responses and to establish their pharmacological characteristics using microiontophoretic application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D,L-2-amino-5-phosphonovaleric acid (APV), the specific antagonists of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors respectively. The number of prefrontal cortex cells which presented short or long latency excitatory responses was smaller in halothane- than in ketamine-anaesthetized rats. Whatever the anaesthetic used, short latency responses were blocked by CNQX and not affected by APV. Long latency responses were mainly blocked by APV and occasionally by CNQX in halothane-anaesthetized rats, while they were only blocked by CNQX in ketamine-anaesthetized animals. Therefore, halothane seems to preferentially reduce evoked responses mediated by AMPA receptors while ketamine completely abolishes evoked responses involving NMDA receptors. Moreover, the present data confirm that excitatory responses resulting from the activation of the mediodorsal thalamic nucleus-prefrontal cortex pathway are mainly mediated by AMPA receptors. In addition, they demonstrate that cortical responses linked to the activation of recurrent collaterals from prefrontal cortex-mediodorsal thalamic nucleus neurons involve both AMPA and NMDA receptors.

Organization of connections between the thalamic reticular and the anterior thalamic nuclei in the rat

The thalamic reticular nucleus (TRN) receives topographically organized input from specific sensory nuclei such as the lateral geniculate nucleus. The present study shows this in the rat. However, the pattern of thalamic connections to the limbic reticular sector is unknown. Injecting biocytin into the ventral parts of anteroventral and anteromedial nuclei labeled neurons and axons in the rostral TRN. Filled axon collaterals and their terminals occupied a rectangular sheet in a plane close to the horizontal, and were confined to the inner zone (the medial portion) of the limbic TRN. Retrogradely filled cells were in the middle of the rostral pole in the same horizontal plane, receiving synapses from surrounding labeled boutons. In electron micrographs, thalamic terminals were found to contain round, densely packed synaptic vesicles and formed asymmetrical synapses onto reticular somata and dendritic profiles. Displacing the injection site along the dorso-ventral and rostro-caudal axis in the anterior nuclei produced corresponding shifts of antero- and retrograde labeling within the inner reticular zone. Projections from the dorsal portions of the anterior nuclei did not follow this pattern. Axons from the anterodorsal nucleus occupied the rostralmost tip of both inner and outer zones of the dorsal limbic sector. In accordance with earlier reports, the limbic sector was found to represent several dorsal thalamic nuclei parallel to each other medio-laterally. A topography is described for the limbic reticulo-thalamic connections, suggesting that the rostral TRN is able to influence circumscribed areas of the limbic thalamus.

Thalamocortical synapses between axons from the mediodorsal thalamic nucleus and pyramidal cells in the prelimbic cortex of the rat

A combined anterograde axonal degeneration and Golgi electron microscopic (Golgi-EM) study was undertaken to identify thalamocortical synaptic connections between axon terminals from the mediodorsal thalamic nucleus (MD) and pyramidal cells in layers III and V of the agranular prelimbic cortex in the rat. The morphological characteristics of thalamocortical synapses from MD were also examined by labeling axon terminals with anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). WGA-HRP labeled axon terminals from MD to the prelimbic cortex were small in size (0.5-1 microns in diameter), contained round synaptic vesicles, and formed axospinous synapses with asymmetrical membrane thickenings. With Golgi-EM methods, gold-toned apical dendrites in layer III were analyzed by reconstruction of serial ultrathin sections. Following lesions in the thalamus, degenerating thalamocortical axon terminals formed asymmetrical contacts exclusively on dendritic spines of the identified apical dendrites. More thalamocortical synapses were found on apical dendrites of layer V pyramidal cells than on apical dendrites of layer III pyramidal cells. In addition to thalamocortical synapses, a very few unlabeled symmetrical synapses were found on apical dendrites and somata of pyramidal cells, but they were not quantified and their sources are unknown.

