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

Neuroinflammation in the anterior cingulate cortex: the potential supraspinal mechanism underlying the mirror-image pain following motor fiber injury

Background

Peripheral nerve inflammation or lesion can affect contralateral healthy structures, and thus result in mirror-image pain. Supraspinal structures play important roles in the occurrence of mirror pain. The anterior cingulate cortex (ACC) is a first-order cortical region that responds to painful stimuli. In the present study, we systematically investigate and compare the neuroimmune changes in the bilateral ACC region using unilateral- (spared nerve injury, SNI) and mirror-(L5 ventral root transection, L5-VRT) pain models, aiming to explore the potential supraspinal neuroimmune mechanism underlying the mirror-image pain.

Methods

The up-and-down method with von Frey hairs was used to measure the mechanical allodynia. Viral injections for the designer receptors exclusively activated by designer drugs (DREADD) were used to modulate ACC glutamatergic neurons. Immunohistochemistry, immunofluorescence, western blotting, protein microarray were used to detect the regulation of inflammatory signaling.

Results

Increased expressions of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6) and chemokine CX3CL1 in ACC induced by unilateral nerve injury were observed on the contralateral side in the SNI group but on the bilateral side in the L5-VRT group, representing a stronger immune response to L5-VRT surgery. In remote ACC, both SNI and L5-VRT induced robust bilateral increase in the protein level of Nav1.6 (SCN8A), a major voltage-gated sodium channel (VGSC) that regulates neuronal activity in the mammalian nervous system. However, the L5-VRT-induced Nav1.6 response occurred at PO 3d, earlier than the SNI-induced one, 7 days after surgery. Modulating ACC glutamatergic neurons via DREADD-Gq or DREADD-Gi greatly changed the ACC CX3CL1 levels and the mechanical paw withdrawal threshold. Neutralization of endogenous ACC CX3CL1 by contralateral anti-CX3CL1 antibody attenuated the induction and the maintenance of mechanical allodynia and eliminated the upregulation of CX3CL1, TNF-α and Nav1.6 protein levels in ACC induced by SNI. Furthermore, contralateral ACC anti-CX3CL1 also inhibited the expression of ipsilateral spinal c-Fos, Iba1, CD11b, TNF-α and IL-6.

Conclusions

The descending facilitation function mediated by CX3CL1 and its downstream cascade may play a pivotal role, leading to enhanced pain sensitization and even mirror-image pain. Strategies that target chemokine-mediated ACC hyperexcitability may lead to novel therapies for the treatment of neuropathic pain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02525-8.

Pathway-specific inhibition of critical projections from the mediodorsal thalamus to the frontal cortex controls kindled seizures

There is a large unmet need for improved treatment for temporal lobe epilepsy (TLE); circuit-specific manipulation that disrupts the initiation and propagation of seizures is promising in this regard. The midline thalamus, including the mediodorsal nucleus (MD) is a critical distributor of seizure activity, but its afferent and efferent pathways that mediate seizure activity are unknown. Here, we used chemogenetics to silence input and output projections of the MD to discrete regions of the frontal cortex in the kindling model of TLE in rats. Chemogenetic inhibition of the projection from the amygdala to the MD abolished seizures, an effect that was replicated using optogenetic inhibition. Chemogenetic inhibition of projections from the MD to the prelimbic cortex likewise abolished seizures. By contrast, inhibition of projections from the MD to other frontal regions produced partial (orbitofrontal cortex, infralimbic cortex) or no (cingulate, insular cortex) attenuation of behavioral or electrographic seizure activity. These results highlight the particular importance of projections from MD to prelimbic cortex in the propagation of amygdala-kindled seizures.

Crossed Connections From Insular Cortex to the Contralateral Thalamus

Sensory information in all modalities, except olfaction, is processed at the level of the thalamus before subsequent transmission to the cerebral cortex. This incoming sensory stream is refined and modulated in the thalamus by numerous descending corticothalamic projections originating in layer 6 that ultimately alter the sensitivity and selectivity for sensory features. In general, these sensory thalamo-cortico-thalamic loops are considered strictly unilateral, i.e., no contralateral crosstalk between cortex and thalamus. However, in contrast to this canonical view, we characterize here a prominent contralateral corticothalamic projection originating in the insular cortex, utilizing both retrograde tracing and cre-lox mediated viral anterograde tracing strategies with the Ntsr1-Cre transgenic mouse line. From our studies, we find that the insular contralateral corticothalamic projection originates from a separate population of layer 6 neurons than the ipsilateral corticothalamic projection. Furthermore, the contralateral projection targets a topographically distinct subregion of the thalamus than the ipsilateral projection. These findings suggest a unique bilateral mechanism for the top-down refinement of ascending sensory information.

Functional reorganization after hemispherectomy in humans and animal models: What can we learn about the brain's resilience to extensive unilateral lesions?

