Ultrastructural analysis of the terminals of various somatosensory pathways in the ventrobasal complex of the rat thalamus: an electron-microscopic study using wheatgerm agglutinin conjugated to horseradish peroxidase as an axonal tracer

Functional and structural synaptic remodeling mechanisms underlying somatotopic organization and reorganization in the thalamus

The somatosensory system organizes the topographic representation of body maps, termed somatotopy, at all levels of an ascending hierarchy. Postnatal maturation of somatotopy establishes optimal somatosensation, whereas deafferentation in adults reorganizes somatotopy, which underlies pathological somatosensation, such as phantom pain and complex regional pain syndrome. Here, we focus on the mouse whisker somatosensory thalamus to study how sensory experience shapes the fine topography of afferent connectivity during the critical period and what mechanisms remodel it and drive a large-scale somatotopic reorganization after peripheral nerve injury. We will review our findings that, following peripheral nerve injury in adults, lemniscal afferent synapses onto thalamic neurons are remodeled back to immature configuration, as if the critical period reopens. The remodeling process is initiated with local activation of microglia in the brainstem somatosensory nucleus downstream to injured nerves and heterosynaptically controlled by input from GABAergic and cortical neurons to thalamic neurons. These fruits of thalamic studies complement well-studied cortical mechanisms of somatotopic organization and reorganization and unveil potential intervention points in treating pathological somatosensation.

Immunocytochemical and ultrastructural organization of the taste thalamus of the tree shrew (Tupaia belangeri)

Ventroposterior medialis parvocellularis (VPMP) nucleus of the primate thalamus receives direct input from the nucleus of the solitary tract, whereas the homologous thalamic structure in the rodent does not. To reveal whether the synaptic circuitries in these nuclei lend evidence for conservation of design principles in the taste thalamus across species or across sensory thalamus in general, we characterized the ultrastructural and molecular properties of the VPMP in a close relative of primates, the tree shrew (Tupaia belangeri), and compared these to known properties of the taste thalamus in rodent, and the visual thalamus in mammals. Electron microscopy analysis to categorize the synaptic inputs in the VPMP revealed that the largest-size terminals contained many vesicles and formed large synaptic zones with thick postsynaptic density on multiple, medium-caliber dendrite segments. Some formed triads within glomerular arrangements. Smaller-sized terminals contained dark mitochondria; most formed a single asymmetric or symmetric synapse on small-diameter dendrites. Immuno-EM experiments revealed that the large-size terminals contained VGLUT2, whereas the small-size terminal populations contained VGLUT1 or ChAT. These findings provide evidence that the morphological and molecular characteristics of synaptic circuitry in the tree shrew VPMP are similar to that in nonchemical sensory thalamic nuclei. Furthermore, the results indicate that all primary sensory nuclei of the thalamus in higher mammals share a structural template for processing thalamocortical sensory information. In contrast, substantial morphological and molecular differences in rodent versus tree shrew taste nuclei suggest a fundamental divergence in cellular processing mechanisms of taste input in these two species.

Roles of GABAA and GABAB receptors in regulating thalamic activity by the zona incerta: a computational study

The posterior thalamic nucleus (PO) is a higher order nucleus heavily implicated in the processing of somatosensory information. We have previously shown in rodent models that activity in PO is tightly regulated by inhibitory inputs from a GABAergic nucleus known as the zona incerta (ZI). The level of incertal inhibition varies under both physiological and pathological conditions, leading to concomitant changes in PO activity. These changes are causally linked to variety of phenomena from altered sensory perception to pathological pain. ZI regulation of PO is mediated by GABAA and GABAB receptors (GABAAR and GABABR) that differ in their binding kinetics and their electrophysiological properties, suggesting that each may have distinct roles in incerto-thalamic regulation. We developed a computational model to test this hypothesis. We created a two-cell Hodgkin-Huxley model representing PO and ZI with kinetically realistic GABAAR- and GABABR-mediated synapses. We simulated spontaneous and evoked firing in PO and observed how these activities were affected by inhibition mediated by each receptor type. Our model predicts that spontaneous PO activity is preferentially regulated by GABABR-mediated mechanisms, while evoked activity is preferentially regulated by GABAAR. Our model also predicts that modulation of ZI firing rate and synaptic GABA concentrations is an effective means to regulate the incerto-thalamic circuit. The coupling of distinct functions to GABAAR and GABABR presents an opportunity for the development of therapeutics, as particular aspects of incerto-thalamic regulation can be targeted by manipulating the corresponding receptor class. Thus these findings may provide interventions for pathologies of sensory processing.

Adenylate cyclase 1 promotes strengthening and experience-dependent plasticity of whisker relay synapses in the thalamus

Synaptic refinement, a process that involves elimination and strengthening of immature synapses, is critical for the development of neural circuits and behaviour. The present study investigates the role of adenylate cyclase 1 (AC1) in developmental refinement of excitatory synapses in the thalamus at the single-cell level. In the mouse, thalamic relay synapses of the lemniscal pathway undergo extensive remodelling during the second week after birth, and AC1 is highly expressed in both pre- and postsynaptic neurons during this period. Synaptic connectivity was analysed by patch-clamp recording in acute slices obtained from mice carrying a targeted null mutation of the adenylate cyclase 1 gene (AC1-KO) and wild-type littermates. We found that deletion of AC1 had no effect on the number of relay inputs received by thalamic neurons during development. In contrast, there was a selective reduction of AMPA-receptor-mediated synaptic responses in mutant thalamic neurons, and the effect increased with age. Furthermore, experience-dependent plasticity was impaired in thalamic neurons of AC1-KO mice. Whisker deprivation during early life altered the number and properties of relay inputs received by thalamic neurons in wild-type mice, but had no effects in AC1-KO mice. Our findings underline a role for AC1 in experience-dependent plasticity of excitatory synapses.