Electron microscopic evidence that axon terminals from the mediodorsal thalamic nucleus make direct synaptic contacts with callosal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and HRP retrograde labeling has shown that there exist monosynaptic connections between afferent fibers from the mediodorsal thalamic nucleus (MD) and callosal cells in the prelimbic cortex of the rat. Degenerating axon terminals from MD made asymmetrical synaptic contacts with dendritic spines from apical dendrites of layer III pyramidal cells that were retrogradely labeled with HRP after its injection into the prelimbic cortex contralateral to MD lesions.

Retrograde labeling of retinal ganglion cells and brain neuronal subsets by [3H]-D-aspartate injection in the Syrian hamster hypothalamus

The circadian pacemaker in the suprachiasmatic nuclei (SCN) is entrained to the environmental light-dark cycle via a direct retinal projection to the hypothalamus. This projection is thought to use glutamate or aspartate as neurotransmitter. [3H]-D-Aspartate was microinjected in the SCN and adjacent hypothalamic nuclei of Syrian hamsters. This neuronal tracer is selectively taken up by terminals of neurons that use glutamate or aspartate as neurotransmitter and retrogradely transported to their perikarya. With autoradiography labeled cells were visualized in the retinal ganglion cell layer. Labeled cells were also found in a subset of brain nuclei known to project to the injection area. Labeled cells were detected in the bed nucleus of the stria terminalis, paraventricular nucleus of the thalamus, lateral septal nucleus, and medial amygdaloid nucleus. No labeled cells were observed in the medial septal nucleus, intergeniculate leaflet, and ventral lateral geniculate nucleus, which are also known to project to the SCN. Our results indicate that glutamatergic/aspartatergic retinal ganglion cells project to the SCN and adjacent medial hypothalamic nuclei. Moreover, the SCN may receive glutamatergic/aspartatergic input from the brain neuronal subsets that were retrogradely labeled with [3H]-D-aspartate.

Nucleus reticularis thalami connections with the mediodorsal thalamic nucleus: a light and electron microscopic study in the monkey

Wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and biotinylated dextran amine (BDA) were used as tracers to study nucleus reticularis (NRT) connections with the mediodorsal nucleus (MD). Injections of WGA-HRP in the MD resulted in retrograde labeling of cells in the anteromedial segment of the NRT, the so-called rostral NRT pole. Injections of WGA-HRP and BDA in this NRT region resulted in dense anterograde labeling in the MD. Labeled NRT fibers gave off several collaterals to different MD regions ending with terminal plexuses of thin varicose fibers. In the neuropil, the varicosities were distributed at random, and no tendency to form pericellular baskets was noted. Postembedding immunocytochemistry for GABA was performed on the tissue containing anterograde WGA-HRP label for identification of NRT boutons under electron microscope. The double-labeled boutons were of small to medium size, contained a large number of pleomorphic vesicles, few mitochondria, and formed multiple symmetric synaptic contacts. The number of contacts established by one bouton ranged from 1 to 4 with an average of 1.8 per bouton. About 60% of these boutons made synapses on distal dendrites of GABAergic local circuit neurons; 33% of synaptic contacts were on distal dendrites of thalamocortical neurons, and the rest on their proximal dendrites and soma. NRT boutons were also found in serial synapses and triads. The results demonstrate that the NRT input to the MD is organized so that a single fiber innervates; different MD regions and its terminals form numerous synaptic contacts mostly on the distal dendrites of a large number of local circuit neurons and projection neurons.

