Brainstem local microglia induce whisker map plasticity in the thalamus after peripheral nerve injury.

Whisker deafferentation in mice disrupts topographic connectivity from the brainstem to the thalamic ventral posteromedial nucleus (VPM), which represents whisker map, by recruiting "ectopic" axons carrying non-whisker information in VPM. However, mechanisms inducing this plasticity remain largely unknown. Here, we show the role of region-specific microglia in the brainstem principal trigeminal nucleus (Pr5), a whisker sensory-recipient region, in VPM whisker map plasticity. Systemic or local manipulation of microglial activity reveals that microglia in Pr5, but not in VPM, are necessary and sufficient for recruiting ectopic axons in VPM. Deafferentation causes membrane hyperexcitability of Pr5 neurons dependent on microglia. Inactivation of Pr5 neurons abolishes this somatotopic reorganization in VPM. Additionally, microglial depletion prevents deafferentation-induced ectopic mechanical hypersensitivity. Our results indicate that local microglia in the brainstem induce peripheral nerve injury-induced plasticity of map organization in the thalamus and suggest that microglia are potential therapeutic targets for peripheral nerve injury-induced mechanical hypersensitivity.

Attenuation of secondary damage and Aß deposits in the ipsilateral thalamus of dMCAO rats through reduction of cathepsin B by bis(propyl)-cognitin, a multifunctional dimer.

Delayed secondary degeneration in the non-ischemic sites such as ipsilateral thalamus would occur after cortical infarction. Hence, alleviating secondary damage is considered to be a promising novel target for acute stroke therapy. In the current study, the neuroprotective effects of bis(propyl)-cognitin (B3C), a multifunctional dimer, against secondary damage in the VPN of ipsilateral thalamus were investigated in a distal middle cerebral artery occlusion (dMCAO) stroke model in adult rats. It was found that B3C (0.5 and 1 mg/kg, ip) effectively improved neurological function of rats at day 7 and day 14 after dMCAO. Additionally, the treatment with B3C alleviated neuronal loss and gliosis in ipsilateral VPN after dMCAO, as evidenced by the higher immunoreactivity of neuron-specific nuclear-binding protein (NeuN) as well as lower immunostaining intensity of glial fibrillary acidic protein (GFAP) and cluster of differentiation 68 (CD68). Most encouragingly, immunohistochemistry and western blotting further revealed that B3C treatment greatly reduced Aß deposits and cathepsin B expression in the VPN of ipsilateral thalamus at day 7 and day 14 after dMCAO. In parallel, we demonstrated herein that the neuroprotective effects of B3C in dMCAO model were similar to L-3-trans-(Propyl-carbamoyloxirane-2-carbonyl)- L-isoleucyl-l-proline methyl ester (CA-074Me), a specific inhibitor of cathepsin B, suggesting that B3C attenuated secondary damage and Aß deposits in the VPN of ipsilateral thalamus after dMCAO possibly through the reduction of cathepsin B. These findings taken together provide novel molecular sights into the potential application of B3C for the treatment of secondary degeneration after cortical infarction.

Neuropathy following spinal nerve injury shares features with the irritable nociceptor phenotype: A back-translational study of oxcarbazepine.

