Alterations in GABAA Receptor Mediated Inhibition in Adjacent Dorsal Midline Thalamic Nuclei in a Rat Model of Chronic Limbic Epilepsy

Two contrasting mediodorsal thalamic circuits target the mouse medial prefrontal cortex

At the heart of the prefrontal network is the mediodorsal (MD) thalamus. Despite the importance of MD in a broad range of behaviors and neuropsychiatric disorders, little is known about the physiology of neurons in MD. We injected the retrograde tracer cholera toxin subunit B (CTB) into the medial prefrontal cortex (mPFC) of adult wild-type mice. We prepared acute brain slices and used current clamp electrophysiology to measure and compare the intrinsic properties of the neurons in MD that project to mPFC (MD→mPFC neurons). We show that MD→mPFC neurons are located predominantly in the medial (MD-M) and lateral (MD-L) subnuclei of MD. MD-L→mPFC neurons had shorter membrane time constants and lower membrane resistance than MD-M→mPFC neurons. Relatively increased hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity in MD-L neurons accounted for the difference in membrane resistance. MD-L neurons had a higher rheobase that resulted in less readily generated action potentials compared with MD-M→mPFC neurons. In both cell types, HCN channels supported generation of burst spiking. Increased HCN channel activity in MD-L neurons results in larger after-hyperpolarization potentials compared with MD-M neurons. These data demonstrate that the two populations of MD→mPFC neurons have divergent physiologies and support a differential role in thalamocortical information processing and potentially behavior.NEW & NOTEWORTHY To realize the potential of circuit-based therapies for psychiatric disorders that localize to the prefrontal network, we need to understand the properties of the populations of neurons that make up this network. The mediodorsal (MD) thalamus has garnered attention for its roles in executive functioning and social/emotional behaviors mediated, at least in part, by its projections to the medial prefrontal cortex (mPFC). Here, we identify and compare the physiology of the projection neurons in the two MD subnuclei that provide ascending inputs to mPFC in mice. Differences in intrinsic excitability between the two populations of neurons suggest that neuromodulation strategies targeting the prefrontal thalamocortical network will have differential effects on these two streams of thalamic input to mPFC.

Inhibition of T-Type calcium channels in mEC layer II stellate neurons reduces neuronal hyperexcitability associated with epilepsy

Temporal lobe epilepsy (TLE) is a form of adult epilepsy involving the entorhinal cortex (EC). Layer II neurons of the medial EC (mEC) are spared and become hyperexcitable in TLE. Studies have suggested a role for T-type calcium channels (T-type Ca²⁺ channels) in facilitating increases in neuronal activity associated with TLE within the hippocampus. We sought to determine if T-type Ca²⁺ channels play a role in facilitating neuronal hyperexcitability of layer II mEC stellate neurons in TLE. TLE was induced in rats by electrical stimulation of the hippocampus to induce status epilepticus (SE). Brain slices were prepared from rats exhibiting spontaneous seizures and compared with age-matched control rats. Action potentials (APs) were evoked either by current injection steps or via presynaptic stimulation of mEC deep layers. The selective T-type Ca²⁺ channel antagonist, TTA-P2 (1 μM), was applied to determine the role of T-type Ca²⁺ channels in maintaining neuronal excitability. Quantitative PCR techniques were used to assess T-type Ca²⁺ channel isoform mRNA levels within the mEC layer II. TLE mEC layer II stellate neurons were hyperexcitable compared to control neurons, evoking a higher frequency of APs and generating bursts of APs when synaptically stimulated. TTA-P2 (1 μM) reduced firing frequencies in TLE and control neurons and reduced AP burst firing in TLE stellate neurons. TTA-P2 had little effect on synaptically evoked AP's in control neurons. TTA-P2 also inhibited rebound APs evoked in TLE neurons to a greater degree than in control neurons. TLE tissue had almost a 3-fold increase in Ca_v3.1 mRNA compared to controls. Ca_v3.2 or Ca_v3.3 levels were unchanged. These findings support a role for T-type Ca²⁺ channel in establishing neuronal hyperexcitability of mEC layer II stellate neurons in TLE. Increased expression of Ca_v3.1 may be important for establishing neuronal hyperexcitability of mEC layer II neurons in TLE.

