Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance

Effects of Axonal Demyelination, Inflammatory Cytokines and Divalent Cation Chelators on Thalamic HCN Channels and Oscillatory Bursting

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that is characterized by the progressive loss of oligodendrocytes and myelin and is associated with thalamic dysfunction. Cuprizone (CPZ)-induced general demyelination in rodents is a valuable model for studying different aspects of MS pathology. CPZ feeding is associated with the altered distribution and expression of different ion channels along neuronal somata and axons. However, it is largely unknown whether the copper chelator CPZ directly influences ion channels. Therefore, we assessed the effects of different divalent cations (copper; zinc) and trace metal chelators (EDTA; Tricine; the water-soluble derivative of CPZ, BiMPi) on hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that are major mediators of thalamic function and pathology. In addition, alterations of HCN channels induced by CPZ treatment and MS-related proinflammatory cytokines (IL-1β; IL-6; INF-α; INF-β) were characterized in C57Bl/6J mice. Thus, the hyperpolarization-activated inward current (I_h) was recorded in thalamocortical (TC) neurons and heterologous expression systems (mHCN2 expressing HEK cells; hHCN4 expressing oocytes). A number of electrophysiological characteristics of I_h (potential of half-maximal activation (V_0.5); current density; activation kinetics) were unchanged following the extracellular application of trace metals and divalent cation chelators to native neurons, cell cultures or oocytes. Mice were fed a diet containing 0.2% CPZ for 35 days, resulting in general demyelination in the brain. Withdrawal of CPZ from the diet resulted in rapid remyelination, the effects of which were assessed at three time points after stopping CPZ feeding (Day1, Day7, Day25). In TC neurons, I_h was decreased on Day1 and Day25 and revealed a transient increased availability on Day7. In addition, we challenged naive TC neurons with INF-α and IL-1β. It was found that I_h parameters were differentially altered by the application of the two cytokines to thalamic cells, while IL-1β increased the availability of HCN channels (depolarized V_0.5; increased current density) and the excitability of TC neurons (depolarized resting membrane potential (RMP); increased the number of action potentials (APs); produced a larger voltage sag; promoted higher input resistance; increased the number of burst spikes; hyperpolarized the AP threshold), INF-α mediated contrary effects. The effect of cytokine modulation on thalamic bursting was further assessed in horizontal slices and a computational model of slow thalamic oscillations. Here, IL-1β and INF-α increased and reduced oscillatory bursting, respectively. We conclude that HCN channels are not directly modulated by trace metals and divalent cation chelators but are subject to modulation by different MS-related cytokines.

Cerebral hierarchies: predictive processing, precision and the pulvinar

This paper considers neuronal architectures from a computational perspective and asks what aspects of neuroanatomy and neurophysiology can be disclosed by the nature of neuronal computations? In particular, we extend current formulations of the brain as an organ of inference—based upon hierarchical predictive coding—and consider how these inferences are orchestrated. In other words, what would the brain require to dynamically coordinate and contextualize its message passing to optimize its computational goals? The answer that emerges rests on the delicate (modulatory) gain control of neuronal populations that select and coordinate (prediction error) signals that ascend cortical hierarchies. This is important because it speaks to a hierarchical anatomy of extrinsic (between region) connections that form two distinct classes, namely a class of driving (first-order) connections that are concerned with encoding the content of neuronal representations and a class of modulatory (second-order) connections that establish context—in the form of the salience or precision ascribed to content. We explore the implications of this distinction from a formal perspective (using simulations of feature–ground segregation) and consider the neurobiological substrates of the ensuing precision-engineered dynamics, with a special focus on the pulvinar and attention.

