Crosstalk between the two sides of the thalamus through the reticular nucleus: a retrograde and anterograde tracing study in the rat

Optogenetic activation of the reticular nucleus of the thalamus attenuates limbic seizures via inhibition of the midline thalamus

OBJECTIVE

The nucleus reticularis of the thalamus (nRT) is most studied in epilepsy for its role in the genesis of absence seizures; much less is known regarding its role in other seizure types, including those originating in limbic structures and the temporal lobe. As it is a major source of inhibitory input to higher order thalamic nuclei, stimulation of the nRT may be an effective strategy to disrupt seizure activity that requires thalamic engagement.

METHODS

We recorded single unit activity from the nRT prior to and after infusion of bicuculline into the area tempestas. We monitored single unit activity time-locked with interictal spikes. We optogenetically activated the nRT in both the area tempestas and amygdala kindling models. We tested a role for projections from the nRT to higher order midline thalamic nuclei through the use of retrogradely trafficked viral vector.

RESULTS

Mean firing rate in the nRT was decreased after infusion of bicuculline into the area tempestas as compared to the preinfusion baseline. nRT unit firing in response to interictal spikes was heterogeneous, with an approximately equal proportion of neurons displaying (1) no change in firing, (2) increased firing, and (3) decreasing firing. Optogenetic activation of the nRT significantly suppressed seizure activity in both the area tempestas and amygdala kindling models. Optogenetic activation of contralaterally targeting projections but not ipsilaterally targeting projections from the nRT to the midline thalamus significantly suppressed seizures in the kindling model.

SIGNIFICANCE

Although the nRT is typically thought of in the context of absence seizures, our data show that it may be a viable target for other seizure types. In two models that recapitulate the seizure types seen in temporal lobe epilepsy, nRT activation suppressed both electrographic and behavioral seizures. These data suggest that the nRT should be considered more broadly in the context of epilepsy.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Activation of Bilateral Secondary Somatosensory Cortex With Right Hand Touch Stimulation: A Meta-Analysis of Functional Neuroimaging Studies

Background: Brain regions involved in processing somatosensory information have been well documented through lesion, post-mortem, animal, and more recently, structural and functional neuroimaging studies. Functional neuroimaging studies characterize brain activation related to somatosensory processing; yet a meta-analysis synthesis of these findings is currently lacking and in-depth knowledge of the regions involved in somatosensory-related tasks may also be confounded by motor influences. Objectives: Our Activation Likelihood Estimate (ALE) meta-analysis sought to quantify brain regions that are involved in the tactile processing of the right (RH) and left hands (LH) separately, with the exclusion of motor related activity. Methods: The majority of studies (n = 41) measured activation associated with RH tactile stimulation. RH activation studies were grouped into those which conducted whole-brain analyses (n = 29) and those which examined specific regions of interest (ROI; n = 12). Few studies examined LH activation, though all were whole-brain studies (N = 7). Results: Meta-analysis of brain activation associated with RH tactile stimulation (whole-brain studies) revealed large clusters of activation in the left primary somatosensory cortex (S1) and bilaterally in the secondary somatosensory cortex (S2; including parietal operculum) and supramarginal gyrus (SMG), as well as the left anterior cingulate. Comparison between findings from RH whole-brain and ROI studies revealed activation as expected, but restricted primarily to S1 and S2 regions. Further, preliminary analyses of LH stimulation studies only, revealed two small clusters within the right S1 and S2 regions, likely limited due to the small number of studies. Contrast analyses revealed the one area of overlap for RH and LH, was right secondary somatosensory region. Conclusions: Findings from the whole-brain meta-analysis of right hand tactile stimulation emphasize the importance of taking into consideration bilateral activation, particularly in secondary somatosensory cortex. Further, the right parietal operculum/S2 region was commonly activated for right and left hand tactile stimulation, suggesting a lateralized pattern of somatosensory activation in right secondary somatosensory region. Implications for further research and for possible differences in right and left hemispheric stroke lesions are discussed.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)

Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for in vivo non-invasive whole-brain mapping of neuronal activity. Mn²⁺ enters active neurons via voltage-gated calcium channels and increases local contrast in T₁-weighted images. Given the property of Mn²⁺ of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn²⁺ dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn²⁺ transport. To this end, we injected mice with low dose MnCl₂ 2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl₂ injections. We assessed changes in T₁ contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn²⁺ content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn²⁺ transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn²⁺ transport. The description of this critical property of Mn²⁺ dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

Sleep onset uncovers thalamic abnormalities in patients with idiopathic generalised epilepsy

The thalamus is crucial for sleep regulation and the pathophysiology of idiopathic generalised epilepsy (IGE), and may serve as the underlying basis for the links between the two. We investigated this using EEG-fMRI and a specific emphasis on the role and functional connectivity (FC) of the thalamus. We defined three types of thalamic FC: thalamocortical, inter-hemispheric thalamic, and intra-hemispheric thalamic. Patients and controls differed in all three measures, and during wakefulness and sleep, indicating disorder-dependent and state-dependent modification of thalamic FC. Inter-hemispheric thalamic FC differed between patients and controls in somatosensory regions during wakefulness, and occipital regions during sleep. Intra-hemispheric thalamic FC was significantly higher in patients than controls following sleep onset, and disorder-dependent alterations to FC were seen in several thalamic regions always involving somatomotor and occipital regions. As interactions between thalamic sub-regions are indirect and mediated by the inhibitory thalamic reticular nucleus (TRN), the results suggest abnormal TRN function in patients with IGE, with a regional distribution which could suggest a link with the thalamocortical networks involved in the generation of alpha rhythms. Intra-thalamic FC could be a more widely applicable marker beyond patients with IGE.

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Sleep onset modifies thalamic FC in generalised epilepsy differently to controls.

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Differences are regionally specific.

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Regions connected to somatomotor/occipital cortices are consistently affected.

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Intra-thalamic FC may be a surrogate marker of thalamic reticular nucleus function.

Asymmetric cross-hemispheric connections link the rat anterior thalamic nuclei with the cortex and hippocampal formation

Dense reciprocal connections link the rat anterior thalamic nuclei with the prelimbic, anterior cingulate and retrosplenial cortices, as well as with the subiculum and postsubiculum. The present study compared the ipsilateral thalamic-cortical connections with the corresponding crossed, contralateral connections between these same sets of regions. All efferents from the anteromedial thalamic nucleus to the cortex, as well as those to the subiculum, remained ipsilateral. In contrast, all of these target sites provided reciprocal, bilateral projections to the anteromedial nucleus. While the anteroventral thalamic nucleus often shared this same asymmetric pattern of cortical connections, it received relatively fewer crossed inputs than the anteromedial nucleus. This difference was most marked for the anterior cingulate projections, as those to the anteroventral nucleus remained almost entirely ipsilateral. Unlike the anteromedial nucleus, the anteroventral nucleus also appeared to provide a restricted, crossed projection to the contralateral retrosplenial cortex. Meanwhile, the closely related laterodorsal thalamic nucleus had almost exclusively ipsilateral efferent and afferent cortical connections. Likewise, within the hippocampus, the postsubiculum seemingly had only ipsilateral efferent and afferent connections with the anterior thalamic and laterodorsal nuclei. While the bilateral cortical projections to the anterior thalamic nuclei originated predominantly from layer VI, the accompanying sparse projections from layer V largely gave rise to ipsilateral thalamic inputs. In testing a potentially unifying principle of anterior thalamic - cortical interactions, a slightly more individual pattern emerged that reinforces other evidence of functional differences within the anterior thalamic and also helps to explain the consequences of unilateral interventions involving these nuclei.

