Ventrobasal Thalamus Research Articles

Antiepileptic effect of electroacupuncture vs. vagus nerve stimulation in the rat thalamus

Our previous study has shown that both electroacupuncture (EA) and vagus nerve stimulation (VNS) can inhibit cortical epileptiform activities induced by pentylenetetrazole (PTZ). The current study compared the effects of EA and VNS on thalamic neuronal responses to PTZ-induced epileptiform activities. Under general anesthesia, extracellular single unit recordings were made from 49 single neurons in the rat ventrobasal (VB) thalamus. The left vagus nerve was stimulated at 30 Hz, 1 or 3 mA for 5 min. For EA, "Dazhui" acupoint (GV14) was stimulated with the same parameters. It was found that (1) the VB thalamic neurons showed epileptiform activities after PTZ injection; (2) VNS and EA could predominantly inhibit the PTZ-induced epileptiform activities in the thalamic neurons. The higher intensity stimulation (3 mA) in either VNS or EA was, however, not associated with a greater inhibition. Our study suggests that both EA and VNS reduce epileptiform activities at the thalamic level, and EA may be an alternative to VNS.

Ethanol Modulates Synaptic and Extrasynaptic GABAA Receptors in the Thalamus

Drinking alcohol is associated with the disturbance of normal sleep rhythms, and insomnia is a major factor in alcoholic relapse. The thalamus is a brain structure that plays a pivotal role in sleep regulation and rhythmicity. A number of studies have implicated GABA(A) receptors (GABA(A)-Rs) in the anxiolytic, amnestic, sedative, and anesthetic effects of ethanol. In the present study, we examined the effects of ethanol on both synaptic and extrasynaptic GABA(A)-Rs of relay neurons in the thalamus. We found that ethanol (> or =50 mM) elicits a sustained current in thalamocortical relay neurons from the mouse ventrobasal thalamus, and this current is associated with a decrease in neuronal excitability and firing rate in response to depolarization. The steady current induced by ethanol was totally abolished by gabazine and was absent in relay neurons from GABA(A)-R alpha(4) subunit knockout mice, indicating that the effect of ethanol is to enhance tonic GABA-mediated inhibition. Ethanol (50 mM) enhanced the amplitude of tonic inhibition by nearly 50%. On the other hand, ethanol had no effect on spontaneous or evoked inhibitory postsynaptic currents (IPSCs) at 50 mM but did prolong IPSCs at 100 mM. Ethanol had no effect on the paired-pulse depression ratio, suggesting that the release of GABA from presynaptic terminals is insensitive to ethanol. We conclude that ethanol, at moderate (50 mM) but not low (10 mM) concentrations, can inhibit thalamocortical relay neurons and that this occurs mainly via the actions of ethanol at extrasynaptic GABA(A)-Rs containing GABA(A)-R alpha(4) subunits.

Prefrontal modulation of tactile responses in the ventrobasal thalamus of rats

Prefrontal cortex (PFC) has been implicated in modulation of sensory information processing in somatosensory cortex. However, it remains unclear whether or not PFC regulates sensory information in thalamus. In the present study, the effect of PFC stimulation on tactile responses of neurons in the ventrobasal thalamus (VB) of the rat was investigated by single-unit recording. PFC stimulation significantly enhanced the signal-noise ratio (tactile responses/background activities) in 16 out of 66 VB neurons (24.2%) that had receptive fields in fore or hind limbs. Such changes can be classified into three different categories: (1) PFC stimulation not only increased the tactile responses, but also suppressed the background activities of neurons (six neurons, 9.1%); (2) PFC stimulation only increased the tactile responses of neurons (five neurons, 7.6%); (3) PFC stimulation only suppressed the background activities of neurons (five neurons, 7.6%). Our results suggest that PFC also modulates somatosensory information at the thalamic level.

