Alterations of GABA(A) and glutamate receptor subunits and heat shock protein in rat hippocampus following traumatic brain injury and in posttraumatic epilepsy

Are alterations in transmitter receptor and ion channel expression responsible for epilepsies?

Neuronal voltage-gated ion channels and ligand-gated synaptic receptors play a critical role in maintaining the delicate balance between neuronal excitation and inhibition within neuronal networks in the brain. Changes in expression of voltage-gated ion channels, in particular sodium, hyperpolarization-activated cyclic nucleotide-gated (HCN) and calcium channels, and ligand-gated synaptic receptors, in particular GABA and glutamate receptors, have been reported in many types of both genetic and acquired epilepsies, in animal models and in humans. In this chapter we review these and discuss the potential pathogenic role they may play in the epilepsies.

Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability

RATIONALE

Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids.

OBJECTIVE

This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior.

CONCLUSION

The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.

Acutely increasing δGABA(A) receptor activity impairs memory and inhibits synaptic plasticity in the hippocampus

Extrasynaptic γ-aminobutyric acid type A (GABA_A) receptors that contain the δ subunit (δGABA_A receptors) are expressed in several brain regions including the dentate gyrus (DG) and CA1 subfields of the hippocampus. Drugs that increase δGABA_A receptor activity have been proposed as treatments for a variety of disorders including insomnia, epilepsy and chronic pain. Also, long-term pretreatment with the δGABA_A receptor–preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) enhances discrimination memory and increases neurogenesis in the DG. Despite the potential therapeutic benefits of such treatments, the effects of acutely increasing δGABA_A receptor activity on memory behaviors remain unknown. Here, we studied the effects of THIP (4 mg/kg, i.p.) on memory performance in wild-type (WT) and δGABA_A receptor null mutant (Gabrd^−/−) mice. Additionally, the effects of THIP on long-term potentiation (LTP), a molecular correlate of memory, were studied within the DG and CA1 subfields of the hippocampus using electrophysiological recordings of field potentials in hippocampal slices. The results showed that THIP impaired performance in the Morris water maze, contextual fear conditioning and object recognition tasks in WT mice but not Gabrd^−/− mice. Furthermore, THIP inhibited LTP in hippocampal slices from WT but not Gabrd^−/− mice, an effect that was blocked by GABA_A receptor antagonist bicuculline. Thus, acutely increasing δGABA_A receptor activity impairs memory behaviors and inhibits synaptic plasticity. These results have important implications for the development of therapies aimed at increasing δGABA_A receptor activity.

γ-aminobutyric acid type A receptors that contain the δ subunit promote memory and neurogenesis in the dentate gyrus

OBJECTIVE

Extrasynaptic γ-aminobutyric acid type A receptors that contain the δ subunit (δGABAA receptors) are highly expressed in the dentate gyrus (DG) subfield of the hippocampus, where they generate a tonic conductance that regulates neuronal activity. GABAA receptor-dependent signaling regulates memory and also facilitates postnatal neurogenesis in the adult DG; however, the role of the δGABAA receptors in these processes is unclear. Accordingly, we sought to determine whether δGABAA receptors regulate memory behaviors, as well as neurogenesis in the DG.

METHODS

Memory and neurogenesis were studied in wild-type (WT) mice and transgenic mice that lacked δGABAA receptors (Gabrd(-/-)). To pharmacologically increase δGABAA receptor activity, mice were treated with the δGABAA receptor-preferring agonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP). Behavioral assays including recognition memory, contextual discrimination, and fear extinction were used. Neurogenesis was studied by measuring the proliferation, survival, migration, maturation, and dendritic complexity of adult-born neurons in the DG.

RESULTS

Gabrd(-/-) mice exhibited impaired recognition memory and contextual discrimination relative to WT mice. Fear extinction was also impaired in Gabrd(-/-) mice, although the acquisition of fear memory was enhanced. Neurogenesis was disrupted in Gabrd(-/-) mice as the migration, maturation, and dendritic development of adult-born neurons were impaired. Long-term treatment with THIP facilitated learning and neurogenesis in WT but not Gabrd(-/-) mice.