Spatiotemporal Organization and Cross-Frequency Coupling of Sleep Spindles in Primate Cerebral Cortex

STUDY OBJECTIVES

The sleep spindle has been implicated in thalamic sensory gating, cortical development, and memory consolidation. These multiple functions may depend on specific spatiotemporal emergence and interactions with other spindles and other forms of brain activity. Therefore, we measured sleep spindle cortical distribution, regional heterogeneity, synchronization, and phase relationships with other electroencephalographic components in freely moving primates.

METHODS

Transcortical field potentials were recorded from Japanese monkeys via telemetry and were analyzed using the Hilbert-Huang transform.

RESULTS

Spindle (12-20 Hz) current sources were identified over a wide region of the frontoparietal cortex. Most spindles occurred independently in their own frequency, but some appeared concordant between cortical areas with frequency interdependence, particularly in nearby regions and bilaterally symmetrical regions. Spindles in the dorsolateral prefrontal cortex appeared around the surface-positive and depth-negative phase of transcortically recorded slow oscillations (< 1 Hz), whereas centroparietal spindles emerged around the opposite phase. The slow-oscillation phase reversed between the prefrontal and central regions. Gamma activities increased before spindle onset. Several regional heterogeneities in properties of human spindles were replicated in the monkeys, including frequency, density, and inter-cortical time lags, although their topographic patterns were different from those of humans. The phase-amplitude coupling between spindle and gamma activity was also replicated.

CONCLUSIONS

Spindles in widespread cortical regions are possibly driven by independent rhythm generators, but are temporally associated to spindles in other regions and to slow and gamma oscillations by corticocortical and thalamocortical pathways.

Acute deep brain stimulation in the thalamic reticular nucleus protects against acute stress and modulates initial events of adult hippocampal neurogenesis

Deep brain stimulation (DBS) is used as an alternative therapeutic procedure for pharmacoresistant psychiatric disorders. Recently the thalamic reticular nucleus (TRN) gained attention due to the description of a novel pathway from the amygdala to this nucleus suggesting that may be differentially disrupted in mood disorders. The limbic system is implicated in the regulation of these disorders that are accompanied by neuroplastic changes. The hippocampus is highly plastic and shows the generation of new neurons, process affected by stress but positively regulated by antidepressant drugs. We explored the impact of applying acute DBS to the TRN (DBS-TRN) in male Wistar rats exposed to acute stress caused by the forced-swim Porsolt's test (FST) and on initial events of hippocampal neurogenesis. After the first session of forced-swim, rats were randomly subdivided in a DBS-TRN and a Sham group. Stimulated rats received 10min of DBS, thus the depressant-like behavior reflected as immobility was evaluated in the second session of forced-swim. Locomotricity was evaluated in the open field test. Cell proliferation and doublecortin-associated cells were quantified in the hippocampus of other cohorts of rats. No effects of electrode implantation were found in locomotricity. Acute DBS-TRN reduced immobility in comparison to the Sham group (p<0.001). DBS-TRN increased cell proliferation (Ki67 or BrdU-positive cells; p=0.02, p=0.02) and the number of doublecortin-cells compared to the Sham group (p<0.02). Similar effects were found in rats previously exposed to the first session of forced-swim. Our data could suggest that TRN brain region may be a promising target for DBS to treat intractable depression.

Unilateral and Bilateral Cortical Resection: Effects on Spike-Wave Discharges in a Genetic Absence Epilepsy Model

Research Question

Recent discoveries have challenged the traditional view that the thalamus is the primary source driving spike-and-wave discharges (SWDs). At odds, SWDs in genetic absence models have a cortical focal origin in the deep layers of the perioral region of the somatosensory cortex. The present study examines the effect of unilateral and bilateral surgical resection of the assumed focal cortical region on the occurrence of SWDs in anesthetized WAG/Rij rats, a well described and validated genetic absence model.

Methods

Male WAG/Rij rats were used: 9 in the resected and 6 in the control group. EEG recordings were made before and after craniectomy, after unilateral and after bilateral removal of the focal region.

Results

SWDs decreased after unilateral cortical resection, while SWDs were no longer noticed after bilateral resection. This was also the case when the resected areas were restricted to layers I-IV with layers V and VI intact.

Conclusions

These results suggest that SWDs are completely abolished after bilateral removal of the focal region, most likely by interference with an intracortical columnar circuit. The evidence suggests that absence epilepsy is a network type of epilepsy since interference with only the local cortical network abolishes all seizures.

Cortico-cortical interactions influence binocularity of the primary visual cortex of adult mice

Electrophysiological studies have revealed that a large proportion of the mouse primary visual cortex (V1) receives input also from the ipsilateral eye. This is surprising as most optic nerve fibers cross at the optic chiasm in mice. Inactivating V1 of one hemisphere has recently demonstrated a strong contribution of one hemisphere's activity on binocularity of single units and visually evoked potentials of V1 in the other hemisphere of young rats and of single units in young adult mice. Here we used intrinsic signal optical imaging to quantitatively study the influence of cortico-cortical connections on the magnitude of neuronal activation in the entire binocular zone of adult mouse V1. We simultaneously measured V1-activity of both hemispheres in adult C57BL/6J mice before and after blocking sensory-driven activity in one hemisphere with muscimol. In V1 contralateral to the inactivation, ipsilateral eye evoked activity was reduced by on average 18% while contralateral eye evoked activity did not change. Our results clearly show that cortico-cortical interactions exert a global amplification of ipsilateral eye evoked activity in adult mouse V1.

Interhemispheric balance sets nostril differences in color-induced nasal thermal judgments

Sniffing out of sight always the same colorless and odorless solution containing no thermal agents while viewing a bottle with colored water increases sensitivity of the left nostril/right hemisphere (RH) for warming sensations and sensitivity of the right nostril/left hemisphere (LH) for cooling sensations. It is likely that engagement in a temperature judgment task and the development of specific expectancies due to the presence of color cues alter and enhance processing in brain areas involved in thermosensory processing. The lateralized patterns thus intimate hemispheric specialization for thermosensory processing probably originating in reciprocal inhibition that confers balance between the hemispheres. If the inhibition-balance hypothesis were correct then the more the left nostril proves sensitive to warming the more the right nostril would prove sensitive to cooling. One hundred and ninety one healthy volunteers were tested here. The left nostril dominance for warming and the right dominance for cooling were replicated once more. The dominance of the left nostril for warming (left minus right nostril) correlated highly with the dominance of the left nostril to cooling (right minus left nostril) and the individual patterns of results were distributed along an axis starting from the expected left nostril/warming - right nostril/cooling pattern and ending at the opposite left nostril/cooling - right nostril/warming pattern. Furthermore, the point where the left nostril dominance for warming responses dropped and inverted perfectly coincided with the point where the right nostril dominance for cooling responses inverted too. Such a good continuum between the expected and the opposite patterns supports the inhibition-balance hypothesis. Finally, 66% of subjects exhibited the expected left-warming/right-cooling pattern suggesting, therefore, that, despite this continuum, there is a dominant lateral specialization for temperature processing.

