Rat Ventrobasal Thalamus Research Articles

Actions of Xanthurenic Acid, a putative endogenous Group II metabotropic glutamate receptor agonist, on sensory transmission in the thalamus

Xanthurenic acid (XA), a molecule arising from tryptophan metabolism by transamination of 3-hydroxykynurenine, has recently been identified as an endogenous Group II (mGlu2 and mGlu3) metabotropic glutamate (mGlu) receptor ligand in vitro. Impairments in Group II mGlu receptor expression and function have been implicated in the pathophysiology of schizophrenia, as have multiple steps in the kynurenine metabolism pathway. Therefore, we examined XA in vivo to further investigate its potential as a Group II mGlu receptor ligand using a preparation that has been previously demonstrated to efficiently reveal the action of other Group II mGlu receptor ligands in vivo. Extracellular single-neurone recordings were made in the rat ventrobasal thalamus (VB) in conjunction with iontophoresis of agonists, an antagonist and a positive allosteric modulator and/or intravenous (i.v.) injection of XA. We found the XA effect on sensory inhibition, when applied iontophoretically and i.v., was similar to that of other Group II mGlu receptor agonists in reducing inhibition evoked in the VB from the thalamic reticular nucleus upon physiological sensory stimulation. Furthermore, we postulate that XA may be the first potential endogenous allosteric agonist (termed ‘endocoid’) for the mGlu receptors. As the Group II receptors and kynurenine metabolism pathway have both been heavily implicated in the pathophysiology of schizophrenia, XA could play a pivotal role in antipsychotic research as this potential endocoid represents both a convergence within these two biological parameters and a novel class of Group II mGlu receptor ligand.This article is part of a Special Issue entitled ‘Metabotropic Glutamate Receptors’.

Glutamatergic input–output properties of thalamic astrocytes

Astrocytes in the somatosensory ventrobasal (VB) thalamus of rats respond to glutamatergic synaptic input with metabotropic glutamate receptor (mGluR) mediated intracellular calcium ([Ca2+]i) elevations. Astrocytes in the VB thalamus also release the gliotransmitter (GT) glutamate in a Ca2+-dependent manner. The tripartite synapse hypothesis posits that astrocytic [Ca2+]i elevations resulting from synaptic input releases gliotransmitters that then feedback to modify the synapse. Understanding the dynamics of this process and the conditions under which it occurs are therefore important steps in elucidating the potential roles and impact of GT release in particular brain activities. In this study, we investigated the relationship between VB thalamus afferent synaptic input and astrocytic glutamate release by recording N-methyl-d-aspartate (NMDA) receptor-mediated slow inward currents (SICs) elicited in neighboring neurons. We found that Lemniscal or cortical afferent stimulation, which can elicit astrocytic [Ca2+]i elevations, do not typically result in the generation of SICs in thalamocortical (TC) neurons. Rather, we find that the spontaneous emergence of SICs is largely resistant to acute afferent input. The frequency of SICs, however, is correlated to long-lasting afferent activity. In contrast to short-term stimulus-evoked GT release effects reported in other brain areas, astrocytes in the VB thalamus do not express a straightforward input–output relationship for SIC generation but exhibit integrative characteristics.

Neural Substrates of Chronic Pain in the Thalamocortical Circuit

AbstractChronic pain (CP), a pathological condition with a large repertory of signs and symptoms, has no recognizable neural functional common hallmark shared by its diverse expressions. The aim of the present research was to identify potential dynamic markers shared in CP models, by using simultaneous electrophysiological extracellular recordings from the rat ventrobasal thalamus and the primary somatosensory cortex. We have been able to extract a neural signature attributable solely to CP, independent from of the originating conditions. This study showed disrupted functional connectivity and increased redundancy in firing patterns in CP models versus controls, and interpreted these signs as a neural signature of CP. In a clinical perspective, we envisage CP as disconnection syndrome and hypothesize potential novel therapeutic appraisal.

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.

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.

Kainate receptor (GluR5)-mediated disinhibition of responses in rat ventrobasal thalamus allows a novel sensory processing mechanism.

