Seizure-induced expression of G proteins in the rat hippocampus

The antiepileptogenic effect of low-frequency stimulation on perforant path kindling involves changes in regulators of G-protein signaling in rat

G-protein coupled receptors may have a role in mediating the antiepileptogenic effect of low-frequency stimulation (LFS) on kindling acquisition. This effect is accompanied by changes at the intracellular level of cAMP. In the present study, the effect of rolipram as a phosphodiesterase inhibitor on the antiepileptogenic effect of LFS was investigated. Meanwhile, the expression of α_s- and α_i-subunit of G proteins and regulators of G-protein signaling (RGS) proteins following LFS application was measured. Male Wistar rats were kindled by perforant path stimulation in a semi-rapid kindling manner (12 stimulations per day) during a period of 6days. Application of LFS (0.1ms pulse duration at 1Hz, 200 pulses, 50-150μA, 5min after termination of daily kindling stimulations) to the perforant path retarded the kindling development and prevented the kindling-induced potentiation and kindling-induced changes in paired pulse indices in the dentate gyrus. Intra-cerebroventricular microinjection of rolipram (0.25μM) partially prevented these LFS effects. Twenty-four hours after the last kindling stimulation, the dentate gyrus was removed and changes in protein expression were measured by Western blotting. There was no significant difference in the expression of α-subunit of G_s and G_i/o proteins in different experimental groups. However, application of LFS during the kindling procedure decreased the expression RGS4 and RGS10 proteins (that reduce the activity of G_i/o) and prevented the kindling-induced decrease of RGS2 protein (which reduces the G_s activity). Therefore, it can be postulated that the G_i/o protein signaling pathways may be involved in antiepileptogenetic effect of LFS, and this is why decreasing the cAMP metabolism by rolipram attenuates this effect of LFS.

Do certain signal transduction mechanisms explain the comorbidity of epilepsy and mood disorders?

It is well known that mood disorders are highly prevalent in patients with epilepsy. Although several studies have aimed to characterize alterations in different types of receptors associated with both disturbances, there is a lack of studies focused on identifying the causes of this comorbidity. Here, we described some changes at the biochemical level involving serotonin, dopamine, and γ-aminobutyric acid (GABA) receptors as well as signal transduction mechanisms that may explain the coexistence of both epilepsy and mood disorders. Finally, the identification of common pathophysiological mechanisms associated with receptor-receptor interaction (heterodimers) could allow designing new strategies for treatment of patients with epilepsy and comorbid mood disorders.

A pentylenetetrazole-induced generalized seizure in early life enhances the efficacy of muscarinic receptor coupling to G-protein in hippocampus and neocortex of adult rat

We have previously shown that exposure to the anti-cholinesterase eserine provokes interictal-like discharges in the CA3 area of hippocampal slices from adult rats in which a generalized seizure has been induced by pentylenetetrazole (PTZ) when immature (at 20 days). Such increased responsiveness to acetylcholine (ACh) was not associated with any change in hippocampal acetylcholine or gamma-aminobutyric acid (GABA) content, GABAergic inhibition or density of ACh innervation, but was blocked by the muscarinic receptor antagonist atropine. We therefore turned to quantitative radioligand binding autoradiography, in situ hybridization and the [35S]GTPgammaS method to assess the properties of hippocampal and neocortical muscarinic receptors in adult rats having experienced a PTZ seizure at P20. The densities of M1 and M2 receptor binding sites, respectively labeled with [3H]pirenzepine and [3H]AFDX-384, as well as the amount of m1, m2 and m3 receptor mRNAs, did not differ from control in the hippocampus and neocortex of these rats. In contrast, in PTZ rats, both brain regions displayed a marked increase in [35S]GTPgammaS incorporation stimulated by ACh, bethanechol and particularly oxotremorine. This finding indicates that a generalized seizure in immature rat can entail a long-term and presumably permanent increase in the efficacy of G-protein coupling to muscarinic receptors in the hippocampus and neocortex of the adult. By analogy, such a mechanism could account for the susceptibility to epilepsy of human adults having suffered from prolonged convulsions in early life.