Epigenetic Suppression of GADs Expression is Involved in Temporal Lobe Epilepsy and Pilocarpine-Induced Mice Epilepsy

Recent studies have shown that histone acetylation is involved with the regulation of enzyme glutamate decarboxylases (GADs), including GAD67 and GAD65. Here, we investigated the histone acetylation modifications of GADs in the pathogenesis of epilepsy and explored the therapeutic effect of a novel second-generation histone deacetylase inhibitor (HDACi) JNJ-26481585 in epilepsy animals. We revealed the suppression of GADs protein and mRNA level, and histone hypoacetylation in patients with temporal lobe epilepsy and pilocarpine-induced epilepsy mice model. Double-immunofluorescence also indicated that the hypoacetyl-H3 was located in hippocampal GAD67/GAD65 positive neurons in epilepsy mice. JNJ-26481585 significantly reversed the decrease of the GAD67/GAD65 both protein and mRNA levels, and the histone hypoacetylation of GABAergic neurons in epilepsy mice. Meanwhile, single-cell real-time PCR performed in GFP-GAD67/GAD65 transgenic mice demonstrated that JNJ-26481585 induced increase of GAD67/GAD65 mRNA level in GABAergic neurons. Furthermore, JNJ-26481585 significantly alleviated the epileptic seizures in mice model. Together, our findings demonstrate inhibition of GADs gene via histone acetylation plays an important role in the pathgenesis of epilepsy, and suggest JNJ-26481585 as a promising therapeutic strategy for epilepsy.

GABAergic transmission in temporal lobe epilepsy: the role of neurosteroids

Modification of GABAergic inhibition is an intensely investigated hypothesis guiding research into mechanisms underlying temporal lobe epilepsy (TLE). Seizures can be initiated by blocking γ amino butyric acid type A (GABAA receptors, GABARs), which mediate fast synaptic inhibition in the brain, and controlled by drugs that enhance their function. Derivatives of steroid hormones called neurosteroids are natural substances that physiologically enhance GABAR function and suppress seizures. GABAR structure, function, expression, assembly, and pharmacological properties are changed in the hippocampus of epileptic animals. These alterations render GABARs less sensitive to neurosteroid modulation, which may contribute to seizure susceptibility. Plasticity of GABARs could play a role in periodic exacerbation of seizures experienced by women with epilepsy, commonly referred to as catamenial epilepsy.

Somatostatin type-2 receptor activation inhibits glutamate release and prevents status epilepticus

Newer therapies are needed for the treatment of status epilepticus (SE) refractory to benzodiazepines. Enhanced glutamatergic neurotransmission leads to SE, and AMPA receptors are modified during SE. Reducing glutamate release during SE is a potential approach to terminate SE. The neuropeptide somatostatin (SST) is proposed to diminish presynaptic glutamate release by activating SST type-2 receptors (SST2R). SST exerts an anticonvulsant action in some experimental models of seizures. Here, we investigated the mechanism of action of SST on excitatory synaptic transmission at the Schaffer collateral-CA1 synapses and the ability of SST to treat SE in rats using patch-clamp electrophysiology and video-EEG monitoring of seizures. SST reduced action potential-dependent EPSCs (sEPSCs) at Schaffer collateral-CA1 synapses at concentrations up to 1μM; higher concentrations had no effect or increased the sEPSC frequency. SST also prevented paired-pulse facilitation of evoked EPSCs and did not alter action-potential-independent miniature EPSCs (mEPSCs). The effect of SST on EPSCs was inhibited by the SST2R antagonist cyanamid-154806 and was mimicked by the SST2R agonists, octreotide and lanreotide. Both SST and octreotide reduced the firing rate of CA1 pyramidal neurons. Intraventricular administration of SST, within a range of doses, either prevented or attenuated pilocarpine-induced SE or delayed the median time to the first grade 5 seizure by 11min. Similarly, octreotide or lanreotide prevented or attenuated SE in more than 65% of animals. Compared to the pilocarpine model, octreotide was highly potent in preventing or attenuating continuous hippocampal stimulation-induced SE in all animals within 60min of SE onset. Our results demonstrate that SST, through the activation of SST2Rs, diminishes presynaptic glutamate release and attenuates SE.