Increased thalamic inhibition in the absence seizure prone DBA/2J mouse

Genetic background plays a significant role in producing variable seizure outcomes in patients and animal models. The neurobiological mechanism underlying this heterogeneity is not clear. Here we compare GABAergic synaptic properties within the thalamocortical circuit of two commonly used inbred mice strains, the C57B/6 and spike-and-wave discharge (SWD) prone DBA/2J. Differences between the strains occur in amplitude, kinetics, and frequency of miniature inhibitory postsynaptic currents (mIPSCs) in a region-specific manner. The biggest difference in synaptic inhibition was seen in the thalamus where DBA/2J mice showed a doubling of mIPSC frequency when compared to C57B/6. EEG analysis revealed higher power in the 6-12 Hz band during non-rapid eye movement sleep in DBA/2J mice. Increased susceptibility of the DBA2/J strain to develop SWDs and increase in the 6-12 Hz EEG power may be due to a hypersynchronous ventrobasal thalamus as a consequence of increased GABAergic input.

Developmental maturation of synaptic and extrasynaptic GABAA receptors in mouse thalamic ventrobasal neurones

Thalamic ventrobasal (VB) relay neurones express multiple GABA(A) receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA(A) receptors of developing VB neurones and investigate the role of the alpha(1) subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and alpha(1)(0/0) mice and mice engineered to express diazepam-insensitive receptors (alpha(1H101R), alpha(2H101R)). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the alpha(4) and delta subunit occurred independently of the alpha(1) subunit. Subsequently (> or = P15), synaptic alpha(2) subunit/gephyrin clusters of WT VB neurones were replaced by those containing the alpha(1) subunit. Surprisingly, in alpha(1)(0/0) VB neurones the frequency of mIPSCs decreased between P12 and P27, because the alpha(2) subunit also disappeared from these cells. The loss of synaptic GABA(A) receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA(A) receptors in VB follows a developmental programme independent of the alpha(1) subunit. Changes to synaptic GABA(A) receptor function and the increased expression of extrasynaptic GABA(A) receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

Taurine Is a Potent Activator of Extrasynaptic GABAAReceptors in the Thalamus

Taurine is one of the most abundant free amino acids in the brain. In a number of studies, taurine has been reported to activate glycine receptors (Gly-Rs) at moderate concentrations (> or = 100 microM), and to be a weak agonist at GABA(A) receptors (GABA(A)-Rs), which are usually activated at high concentrations (> or = 1 mM). In this study, we show that taurine reduced the excitability of thalamocortical relay neurons and activated both extrasynaptic GABA(A)-Rs and Gly-Rs in neurons in the mouse ventrobasal (VB) thalamus. Low concentrations of taurine (10-100 microM) decreased neuronal input resistance and firing frequency, and elicited a steady outward current under voltage clamp, but had no effects on fast inhibitory synaptic currents. Currents elicited by 50 microM taurine were abolished by gabazine, insensitive to midazolam, and partially blocked by 20 microM Zn2+, consistent with the pharmacological properties of extrasynaptic GABA(A)-Rs (alpha4beta2delta subtype) involved in tonic inhibition in the thalamus. Tonic inhibition was enhanced by an inhibitor of taurine transport, suggesting that taurine can act as an endogenous activator of these receptors. Taurine-evoked currents were absent in relay neurons from GABA(A)-R alpha4 subunit knock-out mice. The amplitude of the taurine current was larger in neurons from adult mice than juvenile mice. Taurine was a more potent agonist at recombinant alpha4beta2delta GABA(A)-Rs than at alpha1beta2gamma2 GABA(A)-Rs. We conclude that physiological concentrations of taurine can inhibit VB neurons via activation of extrasynaptic GABA(A)-Rs and that taurine may function as an endogenous regulator of excitability and network activity in the thalamus.