BACKGROUND: The term 'irritable nociceptor' was coined to describe neuropathic patients characterized by evoked hypersensitivity and preservation of primary afferent fibres. Oxcarbazepine is largely ineffectual in an overall patient population, but has clear efficacy in a subgroup with the irritable nociceptor profile. We examine whether neuropathy in rats induced by spinal nerve injury shares overlapping pharmacological sensitivity with the irritable nociceptor phenotype using drugs that target sodium channels. METHODS: In vivo electrophysiology was performed in anaesthetized spinal nerve ligated (SNL) and sham-operated rats to record from wide dynamic range (WDR) neurones in the ventral posterolateral thalamus (VPL) and dorsal horn. RESULTS: In neuropathic rats, spontaneous activity in the VPL was substantially attenuated by spinal lidocaine, an effect that was absent in sham rats. The former measure was in part dependent on ongoing peripheral activity as intraplantar lidocaine also reduced aberrant spontaneous thalamic firing. Systemic oxcarbazepine had no effect on wind-up of dorsal horn neurones in sham and SNL rats. However, in SNL rats, oxcarbazepine markedly inhibited punctate mechanical-, dynamic brush- and cold-evoked neuronal responses in the VPL and dorsal horn, with minimal effects on heat-evoked responses. In addition, oxcarbazepine inhibited spontaneous activity in the VPL. Intraplantar injection of the active metabolite licarbazepine replicated the effects of systemic oxcarbazepine, supporting a peripheral locus of action. CONCLUSIONS: We provide evidence that ongoing activity in primary afferent fibres drives spontaneous thalamic firing after spinal nerve injury and that oxcarbazepine through a peripheral mechanism exhibits modality-selective inhibitory effects on sensory neuronal processing. SIGNIFICANCE: The inhibitory effects of lidocaine and oxcarbazepine in this rat model of neuropathy resemble the clinical observations in the irritable nociceptor patient subgroup and support a mechanism-based rationale for bench-to-bedside translation when screening novel drugs.

Electroacupuncture Treatment Alleviates the Remifentanil-Induced Hyperalgesia by Regulating the Activities of the Ventral Posterior Lateral Nucleus of the Thalamus Neurons in Rats.

Mechanisms underlying remifentanil- (RF-) induced hyperalgesia, a phenomenon that is generally named as opioid-induced hyperalgesia (OIH), still remain elusive. The ventral posterior lateral nucleus (VPL) of the thalamus, a key relay station for the transmission of nociceptive information to the cerebral cortex, is activated by RF infusion. Electroacupuncture (EA) is an effective method for the treatment of pain. This study aimed to explore the role of VPL in the development of OIH and the effect of EA treatment on OIH in rats. RF was administered to rats via the tail vein for OIH induction. Paw withdrawal threshold (PWT) in response to mechanical stimuli and paw withdrawal latency (PWL) to thermal stimulation were tested in rats for the assessment of mechanical allodynia and thermal hyperalgesia, respectively. Spontaneous neuronal activity and local field potential (LFP) in VPL were recorded in freely moving rats using the in vivo multichannel recording technique. EA at 2 Hz frequency (pulse width 0.6 ms, 1-3 mA) was applied to the bilateral acupoints "Zusanli" (ST.36) and "Sanyinjiao" (SP.6) in rats. The results showed that both the PWT and PWL were significantly decreased after RF infusion to rats. Meanwhile, both the spontaneous neuronal firing rate and the theta band oscillation in VPL LFP were increased on day 3 post-RF infusion, indicating that the VPL may promote the development of RF-induced hyperalgesia by regulating the pain-related cortical activity. Moreover, 2 Hz-EA reversed the RF-induced decrease both in PWT and PWL of rats and also abrogated the RF-induced augmentation of the spontaneous neuronal activity and the power spectral density (PSD) of the theta band oscillation in VPL LFP. These results suggested that 2 Hz-EA attenuates the remifentanil-induced hyperalgesia via reducing the excitability of VPL neurons and the low-frequency (theta band) oscillation in VPL LFP.

GABAA receptor in the thalamic specific relay system contributes to the propofol-induced somatosensory cortical suppression in rat.