Kainate receptors have been studied extensively in vitro, but how they might function physiologically remains unclear. We studied kainate receptor modulation of synaptic responses in the rat ventrobasal thalamus using the novel antagonist LY382884 and the agonist ATPA (selective for GluR5-containing kainate receptors) as tools. No evidence could be found for a direct contribution of kainate receptors to responses of thalamic relay cells to lemniscal (sensory) input in thalamic slices studied with the aid of intracellular and field potential recordings, using selective AMPA and NMDA receptor antagonists and LY382884. However, the GluR5 agonist ATPA reduced the IPSPs originating from the thalamic reticular nucleus. Extracellular single-neurone recordings in anaesthetised rats showed that excitatory responses evoked by physiological vibrissa afferent stimulation were reduced by LY382884 applied iontophoretically at the recording site. This action of the antagonist was occluded when GABA receptors were blocked, indicating that the reduction in excitatory sensory responses by LY382884 is due to an action on GABAergic inhibition arising from the thalamic reticular nucleus. Further experiments showed that these actions depended on whether inhibition was evoked during activation of the excitatory receptive field rather than when inhibition was evoked from a surround vibrissa. We suggest that GluR5 is located presynaptically on inhibitory GABAergic terminals of thalamic reticular nucleus neurones, and that it is normally activated by glutamate spillover from synapses between excitatory afferents and relay neurones during physiological stimulation. We propose that this GluR5-activated disinhibition has an important novel role in extracting sensory information from background noise.

Novel mode of nitric oxide neurotransmission mediated via S-nitroso-cysteinyl-glycine.

S-nitroso-cysteinyl-glycine, a novel nitric oxide-adduct thiol compound, can be detected in the brain (2.3+/-0.6 pmol/mg protein), and released following stimulation of sensory afferents to the rat ventrobasal thalamus in vivo (resting conditions 17 nM; stimulation: 186 nM). Iontophoretic application of CysNOGly (20-80 nA) onto thalamic neurons in vivo resulted in enhancements of excitatory responses to either NMDA or AMPA (182+/-13.6% and 244+/-27.8% of control values, n = 15). CysNOGly enhanced responses to stimulation of vibrissal afferents to 132+/-2.2% (n = 7) of control values. In contrast, the dipeptide CysGly reduced responses of ventrobasal neurons to NMDA and AMPA (54+/-8.4% and 55+/-10.8% of control, n = 5). CysNOGly was also a potent activator of soluble guanylate cyclase in vitro. Moreover, we found that NMDA elevated CysNOGly levels in vitro and this stimulatory effect was reduced by inhibitors of the neuronal NO synthase and of the gamma-glutamyl transpeptidase, suggesting that production of NO and CysGly is a prelude to CysNOGly synthesis. These findings suggest that the nitrosothiol CysNOGly plays a role in synaptic transmission in the ventrobasal thalamus. We propose a novel synaptic buffering mechanism where S-nitroso-cysteinyl-glycine serves to restrict the locus of action of nitric oxide and so increase its local availability for target delivery. This could lead to a change in neuronal responses favouring sensory transmission similar to that seen in wakefulness or arousal in order to locally enhance transmission of persistent sensory stimuli.

Contributions of mGlu1 and mGlu5 receptors to interactions with N-methyl-d-aspartate receptor-mediated responses and nociceptive sensory responses of rat thalamic neurons

The nociceptive responses of rat ventrobasal thalamus neurons can be reduced by N-methyl-d-aspartate antagonists and by selective metabotropic glutamate receptor mGlu1 antagonists. The recent development of the mGlu5-selective antagonist 6-methyl-2-(phenylethynyl)-pyridine now allows the direct probing of the possible involvement of mGlu5 receptors in thalamic nociceptive responses. Extracellular recordings were made from single neurons in the ventrobasal thalamus and immediately overlying dorsal thalamic nuclei of adult urethane-anaesthetized rats using multi-barrel electrodes. Responses of neurons to iontophoretic applications of the mGlu5-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine were selectively reduced during continuous iontophoretic applications of 6-methyl-2-(phenylethynyl)-pyridine. Similar applications of 6-methyl-2-(phenylethynyl)-pyridine reduced neuronal responses to noxious thermal stimuli to 53±9.5% of control responses. Co- application by iontophoresis of N-methyl-d-aspartate and metabotropic glutamate receptor agonists resulted in a mutual potentiation of excitatory responses. This effect could be reduced by either 6-methyl-2-(phenylethynyl)-pyridine or the mGlu1 antagonist LY367385.These results, taken together with previous data, suggest that acute thalamic nociceptive responses are mediated by a combination of mGlu1, mGlu5 and N-methyl-d-aspartate receptor activation, and that co-activation of these receptors produces a synergistic excitatory effect. Thus blockade of any of these receptor types would have a profound effect on the overall nociceptive response.