A critical window for experience-dependent plasticity at whisker sensory relay synapse in the thalamus

This study investigated the role of sensory experience in the refinement of whisker sensory relay synapses in the ventral posterior medial nucleus (VPm) of the murine thalamus. Sensory deprivation was done by whisker plucking, and synaptic connectivity was determined by whole-cell patch-clamp recording in acute slices. Sensory deprivation started at P12-P13, but not at P16, disrupted the elimination of VPm relay synapses. The majority of deprived neurons received multiple relay inputs, whereas the majority of nondeprived neurons received a single relay input. Sensory deprivation started a few days earlier at P10, however, had no effect on synapse elimination. The disruption of synapse elimination was associated with a delay in synapse maturation. The AMPA/NMDA ratio of EPSC was significantly smaller in deprived neurons. On the other hand, deprivation had no effect on the peak amplitude or decay time constant of the NMDA component, or the I-V relationship of the AMPA component, nor does it affect the paired-pulse ratio of EPSCs. The reduction in the AMPA/NMDA ratio was already evident within 24 h of whisker plucking, and the effect is associated with a reduction in the amplitude of quantal AMPA events. Thus, P12-P13 is a critical period for experience-dependent refinement at the whisker sensory relay synapse in the VPm.

Neurons in the dorsal column nuclei of the rat emit a moderate projection to the ipsilateral ventrobasal thalamus

The dorsal column nuclei (DCN; gracile and cuneate nuclei) give rise to the medial lemniscus, the fibre system that provides an organised somatosensory input to the thalamus. Unlike the spinothalamic and trigeminothalamic tracts that project, also to the ipsilateral thalamus, the medial lemniscus system is believed to be entirely crossed. We demonstrate that DCN emit a small number of axons that reach the ipsilateral thalamus. As retrograde fluorescent neuronal tracer Fluoro-gold was stereotaxically injected in the ventrobasal thalamus of nine young adult Wistar rats. The injection foci were voluminous and encroached upon adjacent nuclei, but the periphery of the injection halo never spilled over to the contralateral thalamus. All sections of the contralateral gracile and cuneate nuclei and the midline nucleus of Bischoff contained abundant retrogradely labelled neurons. The comparison with the Nissl-stained parallel sections suggests that approximately 70-80% of the DCN neurons project to the contralateral thalamus. Counting of retrogradely labelled neurons in two cases revealed 4,809 and 4,222 neurons in the contralateral and 265 and 214 in the ipsilateral DCN, respectively. Thus, although less prominent than the ipsilateral spinothalamic tract, the lemniscal system also emits an ipsilateral projection that accounts for about 5% of the neuronal population in DCN that innervates the ventrobasal thalamus.

Patterned activity via spinal dorsal quadrant inputs is necessary for the formation of organized somatosensory maps

The normal development of the somatosensory system requires intact sensory inputs from the periphery during a critical window of time early in development. Here we determined how the removal of only part of the ascending spinal inputs early in development affects the anatomical and neurophysiological development of the somatosensory system. We performed spinal overhemisections in rat pups at C3/C4 levels on the third day after birth. This procedure hemisects the spinal cord on one side and transects the dorsal funiculus on the other side. When the rats were 6-8 months old, the responsiveness and somatotopy of the primary somatosensory cortex (S1) contralateral to the hemisection were determined using standard multiunit mapping techniques. Sections of the flattened cortex were processed for cytochrome oxidase activity, Nissl substance, or myelin. We found that histologically apparent modules that are normally present in the regions of the forepaw and the hindpaw representations were absent, whereas the lateral barrel field representing the face was completely normal. The neurons in the forepaw regions of S1 either did not respond to the stimulation of the skin of any region of the body or responded to the stimulation of the upper arm afferents that enter the spinal cord rostral to the site of the lesion. The results show that a lack of normal sensory inputs via ascending pathways in the dorsal spinal cord during early development results in massive anatomical and neurophysiological abnormalities in the cortex. Intact crossed spinothalamic pathways are unable to support the normal development of the forepaw barrels.

Differences in quantal amplitude reflect GluR4- subunit number at corticothalamic synapses on two populations of thalamic neurons

Low-frequency thalamocortical oscillations that underlie drowsiness and slow-wave sleep depend on rhythmic inhibition of relay cells by neurons in the reticular nucleus (RTN) under the influence of corticothalamic fibers that branch to innervate RTN neurons and relay neurons. To generate oscillations, input to RTN predictably should be stronger so disynaptic inhibition of relay cells overcomes direct corticothalamic excitation. Amplitudes of excitatory postsynaptic conductances (EPSCs) evoked in RTN neurons by minimal stimulation of corticothalamic fibers were 2.4 times larger than in relay neurons, and quantal size of RTN EPSCs was 2.6 times greater. GluR4-receptor subunits labeled at corticothalamic synapses on RTN neurons outnumbered those on relay cells by 3.7 times, providing a basis for differences in synaptic strength.

Differential synaptic distribution of AMPA receptor subunits in the ventral posterior and reticular thalamic nuclei of the rat

Although the mechanisms by which the cerebral cortex controls its ascending input are still poorly understood, it is known that cortical control at the thalamic level is via direct glutamatergic projections to relay nuclei and to the reticular nucleus. Here we confirm previous light microscopic reports of a high expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, GluR4, in reticular and ventral posterior thalamic nuclei of the rat, and moderate staining using an antibody recognizing both GluR2 and GluR3. In contrast only low levels of staining for GluR2, and barely detectable levels of GluR1 immunoreactivity were observed. After injections of biotinylated dextran, electron microscopy revealed that anterogradely-labeled cortical synapses in both thalamic nuclei were small with fewer mitochondria and more densely-packed vesicles than terminals likely to arise from intrinsic and ascending pathways. We performed post-embedding immunogold to provide quantitative data on the density of AMPA receptor subunits at morphologically-defined groups of synapses. We found that corticothalamic synapses in the reticular thalamic nucleus contain twice as much GluR2/3, and at least three times more GluR4 protein than do intrathalamic synapses. In the ventral posterior nucleus, corticothalamic synapses contain similar amounts of GluR2/3, but four times more GluR4 than do those from ascending afferents. Corticothalamic synapses in reticular nucleus contain slightly more GluR2/3, and three times more GluR4, than those in ventral posterior nucleus. We conclude that enrichment of GluR4 at morphologically-defined cortical synapses is a feature common to both thalamic nuclei, and those in the reticular nucleus express higher levels of AMPA receptors. The rapid kinetics of GluR4-rich AMPA receptors we suggest indicate that cortical descending control may be more temporally precise than previously recognized.