Hemispherectomy (HS) is an effective surgical procedure aimed at managing otherwise intractable epilepsy in cases of diffuse unihemispheric pathologies. Neurological recovery in subjects treated with HS is not limited to seizure reduction, rather, sensory-motor and behavioral improvement is often observed. This outcome highlights the considerable capability of the brain to react to such an extensive lesion, by functionally reorganizing and rewiring the cerebral cortex, especially early in life. In this narrative review, we summarize the animal studies as well as the human neurophysiological and neuroimaging studies dealing with the reorganizational processes that occur after HS. These topics are of particular interest in understanding mechanisms of functional recovery after brain injury. HS offers the chance to investigate contralesional hemisphere activity in controlling ipsilateral limb movements, and the role of transcallosal interactions, before and after the surgical procedure. These post-injury neuroplastic phenomena actually differ from those observed after less extensive brain damage. Therefore, they illustrate how different lesions could lead the contralesional hemisphere to play the "good" or "bad" role in functional recovery. These issues may have clinical implications and could inform rehabilitation strategies aiming to improve functional recovery following unilateral hemispheric lesions. Future studies, involving large cohorts of hemispherectomized patients, will be necessary in order to obtain a greater understanding of how cerebral reorganization can contribute to residual sensorimotor, visual and auditory functions.

Evaluation of WGA-Cre-dependent topological transgene expression in the rodent brain

Novel neuromodulation techniques in the field of brain research, such as optogenetics, prompt to target specific cell populations. However, not every subpopulation can be distinguished based on brain area or activity of specific promoters, but rather on topology and connectivity. A fascinating tool to detect neuronal circuitry is based on the transsynaptic tracer, wheat germ agglutinin (WGA). When expressed in neurons, it is transported throughout the neuron, secreted, and taken up by synaptically connected neurons. Expression of a WGA and Cre recombinase fusion protein using a viral vector technology in Cre-dependent transgenic animals allows to trace neuronal network connections and to induce topological transgene expression. In this study, we applied and evaluated this technology in specific areas throughout the whole rodent brain, including the hippocampus, striatum, substantia nigra, and the motor cortex. Adeno-associated viral vectors (rAAV) encoding the WGA-Cre fusion protein under control of a CMV promoter were stereotactically injected in Rosa26-STOP-EYFP transgenic mice. After 6 weeks, both the number of transneuronally labeled YFP⁺/mCherry^- cells and the transduced YFP⁺/mCherry⁺ cells were quantified in the connected regions. We were able to trace several connections using WGA-Cre transneuronal labeling; however, the labeling efficacy was region-dependent. The observed transneuronal labeling mostly occurred in the anterograde direction without the occurrence of multi-synaptic labeling. Furthermore, we were able to visualize a specific subset of newborn neurons derived from the subventricular zone based on their connectivity.

Role of thalamic nuclei in the modulation of Fos expression within the cerebral cortex during hypertonic saline-induced muscle nociception

It has been proposed that thalamic mediodorsal (MD) and ventromedial (VM) nuclei form thalamic 'nociceptive discriminators' in discrimination of nociceptive afferents, and specifically govern endogenous descending facilitation and inhibition. The present study conducted in rats was to explore the role of thalamic MD and VM nuclei in modulation of cerebral neuronal activities by means of detection of spatiotemporal variations of Fos expression within the cerebral cortex. Following a unilateral intramuscular injection of 5.8% saline into the gastrocnemius muscle, Fos expression within the bilateral, different areas of the cerebral cortex except S2 was significantly increased (P<0.05). Particularly, the increases in Fos expression within the cingulate cortex and the insular cortex occurred at 0.5h, 4h and reached the peak level at 4h, 16h, respectively. Electrolytic lesion of the contralateral thalamic MD and VM nuclei significantly blocked the 5.8% saline intramuscularly induced increases in Fos expression within the bilateral cingulate and insular cortices, respectively. Additionally, the 5.8% saline-induced Fos expression in the cingulate cortex and the insular cortex were dose-dependently attenuated by microinjection of μ-opioid antagonist β-funaltrexamine hydrochloride into the thalamic MD and VM nuclei. It is suggested that (1) the neural circuits of 'thalamic MD nucleus - cingulate cortex' and 'thalamic VM nucleus - insular cortex' form two distinct pathways in the endogenous control of nociception, (2) mirror or contralateral pain is hypothesized to be related to cross-talk of neuronal activities within the bilateral cerebral cortices modulated by μ-opioid receptors within the thalamic MD and VM nuclei.