The loss of circadian PAR bZip transcription factors results in epilepsy

DBP (albumin D-site-binding protein), HLF (hepatic leukemia factor), and TEF (thyrotroph embryonic factor) are the three members of the PAR bZip (proline and acidic amino acid-rich basic leucine zipper) transcription factor family. All three of these transcriptional regulatory proteins accumulate with robust circadian rhythms in tissues with high amplitudes of clock gene expression, such as the suprachiasmatic nucleus (SCN) and the liver. However, they are expressed at nearly invariable levels in most brain regions, in which clock gene expression only cycles with low amplitude. Here we show that mice deficient for all three PAR bZip proteins are highly susceptible to generalized spontaneous and audiogenic epilepsies that frequently are lethal. Transcriptome profiling revealed pyridoxal kinase (Pdxk) as a target gene of PAR bZip proteins in both liver and brain. Pyridoxal kinase converts vitamin B6 derivatives into pyridoxal phosphate (PLP), the coenzyme of many enzymes involved in amino acid and neurotransmitter metabolism. PAR bZip-deficient mice show decreased brain levels of PLP, serotonin, and dopamine, and such changes have previously been reported to cause epilepsies in other systems. Hence, the expression of some clock-controlled genes, such as Pdxk, may have to remain within narrow limits in the brain. This could explain why the circadian oscillator has evolved to generate only low-amplitude cycles in most brain regions.

Identification of activity-regulated proteins in the postsynaptic density fraction

BACKGROUND

The postsynaptic density (PSD) at synapses is a specialized submembranous structure where neurotransmitter receptors are linked to cytoskeleton and signalling molecules. Activity-dependent dynamic change in the components of the PSD is a mechanism of synaptic plasticity. Identification of the PSD proteins and examination of their modulations dependent on synaptic activity will be valuable for an understanding of the molecular basis of learning and memory.

RESULT

We attempted here to identify proteins in the PSD fraction by two-dimensional (2D) gel electrophoresis and mass spectrometry. About 1.7 x 103 protein spots were detected on 2D gels. A total of 90 spots were identified, containing 47 different protein species. In addition to previously identified PSD proteins such as PSD-95/SAP90, several new proteins were identified in the PSD fraction. They included stomatin-like protein 2 and NIPSNAP1. We also examined activity-dependent modulations of PSD proteins by 2D gel electrophoresis. The spot concentration of G protein beta subunit 5 and NIPSNAP1 increased 2 h after kainate treatment that caused generalized seizures.

CONCLUSION

These results indicate that the combination of 2D gel electrophoresis and mass spectrometry is an excellent tool for the identification of activity-regulated PSD proteins.

Functional significance of stimulatory GTP-binding protein in hippocampus is associated with kindling-elicited epileptogenesis

In order to evaluate the involvement of the stimulatory G-protein (Gs)-related transduction system in the basic mechanisms of epilepsy, we examine the expression levels of Gsalpha mRNA and specific GTP-binding ability in the hippocampus of amygdaloid-kindled rats at various seizure stages. Northern blot analysis showed a significant increase in the Gsalpha mRNA expression level in the bilateral hippocampus at 24h after the last generalized seizure. The [3H]-GTP-binding assay with isoproterenol (IPN), a beta-receptor agonist, revealed a remarkable increase of Bmax values in the sham-operated control and partially kindled groups. However, the IPN-induced increase of Bmax values was abolished on both sides of the hippocampus at 24 h after and at 4 weeks after the last generalized seizure in fully kindled rats. These data suggest that alteration in the Gs function and beta-adrenergic receptor-Gs coupling might be implicated in the neurobiological basis of the induction mechanisms of the generalization of seizures and the mechanisms of the maintenance of enduring epileptogenesis. Conversely, the Gs-related transduction system might have a lesser impact on the acquisition process of epileptogenesis.

Altered expression levels of G protein subclass mRNAs in various seizure stages of the kindling model

The expressions of mRNAs encoding G protein alpha subunits were analyzed in the cerebral cortex of amygdaloid kindled rats. A remarkable increase in Gsalpha mRNA were observed on the bilateral cerebral cortex at 24 h after the last generalized seizure and persisted 3 weeks on the unstimulated side. Gi2alpha mRNA level was also increased on the stimulated side at 24 h and persisted 3 weeks. These result suggest that dysfunction of Gs and Gi2 might relate to the basic mechanisms of seizure generation and the maintenance of epileptogenesis.

The effect of acute kainic acid treatment on mu-opioid receptors in rat brain

There is evidence that acute exposure to kainic acid (KA) induces the release of endogenous ligands for opioid receptors and that mu-opioid agonists intensify KA-induced neurodegeneration. The aim of the present study was to investigate any acute toxic effects of KA upon mu-opioid receptors labelled with [3H]-DAMGO. 200-250 g rats were injected intraperitoneally with either saline or 16 mg/kg KA and brains were removed after 4 h. Membrane homogenates were prepared from the cerebellum, cortex, hippocampus, medulla and pons, midbrain and hypothalamus and striatum and in separate studies, from whole brain. In addition, frozen coronal sections were processed for comparative quantitative autoradiography. KA produced a two-fold increase in receptor affinity for [3H]-DAMGO in all regions and significant increases in receptor number in cortex, medulla and pons and striatum. Quantitative autoradiography showed similar significantly increased mu-labelling of structures comprising these gross anatomical regions. The findings demonstrate region specific changes in rat brain mu-opioid receptors after acute KA treatment which may be functionally related to the convulsant effects of this excitotoxin.