Indirect nucleus accumbens input to the prefrontal cortex via the substantia nigra pars reticulata: a combined anatomical and electrophysiological study in the rat

The nucleus accumbens is a major component of the ventral striatum through which most of the limbic affiliated cortical areas gain access to the basal ganglia circuitry. In this study, the organization of the pathways linking the nucleus accumbens to the thalamus, via the substantia nigra pars reticulata, was examined in the rat using anatomical and electrophysiological methods. Use of anterograde and retrograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase has established that the core of the nucleus accumbens innervates a dorsal region of the substantia nigra pars reticulata which projects to subfields of the mediodorsal and ventral medial thalamic nuclei. These subfields consist of the rostral pole of the mediodorsal nucleus with the exception of its central segment and a region of the ventral medial nucleus, medial to the mammillothalamic tract. Confirming the existence of a nucleus accumbens nigrothalamic link, we have observed that electrical or chemical stimulation of the nucleus accumbens induces an inhibition of the spontaneous discharges of the nigral cells which project to the mediodorsal and ventral medial thalamic nuclei. Finally, the cortical projections of the thalamic subfields involved in the nucleus accumbens nigrothalamic circuit were determined using the anterograde and retrograde axonal transport of wheatgerm agglutinin conjugated with horseradish peroxidase. These subfields innervate mainly the prelimbic and to a lesser degree the orbital areas of the prefrontal cortex. The present data show that the substantia nigra pars reticulata is a major link between the core of the nucleus accumbens and the prefrontal cortex and provide further evidence for the concept of a parallel architecture in the basal ganglia thalamocortical circuits of the ventral striatum.

GABAergic projection from the intercalated cell masses of the amygdala to the basal forebrain in cats

The intercalated cell masses (ICMs) are dense clusters of small GABAergic cells interposed between the basolateral and centromedial nuclear groups of the amygdala. Until now, the ICMs have been largely ignored in anatomical studies of the amygdaloid complex. Thus, this study was undertaken to identify some of their targets by means of tract-tracing methods combined with immunohistochemical techniques. Wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into numerous cortical areas and dorsal thalamic nuclei, in the anterior commissure and/or stria terminalis nuclei, and in the caudate nucleus, as well as into lateral and preoptic hypothalamic areas. Very few retrogradely labeled cells were seen in the ICMs following these injections. In contrast, massive retrograde labeling was found in the rostral groups of ICMs after WGA-HRP injections involving the substantia innominata and horizontal limb of the diagonal band. Furthermore, these retrogradely labeled intercalated cells were also GABA-immunoreactive. Results of iontophoretic injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the rostral ICMs confirmed that they contribute a massive projection to the entire extent of the substantia innominata and horizontal limb of the diagonal band. Electron microscopic observations of ultrathin sections prepared for postembedding GABA or glutamate immunocytochemistry revealed that the ICM terminals labeled with PHA-L displayed GABA, but not glutamate immunoreactivity, and formed symmetric synapses with dendritic profiles. The present findings constitute the first direct demonstration of an amygdalofugal GABAergic projection to the basal forebrain. Considering that the basal forebrain contains a group of cholinergic and GABAergic neurons collectively projecting to the entire cortical mantle, this GABAergic projection of the ICMs could allow the amygdaloid complex to influence the activity of widespread cortical regions to which it is not directly connected, at least in the cat.

Organization of cortical afferents to the rostral, limbic sector of the rat thalamic reticular nucleus

The organization of limbic cortical afferents to the thalamic reticular nucleus (TRN) is described. Wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), biocytin, neurobiotin, or fluorescent dextrans was delivered into the rat cingulate, retrosplenial, and, for comparison, somatosensory cortices. In other species a slab-like arrangement of cortical terminals has been described for sensory TRN sectors. Here this is seen in the rat somatosensory sector. Terminals from limbic cortices did not cluster into slabs but were found to fill the entire thickness of distinct rostral TRN regions. The cingulate and retrosplenial recipient TRN regions overlap, as do the projections from these cortical areas to anterior thalamic nuclei. Retrosplenial fibres contacted the dorsal and rostral TRN, which is known to be connected to the retrosplenial-recipient anteroventral, anterodorsal, and laterodorsal thalamic nuclei. Cingulate terminals occupied more ventral regions of the rostral TRN. This area is connected to thalamic nuclei also innervated by the cingulate cortex: the mediodorsal and anteromedial nuclei. A loose, but clear, topography could be defined for the cingulate-reticular pathway: rostrocaudal and mediolateral directions in the cortex are represented by ventrodorsal and rostrocaudal directions in the TRN, respectively. This organization of limbic corticoreticular pathway corresponds to the arrangement of limbic corticothalamic connections. The ultrastructure of the limbic cortical axon terminals was similar to that of the cortical boutons (D-type) described previously. The labelled terminals formed asymmetrical synapses onto dendritic profiles of reticular neurons. These findings, together with data in the literature, show significant morphological and connectional differences within the TRN that imply functional heterogeneities.