Zn2+ enhances the intrinsic bursting activity of a rat thalamic relay neuron

Although zinc ion (Zn2+) reduced the low-threshold T-type Ca2+ current of a rat thalamic relay neuron (TRN), we observed that Zn2+ increased a bursting activity of TRN by altering the generation and maintenance of low-threshold spike (LTS). Interestingly and importantly, Zn2+ shifted dramatically the voltage-dependence of both steady-state inactivation and activation of the transient A-type K+ current (I(A)) to a depolarizing direction. As I(A) is one of the main factors in shaping thalamic LTS, such alterations of gating properties of I(A) would contribute to the enhancement of TRN excitability under Zn2+.

Contribution of extracellular negatively charged residues to ATP action and zinc modulation of rat P2X2 receptors

Two histidines are known to be essential for zinc potentiation of rat P2X2 receptors, but the chemistry of zinc coordination would suggest that other residues also participate in this zinc-binding site. There is also a second lower affinity zinc-binding site in P2X2 receptors whose constituents are unknown. To assess whether the extracellular acidic residues of the P2X2 receptor contribute to zinc potentiation or inhibition, site-directed mutagenesis was used to produce alanine substitutions at each extracellular glutamate or aspartate. Two electrode voltage clamp recordings from Xenopus oocytes indicated that 7 of the 34 mutants (D82A, E85A, E91A, E115A, D136A, D209A, and D281A) were deficient in zinc potentiation and one mutant (E84A) was deficient in zinc inhibition. Additional tests on cysteine mutants at these eight positions indicated that D136 is the only residue that is a strong candidate to be at the potentiating zinc-binding site, and that E84 is unlikely to be at the inhibitory zinc-binding site.

Zn2+ Slows Down CaV3.3 Gating Kinetics: Implications for Thalamocortical Activity

We employed whole cell patch-clamp recordings to establish the effect of Zn2+ on the gating the brain specific, T-type channel isoform CaV3.3 expressed in HEK-293 cells. Zn2+ (300 μM) modified the gating kinetics of this channel without influencing its steady-state properties. When inward Ca2+ currents were elicited by step depolarizations at voltages above the threshold for channel opening, current inactivation was significantly slowed down while current activation was moderately affected. In addition, Zn2+ slowed down channel deactivation but channel recovery from inactivation was only modestly changed. Zn2+ also decreased whole cell Ca2+ permeability to 45% of control values. In the presence of Zn2+, Ca2+ currents evoked by mock action potentials were more persistent than in its absence. Furthermore, computer simulation of action potential generation in thalamic reticular cells performed to model the gating effect of Zn2+ on T-type channels (while leaving the kinetic parameters of voltage-gated Na+ and K+ unchanged) revealed that Zn2+ increased the frequency and the duration of burst firing, which is known to depend on T-type channel activity. In line with this finding, we discovered that chelation of endogenous Zn2+ decreased the frequency of occurrence of ictal-like epileptiform discharges in rat thalamocortical slices perfused with medium containing the convulsant 4-aminopyridine (50 μM). These data demonstrate that Zn2+ modulates CaV3.3 channel gating thus leading to increased neuronal excitability. We also propose that endogenous Zn2+ may have a role in controlling thalamocortical oscillations.

Selective block of the human 2-P domain potassium channel, TASK-3, and the native leak potassium current, IKSO, by zinc