GAP-43 is critical for normal targeting of thalamocortical and corticothalamic, but not trigeminothalamic axons in the whisker barrel system

Mice lacking the growth-associated protein GAP-43 (KO) show disrupted cortical topography and no barrels. Whisker-related patterns of cells are normal in the KO brainstem trigeminal complex (BSTC), while the pattern in KO ventrobasal thalamus (VB) is somewhat compromised. To better understand the basis for VB and cortical abnormalities, we used small placements of DiI to trace axonal projections between BSTC, VB, and barrel cortex in wildtype (WT) and GAP-43 KO mice. The trigeminothalamic (TT) pathway consists of axons from cells in the Nucleus Prinicipalis that project to the contralateral VB thalamus. DiI-labeled KO TT axons crossed the midline from BSTC and projected to contralateral VB normally, consistent with normal BSTC cytoarchitecture. By contrast, the KO thalamocortical axons (TCA) projection was highly abnormal. KO TCAs showed delays of 1–2 days in initial ingrowth to cortex. Postnatally, KO TCAs showed multiple pathfinding errors near intermediate targets, and were abnormally fasciculated within the internal capsule (IC). Interestingly, most individually labeled KO TCAs terminated in deep layers instead of in layer IV as in WT. This misprojection is consistent with birthdating analysis in KO mice, which revealed that neurons normally destined for layer IV remain in deep cortical layers. Early outgrowth of KO corticofugal (CF) axons was similar for both genotypes. However, at P7 KO CF fibers remained bundled as they entered the IC, and exhibited few terminal branches in VB. Thus, the establishment of axonal projections between thalamus and cortex are disrupted in GAP-43 KO mice.

Nucleus-Specific Abnormalities of GABAergic Synaptic Transmission in a Genetic Model of Absence Seizures

Human and experimental studies indicate that molecular genetic changes in GABA(A) receptors may underlie the expression of spike-and-waves discharges (SWDs) occurring during absence seizures. However, the full spectrum of the genetic defects underlying these seizures has only been partially elucidated, the expression and functional profiles of putative abnormal protein(s) within the thalamocortical network are undefined, and the pathophysiological mechanism(s) by which these proteins would lead to absence paroxysms are poorly understood. Here we investigated GABA(A) inhibitory postsynaptic currents (IPSCs) in key thalamocortical areas, i.e., the somatosensory cortex, ventrobasal thalamus (VB) and nucleus reticularis thalami (NRT), in preseizure genetic absence epilepsy rats from Strasbourg (GAERS), a well-established genetic model of typical absence seizures that shows no additional neurological abnormalities, and compared their properties to age-matched non-epileptic controls (NECs). Miniature GABA(A) IPSCs of VB and cortical layers II/III neurons were similar in GAERS and NEC, whereas in GAERS NRT neurons they had 25% larger amplitude, 40% faster decay. In addition, baclofen was significantly less effective in decreasing the frequency of NRT mIPSCs in GAERS than in NEC, whereas no difference was observed for cortical and VB mIPSCS between the two strains. Paired-pulse depression was 45% smaller in GAERS NRT, but not in VB, and was insensitive to GABA(B) antagonists. These results point to subtle, nucleus-specific, GABA(A) receptor abnormalities underlying SWDs of typical absence seizures rather than a full block of these receptors across the whole thalamocortical network, and their occurrence prior to seizure onset suggests that they might be of epileptogenic significance.

Different composition of glutamate receptors in corticothalamic and lemniscal synaptic responses and their roles in the firing responses of ventrobasal thalamic neurons in juvenile mice