Evidence for excitatory amino acid neurotransmitters in forward and feedback corticocortical pathways within rat visual cortex

It is a commonly accepted notion that cells which make projections between the multiple cortical areas found in the mammalian visual system are excitatory, but there is little direct evidence that this is the case. Here we demonstrate using retrograde tracing with D-[3H]aspartate that connections in the rat which project from lower to higher visual areas (i.e. forward) and those which project from higher to lower areas (i.e. feedback) may use excitatory amino acid neurotransmitters. Following injection into the primary visual cortex, clusters of retrogradely labelled cells were found in several extrastriate areas within the cytoarchitectonic subdivisions 18a ('areas' LM, AL, PX, FLX, RL, AX) and 18b ('area' MX), and in the retrosplenial cortex. In all of these areas D-[3H]aspartate-labelled cells were surrounded by diffuse label which may represent anterograde labelling of axon terminals. This suggests that both legs of reciprocal intracortical circuits have similar chemospecificity. To directly demonstrate excitatory amino acid localization in forward projections, D-[3H]aspartate was injected into extrastriate area LM. As expected, the results revealed retrogradely labelled neurons within area 17. Outside area 17, LM injections labelled neurons in AL, PX, FLX, RL, AX and MX. Taken in the context of the hierarchy of areas in rat cerebral cortex (Coogan and Burkhalter, J. Neurosci., 13, 3749-3772, 1993), these results show that D-[3H]aspartate labels: (1) forward connections from area 17 to LM, AL, PX, RL, AX and MX, (2) feedback connections from LM, AL, FLX, PX, RL, AX and MX to area 17, (3) feedback connections from AL, PX, RL, AX and MX to LM, and (4) lateral connections between FLX and LM. These findings strongly indicate that both forward and feedback connections as well as lateral connections at several different levels of the cortical hierarchy use excitatory amino acid neurotransmitters.

Colocalization of excitatory and inhibitory neurotransmitter markers in striatal projection neurons in the rat

The principle neuronal output of the neostriatum comes from medium spiny neurons that project from the caudate/putamen to the globus pallidus and substantia nigra. Although current evidence generally indicates that gamma-aminobutyric acid (GABA) is the principal neurotransmitter in this pathway, this cannot account for the excitatory synaptic activity present among cultures of striatal neurons or the short latency excitatory postsynaptic potentials which often proceed or obscure inhibitory activity evoked by striatal stimulation. In this study, retrograde transport of [3H]D-aspartate has been used to demonstrate striato-pallidal and striato-nigral neurons that possess a high-affinity uptake system for glutamate and aspartate and are therefore putatively glutamatergic. Injections of [3H]D-aspartate into the globus pallidus or substantia nigra, pars reticularis of the rat retrogradely labeled medium-sized neurons throughout the rostral-caudal extent of the neostriatum. To characterize this population further, adjacent sections were immunoreacted with antibodies to either GABA, glutamic acid decarboxylase (GAD), calbindin, or parvalbumin prior to autoradiographic processing. Under these conditions, autoradiographically labeled neurons displayed positive immunoreactivity for GABA, GAD, or calbindin. Autoradiographic label did not colocalize with parvalbumin immunoreactivity. The colocalization of anatomical markers of GABAergic and glutamatergic neurotransmission raises the possibility that both neurotransmitters are functionally expressed within single striatal projection neurons.