Metabotropic glutamate receptors 2 and 3 expressed by astrocytes in rat ventrobasal thalamus

It has been reported that pharmacologic activation of metabotropic receptors mGluR2/3 may produce inhibition of GABAergic transmission in rat ventrobasal thalamus. We use double immunolabeling to show that: (i) mGluR2/3 is expressed in glial processes surrounding GABAergic terminals, rather than in the terminals themselves, and (ii) mR2/3 positive glial lamellae more frequently surround GABAergic terminals and ascending terminals, than cortical terminals. This suggests involvement of glia in glutamate/GABA interactions in the sensory thalamus.

On the Action of the Anti-Absence Drug Ethosuximide in the Rat and Cat Thalamus

The action of ethosuximide (ETX) on Na+, K+, and Ca2+ currents and on tonic and burst-firing patterns was investigated in rat and cat thalamic neurons in vitro by using patch and sharp microelectrode recordings. In thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (LGN), ETX (0.75-1 mM) decreased the noninactivating Na+ current, INaP, by 60% but had no effect on the transient Na+ current. In TC neurons of the rat and cat LGN, the whole-cell transient outward current was not affected by ETX (up to 1 mM), but the sustained outward current was decreased by 39% at 20 mV in the presence of ETX (0.25-0.5 mM): this reduction was not observed in a low Ca2+ (0.5 mM) and high Mg2+ (8 mM) medium or in the presence of Ni2+ (1 mM) and Cd2+ (100 microM). In addition, ETX (up to 1 mM) had no effect on the low-threshold Ca2+ current, IT, of TC neurons of the rat ventrobasal (VB) thalamus and LGN and in neurons of the rat nucleus reticularis thalami nor on the high-threshold Ca2+ current in TC neurons of the rat LGN. Sharp microelectrode recordings in TC neurons of the rat and cat LGN and VB showed that ETX did not change the resting membrane potential but increased the apparent input resistance at potentials greater than -60 mV, resulting in an increase in tonic firing. In contrast, ETX decreased the number of action potentials in the burst evoked by a low-threshold Ca2+ potential. The frequency of the remaining action potentials in a burst also was decreased, whereas the latency of the first action potential was increased. Similar effects were observed on the burst firing evoked during intrinsic delta oscillations. These results indicate an action of ETX on INaP and on the Ca2+-activated K+ current, which explains the decrease in burst firing and the increase in tonic firing, and, together with the lack of action on low- and high-threshold Ca2+ currents, the results cast doubts on the hypothesis that a reduction of IT in thalamic neurons underlies the therapeutic action of this anti-absence medicine.

Role of N-methyl-D-aspartate and metabotropic glutamate receptors in corticothalamic excitatory postsynaptic potentials in vivo.

The ventrobasal thalamus is the principal somatosensory thalamic relay nucleus, and it receives two major sources of excitatory input: firstly an input from ascending sensory afferents, and secondly a descending projection from the primary somatosensory cortex. There is considerable anatomical evidence to suggest that both of these projections utilise the excitatory amino acid L-glutamate as their neurotransmitter. Previous work from this laboratory has shown that the sensory input to the rat ventrobasal thalamus in vivo is mediated by ionotropic excitatory amino acid receptors of both the N-methyl-D-aspartate and non-N-methyl-D-aspartate type. These findings are consistent with data from other studies in various thalamic relay nuclei. In contrast, there are considerably less data available concerning the synaptic pharmacology of the corticothalamic projection although there have been both speculation and studies concerning the functional significance of this pathway. There is some evidence to suggest an involvement of N-methyl-D-aspartate receptors and metabotropic glutamate receptors. The aim of this study was to determine which excitatory amino acid receptors might mediate cortically-elicited excitatory postsynaptic potential in the ventrobasal thalamus in vivo. Intracellular recordings were made, and neurotransmitter antagonists were applied on to rat ventrobasal thalamus neurons by microiontophoresis. Cortically-elicited excitatory postsynaptic potentials were reduced by the N-methyl-D-aspartate antagonist 3-[(+/-)-2-carboxy-piperazin-4-yl]-propyl-1-phosphonate, or the Group I metabotropic antagonist (S)-4-carboxyphenylglycine. These data indicate that both N-methyl-D-aspartate receptors and Group I (possibly metabotropic glutamate receptors type I) metabotropic receptors are involved in the mediation of corticothalamic transmission. Such a transmitter mechanism would allow a modulatory system that could selectively enhance other excitatory inputs. Some of these data have been reported in abstract form.