Background potassium channels control the resting membrane potential of neurones and regulate their excitability. Two-pore-domain potassium (2-PK) channels have been shown to underlie a number of such neuronal background currents. Currents through human TASK-1, TASK-2 and TASK-3 channels expressed in Xenopus oocytes were inhibited by extracellular acidification. For TASK-3, mutation of histidine 98 to aspartate or alanine considerably reduced this effect of pH. Zinc was found to be a selective blocker of TASK-3 with virtually no effect on TASK-1 or TASK-2. Zinc had an IC(50) of 19.8 microM for TASK-3, at +80 mV, with little voltage dependence associated with this inhibition. TASK-3 H98A had a much reduced sensitivity to zinc suggesting this site is important for zinc block. Surprisingly, TASK-1 also has histidine in position 98 but is insensitive to zinc block. TASK-3 and TASK-1 differ at position 70 with glutamate for TASK-3 and lysine for TASK-1. TASK-3 E70K also had a much reduced sensitivity to zinc while the corresponding reverse mutation in TASK-1, K70E, induced zinc sensitivity. A TASK-3-TASK-1 concatamer channel was comparatively zinc insensitive. For TASK-3, it is concluded that positions E70 and H98 are both critical for zinc block. The native cerebellar granule neurone (CGN) leak current, IK(SO), is sensitive to block by zinc, with current reduced to 0.58 of control values in the presence of 100 microM zinc. This suggests that TASK-3 channels underlie a major component of IK(SO). It has recently been suggested that zinc is released from inhibitory synapses onto CGNs. Therefore it is possible that inhibition of IK(SO) in cerebellar granule cells by synaptically released zinc may have important physiological consequences.

Abnormal benzodiazepine and zinc modulation of GABAA receptors in an acquired absence epilepsy model

Brain cholesterol synthesis inhibition (CSI) at a young age in rats has been shown to be a faithful model of acquired absence epilepsy, a devastating condition for which few therapies or models exist. We employed the CSI model to study cellular mechanisms of acquired absence epilepsy in Long-Evans Hooded rats. Patch-clamp, whole-cell recordings were compared from neurons acutely dissociated from the nucleus reticularis of thalamus (nRt) treated and untreated with a cholesterol synthesis inhibitor, U18666A. In U18666A-treated animals, 91% of rats developed EEG spike-waves (SWs). Patchclamp results revealed that although there was no remarkable change in GABAA receptor affinity, both a loss of ability of benzodiazepines to enhance GABAA-receptor responses and an increase of Zn2+ inhibition of GABAA-receptor responses of nRt neurons occurred in Long-Evans Hooded rats previously administered U18666A. This change was specific, since no significant changes were found in neurons exposed to the GABA allosteric modulator, pentobarbital. Taken collectively, these findings provide evidence for abnormalities in benzodiazepine and Zn2+ modulation of GABAA receptors in the CSI model, and suggest that decreased gamma2 subunit expression may underlie important aspects of generation of thalamocortical SWs in atypical absence seizures. The present results are also consistent with recent findings that mutation of the gamma2 subunit of the GABAA receptor changes benzodiazepine modulation in families with generalized epilepsy syndromes.

Endogenous Zinc Inhibits GABAAReceptors in a Hippocampal Pathway

Depending on their subunit composition, GABAAreceptors can be highly sensitive to Zn2+. Although a pathological role for Zn2+-mediated inhibition of GABAAreceptors has been postulated, no direct evidence exists that endogenous Zn2+can modulate GABAergic signaling in the brain. A possible explanation is that Zn2+is mainly localized to a subset of glutamatergic synapses. Hippocampal mossy fibers are unusual in that they are glutamatergic but have also been reported to contain GABA and Zn2+. Here, we show, using combined Timm's method and post-embedding immunogold, that the same mossy fiber varicosities can contain both GABA and Zn2+. Chelating Zn2+with either calcium-saturated EDTA or N,N,N′ ,N′-tetrakis (2-pyridylmethyl)ethylenediamine had no effect on stratum-radiatum-evoked inhibitory postsynaptic currents (IPSCs), but enhanced IPSCs evoked by stimuli designed to recruit dentate granule cells. We also show that IPSCs recorded in CA3 pyramidal neurons in acute hippocampal slices are depressed by exogenous Zn2+. This depression was of similar amplitude whether the IPSCs were evoked by stimulation in s. radiatum (to recruit local interneurons) or in the s. granulosum of the dentate gyrus (to recruit mossy fibers). These results show for the first time that GABAergic IPSCs can be modulated by endogenous Zn2+and are consistent with GABA release at Zn2+-containing mossy fiber synapses.