Targeted mini-strokes produce changes in interhemispheric sensory signal processing that are indicative of disinhibition within minutes

Most processing of sensation involves the cortical hemisphere opposite (contralateral) to the stimulated limb. Stroke patients can exhibit changes in the interhemispheric balance of sensory signal processing. It is unclear whether these changes are the result of poststroke rewiring and experience, or whether they could result from the immediate effect of circuit loss. We evaluated the effect of mini-strokes over short timescales (<2 h) where cortical rewiring is unlikely by monitoring sensory-evoked activity throughout much of both cortical hemispheres using voltage-sensitive dye imaging. Blockade of a single pial arteriole within the C57BL6J mouse forelimb somatosensory cortex reduced the response evoked by stimulation of the limb contralateral to the stroke. However, after stroke, the ipsilateral (uncrossed) forelimb response within the unaffected hemisphere was spared and became independent of the contralateral forelimb cortex. Within the unaffected hemisphere, mini-strokes in the opposite hemisphere significantly enhanced sensory responses produced by stimulation of either contralateral or ipsilateral pathways within 30-50 min of stroke onset. Stroke-induced enhancement of responses within the spared hemisphere was not reproduced by inhibition of either cortex or thalamus using pharmacological agents in nonischemic animals. I/LnJ acallosal mice showed similar rapid interhemispheric redistribution of sensory processing after stroke, suggesting that subcortical connections and not transcallosal projections were mediating the novel activation patterns. Thalamic inactivation before stroke prevented the bilateral rearrangement of sensory responses. These findings suggest that acute stroke, and not merely loss of activity, activates unique pathways that can rapidly redistribute function within the spared cortical hemisphere.

Relationship between touch impairment and brain activation after lesions of subcortical and cortical somatosensory regions

BACKGROUND

The neural basis underlying somatosensory impairment and recovery poststroke is virtually unexplored.

OBJECTIVE

To investigate the relationship between touch discrimination impairment and task-related brain activation in stroke survivors with somatosensory impairment following subcortical or cortical lesions.

METHODS

A total of 19 stroke survivors with touch impairment were investigated using fMRI and a touch discrimination paradigm 1-month poststroke; 11 had subcortical and 8 cortical sensory lesions; 12 age-matched healthy controls were also studied. Mean task-related contrast images were regressed with sensory impairment using random effects analysis for each subgroup and the total group.

RESULTS

There was no significant difference in touch impairment between stroke subgroups. Touch discrimination of the affected hand correlated negatively with task-related activation in the ipsilesional primary somatosensory cortex (SI; adjacent to the SI hand area activated in healthy controls); ipsilesional secondary somatosensory cortex (SII); contralesional thalamus; and attention-related frontal and occipital regions in the subcortical group. In contrast, the cortical group did not show significant correlated activity. Yet there was no significant between-group difference in a priori somatosensory regions: only in the superior medial frontal gyrus. A negative correlation was observed in the contralesional thalamus for the total group, irrespective of lesion type.

CONCLUSION

The findings provide novel evidence of neural correlates of poststroke touch impairment involving a distributed network of ipsilesional SI and SII, the contralesional thalamus, and frontal attention regions, particularly following subcortical lesions. Further systematic investigation of a modulatory role for ipsilesional SI, the thalamus, and frontal attention regions in sensory processing and recovery is warranted, particularly given implications for rehabilitation.

Involvement of the thalamic parafascicular nucleus in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized by focal seizures, associated with hippocampal sclerosis, and often resistance to antiepileptic drugs. The parafascicular nucleus (PF) of the thalamus is involved in the generation of physiological oscillatory rhythms. It receives excitatory inputs from the cortex and inhibitory inputs from the basal ganglia, a system implicated in the control of epileptic seizures. The aim of this study was to examine the involvement of the PF in the occurrence of hippocampal paroxysmal discharges (HPDs) in a chronic animal model of MTLE in male mice. We recorded the local field potential (LFP) and the extracellular and intracellular activity of hippocampal and PF neurons during spontaneous HPDs in vivo. The end of the HPDs was concomitant with a slow repolarization in hippocampal neurons leading to an electrical silence. In contrast, it was associated in the PF with a transient increase in the power of the 10-20 Hz band in LFPs and a depolarization of PF neurons resulting in a sustained firing. We tested the role of the PF in the control of HPDs by single 130 Hz electrical stimulation of this nucleus and bilateral intra-PF injection of NMDA and GABA(A) antagonist and agonist. High-frequency PF stimulation interrupted ongoing HPDs at an intensity devoid of behavioral effects. NMDA antagonist and GABA(A) agonist suppressed hippocampal discharges in a dose-dependent way, whereas NMDA agonist and GABA(A) antagonist increased HPDs. Altogether, these data suggest that the PF nucleus plays a role in the modulation of MTLE seizures.

Spontaneous fMRI activity during resting wakefulness and sleep

Functional magnetic resonance imaging (fMRI) studies performed during both waking rest and sleep show that the brain is continually active in distinct patterns that appear to reflect its underlying functional connectivity. In this review, potential sources that contribute to spontaneous fMRI activity will be discussed.

Dynamics of coupled thalamocortical modules

We develop a model of thalamocortical dynamics using a shared population of thalamic neurons to couple distant cortical regions. Behavior of the model is determined as a function of the connection strengths with shared and unshared populations in the thalamus, either within a relay nucleus or the reticular nucleus. When the coupling is via the reticular nucleus, we locate solutions of the model where distant cortical regions maintain the same activity level, and regions where one region maintains an elevated activity level, suppressing activity in the other. We locate and investigate a region where both types of solutions exist and are stable, yielding a mechanism for spontaneous changes in global activity patterns. Power spectra and coherence are computed, and marked differences in the coherence are found between the two kinds of modes. When, on the other hand, the coupling is via a shared relay nuclei, the features seen with the reticular coupling are absent. These considerations suggest a role for the reticular nucleus in modulating long distance cortical communication.