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia. The dominant frequency of electrocorticographic (ECoG) recordings was used to determine the depth of halothane or urethan anesthesia while recording extracellular single-unit responses from thalamic ventral posterior medial (VPM) neurons. A piezoelectric stimulator was used to deflect individual whiskers to assess the peak onset latency, magnitude, probability of response, and receptive field (RF) size. There was a predictable increase in the dominant ECoG frequency from deep stage IV to light stage III-1 anesthetic levels. There was no detectable frequency at stage IV, a 1- to 2-Hz dominant frequency at stage III-4, 3-4 Hz at stage III-3, 5-7 Hz at stage III-2, and a dual 6- and 10- to 13-Hz pattern at stage III-1. Reflexes and other physical signs showed a correlation with depth of anesthesia but exhibited too much overlap between stages to be used as a criterion for any single stage. RF size and peak onset latency of VPM neurons to whisker stimulations increased between stage III-4 and III-1. A dramatic increase in RF size and response latency occurred at the transition from stage III-3 (RF size approximately 2 whiskers, latency approximately 7 ms) to stage III-2 (RF size approximately 6 whiskers, latency approximately 11 ms). Response probability and magnitude decreased from stage III-4 to stage III-3 and III-2. No responses were ever evoked in VPM cells by vibrissa movement at stage IV. These changes in VPM responses as a function of anesthetic depth were seen only when the nucleus principalis (PrV) and nucleus interpolaris (SpVi) trigeminothalamic pathways were both intact. Eliminating SpVi inputs to VPM, either by cutting the primary trigeminal afferent fibers to SpVi or cutting axons projecting from SpVi to VPM, immediately reduced the RF size to fewer than three whiskers. In addition, the predictable changes in VPM response probability, response magnitude, and peak onset latency at different anesthetic depths were all absent after SpVi pathway interruption. We conclude that 1) the PrV input mediates the near "one-to-one" correspondence between a neuronal response in VPM and a single mystacial whisker, 2) in contrast, the SpVi input to VPM is primarily responsible for the RF properties of VPM neurons at light levels of anesthesia and presumably in the awake animal, and 3) alterations in VPM responses produced by changing the depth of anesthesia are due to its selective influence on the properties mediated by SpVi inputs at the level of the thalamus.

Visualization of calcium influx through channels that shape the burst and tonic firing modes of thalamic relay cells

Thalamic neurons have two firing modes: "tonic" and "burst." During burst mode, both low-threshold (LT) and high-threshold (HT) calcium channels are activated, while in tonic mode, only the HT-type of calcium channel is activated. The calcium signals associated with each firing mode were investigated in rat thalamic slices using whole cell patch clamping and confocal calcium imaging. Action potentials were induced by direct current injection into thalamic relay cells loaded with a fluorescent calcium indicator. In both tonic and burst firing modes, large calcium signals were recorded throughout the soma and proximal dendrites. To map the distribution of the channels mediating these calcium fluxes, LT and HT currents were independently activated using specific voltage-clamp protocols. We focused on the proximal region of the cell (up to 50 microm from the soma) because it appeared to be well clamped. For a voltage pulse of a given size, the largest calcium signals were observed in the proximal dendrites with smaller signals occurring in the soma and nucleus. This was true for both LT and HT signals. Rapid imaging, using one-dimensional linescans, was used to more precisely localize the calcium influx. For both LT and HT channels, calcium influx occurred simultaneously throughout all imaged regions including the soma and proximal dendrites. The presence of sizable calcium signals in the dendrites, soma, and nucleus during both firing modes, and the presence of LT calcium channels in the proximal dendrite where sensory afferents synapse, have implications for both the electrical functioning of relay cells and the transmission of sensory information to cortex.

Distribution of four types of synapse on physiologically identified relay neurons in the ventral posterior thalamic nucleus of the cat

This study was aimed at providing quantitative data on the thalamic circuitry that underlies the central processing of somatosensory information. Four physiologically identified thalamocortical relay neurons in the ventral posterior lateral nucleus (VPL) of the cat thalamus were injected with horseradish peroxidase and subjected to quantitative electron microscopy after pre- or postembedding immunostaining for gamma-aminobutyric acid to reveal synaptic terminals of thalamic inhibitory neurons. The four cells all had rapidly adapting responses to light mechanical stimuli applied to their receptive fields, which were situated on hairy or glabrous skin or related to a joint. Their dendritic architecture was typical of cells previously described as type I relay cells in VPL, and they lacked dendritic appendages. Terminals ending in synapses on the injected cells were categorized as RL (ascending afferent), F (inhibitory), PSD (presynaptic dendrite), and RS (mainly corticothalamic) types and were quantified in reconstructions of serial thin sections. RL and F terminals formed the majority of the synapses on proximal dendrites (approximately 50% each). The number of synapses formed by RL terminals declined on intermediate dendrites, but those formed by F terminals remained relatively high, declining to moderate levels (20-30%) on distal dendrites. RS terminals formed moderate numbers of the synapses on intermediate dendrites and the majority (> 60%) of the synapses on distal dendrites. Synapses formed by PSDs were concentrated on intermediate dendrites and were few in number (approximately 6%). They formed synaptic triads with F terminals and rarely with RL terminals. On somata, only a few synapses were found, all made by F terminals. The total number of synapses per cell was calculated to be 5,584-8,797, with a density of 0.6-0.9 per micrometer of dendritic length. Of the total, RL terminals constituted approximately 15%, F terminals approximately 35%, PSD terminals approximately 5%, and RS terminals approximately 50%. These results provide the first quantitative assessment of the synaptic architecture of thalamic somatic sensory relay neurons and show the basic organizational pattern exhibited by representatives of the physiological type of relay neurons most commonly encountered in the VPL nucleus.