Zinc reduces low-threshold Ca2+ currents of rat thalamic relay neurons

Whole-cell voltage-clamping of thalamic relay neurons was used to examine the possibility of a Zn(2+)-mediated reduction of the low-threshold transient Ca(2+) current (I(T)) of a central element in thalamocortical oscillations. We found that Zn(2+) reversibly decreased I(T) in a concentration-dependent manner (IC(50)=55 microM), mainly by reducing the number of I(T) channels available for activation. Zn(2+) did not affect the reaction kinetics, but did affect the voltage-dependence of I(T) channel gating. However, the apparent alterations in gating properties were not enough to account for the huge I(T) reduction.

Lack of vesicular zinc in mossy fibers does not affect synaptic excitability of CA3 pyramidal cells in zinc transporter 3 knockout mice

Zinc is found throughout the CNS in synaptic vesicles of glutamatergic neurons and has been suggested to have a modulatory role in the brain because of its interaction with voltage- and ligand-gated ion channels. We took advantage of zinc transporter 3 knockout mice, which lack vesicular zinc, to study the possible physiological role of this heavy metal in hippocampal mossy fiber neurotransmission. We examined postsynaptic responses evoked by mossy fiber activation, recorded in CA3 pyramidal cells in hippocampal slices prepared from zinc transporter 3 knockout and wild-type mice. Field-potential response threshold and amplitude, input-output curves, and paired-pulse evoked responses were the same in slices from zinc transporter 3 knockout and wild-type mice. Furthermore, neither amplitude nor duration of pharmacologically isolated N-methyl-D-aspartate, non-N-methyl-D-aspartate, GABA(A), and GABA(B) receptor-mediated postsynaptic potentials differed between zinc transporter 3 knockout and wild-type mice. There was no difference in the magnitude of epileptiform discharges evoked by repetitive stimulation or kainic acid application. However, in slices from zinc transporter 3 knockout mice, there was greater attenuation of GABA(A)-mediated inhibitory postsynaptic potentials during tetanic stimulation compared with slices from wild-type animals. We conclude that lack of vesicular zinc in mossy fibers does not significantly affect the mossy fiber-associated synaptic excitability of CA3 pyramidal cells; however, zinc may modulate GABAergic synaptic transmission under conditions of intensive activation.

A histidine residue in the extracellular N-terminal domain of the GABA(A) receptor alpha5 subunit regulates sensitivity to inhibition by zinc

The divalent cation zinc is abundant in the brain, particularly in the mossy fibers of the hippocampus. Recent evidence suggests that zinc is packaged into some synaptic vesicles in this region and can be co-released with neurotransmitter. Zinc inhibits the activity of GABA(A) receptors and the sensitivity of the receptor to zinc is influenced by its alpha subunit subtype composition. The alpha4, alpha5 and alpha6 subunits confer greater sensitivity to zinc than receptors containing other alpha subunits. The alpha4 and alpha5 subunits are highly expressed in hippocampal neurons, and likely mediate any effects of zinc on GABAergic neurotransmission in this area. The alpha5 subunit contains a unique histidine residue in the N-terminal extracellular domain while the other alpha subunits have an aspartate residue in this location. Point mutations were created to exchange the histidine and aspartate residues of the alpha1 and alpha5 subunits. Receptors containing the mutated alpha5((H195D)) subunit had reduced sensitivity to zinc, while alpha1((D191H))beta3gamma2L receptors had increased sensitivity to zinc, similar to the alpha5beta3gamma2L wild type receptors. These findings indicate that histidine195 of the alpha5 subunit plays an important role in determining the sensitivity of recombinant GABA(A) receptors to zinc.

Anticonvulsant actions of lamotrigine on spontaneous thalamocortical rhythms

PURPOSE

This study examined the actions of lamotrigine (LTG) on epileptiform discharges resembling generalized absence (GA) and primary generalized tonic-clonic (GTC) seizures in rat thalamocortical (TC) brain slices and attempted to characterize further the cellular mechanisms of action of LTG on neuronal ionic conductances.