Interhemispheric neuroplasticity following limb deafferentation detected by resting-state functional connectivity magnetic resonance imaging (fcMRI) and functional magnetic resonance imaging (fMRI)

Functional connectivity magnetic resonance imaging (fcMRI) studies in rat brain show brain reorganization following peripheral nerve injury. Subacute neuroplasticity was observed 2 weeks following transection of the four major nerves of the brachial plexus. Direct stimulation of the intact radial nerve reveals a functional magnetic resonance imaging (fMRI) activation pattern in the forelimb regions of the sensory and motor cortices that is significantly different from that observed in normal rats. Results of this fMRI experiment were used to determine seed voxel regions for fcMRI analysis. Intrahemispheric connectivities in the sensorimotor forelimb representations in both hemispheres are largely unaffected by deafferentation, whereas substantial disruption of interhemispheric sensorimotor cortical connectivity occurs. In addition, significant intra- and interhemispheric changes in connectivities of thalamic nuclei were found. These are the central findings of the study. They could not have been obtained from fMRI studies alone-both fMRI and fcMRI are needed. The combination provides a general marker for brain plasticity. The rat visual system was studied in the same animals as a control. No neuroplastic changes in connectivities were found in the primary visual cortex upon forelimb deafferentation. Differences were noted in regions responsible for processing multisensory visual-motor information. This incidental discovery is considered to be significant. It may provide insight into phantom limb epiphenomena.

Different patterns of neuronal activation and neurodegeneration in the thalamus and cortex of epilepsy-resistant Proechimys rats versus Wistar rats after pilocarpine-induced protracted seizures

PURPOSE

To analyze cellular mechanisms of limbic-seizure suppression, the response to pilocarpine-induced seizures was investigated in cortex and thalamus, comparing epilepsy-resistant rats Proechimys guyannensis with Wistar rats.

METHODS

Fos immunoreactivity revealing neuronal activation, and degenerating neurons labeled by Fluoro-Jade B (FJB) histochemistry were analyzed on the first day after onset of seizures lasting 3 h. Subpopulations of gamma-aminobutyric acid (GABA)ergic cells were characterized with double Fos-parvalbumin immunohistochemistry.

RESULTS

In both cortex and thalamus, degenerating neurons were much fewer in Proechimys than Wistar rats. Fos persisted at high levels at 24 h only in the Proechimys thalamus and cortex, especially in layer VI where corticothalamic neurons reside. In the parietal cortex, about 50% of parvalbumin-containing interneurons at 8 h, and 10-20% at 24 h, were Fos-positive in Wistar rats, but in Proechimys, Fos was expressed in almost all parvalbumin-containing interneurons at 8 h and dropped at 24 h. Fos positivity in cingulate cortex interneurons was similar in both species. In the Wistar rat thalamus, Fos was induced in medial and midline nuclei up to 8 h, when <30% of reticular nucleus cells were Fos-positive, and then decreased, with no relationship with cell loss, evaluated in Nissl-stained sections. In Proechimys, almost all reticular nucleus neurons were Fos-positive at 24 h.

DISCUSSION

At variance with laboratory rats, pilocarpine-induced protracted seizures elicit in Proechimys limited neuronal death, and marked and long-lasting Fos induction in excitatory and inhibitory cortical and thalamic cell subsets. The findings implicate intrathalamic and intracortical regulation, and circuits linking thalamus and cortex in limbic seizure suppression leading to epilepsy resistance.

Thalamic lesions in a genetic rat model of absence epilepsy: dissociation between spike-wave discharges and sleep spindles

Recent findings have challenged the traditional view that the thalamus is the primary driving source of generalized spike-wave discharges (SWDs) characteristic for absence seizures, and indicate a leading role for the cortex instead. In light of this we investigated the effects of thalamic lesions on SWDs and sleep spindles in the WAG/Rij rat, a genetic model of absence epilepsy. EEG was recorded from neocortex and thalamus in freely moving rats, both before and after unilateral thalamic ibotenic acid lesions. Complete unilateral destruction of the reticular thalamic nucleus (RTN) combined with extensive destruction of the thalamocortical relay (TCR) nuclei, resulted in the bilateral abolishment of SWDs and ipsilateral abolishment of sleep spindles. A suppression of both types of thalamocortical oscillations was found when complete or extensive damage to the RTN was combined with minor to moderate damage to the TCR nuclei. Lesions that left the rostral pole of the RTN and part of the TCR nuclei intact, resulted in an ipsilateral suppression of sleep spindles, but a large increase of bilateral SWDs. These findings demonstrate that the thalamus in general and the RTN in particular are a prerequisite for both the typical bilateral 7-11 Hz SWDs and natural occurring sleep spindles in the WAG/Rij rat, but suggest that different intrathalamic subcircuits are involved in the two types of thalamocortical oscillations. Whereas the whole RTN appears to be critical for the generation of sleep spindles, the rostral pole of the RTN seems to be the most likely part that generates SWDs.

Interhemispheric effect of parietal TMS on somatosensory response confirmed directly with concurrent TMS-fMRI

Transcranial magnetic stimulation (TMS) has been used to document some apparent interhemispheric influences behaviorally, with TMS over the right parietal cortex reported to enhance processing of touch for the ipsilateral right hand (Seyal et al., 1995). However, the neural bases of such apparent interhemispheric influences from TMS remain unknown. Here, we studied this directly by combining TMS with concurrent functional magnetic resonance imaging (fMRI). We applied bursts of 10 Hz TMS over right parietal cortex, at a high or low intensity, during two sensory contexts: either without any other stimulation, or while participants received median nerve stimulation to the right wrist, which projects to left primary somatosensory cortex (SI). TMS to right parietal cortex affected the blood oxygenation level-dependent signal in left SI, with high- versus low-intensity TMS increasing the left SI signal during right-wrist somatosensory input, but decreasing this in the absence of somatosensory input. This state-dependent modulation of SI by parietal TMS over the other hemisphere was accompanied by a related pattern of TMS-induced influences in the thalamus, as revealed by region-of-interest analyses. A behavioral experiment confirmed that the same right parietal TMS protocol of 10 Hz bursts led to enhanced detection of perithreshold electrical stimulation of the right median nerve, which is initially processed in left SI. Our results confirm directly that TMS over right parietal cortex can affect processing in left SI of the other hemisphere, with rivalrous effects (possibly transcallosal) arising in the absence of somatosensory input, but facilitatory effects (possibly involving thalamic circuitry) in the presence of driving somatosensory input.

Contralateral corticothalamic projections from MI whisker cortex: potential route for modulating hemispheric interactions

Rat whisking behavior is characterized by high amounts of bilateral coordination in which whisker movements on both sides of the face are linked. To elucidate the neural substrate that might mediate this bilateral coordination, neuronal tracers were used to characterize the bilateral distribution of corticothalamic projections from primary motor (MI) cortex. Some rats received tracers in the MI whisker region, whereas others received tracers in the MI forepaw region. The MI whisker region projects bilaterally to the anteromedial (AM), ventromedial (VM), and ventrolateral (VL) nuclei, and to parts of the intralaminar nuclei. By contrast, the MI forepaw region sends virtually no projections to the contralateral thalamus. Consistent with these findings, bilateral injections of different tracers into the MI whisker region of each hemisphere produced tracer overlap on both sides of the thalamus. Furthermore, MI whisker projections to the contralateral thalamus terminate in close proximity to the thalamocortical neurons that project to the MI whisker region of that contralateral hemisphere. The terminal endings of the contralateral corticothalamic projections contain small synaptic varicosities and other features that resemble the modulator pathways described for other corticothalamic projection systems. In addition, tracer injections into AM, VM, and VL revealed dense clusters of labeled neurons in layer VI of the medial agranular (Agm) zone, which corresponds to the MI whisker region. These results suggest that projections from the MI whisker region to the contralateral thalamus may modulate the callosal interactions that are presumed to play a role in coordinating bilateral whisking behavior.