Interaction with the gamma-aminobutyric-acid-type-A (GABAA) receptors is recognized as an important component of the mechanism of propofol, a sedative-hypnotic drug commonly used as anesthetic. However the contribution of GABAA receptors to the central nervous system suppression is still not well understood, especially in the thalamocortical network. In the present study, we investigated if intracerebral injection of bicuculline (a GABAA receptor antagonist) into the thalamus ventral posteromedial nucleus (VPM, a thalamus specific relay nuclei that innervated S1 mostly) could reverse propofol-induced cortical suppression, through recording the changes of both spontaneous and somatosensory neural activities in rat's somatosensory cortex (S1). We found that after injection of bicuculline into VPM, significant increase of neural activities were observed in all bands of local field potentials (total band, 182±6%), while the amplitude of all components in somatosensory evoked potentials were also increased (negative, 121±9% and positive, 124±6%).These data support that the potentiation of GABAA receptor-mediated synaptic inhibition in a thalamic specific relay system seems to play a crucial role in propofol-induced cortical suppression in the somatosensory cortex of rats.

[Effect sites of anesthetics in the central nervous system network--looking into the mechanisms for natural sleep and anesthesia].

We showed the effect sites of anesthetics in the central nervous system (CNS) network. The thalamus is a key factor for loss of consciousness during natural sleep and anesthesia. Although the linkages among neurons within the CNS network in natural sleep are complicated, but sophisticated, the sleep mechanism has been gradually unraveled. Anesthesia disrupts the link-ages between cortical and thalamic neurons and among the cortical neurons, and thus it loses the integration of information derived from the arousal and sleep nuclei. It has been considered that anesthesia does not share the common pathway as natural sleep at the level of unconsciousness, because anesthetics have multiple effect sites within CNS network and may induce disintegration among neurons. Recent literatures have shown that the effects of anesthetics are specific rather than global in the brain. It is interesting to note that thalamic injection of anti-potassium channel materials restored consciousness during inhalation anesthesia, and that the sedative components of certain intravenous anesthesia may share the same pathway as natural sleep. To explore the sensitivity and susceptibility loci for anesthetics in the thalamocortical neurons as well as arousal and sleep nuclei within CNS network may be an important task for future study.

The effects of ethosuximide on amino acids in genetic absence epilepsy rat model.

Genetic absence epilepsy rats from Strasbourg (GAERS), a selectively inbred strain of Wistar rats, has been validated as an experimental model for human absence epilepsy. In this model, systemic administration of ethosuximide (ETX) was shown to reduce the spike and wave discharges (SWD). In this study, gamma-aminobutyric acid (GABA) and L-glutamic acid levels in response to ETX injections (i.p., 100 mg/kg) were measured in the microdialysis samples collected from the ventrolateral thalamus (VLT) and the primary motor cortex (M1) area of Wistar rats and GAERS by using HPLC with fluorescent detection. Throughout the microdialysis procedure, continuous EEG recording was performed where ETX was shown to suppress the SWD activity. We demonstrated increased basal GABA levels in the M1 and VLT of GAERS, and ETX treatment did not produce any effect on higher GABA levels in the VLT, but suppressed the increased GABA levels significantly in the M1 of GAERS. All these findings denote the importance of corticothalamic circuitry and the role of increased GABA tonus in primary motor cortex and thalamus of GAERS. The primary motor cortex also seems to be involved in the SWD activity and ETX exerts, at least partially, its neurotransmitter effects through it.

Validation of a method for localised microinjection of drugs into thalamic subregions in rats for epilepsy pharmacological studies.