Differential foci and synaptic organization of the principal and spinal trigeminal projections to the thalamus in the rat

The thalamus is known to receive single-whisker 'lemniscal' inputs from the trigeminal nucleus principalis (PrV) and multiwhisker 'paralemniscal' inputs from the spinal trigeminal nucleus (SpV), yet the responses of cells in the thalamic ventroposteromedial nucleus (VPM) are most similar to and contingent upon inputs from PrV. This may reflect a differential termination pattern, density and/or synaptic organization of PrV and SpV projections. This hypothesis was tested in adult rats using anterograde double-labelling with fluorescent dextrans, horseradish peroxidase (HRP) and choleragenoid, referenced against parvalbumin and calbindin immunoreactivity. The results indicated that PrV's most robust thalamic projection is to the whisker-related barreloids of VPM. The SpV had robust projections to non-barreloid thalamic regions, including the VPM 'shell' encapsulating the barreloid area, a caudal and ventral region of VPM that lacks barreloids and PrV inputs, the posterior thalamic nucleus, nucleus submedius and zona incerta. Within the barreloid portion of VPM, SpV projections were sparse relative to those from PrV, and most terminal labelling occurred in the peripheral fringes of whisker-related patches and in interbarreloid septae. Thus, PrV and SpV have largely complementary projection foci in the thalamus. Intra-axonal staining of a small sample of trigeminothalamic axons with whisker or guard hair receptive fields revealed highly localized and somatotopic terminal aggregates in VPM that spanned areas no larger than that of a single barreloid. In the electron microscopic component of this study, HRP transport to the barreloid region of VPM from left SpV and right PrV in the same cases revealed PrV terminals contacting dendrites with a broad range of minor axis diameters (mean +/- SD: 1.51 +/- 0.10 microns). SpV terminals were indistinguishable from those of PrV, but they had a disproportionate number of contacts on narrow dendrites (1.27 +/- 0.07 microns, P < 0.01). PrV endings were also more likely to contact VPM somata (11.0 +/- 4.2% of all labelled terminals) than those from SpV (3.0 +/- 1.0%, P < 0.01). Insofar as primary dendrites are thicker than distal dendrites in VPM, these data suggest a differential distribution of PrV and SpV inputs onto VPM cells that may account for their relative efficacies in dictating the responses of VPM cells to whisker stimulation. Multiwhisker receptive fields in VPM may also reflect direct transmission of convergent inputs from PrV.

Demyelination, and remyelination by Schwann cells and oligodendrocytes after kainate-induced neuronal depletion in the central nervous system

Excitotoxins are thought to kill neurons while sparing afferent fibers and axons of passage. The validity of this classical conclusion has recently been questioned by the demonstration of axonal demyelination. In addition, axons are submitted to a profound alteration of their glial environment. This work was, therefore, undertaken to reassess axonoglial interactions over time after an excitotoxic lesion in the rat. Ultrastructural studies were carried out in the ventrobasal thalamus two days to 18 months after neuronal depletion by in situ injections of kainic acid. In some cases, lemniscal afferents were identified by using anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase from the dorsal column nuclei. Two and four days after kainate injection, numerous dying axons displaying typical signs of Wallerian degeneration were observed in a neuropile characterized by the loss of neuronal somata and dendrites, an increase in number of microglia/macrophages and the disappearance of astrocytes. Ten and 12 days after kainate injection, degenerating axons were no longer observed although myelin degeneration of otherwise unaltered axons was ongoing with an accumulation of myelin remnants in the neuropile. At 16 and 20 days, the demyelination process was apparently complete and axons of different diameters were sometimes packed together. One and two months after kainate injection, the axonal environment changed again: remyelination of large-caliber axons occurred at the same time as reactive astrocytes, oligodendrocytes and numerous Schwann cells appeared in the tissue. Schwann cell processes surrounded aggregates of axons of diverse calibers, ensheathed small ones and myelinated larger ones. Axons were also remyelinated by oligodendrocytes. Horseradish peroxidase-labeled lemniscal afferents could be myelinated by either of the two cell types. After three months, the neuropile exhibited an increase in number of hypertrophied astrocytes and the progressive loss of any other cellular or axonal element. At this stage, remaining Schwann cells were surrounded by a glia limitans formed by astrocytic processes. These data indicate that although excitotoxins are sparing the axons, they are having a profound and complex effect on the axonal environment. Demyelination occurs over the first weeks, accompanying the loss of astrocytes and oligodendrocytes. Axonal ensheathment and remyelination takes place in a second period, associated with the reappearance of oligodendrocytes and recruitment of numerous Schwann cells, while reactive astrocytes appear in the tissue at a slightly later time. Over the following months, astrocytes occupy a greater proportion of the neuron-depleted territory and other elements decrease in number.(ABSTRACT TRUNCATED AT 400 WORDS)

Diencephalic projections from the superficial and deep laminae of the medullary dorsal horn in the rat