Structural organization, neurochemical characteristics, and connections of the reticular nucleus of the thalamus

This review analyzes current concepts of the structural organization and ultrastructure of the reticular nucleus of the thalamus (RNT) and the neurochemical characteristics of its neurons. The topography, cytoarchitectonics, and neuronal organization of this nucleus are considered in detail, as are questions of its neurogenesis. Neurochemical data clarifying the representation of neurotransmitter systems in the RNT and data on neuropeptides synthesized in its neurons are systematized. The complex ultrastructural organization of the RNT is characterized in terms of recent data from state-of-the-art immunocytochemical methods allowing localization of glutamatergic and GABAergic receptors on synaptic elements. Data on the afferent and efferent connections of the RNT demonstrate its influences on various parts of the brain and the specific features of its interactions with cortical formations.

Improved Golgi-like Visualization in Retrogradely Projecting Neurons after EGFP-Adenovirus Infection in Adult Rat and Monkey

An adenovirus vector was generated using a neuron-specific promoter synapsin I and enhanced green fluorescent protein (EGFP) reporter (AdSynEGFP). In addition, two modifications were identified that resulted in robust and reliable retrograde transport and EGFP expression after injection of the virus into three different brain regions in adult rats (medial prefrontal cortex, posterior thalamic nuclear group, and CA1). These are post-injection survival times of 14 days and addition of high concentrations of NaCl (≥600 mM) to the injection buffer. These modifications resulted in obvious improvement in the intensity of the EGFP signal and in the number of labeled cells. Use of anti-EGFP in immunofluorescence or immunoperoxidase processing further enhanced the signal so that Golgi-like filling of dendritic spines and axon collaterals was routinely achieved. Effectiveness of the AdSynEGFP for Golgi-like filling was confirmed in one rhesus monkey with injections in visual area V4. Because of the long-term viability of the infected neurons (at least up to 28 days in rats and 22 days in monkey), this AdSynEGFP is suitable for use in microcircuitry studies in combination with other fluorescently tagged elements, including anterogradely labeled extrinsic projections. The native EGFP signal (without antibody enhancement) may be sufficient for studies involving cultured cells or slices. (J Histochem Cytochem 54:539-548, 2006)

The neurophysiology of attention-deficit/hyperactivity disorder

Recent reviews of the neurobiology of Attention-Deficit/Hyperactivity Disorder (AD/HD) have concluded that there is no single pathophysiological profile underlying this disorder. Certainly, dysfunctions in the frontal/subcortical pathways that control attention and motor behavior are implicated. However, no diagnostic criteria or behavioral/neuroimaging techniques allow a clear discrimination among subtypes within this disorder, especially when problems with learning are also considered. Two major Quantitative EEG (QEEG) subtypes have been found to characterize AD/HD. Here we review the major findings in the neurophysiology of AD/HD, focusing on QEEG, and briefly present our previous findings using a source localization technique called Variable Resolution Electromagnetic Tomography (VARETA). These two techniques represent a possible objective method to identify specific patterns corresponding to EEG-defined subtypes of AD/HD. We then propose a model representing the distribution of the neural generators in these two major AD/HD subtypes, localized within basal ganglia and right anterior cortical regions, and hippocampal, para-hippocampal and temporal cortical regions, respectively. A comprehensive review of neurochemical, genetic, neuroimaging, pharmacological and neuropsychological evidence in support of this model is then presented. These results indicate the value of the neurophysiological model of AD/HD and support the involvement of different neuroanatomical systems, particularly the dopaminergic pathways.

Cortical and limbic excitability in rats with absence epilepsy

The classical cortico-reticular theory on absence epilepsy suggests that a hyperexcitable cortex is a precondition for the occurrence of absence seizures. In the present experiment seizure thresholds and characteristics of cortical and limbic epileptic afterdischarges (AD) were determined in a comparative cortical stimulation study in young and old adult genetically epileptic WAG/Rij, congenic ACI and Wistar rats. Fifteen-second series of 8Hz stimulation of the sensory-motor cortex were applied in 80- and 180-day-old rats with implanted electrodes. Strain differences were found for the threshold for movements directly induced by stimulation, low frequency spike-and-wave AD, maximal clonic intensity of seizures accompanying direct stimulation, and frequency characteristics of low frequency AD. None of these results agreed with a higher cortical excitability exclusively in WAG/Rij rats. However, WAG/Rij rats had the longest duration of the low frequency AD, and the lowest threshold for the transition to the limbic type of AD. The decrease of this threshold correlated with the increase of the incidence and total duration of spontaneous SWDs in WAG/Rij rats. It is concluded that the elevated excitability of the limbic system or pathways mediating the spread of the epileptic activity into this system can be attributed to the development of genetic epileptic phenotype in WAG/Rij rats.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Differential distribution of the KCl cotransporter KCC2 in thalamic relay and reticular nuclei

In the thalamus of the rat the reversal potential of GABA-induced anion currents is more negative in relay cells than in neurones of the reticular nucleus (nRt) due to different chloride extrusion mechanisms operating in these cells. The distribution of KCl cotransporter type 2 (KCC2), the major neuronal chloride transporter that may underlie this effect, is unknown in the thalamus. In this study the precise regional and ultrastructural localization of KCC2 was examined in the thalamus using immunocytochemical methods. The neuropil of all relay nuclei was found to display intense KCC2 immunostaining to varying degrees. In sharp contrast, the majority of the nRt was negative for KCC2. In the anterior and dorsal part of the nRt, however, KCC2 immunostaining was similar to relay nuclei and parvalbumin and calretinin were found to colocalize with KCC2. At the ultrastructural level, KCC2 immunoreactivity was mainly located in the extrasynaptic membranes of thick and thin dendrites and the somata of relay cells but was also found in close association with asymmetrical synapses formed by cortical afferents. Quantitative evaluation of KCC2 distribution at the electron microscopic level demonstrated that the density of KCC2 did not correlate with dendritic diameter or synaptic coverage but is 1.7 times higher on perisynaptic membrane surfaces than on extrasynaptic membranes. Our data demonstrate that the regional distribution of KCC2 is compatible with the difference in GABA-A reversal potential between relay and reticular nuclei. At the ultrastructural level, abundant extrasynaptic KCC2 expression will probably play a role in the regulation of extrasynaptic GABA-A receptor-mediated inhibition.