Development of firing patterns and electrical properties in neurons of the rat ventrobasal thalamus

The electrophysiological properties of thalamic neurons of the rat ventrobasal complex (VB) in vitro were studied during early postnatal development. Current clamp recordings using the whole cell patch clamp method revealed that immature thalamic neurons had less negative membrane potential and higher input resistance than mature neurons. One of the most remarkable differences was the absence of spike bursts riding on the low threshold calcium spike (LTS) in VB neurons before postnatal day 12 (P12). Action potentials recorded from immature neurons had longer duration than those of mature cells and were followed by a longer afterhyperpolarization (spike-ahp). The spike-ahp became shorter as maturation progressed, reaching mature characteristics around P12, coinciding with the appearance of spike bursting on the LTS. The calcium activated potassium conductance, I C, played a prominent role in the spike-ahp in immature neurons. In conclusion, the major differences in intrinsic membrane properties of VB neurons occur during the first 12 postnatal days. The appearance of spike bursting riding on the LTS at P12 is consistent with the emergence of synchronized thalamocortical oscillations in rats around that age.

Calcium influx in rat thalamic relay neurons through voltage-dependent calcium channels is inhibited by enkephalin

High and low voltage-activated, transient (HVA and LVA,T) Ca 2+ currents are crucial in determining the characteristic thalamic firing pattern, during the oscillatory mode. The modulatory effects induced by d-ala 2- d-leu 5-enkephalin (DADLE) on voltage-dependent Ca 2+ channels have been investigated on acutely dissociated neurons from rat ventro-basal (VB) thalamus, by means of whole cell patch-clamp technique. DADLE (400 nM) reduced HVA Cat+ channel currents in 37 out of 44 cells tested (−53 ± 5.3% to 0 mV test potential, n = 24,). In 50% of the cases DADLE induced an effect which was persistent at all the potentials tested, i.e. a voltage-independent one. In the remaining neurons, the inhibition partially or totally disappeared on the currents evoked at the highest potentials. DADLE was also able to inhibit LVA Ca 2+ channels (−40% in five out of 12 cells). In conclusion, thalamic relay neurons present opioid receptors negatively coupled to both HVA and LVA Ca 2+ channels. The presence of two inhibitory effects of DADLE on the total HVA Ca 2+ channels has been observed, and they are distinguishable on the basis of their sensitivity to voltage. It is suggested that Ca 2+ current modulation may play a role in the production and tuning of the rhythmic burst discharge in these neurons.

Electrophysiological evidence that morphine can exert an antinociceptive effect in a neuropathic state: a study in the ventrobasal thalamus of rats after moderate ligation of one sciatic nerve

This study was performed in rats with a mononeuropathy induced by loose ligatures around the common sciatic nerve 2 weeks before the recording session, and exhibiting clear alterations of several pain-related behaviours. Morphine injected intravenously (0.6 and 1 mg/kg) strongly depressed the ventrobasal thalamic neuronal responses to pinch applied to the lesioned or the non-lesioned hindpaw. The effect, comparable on the both sides, was dose-related and reversed by naloxone (0.1 mg/kg i.v.).

Function of non-NMDA receptors and NMDA receptors in synaptic responses to natural somatosensory stimulation in the ventrobasal thalamus

Sensory synaptic responses of rat ventrobasal thalamus neurones were challenged with iontophoretic applications of the excitatory amino acid antagonists CNQX and CPP. CNQX, applied with currents which were selective for non-NMDA receptors, antagonised responses of VB neurones to both 10 ms and 2000 ms air jet stimulation of the peripheral receptive field. In contrast, CPP only antagonised the latter type of response. These results suggest a differential involvement of excitatory amino acid receptors in sensory synaptic transmission to the ventrobasal thalamus, with an initial synaptic component being mediated by non-NMDA receptors (including kainate receptors), and a further NMDA receptor-mediated component being manifested upon maintained sensory stimulation. The expression of this latter component appears to be largely dependent upon the integrity of the non-NMDA receptor-mediated component.