Thalamic ventrobasal (VB) relay neurons receive information via two major types of glutamatergic synapses, that is, from the medial lemniscus (lemniscal synapses) and primary somatosensory cortex (corticothalamic synapses). These two synapses influence and coordinate firing responses of VB neurons, but their precise operational mechanisms are not yet well understood. In this study, we compared the composition of glutamate receptors and synaptic properties of corticothalamic and lemniscal synapses. We found that the relative contribution of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) to non-NMDA receptor-mediated EPSCs was significantly greater in corticothalamic synapses than in lemniscal synapses. Furthermore, NMDA receptor 2B-containing NMDA receptor- and kainate receptor-mediated currents were observed only in corticothalamic synapses, but not in lemniscal synapses. EPSCs in corticothalamic synapses displayed the postsynaptic summation in a frequency-dependent manner, in which the summation of the NMDA receptor-mediated component was largely involved. The summation of kainate receptor-mediated currents also partially contributed to the postsynaptic summation in corticothalamic synapses. In contrast, the contribution of NMDA receptor-mediated currents to the postsynaptic summation of lemniscal EPSCs was relatively minor. Furthermore, our results indicated that the prominent NMDA receptor-mediated component in corticothalamic synapses was the key determinant for the late-persistent firing of VB neurons in response to corticothalamic stimuli. In lemniscal synapses, in contrast, the onset-transient firing in response to lemniscal stimuli was regulated mainly by AMPA receptors.

Feedforward Inhibitory Connections From Multiple Thalamic Cells to Multiple Regular-Spiking Cells in Layer 4 of the Somatosensory Cortex

Thalamocortical (TC) cells in the ventrobasal thalamus make direct excitatory connections with regular-spiking (RS) cells in layer 4 of the somatosensory cortex, but also make disynaptic feedforward inhibitory connections with the RS cells by layer 4 fast-spiking (FS) cells. In this study, we investigated connection rules of the feedforward inhibitory circuit from multiple TC cells to multiple RS cells, at the level of synaptic potentials. Using thalamocortical brain slices of young mice (postnatal days 12-16), we made simultaneous patch-clamp recordings from three adjacent cortical cells (two RS cells and one FS cell), combined with minimal stimulation of presumed single TC fibers. We found that nearly all (97%) of TC fibers, which generated excitatory inputs onto RS cells, also generated divergent excitatory inputs onto adjacent FS cells. Some 44% of TC fibers generated divergent excitatory inputs onto adjacent pairs of RS cells. We then combined the triple patch-clamp recording with multisite (two to three) minimal stimulation of single TC fibers and found that 86% of FS cells received convergent inputs from all of the stimulated TC fibers. We also found that 68% of FS cells generated divergent inhibitory inputs onto adjacent pairs of RS cells. The results indicate that spikes in TC cells, which excite RS cells, also excite adjacent FS cells with high fidelity. The results also indicate that FS cells receive convergent excitatory inputs from multiple TC cells and then send divergent inhibitory outputs to multiple RS cells.

Distinctive Glycinergic Currents With Fast and Slow Kinetics in Thalamus

We examined functional properties of inhibitory postsynaptic currents (IPSCs) evoked by medial lemniscal stimulation, spontaneous IPSCs (sIPSCs), and single-channel, extrasynaptic currents evoked by glycine receptor agonists or gamma-aminobutyric acid (GABA) in rat ventrobasal thalamus. We identified synaptic currents by reversal at E(Cl) and sensitivity to elimination by strychnine, GABA(A) antagonists, or combined application. Glycinergic IPSCs featured short (about 12 ms) and long (about 80 ms) decay time constants. These fast and slow IPSCs occurred separately with monoexponential decays, or together with biexponential decay kinetics. Glycinergic sIPSCs decayed monoexponentially with time constants, matching fast and slow IPSCs. These findings were consistent with synaptic responses generated by two populations of glycine receptors, localized under different nerve terminals. Glycine, taurine, or beta-alanine applied to excised membrane patches evoked short- and long-duration current bursts. Extrasynaptic burst durations resembled fast and slow IPSC time constants. The single, intermediate time constant (about 22 ms) of GABA(A)ergic IPSCs cotransmitted with glycinergic IPSCs approximated the burst duration of extrasynaptic GABA(A) channels. We noted differences between synaptic and extrasynaptic receptors. Endogenously activated glycine and GABA(A) receptor channels had higher Cl- permeability than that of their extrasynaptic counterparts. The beta-amino acids activated long-duration bursts at extrasynaptic glycine receptors, consistent with a role in detection of ambient taurine or beta-alanine. Heterogeneous kinetics and permeabilities implicate molecular and functional diversity in thalamic glycine receptors. Fast, intermediate, and slow inhibitory postsynaptic potential decays, mostly attributed to cotransmission by glycinergic and GABAergic pathways, allow for discriminative modulation and integration with voltage-dependent currents in ventrobasal neurons.