Double retrograde tracer study of the corticostriatal projections to the cat caudate nucleus

The distribution of corticostriatal neurons projecting to the caudate nucleus was examined in the cat by retrograde fluorescent tracers. Thus, Fast Blue and Diamidino Yellow were concomitantly injected in different rostrocaudal, dorsoventral, or mediolateral sectors of the caudate nucleus. The main findings of this study are: 1) few double-labeled cells were found after two injections in different sectors of the caudate nucleus; 2) double-labeled neurons were more abundant after adjacent injections and they were mainly located in 6 alpha beta, dorsolateral prefrontal, dorsomedial prefrontal, prelimbic, anterior limbic, sylvian anterior, and rostral part of cingulate cortical areas; and 3) there were variations in the spatial organization of the corticostriatal neurons in different cortical areas projecting to various parts of the caudate nucleus.

Basal ganglia involvement in memory-guided movement sequencing

The basal ganglia (BG) are thought to play a critical role in motor planning and movement sequencing. While electrophysiological and imaging studies have shown that the dorso-lateral prefrontal cortex (DLPFC) is involved in working memory (WM), the involvement of the BG in this process is not well understood. We used a motor sequencing task to investigate the differential role of BG nuclei in memory-guided movement. Significant activation was observed in the DLPFC and posterior putamen and globus pallidus (GP), with a trend in the caudate and no differences in the anterior putamen. We then investigated the effect of BG outflow on thalamic activation using functional connectivity analysis. Activation in the posterior putamen + GP was found to be correlated with thalamic activation only in the hemisphere contralateral to movement. These results provide the first fMRI evidence that the BG may modulate activity in the thalamus during working memory-guided movement sequencing. Our findings suggest that the BG activation may reflect increased motor sequencing demands during the memory-guided movement condition and, specifically, that the posterior putamen and GP may play a role in maintenance of representations in WM in a manner that contributes to planning and temporal organization of motor sequencing.

Effects of medial thalamotomy and pallido-thalamic tractotomy on sleep and waking EEG in pain and Parkinsonian patients

OBJECTIVES

Investigation of sleep and sleep EEG before and after stereotactic neurosurgery.

METHODS

All-night polysomnographic recordings were obtained in 3 neurogenic pain patients and 3 parkinsonian patients. One subject of each group was recorded in addition 3 months after surgery. Stereotactic operations were performed in the medial thalamus and on the pallido-thalamic tract to relieve neurogenic pain and parkinsonian symptoms, respectively.

RESULTS

Sleep efficiency was little affected by the surgical intervention in neurogenic pain patients and a dramatic reduction in REM sleep occurred, which had recovered in the subject recorded after 3 months. After the surgery parkinsonian patients showed an increase in total sleep time and in sleep efficiency, and a decrease in REM sleep latency. Sleep efficiency remained elevated in the 3 months follow-up. Medial thalamotomy abolished spindle frequency activity (SFA) in the power and coherence spectra in non-REM sleep stage 2 systematically. Pallido-thalamic tractotomy attenuated SFA only to varying degrees. After 3 months SFA had reemerged. The alpha peak of the waking EEG was shifted to lower frequencies after surgery in 5 of 6 patients and had reverted to the original frequency 3 months later.

CONCLUSIONS

Medial thalamotomy or pallido-thalamic tractotomy had acute and reversible effects on the EEG and long-term deleterious side effects of stereotactic surgery on sleep and sleep EEG are improbable. The results provide further evidence for the involvement of the human thalamus in the generation of sleep spindles.

The role of diaschisis in stroke recovery

BACKGROUND AND PURPOSE

Recovery from hemiparesis after stroke has been shown to involve reorganization in motor and premotor cortical areas. However, whether poststroke recovery also depends on changes in remote brain structures, ie, diaschisis, is as yet unresolved. To address this question, we studied regional cerebral blood flow in 7 patients (mean+/-SD age, 54+/-8 years) after their first hemiparetic stroke.

METHODS

We analyzed imaging data voxel by voxel using a principal component analysis by which coherent changes in functional networks could be disclosed. Performance was assessed by a motor score and by the finger movement rate during the regional cerebral blood flow measurements.

RESULTS

The patients had recovered (P<0. 001) from severe hemiparesis after on average 6 months and were able to perform sequential finger movements with the recovered hand. Regional cerebral blood flow at rest differentiated patients and controls (P<0.05) by a network that was affected by the stroke lesion. During blindfolded performance of sequential finger movements, patients were differentiated from controls (P<0.05) by a recovery-related network and a movement-control network. These networks were spatially incongruent, involving motor, sensory, and visual cortex of both cerebral hemispheres, the basal ganglia, thalamus, and cerebellum. The lesion-affected and recovery-related networks overlapped in the contralesional thalamus and extrastriate occipital cortex.

CONCLUSIONS

Motor recovery after hemiparetic brain infarction is subserved by brain structures in locations remote from the stroke lesion. The topographic overlap of the lesion-affected and recovery-related networks suggests that diaschisis may play a critical role in stroke recovery.

The pallidofugal projection system in primates: evidence for neurons branching ipsilaterally and contralaterally to the thalamus and brainstem

This paper summarizes the results of some of our previous neuroanatomical studies on the pallidofugal projections in squirrel monkeys and also reports more recent data obtained with double retrograde and single axon tracing methods. Injections of anterograde tracers in the internal pallidum label axons that reach the ventral tier, centromedian and lateral habenular thalamic nuclei, as well as the pedunculopontine tegmental nucleus. The pallidofugal projections are composed of axons that branch to the ventral tier and pedunculopontine nuclei, and to ventral tier and centromedian nuclei. Double retrograde labeling with fluorescent tracers and single axon tracing confirm this high degree of collateralization. Furthermore, some pallidal labeled axons cross the midline and arborize contralaterally in the major pallidal targets. Double retrograde fluorescent labeling experiments support these findings. Pallidal axons that branch ipsilaterally as well as contralaterally to the thalamus and brainstem could play a crucial role in the functional organization of primate basal ganglia.

Early activation of inhibitory neurons in the thalamic reticular nucleus during focal neocortical seizures

The neurons of the thalamic reticular nucleus are among the main targets of corticothalamic projections and their vulnerability in pathological conditions is well established. The present experiments aimed at the description and immunocytochemical characterization of the neurons of the thalamic reticular nucleus activated in neocortical seizures. Focal seizures were induced by the topical application of isotonic, isohydric 4-aminopyridine solution to the sensorimotor neocortex of adult, anesthetized Wistar rats. The animals were perfused with fixative after 1 and 2 h of recorded seizure activity. Coronal plane vibratome sections were incubated with cocktails of polyclonal c-fos and monoclonal parvalbumin antisera. Labeled cells in the thalamic reticular nucleus were counted and related to total cell counts. Neurons and neuropil showed strong parvalbumin immunoreactivity. Double-stained sections revealed that 55.32% of the parvalbumin-positive cell population expressed c-fos protein in their cell nuclei at the end of the 1 h seizure period. This ratio decreased to 43.5% following 2 h seizure. Labeled cells, although less in number were also observed in the contralateral thalamic reticular nucleus. Since parvalbumin labels GABAergic cells, it is tempting to speculate that this activation of intrathalamic inhibiton might exert an important anticonvulsant protection on other thalamic nuclei.