An important function of the medullary dorsal horn (MDH) is the relay of nociceptive information from the face and mouth to higher centers of the central nervous system. We studied the central projection pattern of axons arising from the MDH by examining the axonal transport of Phaseolus vulgaris-leucoagglutinin (PHA-L). Labeled axon and axon terminal distributions arising from the MDH were analyzed at the light microscopic level. After large injections of PHA-L into both superficial and deep laminae of the MDH in the rat, labeled axons were observed in the nucleus submedius of the thalamus (SUB), ventroposterior thalamic nucleus medialis (VPM), ventroposterior thalamic nucleus parvicellularis (VPPC), posterior thalamic nuclei (PO), zona incerta (ZI), lateral hypothalamic nucleus (LH), and posterior hypothalamic nucleus (PH). Restriction of PHA-L into only the superficial laminae resulted in heavy axon and varicosity labeling in the SUB, VPM, PO, and VPPC and light labeling in LH. In contrast, after injections into deep laminae, labeled axons were mainly distributed in ZI and PH; some were also in VPM and LH, and fewer still in PO and SUB. Varicosities in VPM, SUB, and PO were significantly larger than those in VPPC, ZI, LH, and PH. Varicosity density was highest in SUB and lowest in the VPPC. We concluded that there are two distinct nociceptive pathways, one originating from the superficial MDH and terminating primarily in the dorsal diencephalon and the second originating from deep laminae of the MDH and terminating primarily in the ventral diencephalon. We propose that in the rat, input from the deeper laminae is primarily involved in the motivational-affective component of pain, whereas input from the superficial MDH is related to both the sensory-discriminative and motivational-affective component of pain.

Is there capacity for anatomical and functional repair in the adult somatosensory thalamus?

The capacity for structural and functional remodeling in damaged adult CNS sensory systems can be studied by replacement of neurons in damaged structures by fetal cells from these anatomical origins. For integration to take place, the replacement paradigm assumes that (a) reconnection of adult host afferent fibers onto developing neurons is possible and (b) that the correct molecular signals exist also in the adult brain for fetal neurons to extend axons and pattern synaptic contacts. We have tried to answer some of these fundamental questions by using neuronal depletion models followed by neuronal replacement in the adult rat CNS (Isacson et al. 1984. Nature (London) 311: 458-460; Isacson et al. 1988. Prog. Brain Res. 78: 13-27; Nothias et al. 1988. Brain Res. 461: 349-354; Peschanski and Isacson. 1988. J. Comp. Neurol. 274: 449-463; Sofroniew et al. 1990. Prog. Brain Res. 82: 313-320). In one such model, kainic acid infusions deplete the ventrobasal complex (VB) of all neurons projecting to the somatosensory cortex, while afferent axons from the lemniscal and monoaminergic systems remain in the area. Direct implantation of fetal neurons (gestation age 15-16) of ventrobasal destination allows reconnection of circuitry to take place at the thalamic level, as studied by anatomical tracers, electron microscopy, and functional 2-deoxyglucose studies, while fetal thalamic VB neurons appear less likely to grow through the internal capsule toward the cortical level.

Thalamic processing of vibrissal information in the rat: II. Morphological and functional properties of medial ventral posterior nucleus and posterior nucleus neurons

Extracellular recording, intracellular recording, intracellular horseradish peroxidase injection, and receptive field mapping techniques were employed to evaluate the physiological and morphological properties of medial ventral posterior nucleus (VPM) and posterior nucleus (POm) neurons in normal adult rats. Overall, we physiologically characterized 148 VPM and 121 POm neurons. Over 82% of the VPM cells were excited only by deflection of one or more mystacial vibrissae, 10% were activated by displacement of guard hairs, and the remainder were either excited by indentation of the skin or were unresponsive. Less than 40% of the POm cells were activated by vibrissa deflection, 18% were excited by displacement of guard hairs, and another 17% were unresponsive. Most of the rest of the POm cells were excited by stimulation of skin, mucosa, or activation of muscle-related afferents. Small percentages of POm cells responded only to noxious stimulation, were classified as having a wide dynamic range, or were inhibited by peripheral stimulation. Electrical stimulation of either PrV or SpI activated most neurons in both VPM and POm. This excitation was almost invariably followed by a long-lasting hyperpolarization which was generally strong enough to prevent responses to either electrical stimuli delivered in the brainstem or mechanical stimulation of the periphery. The receptive fields of vibrissa-sensitive cells in POm were generally much larger than those of cells in VPM. Data obtained with extracellular recording indicated that VPM and POm cells responded to an average of 1.4 and 4.0 vibrissae, respectively. Intracellular recording from smaller samples of VPM and POm cells demonstrated the existence of inputs that were insufficient to produce spikes from the cell, but did yield epsp's. When both sub- and suprathreshold excitation were considered, the average number of vibrissa in the receptive field of a VPM cell was 2.7 and the value for POm cells became 7.8. HRP-filled neurons recovered in POm (N = 20) generally had much larger dendritic arbors than neurons in VPM (N = 31). For the former cells, the size of the dendritic tree was significantly correlated with the number of vibrissa to which the cell responded; for the latter neurons, it was not.

Morphology of corticothalamic terminals arising from the auditory cortex of the rat: a Phaseolus vulgaris-leucoagglutinin (PHA-L) tracing study

Phaseolus vulgaris-leucoagglutinin (PHA-L) injection in the auditory cortex of the rat labeled anterogradely corticothalamic axons whose trajectory, morphology of terminals and their distribution were analyzed in light microscopy. From the primary auditory cortex, corticofugal axons ran in a rostral direction in the white matter (external capsule), and reached the internal capsule by crossing the caudate putamen. Then, they turned caudally, crossed the reticular nucleus (RE) of the thalamus, where some of them were seen to give off collaterals, ramifying in the 'auditory sector' of RE. From RE, the parent corticofugal axons continued in a caudal and medial direction to enter in the medial geniculate body (MGB). Corticofugal axons from the auditory cortex gave rise to 2 distinct types of terminals in the thalamus. First, small boutons (about 1 micron in diameter) were observed in the ventral division of the MGB (v-MGB; the main auditory relay nucleus in the thalamus), in RE, in the lateral part of the posterior thalamic nucleus, in the dorsal division of the MGB (d-MGB), as well as occasionally in the medial division of the MGB. Giant terminals (5-10 microns in diameter) formed the second type of cortical terminals, only present in a restricted zone of the ventral portion of d-MGB. Both types of terminals were observed as boutons 'terminaux' and 'en passant'. The zone of termination in v-MGB and RE varied as a function of the site of cortical injection. The similarity in the morphology and distribution of the terminals of corticothalamic axons arising from the primary auditory cortex with those of the primary somatosensory cortex of the mouse is striking and points to the existence of a basic pattern of connectivity used in corticothalamic processing of sensory information in rodents.