Chronic changes in synaptic responses of entorhinal and hippocampal neurons after amino-oxyacetic acid (AOAA)-induced entorhinal cortical neuron loss

Chronic changes in synaptic responses of entorhinal and hippocampal neurons after amino-oxyacetic acid (AOAA)-induced entorhinal neuron loss. J. Neurophysiol. 80: 3031-3046, 1998. Synaptic responses of entorhinal cortical and hippocampal neurons were examined in vivo and in vitro, 1 mo to 1.5 yr after a unilateral entorhinal lesion caused by a focal injection of amino-oxyacetic acid (AOAA). It has been shown previously that injection of AOAA into the medial entorhinal cortex produces cell loss in layer III preferentially. Although behavioral seizures stopped approximately 2 h after AOAA treatment, abnormal evoked responses were recorded as long as 1.5 yr later in the entorhinal cortex and hippocampus. In the majority of slices from AOAA-treated rats, responses recorded in the superficial layers of the medial entorhinal cortex to white matter, presubiculum, or parasubiculum stimulation were abnormal. Extracellularly recorded responses to white matter stimulation were prolonged and repetitive in the superficial layers. Intracellular recordings showed that residual principal cells in superficial layers produced prolonged, repetitive excitatory postsynaptic potentials (EPSPs) and discharges in response to white matter stimulation compared with brief EPSPs and a single discharge in controls. Responses of deep layer neurons of AOAA-treated rats did not differ from controls in their initial synaptic response. However, in a some of these neurons, additional periods of excitatory activity occurred after a delay. Abnormal responses were recorded from slices ipsilateral as well as contralateral to the lesioned hemisphere. Recordings from the entorhinal cortex in vivo were abnormal also, as demonstrated by prolonged and repetitive responses to stimulation of the area CA1/subiculum border. Evoked responses of hippocampal neurons, recorded in vitro or in vivo, demonstrated abnormalities in selected pathways, such as responses of CA3 neurons to hilar stimulation in vitro. There was a deficit in the duration of potentiation of CA1 population spikes in response to repetitive CA3 stimulation in AOAA-treated rats. Theta activity was reduced in amplitude in area CA1 and the dentate gyrus of AOAA-treated rats, although evoked responses to angular bundle stimulation could not be distinguished from controls. The results demonstrate that a preferential lesion of layer III of the entorhinal cortex produces a long-lasting change in evoked and spontaneous activity in parts of the entorhinal cortex and hippocampus. Given the similarity of the lesion produced by AOAA and entorhinal lesions in temporal lobe epileptics, these data support the hypothesis that preferential damage to the entorhinal cortex contributes to long-lasting changes in excitability, which could be relevant to the etiology of temporal lobe epilepsy.

Evidence for Fos involvement in the regulation of proenkephalin and prodynorphin gene expression in the rat hippocampus

For a long time Fos has been proposed to play some role in regulation of the proenkephalin (PENK) and prodynorphin (PDYN) gene expression. In recent years, however, evidence has accumulated that the transcription of both genes in several brain regions in vivo is transactivated by the transcription factor CREB rather than by Fos. In the present study, involvement of Fos in the mechanism of the PENK and PDYN gene induction in the hippocampal dentate gyrus during seizures elicited by kainic acid was studied using a knock-down technique. Pretreatment with an antisense oligonucleotide complementary to c-fos mRNA did not influence the kainic acid-elicited convulsions. It inhibited, by about 50%, the induction of Fos protein in the dentate gyrus during seizures. The subsequent induction of PENK and PDYN mRNAs was reduced by more than 60% by the c-fos antisense oligonucleotide, while constitutive expression of three other genes (alpha-tubulin, NMDA receptor-1, and GS protein alpha-subunit) was not affected. The obtained results support the view that Fos may be involved in regulation of the PENK and PDYN gene expression in the dentate gyrus during seizures, which further suggests that the mechanisms triggering the up-regulation of both these genes in the dentate gyrus may differ from these working in other brain regions, such as the striatum and hypothalamus.