Calretinin in the thalamic reticular nucleus of the rat: distribution and relationship with ipsilateral and contralateral efferents

In order to investigate the existence of anatomical subdivisions within the thalamic reticular nucleus (Rt), the distribution of reticular neurons expressing the calcium binding protein calretinin was investigated in the rat by means of immunocytochemistry. Calretinin immunoreactive (Cr-ir) neurons were mainly distributed in the lateral and ventral regions, and along the medial border of the Rt rostral pole. Caudal to the rostral pole, many neurons were Cr-ir in the more dorsal part of the rostral two-thirds (the "dorsal cap") of the Rt. Fewer Cr-ir neurons were present more caudally along the lateral and medial borders, and in the caudalmost part of the nucleus, related to the acoustic thalamus. The distribution of Cr-ir neurons in the rostral Rt was compared with that of neurons projecting to the ipsilateral and contralateral anterior, intralaminar, midline, and mediodorsal nuclei, or to the contralateral rostral Rt. The retrograde transport of Fluorogold revealed a remarkably precise topography of the rostral Rt: different reticular areas were found to project to different thalamic nuclei, or to different rostrocaudal or mediolateral portions of the same thalamic nucleus, with a limited degree of overlap. The double-labeling experiments demonstrated that the reticular neurons projecting to the ipsilateral anterodorsal, midline, mediodorsal, and anterior intralaminar nuclei frequently expressed calretinin; by contrast, the majority of the reticular commissural neurons did not express the protein, with the exception of neurons projecting to the contralateral mediodorsal and midline nuclei. The ipsilaterally projecting calretinin-positive neurons were frequently located along the medial edge of the rostral pole and in the dorsal cap of the nucleus, segregated from the commissural calretinin-negative neurons. The combined analysis of calretinin expression patterns and tract tracing data provided further insight in the anatomical organization of the thalamic reticular nucleus, suggesting a different neurophysiological role for the ipsilaterally vs. the contralaterally projecting reticular neurons in the modulation of the synaptic activity of the dorsal thalamus.

Organisation of the reticular thalamic projection to the intralaminar and midline nuclei in rats

This study examines the projection of the reticular thalamic nucleus to the classic "nonspecific" dorsal thalamic nuclei of rats. Individual nuclei of the intralaminar (central-lateral, paracentral, central-medial, parafascicular) and the midline (reuniens/rhomboid, parataenial) nuclear groups, together with the reticular nucleus itself, were injected with the neuronal tracers biotinylated dextran or fluorescent latex microspheres (red or green). Reticular cells projecting to the intralaminar and midline nuclei are limited largely to the rostral pole of the nucleus. Within the rostral pole, most reticular cells projecting to the intralaminar and midline nuclear groups are found in largely distinct sectors; cells that project to the intralaminar nuclei tend to lie more laterally, whereas those projecting to the midline nuclei lie more medially within the pole. Among the individual nuclei of both the intralaminar and midline nuclear groups, however, the segregation is far less distinct. For instance, the reticular cells that project to the intralaminar central-lateral, central-medial, paracentral, and parafascicular nuclei are intermixed completely on the lateral edge of the rostral pole. After separate injections of different colored latex microspheres into individual intralaminar nuclei, the incidence of double-labelled reticular cells is about 37%, a percentage much higher than among the "specific" dorsal thalamic nuclei (< 1%). All the above-mentioned results refer to the reticular labelling seen on the side ipsilateral to the injection. After separate injections into the intralaminar central-medial nucleus, the midline nuclei, and the reticular nucleus itself, we also see a very small group of reticular cells labelled on the contralateral side. In general, our results indicate that the reticular projection to the intralaminar and midline nuclei is far more diffuse than the reticular projection to the specific dorsal thalamic nuclei.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

Calbindin-containing non-specific thalamocortical projecting neurons in the rat

Immunoreactivity for calcium binding proteins was used to demonstrate the neurochemical profiles of non-specific thalamocortical neurons located in the ventromedial nucleus, the centrolateral nucleus, and the nucleus reuniens that project to the somatosensory cortex in the adult rat. Cortical injections of fluorescent tracers combined with immunohistochemistry for calcium binding proteins revealed that retrogradely labeled neurons in these three thalamic nuclei are immunoreactive for calbindin. The present results suggest the presence of a chemically distinct non-specific thalamocortical system which terminates in the neocortex.

Unilateral nigrostriatal lesions induce a bilateral increase in glutamate decarboxylase messenger RNA in the reticular thalamic nucleus

The reticular thalamic nucleus consists of densely packed neurons containing the neurotransmitter GABA. It surrounds the lateral border of the thalamus, has extensive reciprocal connections with thalamocortical neurons, and is thought to be involved in attentional processes. The reticular thalamic nucleus also receives direct and indirect inputs from the basal ganglia, suggesting that it may be involved in relaying motor information to the thalamus and cortex. We examined the possibility that decreased dopaminergic transmission in the basal ganglia indirectly affects the reticular thalamic nucleus. Rats received unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta and were killed two or three weeks after the lesion. Sections of the reticular thalamic nucleus were processed for in situ hybridization histochemistry at the single cell level with RNA probes for both isoforms of glutamate decarboxylase (M(r) 65,000: glutamate decarboxylase 65 and M(r) 67,000: glutamate decarboxylase 67), the rate limiting enzyme of GABA synthesis. Unilateral nigrostriatal dopaminergic lesions induced a topographically specific, bilateral increase in glutamate decarboxylase 67 messenger RNA in neurons of the lateral and ventral reticular thalamic nucleus. A much smaller increase in glutamate decarboxylase 65 messenger RNA was observed which was significant only ipsilateral to the lesion. Short- (seven day) and long-term (eight month) treatments with the antipsychotic drug haloperidol, in regimens that preferentially block D2 dopamine receptors, induced catalepsy and orofacial dyskinesia, respectively, but did not alter glutamate decarboxylase 67 messenger RNA levels in the reticular thalamic nucleus. Thus, loss of dopaminergic terminals, but not blockade of D2 dopamine receptors, induced the effects observed in the reticular thalamic nucleus. The results reveal a novel bilateral effect of unilateral dopamine depletion. In view of the role of the reticular thalamic nucleus in tremor and attentional processes, which are altered in Parkinson's disease, this effect may contribute to the clinical manifestations of nigrostriatal dopamine depletion.