OBJECTIVES: To validate a method for the chronic implantation of micro-cannulae to examine the effect of drug administration to two small brain regions critical to the control of generalised seizures, the reticular nucleus of the thalamus (Rt) and the ventrobasal thalamus (VB), in a genetically epileptic rat model. METHOD: Micro-cannulae guides (length 9 mm, 26G, i.d. 0.24 mm, o.d. 0.46 mm) were implanted bilaterally into either the Rt or the VB of 11- to 13-week-old Genetic Absence Epilepsy Rats from Strasbourg (GAERS) using a stereotaxic head frame. After a seven-day recovery period the animals were injected with 0.2 microl of methylene blue. The animals were allowed to move freely in their cages for a further 90 min while a post-drug EEG recording was acquired and then brains were perfused with 4% paraformaldehyde and extracted. Twenty-micrometer slices were cut on a cryostat and the site and extent of the methylene blue staining in the brain determined. The implantation co-ordinates were adjusted accordingly, and then a validation study was performed on a further cohort of rats (n=8 Rt, n=7 VB). RESULTS: The co-ordinates that were found to most accurately localise the Rt were: AP -3mm, ML 3.6mm, DV -5.8mm (relative to Bregma). Those that accurately localised the VB were: AP -3mm, ML 2.6mm, DV -5.5mm. In the validation study, the dye staining was measured to diffuse an average radius of 520+/-120 microm from the centre of the injection site for the 0.2 microl injection in both brain hemispheres. For the VB injections the dye remained confined within the structure, however, for the smaller Rt there was spread to surrounding structures in the axial plane. The radial diffusion for the 0.5 microl injection was similar, but more of the dye was found to spread back up the cannula tract away from the target zone. CONCLUSION: These studies have validated a method for accurate and localised injection of drugs into the VB and Rt for neuropharmacological studies in a rat model of generalised epilepsy. This method allows the measurement of localised drug effects on EEG and generalised seizure activity at these sites.

Convergence of cutaneous, muscular and visceral noxious inputs onto ventromedial thalamic neurons in the rat.

We have recently described a population of neurons in the lateral part of the ventromedial thalamus (VMl), that respond exclusively to noxious cutaneous stimuli, regardless of which part of the body is stimulated. The purpose of the present study was to investigate the convergence of cutaneous, muscular and visceral noxious inputs onto single, VMl neurons in anesthetized rats. VMl neurons were characterized by their responses to Adelta- and C-fiber activation as well as noxious heat applied to the hindpaw. We investigated whether they responded also to colorectal distensions. In an additional series of experiments, we tested the effects of colorectal, intraperitoneal, intramuscular and subcutaneous applications of the chemical irritant mustard oil (MO). The present study shows that a population of neurons located within the thalamic VMl nucleus, carries nociceptive somatosensory signals from the entire body. All these neurons responded to noxious cutaneous and intramuscular stimuli but not to levels of distension that could be considered innocuous or noxious, of the intact and inflammed colon and rectum. Although colorectal distension did not elicit VMl responses, convergence of visceral as well as muscle and cutaneous nociceptors was demonstrated by the increases in ongoing (background) discharges following intracolonic MO. A distinct effect is seen after MO injection into the lumen of the colon: an increase in ongoing activity for 15min but still a lack of effect of colorectal distension. Moreover, following inflammation induced by subcutaneous injections of MO VMl neurons developed responses to both thermal and mechanical innocuous skin stimulation, reminiscent of allodynia phenomena. It is suggested that the VMl contributes to attentional aspects of nociceptive processing and/or to the integration of widespread noxious events in terms of the appropriate potential motor responses.

Inhibitory effect of 17beta-estradiol in the parabrachial nucleus is mediated by GABA.

In the present investigation, electrophysiological recordings of thalamic relay neurons were used to investigate the role of estrogen as a modulator of visceral afferent information through the PBN to forebrain structures. Experiments were done in anaesthetized (sodium thiobutabarbitol; 100 mg/kg) male and ovariectomized female rats supplemented for 7 days prior with either 17beta-estradiol (OVX-E(2)) or saline (OVX-S). A portion of the right cervical vagus was isolated for the electrical activation (0.8 Hz, 2 ms duration) of visceral afferents. The evoked single and multi-unit activity was recorded via a recording electrode in the ventrobasal thalamus. Exogenous microinjection of 17beta-estradiol (0.1, 0.25 and 0.5 microM; 200 nl) into the parabrachial nucleus (PBN) produced a significant, dose-dependent attenuation in the magnitude of visceral afferent activation-evoked responses of neurons recorded in the thalamus in both male and OVX-E(2) groups. No effect on evoked thalamic activity was observed following injection of estrogen into the PBN of OVX-S animals. Co-injection of estrogen with the GABA(A) receptor antagonist, bicuculine (0.1 microM; 200 nl) but not phaclofen (GABA(B); 0.1, 0.5 or 1 microM; 200 nl) resulted in an increase in the evoked thalamic response in males (55+/-11%) and OVX-E(2) female (68+/-15%) rats. These studies suggest that estrogen inhibits neurotransmission in the PBN via an interaction with the GABA(A) receptor to modulate the flow of visceral information to the thalamus.