Thalamic infarctions cause side-specific suppression of vestibular cortex activations

H2O15-PET was performed during caloric vestibular stimulation of the right and left external ears in eight right-handed patients with acute unilateral infarctions or haemorrhages of the posterolateral thalamus (four right, four left). The posterolateral thalamus is the relay station for ipsi- and contralateral ascending vestibular input to the multiple multisensory vestibular cortex areas. The aim of this study was to evaluate the differential effects of unilateral vestibular thalamic lesions on thalamo-cortical projections, right hemispheric dominance and reciprocal inhibitory visual-vestibular interaction, as well as perceptual and ocular motor consequences during caloric irrigation. The major findings of the group analyses of the patients with right-sided and those with left-sided lesions were as follows: (i) activation of the multisensory vestibular temporo-parietal cortex was significantly reduced in the hemisphere ipsilateral to the thalamic lesion when the ipsilesional or contralesional ear was stimulated; (ii) activation of multisensory vestibular cortex areas of the hemisphere contralateral to the irrigated ipsilesional ear was also diminished; and (iii) the right hemispheric dominance in right-handers described above was preserved in those with right and left thalamic lesions. Simultaneous deactivations were often restricted to only one hemisphere--the one contralateral to the stimulation and contralateral to the vestibular cortex areas activated. There was, however, one area in the inferior insula which was also activated by either right or left ear stimulation in the hemisphere ipsilateral to the lesion. This supports the assumption that there is a bilateral direct ascending vestibular projection from the vestibular nuclei to the inferior part of the insula, which bypasses the posterolateral thalamus and is stronger in the right hemisphere. The cortical asymmetry of the pattern of activation during horizontal semicircular canal stimulation by calorics was not associated with a significant direction-specific asymmetry of caloric nystagmus or perceived body motion. Thus, the data demonstrate the functional importance of the posterolateral thalamus as a unique relay station for vestibular input to the cortex, of the dominance of the right hemisphere in right-handedness, and of ipsilateral ascending pathways. Furthermore, the normal interaction between the two sensory systems--the vestibular and the visual--appears to be impaired.

Rostral agranular insular cortex and pain areas of the central nervous system: a tract-tracing study in the rat

The rostral agranular insular cortex (RAIC) has recently been identified as a site where local changes in GABA and dopamine levels, or application of opioids, can alter nociceptive thresholds in awake animals. The connections of the cortex dorsal to the rhinal fissure that includes the RAIC have been examined previously, with emphasis on visceral and gustatory functions but not nociception. Here we examined the afferent and efferent connections of the RAIC with sites implicated in nociceptive processing. Sensory information from the thalamus reaches the RAIC via the submedius and central lateral nuclei and the parvicellular part of the ventral posterior nucleus. The RAIC has extensive reciprocal cortico-cortical connections with the orbital, infralimbic, and anterior cingulate cortices and with the contralateral RAIC. The amygdala, particularly the basal complex, and the nucleus accumbens are important targets of RAIC efferent fibers. Other connections include projections to lateral hypothalamus, dorsal raphe, periaqueductal gray matter, pericerulear region, rostroventral medulla, and parabrachial nuclei. The connectivity of the RAIC suggests it is involved in multiple aspects of pain behavior. Projections to the RAIC from medial thalamic nuclei are associated with motivational/affective components of pain. RAIC projections to mesolimbic/mesocortical ventral forebrain circuits are likely to participate in the sensorimotor integration of nociceptive processing, while its brainstem projections are most likely to contribute to descending pain inhibitory control.

A comparative analysis of the morphology of corticothalamic projections in mammals

Recent anatomical tracing methods have revealed new principles underlying the organization of corticothalamic connections in the mammalian nervous system. These data demonstrated the distribution of two types of synaptic contacts in the corticothalamic projection: small (<1 microm) and giant (2-10 microm) axon terminals. We compare the organization of corticothalamic projections in the auditory, somatosensory, visual, and motor systems of a variety of mammalian species, including the monkey. In all these systems and species, both types of corticothalamic terminals have been observed. Small endings formed the major corticothalamic terminal field, whereas giant terminals were less numerous and formed additional terminal fields together with small terminals. After comparing their spatial distribution, as well as the degree of reciprocity between the corticothalamic and thalamocortical projections, different roles are proposed for small and giant endings. Small terminals are typically present in the projection serving the feed-back control of the cerebral cortex on the thalamic nucleus from which it receives its main projection. In contrast, giant terminals are involved in feed-forward projections by which activity from a cortical area is distributed, via the thalamus, to other parts of the cerebral cortex. The cross-species and cross-systems comparison reveals differences in the mode of feed-forward projection, which may be involved in the activation of other parts of the same cortical area or form part of a projection that activates other cortical areas.