Dissimilar responses of adult thalamic monoaminergic and somatosensory afferent fibers to implantation of thalamic fetal cells

It is generally accepted that transplanted fetal neurons can, after several weeks to months, establish connections with the host CNS. Host afferent systems seem, however, to show different types of responses to the presence of grafted fetal neurons. The present study is a preliminary step to identify mechanisms involved in the reactions of adult axons to transplanted fetal neurons. The right ventrobasal thalamus of adult rats was depleted of neurons by in-situ injection of kainic acid and cell suspensions from homotopic thalamic embryonic primordia which were injected into the lesioned area. After various post-implantation delays, ranging from five to 30 days, two types of experiments were performed: (i) noradrenaline and serotonin immunohistochemistry with specific antibodies on alternate sections; and (ii) anterograde tracing using wheat germ agglutinin conjugated to horseradish peroxidase from the dorsal column nuclei and the principal sensory trigeminal nucleus. Five days after transplantation, host monoaminergic fibers (either noradrenergic or serotoninergic) had already grown into the transplants. Ingrowing fibers were thin and poorly varicose, exhibiting endings morphologically similar to the growth cones observed during axogenesis. Seven days after grafting, growth cones were no longer visible and monoaminergic fibers exhibited either normal-sized or very large varicosities. Large varicosities progressively decreased in number and, after three weeks, the fibers displayed a normal adult morphology, forming a dense network all over the transplants. In contrast, host somatosensory afferents, labeled by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, did not grow into the transplants. Intermingling of somatosensory afferents and transplanted cells was observed only after 10 days, when grafted neurons extended outside the original transplantation site into the neuron-depleted area containing the somatosensory afferents. The present results demonstrate that adult monoaminergic and somatosensory afferents, when deprived of their usual target, do not react in a similar way to the addition of fetal neurons. It is proposed that adult monaminergic fibers have the ability to regain morphological (and probably functional) immature forms which were considered to be restricted to the period of axogenesis or to lesion-induced regeneration. In contrast, fetal transplants do not seem to induce, by themselves, a similar alteration of genetic expression in adult somatosensory neurons. It has been proposed that "diffuse" and "point-to-point" axonal systems may be differentiated in the CNS on anatomical bases. The present results add to the identification of two different systems by demonstrating that, in the thalamus, they present dissimilar responses to the implantation of fetal cells.

Vascularization of fetal cell suspension grafts in the excitotoxically lesioned adult rat thalamus

Several studies have considered the establishment of vascularization in intracerebral solid transplants of neural tissue. The widely supported interpretation of the results is that the vascular network of the solid grafts is already present before implantation into the host brain. The situation is different when dissociated fetal tissue is transplanted as a cell suspension because in these conditions the fetal vascular network is disrupted. The present study has, therefore, been undertaken to follow the angiogenesis in a transplant of dissociated fetal cells implanted into the excitotoxically neuron-depleted thalamus. The vascular network is compared to that observed in the intact and in the lesioned thalamus both in terms of morphology of the capillaries and of the function of the blood-brain barrier (BBB). In the transplant, capillaries, stained by Indian ink, are very few in number and have very fine calibers during the first 20 days after grafting. Some structures can be identified as immature blood vessels at the electron microscopic level. The blood vessels are progressively more numerous in the graft and they demonstrate mature ultrastructural features 2 months after grafting. Last, there is no leakage of the BBB for peroxidase. The vascularization seems to follow a pattern of maturation comparable to that described during development in the literature. In contrast, in the lesioned area, there is a reactive angiogenesis: 10 days after the excitotoxic injection (shortest time studied), there are many wide caliber vessels with expanded perivascular spaces engorged with mesodermal cells. A microvascularization also develops transiently during the first two months. Capillaries are abnormal from the functional point of view, since there is a leakage of the BBB to macromolecules. The use of an experimental model in which transplant had to grow in a lesioned area permits to determine two types of vascularization: an apparently normal developmental timetable, normal morphological and functional characteristics, in the transplant; a reactive angiogenesis, in the lesioned area.

Light and electron microscopic study of galanin-immunoreactive nerve fibers in the rat posterior thalamus

Light and electron microscopic immunocytochemistry was used to study certain cell groups in the posteromedial thalamus which contain galanin-immunoreactive (GAL-IR) fibers. The nuclei subparafascicularis pars parvicellularis (SPFpc) and parafascicularis (PF) contain a dense network of GAL-IR fibers which form basketlike structures around unstained cells. The periventricular area also contains numerous GAL-IR fibers and these also occasionally form basketlike structures. The GAL-IR terminal fields continue caudally in the mesodiencephalic junction and merge with other GAL-IR fibers in the dorsal aspects of the substantia nigra and around the dorsolateral tip of the medial lemniscus. Ultrastructural analysis of the GAL-IR basketlike structures revealed that GAL-IR terminals make numerous synapses with the cell bodies and proximal dendrites of SPFpc neurones. These results suggest that the activity of cells in the SPFpc and PF nuclei may be strongly influenced by galanin-containing nerve fibers probably originating in the spinal cord.

Morphological alteration of thalamic afferents in the excitotoxically lesioned striatum

Excitotoxic lesions of the neostriatum cause anatomical and biochemical changes resembling those occurring in Huntington's disease. One major characteristic of these lesions is that they acutely spare axons of passage and afferent fibers. However, evidence is accumulating that afferent axonal systems decrease their fiber density in the long-term excitotoxic lesion. Ultrastructural changes of neuron-deprived terminals may also occur. A parallel study considering changes in afferent fibers to the excitotoxically lesioned thalamus showed that, a few weeks after neuron-depletion, specific 'point-to-point' systems formed regenerating axonal growth cone-like structures. The present study used the anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to determine whether specific thalamostriatal afferents form the same kind of regenerating structures following excitotoxic lesion of their target neurons. Thalamostriatal afferents decreased in density over months after lesion, but some were still labeled as long as 4 months after ibotenic acid injection. Remaining afferents formed axonal growth cone-like structures, identified at both light and electron microscopic levels, similar to those observed in the lesioned thalamus. These results demonstrate that in the striatum as in the thalamus, neuron depletion is followed by a long-term alteration of the morphology of some afferent fibers which form regenerating growth cone-like structures. These results are discussed with regard to the possible functional integration of fetal neurons transplanted into previously excitotoxically lesioned areas.