Temporary inactivation in the primate motor thalamus during visually triggered and internally generated limb movements.

To better understand the contribution of cerebellar- and basal ganglia-receiving areas of the thalamus [ventral posterolateral nucleus, pars oralis (VPLo), area X, ventral lateral nucleus, pars oralis (VLo), or ventral anterior nucleus, pars parvicellularis (VApc)] to movements based on external versus internal cues, we temporarily inactivated these individual nuclei in two monkeys trained to make visually triggered (VT) and internally generated (IG) limb movements. Infusions of lignocaine centered within VPLo caused hemiplegia during which movements of the contralateral arm rarely were performed in either task for a short period of time ( approximately 5-30 min). When VT responses were produced, they had prolonged reaction times and movement times and a higher incidence of trajectory abnormalities compared with responses produced during the preinfusion baseline period. In contrast, those IG responses that were produced remained relatively normal. Infusions centered within area X never caused hemiplegia. The only deficits observed were an increase in reaction time and movement amplitude variability and a higher incidence of trajectory abnormalities during VT trials. Every other aspect of both the VT and IG movements remained unchanged. Infusions centered within VLo reduced the number of movements attempted during each block of trials. This did not appear to be due to hemiplegia, however, as voluntary movements easily could be elicited outside of the trained tasks. The other main deficit resulting from inactivation of VLo was an increased reaction time in the VT task. Finally, infusions centered within VApc caused IG movements to become slower and smaller in amplitude, whereas VT movements remained unchanged. Control infusions with saline did not cause any consistent deficits. This pattern of results implies that VPLo and VLo play a role in the production of movements in general regardless of the context under which they are performed. They also suggest that VPLo contributes more specifically to the execution of movements that are visually triggered and guided, whereas area X contributes specifically to the initiation of such movements. In contrast, VApc appears to play a role in the execution of movements based on internal cues. These results are consistent with the hypothesis that specific subcircuits within the cerebello- and basal ganglio-thalamo-cortical systems preferentially contribute to movements based on external versus internal cues.

Origin of synchronized oscillations induced by neocortical disinhibition in vivo.

During disinhibition, the neocortex generates synchronous activities. Block of GABA(A) receptors in neocortex transforms cortical slow-wave oscillations into large-amplitude approximately 1 Hz discharges consisting of a negative spike or multiple negative spikes riding on a positive wave. Further block of GABA(B) receptors in neocortex slows the discharges to approximately 0.5 Hz and increments the number of negative spikes forming rhythmic approximately 10 Hz neocortical oscillations. Although the thalamus responds robustly to these neocortical discharges, these are unaffected by thalamic inactivation using tetrodotoxin. Thus, an important problem relates to the origin of these activities within the neocortex. Current source density analysis and intracellular recordings revealed that the first negative spike in a discharge corresponded to a current sink that reflected a paroxysmal depolarizing shift (PDS) and could originate in the lower layers or in the upper layers. Regardless of the origin (upper or lower layer), the initial current sink always spreads to the same site in upper layer V-IV. In contrast, the approximately 10 Hz oscillation that follows the initial negative spike corresponds to current sinks that always originate in the lower layers but do not spread to upper layer V-IV, jumping directly to the upper layers. Each current sink in the approximately 10 Hz oscillation reflects a small PDS and is followed by a current source that reflects the repolarization after each PDS.