Prolonged cannabinoid exposure alters GABA(A) receptor mediated synaptic function in cultured hippocampal neurons

Developing cannabinoid-based medication along with marijuana's recreational use makes it important to investigate molecular adaptations the endocannabinoid system undergoes following prolonged use and withdrawal. Repeated cannabinoid administration results in development of tolerance and produces withdrawal symptoms that may include seizures. Here we employed electrophysiological and immunochemical techniques to investigate the effects of prolonged CB1 receptor agonist exposure on cultured hippocampal neurons. Approximately 60% of CB1 receptors colocalize to GABAergic terminals in hippocampal cultures. Prolonged treatment with the cannabinamimetic WIN 55,212-2 (+WIN, 1 μM, 24 h) caused profound CB1 receptor downregulation accompanied by neuronal hyperexcitability. Furthermore, prolonged +WIN treatment resulted in increased GABA release as indicated by increased mIPSC frequency, a diminished GABAergic inhibition as indicated by reduction in mIPSC amplitude and a reduction in GABA(A) channel number. Additionally, surface staining for the GABA(A) β(2/3) receptor subunits was decreased, while no changes in staining for the presynaptic vesicular GABA transporter were observed, indicating that GABAergic terminals remained intact. These findings demonstrate that agonist-induced downregulation of the CB1 receptor in hippocampal cultures results in neuronal hyperexcitability that may be attributed, in part, to alterations in both presynaptic GABA release mechanisms and postsynaptic GABA(A) receptor function demonstrating a novel role for cannabinoid-dependent presynaptic control of neuronal transmission.

Mediodorsal thalamic stimulation is not protective against seizures induced by amygdaloid kindling in rats

Deep brain stimulation (DBS) is now emerging as a new option for treating intractable epilepsy. Cumulative studies suggest that the mediodorsal thalamic nucleus (MD) is involved in limbic seizure activity. This study aims to investigate whether DBS of the MD can protect against seizures induced by amygdaloid kindling. We studied the effect of low-frequency stimulation (LFS, 1 Hz) or high-frequency stimulation (HFS, 100 Hz) in the MD on amygdaloid kindling seizures. During the kindling acquisition, DBS in the MD was daily administered immediately after the kindling stimulus or before the kindling stimulus (preemptive DBS). The effects of both post-treatment of DBS and preemptive DBS in the MD on the expression of amygdaloid kindling seizures were evaluated. We found the DBS or preemptive DBS in the MD, either LFS or HFS, did not significantly change the rate of amygdaloid kindling. Similarly, DBS or preemptive DBS in the MD did not significantly change any parameters representing the expression of amygdaloid kindling. Our study suggests that DBS in the MD may have no significant effect on limbic seizures.

Nitric oxide alters GABAergic synaptic transmission in cultured hippocampal neurons

Nitric oxide (NO) production increases during hypoxia/ischemia-reperfusion in the immature brain and is associated with neurotoxicity. NO at physiologic concentrations has been shown to modulate GABAergic (gamma-aminobutyric acid) synaptic transmission in the adult brain. However, the effects of neurotoxic concentrations of NO (relevant to hypoxia-ischemia) on GABAergic synaptic transmission remain unknown. The present study tests the hypothesis that nNOS is expressed at GABAergic synapses and that exposure to neurotoxic concentrations of NO results in enhanced GABAergic synaptic transmission in cultured hippocampal neurons (days-in-vitro 10-14) prepared from fetal rats. Using double immunocytochemistry techniques, we were able to demonstrate that nNOS is co-localized to both presynaptic and postsynaptic markers of GABAergic synapses. The effects of NO on GABAergic synaptic transmission were then studied using whole cell patch-clamp electrophysiology. Spontaneous and miniature inhibitory postsynaptic currents (sIPSCS and mIPSCs) were recorded prior to and after exposure to 250 microM of the NO donor diethyleneamine/nitric oxide adduct (DETA-NO). Exposure to DETA-NO resulted in increased sIPSCs and mIPSCs frequency, indicating that neurotoxic concentrations of NO enhance GABAergic synaptic transmission in cultured hippocampal neurons. Because GABA synapses appear to be excitatory in the immature brain, this effect may contribute to overall enhanced synaptic transmission and hyperexcitability. We speculate that NO represents one of the mechanisms by which hypoxia-ischemia increases seizure susceptibility in the immature brain.