METHODS

Rat TC slices generated spontaneous generalized epileptiform discharges after perfusion with a medium containing no added Mg(2+). Using multiple channel extracellular field-potential recordings in thalamus and cortex, the effects of LTG were characterized on two principal variants of activity that are similar to spike-wave discharges (SWDs) of GA epilepsy and GTC seizure discharges. These were termed simple TC burst complexes (sTBCs) and complex TC burst complexes (cTBCs), respectively. With whole-cell patch-clamp recording techniques in acutely dissociated TC neurons, the effects of LTG on GABA (gamma-aminobutyric acid)(A)-receptor-mediated currents and the low-threshold calcium current (I(T)) were examined.

RESULTS

In field-potential recording studies in TC slices, both sTBCs and cTBCs were blocked by clinically relevant concentrations of LTG. In patch-clamp recording studies, LTG was found to be ineffective in the modulation of both GABA(A) receptors (GABARs) and I(T) in TC neurons.

CONCLUSIONS

The efficacy of LTG on both variants of epileptiform discharges in TC slices clearly parallels its broad human clinical spectrum of action. This demonstrates that neurons within the TC system constitute one probable therapeutic target of LTG. However, LTG did not block either GABAR-mediated responses or I(T) in TC neurons. Modulation of these conductances represent likely cellular mechanisms of action of other antiepileptic drugs effective in the control of GA epilepsy. This suggests that LTG may have as yet uncharacterized effects that could combine with its previously defined sodium channel-blocking actions to explain its clinical utility in the control GA seizures.

Kinetic and pharmacological properties of GABA(A) receptors in single thalamic neurons and GABA(A) subunit expression

Synaptic inhibition in the thalamus plays critical roles in sensory processing and thalamocortical rhythm generation. To determine kinetic, pharmacological, and structural properties of thalamic gamma-aminobutyric acid type A (GABA(A)) receptors, we used patch-clamp techniques and single-cell reverse transcriptase polymerase chain reaction (RT-PCR) in neurons from two principal rat thalamic nuclei-the reticular nucleus (nRt) and the ventrobasal (VB) complex. Single-channel recordings identified GABA(A) channels with densities threefold higher in VB than nRt neurons, and with mean open time fourfold longer for nRt than VB [14.6 +/- 2.5 vs. 3.8 +/- 0.7 (SE) ms, respectively]. GABA(A) receptors in nRt and VB cells were pharmacologically distinct. Zn(2+) (100 microM) reduced GABA(A) channel activity in VB and nRt by 84 and 24%, respectively. Clonazepam (100 nM) increased inhibitory postsynaptic current (IPSC) decay time constants in nRt (from 44.3 to 77.9 ms, P < 0.01) but not in VB. Single-cell RT-PCR revealed subunit heterogeneity between nRt and VB cells. VB neurons expressed alpha1-alpha3, alpha5, beta1-3, gamma2-3, and delta, while nRt cells expressed alpha3, alpha5, gamma2-3, and delta. Both cell types expressed more subunits than needed for a single receptor type, suggesting the possibility of GABA(A) receptor heterogeneity within individual thalamic neurons. beta subunits were not detected in nRt cells, which is consistent with very low levels reported in previous in situ hybridization studies but inconsistent with the expected dependence of functional GABA(A) receptors on beta subunits. Different single-channel open times likely underlie distinct IPSC decay time constants in VB and nRt cells. While we can make no conclusion regarding beta subunits, our findings do support alpha subunits, possibly alpha1 versus alpha3, as structural determinants of channel deactivation kinetics and clonazepam sensitivity. As the gamma2 and delta subunits previously implicated in Zn(2+) sensitivity are both expressed in each cell type, the observed differential Zn(2+) actions at VB versus nRt GABA(A) receptors may involve other subunit differences.