Organization of the output of the ventral striatopallidal system in the rat: ventral pallidal efferents

The efferent projections of the ventral pallidum in the rat were studied using anterograde tracing of Phaseolus vulgaris-leucoagglutinin and retrograde tracing of choleratoxin subunit B. The main aim of this study was to determine the degree of topographical organization in the outputs of the ventral pallidum. In the telencephalon, ventral pallidal fibers reach the prefrontal cortex, the ventral striatum, the lateral septum, the basolateral, lateral, and central amygdaloid nuclei, and the lateral entorhinal area. Diencephalic targets of ventral pallidal fibers are the lateral hypothalamus, the reticular nucleus of the thalamus, the mediodorsal thalamic nucleus, the dorsomedial part of the subthalamic nucleus, the medial part of the parafascicular nucleus and the lateral habenula. In the mesencephalon, ventral pallidal fibers terminate in the ventral tegmental area, the substantia nigra, the retrorubral area, the median raphe nucleus, the nucleus raphe magnus, the peribrachial area, the ventromedial part of the central gray substance and the locus coeruleus. The results of the experiments in which retrograde tracers were injected in different nuclei in the mesencephalon allow the distinction of two main areas in the ventral pallidum. Deposits of retrograde tracers in the substantia nigra, pars reticulata result in labeling of cells in the dorsolateral part of the ventral pallidum, located immediately ventral to the anterior limb of the anterior commissure. Retrograde tracer injections in other targets of the ventral mesencephalon, i.e. the dopaminergic cell groups A10, A9 or A8, or nuclei in the peribrachial area result in labeling of neurons in an extensive ventromedial and ventrolateral zone of the ventral pallidum. The medial part of this ventral pallidal zone projects to the ventral tegmental area, whereas ventral and lateral parts connect with more lateral and caudal mesencephalic targets. The projections from the ventral pallidum to the ventral striatum, the subthalamic nucleus and adjacent lateral hypothalamic area, and the mediodorsal thalamic nucleus are distinctly topographically organized. The ventral pallidostriatal projections preserve a medial-to-lateral, a dorsal-to-ventral and, to a lesser degree, a rostral-to-caudal topography. With respect to the subthalamic region, the dorsolateral part of the ventral pallidum projects to the dorsomedial part of the subthalamic nucleus, whereas the ventromedial and ventrolateral parts of the ventral pallidum are topographically connected with the area of the lateral hypothalamus medially adjacent to the subthalamic nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct synaptic connections between thalamocortical axon terminals from the mediodorsal thalamic nucleus (MD) and corticothalamic neurons to MD in the prefrontal cortex

A combined anterograde axonal degeneration with ibotenic acid and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing study revealed that some degenerating thalamocortical axon terminals from the mediodorsal thalamic nucleus (MD) directly formed asymmetrical synaptic contacts predominantly with dendritic spines of apical dendrites of WGA-HRP-labeled corticothalamic projection neurons to MD in the prelimbic cortex of the rat. This result suggests that there is a monosynaptic feedback loop from and to MD via deeper layer neurons in the prelimbic cortex.

Neuronal and synaptic composition of the mediodorsal thalamic nucleus in the rat: a light and electron microscopic Golgi study