Fine structure of the dorsal part of the nucleus submedius of the rat thalamus: an anatomical study with reference to possible pain pathways

The dorsal portion of the nucleus submedius of the rat thalamus receives spinal and trigeminal projections which may convey noxious inputs. The present study was undertaken to analyse the fine structure of the nucleus with particular reference to a possible trigemino-thalamo-prefrontal cortical pathway relaying in nucleus submedius. Presynaptic terminals in the dorsal portion of the nucleus submedius were classified into three broad categories usually observed in thalamic nuclei: "small round", "flat" and "large round" types. Axonal tracing using either anterograde transport of horseradish peroxidase or degeneration methods indicated that some "small round" terminals originate from the pre-frontal cortex. Some "large round" terminals were labelled from the trigeminal subnucleus caudalis. These "large round" terminals exhibited distinct morphological features when compared with trigeminal terminals in other thalamic nuclei. In particular they made synaptic contacts predominantly with dendritic protrusions and were surrounded by multilamellate astrocytic processes. Double-labelling experiments were performed by means of the combined retrograde transport of horseradish peroxidase and Wallerian degradation techniques. Terminals degrading after lesion of the trigeminal subnucleus caudalis contacted submedius neurons labelled retrogradely from the prefrontal cortex. These observations demonstrate the existence of a direct monosynaptically relayed pathway from subnucleus caudalis to prefrontal cortex which relays in the dorsal part of nucleus submedius.

The intrinsic organization of the ventroposterolateral nucleus and related reticular thalamic nucleus of the rat: a double-labeling ultrastructural investigation with gamma-aminobutyric acid immunogold staining and lectin-conjugated horseradish peroxidase

An electron-microscopic investigation of the synaptic organization of the rat's ventroposterolateral nucleus (VPL) and of a reticular thalamic nucleus (RTN) area related to somatosensory thalamic nucleus was performed. In a group of 11 rats, wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) was injected either in the first somatosensory area of cortex (SI) or in the dorsal column nuclei (DCN). The retrogradely and/or anterogradely transported enzyme was visualized using paraphenylenediamine-pyrocatechol (PPD-PC) as substrate. In a second series of six experiments, an immunocytochemical procedure using a specific anti-gamma-aminobutyric acid (anti-GABA) was employed. Postembedding localization of GABA was performed for ultrastructural observation by means of the colloidal gold immunostaining procedure. Thin sections of recognized VPL and RTN areas from WGA:HRP-injected animals were further processed for immunocytochemistry in order to localize simultaneously, at the electron-microscopic level, the transported enzyme and GABA. The results obtained with this procedure demonstrated that HRP-labeled terminals from DCN contacted the soma and proximal dendrites of VPL neurons, while the terminals labeled after SI cortical injections were predominantly localized to the distal portion of the dendrites. The same cortical injection also determined the presence of labeled synaptic boutons contacting the soma, and both proximal and distal dendrites of RTN neurons. GABA-immunolabeled terminals were observed in VPL in a number larger than those observed with other methods, since not only typical F terminals were labeled but also terminals containing round and/or pleomorphic vesicles. GABA-ergic terminals contacted the soma and the proximal and distal dendrites of VPL neurons, while in RTN cells they made synaptic contact mainly with the soma and proximal dendrites. In the double-labeling experiments, terminals containing both HRP and specific immunogold GABA staining were never observed. The present data provide a direct demonstration of the presence of a strong inhibitory input from RTN upon VPL neurons and of the existence of autoinhibition within RTN neurons.

Spinal and trigeminal projections to the parabrachial nucleus in the rat: electron-microscopic evidence of a spino-ponto-amygdalian somatosensory pathway

The fine structure of spinal and trigeminal projections to the parabrachial area (PB) of the rat was studied using either the anterograde transport of a lectin-peroxidase conjugate or the degeneration technique. Two morphologically different types of terminals were observed. Most labeled terminals contained round vesicles (R type) and formed asymmetrical synapses, usually with large dendrites. Others contained pleomorphic vesicles (P type) and usually made symmetrical contacts with large or medium-size dendrites. A double-labeling strategy was used, combining the retrograde labeling of PB neurons with lectin-peroxidase conjugate from the amygdala and the identification of degenerating terminals after lesions of spinal or trigeminal pathways. These experiments demonstrated that spinal and trigeminal terminals contact PB neurons that project to the central nucleus of the amygdala. The role of this spino(trigemino)-ponto-amygdalian pathway is discussed in relation to some aspects of pain.

The differential synaptic organization of the spinal and lemniscal projections to the ventrobasal complex of the rat thalamus. Evidence for convergence of the two systems upon single thalamic neurons