Different patterns of neuronal activation and neurodegeneration in the thalamus and cortex of epilepsy-resistant Proechimys rats versus Wistar rats after pilocarpine-induced protracted seizures

PURPOSE

To analyze cellular mechanisms of limbic-seizure suppression, the response to pilocarpine-induced seizures was investigated in cortex and thalamus, comparing epilepsy-resistant rats Proechimys guyannensis with Wistar rats.

METHODS

Fos immunoreactivity revealing neuronal activation, and degenerating neurons labeled by Fluoro-Jade B (FJB) histochemistry were analyzed on the first day after onset of seizures lasting 3 h. Subpopulations of gamma-aminobutyric acid (GABA)ergic cells were characterized with double Fos-parvalbumin immunohistochemistry.

RESULTS

In both cortex and thalamus, degenerating neurons were much fewer in Proechimys than Wistar rats. Fos persisted at high levels at 24 h only in the Proechimys thalamus and cortex, especially in layer VI where corticothalamic neurons reside. In the parietal cortex, about 50% of parvalbumin-containing interneurons at 8 h, and 10-20% at 24 h, were Fos-positive in Wistar rats, but in Proechimys, Fos was expressed in almost all parvalbumin-containing interneurons at 8 h and dropped at 24 h. Fos positivity in cingulate cortex interneurons was similar in both species. In the Wistar rat thalamus, Fos was induced in medial and midline nuclei up to 8 h, when <30% of reticular nucleus cells were Fos-positive, and then decreased, with no relationship with cell loss, evaluated in Nissl-stained sections. In Proechimys, almost all reticular nucleus neurons were Fos-positive at 24 h.

DISCUSSION

At variance with laboratory rats, pilocarpine-induced protracted seizures elicit in Proechimys limited neuronal death, and marked and long-lasting Fos induction in excitatory and inhibitory cortical and thalamic cell subsets. The findings implicate intrathalamic and intracortical regulation, and circuits linking thalamus and cortex in limbic seizure suppression leading to epilepsy resistance.

Altered pharmacology and GABA-A receptor subunit expression in dorsal midline thalamic neurons in limbic epilepsy

The mediodorsal (MD) and paraventricular (PV) thalamic nuclei play a significant role in limbic epilepsy, and previous reports have shown changes in GABA-A receptor (GABAAR) mediated synaptic function. In this study, we examined changes in the pharmacology of GABAergic drugs and the expression of the GABAAR subunits in the MD and PV neurons in epilepsy. We observed nucleus specific changes in the sensitivity of sIPSCs to zolpidem and phenobarbital in MD and PV neurons from epileptic animals. In contrast, the magnitude of change in electrically evoked response (eIPSC) to zolpidem and phenobarbital were uniformly diminished in both MD and PV neurons in epilepsy. Immunohistochemical studies revealed that in epilepsy, there was a reduction in GAD65 expression and NeuN positive neurons in the MD neurons. Also, there was a decrease in immunoreactivity of the alpha1 and beta2/3 subunit of GABAARs, but not the gamma2 of the GABAAR in both MD and PV in epilepsy. These findings demonstrate significant alterations in the pharmacology of GABA and GABAARs in a key region for seizure generation, which may have implications for the physiology and pharmacology of limbic epilepsy.