The distribution and dendritic domain of neurons in each segment of the mediodorsal thalamic nucleus (MD) have been studied in the rat with the Golgi technique. In addition, a combined Golgi method-electron microscopic (Golgi-EM) study was undertaken to determine the distribution of morphologically distinct synapse types along the dendrites of individual identified neurons in MD. All the subdivisions or "segments" of MD (medial, central, lateral) contained both stellate and fusiform cells. The dendritic domain of both types of cells was predominantly restricted to the same segment of MD that contained the cell body of the neuron. Typical stellate neurons were found near the center of each segment, with radiating dendrites that extended to but not across the boundaries of the segment. Fusiform cells were usually located close to the segmental or nuclear boundaries and tended to have dendrites oriented parallel to those borders; again, the dendrites tended not to extend across borders between segments or at the outer edge of MD. In the medial segment of MD many fusiform cells had especially bipolar dendritic configurations, generally with a dorsoventral orientation. Because no small neurons were identified that might correspond to thalamic interneurons, all the impregnated cells in MD are presumed to be thalamocortical projection neurons. These results indicate that cells and their major dendrites are confined to a single segment of MD, with little dendritic overlap across segmental or nuclear borders. The segments of MD may therefore be considered to be relatively independent subnuclei. The distribution of the four types of synapses previously identified in MD (Kuroda and Price, J. Comp. Neurol., 303:513-533, 1991) was determined along several identified dendrites studied with the Golgi-EM method. Primary dendrites were contacted mostly by large axon terminals, including both large, round vesicle (LR) terminals and large, pleomorphic vesicle (LP) terminals, as well as a few small to medium sized terminals with pleomorphic vesicles (SMP). No small terminals with round vesicles (SR terminals) were observed to make synapses with primary dendrites. Secondary and tertiary dendrites received synapses from all types of axon terminals. Higher order dendrites were contracted predominantly by SR boutons, but they also carried some LR and SMP terminals. In addition, SMP boutons were often found to form symmetric contacts with cell somata.

The organization of the thalamocortical connections of the mediodorsal thalamic nucleus in the rat, related to the ventral forebrain-prefrontal cortex topography

The medial and central segments of the mediodorsal nucleus of the thalamus (MD) receive afferents from the ventral forebrain, including the piriform cortex, the ventral pallidum, and the amygdaloid complex. Because MD is reciprocally interconnected with prefrontal and agranular insular cortical areas, it provides a relay of ventral forebrain activity to these cortical areas. However, there are also direct projections from the piriform cortex and the amygdala to the prefrontal and agranular insular cortices. This study addresses whether this system has a "triangular" organization, such that structures in the ventral forebrain project to interconnected areas in MD and the prefrontal/insular cortex. The thalamocortical projections of MD have been studied in experiments with injections of retrograde tracers into prefrontal or agranular insular cortical areas. In many of the same experiments, projections from the ventral forebrain to MD and to the prefrontal/insular cortex have been demonstrated with anterograde axonal tracers. The connections of the piriform cortex (PC) with MD and the prefrontal/insular cortex form an organized triangular system. The PC projections to the central and medial segments of MD and to the lateral orbital cortex (LO) and the ventral and posterior agranular insular cortices (AIv and AIp) are topographically organized, such that more caudal parts of PC tend to project more medially in MD and more caudally within the orbital/insular cortex. The central and medial portions of MD also send matching, topographically organized projections to LO, AIv and AIp, with more medial parts of MD projecting further caudally. The anterior cortical nucleus of the amygdala (COa) also projects to the dorsal part of the medial segment of MD and to its cortical targets, the medial orbital area (MO) and AIp. The projections of the basal/accessory basal amygdaloid nuclei to MD and to prefrontal cortex, and from MD to amygdaloceptive parts of prefrontal cortex, are not as tightly organized. Amygdalothalamic afferents in MD are concentrated in the dorsal half of the medial segment. Cells in this part of the nucleus project to the amygdaloceptive prelimbic area (PL) and AIp. However, other amygdaloceptive prefrontal areas are connected to parts of MD that do not receive fibers from the amygdala. Ventral pallidal afferents are distributed to all parts of the central and medial segments of MD, overlapping with the fibers from the amygdala and piriform cortex. Fibers from other parts of the pallidum, or related areas such as the substantia nigra, pars reticulata, terminate in the lateral and ventral parts of MD, where they overlap with inputs from the superior colliculus and other brainstem structures.(ABSTRACT TRUNCATED AT 400 WORDS)