Adenosine A1 receptors decrease thalamic excitation of inhibitory and excitatory neurons in the barrel cortex

Caffeine is consumed worldwide to enhance wakefulness, but the cellular mechanisms are poorly understood. Caffeine blocks adenosine receptors suggesting that adenosine decreases cortical arousal. Given the widespread innervation of the cerebral cortex by thalamic fibers, adenosine receptors on thalamocortical terminals could provide an efficient method of limiting thalamic activation of the cortex. Using a mouse thalamocortical slice preparation and whole-cell patch clamp recordings, we examined whether thalamocortical terminals are modulated by adenosine receptors. Bath application of adenosine decreased excitatory postsynaptic currents elicited by stimulation of the ventrobasal thalamus. Thalamocortical synapses onto inhibitory and excitatory neurons were equally affected by adenosine. Adenosine also increased the paired pulse ratio and the coefficient of variation of the excitatory postsynaptic currents, suggesting that adenosine decreased glutamate release. The inhibition produced by adenosine was reversed by a selective antagonist of adenosine A1 receptors (8-cyclopentyltheophylline) and mimicked by a selective A1 receptor agonist (N6-cyclopentyladenosine). Our results indicate that thalamocortical excitation is regulated by presynaptic adenosine A1 receptors and provide a mechanism by which increased adenosine levels can directly reduce cortical excitability.

Reorganization of barrel circuits leads to thalamically-evoked cortical epileptiform activity

We studied circuit activities in layer IV of rat somatosensory barrel cortex containing microgyri induced by neonatal freeze lesions. Structural abnormalities in GABAergic interneurons are present in the epileptogenic paramicrogyral area (PMG) and we therefore tested the hypothesis that decreased postsynaptic inhibition within barrel microcircuits occurs in the PMG and contributes to epileptogenesis when thalamocortical afferents are activated. In thalamocortical (TC) slices from naïve animals, single electrical stimuli within the thalamic ventrobasal (VB) nucleus evoked transient cortical multi-unit activity lasting 65±42 ms. Similar stimuli in TC slices from lesioned barrel cortex elicited prolonged 850 ±100 ms paroxysmal discharges that originated in the PMG and propagated laterally over several mm. Paroxysmal discharges were shortened in duration by ~70 % when APV was applied, and were totally abolished by CNQX. The cortical paroxysmal discharges did not evoke thalamic oscillations. Whole cell patch clamp recordings showed that there was a shift in the balance of TC evoked responses in the PMG that favored excitation over inhibition. Dual whole-cell recordings in layer IV of the PMG indicated that there was selective loss of inhibition from fast-spiking interneurons to spiny neurons in the barrel circuits that likely contributed to unconstrained cortical recurrent excitation with generation and spread of paroxysmal discharges.