The synaptic organization of terminals originating either from the spinal cord (spinothalamic) or from the dorsal column nuclei (lemniscal) was investigated in the ventrobasal complex of the rat thalamus. Wheatgerm agglutinin conjugated to horseradish peroxidase was used as an anterogradely transported axonal tracer, using benzidine dihydrochloride as a chromogen for the identification by electron microscopy of spinal and lemniscal projections to the ventrobasal thalamus. A double anterograde tract tracing strategy, based labeling by wheatgerm agglutinin conjugated to horseradish peroxidase of spinal terminals and simultaneous visualization of lemniscal terminals identified by Wallerian degeneration induced by lesion of the neurons of origin in the dorsal column nuclei, was used to compare the postsynaptic elements contacted by the two pathways and to look for a possible convergence of the two pathways onto single thalamic neurons. Spinal and lemniscal terminals are large (2-2.5 microns mean average diameter) terminals containing several mitochondria and numerous rounded vesicles. A quantitative analysis of the mean average diameters of the terminals revealed that one could not differentiate between synapses formed by the two pathways on a morphological basis. Terminals of the two pathways make asymmetrical contacts (Gray type I) with dendrites of varying diameter, dendritic protrusions, and cell somata. A quantitative analysis of the least diameter of the postsynaptic elements demonstrates projections of the two systems to different, partially overlapping regions of thalamic neurons. Lemniscal terminals originating from the dorsal column nuclei frequently contact cell somata; axosomatic spinothalamic contacts are uncommon. In addition, lemniscal projections tend to contact more proximal dendrites than do spinal projections, and this differential synaptic organization is statistically significant. From a functional point of view, this differential synaptic organization might indicate that lemniscal inputs have greater influence than spinal inputs in affecting the activity of thalamic neurons. Labeled spinothalamic terminals contact the same dendritic profile as do degenerating lemniscal terminals in about 10% of single sections. Because the present study did not include a complete reconstruction of ventrobasal complex neurons of the thalamus or even regions of dendritic arbors, the degree of convergence is likely to be significantly underestimated. These findings indicate that the anatomical basis exists for an interaction between nociceptive and non-nociceptive somesthetic systems at the level of single ventrobasal neurons of the thalamus of the rat.

Synaptic glomeruli in the nucleus submedius of the rat thalamus

Synaptic glomeruli in the nucleus submedius of the rat are described and the source of some of the component terminals identified. The glomeruli consist of large terminals with round synaptic vesicles establishing Gray type I contacts with dendrites and surrounded by layers of astrocyte derived membranes. The astrocyte processes may be composed of cell membranes with minimal interventing cytoplasm or, less frequently, contain larger amounts of cytoplasm. Horseradish peroxidase injected into the trigeminal nucleus caudalis labels some of the large astrocyte-enclosed terminals in nucleus submedius.

Fine structure of the spinothalamic projections to the central lateral nucleus of the rat thalamus

The fine structure of labelled spinothalamic terminals in the central lateral nucleus has been studied in the rat following injection of wheat germ agglutinin-horseradish peroxidase into the spinal cord. Myelinated axons gave rise to the labelled terminals, which were large profiles which contained round vesicles, numerous mitochondria, and formed asymmetrical contacts with large dendrites or dendritic protrusions. These profiles are similar to those described in other somatosensory thalamic nuclei, and in many other nuclei of the thalamus.

Structural alteration and possible growth of afferents after kainate lesion in the adult rat thalamus

Afferents to the thalamic ventrobasal complex (VB) originating from the spinal cord, the dorsal column nuclei, and the somatosensory cortex were anterogradely labeled by WGA-HRP 30 days after an injection of kainic acid (KA), which produced a complete unilateral neuronal loss in the VB, the opposite side being used as a control. At the light microscopic level, there was no obvious rerouting of spinal afferents away from the lesioned areas towards unlesioned parts of VB. There was an apparent decrease in the number of lemniscal afferents to the lesioned side, which may indicate a progressive retrograde degeneration. At higher magnification, all three afferent systems studied demonstrated morphological changes, predominantly manifested by terminal swellings that reached up to 25 micron in diameter. Control experiments suggested that these morphological alterations were related neither to a direct action of the excitotoxin nor to the absence of a different afferent system but to the loss of neuronal postsynaptic targets. At the electron microscopic level, the normal ultrastructural features of VB were not observed after a KA lesion. No neuronal somata, dendrites, or normal presynaptic elements were observed. Neural elements, some of which were labeled from the somatosensory cortex or the dorsal column nuclei, were essentially of two types: varicosities and unmyelinated axonal profiles. Varicosities could be separated into two broad classes: The majority were large structures derived from large, sometimes myelinated, axons and containing a wealth of organelles. Since they were not completely surrounded by glial elements, we have denoted them unensheathed varicosities. Among the organelles, the most obvious features were vesicles and tubules of smooth endoplasmic reticulum, microtubules, mitochondria, and various lysosome-like inclusions. These unensheathed varicosities gave rise to large, mound-like protrusions containing large vacuoles and thin long protrusions either filled with neurofilaments or resembling unmyelinated axonal profiles. Others were completely surrounded by a glial sheet and were therefore called ensheathed varicosities. These ensheathed varicosities presented several characteristics typical of degenerating profiles, including neurofilamentous proliferation and morphological alterations of the mitochondria. Unmyelinated axonal profiles occupied a substantial territory in the lesioned area. They were most often grouped in bundles sometimes wrapped by glial processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Time course of reactive synaptogenesis in the subcortical somatosensory system

These experiments were designed to determine when synaptogenesis begins in the adult rat ventral posterolateral nucleus of the thalamus following lesions of the dorsal column nuclei. Given the relatively uncomplicated structure of the neuropil in the ventral posterolateral nucleus of the rat, the specificity of reactive synaptogenesis of the lemniscal input and the effect of the loss of lemniscal terminals on terminals from other sources could be determined. By use of morphometric analysis of electron micrographs, the numerical density of the 3 terminal types in the neuropil was determined at a series of postlesion survival times ranging from 12 hours to 50 days. Synaptogenesis began about 30 days after the lesions of the dorsal column nuclei and was complete by 50 days. The slow onset of synaptogenesis was in response to a loss of the lemniscal terminals, which account for only 3% of the total number of synapses in the ventral posterolateral nucleus. The low level of synaptogenesis early in the recovery process differs from the recovery seen in other central nervous system sites, which show an early rapid increase in synapses in response to much greater denervation. The loss of lemniscal terminals has relatively little effect on the numerical density or distribution of the terminals of other types. The new terminals that are formed come both from axons that originate from the undamaged portion of the dorsal column nuclei and from axons originating in the spinal cord.