Modulation of NMDA receptor-mediated responses by glycine and d-serine in the rat thalamus in vivo

The effects of iontophoretically applied glycine and d-serine upon responses of rat ventrobasal thalamus neurones to excitatory amino acids and sensory stimulation were investigated. Glycine inhibited excitatory responses to kainate, quisqualate and N-methylaspartate (NMA). Strychnine antagonised inhibitory responses to glycine, but in only a small number ( 2 7 ) of cases did such an antagonism reveal a facilitatory action of glycine on NMA responses. d-Serine inhibited responses of almost all neurones to either kainate or quisqualate, but enhanced responses to NMA on nearly half of the tested neurones, whilst having a weaker inhibitory action on the remainder. It is thus possible that d-serine has a dual action: (i) a facilitation of N- methyl- d-aspartate (NMDA) receptor-mediated responses and (ii) a non-selective inhibitory action.

Gamma-aminobutyric acid and afferent inhibition in the cat and rat ventrobasal thalamus

Extracellular single neuron recordings were made in the ventrobasal thalami of anaesthetized rats and cats. Physiological stimulation of vibrissa and hair follicle afferents was performed with an air jet (10–20 ms duration) directed at a single vibrissa or small area of hairy skin. Paired conditioning and test air jets delivered to the excitatory portion of receptive fields revealed inhibition of the response of ventrobasal thalamic neurons to test stimuli following the excitatory response to the conditioning stimulus. Such inhibitions could last up to 500 ms. An increase in neuronal excitability was sometimes observed following this inhibitory period. In addition, it was possible to produce inhibition without an excitatory response using conditioning stimuli delivered adjacent to the excitatory receptive field. Iontophoretic application of bicuculline methochloride, with currents that were adequate to antagonize iontophoretically applied GABA, was found to reduce the inhibition of test responses evoked by conditioning stimuli in almost all of the neurons studied. In most cases, no excitatory responses to conditioning stimuli directed outside the original excitatory receptive field were revealed by application of the GABA antagonist. In rats, bicuculline also led to a decrease in the post-inhibitory excitation, whereas in cats the converse appeared to be the case. These results suggest that GABAergic transmission may underlie inhibitory responses of cat and rat ventrobasal thalamus neurons to physiological stimulation of somatosensory afferents. Furthermore, removal of such inhibition does not appear to reveal excitatory inputs from outside of the original excitatory receptive field.

Modulatory effects of serotonin on excitatory amino acid responses and sensory synaptic transmission in the ventrobasal thalamus

Excitatory amino acid receptors are thought to mediate sensory input to the ventrobasal thalamus. There is evidence for a brainstem serotonergic projection to the ventrobasal thalamus which may have a modulatory role. The possibility that serotonin may selectively modulate responses to excitatory amino acid receptor agonists and its effects on sensory synaptic transmission has been examined in the rat ventrobasal thalamus in vivo. Iontophoretic ejection of serotonin at low currents produced a marked facilitation of responses to excitatory amino acids. In contrast, excitatory responses to cholinomimetic agonists were attenuated. Synaptic transmission was concomitantly enhanced or unchanged in these circumstances. Higher serotonin ejection currents reversed the facilitation, or inhibited excitatory amino acid responses and synaptic transmission. It is concluded that serotonin can modulate responses to excitatory amino acids relatively selectively and that synaptic transmission of somatosensory information through the ventrobasal thalamus may be susceptible to brainstem serotonergic modulation.

Antagonism of N-methylaspartate and synaptic responses of neurones in the rat ventrobasal thalamus by ketamine and MK-801.

1. Extracellular single neurone recordings were made in the ventrobasal thalamus of urethane-anaesthetized rats. 2. Iontophoretically and intravenously administered ketamine and MK-801 were found to be selective antagonists of responses of neurones to iontophoretically applied N-methylaspartate. 3. Both antagonists, administered in N-methylaspartate-selective quantities, reduced the synaptic responses of ventrobasal thalamus neurones to a two-second air jet directed at the peripheral receptive field.

Chronic sensory deprivation affects cytochrome oxidase staining and glutamic acid decar☐ylase immunoreactivity in adult rat ventrobasal thalamus

The rodent ventrobasal thalamus (VB) contains groups of vibrissa-related neurons (barreloids) that are highly reactive for the enzyme cytochrome oxidase. The present experiments show that each barreloid also contains a dense accumulation of glutamic acid decar☐ylase (GAD) immunoreactive terminals. Chronic vibrissa trimming results in parallel declines in staining for both cytochrome oxidase (CO) and GAD in barreloids associated with the trimmed hairs. Thus, thalamic metabolism like that in the cortex is dependent upon normal sensory input. This includes projection neurons as well as neurons in the reticular nucleus, which are the major source of γ-aminobutyric acid (GABA)ergic input to the rat VB.