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

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

An Extrasynaptic GABAA Receptor Mediates Tonic Inhibition in Thalamic VB Neurons

Whole cell patch-clamp recordings were obtained from thalamic ventrobasal (VB) and reticular (RTN) neurons in mouse brain slices. A bicuculline-sensitive tonic current was observed in VB, but not in RTN, neurons; this current was increased by the GABA(A) receptor agonist 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridine-3-ol (THIP; 0.1 microM) and decreased by Zn(2+) (50 microM) but was unaffected by zolpidem (0.3 microM) or midazolam (0.2 microM). The pharmacological profile of the tonic current is consistent with its generation by activation of GABA(A) receptors that do not contain the alpha(1) or gamma(2) subunits. GABA(A) receptors expressed in HEK 293 cells that contained alpha(4)beta(2)delta subunits showed higher sensitivity to THIP (gaboxadol) and GABA than did receptors made up from alpha(1)beta(2)delta, alpha(4)beta(2)gamma(2s,) or alpha(1)beta(2)gamma(2s) subunits. Western blot analysis revealed that there is little, if any, alpha(3) or alpha(5) subunit protein in VB. In addition, co-immunoprecipitation studies showed that antibodies to the delta subunit could precipitate alpha(4), but not alpha(1) subunit protein. Confocal microscopy of thalamic neurons grown in culture confirmed that alpha(4) and delta subunits are extensively co-localized with one another and are found predominantly, but not exclusively, at extrasynaptic sites. We conclude that thalamic VB neurons express extrasynaptic GABA(A) receptors that are highly sensitive to GABA and THIP and that these receptors are most likely made up of alpha(4)beta(2)delta subunits. In view of the critical role of thalamic neurons in the generation of oscillatory activity associated with sleep, these receptors may represent a principal site of action for the novel hypnotic agent gaboxadol.

Glycinergic inhibition in thalamus revealed by synaptic receptor blockade

Using juvenile rat brain slices, we examined the possibility that strychnine-sensitive receptors for glycine-like amino acids contributed to synaptic inhibition in ventrobasal thalamus, where γ-aminobutyrate (GABA) is the prevalent inhibitory transmitter. Ventrobasal nuclei showed staining for antibodies against α 1 and α 2 subunits of the glycine receptor. Exogenously applied glycine, taurine and β-alanine increased membrane conductance, effects antagonized by strychnine, indicative of functional glycine receptors. Using glutamate receptor antagonists, we isolated inhibitory postsynaptic potentials and currents (IPSPs and IPSCs) evoked by high-threshold stimulation of medial lemniscus. Like the responses to glycine agonists, these synaptic responses reversed near E Cl. In comparative tests with GABA receptor antagonists, strychnine attenuated inhibition in a majority of neurons, but did not alter slow, GABA B inhibition. For complete blockade, the majority of fast IPSPs required co-application of strychnine with bicuculline or gabazine, GABA A receptor antagonists. Strychnine acting with an IC 50≅33 nM, eliminated residual fast inhibition during selective GABA A receptor blockade with gabazine. The latency of onset for IPSPs was compatible with polysynaptic pathways or prolonged axonal propagation time. Strychnine lacked effects on monosynaptic, GABAergic IPSPs from zona incerta. The specific actions of strychnine implicated a glycine receptor contribution to fast inhibition in somatosensory thalamus.

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, 26 G, 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 μl 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 −3 mm, ML 3.6 mm, DV −5.8 mm (relative to Bregma). Those that accurately localised the VB were: AP −3 mm, ML 2.6 mm, DV −5.5 mm. In the validation study, the dye staining was measured to diffuse an average radius of 520 ± 120 μm from the centre of the injection site for the 0.2 μl 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 μl 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.

Altered glutamate and GABA release within thalamocortical circuitry in metabotropic glutamate receptor 4 knockout mice

The metabotropic glutamate receptor 4 is highly expressed presynaptically on thalamocortical neurons that are involved in the pathogenesis of generalized absence seizures. Mutant mice devoid of metabotropic glutamate receptor 4 are completely resistant to absence seizures induced by low doses of GABA type A receptor antagonists. The purpose of this study was to test the hypothesis that there is altered glutamate and GABA release within thalamocortical circuitry in mice devoid of metabotropic glutamate receptor 4. Extracellular GABA and glutamate release were determined in ventrobasal thalamus, the nucleus reticularis thalami and laminae I–III, and IV–VI of cerebral cortex (laminae I–III of cerebral cortex, and laminae IV–VI of cerebral cortex) using in vivo microdialysis techniques on awake, free moving mice. A significant increase of both basal and K +-evoked glutamate release was detected in the ventrobasal thalamus, the nucleus reticularis thalami and laminae IV–VI of cerebral cortex of mice devoid of metabotropic glutamate receptor 4 mice. There also was a significant increase in both basal and K +-evoked GABA release in the mice devoid of metabotropic glutamate receptor 4, but a significant decrease of GABA release in laminae IV–VI of cerebral cortex. However, there was no alteration of either GABA or glutamate release in laminae I–III of cerebral cortex, cortical laminae that are not involved in absence seizures. These data indicate that deletion of the metabotropic glutamate receptor 4 gene results in a selective perturbation of glutamate and GABA release within the thalamocortical circuitry involved in the pathogenesis of absence seizures.