INTERPRETATION

δGABAA receptors promote the performance of certain DG-dependent memory behaviors and facilitate neurogenesis. Furthermore, δGABAA receptors can be pharmacologically targeted to enhance these processes.

Rapamycin attenuates the development of posttraumatic epilepsy in a mouse model of traumatic brain injury

Posttraumatic epilepsy is a major source of disability following traumatic brain injury (TBI) and a common cause of medically-intractable epilepsy. Previous attempts to prevent the development of posttraumatic epilepsy with treatments administered immediately following TBI have failed. Recently, the mammalian target of rapamycin complex 1 (mTORC1) pathway has been implicated in mechanisms of epileptogenesis and the mTORC1 inhibitor, rapamycin, has been proposed to have antiepileptogenic effects in preventing some types of epilepsy. In this study, we have tested the hypothesis that rapamycin has antiepileptogenic actions in preventing the development of posttraumatic epilepsy in an animal model of TBI. A detailed characterization of posttraumatic epilepsy in the mouse controlled cortical impact model was first performed using continuous video-EEG monitoring for 16 weeks following TBI. Controlled cortical impact injury caused immediate hyperactivation of the mTORC1 pathway lasting at least one week, which was reversed by rapamycin treatment. Rapamycin decreased neuronal degeneration and mossy fiber sprouting, although the effect on mossy fiber sprouting was reversible after stopping rapamycin and did not directly correlate with inhibition of epileptogenesis. Most posttraumatic seizures occurred greater than 10 weeks after TBI, and rapamycin treatment for one month after TBI decreased the seizure frequency and rate of developing posttraumatic epilepsy during the entire 16 week monitoring session. These results suggest that rapamycin may represent a rational treatment for preventing posttraumatic epilepsy in patients with TBI.

Myo-inositol treatment and GABA-A receptor subunit changes after kainate-induced status epilepticus

Identification of compounds preventing the biochemical changes that underlie the epileptogenesis process is of great importance. We have previously shown that myo-Inositol (MI) daily treatment prevents certain biochemical changes that are triggered by kainic acid (KA)-induced status epilepticus (SE). The aim of the current work was to study the further influence of MI treatment on the biochemical changes of epileptogenesis and focus on changes in the hippocampus and neocortex of rats for the following GABA-A receptor subunits: α1, α4, γ2, and δ. After SE, one group of rats was treated with saline, while the second group was treated with MI. Control groups that were not treated by the convulsant received either saline or MI administration. 28-30 h after the experiment, a decrease in the amount of the α1 subunit was revealed in the hippocampus and MI had no significant influence on it. On the 28th day of the experiment, the amount of α1 was increased in both the KA- and KA + MI-treated groups. The α4 and γ2 subunits were strongly reduced in the hippocampus of KA-treated animals, but MI significantly halted this reduction. The effects of MI on α4 and γ2 subunit changes were significantly different between hippocampus and neocortex. On the twenty-eighth day after SE, a decrease in the amount of α1 was found in the neocortex, but MI treatment had no effect on it. The obtained results indicate that MI treatment interferes with some of the biochemical processes of epileptogenesis.

GABA(A) receptor regulation after experimental traumatic brain injury

The gamma-aminobutyric acid (GABA) type A receptor (GABA(A)R) is responsible for most fast synaptic inhibition in the adult brain. The GABA(A)R protein is composed of multiple subunits that determine the distribution, properties, and dynamics of the receptor. Several studies have shown that the Janus kinase/signal transducer and activator of transcription (JaK/STAT) and early growth response 3 (Egr3) signaling pathways can alter GABA(A)R subunit expression after status epilepticus (SE). In this study we investigated changes in these pathways after experimental TBI in the rat using a lateral fluid percussion injury (FPI) model. Our results demonstrated changes in the expression of several GABA(A)R subunit levels after injury, including GABA(A)R α1 and α4 subunits. This change appears to be transcriptional, and there is an associated increase in the phosphorylation of STAT3, and an increase in the expression of Egr3 and inducible cAMP element repressor (ICER) after FPI. These findings suggest that the activation of the JaK/STAT and Egr3 pathways after TBI may regulate injury-related changes in GABA(A)R subunit expression.