Stimulus frequency affects c-fos expression in the rat visual system

We have characterised the c-fos expression patterns in various centers of the visual pathway of adult rats monocularly stimulated either by continuous or flickering light at different frequencies. Results show different immunocytochemical patterns in all centers studied, the geniculate lateral complex (LGC), superior colliculus (SC) and primary visual cortex (Oc1), depending on the physical characteristics of the stimulus (blinking frequency and light wavelength). After stimulation of the left eye, the ipsilateral pathway presents a substantial density of immunoresponsive cells, which is greater than expected with respect to the number of fibers that project ipsilaterally from the retina to the LGC and the superficial layers of the SC. A surprisingly high positive immunoresponsiveness is obtained in all cases with coherent light stimulation in the red spectrum (634 nm).

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.

Histological markers of neuronal, axonal and astrocytic changes after lateral rigid impact traumatic brain injury

The model of lateral, rigid impact traumatic brain injury is widely used but remains relatively poorly characterized by comparison with fluid percussion injury models. Thus, whilst the gross morphological changes that occur over the short- and long-term post-injury have been described, more subtle measures of neuronal injury and activation, and markers of axonal and glial reactions have not been investigated, complicating interpretation of data from this model. To address this issue, a variety of neurohistological markers were examined in adult male rats which had been subjected to open brain, lateral rigid impact injury. A piston device was unilaterally driven 3.0 mm into the somatosensory cortex at a speed of 3.2 m/s. Neuronal activation evidenced by Fos-like immunoreactivity showed a complex pattern at 3 h after injury which appeared to be related both to proximity to the impact site and cortical efferent connectivity. At 24 h after injury, acid fuchsin staining demonstrated dying neurons in the margin of the injury and in ipsilateral hippocampus and dorsal thalamus. Injured cells identified by heat-shock protein immunoreactivity showed a similar distribution. Axonal injury demonstrated with 68 kDa neurofilament immunoreactivity was more widely distributed. Less axonal damage was found with increasing distance from the injury site. At 7 days post-injury, glial fibrillary acidic protein immunoreactive astrocytes were prolific in the ipsilateral thalamus, hippocampus and striatum and throughout the injured cortex. In general, controlled, lateral rigid impact injury provides a more focused injury than is seen with lateral fluid percussion which may have implications for the behavioral deficits seen in this injury model.

Extracellular levels of glutamate and aspartate in the entopeduncular nucleus of the rat determined by microdialysis: regulation by striatal dopamine D2 receptors via the indirect striatal output pathway?

This study used intracerebral microdialysis to monitor the outputs of excitatory amino acids in the entopeduncular nucleus (EPN) of conscious or halothane-anaesthetized rats, in an attempt to obtain direct biochemical evidence for the theory that neuronal inputs to the EPN by the indirect striatal output pathway are glutamatergic and regulated primarily by dopamine D2 receptors in the striatum. In dopamine-intact animals, both glutamate and asparate were readily detectable in EPN dialysates. Recoveries of both amino acids were increased bilaterally by local perfusion with veratridine (100 microM, given under halothane anaesthesia), pretreatment with reserpine (4 mg/kg, i.p., 24 h beforehand), unilateral pretreatment of the medial forebrain bundle with 6-OHDA (8 microg/4 microl), and by the systemic (1 mg/kg, i.p.) or bilateral intrastriatal (7 microg/0.5 microl under halothane anaesthesia) administration of the dopamine D2 receptor antagonist haloperidol, but not raclopride (2 mg/kg, i.p.). The dopamine D1 receptor antagonist SCH 23390 was ineffective both systemically (0.25 mg/kg, i.p.) and intrastriatally (0.125 microg/0.5 microl/side), as also were control intrastriatal injections of saline (0.5 microl/side). By contrast, the dopamine D2/3 receptor agonist quinpirole (4 mg/kg, i.p.) lowered the outputs of glutamate and aspartate in the EPN of reserpine-treated and normal individuals, whilst the dopamine D1 receptor agonist SKF 38393 (30 mg/kg, i.p.) was inactive; however, both drugs caused behavioural arousal. The dopamine D2/3 receptor agonist RU 24213 reversed reserpine-induced akinesia, yet paradoxically increased glutamate (not aspartate) output in the EPN still further. The combination of benserazide (30 mg/kg, i.p.) and L-DOPA (50 mg/kg, i.p.) evoked intense contraversive circling in unilaterally 6-OHDA-lesioned rats, together with a drop in EPN glutamate (but not aspartate) output in the intact but not lesioned hemisphere. These results offer biochemical support for the hypothesis that excitatory neurones innervating the EPN via the indirect striatal output pathway, may utilise glutamate and/or aspartate as their neurotransmitter. They further endorse the view that the EPN receives information from striatal D2 and not D1 receptors via excitatory synapses, which become hyperactive following dopamine depletion or inactivation, and which are subject to control by the contralateral as well as by the ipsilateral hemisphere. The results obtained with RU 24213 and L-DOPA, however, indicate that dopaminergic behaviours can also occur independently of glutamate or aspartate release in the EPN.