Crossed thalamo-cortical and cortico-thalamic projections in adult mice

The crossed thalamo-cortical and cortico-thalamic connections of the mouse are drawn using the tracer wheat germ agglutinin-horseradish peroxidase. After injections in the frontal cortex of the right hemisphere cells labeled retrogradely and axons labeled anterogradely are observed in the thalamus ipsilateral and contralateral to the cortical injections. The retrograde and anterograde labeling in the contralateral thalamus is less intense than ipsilaterally and involves the mediodorsal, ventral medial, central medial, and paracentral nuclei. Crossed fronto-thalamic axons reach more lateral regions than those containing contralateral thalamo-frontal neurons. Our results demonstrate that the thalamo-cortical system of mice has a bilateral component. The functional significance of this pathway and analogies with crossed thalamo-cortical connections in other species are discussed.

Nucleus reticularis thalami and neocortical paroxysms in the rat

The role of the nucleus reticularis thalami in spike-wave discharges in rats with genetic absence epilepsy has already been demonstrated. This study further investigated the role of the nucleus reticularis thalami in paroxysmal synchronizations in Sprague-Dawley rats; this strain shows no propensity to epileptic activity. Electroencephalographic patterns were followed in chronically implanted, unrestrained rats. After both electrolytic and chemical unilateral lesions, stereotaxically placed in the anterolateral sectors of this nucleus (verified post mortem), abnormal electroencephalographic rhythms (high-voltage polyspikes and spike-wave complexes) were recorded from the frontoparietal cortex, primarily in the contralateral hemisphere. Stereotyped discharges at 3 Hz developed progressively from multiple spikes within the alpha frequency range through the lengthening of the wave component. The excessive synchronized activity recorded from the intact hemisphere was of greater amplitude and occurred slightly earlier than from the lesioned hemisphere. These EEG patterns were associated with behavioural manifestations closely resembling those seen during absence seizures in humans. Bilateral lesions did not induce paroxysmal activity, both hemispheres being characterized by dominant delta/theta activity without signs of EEG-synchronized sleep. The seizures may thus have been due to disinhibition of the contralateral reticularis nucleus, recently shown to project to the reticularis nucleus of the other side in rats. This working hypothesis is supported by callosal cuts. The results indicate that the reticular neurons exert a control over neocortical paroxysmal activity even in animals which do not present genetic absence epilepsy.

Functional anatomy of the thalamic centrolateral nucleus as revealed with the [14C]deoxyglucose method following electrical stimulation and electrolytic lesion

The effects of electrical stimulation and electrolytic lesion of the thalamic intralaminar centrolateral nucleus were studied in the rat brain by means of the quantitative autoradiographic [14C]deoxyglucose method. Unilateral electrical stimulation of the centrolateral nucleus induced: (i) local increase in metabolic activity within the stimulated centrolateral nucleus and the ipsilateral thalamic mediodorsal nucleus, (ii) metabolic depression in all layers of the ipsilateral frontal cortex, (iii) bilateral increase in glucose consumption within the periaqueductal gray, pedunculopontine nucleus, and pontine reticular formation, and (iv) contralateral metabolic activation in the deep cerebellar nuclei. The unilateral electrolytic lesion of the thalamic centrolateral nucleus elicited metabolic depressions in several distal brain areas. The metabolic depression elicited in the mediodorsal, ventrolateral, and lateral thalamic nuclei, as well as in the caudate nucleus, the cingulate, and the superficial layers of forelimb cortex were ipsilateral to the lesioned side. The metabolic depression measured in the medulla and pons (medullary and pontine reticular formation, periaqueductal gray, locus coeruleus, dorsal tegmental, cuneiformis, raphe and pedunculopontine tegmental nuclei), the cerebellum (molecular and granular layers of the cerebellar cortex, interpositus and dentate nuclei), the mesencephalon (substantia nigra reticulata, ventral tegmental area and deep layers of the superior colliculus), the diencephalon (medial habenula, parafascicular, ventrobasal complex, centromedial and reticular thalamic nuclei), the rhinencephalon (dentate gyrus and septum), the basal ganglia (ventral pallidum, globus pallidus, entopeduncular and accumbens nuclei) and the cerebral cortex (superficial and deep layers of the frontal and parietal cortex, deep layers of the forelimb cortex) were bilateral. These functional effects are discussed in relation to known anatomical pathways. The bilateral effects induced by the centrolateral nucleus lesion reflect an important role of the centrolateral nucleus in the processing of reticular activating input and in the interhemispheric transfer of information. The cortical metabolic depression induced by centrolateral nucleus stimulation indicates the participation of this nucleus in attentional functions.

The thalamic reticular nucleus does not send commissural projection to the contralateral parafascicular nucleus in the rat

The reticular nucleus of the thalamus (NRT) projects to virtually all thalamic nuclei ipsilaterally. In addition, recent studies suggest that NRT sends contralateral projections through an intrathalamic commissural fiber system to several thalamic nuclei, including the NRT itself. In the present study we used retrograde cell labeling, multi-unit anterograde labeling and immunohistochemical methods to study both ipsi- and contralateral NRT projection to the parafascicular nucleus (Pf) in the rat. Injections of the fluorescent tracers true blue or fluorogold in Pf led to massive retrograde cell labeling in rostral and dorsal portions of the ipsilateral NRT, whereas the same sectors of the contralateral NRT were devoid of labeling. Some retrogradely labeled cells were nevertheless present on the contralateral side in the borderline region between NRT and the zona incerta (ZI). Retrograde cell labeling experiments with cholera toxin B subunit (CTb) combined to immunohistochemistry for parvalbumin (PV) and calbindin D-28k (CB) indicated that the few retrogradely labeled cells encountered at the border between NRT and ZI displayed immunoreactivity for CB but not for PV. Since PV and CB label neurons belonging to NRT and ZI, respectively, it is concluded that these contralateral retrogradely labeled cells belong to ZI and not to NRT. Multi-unit cell anterograde labeling experiments with biocytin showed that NRT cells that project to Pf arborize extensively only on the ipsilateral side. The same approach, however, has revealed NRT cells projecting to both ipsi- and contralateral ventromedial thalamic nuclei. The axon of these NRT neurons arborizes more profusely ipsilaterally than contralaterally. These results reveal that the NRT projection to Pf in rodents is strictly unilateral. These findings are at variance with the emerging concept that NRT exerts a prominent bilateral influence upon most thalamic nuclei.

Crossed thalamocortical connections in the Madagascan hedgehog tenrec: dissimilarities to erinaceous hedgehog, similarities to mammals with more differentiated brains

The adult erinaceous hedgehog, unlike other mammals, has recently been shown to have prominent crossed projections from the thalamus to the motor cortex. There are suggestions relating this unique pattern of connectivity to the overall degree of brain differentiation and/or the poorly developed corpus callosum. The present tracing study demonstrates that the Madagascan lesser hedgehog tenrec, with its tiny corpus callosum and one of the lowest neocorticalization indices among insectivores, has extensive crossed cortico-thalamic projections, but essentially the same sparse thalamic projections to the contralateral cortex as have placental mammals with more differentiated brains. The implications of the findings and the relevance of extracallosal pathways are discussed.