Positive α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Modulators Have Different Impact on Synaptic Transmission in the Thalamus and Hippocampus

Earlier studies showed that positive modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors enhance synaptic responses and facilitate synaptic plasticity. Those studies focused mainly on hippocampal functions. However, AMPA receptors have regionally distinct subunit compositions and thus potencies and efficacies of modulators may vary across the brain. The present study compared the effects of CX546 [1-(1,4-benzodioxan-6-ylcarbonyl) piperidine], a benzamide-type modulator, on synaptic transmission in neurons of the reticular thalamic nucleus (RTN), which regulates the firing mode of relay cells in other thalamic nuclei, and on hippocampal CA1 pyramidal cells. CX546 greatly prolonged synaptic responses in CA1 pyramidal cells, but at the same concentration it had only weak modulatory effects in RTN neurons. Effects on miniature excitatory postsynaptic currents (EPSCs) were similar to those on EPSCs in both regions, suggesting that variations in neuronal morphology and transmitter release kinetics do not account for the differences. Relay cells in the ventrobasal thalamus also exhibited weak modulatory effects that were comparable with those in RTN neurons. Regionally different effects on response duration were also observed with CX516 [BDP-12, 1-(quinoxalin-6-ylcarbonyl)piperidine], a second benzamide drug. In contrast, 100 microM cyclothiazide produced comparable synaptic enhancements in hippocampus and RTN. The regional selectivity of benzamide drugs (ampakines) may be explained, at least in part, by a lower potency at thalamic AMPA receptors, perhaps due to the prevalence of the subunits GluR3 and 4. Although regional preferences of the ampakines were modest in their extent, they may be sufficient to be of relevance when considering future therapeutic applications of such compounds.

Role of Group I Metabotropic Glutamate Receptors mGlu1 and mGlu5 in Nociceptive Signalling

Distribution, pharmacological and knockdown studies point to a role of group I receptors in nociceptive processing. mGlu1 and mGlu5 receptors are expressed on peripheral sensory afferents, in dorsal root ganglion neurons, dorsal and ventral horn of spinal cord, rostral ventromedial medulla, periaqeductal gray, amygdala, and ventrobasal thalamus. These receptors are functional: sensory neurons in these regions respond to pain stimuli and / or to mGlu1 and mGlu5 agonists. Intrathecal administration of group I agonists evokes spontaneous pain behaviour. Whereas mGlu1 / 5 antagonists do not alter normal sensation, their effect on acute nociception is controversial. Activation of spinal and peripheral mGlu1 or mGlu5 receptors is sufficient to evoke hyperalgesia and allodynia. Blockade of spinal or peripheral mGlu1 or peripheral mGlu5 receptors was antihyperalgesic in animal models of inflammatory pain. Although the role of supraspinal, spinal and peripheral mGlu1 and mGlu5 receptors in the development and maintenance of neuropathic pain states is not fully elucidated, spinal mGlu1 receptor blockade is effective in all pain states tested. In the periaqueductal gray, a group I agonist attenuates nociceptive responses, indicating that mGlu1 and / or mGlu5 receptors in this structure can activate the descending pathway to dampen pain. The data indicate that mGlu1 and mGlu5 antagonists have therapeutic potential for the treatment of chronic inflammatory and neuropathic pain states. More potent and bioavailable mGlu1 versus mGlu5 antagonists are needed to provide proof of concept. Keywords: mglu1, mglu5, metabotropic glutamate, nociception, pain, hyperalgesia