Clinical Relevance of Biomarkers for Traumatic Brain Injury

Approximately 1.4 million people in the United States sustain a traumatic brain injury (TBI) each year, resulting in more than 235 000 hospitalizations and 50 000 deaths. An estimated 5.3 million Americans have current long-term disabilities as a result of TBI, which results in an estimated $60 billion in healthcare expenditures. Mild TBI (mTBI), which accounts for 80% to 90% of all cases, is the most prevalent form of brain injury in athletes. Many of these traumas still remain undetected, as they are difficult to diagnose. New biomarkers of TBI may allow more rapid diagnosis of TBI, improving early identification and treatment, and could help to predict clinical outcome. The field of TBI biomarkers is rapidly evolving. This chapter will discuss some of the most clinically relevant biomarkers for TBI that have been recently studied in human subjects.

Aestivation and hypoxia-related events share common silent neuron trafficking processes

Background

The availability of oxygen is a limiting factor for neuronal survival since low levels account not only for the impairment of physiological activities such as sleep-wake cycle, but above all for ischemic-like neurodegenerative disorders. In an attempt to improve our knowledge concerning the type of molecular mechanisms operating during stressful states like those of hypoxic conditions, attention was focused on eventual transcriptional alterations of some key AMPAergic silent neuronal receptor subtypes (GluR1 and GluR2) along with HSPs and HIF-1α during either a normoxic or a hypoxic aestivation of a typical aquatic aestivator, i.e. the lungfish (Protopterus annectens).

Results

The identification of partial nucleotide fragments codifying for both AMPA receptor subtypes in Protopterus annectens displayed a putative high degree of similarity to that of not only fish but also to those of amphibians, birds and mammals. qPCR and in situ hybridization supplied a very high (p < 0.001) reduction of GluR1 mRNA expression in diencephalic areas after 6 months of aerial normoxic aestivation (6mAE). Concomitantly, high (p < 0.01) levels of HSP70 mRNAs in hypothalamic, mesencephalic and cerebellar areas of both 6mAE and after 6 months of mud hypoxic aestivation (6mMUD) were detected together with evident apoptotic signals. Surprisingly, very high levels of GluR2 mRNAs were instead detected in thalamic along with mesencephalic areas after 6 days of normoxic (6dAE) and hypoxic (6dMUD) aestivation. Moreover, even short- and long-term hypoxic states featured high levels of HIF-1α and HSP27 transcripts in the different brain regions of the lungfish.

Conclusions

The distinct transcriptional variations of silent neurons expressing GluR1/2 and HSPs tend to corroborate these factors as determining elements for the physiological success of normoxic and hypoxic aestivation. A distinct switching among these AMPA receptor subtypes during aestivation highlights new potential adaptive strategies operating in key brain regions of the lungfish in relation to oxygen availability. This functional relationship might have therapeutic bearings for hypoxia-related dysfunctions, above all in view of recently identified silent neuron-dependent motor activity ameliorations in mammals.

Protection from glutamate-induced excitotoxicity by memantine

This study investigates whether the uncompetitive N-methyl-D-aspartic acid receptor antagonist, memantine, is able to protect dissociated cortical neurons from glutamate-induced excitotoxicity (GIE). Treatment with glutamate resulted in a significant loss of synchronization of neuronal activity as well as a significant increase in the duration of synchronized bursting events (SBEs). By administering memantine at the same time as glutamate, we were able to completely prevent these changes to the neuronal activity. Pretreatment with memantine was somewhat effective in preventing changes to the culture synchronization but was unable to fully protect the synchronization of electrical activity between neurons that showed high levels of synchronization prior to injury. Additionally, memantine pretreatment was unable to prevent the increase in the duration of SBEs caused by GIE. Thus, the timing of memantine treatment is important for conferring neuroprotection against glutamate-induced neurotoxicity. Finally, we found that GIE leads to a significant increase in the burst duration. Our data suggest that this may be due to an alteration in the inhibitory function of the neurons.